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TRANSCRIPT
COYNEPRACTICAL APPLIED
ELECTRICITY
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COYNEPRACTICAL APPLIEDELECTRICITY
A Set of Complete Practical Books
For Home Study
and
Field Reference
On
Electrical principles, telephones, wiring, meters,
D.C. and A.C. motors, controls, and equip-ment, household appliance repair, Rural Elec-
trification, Armature winding, Generators,Diesel Electric Plants, Automotive Electricity,Batteries, Electrical Refrigeration and AirConditioning, Industrial Electronics, RadioElectric Welding - laws, rules, etc. - Over3,000 subjects, 5,000 Electrical facts, thou-sands of photos and diagrams.
By
THE TECHNICAL STAFF
of the
COYNE ELECTRICAL SCHOOL
Copyright 1945 byCOYNE ELECTRICAL SCHOOL
500 So. Paulina St.Chicago, Illinois
All rights reserved. This book, or any parts thereof may not be reproduced in any formwithout written permission of the Publishers.
Printed in United States of America
FOREWORD
ELECTRICITY is the greatest force known tomankind.
ELECTRICITY is leaping ahead at an unbeliev-able rate. It is moving into practically every partof the world and has become the most importantfactor in modern civilization. Practically everymonth the Kilowatt hour demand sets a new high-it has been doing this for the past 25 years.ELECTRICITY, even though it is one of Amer-ica's youngest industries, already employs directlyand indirectly over 3 million people.
All of our marvelous developments in Radio, Tele-vision, Electronics, Radar, etc., employ electricalpower and the principles of. Electricity. It is truly"one of the world's greatest industries.
Because of the tremendous opportunity in thefield of Electricity, there have been many bookswritten on the subject. Most treat with one specificphase of Electricity. This set of books - CoynePractical Applied Electricity (of which this yolumeyou now read is an integral part)-covers the en-tire field.
This set is NEW. It includes the very latestmethods and explanations of Electrical installation,operation and maintenance.
COYNE PRACTICAL APPLIEDELECTRICITY WRITTEN BY A STAFF OF
EXPERTSMost Electrical publications are written by one
man and can therefore only cover his own specificknowledge of a subject. COYNE PRACTICALAPPLIED ELECTRICITY, however, represents
FOREWORD
the combined efforts of the entire Coyne ElectricalSchool Teaching Staff and the assistance of otherauthorities on the subject. These men have a widefield and teaching experience and practical knowl-edge in electricity and its allied branches.
HOW THIS SET WAS DEVELOPEDIn submitting any material for these books these
experts kept two things in mind-i. MAKE ITSIMPLE ENOUGH FOR THE "BEGINNER" -2. MAKE IT COMPLETE, PRACTICAL andVALUABLE FOR THE "OLD TIMER". Allmaterial that was submitted for these books by anyindividual was then rewritten by an editorial groupso that added explanations for' the benefit of clar-ity and easier understanding could be included.
Coyne Practical Applied Electricity can pay youbig dividends every day "on the job". However, ifyou only use the set occasionally when you MUSTBE SURE before going ahead on a job-the setwill pay for itself many times over.
Coyne Practical Applied Electricity is to anelectrician what a set of complete law books is toa lawyer or a set of medical books is to a doctor.Regardless of whether a lawyer or a doctor is "juststarting out" or is an "old timer" and has beenpracticing his profession for many years he hasmany occasions to refer to his reference books.Many doctors and lawyers spend thousands of dol-lars on complete sets of reference books-they findit a very wise investment.
In ELECTRICITY the need for good referencebooks is just as great. So, when you make a pur-chase of this set you are not just buying a set ofbooks-you are making an investment in your fu-ture that can pay dividends all your life.
COYNE ELECTRICAL SCHOOL
CONTENTSD -C Power
Field frames & poles-armatures-speeds-drives - brushes - commutators - bear-ings - questions.
1 - 26
GeneratorsBuilding up voltage-voltage drop-types-shunts -.compound types - operation -paralleling - starting - testing - adjust-ing & equalizing - questions.
27 - 72
3 Wire Systems 73 - 83
Balancing-principle of operation-motorgenerator & balancer sets -3 wire gener-ators-effects of shunt and series field onbalancer generator-questions.
Commutation - Interpoles - Brushes 84 - 116Process - shifting brushes with varyingload-adjustment-changing the air gap-polarity of interpoles - carbon brushes -materials-requirements-special types ofbrushes - pressure & tension - experi-ments - questions.
DC Switchboards 117 - 144Types-feeder panels-frames-switches-circuit breakers - coils - bus bars -wiring - questions.
Meters 145 - 198Types - voltmeters & ammeters - damp-ing - care & adjUstment -.connections-wattmeters - compensating coils - read-ing - recording instruments - creeping -Wheatstone bridge - load demand indi-cators - meggers - questions.
DC Motors, Starters and Controllers 199 - 320Operation - rotation - torque - types -speeds & Hp - ratings - counter EMF -prony brake test - efficiency tests - Hpcalculation - rheostats - methods of con-trolling speed - construction features ofstarters & controllers - 3 & 4 pointstarters - release coils - remote control -magnetic starters 1- overload protection -reversing - hoist controller -regenerativebraking -questions.
Maintenance of DC Machines 321 - 365Oiling - cleaning - mica undercutting-generator troubles & remedies - use ofmeters, test lamps, buzzers & magnets inmaintenance - tools needed-schedule forefficient maintenance-questions.
ACKNOWLEDGMENTSWe wish to acknowledge and express our appre-ciation for the assistance and co-operation givenby the following companies, in supplying data andillustrations for the preparation of this ElectricalSet.
GENERAL ELECTRIC COMPANYWESTINGHOUSE ELECTRIC & MFG. CO.ALLIS CHALMERS MFG. CO.POWER PLANT ENGINEERING JOURNALAMERICAN BROWN ROVERI CO.CUTLER HAMMER, INC.PHILADELPHIA ELECTRIC CO.EDISON STORAGE BATTERY CO.PHILADELPHIA BATTERY CO.WALTER BATES STEEL CORP.FAIRBANKS MORSE CO.HOSKINS MFG. CO.ALLEN -BRADLEY CO.DELTA STAR MFG. CO.NATIONAL CARBON CO.CENTRAL SCIENTIFIC CO.OHIO BRASS CO.GRAYBAR ELECTRIC CO.WELSH SCIENTIFIC CO.CENTRAL SCIENTIFIC CO.
You will note that in some places in this Set we have explainedand shown illustrations of some of the earlier types of Elec-trical equipment.WE HAVE A DEFINITE REASON FOR DOING THIS,namely, many of the earlier units are much easier to un-derstand. An important point to keep in mind is that theBASIC PRINCIPLES of these earlier machines are the same asthose of the modern equipment of today.Modern equipment has not materially changed in principle-IT IS MERELY REFINED AND MODERNIZED. It is fromthe earlier basic theories and simple beginnings that the com-p.icated mechanisms of today have been developed. IT IS TOTHESE EARLY BEGINNINGS WE MUST OFTEN TURN INORDER TO GET A FULL UNDERSTANDING OF THEPRESENT ADVANCED TYPES OF EQUIPMENT.In the early days many of the parts and mechanism of Elec-trical equipment were visible whereas today much of it is not.However, the PRINCIPLES OF THE EARLY EQUIPMENTARE SIMILAR TO THOSE OF MODERN ELECTRICAL AP-PARATUS.
SO IN VARIOUS PLACES IN THIS SET, WE SHOW YOUSOME OF THIS EARLIER EQUIPMENT BECAUSE ITS CON-STRUCTION IS SIMPLER AND EASIER TO UNDERSTANDAS YOU STUDY THE MODERN EQUIPMENT. THEN FROMTHESE EARLIER TYPES OF EQUIPMENT WE CARRYYOU ON TO THE VERY LATEST DEVELOPMENTS INTHE FIELD.
HOW TO USETHIS SET OF BOOKS
Coyne Practical Applied Electricity will be ofuse and value to you in exact proportion to thetime and energy you spend in studying and using it.
A Reference Set of this kind is used in twodistinct ways.
FIRST, it is used by the fellow who wishes tomake Electricity his future work and uses this Ref-erence Set as a home training course.
SECOND, it is especially valuable to the manwho wishes to use it strictly as a Reference Set.This includes electricians, mechanics or anyoneworking at any trade who wishes to have a set ofbooks so that he can refer to them for information
A
in Electrical problems at any time.
You, of course, know into which group you falland this article will outline how to properly usethis Set to get the most value for your own per-sonal benefit.
How To Use This Set As A Home TrainingCou -e In Electricity
The most important advice I can give the fellowwho wishes to study our set as a home trainingcourse in Electricity is to start from the begin-ning in Volume 1, and continue in order throughthe other 6 volumes. Don't make the mistake ofjumping from one subject to another or taking aportion of one volume and then reverting back toanother. Study the set as it has been written andyou'll get the most out of it.
Volume 1 is one of the most important of theentire Set. Every good course of training must havea good foundation. Our first volume is the founda-tion of our course and is designed to explain insimple language terms and expressions, laws and
How To Use This Set of Books
rules of Electricity, upon which any of the big instal-lations, maintenance and service jobs are based.So, become thoroughly familiar with the subjectscovered in the first volume and you will be able tomaster each additional subject as you proceed.
One of the improvements we made in this setwas to add "review" questions throughout thebooks. You will find these questions in most casesat the end of a chapter. They are provided so "be-ginners" or "old timers" can check their progressand knowledge of particular subjects. Our mainpurpose in including the "review" questions is toprovide the reader with a "yardstick" by which hecan check his knowledge of each subject. This fea-ture is a decided improvement in home study ma-terial.
Above all, do not rush through any part .of thesebooks in order to cover a large amount at one time.You should read them slowly and understand eachsubject before proceeding to the next and in this wayyou will gain a thorough understanding as you readand think it out.
For the special benefit of the fellow desiring tolearn Electricity at home, we have prepared a greatnumber of diagrams and illustrations. Refer to thesepictures and diagrams in our books regularly.
How To Use Coyne Practical Applied ElectricityStrictly As A Reference Set
The man who is interested in using these booksmainly for reference purposes will use it in a littledifferent way than the fellow who is trying to learnElectricity as a trade. Some of the types of fellowswho use this set strictly for reference purposes are:home owners, electricians or mechanics, garageowners or workers, hardware store owners, farmersor anyone who has an occasional use for electricalknowledge. Those types of fellows should use thisset in the following manner.
If some particular type of 'electrical problem pre-sents itself, refer immediately to the Index-it willgive you the section in which the subject is covered.
How To Use This Set of Books
Then, turn to that section and carefully read theinstructions outlined. Also read any other sectionsof the set mentioned in the article. As an example,in checking over some information on electric mo-tors, some reference Might be made to an electricallaw of principles contained in Volume 1 of the Set.In order to thoroughly understand the procedure tofollow in working out the electrical problem, youshould refer to Volume 1 and get a better under-standing of the electrical law on principles involved.
Use The Master Index To LocateElectrical Subjects
Thousands of men use this Set in their dailyproblems, both on the job and around the homeas well. If you follow the instructions outlined youwill be able to locate any information you maywant at any time on your own electrical problems.
And here's a very important point. Althoughthis set of books starts in Volume 1 and proceedsthrough the other 6 volumes in order, it makes anideal home study course-nevertheless, any individ-ual book in the series is independent of the others andcan be studied separately. As an example, Volume3 covers D.C. motors and equipment. If a manwanted to get some information on D.C. machinesonly he could find it completely covered in thisvolume and it would not be essential to refer to anyother volume of the set unless he wanted someadditional information on some other electricalprinciple that would have a bearing on his problem.
This feature is especially beneficial to the "oldtimer" who plans to use the set mainly for fieldreference purposes.
We believe, however, that the entire set of 7 vol-umes should be read completely by both the "begin-ner" or the expert. In this way you get the greatestbenefit from the set. In doing so the experiencedElectrician will be able to get very valuable informa-tion on subjects that he may have thought he wasfamiliar with, but in reality he was not thoroughlyposted on a particular subject.
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DIRECT CURRENT
Direct current energy and machines are very ex-tensively used for traction work and certain classesof industrial power drives.
The principal advantages of D. C. motors aretheir very excellent starting torque and wide rangeof speed control_
D. C. motors are ideal for operating certain classesof machines which are difficult to start under load,and must be driven at varying speeds, or perhapsreversed frequently. Their speed can be varied overa very wide range, both above and below normalspeed.
Many thousands of factories and industrial plantsuse electric motors exclusively for driving variousmachines, and in certain classes of this work D. C.motors are extensively used. They are made insizes from 1/10 h. p. to several thousand horsepower each, and are used both for group drive andindividual drive of various machines.
Fig. 1. This photo shows a group of large D. C. motors in use in asteel mill. Machines of this type, ranging from several hundredto several thousand horsepower each, are used in this work.
Fig. 1 shows an installation of large D. C. motorsin use in a steel mill. These motors are located inthe power room as shown, and are connected to
1
D. C. Power
shafts extending through the wall at the right, todrive the great rolls which roll out the hot steel inthe adjoining room.
LARGE MACHINES LIKE THIS USED TO R0.1.1. OUT STEEL INTOSTRIPS ARE POWERED BY HEAVY OUTY ELECTRICAL-EOUIPMENT.
For operation of street cars and elevated trains incities, and electric railways across the country,series D. C. motors are extensively used, becausetheir great starting torque enabled them to easilystart a loaded car or train from a standing position,and quickly bring it up to very high speeds.
Fig. 3 shows a powerful electric locomotive whichis driven by several electric motors of several hun-dred horse power each.
D. C. motors are commonly made to operate onvoltages of 110, 220, and 440, for industrial service ;and from 250 to 750 volts for railway service.
Elevators in large skyscraper office and storebuildings also use thousands of powerful D. C.motors, to smoothly start the loaded cars andswiftly shoot" them up or down; ten, forty, or 70.stories as desired.
2
Direct Current
Here again their good starting torque, smooth-ness of operation, and accurate control for stoppingexactly at floor levels, make them very desirable.
Fig. 3. Electrical locomotives of the above type often use six or eightpowerful D. C. motors to turn their driving wheels.
One modern type of elevator equipment uses di-rect current motors and xvhat is known as variablevoltage control. The variable voltage for each ele-vator motor supplied by a separate D. C. genera-tor, which is designed to vary its voltage as theload on the car varies. thus providing even speedregulation and extremely smooth starting and stop-ping.
Fig. 4 shows a large I). C. elevator motor withits cable drum and magnetic brake attached on theright hand side.
Because of the extensive use of direct currentfor elevators in large buildings, and for tractionpurposes, some large cities have their central busi-ness districts supplied with I). C.. and the outlyingdistricts where the power must be transmittedfarther are supplied with A. C.
Direct current generators are used to supply thedirect current wherever it is extensively used; andmany privately owned power plants use D. C. gen-erators because of the simplicity of their operationin parallel, where several are used.
In the operation of D. C. generators the speed atwhich they are driven is not as critical as it is with
3
D. C. Power
A. C. generators. Small D. C. generators can bebelt driven ; but this is not practical with A. C.generators, because a slight slip of the belt wouldcause their speed to vary, and make trouble in theirparallel operation
Fig. 4. This photo shows a U. C. elevator motor with the magneticbrake on the right end of the cable drum.
D. C. generators are made in sizes from 60 wattsfor automotive use, up to those of several thousandkilowatts for industrial and railway power plants.Their voltages range from 6 volts on automotivegenerators to 440 volts for industrial purposes ; andon up to 600 and 750 volts for railway work.
The smaller sizes for belt drive operate at speedsfrom 300 to 1800 R. P. M., while the larger sizeswhich are direct connected to steam, oil, or gasengines, run at speeds from 60 to 250 R. P. M.
When these generators are driven by direct shaftconnections to reciprocating steam engines, a largeflywheel is usually provided on the same shaft toproduce a more even speed. It will also deliverpower to the generator during sfiddenly increasedloads, until the engine governor can respond.
4
Direct Current
D. C. generators are not so well adapted for directconnection to steam turbines, because of the veryhigh speeds of the turbines, and the great stressthese speeds would set up in the commutators andwindings of the generators.
When driven by turbines, they are usuallycoupled together by gears. For example a 360 R. P.M. generator can be driven by a 3600 R. P. M. tur-bine. through speed reducing gears with a ratio of10 to 1.
Fig. S. Small engine -driven D. C. generators of the above type areused in a great number of privately owned power plants.
Fig. 5 shows a small D. C. generator driven by avertical steam engine. Note the flywheel used tomaintain even speed and voltage, and also note thecommutator and brushes which are in plain view onthis generator.
Direct current is not much used where the energymust be transmitted over distances more than one-half mile to a mile, as it requires high voltage totransmit large amounts of power over longer dis-
5
D. C. Power
tances ; and it is usually not practical to operatelarge D. C. generators at voltages above 750.
Where large amounts of power are used in a com-pact area, such as in a large factory, in mines, steelmills, or densely built up business section of cities,D. C. finds its greatest use.
Where direct current is desired ftr use at a con-siderable distance from the location of the powerplant, alternating current may be used to transmitthe energy at high voltage, to a substation in whicha motor generator set is used to produce D. C.
Fig. 7 shows a motor generator of this type, con-sisting of an A. C. motor on the left, driving a D. C.generator on the right. In this case the two ma-chines are coupled directly together on the sameshaft.
Other common uses for Direct Current are forelectro-plating. electrolytic metal refining, batterycharging, operation of electro-magnets, farm light-ing plants, and automotive equipment.
D. C. generators for electro-plating and electro-lytic refining, are made to produce low voltages,from 6 to 25 volts, and very heavy current of sev-eral thousand amperes on the larger machines.
Garages use thousands of small motor generators;to produce D. C. for battery charging ; and storesand plants using large fleets of electric trucks,charge their batteries with D. C. from larger charg-ing generators.
Train lighting with the thousands of batteries andgenerators for this work is another extensive fieldfor D. C. equipment.
Many thousands of D. C. farm lighting plants arein use throughout this country, supplying directcurrent at either 32 volts or 110 volts for light andpower on the farms.
Powerful electro-magnets requiring direct cur-rent for their operation, are used by the thousandsto speed up the handling of iron and steel materialsin industrial plants, railway shops, etc. Fig. 8 showsa large magnet of this type which is used for liftingkegs of nails and bolts. This illustration also showshow the magnetism acts through the wooden kegs,
6
Direct Current
proving what we have learned in an earlier lesson-that magnetism cannot be insulated.
STRANGE JOBS
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The automotive field is an enormous user of di-rect current equipment. Each modern automobilehas a complete little power plant of its own, con-sisting of its D. C. generator, series D. C. startingmotor, battery, lights, ignition coil, horn, etc. M'anymillions of D. C. generators and motors are in useon cars and trucks in this country alone.
Many powerful busses also use gas electric drive,having a gasoline engine to drive a D. C. generator,
7
D. C. Power
which in turn supplies current to D. C. motorsgeared to the axles. This form of drive providessmoother starting and stopping, greater hill climb -
Fig. 7. Motor generator sets of the above type are very extensivelyused for changing A. C. to D. C. The D. C. generator is shownon the right and is driven by the A. C. motor on the left.
F.g. 8. Direct current is used to operate powerful electro-magnets ofthe above type for handling metal materials in industrial plants,warehouses, foundries, and iron yards. Note the manner in whichthese kegs of bolts are lifted by the magnet, even though thewooden heads of the kegs are between the magnet and the metalto be lifted. In plants where the principal supply of electricity isalternating current small motor generators are often used to supplythe direct current for magnets of this type.
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Direct Current
ing ability, higher speeds on level roads, and elimin-ates gear shifting. Diesel -Electric trains also useD. C. generators
With this great variety of uses for direct currentand D. C. machines, you ,can readily see the valueof making a thorough study of the equipment andprinciples covered in this lesson. The opportunitiesopen to a trained man are certain to be much greaterif he has a good knowledge of the operation, careand testing of direct current. machines of all com-mon types.
D. C. GENERATORS
It has already been stated in earlier lessons thatD. C. generators and motors are almost exactlyalike in their mechanical construction, and that inmany cases the same machine can be used either asa motor or a generator, with only slight changes inthe field connections, brush adjustment, etc. Thisis a very good point to keep in mind while studyingthe following material, as many of the points cov-ered on construction, operation, load ratings, tem-peratures, etc., will apply to either a motor or a gen-erator.
2. GENERATOR RATINGSD. C. generators are always rated in kilowatts, a
unit of electric power with which you are alreadyfamiliar. It will be yell to recall at this point, how-ever, that one kilowatt is equal to 1000 watts, and
,approximateSly 1.34 h. p. You will also recall thatthe watts or kilowatts consumed in any circuit areequal to the product of the volts and amperes.Therefore. with a machine of any given voltage, thegreater the load in K. NV., the greater will be theload in amperes of current carried by the windingsof that -machine.
The K. \V. rating of a D. C. generator is thepower load that it will carry continuously withoutexcessive heating, sparking, or internal voltage drop.
If a load greater than a machine is designed orrated for is placed upon it for an extended period,
9
D. C. Generators
it will probably give trouble due to one of the threecauses mentioned ; and if the overload is very greatand left on too long it will cause the armature wind-ing to burn out.
Nearly all generators are designed to be able tocarry some overload for short periods without in-jury to the machine. This is usually from 15 to 25per cent, for periods not longer than an hour or so.
3. OPERATING TEMPERATURESThe safe temperature rises in electrical machinery
are determined by the temperatures the insulatingmaterials will withstand without damage. All othermaterials in the machine are metals which may besubjected to quite high temperatures without muchdamage.
Of course the higher the temperature of the cop-per windings the greater their resistance will be,and the higher will be the losses due to voltage dropin the machine.
Ordinary combustible insulations such as silk,cotton, and paper, should never be subjected to tem-peratures higher than 212° F. (or 100° C). Mica,asbestos, and other non-combustible insulations maybe subjected to temperatures as high as 257° F.,or 125° C.
In establishing temperature rise ratings for elec-trical machinery, it is assumed that the temperaturein the rooms where the machines are installed willnever be over 104° F. or 40° C. This gives, for theordinary insulations, a permissable rise of 212 -104, or 108° F. or 60° C. For non-combustible in-sulations the permissable rise is 257 - 104. or 153°F. or 85° C.
Ordinary generators and motors are usually guar-anteed by the manufacturers to operate continu-ously at full load, without exceeding a temperaturerise of 35° C., 40° C., or 50° C., as the case may be.
The temperatures of machines can be checked byplacing small thermometers in between, or clpse to,the ends of their windings. A good general rule toremember, is that if the hand can be held on theframe of the machine near the windings withoutgreat discomfort from the heat, the windings arenot dangerously hot.
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Direct Current
4. GENERATOR SPEEDSThe speeds at which generators are operated de-
pends upon their size, type of design, and method ofdrive. -The speed is of course rated in R. P. M.(revolution per min.) but another expression com-monly used in referring to the rotating armaturesof electrical machines is the Peripheral Speed. Thisrefers to the travelling speed of the outside or cir-cumference of the rotating element, and this sur-face is commonly known as the Periphery. Thisspeed is expressed in feet per second or feet perminute.
The centrifugal force exerted on the armatureconductors or commutator bars depends on theperipheral speed of the armature or commutator.This speed, of course, depends on the R. P. M., andthe diameter of the rotating part.
The larger the armature, the farther one of itsconductors will travel in each revolution. When acoil of a bi-polar, (two pole) machine makes onerevolution, it will have passed through 360 actualor mechanical degrees and 360 electrical degrees.But a coil of a six pole machine will only have torotate 120 mechanical degrees to pass two poles,and through 360 electrical degrees.
So we find that with the same flux per pole in thelarger machine as in the two pole one, the sameE. M. F. can be generated at a much lower speedWith the multipolar machine.
Small generators of two or four poles and forbelt drive, have long armatures of small diameterand may be operated at speeds from 120 to 1800R. P. M. Larger machines for slower speed driveby direct connection to the shafts of low speed re-ciprocating engines. may have as many as 24 ormore field poles, and operate at speeds of 60 to 600R. P. M. Armatures for these lower speed machinesare made shorter in length and much larger in diam-eter, so their conductors cut through the field fluxat high speeds, even though the R. P. M. of thearmature is low.
The peripheral speeds of armatures not only de-termine the voltage induced and the stresses on the
11
D. C. Generators
and commutator bars, but also determine the-ar on brushes and the type o,f brushes needed, as
will be explained later.
5. TYPES OF DRIVESBelt driven generators are not much used in large
plants any more because of possible belt slippage.and the danger of high speed belts. A number ofolder plants and many small ones use belt drivenmachines, and with fairly satisfactory results if theproper belts and pulleys are used.
One advantage of small belt driven generators isthat they can be designed for high speeds; highspeed generators are much smaller in size than lowspeed generators of the same kilowatt capacity ;
therefore, high speed generators are used whereverpossible, because the purchase cost per kilowatt isconsiderably lower than with the low speed type.
NILFig. 10. An early type of D. C. generator developed by Thomas
Edison. Note the construction of the field magnets of this machine.
The engine type generator with the large diam-eter, s,low-soeed armature, direct connected to the
12
Direct Current
engine shaft, is more commonly used. Steam en-gines are a very desirable form of prime mover forgenerators, because of their high efficiency andsimple operation, and because they can be -operatedon the ordinary steam* pressure.
Steam turbines are used to drive D. C. generatorsin plants where space is limited, because they areso small and compact.
Water wheels are used for prime movers whereconvenient water power is available. Generatorsfor water wheel drive may he either low or highspeed type, according to the water pressure andtype of water wheel used.
6. MECHANICAL CONSTRUCTION OF D. C.GENERATORS
We have already learned that a generator is adevice used to convert mechanical energy into elec-trical energy. We also know that the principalparts of a D. C. generator are its field frame, fieldpoles, armature, commutator, brushes, bearings, etc.
The purpose of the field poles is to supply astrong magnetic field or flux, through which thearmature conductors. are rotated to generate thevoltage in them.
D. C. generators were the first type commerciallyused, and the early types were very simply con-structed with two large field poles in the shape of ahuge bipolar electroc-magnet. The armature waslocated between the lower ends of these magnets, asshown in Fig. ba This figure shows one of the.early types of Edison generators of 100 K. W. size.
7. FIELD FRAMESModern generators and motors have their field
poles mounted in a circular frame, as shown in Fig.11. This figure shows a two -pole field frame withthe two large poles mounted on the inside of theframe. The field coils can be plainly seen on thepoles.
The circular frame, in addition to providing asupport for the field poles, also provides a completeclosed path of magnetic material for the flux circuit
13
D. C. Generators
between the pole. For this reason the frames areusually made of soft iron.
For the smaller and medium sized machines, theyare generally cast in one piece with feet or exten-sions for bolting to a base. The inner surface isusually machined smooth where the poles are boltedto it, or in some cases the poles are cast as a part ofthe frame. The ends of the frame are machined toallow the bearing brackets to fit properly.
Fig. 11. Field frame of a modern generator or motor. Field coilslocated on the poles set up powerful magnetic flux in which thearmature rotates.
The frames for larger generators are usually castin two pieces for more convenient handling duringinstallation and repairs. They can be split eitherhorizontally or vertically. Fig. 12 shows a frameof this type for an eight -pole machine. Note wherethe halves of the frame are joined together andbolted at each side.
8. FIELD POLESThere may be any equal number of field poles in
a generator or motor frame, according to its size
14
Direct Current
and speed. These poles are made of soft iron tokeep the magnetic reluctance as low as possible.
The poles can be cast as a part of the frame onsmaller machines, but are usually bolted into thelarger frames. It is very important that they should
Fig. 12. Field frame for an eight -pole D. C. generator. Note themanner in which the frame is built in two sections for conveniencewhen installing and making repairs.
fit tight to the frame to prevent unnecessary airgaps and reluctance in the magnetic circuit.
The ends of the poles which are next to thearmature are usually curved and ,flared out intowhat are called Pole Shoes or Faces. This providesa more even distribution of the field flux over thearmature core and conductors. These pole shoesare generally machined to produce an even air gapbetween them and the armature core.
Pole shoes are often made of laminated strips tokeep down the induced eddy currents from the fluxof the moving armature conductors. These lamin-ated pole shoes are then bolted to the field poles.The machine in Fig. 11 has laminated pole shoes ofthis type.
15
D. C. Generators
In some machines the field poles or sometimes theentire magnetic circuit is laminated to reduce eddycurrents and also to permit the rapid changes infield flux that must take place when the machine isoperating under conditions of highly fluctuatingload. If the field flux cannot change as rapidly asthe armature flux, severe sparking at the brushesmay occur when the load rapidly changes. As fluxchanges can take place much more rapidly in alaminated structure - because the eddy' currentswhich tend to sustain the flux are suppressed, gen-erators and motors designed for loads_ that varysuddenly through wide limits usually have a lam-inated magnetic circuit.
9. ARMATURESWe have already learned a great deal about this
very important part of D. C. machines, as armatureconstruction and winding were thoroughly coveredin the preceding lessons. A few of the points thatare particularly good to keep in mind throughoutthe study of D. C. motors and generators will bebriefly reviewed here.
The function of the armature, we know, is tocarry the rotating conductors in its slots and movethese swiftly through the magnetic flux of the field,in order to generate the voltage in them.
The very soft iron .and steel in armature coresand its excellent magnetic properties also greatlyreduce hysteresis loss. Also remember that thenumber of turns per coil and the method of con-necting these coils will determine the voltage thatis induced in a generator armature, or the counter -voltage in a motor armature.
Armature cores are made of thin laminations ofsoft iron which are partially insulated from eachother either by a thin coating of oxide which isformed on their surface when they are being heattreated or by a thin layer of insulating varnish.This laminated construction prevents to a greatextent the eddy currents which would otherwise beinduced in the core as it revolves through the fieldflux.
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Direct Current
10. COMMUTATORS
A commutator, we already know, is a device usedto rectify or change the alternating E. M. F. whichis induced in the armature, to a direct E. M. F. orcurrent in the external circuit. A commutator might
Fig. 14. The above photo shows an excellent sectional view of acommutator for a D. C. machine.
also be called a sort of rotating switch which quicklyreverses the connections of the armature coils tothe external circuit as these coils pass from one poleto the next.
The manner in which the commutators are con-structed of forged copper bars and insulated fromeach other by mica segments, was covered in a pre-ceding lesson under D. C. armatures.
Figs. 14 and 15 show two excellent views of tom-mutators of slightly different types. The smallerone in Fig. 14 is held together by the ring nutshown on the right, while the larger one is knownas a "bolted type" commutator, and has bolts whichdraw the V -rings tightly into the grooves in thebars.
17
D. C. Generators
11. BRUSHESThe brushes slide on the commutator bars and
deliver the current from a generator winding to theline ; or, in the case of a motor, supply the currentfrom the line to the winding. Most of these brushes
Fig. 15. This view shows another type of commutator in which thebars are held in place by bolts that are used to draw the clampingrings tight.
are made of a mixture of carbon and graphitemolded into blocks of the proper size. While thismaterial is of fairly high resistance, the very shortlength of the brushes dosen't introduce enough re-sistance in the circuit to create much loss. Theproperties of the carbon and graphite tend to keepthe commutator clean and brightly polished as thebrushes slide on its surface. Some resistance in thebrush material is an advantage, as it tends to pre -
18
Direct Current
vent severe sparking during the period the commu-tator bars are short circuited. This will he ex-plained in a later section on brushes.
Brushes must be of the proper size and materialto carry without undue heat the full load currentsof either a generator or motor. The carrying capac-ity of the brushes is a figure generally set by themanufacturers to indicate the number of amperesthe brush will carry per square inch of cross-sec-tional area. This figure takes into account the heatdue to overloads, friction, short. circuit currents inthe coils, voltage drop at the contact, and the heatproduced by sparking.
Fig. 16 shows two common types of generatorbrushes to which are attached Pig Tails of softstranded copper. These copper pig -tails are used formaking a secure connection to carry the currentfrom the brush to the holder and line.
Fig. 16. Two common types of carbon brushes used for D. C. machines.Note the flexible copper leads used for connecting them to thebrush holders.
12. BRUSH HOLDERSBrushes are held firmly in the correct position
with relation to the commutator by placing themin brush holders. The brush holders in commoncommercial use' today may be classed under threegeneral types, called Box Type, Clamp Type, andReaction Type.
19
D. C. Generators
The box type holder was one of the first to bedeveloped and used, while the clamp type has beendeveloped in two forms known as the "swivel" and"parallel" motion types. Fig. 17 shows sketchesof these several types of brush holders. The upperviews in each case show the holders assembled onround studs, while the lower views show thembolted to rectangular studs.
A brush holder, in addition to providing a boxor clamp to hold the brush in place, also has springs
a
a
Fig. 17. The above sketches show several common types of brushholders. At "A" are two views of box -type holders. "B" and "C"are known as clamp -type holders; while "D" is a brush holder ofthe reaction -type.
to hold the brush against the commutator surfaceand under the proper tension. Fig. 18 shows a box -type brush holder and the springs which apply thetension on the brush, and the view on the rightshows this brush holder from the opposite side,mounted in the rocker ring. The requirements ofgood brush holders are as follows:
1. To provide means for carrying the currentfrom the brush to the holder stud, either with aflexible copper connection or by direct contact be-tween the brush and the holder. This must beaccomplished without undue heating or sparking
20
Direct Current
between the brush and holder, as this would resultin a rapid burning and damage to the holders.
2. To provide means for accurately adjustingthe brush on the commutator or ring.
3. To hold the brush firmly at the proper angle.4. To permit free and quick movement of the
brush in order -that it may follow any uneven sur-face of the cor nnutator or ring.
Fig. 18. Above are shown two box -type brush holders. The one atthe left is simply attached to its holder stud sleeve and springs,while the one at the right is mounted on the holder stud which isfastened in a brush rocker arm.
5. To provide a tension spring of such lengthor shape that the tension on the worn brush willbe very little less than that on a new brush.
6. To have a brush hammer so constructed thatit will bear directly on the top of the brush and notgive a side push either when the brush is full lengthor nearly worn out.
Fig. 19 shows a brush holder of the ReactionType, in which the brush is held securely betweenthe commutator surface and the Brush Hammershown on the top in this view. The spring usedwith this brush holder is a coiled steel wire andcan be seen on the back of the holder near thehammer hinge.
Brush holders are generally mounted or attachedto a Rocker Ring by .means of holder studs, asshown in Fig. 20. The holders can usually be ad-justed on these studs both sidewise and up anddown, to provide the proper spacing and tension.The purpose of the rocker frame or ring is to allow
21
D. C. Generators
the entire group of brushes to be rotated througha small arc, so their position can be adjusted forvarying current loads on the machine. This is oftennecessary on machines that do not have interpoles-as will be explained later.
Fig. 19. Reaction -type brush holders keep the brush in place by thepressure of a "brush hammer", as shown on top of the brush inthis view.
Frequently there are two or more brushesmounted on each stud, as several small brushes are.more flexible and will fit themselves to uneven com-mutator surfaces much better than one large brush.The brush holder studs are, of course, insulated fromthe rocker frames by means of fibre washers andbushings.
Fig. 21 shows six sets of brushes mounted on thebrush holder studs and rocker frame, which in turnis mounted in the end bracket of the machine.
22
Direct Current
13. BEARINGS
As previously mentioned, the bearings of motorsand generators are to support the armature prop -
Fig. 20. Above are shown two brushes in their holders which aremounted on a brush rocker arm for a four -pole machine. Notethe coil springs by which the brush tension on the commutatorcan he adjusted.
erly centered between the field poles and to allowit to rotate freely when the machine is in operation.These bearings are mounted in bearing brackets andheld firmly at the ends of the machine ; or, in somecases, they may be mounted in pedestals which areseparate from the field frame.
These bearings are of two common types, calledsleeve bearings and ball bearings. Roller bearingsare also used in some cases. Sleeve bearings aremade of babbit metal on the medium and largersized machines, while bronze is used for very small,high-speed machines. Bearing metal must alwaysbe of a different grade than that in the shaft, be-cause two similar metals will rapidly wear awayor eat into each other when they are rubbed to-gether.
Sleeve -type bearings are commonly oiled by oilrings or chains which rotate in the oil well andcarry, a small amount of oil up on to of the shaft
23
D. C. Generators
continuously while it is rotating. In other cases,on smaller machines, the oil is fed to the shaft bya cotton wick. Ball and roller bearings are lubri-cated with a light grade of grease.
Fig. 21. This view shows a complete set of brushes and holdersmounted on the rocker arm, which in turn is fastened in the endbracket of the machine.
A more thorough study of bearings will be givenin a later section. The principal point to rememberat this time in connection with bearings is the im-portance of keeping all bearings properly lubri-cated with clean oil. There should always beenough oil to be sure that the bearings are receiv-ing it ; but never oil them excessively and thuscause an overflow which may run into the windingand damage their insulation or get on the commu-tator and destroy its clean, bright surface.'
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Direct Current
EXAMINATION QUESTIONS
1. Where are D. C. machines extensively used?
2. What are several advantages of D. C. motors?
3. What speeds and voltages are D. C. generatorscommonly made for?
4. What are the principal parts of a D. C. motoror generator?
5. Can a D. C. motor be used as a generator?
6. What temperature rise is permissible or safeon D. C. windings with cotton insulation onthe windings?
7. What would muse a D. C. Generator or motorto over heat?
8. (a) What is the function' or purpose of thefield poles in a D. C. generator?(b) Why are field pole faces or shoes oftenlaminated?
9. What is the function of the commutator on aD. C. generator?
10. Name three common types of brush holders.What are several important requirements of agood brush holder?
25
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Direct Current
OPERATING PRINCIPLES OFD. C. GENERATORS
It is important to remember that the electricalgenerator is a device designed to change mechanicalenergy into electrical energy. Note that the machinedoes not generate electrical energy ; instead it con-verts mechanical energy into electrical energy.Since such a machine is merely a conversion device,it is logical to assume that the conversion can bereversed and the machine used to change electricalenergy into mechanical energy. Actually, this is thecase. Therefore any electrical generator may beused as a motor, and any electrical motor may beused as a generator.
We have learned that the E. M. F. or _voltage ina generator is produced by electro-magnetic induc-tion when the conductors of the armature are ro-tated through the lines of force of the field.
We also know that the amount of voltage pro-duced depends on the number of lines of force whichare cut per second. This in turn depends on thestrength of the field, the speed of armature rotationand the number of turns or coils in series betweenbrushes.
The voltage that will be produced by a generatorcan be calculated by the formula :
P X Op x Cr X RPME =
108 X 60 X M.in which
P = No. of field poles= Total useful flux per pole
Cr = Total No. of inductors on armature108 = 100,000,000 lines of flux to he cut per sec.
by one conductor60 = 60 sec. per min.M = No. of parallel conducting paths between
the + and - brushes.For example, suppose we have a machine with 4
poles and with 200 armature inductors (conductors)in four parallel circuits between the brushes. Themachine runs at 1200 R.P.M., and we will assume
27
D. C. Generators
that the useful flux per pole is 3,000,000 lines.
4 X 3,000.000 X 200 X 1200Then E = , or 120 volts.
100,000,000 X 60 X 4You may not need to use this formula often, but
Fig. 1. This figure shows a method of holding the fingers to usethe right-hand rule for determining direction of induced voltagein generators.
it serves to show what the voltage of generatorsdepends on in their design and also to illustrate thefactors of greatest importance in regulating thevoltage of a generator.
Note that once the 'machine has been constructedthe only quantities that can be varied are the R.P.M.and the flux per pole. Therefore, the voltage of agenerator can be changed only by varying the speed,or the field strength, or both.
It is an easy matter to determine the directionof induced voltage in the conductors of a generatorby the use of Fleming's Right Hand Rule, whichhas been previously stated and explained.
The rule is one that you will have a great dealof use for in connection with generators, so we willrepeat it here.
28
Direct Current
Place the first finger, thumb, and remaining fing-ers of the right hand all at right angles to each other.(See Fig. 1.) Let the first finger point in the direc-tion of magnetic flux from the field poles, the thumbin the direction of conductor rotation, and the re-maining fingers will indicate the direction of in-duced voltage.
This rule can be used either with diagrams or atthe machine to quickly determine the direction ofinduced voltage in any conductor, where the direc-tion of conductor movement and field polarity areknown.
Macirtettc. Circuit of Z Poles.Fig. 2. The above diagram shows the magnetic circuit or path of
the field flux in a simple two -pole machine.1. MAGNETIC CIRCUIT IN A GENERATOR
The number of conductors in the armature of agenerator usually remains unchanged once it is built,and while the speed can be varied somewhat, themachine is generally- operated at about the speedfor which it is designed. So we find that the voltageadjustment or variation during the operation of agenerator will depend largely upon the fieldstrength. It would be well, therefore, to considermore in detail some of the factors upon which thisfield strength depends, and also the methods bywhich it can be varied.
Every generator or motor has what is called aMagnetic Circuit. This is the path followed by the
29
D. C. Generators
flux of its field poles through the poles themselves,and through the armature core, and field frame-as shown in Fig. 2.
Magnetic Circuit of 4 Po% et,Fig. 3. Magnetic circuits in a four -pole machine. Note the direction
of flux from N. to S. poles in the external circuit and from S.to N. poles in the internal circuit of the field.
There are always as many magnetic circuits ina generator as it has poles. That is, a two -polegenerator will have two magnetic circuits. A four -pole generator four magnetic circuits, etc. Thesemagnetic paths must be continuous and will com-plete themselves through air unless iron or steel isprovided. It is advisable, therefore, to have asmuch of the magnetic circuit through iron as pos-sible, in order to reduce the reluctance of the circuitand increase the strength of the field.
The magnetic paths of commercial generators arecompleted through an all -iron or steel path, withthe exception of the'air gap between the armaturecore and field poles. Assuming the generator R.P.M.to be constant, increasing this air gap will weakenthe strength of the field and reduce the generatorvoltage proportionately.
Fig. 3 shows a sketch of a four -pole generatorframe and the four magnetic circuits which it willhave. It is very easy to determine the direction offlux at any pole of a generator if we know which
30
Direct Current
ends of the pole are N. and S., and simply rememberthe rule that magnetic flux always travels from anorth to a south pole in the external circuit. Exam-ining Fig. 3 again, we find that the flux from eithernorth pole divides and half of it goes to each of thesouth poles, then through the air gap and armaturecore which form the external circuit for the fieldpoles. The internal circuit from the south pole backto the north pole is completed through the fieldframe. From this we see that each pair of field polesof a generator form a sort of horse -shoe magnet.
The area of the field poles and frame must begreat enough to carry the flux without saturation.For highest efficiency, generators are operated atfield densities considerably less than saturation, andgenerally at about 20,000 to 40,000 lines per sq. inch.
2. FIELD EXCITATIONWe know that the strong magnetic field of the
poles in a generator is set up by direct currentflowing through the coils on these iron poles. Thiscurrent is called the Field Exciting Current. Thestrength of the field will, of course, depend on thenumber of turns in the field coils and the amountof current which is passed through them. So, bycontrolling excitation current with a rheostat, wecan readily adjust the strength of the field and thevoltage of the generator.
Generators are classed as either Separately Ex-cited or Self -Excited, according to the manner inwhich the field coils obtain the exciting current.
A separately excited generator is one that has itsfield excited from some source other than its ownarmature. This source may be either a storage bat-tery or another small D.C. generator. Alternatingcurrent cannot be used to excite the field poles ofeither a D.C. or A.C. generator. So alternators arepractically always separately excited by currentfrom storage batteries or D. C. generators. Sepa-rately excited D.C. generators are sometimes usedfor electro-plating machines, and work of this type,and have their field coils wound for a certain voltage.This voltage may range from. 6 to 25 for batteryexcitation ; and from 110 to 220 when excited fromanother generator.
31
D. C. Generators
rig. 4 shows a sketch of a simple two -pole D.C.generator which has its field separately excited froma storage battery. Note the field rheostat which isprovided to vary the field current and the generatorvoltage.
This type generator may be driven in either di-rection for the field excitation is independent of thearmature polarity. However, the polarity of thebrushes will reverse when the armature rotation isreversed, the positive brush becoming negative andvice versa.
SELF EXCITED
D.C.GENERATOR.
Fig. 4. This diagram shows a simple D. C. generator which has itsfield separately excited from a storage battery.
A self-excited generator is one that receives itsfield current from its own armature winding. Fig.5 shows a sketch of as simple generator of this type.You will note that the field coils are connectedacross the positive and negative brushes of thearmature in parallel with the line and load. There-fore the field has applied to it at all times the voltagegenerated by the armature, so a current will flowthrough the field coils as long as the armature is inmotion. Most commercial D.C. generators are self-excited.
32
Direct Current
3. BUILDING UP VOLTAGE IN AGENERATOR
With a separately excited generator, as soon asthe circuit is closed from the source of direct cur-rent for the field, the field will be magnetized at fullstrength, and the generator voltage will build uprapidly as soon as the machine comes up to fullspeed.
A self-excited generator must build up its voltagemore gradually from the small amount of residualmagnetism in the poles when the machine is started.You will recall that residual magnetism is thatwhich remains in or is retained by the iron of thefield poles even when their current is shut off. Thisresidual magnetism, of course, produces only a veryweak field.
When the machine is first started up and thearmature conductors begin to cut this weak residualfield, a very low voltage is generated in them. Asthe field is connected to the armature, this low ar-mature voltage forces a small current through thefield coils ; this small current slightly increases thestrength of the field. Then as the conductors cutthrough this slightly stronger field a still highervoltage is induced in them. This increases the fieldstrength still more, which in turn builds up a greatervoltage in the armature and still further fieldstrength. This continues, and the strength of thefield as well as the armature voltage keep on gettinggreater, until the point of Saturation is reached inthe field poles.
The saturation point, you will remember, is whena magnetic circuit is carrying its maximum practicalload of flux. When this point is reached it wouldrequire a considerable increase of current in the fieldcoils to make even a small increase in the flux ofthe poles. So we find that self-excited generatorsbuild up their voltage gradually from residual mag-netism as the machine comes up to speed.
Some times it may require a few seconds after themachine has reached full speed for its voltage tocome up to normal value.
33
D. C. Generators
4. FAILURE TO BUILD UP VOLTAGEWith self-excited generators, it is, of course, nec-
essary that the flux lines produced by the field coilsbe of the same polarity as the residual magnetismin the iron of the poles. Otherwise, the first lowvoltage applied to the field coils would tend toneutralize the residual magnetism and cause thegenerator to fail to build up its voltage. For thisreason, self-excited generators will build up voltageonly when rotated in the proper direction. Gen-erators may, however, be made to build up voltagewhen rotated in the opposite direction by changingthe field connections.
SEPARATELY EXCITED
D.C.GENERATOR.
Fig. 5. This simple two -pole machine has its field coils self-excitedby connection to its own armature brushes.
After a generator has been idle for quite a periodit sometimes loses its residual magnetism to suchan extent that it will not build up voltage until itis first separately excited. Some of the causes of agenerator failing to build up voltage are as follows :
Weak or dead residual magnetism, low speed, poorbrush -contact on the commutator, severe overloads.open field circuits, or high resistance connections.Weak or dead residual magnetism, low speed, poorbrush -contact on the commutator, severe overloads,open field circuits, or high resistance connections,in the armature or field circuit improper brush posi-tion or wrong rotation.
34
Direct Current
Removing the cause of the trouble will usuallystart the machine generating, but if it does not alow voltage storage battery or some other sourceof direct current applied to the field coils momen-tarily and in the proper direction will generallycause the machine to promptly build up voltageagain.
On some generators it is necessary to cut outpart or all of the resistance of the field rheostatbefore the machines will build up voltage.
5. VOLTAGE ADJUSTMENT ANDREGULATION
When a generator is running at normal speed, itsvoltage can be conveniently controlled and adjustedby means of the field rheostat, as shown in Figs.4 and 5. On most D.C. generators this adjustmentis made manually or by the operator, putting resist-ance in or out of the field circuit by means of thisrheostat. In some cases automatic voltage regu-lators are used to control this voltage accordingto the load on the machine. This automatic regu-lating device will be explained later.
The terms "control" and "adjustment" refer tochanges made in the voltage by the operator or bythe automatic device. The term "voltage regulation"refers to some change in the voltage which the ma-chine makes of its own accord as the load is changed.or varied. This change is inherent in the machineand is determined by its design and construction.
6. NEUTRAL PLANE
The neutral plane in a generator is that pointbetween adjacent field poles at which the armatureconductors are traveling parallel to the lines offorce, and in a very weak field. Normally, when thegenerator is not carrying a load this neutral planeis half way between adjacent poles of oppositepolarity, as shown in Fig. 6.
When the conductors are passing through thispoint they do not generate any voltage, as they arenot cutting across the lines of force. It is at thispoint that the commutator bars attached to the
35
D. C. Generators
conductors usually 'pass under the brushes, wherethey are momentarily short circuited by thebrushes. If the brushes were allowed to short cir-cuit coils while they were passing through a strongflux under a pole, and generating appreciable volt-age, it would cause very severe sparking at thebrushes. So it is important that the brushes beadjusted properly for this neutral plane.
Fig. 6. This diagram shows the normal path of flux through thearmature of the generator when the machine is not operating underload. Note the position of the normal neutral plane and also theposition this plane takes when a machine is loaded.
7. ARMATURE REACTIONIn addition to the flux which is set up between
the field poles from their coils and exciting current,there is also to be considered the flux around thearmature conductors. When a load of any kind isconnected to a generator and its voltage beginsto send current out through the line and load, thiscurrent, of course, flows through the armature con-ductors of the generator as well.
The greater the load placed on the machine thegreater will be the current in the armature con-ductors and the stronger will be the flux set uparound them
The armature flux is set up at right angles to theflux of the field poles, and therefore tends to distortthe field flux out of its straight path between poles.This effect is known as Armature Reaction.
36
Direct Current
Fig. 7 shows the position of the armature fluxas it would be when set up by current in the con-ductors, if there were no field flux to react with it.In actual operation, however, the armature and
Fig. 7. This sketch shows the magnetic flux set up around thearmature conductors of the simple two -pole machine when currentis passing through them.
field flux of the generator are more or less mixedtogether or combined to produce the distorted fieldshown in Fig. 8. Here we see that the lines of forcefrom the field poles have been shifted slightly out oftheir normal path and are crowded over towardthe tips of the poles which lie in the direction ofthe rotation of the armature. This causes the fieldstrength to be somewhat uneven over the pole faces,and more dense on the side toward which the arma-ture is rotating.
You will also note that this distortion of the fieldhas shifted the neutral plane, which must remain atright angles to the general path of the field flux.
As the armature flux depends on the amount ofcurrent through its conductors, it is evident that thegreater the load on the machine, the greater willbe the armature reaction and field distortion ; andthe farther the neutral plane will be shifted fromits original position. So unless a generator is pro-vided with some means of overcoming the effect ofarmature reaction, it will be necessary to shift the
37
D. C. Generators
brushes with varying loads in order to obtainsparkless commutation.
Some machines are provided with commutating
Fig. 8. This view shows the manner in which the magnetic linesof the field are distorted from their normal path by the effect ofarmature reaction. The neutral plane is shifted counter clockwise,or in the direction of rotation as shown by the dotted line.
poles or interpoles, as they are sometimes called,which are placed between the main field -poles toneutralize this feature of armature reaction andthereby eliminate the necessity of shifting thebrushes with changes of load. These poles and theiroperation will be more fully explained later.
When current flows through the armature wires,magnetic poles are produced on the armature sur-face and these poles are located midway betweenthe main poles as shown in Fig. G and H. As thecurrent drawn from the generator armature in-creases, the strenth of the armature poles becomesgreater in proportion. Note that the relation be-tween the armature poles and the field poles is suchthat rotation of the armature is opposed. As theelectrical load on the generator increases, this op-position to rotation becomes greater; this explainswhy a generator is harder to drive as the load is in-creased. Actually the mechanical power provided by
38
Direct Current
the prime mover is utilized in overcoming the mag-netic effect of the armature and field pole attractionfor each other.
H
GENERATORSDiagrams G and H show two generators, one
arranged for clockwise and the other for counterclockwise rotation. Note that poles are set up ongenerator armatures but that in this case the polesoppose rotation.
8. ARMATURE RESISTANCE AND I. R.LOSS
All armature windings offer some resistance tothe flow of the load current through them. Whilethis resistance is very low and usually only 'a frac-tion of an ohm, it nevertheless causes a certainamount of voltage drop in the internal circuit ofthe armature. In other words, a certain smallamount of the generated voltage is used to forcethe load current through the resistance of the arma-ture winding. The greater the load on a generator,the greater will be the voltage drop through thearmature.
As we know, this voltage drop is always propor-tional to the product of the amperes and ohms ; andfor this reason it is often referred to as I. R. Drop.
We can also determine the watts lost in an arma-ture, or converted into heat because of its resistance,by squaring the current and multiplying that bythe resistance, according to the watts law, formula.Therefore, 12 X R will equal the watts lost in an
39
D. C. Generators
armature due to its resistance. In which :I = the load currentR = the resistance of the armature only.
The armature resistance may be measured withan ohmmeter, a Wheatstone bridge, or by sendinga current through the winding and measuring thevoltage drop across the armature with a millivolt -meter. Knowing the current flowing through thearmature and the voltage drop across it, the resist-ance of the winding can then be determined byOhm's Law.know the size of the wire, the length of the turnsin the coils, and the number of paths in parallel inthe armature.
9. VOLTAGE DROP IN BRUSHESAND LINES
There is also a certain amount of voltage dropat the brushes of a generator which is due to theresistance of the brushes themselves and also theresistance of the contact between the brushes andcommutator. This resistance is also very low andwill cause a voltage drop of only about one or twovolts on ordinary machines under normal load.
In addition to the voltage drop encountered inthe generator, we also have the drop in the linewhich leads from the generator to the devices whichuse the current produced by the generator.
Knowing that the voltage drop in both the line.or external circuit, and the generator internal circuitwill vary with the amount of load in amperes, wecan see the desirability and need of some voltageadjustment or regulation at the generator, to keepthe voltage constant at the devices using the energy.
10. GENERATOR TYPES
Direct current generators can be divided intoseveral classes, according to their field constructionand connections. They are called respectively :Shunt Generators, Series Generators, and Com-pound Generators.
The shunt generator has its field coils connectedin shunt or parallel with the armature. Shunt field
40
Direct Current
coils consist of a great many turns of small wireand have sufficient resistance so that they can bepermanently connected across the brushes and havefull armature voltage applied to them at all timesduring operation. The current through these coilsis, therefore, determined by their resistance and thevoltage of the armature.
Series generators have their field coils connectedin series with the armature, so they carry the fullload current. Such coils must, of course, be woundwith heavy wire in order to carry this current andthey usually consist of only a very few turns.
Compound generators are those which have botha shunt and series field winding.
Fig. 10. This diagram shows the connections of a shunt generator.The shunt field winding "F" is connected in series with the fieldrheostat and then across the brushes. Note that this field windingis also in paralle' with the load on the line.
Each of these machines has certain characteristicswhich are particularly desirable for certain classesof work, as will be explained in detail in the fol-lowing paragraphs.11. SHUNT GENERATORS
Fig. 10 is a simple sketch showing the method ofconnecting the field winding of a shunt generatorin parallel with its armature. The field rheostat,F.R., is connected in series with the shunt fieldwinding to regulate the field strength, as previouslyexplained.
41
D. C. Generators
It is well to note at this point that, in variouselectrical diagrams, coils of windings are commonlyrepresented by the turns or loops shown for theshunt field at "F", while resistance wires or coilsare commonly shown by zigzag lines such as thoseused for the rheostat at "F.R."
Fig. 11 shows the connections of a shunt genera-tor as they would appear on the machine itself.By comparing this diagram with the one in Fig. 10and tracing the circuits of the field and armatures,you will find they are connected the same in eachcase.
The shunt generator, being a self-excited ma-chine, will start to build up its voltage from residualmagnetism as soon as the armature commences torotate. Then, as the armature develops a smallamount of voltage, this sends some current throughthe field, increasing the lines of force and buildingup the voltage to full value; as previously explained.However, if there is a low resistance connectedacross the generator terminals, the generator mayfail to build up because this low resistance willshunt most of the armature current away from thefield winding and so prevent the increase in fieldcurrent essential to the normal building up process.This explains why a shunt generator sometimes failsto build up when it is run up to speed with thenormal load connected to the armature circuit ; italso shows why the voltage of a shunt generatorwill fall to zero if the machine is short circuited.
As the field circuit and the load circuit are bothconnected across the generator terminals, any cur-rent generated in the armature must divide betweenthese two paths in inverse proportion to their re-sistance and, as the resistance of the field circuit isrelatively high, most of the current goes throughthe load circuit thereby preventing the increase infield strength essential to the production of normalterminal voltage.
12. VOLTAGE CHARACTERISTICS OFSHUNT GENERATORS
The voltage of the shunt genefator will vary in-versely as the load, due to the same reason men -
42
Direct Current
tioned in the preceding article. Increasing the loadcauses increased voltage drop in the armature cir-cuit, thus reducing the voltage applied to the field.This reduces the field strength and thereby reducesthe generator voltage.
If the load on a shunt generator is suddenly in-creased, the voltage drop may be quite noticeable ;while, if the load is almost entirely removed, the
FR
Line
Shunt GeneratorFig. 11. This sketch shcws the wiring and connections of the brushes
and field coils for a four -pole, shunt generator.
the voltage regulation of a shunt generator is verypoor, because it doesn't inherently regulate or main-tain its voltage at a constant value.
The voltage may be maintained fairly constantby adjusting the field rheostat, provided the loadvariations are not too frequent and too great.
Shunt generators are, therefore, not adapted. toheavy power work but they may be used for incan-descent lighting or other constant potential deviceswhere the load variations are not too severe.
Shunt generators are difficult to operate in paral-lel because they don't divide the load equally be -
43
D. C. Generators
tween them. Due to these disadvantages shuntgenerators are very seldom installed in new plantsnowadays, as compound generators are much moresatisfactory for most purposes.
1a
O-
50
40
30
10
10
00
SO
GO
70
t0
50
40
30
20
10
17423
? ,I 3S8'isZ888,1.7:: gn
LOAD IN AMPERESFig. 12. This curve illustrates the voltage characteristic of a shunt
generator. Note how the voltage drops as the load in kilowatts isincreased. Full load in this case is 240 amperes.
Fig. 12 shows a voltage curve for a shunt genera-tor and illustrates the manner in which the voltageof these machines varies inversely with the load.You will note that at no load the voltage of thegenerator is normal or maximum, while as the loadin kilowatts increases the generator voltage grad-ually falls off to a lower and lower value.
13. SERIES GENERATORS
These machines have their field coils connectedin series with the armature and the load, as shownin Fig. 13. The field winding is usually made ofvery heavy wire or strip copper, so that it will carrythe full load current without overheating.
44
Direct Current
By referring to Fig. 13 we can see that with noload connected to the line, it would be impossiblefor any current to flow through the series field andtherefore the generator couldn't build up voltage.So, in order for a series generator to build up volt-age when it is started, we must have some load con-nected to the line circuit.
Fig. 13. This sketch sfiows the connections of a series generator.The series field at "F" is connected in series with the armatureand the line. Note that no current could flow through this fieldif there was no load connected to the line.
14 VOLTAGE CHARACTERISTIC OFSERIES GENERATORS
The greater the load connected to such a genera-tor, the heavier will be the current flowing throughthe field winding and the stronger the field flux.This causes the voltage of a series generator tovary directly with the load ; or to increase as theload is increased and decrease as the load decreases.This, you will note, is exactly the opposite charac-teristic to that of a shunt generator.
As most electrical equipment is to be operatedon constant voltage and is connected to -the line inparallel, series generators are not used for ordinarypower purposes or for incandescent lighting. Theirprincipal use has been in connection with series arclights for street lighting.
With a load of this kind, the current must alwaysremain at the same value for the series lamps and,therefore, the generator field and voltage will re -
45
D. C. Generators
main fairly constant. You can readily see that aseries generator would be entirely impractical forordinary power and light circuits, because, if theload is decreased by disconnecting some of the de-vices, the voltage on the rest will drop,15. SERIES FIELD SHUNTS
Fig. 14 shows a curve illustrating the voltageregulation of a series generator. The voltage of
A
O
00
20
30
10
10
00
SO
00
70
30
40
30
20
10
40
.
47, .4. 3 2 8 0 3 0 0* 3 2 3 3 2 0.. it C. N. ..,LOAD IN AMPERE)
ig. 14. This curve shows the voltage characterisf c of a series gen-erator. Note that the voltage increases rapidly as the load on themachine is increased up to about full load. Full load in this caseis 125 amperes.
such machines can be adjusted by the use of alow -resistance shunt connected in parallel with theseries field coils, as shown in Fig. 15. This figureshows the connections of a series generator as theywould appear on the machine. By tracing the cir-cuit you will find that the field coils are connectedin series with the armature and load.
The purpose of the shunt is to divide the loadcurrent, allowing part of it to flow through theseries field and the rest through the shunt. By
46
Direct Current
varying the resistance of this shunt, we can causemore or less of the total load current to flowthrough it, thus either weakening or strengtheningthe series field.
These shunts are generally made of very lowresistance material, such as copper ribbon or stripsof metal alloy IA ith higher resistance than copper,
Fig. 15. Connections of brushes and field coils of a four -pole, seriesgenerator.
in order to make them short in length and compactin size.
By referring again to the curve in Fig. 14, youcan see that the voltage regulation of a series gen-erator is also very poor.
16. COMPOUND GENERATORSThe fields of a compound generator are composed
of both shunt and series windings, the two separatecoils being placed on each pole. Fig. 16 shows theconnections of both the series and shunt fields ofa compound generator.
The shunt field is connected in parallel with thearmature and therefore it maintains a fairly con -
47
D. C. Generators
stant strength. The series field, being in series withthe armature and load, will have its strength variedas the load varies. These machines will thereforehave some of the characteristics of both shunt andseries generators.
Fig. 16. This sketch shows the connections of a compound generator.The shunt field is connected across the brushes. The series fieldis connected in series with the line.
We have found that the shunt generator tendsto decrease its voltage as the load increases andthat the series generator increases its voltage withincreases of load. Therefore, by designing a com-pound generator with the proper proportions ofshunt and series fields, we can build a machine thatwill maintain almost constant voltage with anyreasonable variations in load.
The shunt field winding of a compound generatoris usually the main winding and produces by farthe greater portion of the field flux. The seriesfield windings usually consist of just a few turns,or enough to strengthen the field to compensatefor the voltage drop in the armature and brushesas the load increases.
Compound generators can have the shunt fieldstrength adjusted by a rheostat in series with thewinding, and may also have a shunt in parallelwith the series field for its adjustment. The shuntfield rheostat on these machines, however, is notgenerally used for making frequent adjustments intheir voltage, but is intended for establishing theproper adjustment between the series and shuntfield strengths when the generators are placed inoperation.
48
Direct Current
The variation in the strength of the series field,which compensates for the voltage drop with vary-ing load, makes unnecessary the frequent use of theshunt field rheostat employed on these machines, asis done with shunt generators.
Fig. 17. Connections of brushes and field coils for a four -pole, cumula-tive compound generator. Note that the direction of currentthrough the series field winding is the same as that through theshunt coils.
Fig. 17 shows the complete connections for thearmature and fields of a compound generator. You;vill note that the series winding is composed ofjust a few turns of very heavy wire on each poleand is in series with the armature and line. Theshunt winding is composed of a far greater numberof turns of small wire and is connected in parallelwith the armature brushes.
By referring back to Fig. 12, in lesson 33, you willnote the series coils wound on the poles over oroutside of the shunt winding, which is wound nextto the pole cores_17. CUMULATIVE AND DIFFERENTIAL
COMPOUND GENERATORS.In the type of generator just described, the shunt
and series field coils are so arranged that the cur -
49
D. C. Generators
rents flowing in both windings circulate around thepoles in the same direction ; therefore the windingsaid each other in setting up the field flux. Since theaction of the fields is cumulative, this type of gen-erator is known as the Cumulative Compound type.The cumulative compound generator is the mostwidely used type of D.C. machine.
Some compound generators have the series fieldswound in the opposite direction, so that their fluxopposes that of the shunt field. Such machines areknown as Differential Compound generators. Theiruses will be explained later.
18. FLAT COMPOUND GENERATORS.VOLTAGE CHARACTERISTICS
When a compound generator has just enough ofseries field to compensate for the voltage drop inits own armature and brushes, and to maintain anearly constant voltage from no load to full loadon the generator, it is known as a Flat Compoundmachine.
The voltage regulation of such a machine is verygood, as it automatically maintains almost constantvoltage at its terminals with all normal load varia-tions. Such machines are very commonly used forsupplying current to general power and light cir-cuits where the load is not located too far from thegenerator and the line drop is small. Fig. 18 showsthe voltage curve of a flat compound generator at F.19. OVER COMPOUND GENERATORS.
VOLTAGE CHARACTERISTICS
Where the load is located some distance fromthe generator or power plant and the line drop issufficient to cause considerable reduction of voltageat the current -consuming devices when the load isheavy, the generators are commonly equipped withseries field windings large enough to compensatefor this line drop as well as their own armatureand brush voltage drop. Such machines are calledOver Compound generators .and are by far the mostcommon type used in power work
The voltage of an over compound generator willincrease slightly at the generator terminals with
50
Direct Current
every increase of load. Every increase of load in-creases the current through these series turns,thereby strengthening the field enough to actuallyraise the voltage a little higher at full load than atno load.
This voltage increase at the generator terminalsmakes up for the additional voltage drop in the
150
140
1.30
110
110
100
90
80
70
60
50
40
JO
20
10
v0a4.--,10
C/
..
00000 0oe * 0 0 o
o o?e 2 2 . el
0 0 0 0f 41 0 0LOAD IN AIM P R41
OANN.0Fig. 18. These curves show the voltage characteristic of a flat com-
pound generator at F, over compound at 0, and under compoundat U. Full load in this case is 220 amperes.
line when the load is increased. Therefore, if theseries and shunt fields of such a machine areproperly adjusted, it will maintain a very constantvoltage on the equipment at the end of the line.
The adjustment of the shunt and series fields ofthese machines can be made with the usual shuntfield rheostat and series field shunts.
The voltage regulation of an over compoundgenerator is very good, and for ordinary powerpurposes they don't require frequent adjustment ofthe rheostat or any special voltage regulating equip -
51
D. C. Generators
ment, because this regulation is inherent in thedesign and operation of the machine. Over com-pound generators are usually made and adjustedso that the terminal voltage will be from 4Y2% to6% higher at full load than at no load.20. DIFFERENTIAL COMPOUND
GENERATORS
Any compound generator .can he connected eithercumulative or differential, by simply reversing theconnections of the series field windings so that thesecoils will either aid or oppose the flux of the shuntfield.
Compound generators are usually designed to op-erate cumulative unless otherwise ordered forspecial purposes.
When the series field coils are connected differen-tial, and so that their flux opposes that of the shunt
Fig. 19. Connections of brushes and field coils for a four -pole, dif-ferential, compound generator. Note that the direction of currentthrough the series field coils is opposite to that in the shunt coils.
field, each increase in the load on the machine willcause quite a decided voltage drop, as it increases
52
Direct Current
the current in the differential winding and therebyweakens the field flux.
The voltage of these machines, therefore, willvary inversely with the load and considerably morethan it varies with the shunt generator. The volt-age regulation of differential compound generatorsmay be classed as very poor, but they have advan-tages in certain classes of work.
For the generators used in welding, where suddenand severe overloads are placed on the machine instarting the arcs, or for any machines that havefrequent severe overloads or the, possibility of short
20
i
00
20
50
70
60
50
40
20
10
OVER COJIROUS O
_ ___
-411111111141P4k
-- - - -
.7
DIP 2ERENTIAL
: 0 0 2 02 2 r '
0 o o 0r, 0 .^4 4 III
0e ,-on
0 0.6LOAD tN KILOWAT T5
Fig. 20. This chart shows the curves of several types of generatorsall together so they can be easily compared.
circuits, the differential compound winding is agood protective feature.
When an overload is placed on the line, the ad-ditional current in the differential series coils tendsto neutralize the shunt field flux and thereby re-duces the generator voltage considerably. This also
53
D. C. Generators
reduces the amount of current which will flowthrough the armature, and therefore protects itfrom overheating.
The shunt field winding of the differential gen-erator should be the main field winding and shoulddetermine the polarity of the pole. The series fieldshould at no time determine the. polarity of thepoles, unless the shunt field circuit is open, or ex-cept in a case of a short circuit across the brushes.
Fig. 19 shows the connections of a differentialcompound generator. Note that the current flowsin opposite directions in the shunt and series wind-ings around the field poles.
Fig. 20 shows the curves for the several typesof generators just described and provides a goodopportunity to compare the voltage characteristicsof shunt, series, and compound generators. Notehow rapidly the voltage of the differential machinefalls off as the kilowatt load increases.
It will be well to keep in mind the differentvoltage characteristics of these machines and theprinciples by which their voltage regulation is ob-tained, because you will encounter all types invarious plants in the field. Therefore a knowledgeof their field connections and adjustment, and theproper methods by which these connections can bechanged to obtain different characteristics, willoften be very valuable to you.
54
Direct Current
EXAMINATION QUESTIONS
1. On what three factors does the voltage of aD.C. generator depend?
2. Explain the difference between a self excitedand a separately excited generator.
3. How can the voltage of a D.C. generator beconveniently controlled or adjusted?
4. a. How does a self excited generator start tobuild up its voltage.
b. What are several possible causes of failureof such a machine to build up voltage?
5. a. What is meant by the term "Armature re-action"?
b. What is meant by the term "Neutral plane"?
6. What is meant by the term I R drop in a gen-erator armature, and how is it affected by variationsin load on the machine?
7. a. Name tnree common types of D.C. genera-tors as classified according to their field connections.
b. Which type is most commonly used for gen-eral power service?
8. a. How does the voltage of a shunt generatorvary with load changes?
b. Is the voltage regulation of a shunt generatorgood or poor?
9. For what purpose is a series field shunt used ?
10. What is the difference between a "flat com-pound" and an "over compound" generator?
55
D. C. Generators
soy
Direct Current
GENERATOR OPERATION
In commencing the study of the operation ofgenerators, it will be well to first consider primemovers, or the devices used to drive the generators.
The term Prime Mover may apply to any formof mechanical power device, such as a steam en-gine, steam turbine, gas or oil engine, or waterwheel. These devices, when used to drive electricgenerators, are designed to operate at a constantspeed at all loads up to full load. They are usuallyequipped with governors which maintain this con-stant speed by allowing the correct amount ofpower in the form of steam, gas, or water to enterthe prime mover, according to the variations of cur-rent load on the generator.
The prime mover should always be large enoughto drive the generator when it is fully loaded, with-out any reduction in speed which would be notice-able in the generator voltage output.
It is not our purpose at this time to discuss indetail the design or operation of prime movers, al-though in a later lesson they will be covered to agreater extent with regard to their operation.
1. CALCULATION OF PROPER H. P. FORPRIME MOVERS
To determine the proper size of engine or primemover to drive a D.C. generator of a given ratingin kilowatts, we can easily caluculate the horsepower by multiplying the number of kilowatts by1.34.
You will recall that one h. p. is equal to 746 watts,and one kilowatt, or one thousand watts, is equalto 1.34 h. p.
Multiplying the kilowatt rating of the generatorby 1.34 gives the horse power output of the machine.This horse power output can also be determined bythe formula :
E XH. P. =
746
57
Horse Power Calculations
In which :E = the generator voltageI = the maximum current load rating
746 = the number of watts in one h. p.In addition to the electrical hore power output
of the generator, we must also consider its effi-ciency, or the loss which takes place in its windingsand bearings.
If the efficiency of a generator is known to be80%, the formula to determine the horse powerrequired to drive it will be as follows :
E X IH. P.=
e X 746In which :
e = the efficiency of the generator, ex-pressed decimally.
We should also allow a certain amount for anyoverload that the generator is expected to carry. Aconvenient rule for determining the approximatehorse power required to drive any generator, is tomultiply the kilowatt rating of the machine by 1.5,which will usually allow enough extra power tomake up for the loss in .the generator.
For example, if we have a generator which israted at 250 volts and 400 amperes, and this ma-chine has an efficiency of 90%, we can determinethe necessary horse power by the formula, as fol-lows :
250 X 400H. P. = , or 148.94 h. p.
.90 X 746
The kilowatt rating of this same generator wouldbe 100 KW, as can be proven by multiplying thevolts by the ampeies. So, if we simply multiply100 X 1.5, according to our approxirriate rule, wefind that 150 h. p. will be required. This is approxi-mately the same figure as obtained by the use ofthe other formula.
If the generator has less than 90% efficiency andif it is known that the load will be up to the fullcapacity of the generator at practically all times, andoccasionally a little overload, then it is better to
58
Direct Current
allow slightly greater horse power than in the prob-lem just given. '
Fig. 1. This photo shows a large modern D. C. generator with awelded frame. The capacity of this generator is 1000 KW. Whathorse power will be required to drive it and satisfactorily maintainthe speed when the generator is 10% overloaded? Assume theefficiency of the generator to be 93%.
Although generators of 5 kilowatt capacity orabove will usually have efficiencies of 80 per centor higher, which means that 80 per cent of the me-chanical input to such machines will appear as elec-trical power at the brushes, it must be noted that inthe smaller machines efficiencies of a much lowervalue will be the rule, for in the fractional horsepowerfield the efficiencies of both motors and generatorswill frequently run below 50 per cent. Such ma-chines convert less than half the energy input intouseful energy in the output. This will explain whya small D.C. generator with an output of 100 wattsmay require a one-half H.P. motor to drive it. Ifboth machines have an efficiency of 50 per cent, theoverall efficiency of this combination is 25 per cent.This means that every watt output of the generatorrequires 4 watts input to the driving motor. Thiscombination is encountered frequently when an A.C.
59
Starting Generators
motor is used to drive an automobile generator forbattery charging purposes.
Prime movers for the operation of generatorsshould be equipped with governors which are quickenough in their response so that they do not allowthe generator to slow up noticeably when additionalload is applied.
There is generally some adjustment provided onthese governors which can be used to set the primemover to run the generator at the proper speed tomaintain the proper voltage.
As the voltage of the generator depends upon itsspeed, we should keep in mind that its voltage canbe adjusted by adjusting the governors or throttleof the prime mover.
2. INSPECTION BEFORE STARTINGGENERATORS
When starting up a generator we should firstmake a thorough examination, to make sure thatthe prime mover and generator are both in properrunning order. The oil wells should be examinedto see that there is sufficient oil in all main bearingsand that the oil rings are free to turn. Be careful,however, not to flood oil wells, because excess oilallowed to get into the windings of the generatoris very damaging to the insulation, and may neces-sitate rewinding the machine.
On small and medium-sized machines only a littleoil need be added from time to time, unless the oilwells leak. On large machines, where the armatureis very heavy, forced lubrication is necessary tomaintain the film of oil between the shaft and bear-ings. With these machines an oil pump is used toforce oil to the bearings at a pressure of 20 to 30lbs. per square inch to insure proper lubrication.Some bearings are also water cooled, having open-ings through the metal around the bearing for waterto flow through and carry away excessive heat.
If there are auxiliaries of this kind, they shouldbe carefully examined and checked before runningthe machine.
Direct Current
3. STARTING GENERATORSBefore starting up a generator it is usually best
to see that the machine is entirely disconnected fromthe switchboard. This is not always necessary, butit is safest practice. Next start the prime mover andallow the, generator armature to come gradually tipto full speed. Never apply the power jerkily orirregularly.
Power generators are always rotated at their fullspeed when operating under load. When the ma-chine is up to full speed the voltage can be adjustedby means of the field rheostat which is connectedin series with the shunt field.
The machine voltage can be checked by means ofthe switchboard voltmeter, and it should be broughtup to full operating voltage before any switches areclosed to place load on the generator.
This photo shows heavy Direct Current Electric Motors and control,equipment.
After the voltage is adjusted properly, the ma-chine may be connected to the switchboard by
of the circuit breakers and switches. Wherecircuit breakers are used they should always beclosed first, as they are overload devices and shouldhe free to drop out in the event there is an overloador short circuit on the line.
61
Starting Generators
After closing die circuit breaker the machineswitch may be closed, completing the connection ofthe generator to the switchboard. As the switch isclosed the operator should watch the ammeter andvoltmeter to see that the load is normal and tomake any further necessary adjustments with thefield rheostat.
If the generator is operating in pa,rallel withothers, the ammeter will indicate whether or not itis carrying its proper share of the load. The load onany generator should be frequently checked bymeans of an ammeter or wattmeter to see that themachine is not overloaded.
The temperature of the machine windings andbearings should also be frequently observed in orderto check any overheating before it becomes serious.
4. CARE OF GENERATORS DURINGOPERATION
After the machine is running, the most importantobservations to be made frequently are to check thebearing oil and temperature, winding temperaturesand ventilation, voltage of the machine as indicatedby the volt meter, and the load in amperes shownby the ammeter. Commutator and brushes shouldalso be observed to see that no unusual sparking orheating is occurring.
Commutators should be kept clean and free fromdirt, oil or grease at all times. Brushes should bekept properly fitted and renewed when necessary,and the commutator surface kept smooth and evenfor the best results.
All parts of an electric generator should be keptclean at all times as dust and oil tend to clog theventilating spaces in the windings, destroying thevalue of the insulation, and also interfering withproper commutation.
The supply of ventilating air in the generatorroom should be frequently checked to see that it isnot restricted, and that the temperature of the arma-ture is not allowed to become too high. Moistureis very detrimental to the generator winding andwater in or around the generator is very dangerous,
62
Direct Current
unless confined in the proper pipes for such pur-poses as cooling bearings, etc. Never use water toextinguish fire on any electrical equipment.
5. PARALLELING D. C. GENERATORSWhen direct current is used in large quantities
the power is usually furnished by several generatorsoperating in parallel, rather than by one or two verylarge machines. The larger machines when operatedat full load, are, of course, more efficient thansmaller ones, but the use of several machines in-creases the flexibility and economy of operation inseveral ways.
If only one large generator is used and the loadis small during a considerable part of the time, itis then necessary to operate the machine partlyloaded. The efficiency of any generator is generallyless when operating at less than full load, as theyare designed to operate at highest efficiency whenthey are fully loaded. or nearly so.
When several machines are used, the requirednumber can be kept in operation to carry the exist-ing load at any time. Then if the load is increasedadditional machines may be put in operation, or ifit is decreased one or more machines may be shutdown.
In a plant of this kind if any generator developstrouble it can be taken out of service for repairs, andits load carried by the remaining machines for ashort period, if it doesn't overload them more thanthe amount for which they are designed.
6. IMPORTANT RULES FOR PARALLELOPERATION
As we learned in the previous section on seriesand parallel circuits, when generators are -connectedin parallel their voltages will be the same as that ofone machine. The current capacity of the numberof generators in parallel, however, will be equal tothe capacity of all of them, or the sum of their ratedcapacities in amperes.
To operate generators in parallel, their voltagesmust be equal and their polarities must be alike.
63
Paralleling Generators
The positive leads of all machines must connect tothe positive bus bar and the negative leads of allmachines must connect to the negative bus bar.This is illustrated by the sketch in Fig. 3, whichshows two D. C. generators arranged for paralleloperation. You will note that if the switches areclosed the positive brushes of both machines willconnect to the positive bus bar, and the negativebrushes are both connected to the negative bus bar.
The voltmeters connected to each machine can beused to check the voltage as the machine is broughtup to speed, in order to be sure that it is equal tothe voltage of the other machine which may adreadybe running and connected to the bus. If the voltagesare unequal to any great extent, the machine of
Fig. 3. This simple sketch shows a method of connecting two D. C.generators in parallel. Note the polarity of the generator brushesand bus bars.
higher voltage will force current backward throughthe one of lower voltage and tend to operate it asa motor.
It is, therefore, very important that the voltagebe carefully checked before closing the switch whichconnects a generator in parallel with others.
If the polarity of one machine were reversed, thenwhen they are connected together it would result
64
Direct Current
in a dead short circuit with double voltage or thevoltage of both machines applied in series.
Just try making a sketch similar to Fig. 3 andreverse the polarity of one generator and see whatwould happen. You will find that the positive ofone machine feeds directly into the negative of theother, and so on around a complete short circuit.
The resistance of the machine windings, bus bars,ammeters and connections is so low that an enor-mous current would flow, until circuit breakers orfuses opened the circuit. If no such protective de-vices were provided. the windings would be burnedout or possibly even thrown out of the slots, bythe enormous magnetic stresses set up by the severeshort circuit currents.
You can readily see that in such matters as theseyour training an electrical principles and circuitsbecomes of the greatest importance, as you shouldat all times know the results of your movementsand operations in a power plant, and know theproper methods and precautions to follow.
7. CORRECTING WRONG POLARITYIf the polarity of a generator should build up
wrong, or in the reverse direction, it will be indicatedby the voltmeter reading in the wrong direction,and these meters should always be carefully ob-served when starting up machines.
Sometimes the generator will build up wrongpolarity because its residual magnetism has reversedwhile the machine was shut down. Sometimes stop-ping and starting the machine again will bring it upin the right polarity. If it doesn't the polarity canbe corrected by momentarily applying a low voltagesource of direct current to the field coils and send-ing current through them in the proper direction.
In power plants where several generators are con-nected to the same switchboard, the polarity of areversed machine may be easily corrected by raisingthe brushes on the reversed generator, to open itsarmature circuit, and then closing the switch whichconnects this machine in parallel with the operatingunits. This procedure will allow current to flow
65
Paralleling Generators
through the generator shunt field in the correct di-rection, thereby establishing the proper field po-larity. The main switch is then opened, the brusheslowered onto the commutator, and the machineparalleled in the usual way.8. COMPOUND MACHINES BEST FOR
GENERAL SERVICEShunt wound generators will operate quite satis-
factorily in parallel if their voltages are kept care-fully adjusted to keep the load divided properlybetween them. If the voltage of one machine isallowed to rise or fall considerably above or belowthat of the others, it will cause the machine of lowervoltage to motorize and draw excessive reversecurrent, and trip open the circuit breakers.
If the voltage of one machine falls ionly a littlebelow that of the others, the back current may notbe sufficient to open the breakers, but would be in-dicated by the ammeter of this machine reading inthe reverse direction.
Shunt genefators are not very often used in largepower plants, because of their very poor voltageregulation and the considerable drop in their voltagewhen a heavy load is applied. A plain shunt gen-erator can usually be changed for compound opera-tion by simply adding a few turns of heavy wirearound the field poles, and connecting them in serieswith the armature, with the right polarity to aidthe shunt field flux.
The compound generator is best suited to mostloads and circuits for power and lighting service andis the type generally used where machines are oper-ated in parallel in D. C. power plants.
Series generators are not operated in parallel andin fact they are very little used, except for welders,test work or in older street lighting installations.
9. SIMILAR VOLTAGE CHARACTERISTICSNECESSARY FOR PARALLELOPERATION
Compound generators can be readily paralleled ifthey are of the same 'design and voltage. They usu
66
Direct Current
ally have similar ele'ctrical and voltage character-istics and should be made with the same compound-ing ratios. That is the compounding effects of themachines must be equal even though they are ofunequal size.
Machines of different kilowatt ratings can be sat-isfactorily operated in parallel, if they are made bythe same manufacturer or of the same general de-sign, so that each will tend to carry its own shareof the load. If their compounding is properly pro-portioned, the voltage rise of each generator shouldbe the same for a similar increase of load.
When a D. C. generator is operated in parallelwith others and its voltage is increased, it will im-mediately start to carry a greater share of the cur-rent load. We can, therefore, adjust the load on thevarious machines by increasing or decreasing theirvoltages the proper amount.
10. TESTING AND ADJUSTING COM-POUNDING OF GENERATORS
The compounding effects of different generatorscan be tested or compared by separately loadingthem different amounts and observing their volt-meters. This can be done by connecting one of themachines to the switchboard, or to a special loadingrheostat, and operating the machine under normalvoltage. Then apply a certain amount of load to itand observe the voltmeter closely, to note theamount of increase in the voltage due to the com-pounding effect.
It will probably be well to check the voltage in-crease as the load is changed from one-fourth toone-half, and then to three -fourths and full loadvalues. By testing each generator in this manner wecan determine which of them has the greatest over -compounding effect, or produces the highest in-crease in voltage for the various increases in load.
If this compounding is found to be different onthe various machines, it can be adjusted by meansof the series field shunt. which will allow more orless of the total load current to flow through theseries winding of the compound field.
67
Paralleling Generators
When a number of machines of similar design arethus properly adjusted they should operate satis-factorily in parallel under all normal load changes.
In case the machines do not properly divide theirloads and one is found to be taking more than itsshare of any load increases, this can be correctedby very slightly increasing the resistance of itsseries field circuit by adding a few feet of cable inthe series field connection.
The series field windings may be connected toeither the positive or negative brush leads of thearmature ; but where compound generators areoperated in parallel, the series field lead of each ma-chine must be connected to the same armature lead,either positive or negative, on all generators.11. EQUALIZER CONNECTIONS
When compound generators are operated in par-allel, an equalizer connection should be used toequalize the proportion of currents through theirseries fields and to balance their compounding ef-fects.
This equalizer connection, or bus, is attached tothe end of the series field next to the armature.Its purpose is to connect the series fields of allgenerators directly in parallel by a short path ofvery low resistance, and to allow the load to divideproperly between them. When this connection isproperly made the current load will divide betweenthe series fields of the several machines in propor-tion to their capacity.
The equalizer allows the total load current todivide through all series fields in inverse proportionto their resistance, independently of the load on thearmature of the machine and of the armature resist-ance and voltage drop. This causes an increase ofvoltage on one machine to build up the voltage ofthe others at the same time, so that no one machinecan take all the increased load.
The connecting cables or busses used for equal-izer connections between compound generatorsshould be of very low resistance and also of equalresistance. This also applies to the series field con-nections from the generators to the main buss, ifmachines are of the same type.
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Direct Current
If the machines are located at different distancesfrom the switchboard, bus cables of slightly differ-ent size can be used, or an additional low resistanceunit can be inserted in the lower resistance leads.
Whenever possible, leads of equal length should,be used ; and, in the case of cables, it is sometimesadvisable to loop them or have several turns in thecable to make up the proper length. If these cablesor busses were of unequal resistance on machinesof the same size, there would be an unequal divisionof the load through the machines, and the machinehaving the lowest resistance in its series field cir-cuit would take more than its share of the load.When machines of unequal size are to be paralleledthe resistance of the series field leads should be inproportion to the resistance of the series windings.
Fig. 5 shows a wiring diagram for two com-pound generators to be operated in parallel. Notethe series and shunt field windings, and also theseries field shunts and shunt field rheostat. Theequalizer connections are shown properly made atthe point between the series field lead and the nega-tive brush. From this point they are attached tothe equalizer bus on the switchboard. The volt-meters are connected directly _across the positiveand negative leads of each generator, and the am-meters are connected across ammeter shunts whichare in series with the positive leads of each machine.These shunts will be explained later.
The machine switches for connecting the genera-tors to the bus bars are also shown in this diagram ;but the circuit breakers, which would be connectedin series with these switches, are not shown.
12. LOCATION OF EQUALIZER SWITCHESOn machines of small or medium sizes and up to
about 1,000 amperes capacity, the equalizer switch isoften the center pole of the three -pole switch, asshown in Fig. 5.
The two outside switch blades are in the positiveand negative leads of the machine. For machinesrequiring larger switches, three separate single -poleswitches may be used for greater ease of operation.
69
Paralleling Generators
In this case the center one is usually the equalizerswitch.
It is quite common practice to mount all of theseswitches on the switchboard, although in some in-stallations the equalizer switch is mounted on apedestal near the generator. In this case, the equal -
9 fa e eBu -s Bar
Bus Bar.
,,,,,,,,,F. R.
Ectualtz.er.
-4---F.
...,
1
.
OIQc.=.
..
E.
04-- ,
GG
a
+
41.----
.
Shunt FieldI Shunt Field1
/Series Field Shunt/
Fig. 5. This diagram shows the connections for two compound D. C.generators to be operated in parallel. Note carefully the connec-tions of the equalizer leads, series and shunt fields and instruments.
izer cable or bus is not taken to the switchboard butis run directly between the two machines.
Regardless of the location of the equalizerswitches, they should be closed at the same time orbefore the positive and negative machine switchesare closed.
Where three -pole switches are used, all of thepoles are, of course, closed at the same time ; but,if three single -pole switches are used, the equalizershould be closed first. If the positive and negativeswitches are closed one at a time, the switch on the
70
Direct Current
same side of the armature from which the equalizerconnection is taken should be closed second.
The series field should always be paralleled beforeor at the same instant that the generator armatureis paralleled with the main bus, in -order to insureequalization of the compounding effects and toallow the machine to take its proper share of theload at once.
13. INSTRUMENT CONNECTIONS WITHPARALLEL GENERATORS
Current instruments and devices-such as am-meters, overload coils on circuit breakers, currentcoils of wattmeters, etc.-should always be con-nected in the armature lead which doesn't containthe series field winding. This is shown by the am-meter shunts in Fig. 5, which are properly con-nected in the positive lead.
If these devices are connected in the lead whichhas the series field in it, the current indications willnot be accurate, because current from this side of themachine can divide and flow through either theequalizer bus or the armature.
Ammeters and other current devices should indi-cate the amount of current through the armature ofthe machine. It is not necessary to measure the cur-rent through the series fields, since they are all inparallel with each other.
The voltage generated in the armature will deter-mine the amount of current which is carried throughit, and it is possible to control the armature voltageof any machine by the adjustment of the shunt fieldrheoptat and thus vary the load carried by eachgenerator.
Voltmeters should_ be connected, as shown in Fig.5, at a point between the generator brushes andthe main switch, so that the voltage readings canbe obtained before this switch is closed. This isnecessary because we must know the voltage of thegenerator before it is connected in parallel with theothers
71
Paralleling Generators
14. STARTING, PARALLELING and ADJUST-ING LOAD ON GENERATORS
In starting up a generator plant with several ma-chines, the first generator can be started by the pro-cedure previously described and connected to thebus as soon as its voltage is normal. The secondgenerator should then be brought up to speed andits voltage then carefully checked and adjusted tobe equal to that of the first machine. Then thissecond machine can be connected to the bus. Theammeters of both machines should then be read tosee that they are dividing the load equally or inproportion to their sizes.
By adjusting the voltage of any generator withits field rheostat, it can be made to take its propershare of the load. After this adjustment is made,the same procedure can be followed on the remain-ing machines. If there are a number of branch cir-cuits and switchboard panels feeding to the linesand load, the switches on these panels can be closedone at a time, applying the load to the generatorsgradually.
To shut down any machine adjust its shunt fieldrheostat to cut in resistance and weaken its field,lowering the.voltage of that generator until its am-meter shows that it has dropped practically all ofits load. The circuit breaker can then be openedand the machine shut down.
72
Direct Current
THREE -WIRE D. C. SYSTEMS
The Edison three -wire D.C. system is used chieflywhere the generating equipment is to supply energyfor both power and lighting. The advantages of thissystem are that it supplies 110 volts for lights and220 volts for motors and also saves considerablyin the amounts and cost of copper, by the use of thehigher voltage and balancing of the lighting cir-cuits. Some of these features of the three -wire sys-tem were also explained in lesson 19.
One of the most simple and common methods ofobtaining the twe voltages on three -wire D. C. cir-cuits is by connecting two 110 -volt generators inseries, as shown in Fig. 6.
We know that when generators are connected inseries in this manner their voltages add together,so these two 110 -volt machines will produce 220volts between the outside or positive and negativewires. The third, or neutral, wire is connected tothe point between the two generators where thepositive of one and negative of the other are con-nected together. The voltage between the neutralwire and either outside wire will be 110 volts, or thevoltage of one machine.
Generators for this purpose may be either shuntor compound, but the compound machines are moregenerally used. They can be driven by separateprime movers or both driven by the same primemover if desired ; and the drive may be either bybelt or direct coupling.
In general the operation of a three -wire system ispractically the same as for a two -wire machine. Thevoltage of each generator may be adjusted by meansof the shunt field rheostat.
A4 these machines are operated in series insteadof parallel, it is not necessary to have their voltageexactly even:, but they should be kept properly ad-justed in order to maintain balanced voltages on thetwo sides of the three -wire system.
There is no division of the current load betweenthese generators-as in the case of parallel ma-chines-as the main current flows through both ma-chines in series. When the voltage of both machines
73
Three -Wire Systems
is properly adjusted, they can be connected to theswitchboard busses. The ammeters should then beobserved to note the current in each wire.
15. DIRECTION AND AMOUNT OF CUR-RENT IN. THE NEUTRAL WIRE
The ammeter in the neutral wire is of the double -
reading type, with the zero mark in the center ofthe scale, and it will read the amount of currentflowing in either direction.
When the load on a three -wire system is perfectlybalanced, the neutral wire will carry no current andthe set operates on 220 volts. In this case the twooutside ammeters will read the same and the centerammeter will read zero. When there is an unequalamount of load in watts on each side of the systemit is said to be unbalanced, and the neutral wirewill carry current equal to. the difference betweenthe current required by the load on one side andthat on the other.
Fig. 6. This sketch shows two D. C. generators connected in seriesfor providing three -wire, 110 and 220 volt service.
This current may, therefore, flow in either direc-tion, according to which side of the system has theheaviest load. Referring to Fig. 6-if the greater
74
Direct Current
load were on the lower side, the extra current re-quired would be furnished by the lower generator;and the current in the neutral wire would be flow-ing to the right, or away from the generators. Ifthe heavier load were placed on the upper side ofthe system, the extra current would be supplied bythe upper machine, flowing out on its positive wireand back to the line on the neutral wire.16. BALANCED SYSTEM MORE
ECONOMICALFor efficient operation, the amount of unbalance
should not exceed 10% of the total load. In manycases, however, it is allowed to exceed 15% or more.If the load could always be kept perfectly balanced,no neutral wire would be required as all of the loaddevices would be operated two in series on 220 volts.
Without the neutral wire, if one or more of thelamps or devices should be disconnected, the re-maining ones on the other side of the system wouldoperate at more than normal voltage.
In most systems it is practically impossible tokeep the load balanced at all times, and, therefore,the neutral wire is necessary to carry the unbalancedload and keep the voltages equal on both sides ofthe system. It is very seldom, however, that theneutral wire will have 'to carry as much current asthe outside wires. Therefore, it may be made smaller,than the positive and negative wires.
Quite often the neutral wire is made one-half thesize of either of the outer wires, unless local rulingsrequire it to be of the same size. If the neutral wireis made one-half the size of the outer ones, a three -wire system of this type will require only 31.3% ofthe copper required for the same load on a two -wire,110 -'volt system.
The neutral wire is generally grounded, as shownin Fig. 6.7. THREE -WIRE GENERATORS
In some cases a special three -wire generator isused, instead of the two machines in series, to pro-duce a three -wire D.C. system. An early type ofthree -wire generator, and one which is still used for
75
Three -Wire Systems
certain installations, consists of a 220 -volt armatureequipped with both a commutator and slip rings.
The armature coil connections are made to thecommutator in the usual manner, and 220 volts isobtained from the brushes on the commutator. Inaddition to the leads from each coil to the commuta-tor bars, other leads are taken from points spaced180° apart around the winding and are connectedto a pair of slip rings mounted on the shaft nearthe end of the commutator. This supplies single-phase alternating current at 220 volts to the sliprings.
From the brushes on these slip rings two leadsare taken to opposite ends of a choke coil, whichconsists of a number of turns of heavy wire woundon an iron core similar to a transformer core. Thisconnection is shown in Fig. 7.
A tap is made at the exact center of this chokecoil for the third or neutral wire. In some cases achoke coil is mounted on the armature shaft androtated with it ; but in most cases this coil is sta-tionary and outside of the machine, having its con-nections made through the slip rings and brushesThese coils are often referred to as three -wire trans-formers or compensators
18. PRINCIPLE OF THE BALANCE COILThe neutral wire, being connected to the center
of the coil, is always at a voltage about one-halfthat between the positive and negative brushes.Therefore, if 220 volts are obtained between thesebrushes, 110 volts are obtained between either thepositive and negative wire and the neutral.
When the load on a three -wire generator of thistype is perfectly balanced, no current flows in theneutral wire and all of the load current is suppliedfrom the commutator by the positive and negativeD. C. brushes. There is, however, a small amountof alternating current flowing through the choke orbalance coil at all times, as there is an alternatingvoltage applied to it from the slip rings as long asthe machine is operating. This current will be verysmall, as a choke coil of this type offers a very high
76
Direct Current
impedance or opposition to the flow of alternatingcurrent.
This impedance, or opposition, is composed of theohmic resistance of the conductors in the coil, andalso of the counter -voltage generated by self-induc-tion whenever alternating current is passed throughsuch turns of wire wound on an iron core.
Direct current, however, can flow through a coilof this type with only the opposition of the copperresistance, as the flux of direct current is not con-stantly expanding and contracting like that of alter-nating current, and so doesn't induce the highcounter -voltage of self-induction.
19. UNBALANCED LOAD ON THREE -WIREGENERATORS
When a system such as that shown in Fig. 7 is
Fig. 7. The above diagram shows the commutator, slip rings, andbalance coil of a three -wire D. C. generator.
unbalanced and has, we will say, a heavier load be-tween the positive wire and neutral, the unbalancedcurrent flowing in the neutral wire will return tothe center tap of the choke coil. From this point itwill flow first in one direction and then in the other,as the alternating current reverses in directionthrough the coil. Thus it returns to the armature
77
Three -Wire Systems
winding, through first one slip ring and then theother.
If the lower side of the circuit is loaded the heavi-est the unbalanced current will flow out throughthe choke coil in the same manner, passing firstthrough one half and then the other, to reach theneutral wire.
The choke coil must, of course, be wound withwire large enough to carry the maximum un-balanced current that the neutral wire is expected
,to carry. It must also have a sufficient number ofturns to limit the flow of alternating current fromthe slip rings to a very low value, in order to pre-vent a large waste of current through this coil.
Three -wire generators of this type can stand con-siderable unbalanced load without much effect onthe voltage regulation. They are very compact andeconomical and are used to some extent in smallisollted D.C. plants, where the circuits carry a loadof 110 -volt lamps and equipment, and also 220 -voltmotors.
20. THREE -WIRE MOTOR GENERATORSOR BALANCER SETS
Three -wire circuits may also be obtained bymeans of a 220 -volt D.C. generator in combinationwith a motor -generator or balancer set. Thesebalancer sets consist of two 110 -volt machinesmounted on the same bed plate and directly con-nected together by their shafts. See Fig. 11. Thearmatures of both machines are connected in serieswith each other, and across the positive and nega-tive leads of the 220 -volt generator, as shown inFig. 10.
This allows 110 volts to be applied to each arma-ture and operates both machines as motors whenthe load is perfectly balanced. Either machine can,however, be operated either as a motor or as a gen-erator, if the load on the system becomes un-balanced.
If one side of the system has a heavier load con-nected to it, the machine on this side automaticallystarts to operate as a generator and is driven by the*+achine on the other side, which then operates as
78
Direct Current
a motor. This condition will immediately reverseif the greater load is placed on the opposite side ofthe system. A balancer set of this type will, there-fore, supply the unbalanced current in either direc-tion, and will maintain 110 volts between the neu-tral and either outside wire.
Fig. 8. This view shows a three -wire generator disassembled. Youwill note the slip rings mounted on the end of the commutator.
Where these machines are larger than one ortwo kilowatts, a starting rheostat should be used tolimit the flow of current through their armaturesuntil the machines attain full speed. After theyreach full speed, they generate sufficient counter-voltage to limit the current flow through their arma-tures while operating as motors.
The neutral wire is connected between the arma-tures of the motor generator set where their posi-tive and negative leads connect together.21. EFFECTS OF SHUNT AND SERIES
FIELDS OF BALANCER GENERATORSEither shunt or compound machines may be used
for these equalizers, but compound machines areused more extensively. The number of turns in theseries field coils must be carefully selected to pro-vide the proper compounding effects. Generally the
79
Three -Wire Systems
number of turns is very small, so that the voltagerise clue to compounding will not he very great.
If this series field produces too great a voltagerise on either machine, that machine will be aptto take more than the unbalanced part of the load.The machines shown in Fig. 10 are of the com-pound type and have their series fields connectedin series with the armatures and the positive andnegative line wires.
The shunt fields are connected in parallel withtheir armatures and are both in series with a fieldrheostat, which can be used to increase the strengthof the field of one machine and decrease that of thtother at the same time.
Fig. 10. This diagram shows the connections for the main generatorand two balancer machines of a three -wire system.
The series fields are connected so that they in-crease the field strength when either machine isoperating as a generator, but tend to decrease oroppose the flux of the shunt field on either machinewhen it operates as a motor. This is caused by thereversal of the direction of current through theseries field and armatures as the unbalanced load
80
Direct Current
is shifted from one side of the system to the other.Current through the shunt fields, however, con-tinues to flow in the same direction at all times, be-cause they are connected across the positive andnegative leads from the main generator.
If the compounding effect of the balancer ma-chines tends to strengthen the field of either oneoperating as a generator, the voltage of that ma-chine will rise slightly; while the compound effecton the machine operating as a motor weakens itsfield and tends to make it speed up.
As long as the load on the system is perfectlybalanced, both machines operate as differentialmotors without any mechanical load. The currentthrough their armatures at such times is very small,being only sufficient to keep the armatures turningagainst the bearing and friction losses and to supplythe small electric losses in the machines.
22. BALANCING OF UNEQUAL LOADS
When a system is unbalanced, the neutral currentdivides between the two armatures, driving the oneon the lightly loaded side as a motor and passingthrough the other as a generator. In Fig. 10 theupper side of the system has the heaviest load, andthe lower side has the highest resistance. This willcause the excess current from the greater load toflow back through the neutral wire and through theseries of the lower machine, in a direction opposingits shunt field. This weakens the field flux andcauses this machine to speed up and tend to act asa motor to drive the upper machine as a generator.
As the voltage of the generator unit rises slightlywith the increased speed, it causes part of the un-balanced load to flow through it, and its series field,in a direction aiding the flux of the shunt field.
This increases its voltage still more, which en-ables it to take its proper share of the unbalancedcurrent and to compensate for the voltage drop onthe heavily loaded side of the system.
If the heaviest load is placed on the other side ofthe system, the current through the series fields ofboth machines will reverse, and cause the one which
81
Three -Wire Systems
was operating as a generator to speed up and oper-ate as a motor.
The motor armature must take enough more thanone-half of the neutral current to supply the lossesof both armatures.
Referring again to Fig. 10, we find that the con-nections of the field rheostat, F. R., are such thatwhen the handle or sliding contact is moved up-ward it will cut resistance out of the shunt field ofthe upper machine and add resistance in series withthe shunt of the lower machine. This would pro -
Fig. 11. Photo of a motor -generator balancer set used for three wiresystem machines of this type are used considerably, where theunbalanced load is small compared to the total load on the maingenerator.
duce the desired effects when the upper machine isoperating as a generator and the lower one as amotor.
As this change of resistance increases the fieldstrength voltage of the generator, it weakens thefield strength and increases the speed of the motor.
The shunt fields can he controlled separately, ifdesired, by connecting a separate rheostat in serieswith each field. In Fig. 6 the shunt fields of eachmachine are connected in parallel with their ownarmatures. By changing these connections so thatthe shunt field of each machine is connected acrossthe armature of the other machine, the machinewhich is operating as a generator will increase thecurrent flow through the motor field and improvethe torque of the motor armature.
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Direct Current
EXAMINATION QUESTIONS
1. For what purpose is a Prime Mover used?
2. What size engine in H. P. rating would be re-quired to operate a 110 volt generator capable offurnishing current at the rate of 30 amperes? As-sume the generator to be 80% efficient.
3. a. In what order are the circuit breakers andswitches closed when connecting the generator tothe switch board?
Why?
4. What are two advantages in operating two ormore generators in parallel rather than using onegenerator large enough to carry the entire load?
5. What effect does a parallel connection of gen-erators have on the combined voltage and currentcapacity?
6. In what respect must generators be alike whenthey are connected in parallel?
7. What is the purpose of an equalizer connectionwhen paralleling compound generators?
8. In what three ways may a three wire D.C. sys;tem be produced?
9. Does current flow in the neutral wire when athree wire system is operating with a balanced load?
10. What may happen on a three wire system ifthe neutral wire is broken?
83
Three -Wire Systems
On this 10 -Ton railroad repair shop floor -operated crane, DynamicLowering Control gives extreme accuracy, enabling the ma-chine operator to guide the work carefully into position onthe machine tool centers.
84
Direct Current
COMMUTATION AND INTERPOLES
The term "Commutation" applies to the processof reversing the connections of the coils to thebrushes, as the coils pass from one pole to anotherin rotation.
The function of the commutator, as we. alreadyknow, is to constantly deliver to the brushes voltagein one direction only, and thereby rectify or changethe alternating current generated in the windingto direct current for the line.
We have also learned that commutation for thevarious coils, or the contact of their bars with the)rushes, should take place when the coils are inhe neutral plane between adjacent poles; at whichPoint there is practically no voltage generated inhem.The reason for having commutation take place
while the coils are in the neutral plane is to preventshort-circuiting them while they have a high volt-age generated in them. This would cause severesparking, as will be more fully explained later.
1. PROCESS OF COMMUTATIONThe process of commutation, or shifting of coils
in and out of contact with the brushes, isjllustratedin Fig. 1. Here we have a sketch of a simplering -type armature with the ends of the coils shownconnected to adjacent commutator bars. Thiswinding is not the kind used on modern powergenerators, but it illustrates the principles of com-mutation very well, and is very easily traced.
We will assume that the armature in this figureis rotating clockwise. All of the coils which arein front of the north and south poles will be gen-erating voltage, which we will assume is in thedirection shown by the arrows inside the coils.
As the coils are all connected in series throughtheir connections to the commutator bars, thevoltage of all of the coils on each side of the arma-ture will add together. The voltages from bothhalves of the winding cause current to flow to thepositive brush, out through the line and load, and
85
Commutation
back in at the negative brush where it againdivides through both sides of the winding.
Now let us follow the movement of one coilthrough positions A, B, and C; and see what actiontakes place in the coil during commutation.
Fig. 1. This diagram illustrates the principles of commutation in agenerator. Examine each part of it very carefully while readingthe explanation given on these pages.
We will first consider the coil in position A,which is 'approaching the positive brush. This coilis carrying the full current of the left half of thewinding, as this current is still flowing through itto commutator bar 1 and to the positive brush.The coil at "A" also has a voltage generated in it,because it is still under the edge of the north field-pole.
An instant later when the coil has moved intoposition B, it will be short-circuited by the brushcoming in contact with bars 1 and 2.
2. SELF INDUCTION IN COILS SHORTEDBY BRUSHES
As soon as this coil is shorted by the brush,the armature current stops flowing through it, andflows directly through the commutator bar to thebrush. When this current stops flowing through
86
Direct Current
the coil, the flux around the coil collapses and cutsacross the turns of its winding, inducing a voltagein this shorted coil. This is called voltage of self-induction, and it sets up a considerable current flowin the shorted coil, as its resistance is so low. Notethat the voltage of self-induction always tends tomaintain current in an armature coil in the directionit was last flowing when generated from the fieldpole.
As long as the coil remains shorted, the currentset up by self-induction flows around through thecoil, bars, and brush. But as the coil moves farenough so bar 2 breaks contact with the brush, thisinterrupts the self-induced current and causes anarc. Arcing or sparking will tend to burn and pitthe commutator, and is very detrimental to thecommutator surface and brushes. Methods of pre-venting arcing will be explained later.
As the coil which we are considering moves oninto position C, its short circuit has been removedand it is now cutting flux under a north pole. Thiswill generate a voltage in the opposite directionto what it formerly had, and it still feeds its currentback to the positive brush through bar 2.
So we find that, by shifting the contact from oneend of the coils to the other as they pass from poleto pole and have their voltages reversed, the samebrush always remains positive.
3. IMPORTANCE OF PROPER BRUSHSETTING FOR NEUTRAL PLANE
The time allowed for commutation is extremelyshort, because when a motor armature is turningat high speed, the bars attached to any coil are incontact with a brush for only a very small fractionof a second.
The reversal of the coil leads to the brushes musttake place very rapidly as the coils are revolvedat high speed from one pole to the next. On anordinary four -pole generator each coil must passthrough the process of commutation several thou-sand times per minute. Therefore, it is very impor-tant that commutation be accomplished without
87
Commutation
sparking, if we are to preserve a smooth surfaceon the commutator and prevent rapid wear of thebrushes.
Brushes are made in different widths accordingto the type of winding used in the machine; but,regardless of how narrow the brushes may be, therewill always be a short period during which adjacentcommutator bars will be shorted together by thebrushes as they pass under them.
We have found that, in order to avoid severe,sparking during commutation, the coils must beshorted only while they are in the neutral plane,when the coil itself is not generating voltage fromthe flux of the field poles. Therefore, the brushesmust be accurately set so they will short circuitthe_coils only while they are in this neutral plane.4. SHIFTING BRUSHES WITH VARYING
LOAD ON MACHINES WITHOUTINTERPOLES
The neutral plane tends to shift as the load ona generator is increased or decreased. This is dueto the fact that increased load increases the currentthrough the armature winding and the additionalarmature flux will cause greater distortion of thefield flux. The greater the load; the further theneutral plane will move in the direction of armaturerotation
If the brushes are shifted to follow the movementof this neutral plane with increased load, commu-tation can still be accomplished without severesparking. For this reason, the brushes are usuallymounted on a rocker arm which allows them to beshifted or rotated a short distance in either direc-tion around the commutator.
In addition to the sparking which is caused byshorting coils which are not in the neutral plane,the other principal cause of sparking is the self-induced current which is set up in the coils by thecollapse of their own flux when the armature cur-rent through them is interrupted.
We have previously stated that this self-inductionwill set up a considerable flow of current in the
88
Direct Current
shorted coils. Then, when the coil moves on andone of its bars moves out from under the brushand thus opens the short circuit, this current formsan arc as it is interrupted.
Sparking from this cause can be prevented to alarge extent by generating in the coil a voltagewhich is equal and opposite to that of self-induction.
Shifting the brushes also helps to accomplish this,by allowing commutation to take place as the coilis actually approaching the next field pole.
This is illustrated in Fig. 2. In this figure youwill note that the brushes have been shifted so thatthey do not short circuit the coils until they areactually entering the flux of the next pole beyondthe normal neutral plane.
The voltage of self-induction always tends to setup current in the same direction as the currentinduced by the field pole which the coil is justleaving. If, at the time the short circuit on the
S
To Load
Fig. 2. This sketch shows the method of shifting the brushes toshort circuit coils in a position where they will be generating thevoltage to neutralize that of self-induction.
coil is broken, the coil is entering the flux of thenext pole, this flux will induce in the coil a voltagein the opposite direction to that of self-induction.This will tend to neutralize the voltage and currents
89
Commutation
of self-induction and enable the short circuit to bebroken when there is practically no voltage or cur-rent in the shorted coil.
Keep in mind that this is the required conditionfor most satisfactory commutation.
If the load on generators doesn't change oftenor suddenly, manual shifting of the brushes witheach change of load and new position of the neutralplane, may be all that is required to prevent spark-ing; but when the load changes are frequent andconsiderable, it would be very difficult to maintainthis adjustment by hand.
Where the manual method is used to maintainproper commutation, it is common practice toadjust the brushes to a position where they willspark the least for the average load. Then, eventhough a certain amount of sparking results whenthe load rises above or falls below this value, thebrushes are not changed unless the sparking be-comes too severe.
Fig. 3 shows a D.C. generator without the shaftor bearing post. The brushes of this machine areall attached to the ring framework as shown, andthis entire assembly can be rotated to shift thebrushes, by means of the hand wheel at the left.
Referring again to Fig. 2, the solid arrows showthe direction of the voltage of self-induction, andthe dotted arrows show the direction of the voltagewhich is induced by the flux of the field pole whichthe coil is approaching. These two voltages, beingin opposite directions, tend to neutralize each other,as has previously been explained.
5. USE OF COMMUTATING POLES TOPREVENT SPARKING
On the more modern D.C. machines commutatingpoles, or interpoles, are employed to hold the neu-tral plane in its normal position between the mainpoles, and to neutralize the effects of self-inductionin the shorted coils. These interpoles are smallerfield poles which are mounted in between the mainpoles of the machine, as shown in Fig. 4.
The interpoles are wound and connected so theywill set up flux of proper polarity to generate
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Direct Current
voltage in the opposite direction to that of self-induction, as the armature coils pass under them.Fig. 5 shows a ketch of a simple generator withinterpoles, or commutating poles, placed betweenthe main field poles.
We will assume that this armature is rotated ina clockwise direction, and that its armature con-ductors have generated in them voltage which tendsto send current in through the conductors on theleft side, and out through those on the right side
Fig. 3. This end view of a generator with the pedestals, bearings,and shaft removed shows very clearly the brush ring mounted ingrooved rollers on the side of the field frame. The hand -wheel atthe left can be used for rotating this ring to shift the brushes tothe proper neutral plane.
of the winding. Recalling that the voltage of self-induction tends to maintain current in the' samedirection in the conductor as it was under the lastfield pole, we find that this voltage is generated"in" at the top conductor in the neutral plane and"out" at the lower one.
91
I nterpoles
6. POLARITY OF INTERPOLES FOR AGENERATOR
If you will check the polarity of the interpoles,you will find that their flux would he in a directionto induce voltages opposite to those of self-induc-tion in each of these two conductors. The directionof these voltages is shown by the symbols placedjust outside of the conductor circles. So we findthat, if these commutating poles are made to set upflux of the right polarity and in the right amount,they can be caused to neutralize the effects of self-induction and distortion of the neutral plane almostentirely.
These poles are called "commutating poles" be-cause their principal purpose is to improve commu-tation and reduce sparking at the commutator andbrushes.
Diagrams G and H show two generators equippedwith interpoles. G is arranged for clockwise rotationand H for counter clockwise rotation. Note that thepolarity of the interpole is the same as that of thearmature pole immediately adjacent to it. This ruleapplies to both generators and motors and is trueregardless of the number of poles, the type of ma-chine, or the direction of rotation. Note too, thatthe armature poles on the generators oppose rota-tion and thus produce the force against which theprime mover must work to maintain rotation.7. STRENGTH OF COMMUTATING FIELD
VARIES WITH LOAD
In order that these commutating poles may pro-duce fields of the proper strength for the varying
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Direct Current
loads on the generator armature, their windings areconnected in series with the armature, so that theirstrength will at all times be ,proportional to theload current. In this manner, the strength andneutralizing effect of the interpoles increases as theload increases, and thereby tends to counteract theeffect of increased load on field distortion and self-induction.
In this manner, interpoles can be made to main-tain sparkless commutation at all loads and thusmake unnecessary the shifting of the brushes forvarying loads.
Referring again to Fig. 4, you will note thatthe windings on the interpoles consist of a fewturns of very heavy cable, so that they will beable to carry the armature current of the machine.The strength of the interpoles can be varied by theuse of an interpole shunt, which is connected inparallel with the commutating field to shunt part
Fig. 4. This photo shows a four -pole, D. C. generator with co nmu-tating poles. These commutating poles or interpoles are thesmaller ones shown between the main field poles.
of the armature current around these coils. Theconnections of this shunt are shown in Fig. 6-A.The interpole shunt is usually made of low re -
93
Interpoles
sistance materials, such as bronze or copper, so itwill carry the current readily without undue heat-ing.
This method of weakening the strength of thecommutating field is quite commonly used on thelarger machines. The terminals of the commu-tating field are usually connected directly to thebrushes, to eliminate confusion when making ex-ternal connections to the machine.
8. ADJUSTMENT OF' BRUSHES ONINTERPOLE MACHINES
On machines of small and medium sizes, the endplate or bracket on the generator is sometimes
Fig. 5. This sketch illustrates the manner in which interpoles generatevoltage opposite to that of self-induction in the conductors whichare shorted by the brushes.
slotted, as shown in Fig. 6-B, to allow, the brushesto be rotated or shifted within a very limited range.With such machines, the brush -holder studs aremounted rigidly in the bracket but are, of course,insulated from the metal with fibre sleeves andwashers.
When the. brushes are to be rotated the boltswhich hold the end plate to the field frame areloosened slightly and the entire end plate is shifted.
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Direct Current
This allows the armature coils to he commutatedat a point where the effects of the interpole arejust great enough to neutralize or balance self-induction.
Before removing the end plate to make repairson a machine of this type, it is well to mark itsposition, so that you can be sure to get it replacedin the correct position. This can be done by makingone or two small marks in line with each other onboth the field frame and the end plate. The markscan be made with a file or prick punch.
Fig. 6. At "A" is shown the connection of an interpole shunt forvarying the strength of the commutating field. "B" shows anend bracket with slots to allow it to be rotated slightly to shiftthe brushes. The brush holders on this machine would be mountedon this end bracket.
9. ADJUSTING INTERPOLES BYCHANGING THE AIR GAP
If the position of the brushes is in question ona generator, connect a voltmeter from the positiveto the negative brush and operate the machine.Move the brushes carefully back and forth acrossthe neutral plane until the position is found thatgives the highest reading on the voltmeter; lockthe brush rigging in that position.
A simple test for determining the proper brushposition on a motor is to run the machine with anammeter connected in series with it and move thebrushes to the position that gives the lowest read-ing on the ammeter.
95
Interpoles
In the field analysis it should be remembered thatthe brushes are to set in a position that resultsin minimum sparking under load; therefore, if aslight departure from the brush position as foundabove provides less sparking when the machine isin operation, then that is the position in which thebrushes should be set.
The strength of interpoles can also be variedby placing iron shims or thin strips between theinterpole and the field frame of the machine, asshown in Fig. 7. It is possible in this mannerto vary the width of the air gap between the faceof the interpoles and the armature core.
Decreasing the air gap reduces the magneticreluctance of the interpole field path, therebystrengthening its flux and increasing its effect oncommutation. This method can be used on ma-chines of any size and when no other visible meansof varying the interpole strength is provided, shimsare probably used.
On some machines, the number of interpoles may3nly be one-half the number of main field poles ;in which case they will be placed in every otherneutral plane and will all be of the same polarity.By making these interpoles of twice the strengthas would be used when a machine has one for eachmain pole, we can still effectively neutralize the
Fig. 7. Thin iron shims can be used under an interpole to vary i sstrength by changing the air gap between the pole and thearmature.
self-induction in the coils. This is true because,with a modern drum -wound armature, when one
96
Direct Current
side of any coil is in one neutral plane the otherside will be in the adjacent neut>Cal plane.
For this reason, if interpoles are placed in everyother neutral plane, one side of any coil will alwaysbe under the interpole while this coil is undergoingcommutation. This is illustrated by the sketch inFig. 8, which shows a four -pole generator withonly two interpoles.
As both sides of any coil are in series, the doublestrength of the interpoles over one side will neu-tralize the effects of self-induction in the entire coil.This type of construction reduces the cost of thegenerator considerably and is often used on ma-chines ranging up to six -pole size.
10. COMMUTATION ON MOTORSThe problem of obtaining sparkless commutation
on D. C. motors is practically the same as withD.C. generators.
Fig. 8. This simple sketch illustrates the manner in which twointerpoles can be used to neutralize self-induction in the coils ofa four -pole machine.
Motors as well as generators must have the con-nections from the brushes to the coils reversed as
97
Interpoles
the coils pass from one pole to another of oppositepolarity. This is necessary to keep the current fromthe line flowing in the right direction in all coilsin order to produce torque in the same directionunder all field poles.
During commutation, the coils of a motor arma-ture are momentarily short circuited by the brushes,the same as with a generator.
This shorting and commutation should take placewhile the coils are in the neutral plane betweenthe field poles, where they are doing the least workor producing the least torque.
We also know that the coils of any motor arma-ture have a high counter -voltage generated in themas they rotate under the field poles. This counter -voltage will be at its lowest value while the coilsare passing through the neutral planes; which isanother reason for having commutation take placeat this point in a motor.
11. POSITION OF NEUTRAL PLANE INMOTORS
The neutral plane of a D.C. motor will also shiftwith load variations and changes in armature cur-rent, but this shift will be in the opposite directionto what it is in a generator. This is due to thefact that the rotation of a motor will be oppositeto that of a generator if the current direction is thesame in the motor armature as in the generatorarmature.
Motor coils also have counter -voltage of self-induction produced in them when they are shortedby the brushes. In a motor, the direction of thisself-induced voltage will be opposite to that in agenerator, as the motor armature currents are inthe opposite direction to those in a generator arma-ture of the same direction of rotation.
We can, therefore, improve commutation on amotor by shifting the brushes in the opposite direc-tion to that used for a generator. Motor brushesshould be shifted against the direction of rotation,when the load is increased.
98
Direct Current
Fig. 9 is a sketch of the armature conductorsand field poles, of a simple D.C. motor, showingthe position of the neutral plane with respect tothe direction of rotation.
The heavy symbols in the six armature conduc-tors on each side show the direction of the appliedcurrent from the line, which is flowing "in" on theconductors at the right and "out" on those on theleft side. The lighter symbols in the single con-ductors at the top and bottom show the direction
Fig. 9, This sketch shows the position of the neutral plane withrespect to rotation in a motor. Compare this with Fig. 5 fora generator.
of the currents set up in this coil by self-inductionwhen the coil is shorted. The symbols shown out-side of the conductor circles indicate the directionof the counter E.M.F. produced in the motor wind-ing. This counter -voltage always opposes the di-rection of the applied line voltage.12. POLARITY OF INTERPOLES FOR
MOTORS
To minimize sparking at the brushes, most D.C.motors are equipped with small poles placed mid -
99
Interpoles
way between the main poles and called interpolesor commutating poles. For proper operation, thesesmall poles must have the correct polarity. Ref-
erence to any of the diagrams will show that thepolarity of the interpole is always the same as thearmature pole adjacent to it.
REVERSING ROTATIONThe windings on the interpoles are always con-
nected in series with the armature winding and areconsidered a part of the armature circuit. There-fore, when current thiough the armature is reversed,the interpole polarity is also reversed. This arrange-ment automatically preserves the proper relationbetween the armature poles and the interpoles whenthe armature current is reversed.
NUMBER OF INTERPOLESMachines equipped with interpoles may have as
many interpoles as main poles or one-half as many
100
Direct Current
interpoles as main poles. As the interpole windingis always' connected in series with the armature, theinterpole strength will vary with the value ofarmature current.
Fig. 10 shows the connections of the interpolesfor a two -pole D.C. motor. You will note that oneline or armature lead is connected directly to thenegative brush. while the other lead connects firstto the commutating field and then, through thesepoles, to the positive brush.
Fig. 10. This diagram shows the connections of the interpoles for atwo -pole generator or motor.
If this connection is properly made when themachine is assembled, it is not necessary to makeany change in the connections of the commutating
101
Carbon Brushes
field wnen the motor is reversed. Either the arma-ture current or field poles must be reversed toreverse the rotation, so that the relation of thecommutating poles will still be correct.
This connection can be the same whether themachine is operated as a motor pr generator, be-cause a generator rotated in the same direction asa motor will generate current in the opposite direc-tion through the armature. This is shown by thedotted arrows in Fig. 10 while the solid arrowsshow the direction of motor current.
As the commutating poles are in series with thearmature, this reversed current will also reversethe polarity of the commutating field, and maintainthe proper polarity for generator operation.
These principles of commutation and interpolesshould be kept well in mind, as an efficient main-tenance electrician or power plant operator willnever allow unnecessary sparking to damage thebrushes and commutator of machines he has chargeof.
CARBON BRUSHES
The brushes play a very important part in theoperation of any D. C. motor or generator, and arewell worth a little special artention and study atthis time.
The purpose of the brushes, as we already know,is to provide a sliding contact with the commutatorand to convey the current from a generator arma- -ture to the line, or from the line to a motor arma-ture, as the case may be. We should also keepin mind that the type of br.ush used can have agreat effect on the wear on commutators and inproducing good or bad commutation.
It should not be assumed, just because any brushwill carry current, that any piece of carbon or anytype of brush will do for the replacement of wornbrushes on a D.C. generator or motor. This istoo often done by untrained maintenance men, andit frequently results in poor commutation and some-times serious damage to commutators and ma-chines.
102
Direct Current
Many different grades of brushes are made foruse on machines of various voltages and commu-tator speeds and with different current 'loads.
In order to avoid sparking, heating, and possibledamage to commutators, it is very important whenreplacing worn brushes to select the same type ofbrushes or brush materials as those which are re-moved.
In some special cases, brushes other than thosesupplied by the manufacturers for that particularmachine, may not work satisfactorly. In case thebrushes supplied by the manufactures are not avail-able and difficulty is encountered in obtaining theproper brushes, it may be advisable to get in touchwith some dependable brush manufacturing com-pany. They will be able to render valuable assis-tance in the selection of proper brushes for any ma-chine.
The information contained in the following para-grahps should be carefully studied in order that youwill be able to select the proper brushes for vari-ous uses.13. BRUSH REQUIREMENTS
A good brush should be of low enough resistancelengthwise and of great enough cross-sectional areato carry the load current of the machine withoutexcessive heating. The brush should also be ofhigh enough resistance at the face or contact withthe commutator to keep down excessive currentsdue to shorting the armature coils during commu-tation. In addition, the brush should have justenough abrasive property to keep the surface ofthe commutator bright and the mica worn down,but not enough to cut or wear the commutator sur-face unnecessarily fast.
Figs. 11 and 12 show several carbon brushesof different shapes, with the "pigtail" connectionsused for carrying the current to the brush -holderstuds.14. BRUSH MATERIALS
The most commonly used brushes are made ofpowdered carbon and graphite, mixed with tarrypitch for a binder, and molded under high pressureinto the shapes desired. This material can be
103
Carbon Brushes
molded into brushes of a certain size, or into blocksof a standard size from which the brushes can becut.
The molded material is then baked at high tem-peratures to give it the proper strength and hard-ness and to bake out the pitch and volatile matter.
Fig. 11. Above are shown several carbon brushes of common types,such as used on D. C. motors and generators.
Carbon is a very good brush material because itis of low enough resistance to carry the load cur-rents without too great losses, and yet its resistanceis high enough to limit the short circuit currentsbetween commutator bars to a fairly low value.Carbon also possesses sufficient abrasiveness tokeep commutator mica cut down as the commutatorwears.
104
Direct Current
Graphite mixed with carbon in the brushes pro-vides a sort of lubricant to reduce friction with thecommutator surface. It also provides a brush oflower contact resistance and lower general resist-ance, and one with greater current capacity.
Powdered copper is sometimes added or mixedwith graphite to produce brushes of very high cur-rent capacity and very low resistance. Thesebrushes are used on low -voltage machines such asautomobile starting motors, electro-plating genera-tors, etc. They often contain from 30 to 80 per centof copper, and such brushes will carry from 75 to200 amperes per sq. in.
Lamp -black is added to some brushes to increase
Fig. 12. Two carbon brushes of slightly different shapes, such asused with reaction type brush holders.
their resistance for special brushes on high -voltagemachines.15. COMMON BRUSH MATERIAL. BRUSH
RESISTANCEA very common grade of carbon -graphite brush
is made of 60% coke carbon and 40% graphite,and is known as the "utility grade''. Brush materialof this grade can be purchased in standard blocks4" wide and 9" long, and in various thicknesses.
Brushes for repairs and replacements can thenbe cut from these blocks. They should always becut so that the thickness of the block forms the
105
Carbon Brushes
thickness of the brush, as the resistance per inchthrough these blocks is higher from side to sidethan it is from end to end or edge to edge. Thisis due to the manner in which the brush materialis molded and the way the molding pressure isapplied, so that it forms a sort of layer effect or"grain" in the carbon particles.
This higher "cross resistance" or lateral resist-ance is a decided advantage if the brushes areproperly cut to utilize it, as it helps to reduce short-circuit currents between commutator bars whenthey are shorted by the end of the brush.
Fig. 13 shows how brush measurements shouldbe taken and illustrates why it is an advantage tohave the highest resistance through the thicknessof the brush and in the circuit between the shortedcommutator bars.
The resistance of ordinary carbon -graphite brushesusually ranges .001 to .002 ohms per cubic inch,and these brushes can be allowed to carry from30 to 50 amperes per sq. in. of brush contact area -
These brushes can be used on ordinary 110, 220,and 440 -volt D.C. motors ; and on small, medium,and large sized generators which have either flushor undercut mica, and commutator surface speedsof not over 4000 feet per min.
T ThicknessW- WidthL- Length
Fig. 13. This sketch illustrates the method of taking measurementfor the length, width, and thickness of carbon brushes.
16. HARDER BRUSHES FOR SEVERESERVICE
These utility grade carbon -graphite brushes canbe obtained in a harder grade, produced by special
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Direct Current
processing, and suitable for use on machines whichget more severe service and require slightly moreabrasiveness. These harder brushes are used forsteel mill motors, crane motors, elevator motors,mine and mine locomotive motors, etc.
Brushes with a higher percentage of carbon canbe used where necessary to cut down high mica,and on machines up .to 500 volts and with commu-tator speeds not over 2500 feet per minute. Thistype of brush is usually not allowed to carry over35 amperes per square inch, and is generally usedon machines under 10 h. p. in size.
17. GRAPHITE USED TO INCREASECURRENT CAPACITY
Brushes of higher graphite content are usedwhere high mica is not encountered, and for heaviercurrent capacity. Such brushes are generally usedonly on machines which have the mica undercut ;and are particularly adapted for use on older typesof generators and motors, exhaust fans, vacuumcleaners, washing machines, and drill motors.
Brushes with the higher percentage of graphitedo not wear or cut the commutators much, butthey usually provide a highly -polished surface onboth the commutator and brush face. After aperiod of operation with these brushes the surfaceof the commutator will usually take on a sort ofblown or chocolate -colored glaze which is verydesirable for long wear and good commutation.
18. SPECIAL BRUSHES
Some brushes are made of practically puregraphite and have very low contact resistance andhigh current -carrying capacity. Brushes of thisnature will carry from 60 to 75 amperes per squareinch and they can be used very satisfactorily onmachines of 110 or 220 volts, or on high-speedslip rings with speeds even as high as 10,000 feetper minute.
The greater amount of graphite offers the neces-sary lubrication properties to keep down frictionat this high speed.
107
Carbon Brushes
Another type of brush consisting of graphite andlamp -black, and known as the electro-graphiticbrush, is made for use with high -voltage machineswhich have very high commutator speeds. Thesebrushes have very high contact resistance, whichpromotes good commutation. They can be used tocarry up to 35 amperes per square inch and oncommutators with surface speeds of 3000 to 5000feet per minute.
These brushes are made in several grades, ac-cording to their hardness; the harder ones are welladapted for use on high-speed fan motors, vacuumcleaners, drill -motors with soft mica, D. C. genera-tors, industrial motors, and the D.C. side of rotaryconverters. They are also used for street railwaymotors and automobile generators, and those of aspecial grade are used for high-speed turbine -drivengenerators and high-speed converters.19. BRUSH PRESSURE OR TENSION
It is very important to keep the springs of brushholders or brush hammers properly adjusted sothey will apply an even amount of pressure on allbrushes. If the pressure is higher on one brushthan an another, the brush with the higher pressuremakes the best contact to the commutator surfaceand will carry more than its share of the current.This will probably cause that brush to become over-heated.
To remedy this, the spring tension should beincreased on the brushes which are operating cool,until they carry their share of the load.
Brush pressure should usually be from 1Y4 to 3lbs. per square inch of brush contact surface. Thisbrush tension can be tested and adjusted by theuse of a small spring scale attached to the end ofthe brush spring or hammer, directly over the topof the brush. Then adjust the brush holder springuntil it requires the right amount of pull on thescale to lift the spring or hammer from the headof the brush. One can usually tell merely by liftingthe brushes by hand, whether or not there are somebrushes with very light tension and others with tooheavy tension or pressure.
108
Direct Current
Motors used on street cars, truckS, and movingvehicles, or in places where they are subject tosevere vibration, will usually require a higherbrush -pressure to keep the brushes well seated. Onsuch motors the pressure required may even rangeas high as 4 lbs. per square inch.
20. BRUSH LEADS OR SHUNTS
All brushes should be provided with flexible cop-per leads, which are often called "pigtails" or brushshunts. These leads should be securely connectedto the brushes and also to the terminal screws or
Fig. 14. Above are shown several types of terminals for brushshunts or leads.
bolts on the brush holders, and their purpose isto provide a low -resistance path to carry the cur-rent from the brush to the holder studs.
If these brush shunts or leads become loose orbroken, the current will then have to flow fromthe brush through the holder or brush hammersand springs. This will often cause arcing that willdamage the brush and holder and, in many caseswill overheat the springs so that they becomesoftened and weakened and don't apply the propertension on the brushes.
The brushes shown in Figs. 11 and 12 are equip-ped with leads or brush shunts of this type andFig. 14 shows a number of the types of copper ter-minals or clips that are used to attach these leads
109
Carbon Brushes
to the brush holders by means of terminal screws orbolts.
' When new brushes are cut from standard blocksof brush material, the pigtails can be attached bydrilling a hole in the brush and either screwing theend of the pigtail into threads in this hole or pack-ing the strands of wire in the hole with a specialcontact cement.
ar.,4111164
1111171zweasimao
fgrirIEZ222rorsZMO
65111%imalv"...easimc.a
t1111111111 emensowioalwassamm0641C
Fig. 15. Brush shunts can be obtained with threaded plugs on theirends for securely attaching them to the brushes.
Fig. 15 shows a number of brush shunts or leadswith threaded plugs attached to them. These leadscan be purchased in different sizes already equippedwith threaded end plugs.
Fig. 16 illustrates the method of preparing abrush and inserting the threaded ends of the brushshunts. The top view shows a bar of brush stockfrom which the brushes may be cut, and in thecenter is shown the manner of drilling a hole inthe corner of the brush.
Carbon graphite brushes are soft enough to bedrilled easily with an ordinary metal drill, and theyare then tapped with a hand tap, as shown in theleft view in Fig. 16. The threaded plug on theend of the copper lead can then be screwed intothis hole in the brush by means of pliers, as shownin the lower right-hand view of the same figure.
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Direct Current
Brush leads of this type save considerable timein preparing new brushes, and insure a good lowresistance connection to the brush. These leadscan be unscrewed and removed from worn brushes,and used over a number of times.21. CEMENT FOR ATTACHING LEADS
TO BRUSHESWhen brush leads with threaded tips are not
available, a special compound or cement can bemade by mixing powdered bronze and mercury.The bronze powder should first be soaked in muri-atic or hydrochloric acid, to thoroughly clean it.
The acid should then be washed from the powderwith lukewarm water. The bronze powder can thenbe mixed with mercury to form a thick paste. Thispaste is tamped solidly around the copper strandsof the brush lead in the hole in the carbon brush,and will very soon harden and make a secure con-nection of low resistance.
Care must be used not to make this cement toothick or it may harden before it can be tampedin 'place. It is usually advisable to mix only a verysmall quantity of the paste at oile time, becauseit may require a little experimenting to get it justthe right consistency.22. DUPLICATING AND ORDERING
BRUSHESWorn or broken brushes should always be
promptly replaced, before they cause severe spark-ing and damage to the commutator. Always re-place brushes with ,others of the same grade ofmaterial if possible. The new brushes should alsobe carefully cut to the same size, so they will spanjust the same width on the commutator bar andwill not fit too tight or too loose in the holders.
If it is necessary in emergencies to replace oneor more brushes with others of a slightly differentgrade, place all those of one grade in the positivebrush holders, and those of the other grade in thenegative holders. If brushes of different grades areplaced in the same set of holders, the current willdivide unequally through them and cause heatingof certain ones, and it may also cause unequal wearon the commutator.
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Carbon Brushes
When ordering new brushes for any certain ma-chine, careful measurements should be taken ofthe brush width, thickness, and length. The brushthickness is measured in the direction of travel ofthe commutator or slip rings ; the width is measuredparallel to the armature shaft or commutator bars ;and the length is measured perpendicularly to thecommutator or slip rings surface. These measure-ments are shown in Fig. 13. Any other specialmeasurements should also be given, and in somecases it is well to send the old brush as a sample.
The length of brush leads or shunts 'should alsobe specified when ordering new brushes. They areusually provided in standard lengths of five inches,but can be furnished shorter or longer where re-quired.
The style of terminal or end -clip should also begiven, along with the diameter of the slot or holeby which they are attached to the bolts on thebrush -holder studs.
Fig. 16. These views illustrate the method of preparing a carbonbrush to attach the threaded end of the brush lead or pigtail.
It is generally advisable to have on hand a fewof the brushes most commonly required for re-placement on any machines you may be maintain-ing. It is also well to have a catalogue of some
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Direct Current
reliable brush manufacturer, to simplify orderingby giving the number and exact specifications ofthe brushes required.23. FITTING NEW BRUSHES TO THE
COMMUTATORNew brushes should always be carefully fitted
to the surface or curvature of the commutator.This can be done by setting the brush in the holderwith the spring tension applied, and then drawinga piece of sandpaper under the contact surface orface of the bruSh, as. shown in the center view inFig. 17. Note :..Never use emery cloth because themetallic nature of the emery tends to short circuitthe commutator.
The sandpaper should be laid on the commutatorwith the smooth side next to the bars and therough or sanded side against the face of the brush.Then, with the brush held against the paper bythe brush spring, draw the paper back and forthuntil the face of the brush is cut to the same shapeas the commutator surface.
Be sure to hold the ends of the sandpaper downalong the commutator surface so these ends willnot cut the edges of the brush up away from thecommutator bars.
On small- machines where it is difficult to usesandpaper in the manner just described, a brush -seater stone can be used. These stones consist offine sand pressed in block or stick form with acement binder.
The brush seater is held against the surface ofthe commutator in front of the brush, as shown onthe left in Fig. 17, and as the''commutator revolvesit wears off sharp particles of sand and carries themunder the brush, thus cutting out the end of thebrush until it fits the commutator.
When fitting a number of brushes, a brush jigsuch as shown on the right in Fig. 17 can be usedto save considerable time. This jig can be made ofeither metal or wood, and in the form of a box intowhich the brush will fit. The open end of the boxor jig has its sides cut to the same curve as thecommutator surface.
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Carbon Brushes
A new brush can then be dropped in this boxand its face cut out to the curve or edge of thebox my means of a file. The bulk of the carboncan be cut out very quickly in this manner, andthe brush can then be set in the holder and givena little final shaping with sandpaper as previouslyexplained.
Graphite brushes should generally be used oniron slip rings on three -wire generators, and metal -graphite brushes on copper rings.
On certain very low voltage machines whereheavy currents are handled, "copper leaf" brushes
Fig. 17. The above sketches show several methods of cutting brushfaces to fit the commutator surface. Each is explained in theaccompanying paragraphs.
Fig. 18. Brushes made of copper strips or copper "leaf" constructionare often used for low voltage machines which handle very heavycurrents.
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are used. These are made of a number of thinflat strips of hard drawn copper, with the end ofthe group beveled as shown in Fig. 18.
When brushes of too low resistance are used,they will generally cause long, yellow, trailingsparks at the commutator surface behind thebrushes.
Brushes of too high resistance will cause bluesparks and will also cause the brushes to overheat.
If the commutator mica is not being cut downby the brushes and becomes too high, it will causesparking and burned, spaces to the rear of the micasegments, on the leading edges of the commutatorbars.
The proper type of brushes and their properfitting, well deserve thorough attention on the partof any electrical maintenance man or power plantoperator; as a great many troubles in motors andgenerators can be prevented or cured by intelligentselection and care of brushes.
EXPERIMENTS
Some valuable experience in making brushes canbe obtained by using an old piece of carbon, suchas may be taken fram an old worn out dry cell.
We do not recommend the use of this carbon forbrushes to be used on regular jobs, but the experi-ence gained in working on carbon of this kind willbe just as good as when using expensive brushcarbon.
Brushes of several different dimensions may becut from an old battery. carbon. As an example, ifyou wish to make a brush Y2" thick by /4" wide by3/4 long, you would first cut a Y2" piece from theend of the rod and then it would be a simple matterto lay this piece flat on the bench and measure offthe proper width and length. A hack saw shouldbe used in making the cuts and then the piece maybe smoothed up with a piece of sand paper or a file.
After the pieces are cut to the proper size, theyshould be drilled and tapped for the "pigtail" leads(sometimes'called shunts or leaders)
The next step is to practice fitting the brush tothe commutator. If a commutator is not at hand,
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Carbon Brushes
you may use any round object such as a bottle or atomato can.
The instructions given in the lesson should becarefully followed until you are capable of doinga good job of making and fitting a new brush.
You are then ready to use a good grade of brushcarbon and construct brushes for actual use onmotors or generators.
EXAMINATION QUESTIONS
1. Why should commutation take place while thecoils are in the neutral plane?
2. What happens to the position of the neutralplane on a generator as the load is increased?
3. Why are interpoles used on D.C. machines?
4. How does the polarity of generator interpolescompare to the polarity of adjacent main poles?
5. How are interpoles connected so that theirstrength will be proportional to the load current?
6. In what two ways may the strength of inter -poles be varied?
7. Which direction should motor brushes be shift-ed to improve commutation when the load is in-creased?
8. Give three requirements for a good brush.
9. What is the resistance per cu. inch of an or-dinary carbon brush?
10. In cases where threaded "pigtail" leads are notavailable, how may the wire be fastened to thebrush?
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Direct Current
D. C. SWITCHBOARDS
In power plants, substations, and industrial plantswhere large amounts of electric power are generatedor used, it is necessary to have some central pointfrom which to control and measure this power. Forthis purpose switchboards are used.
The function of the switchboard is to provide aconvenient mounting for the knife switches, circuitbreakers, rheostats, and meters which are used tocontrol and measure the current. The equipmentlocated on the switchboard is generally calledswitchgear.
1. TYPES OF SWITCHBOARDSSwitchboards are of two common types, known
as panel boards and bench boards. The latter arealso called Desk -type boards.
Panel -type boards consist of vertical panels of theproper height and width, on the face of which theswitchgear is mounted. On the rear of the boardare located the bus bars and wires which connectthe switches, circuit breakers, and meters to thevarious power circuits. Fig. 10 on pages 18 and 19shows a panel -type switchboard for a D. C. powerplant. Examine it carefully and note its construc-tion and the arrangement of the equipment mountedon it.
Bench -type switchboards have the lower sectionbuilt like a bench with a sloping top, and above therear edge of the bench section is a vertical panelwhich -contains the instruments.
The sketch in Fig..1 shows an end view of abench -type switchboard with the panels mounted ona pipework frame. Boards of this type are usedmostly for remote -control switchboards, where theswitches and circuit breakers ,are operated by elec-tro-magnets and solenoids, which are controlled bysmall push-button or knife switches on the benchportion of the board.2. SWITCHBOARD PANEL MATERIALS
Switchboard panels are sometimes made of slate
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Generator Panels
or marble, as these materials are good insulatorsand have good mechanical strength as supports forthe switchgear.
Slate is cheaper than marble and is easier to drilland cut for mounting on the frames and for mount-ing the switchgear. Slate is not' quite as good an in-sulator, however, and is usually not used for vol-tages over 500 or 750.
Marble is a better insulator 'and can be used onvoltages up to 1100. Marble presents an excellentappearance, but it is more difficult to keep clean. Itis also very hard to drill or cut.
A newer material recently developed for switch-board panels, and known as ebony asbestos, has anumber of very important advantages for this work.It is made of a composition material in which asbes-tos fibre and electrical insulating compounds aremixed and formed under great pressure into smooth -surfaced panels.
This material has a beautiful natural black finish,is lighter in weight, and has better insulating quali-ties and mechanical strength than either slate ormarble. In addition to these advantages, ebony as-bestos is also much easier to drill and cut, whichmakes it easy and economical to install.
Steel panels are also coming into use for switch-boards, and have the advantage of great strengthand durability. The switchgear on steel panelsmust, of course, be insulated from the metal at allpoints.3. GENERATOR AND FEEDER PANELS
The common panel -type switchboards are usuallymade about ninety inches high, and as wide asnecessary to provide the required space for theequipment needed. They are practically alwaysbuilt up in vertical sections or panels, each of whichis used for the control of separate circuits. Panelsof greater width are used for the main circuits orgenerator circuit control, and sub -panels of nar-rower width are used to control the separate feedercircuits, which supply the energy to the variouslines or power circuits controlled from the switch-board.
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Fig. 10 shows two generator panels on the right,and six feeder panels on the left. Note the differencein the size of the switches and circuit breakers onthe main panels and sub -panels. By referring tothis same figure, you will also note that each verti-cal panel is divided into three sections. This typeof construction facilitates repairs and changes ofcertain equipment, without disturbing the rest ofthe equipment on that panel.
Fig. 1. The above diagram shows an end -view of a "bench -type"switchboard mounted on pipe frame work. This type of board isoften referred to as "desk type".
For example, if the switches on a panel are to bechanged to others of different size or type, the sec-tion containing them can be removed and a new onedrilled and inserted. It is not necessary to disturb
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Switchboard Frames
the other two sections, or to leave unsightly holesin the board where the old switches were removed.
Sectional construction of panels also reduces thedanger of cracked panels which might result frommechanical strains or vibration if larger singlepanels were used.
Switchboard panel material can be obtained inthicknesses from 1/2", for very small boards for lightduty, to 2" or more for large heavy-duty boards.These panels are usually beveled on the corners ofthe front side, for better appearance.
4. SWITCHBOARD FRAMESSwitchboard panels are commonly mounted on
either angle iron or pipe -work frames.Where angle iron is used, it should be of the
proper size to give the required strength and rigidityfor proper support of the panels and switchgear.
SLAW
hZ- .92
Fig. 3. The above sketches show the method of attaching switch boardpanels to the angle iron frame work. Note how the panels areDotted to angle irons, and the angles bolted together betweenpanels. Also note the type of bolts, nuts, and washers used withthis construction.
The board should not bend or vibrate noticeablyduring operation of heavy knife -switches or circuit -breakers.
Angle iron of 11/2" to 3" is commonly used. Itcan be cut to proper length by means of a hack saw,
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and drilled for the bolts with which the panels areattached, and also for the bolts which hold the angleirons of adjoining panels together.
Fig. 3 shows how the panels should be boltedto the angle irons at h, and the method of boltingthe angle irons together at "h2". The panels shouldbe carefully marked for drilling, so they will line upneatly and give the proper appearance when fin-ished.
Short bolts of the proper length, with washersand nickle-plated cap nuts, can be used to providegood appearance of the front surface of the board.
These bolts and nuts should be tightened suffi-ciently to hold the panels securely, but not tightenough to crack the corners of the panels.
The bolt holes can be drilled in the panels withordinary metal drills used in a breast drill or yankeedrill. Where a large number of holes are to bedrilled an electric drill is faster and more conveni-ent. Slate and marbles are hard and should, there-fore, he drilled slowly or the drill should be cooledwhile it is cutting. Ebony asbestos is very easy todrill ; in fact, nearly as easy as hardwood.
The lower ends of the angle irons should have"feet" bent in them or attached with bolts, forsecure anchorage to the floor. The upper endsshould be braced to keep the switchboard rigid.
5. PIPE FRAMES AND THEIRADVANTAGES
Pipe -work frames are very convenient to install,as they do not require drilling as angle iron does.The pipe frame -work is held together by specialclamps, as shown in Fig. 1. Fittings with holes forthe panel bolts are also provided to clamp on thepipes. The pipes are attached to the floor withthreaded floor -flanges.
Standard pipe sizes can be used ; the commonsizes being PA" to 2", or larger for very heavyboards. Special clamp fittings can be obtained formounting bus insulators and various devices on therear of the board. Other fittings are used for at-taching brace pipes to secure the framework andboard in a vertical position.
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Switchboard Frames
Pipe -work frames are very popular and are ex;tensively used, as they provide a very flexible framewhich can easily be adjusted to fit various panelsand devices by merely sliding the clamp fittings.One of the pipes of the frame can be seen on theleft end of the board shown in Fig. 10.
6. KNIFE SWITCHES. TYPESKnife switches, used for controlling the various
Fig. 4. Above are shown a double -pole and three -pole knife switch.Note carefully the construction of the switch plates, hinges, andclips.
circuits on switchboards, are made in single, double,and three -pole types. ,The smaller and medium sizesare generally two or three -pole ; but the larger onesare generally single -pole, for greater ease of opera-tion. Three -pole switches or three single -poleswitches are used to control the circuits of com-pound generators, the three poles being used in thepositive, negative, and equalizer leads.
Three -pole switches are also used for circuits ofthe Edison three -wire system. Other D. C. circuitsare usually two -wire, and they use either one two -pole or two single -pole switches.
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Direct Current
Equalizer switches are sometimes mounted onsmall panels on pedestals near the generators, toeliminate the necessity of running equalizer bussesto the switchboard. In such cases, the main panelsfor compound generators will also use two -pole
itch CS.
Fig. 5. Single -pole and double -pole switches of a modern type. Notethe manner in which the hinges and clips are attached to theboard and the method of making cable or bus connections to thestuds on the back of the board.
7. CONSTRUCTION OF SWITCHESKnife switches consist of three essential parts
called the blade, hinge, and clips. The blades aremade of flat copper bus bar material and are at-tached to the hinges by means of short bolts andspring washers. This fastening gives the requiredtension for good contact between the blades andhinges, and yet allows freedom of operation. SeeFig. 4.
Switch clips are made of two or more thin,springy pieces of copper, mounted in a block. Theblades are inserted between these clips when theswitch is closed. The clips are usually slotted tomake them more flexible and allow them to makebetter contact with the blade of the switch. Thesedetails of construction can be observed by examina-tion of the switches shown in Fig. 4, and also thoseon the switchboard in Fig. 10.
Switch blades are equipped with insulatinghandles and guards on their free ends. The hinges
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Knife Switches
and clips usually have threaded studs of copper at-tached directly to them, for convenient mounting onthe switchboard panels. Bus bars or cable lugs onthe rear of thq board are attached to these studs bymeans of extra nuts provided with them.
The switch at the left in Fig. 5 shows the studsand the nuts used both for holding the switch onthe board and for attaching cable lugs or bus bars.This switch and also the double -pole switch on theright in this figure, are both of a newer type whichhas double blades and single clip prongs.
Knife switches on switchboards are practically al-ways mounted with the blades in a vertical positionand the clips at the top. This allows easier opera-tion and prevents danger of the switch falling closedby gravity.
8. SWITCH MOUNTING AND CURRENTRATINGS
In mounting switches on the panels, the hingesand clips should be carefully lined up so that theblades will fit well and make good electrical con-tact.
All knife switches are rated in amperes accordingto the copper area of their blades and the contactarea of clips and hinges. They are commonly madein sizes from 50 amperes to one thousand amperecapacity ; and for heavy power circuits they aremade to carry 6000 amperes or more.
You will note that a number of the switches onthe right-hand side of the switchboard in Fig. 10have multiple blades in each pole. This gives a muchgreater contact area between the blade surfaces andhinges and clips, and also allows air to circulatethrough the switches to cool them.
Switches should never be loaded above their ratedcapacity in amperes for any great length of time, orthey will overheat. Hinges or clips which are looseor poorly fitted will also cause overheating of theswitch at these points. If switches are allowed tooverheat too much, the copper will become soft andlose the springy qualities which are necessary fortight fitting of the clips. Overheated switches often
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cause the copper clips or blades to turn a bluishcolor. Switches that have been heated to this ex-tent will probably need to be replaced.
9. CARE AND OPERATION OF SWITCHES
New switches should be carefully fitted and"ground in" before loading. "Grinding in" can bedone by coating the switch -blades with vaseline oroil mixed with abrasive powder, and then openingand closing the switch a number of times. Thisgrinds and polishes the sides of the blades and clipsto make their surfaces perfectly parallel and pro-vide a good contact between them.
Never open knife -switches under heavy load, ifthey have a circuit -breaker in series with them.Opening the switch under load will draw an arc atthe point where the blades leave the clips. Thesearcs tend to burn and roughen the blades and clips,making the switch hard to operate and also destroy-ing the good contact bete cen the blade and clips.
Fig. 6. A number of special types of knife switches are made wi hauxiliary clips and blades, as shown above. These two switchesare used as field discharge switches for generators.
Where circuit -breakers are provided they shouldalways be tripped open first and the knife -switchopened afterward. This prevents arcing at the
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Current Breakers
switch and is also much safer for the operator, asthe arcs drawn by opening switches under heavycurrent load may be very dangerous.
Knife -switches should be kept lubricated with athin film of petroleum jelly or light vaseline.
Special types of knife -switches, with snap -actionblades operated by springs, are made for use in theshunt field circuits of generators. Field circuitswitches often have auxiliary blades to close thefield across a resistance just before the main bladesopen. Such switches are called field dischargeswitches. Two types of these switches are shownin Fig. 6. Their purpose is to prevent the settingup of high voltages by self-induction due to thecollapse of the flux around the shunt, field coils whenthis circuit is opened.
10. CIRCUIT BREAKERSFor opening heavy power -circuits in case of over-
load or short circuits, automatic circuit breakers arecommonly used. These are divided into two generalclasses, known as air circuit -breakers and oil cir-cuit -breakers. Air breakers will be described hereand oil breakers will be covered in a later lesson.
An air circuit -breaker is a type of electric switchequipped with special contacts and a trip coil toopen them automatically in case of overload on thecircuit. Thus they provide for the same protectionwhich would be afforded by fuses.
For circuits which frequently require overloadprotection, circuit -breakers are much more suitablethan fuses, as the breakers can be quickly closed assoon as the fault is removed from the circuit.
Circuit breakers are commonly made in singlepole, double -pole, and three -pole types, and for vari-ous current ratings', the same as knife switches.Figures 7 and 8 show two views of a single -polecircuit -breaker. The view in 7 shows the breakerin closed position, and in 8 it is shown open.
The main current -carrying element or bridgingcontact is made of a number of thin strips of coppercurved in the form of an arch and fitted closely to-gether. This copper leaf construction permits theends of this main contact to fit evenly over the sur-
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Direct Current
face of the two lugs, or stationary contacts, whichare mounted in the switchboard and attached to thebus bars.
Fig. 7. This photo shows a common type of air circuit breaker inclosed position. Note the manner in which the main contacts andauxiliary contacts connect with the stationary contacts on the panel.
11. CIRCUIT -BREAKER OPERATION
When the breaker is closed by means of thehandle, a lever action is used to force the main con-tact tightly against the stationery contacts underconsiderable pressure.
Auxiliary arcing contacts and tips are providedabove the main contact, as shown in the figures.The intermediate contact, or the one directly abovethe main contact, consists of the heavy copperspring with a removable copper tip. The top arcingcontact on the movable element is carried by a longcopper spring and has a removable carbon tip.
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Current Breakers
When the breaker is opened, the main contactopens first and allows the current to continue flow-ing momentarily through the auxiliary contacts.This prevents drawing an arc at the surface of the
Fig. 8. This view shows the same circuit breaker as in Fig. 7, exceptthat it is now in open position. Again note carefully the con-struction and position of the main contacts and arcing contacts.Also note the trip adjustment on the bottom of the breaker.
main contact and eliminates possible damage to thiscontact surface, which must be kept bright andsmooth and of low resistance, in order to carrythe full load current without loss.
The intermediate contact opens next and it maydraw a small arc, because the remaining circuitthrough the carbon tips is of rather high resistance.
The carbon contacts open last and the most severearc is always drawn from these points. Carbonwithstands the heat of the arc fairly well, and thesecontacts are easily and cheaply renewed wheneverthey have been burned too badly by repeated arcs.
Circuit -breakers of this type can usually be trip-ped open by means of a small lever or button, aswell as by the automatic trip coil. When released
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Direct Current
they are thrown quickly open by the action ofsprings or gravity on their moving parts.
Fig. 9 is a sketch showing a side view of an airbreaker in which can be seen the leaf constructionof the main contact, and also the bus stubs to whichthe connections are made at the rear of the board.
Fig. 9. This sketch shows a side -view of a circuit breaker in closedposition and illustrates the copper "leaf" construction of the maincontact. Note the copper stubs which project through the boardfor connections to bus bars.
When a circuit -breaker is closed the contacts closein the reverse order, the carbon tips closing first,intermediate contact second, and the main contactlast. This construction and operation eliminatespractically all arcing and danger of pitting at theends of the main contacts. It is very important,however, to keep the auxiliary contacts and carbonarcing tips properly adjusted and occasionally re-newed, so that they make and break contact in theproper order.
12. CIRCUIT -BREAKER TRIP COILS OROVERLOAD RELEASE
Fig. 11 shows a single -pole and a double -pole cir-cuit breaker which are both in closed position. Theoverload coils, or trip coils, can be seen on each ofthe breakers in this figure. These coils are of the
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Circuit Breakers
series type and consist of a very few turns of heavycopper bar or cable, inside of which is located aniron plunger.
When the coil is connected in series with the lineand breaker contacts, any overload of current willincrease its strength and cause it to draw up theplunger. The plunger then strikes the release latchand allows the breaker to open.
An adjustment is provided for raising and lower-ing the normal or idle position of the plunger sothat the breaker can be set to trip at different cur-rents and loads. Trip coils of this type are knownas series -type overload release coils and are corn-
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Direct Current
inonly used on breakers up to 500 amperes capacity.The circuit -breakers. shown in Figures 7 and 8have electro-magnets and armatures which trip theholding latches, and also an oil dash -pot to delaythe opening of the breaker on light overloads. Theadjustments for these devices can be seen below thebreaker in the figures.
13. SHUNT TRIP COILS AND OVERLOADRELAYS
For circuit breakers of 500 amperes and more, itis not usually practical to use series overload -coils,because of the large sized conductor which wouldbe needed to carry the current.
On these larger breaker, shunt trip coils are used,and these coils are wound with a greater number ofturns of small wire and are operated from an am-meter shunt. Shunt trip coils are not connected.directly to the ammeter shunts, but are operated bya relay which obtains from the ammeter shunt thesmall amount of energy needed for its coil.
The greater the current flow through ammetershunts, the greater will bethe voltage drop in them.This voltage drop is usually only a few milli -volts.and as it is difficult to wind the shunt trip coils tooperate on this small fraction of a volt, so overloadrelays are generally used to close a circuit to thesecoils.
The overload relay is a very sensitive instrument,having a small coil designed to operate on a verylow voltage of 50 to 100 milli -volts ; and this coil isconnected across the ammeter shunt.
The tension spring on the armatures of these re-lays is adjustable so the relay can be made to closeits contact and energize the shunt trip coil on thebreaker, at any desired current load within the rangefor which the relay and breaker are designed.
14. REVERSE CURRENT RELAYSSome circuit -breakers are also equipped with re-
verse current protection to cause them to open incase of reversed polarity of a generator or reversedcurrent flow in the line.
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Circuit Breakers
Reverse -current relays are used to trip the break-ers to obtain this protection. These relays have twoelements or windings similar to the field and arma-ture of a simple motor. One is called the potentialor voltage element, and the other the current ele-ment.
The current coil or element is connected acrossthe terminals of the ammeter shunt. The potentialcoil is connected directly aross the positive andnegative leads or busses and serves to maintain acontant field flux.
The direction of current through the current ele-ment or moving coil of the relay is determined bythe direction of current through the ammeter shunt.When the current through the ammeter shunt is inthe normal direction, the moving coil tends to holdthe relay contacts open and keep the shunt trip -coilof the circuit -breaker de -energized.
If the current through the ammeter shunt is re-versed this will reverse the polarity of the voltagedrop across the shunt and send current through themovable elements of the relay in the opposite direc-tion. This reverses its torque and causes the coilto turn in a direction which closes the relay con-tacts and energizes the shunt trip -coil which tripsthe breaker.
These relays are also adjustable so they can beset to open the circuit -breaker at the desired amountof reversed current.
15. CIRCUIT -BREAKER CARE ANDMOUNTING
Circuit -breakers are one of the most importantpieces of switchgear and afford a great deal of pro-tection to the electrical machinery on their circuitsas well as to operators. They should be kept ingood repair and adjustment, and should be frequ-ently tested to be sure that they will open freelyand. quickly when necessary. The main contactsshould be kept clean and well fitted, and arcing con-tacts should be renewed when badly burned. Op-erating springs and trip coils should be kept care-fully adjusted.
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Direct Current
Heavy-duty circuit -breakers require considerableforce on the handle to close them, and also deliverquite a shock to the switchboard when they flyopen. For this reason, switchboard panels carryingheavy breakers should be thick enough and suffi-ciently well braced to provide a rugged mountingfor the breaker, and to prevent vibration of theboard when the breaker is operated.
0
Fig. 12. Large, heavy duty circuit breaker, equipped with a motor forautomatic reclosing, after it has been tripped either by an overload,or by remote control.
Fig. 12 shows a large circuit -breaker which alsohas a motor for automatically reclosing it. Such
.133
Circuit Breakers
breakers can be equipped for remote control by theoperator or for automatic reclosing by a time ele-ment or relay, after the beaker has been trippedopen for a certain definite period.16. INSTRUMENT SWITCHES
In addition to the knife switches and circuit -breakers, special switches are used for the switch-ing and control of motor circuits. These may be of
Fig. 13. Instrument switches of the above types are frequently usedfor changing the connections of various meters to different switch-board panels and busses.
the plug type, pull and push button type, or rotarybutton type. These switches are mounted in open-ings drilled through the board, so that the handlesor buttons project from the face of the board; andthe switch element is mounted on the rear for con-venient connections to the smaller wires of instru-ments and relays.
Fig. 13 shows two instrument switches of the pulland push type in the upper view and one of the plugtype in the lower view.
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17. BUS BARS, MATERIALS AND MOUNT-ING
Copper bus bars are commonly used for connect-ing together the various switches, circuit breakers,and heavy power circuits on switchboards. Long
Fig. 14. The above diagrams show methods of mounting and insta ingbus bars on the back of switchboards.
busses are usually mounted on insulators attachedto the rear of the switchboard frame or panels,while short lengths may be supported by the studsor bolts to which they connect.
Bus bars are generaly run bare for the lowervoltages up to 750 or, in some plants, even higher.Busses for higher voltages can be wrapped withvarnished cloth or friction tape after they are in-stalled.
Copper bus bar materials can usually be obtainedin sizes from to Y2" in thickness, and from 1" to4", or even 6" wide. When very heavy currents areto be carried, several bus bars are usually run inparallel and mounted with their flat sides vertical,as shown in the right-hand view in Fig. 14.
This arangement of the busses allows air to cir-culate freely through them and helps to keep themcool. The view on the right in. Fig. 14 shows twoseparate busses, "A" and "B", each consisting ofthree bars. One set is the positive bus and one isnegative. Both sets are mounted in a base of in-sulating material, shown at "C", and supported bymetal brackets attached to the switchboard frame.
135
Bus Bars
The insulation used for mounting and spacingthe bars can be hard fibre, slate, bakelite, or ebonyasbestos.
cYdIg 577.47 NAVA'
In the left view in Fig. 14 is shown a single busbar supported by a porcelain bus insulator.
136
Direct Current
Busses opposite polarity and for voltages up to750 should be spaced several inches apart whereverpossible. When they are run closer together theyshould be well mounted and braced so they cannoteasily be bent or vibrated together.
18. CONNECTING BUS BARS TOGETHER
Where bus bars are joined together, they can befastened either by means of bolts through holesdrilled in the copper or by bus clamps which do notrequire drilling the bars.
Fig. 15 illustrates the use of a common type busclamp, consisting of two triangular pieces with threeholes for the bolts which draw the parts of theclamp up tightly and grip the bars together. Theseclamps are very easy to install, as they do not re-quire any drilling of the bars.
Copper bus bars can be cut to the proper lengthwith a hack saw; and where bolts are used for con-nections the bars can be drilled with an ordinarymetal drill.
Fig. 16 shows the method of connecting bus barsto the studs of switches or circuit -breakers, bymeans of two nuts and a short strip of bar con-nected to the main bus by a clamp. All joints andconnections in bus bars should be made tight andsecure, to avoid overheating when the current flowsthrough them. Where the sections join the coppershould be well cleaned of all dirt and oxide.
Copper bus bars of the smaller medium sizescan be easily bent to various angles when neces-sary, but care should be used not to bend the cor-ners too sharply and cause the bar to crack.
In locations where the busses are well ventilatedit is common practice to allow about 1000 amperesper square inch of cross-sectional area of the bars.
19. EXPANSION JOINTS OR LOOPS INBUSSES
Where long busses are run, some allowanceshould be made for expansion and contraction withchanges in temperature, or sufficient strains may be
137
Switchboard Circuits
set up to warp the busses or crack the switchboardpanels by twisting the studs.
A special loop or "U" -bend is sometimes put in along bus to absorb this expansion in the spring ofthe bend. In other cases, bus ends can be over-lapped and held fairly tight with two bus clamps,but not tight enough to prevent the lapped endsfrom sliding on each other under heavy strains. Oneor more short pieces of flexible cable can then beconnected around this joint to carry the currentwithout heating. The cable ends should he solderedinto copper lugs, 'and these securely bolted to thebus on each side of the slip joint.20. SWITCHBOARD LAYOUT AND
CIRCUITS
It is not a difficult matter to lay out and erect anordinary switchboard for a small power plant ordistribution center.
A plan should be laid out on paper for the re-quired number of circuits. The desired switches.circuit -breakers, and meters for the control andmeasurement of the power, should be included inthis sketch or plan.
After the load has been determined for the vari-ous circuits, the size of the switches and devicesfor the proper current ratings can be obtained fromthe manufacturer's specifications.
Panels can then be selected large enough to holdthese devices in neat, uncrowded arrangement.
2'115=
pi
011 Ce
ell 11110101111111 1111111111fIllEiin i
aliiiisi.ummintrumuim
SIDAY ///N62
C7
Fig. 16. Two views showing the method of connecting bus bars to thestuds of switches and circuit breakers.
The simplest type of switchboard would at leastcontain switches for each of the main circuits and
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Direct Current
feeder circuits. There should also be on each ofthese circuits some form of overload protection,such as fuses or circuit -breakers.
On circuits of not over 500 amperes capacity andwhich are very seldom subject to overload, car-tridge fuses will provide economical overload pro-tection.
On heavy power circuits or any circuits which are subject to frequent overloads or occasional short
circuits, circuit -breakers should be used. Circuit -breakers eliminate the replacement of fuse linksand enable the circuit to be closed back into opera-tion more quickly.
Usually it will be desired to measure the load inamperes on some of the circuits, if not on all ofthem. Ammeters of the proper size should be usedfor this purpose.
Where only one generator is used, one voltmetermay be sufficient to check the voltage of the mainbusses. Where several generators are operated in'parallel, we will need one voltmeter for the mainbus and probably one for each generator, in orderto check their voltages before connecting them inparallel.
Sometimes one extra voltmeter is used for check-ing the voltage of any one of the generators whichis being started up. This is done by the use of avoltmeter bus and plug switches for connecting themeter to whichever machine is being started up. Ameter used in this manner is often mounted on ahinged bracket at the end of the switchboard, asshown in Fig. 10.
Wattmeters are often used to obtain instantane-ous readings of the power in certain circuits. Watt-hour meters may be installed for showing the totalpower consumed per hour, per day, or per month,on any circuit.
In medium and larger sized plants, recordingvoltmeters and ammeters are often used to keep adaily record of the voltage and current variations.These instruments will be explained in a later les-son on D.C. meters.
139
Switchboard Wiring
21. SWITCHBOARD WIRING
Fig. 17 shows a wiring diagram for a simple D.C.switchboard with three panels, as shown by thedotted lines. The main generator -control panel ison the left, and contains the main switch, circuit -breaker, voltmeter, ammeter and shunt, and theshunt field rheostat.
The two feeder panels on the right merely have.switches and circuit -breakers in each circuit.
Note that the circuit -breakers and knife switchesare in series in each circuit ; so that, when thebreaker in any circuit is tripped, there will be nocurrent flowing through the switch.
The coils in series with each pole of the circuit -breakers are the series overload-relase coils, whichtrip the breakers in case of an overload of current.
Note that the voltmeter is connected on the gen-erator side of the main switch, so a reading of the
Fig. 17. The above diagram shows the wiringsimple switchboard with one generator panelpanels.
generator voltage can be obtainedchine is connected to the busses.
and equipment for aand two distribution
before the ma -
140
Direct Current
Fig. 18 shows -awirng diagram for a switchboardwith two generator panels and two sub -panels orfeeder panels. A number of feeder panels could beadded to either side of this board if necessary.
Equalizer connections are shown for the genera-tors, which are compound and are to be operated inparallel.
The circuit -breaker trip -coils are not shown inthis diagram.
Circuits for switchboard instruments and meterswhich do not require heavy current, are usuallymade with No. 12 or No. 14 switchboard wire,which has white colored slow -burning insulation.These wires can be held on the back of the boardwith small metal clamps and screws.
Fig. 18. Wiring diagram for a D. C. sw.tchboard with two main gen-erator panels and two or more feeder panels. Additional feederpanels would be connected to the boa d and busses the same as thetwo which are shown. Note careful y the arrangement of all ofthe parts and circuits shown in this diagram.
Examine the wiring and check the locations andconnections of the various devices shown in Fig. 18.22. LOCATION OF METERS AND S*ITCH-
GEAR
Refer again to Fig. 10 and note the positions andarrangement of the various switch -gear and devices
141
Switchboard Wiring
On the board. Knife switches are usually mountedso their handles come about in the center of theboard height, or a little lower, as this height is veryconvenient for their operation. Watthour and re-cording meters are frequently mounted along thelower panel -sections, underneath the knife switches.
Voltmeters and ammeters are usually placedabove the knife switches, at about eye level or alittle above, so they can be easily read.
Circuit -breakers are commonly placed at the topof the board, so any smoke or flame from their arcscannot reach other instruments or blacken and burnthe switchboard.
When air circuit -breakers open under severeover -loads or short circuits, they often draw long,hot arcs. The flame, heat, and smoke from thesearcs are driven upward by their own heat. There-fore, if meters or instruments were located abovethe breakers and close to them, they would belikely to be damaged.
Mounting the breakers at the top of the boardsalso places them up high enough so operators arenot likely to be bumped or burned when the break-ers fly open or, as we say, "kick -out."
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Direct Current
EXAMINATION QUESTIONS
1. Name three common types of switchboards.2. What four materials are commonly used for
switchboard panels?3. In what position are knife switches generally
mounted on switchboards?4. a. Explain what is meant by "grinding in" a
switch.
b. How should this be done?5. a. Name two common types of circuit -
breakers.
b. What is the function and advantage ofcircuit -breakers?
6. a. What is the purpose of the overload tripcoil on a circuit -breaker?
b. A reverse current relay?c. The arcing contact?
7. What care and attention should circuit -break-ers receive?
8. a. How are bus bars generally mounted andattached to switchboards?
b. Why should expansion joints be used onlong busses?
9. How many 1/4 x 2" copper buss bars shouldbe used in parallel to carry 1500 amperes on aswitchboard circuit?
10. Draw a switchboard circuit diagram similarto the one in Fig. 17, but with two generators andfour feeder panels, and include the equalizer con-nections.
143
Switchboard Wiring
144
Direct Current
DIRECT CURRENT METERS
Electrical meters are used for accurately meas-uring the pressure, current, and power in variouselectrical circuits. There are a great number oftypes of meters, some of which are used only inlaboratory work and others that are more com-monly used in every -day work by the practical man.
These latter types are the ones which we willprincipally consider in this section. The metersmost frequently used by electricians and operatorsare the voltmeter, ammeter, and wattmeter. Theseinstruments are made both in portable types andfor switchboard mounting.
1. TYPES OF METERSThe portable meters are used for convenient test-
ing of machines and equipment wherever they arelocated, while the switchboard types are perma-nently mounted on switchboards for measuring theenergy of certain circuits on these boards.
Voltmeters and ammeters are also made in re-cording types, which keep a record of their variousreading throughout certain periods of time.
Wattmeters are divided into two general classes,called indicating and integrating.
The indicating instrument merely indicates thepower in the circuit at any instant at which it isread. Integrating wattmeters, or watthour meters,as they are commonly called, keep summing up thetotal amount of energy in kilowatt hours which isused throughout any certain period of their opera-tion.
2. PARTS AND CONSTRUCTION OF D. C.METERS
Most meters operate on magnetic principles oruse the magnetic effect of electric currents to pro-duce the movement of the meter needle.
145
D. C. Meters
Ordinary D. C. voltmeters and ammeters consistof a permanent magnet of horse -shoe shape whichsupplies a magnetic flux or field, a delicately bal-anced coil of fine wire which is rotated in this field,a pointer, scale, and case.
Fig. 1. The above view shows the important parts of a D. C. volt-meter. Note the horse -shoe magnet which provides the magneticfield in which the movable coil rotates. The movable coil with thepointer attached can also be seen between the magnet poles.
Fig. 1 shows the principal parts of a meter ofthis type, with the case or cover removed. Thepoles of the permanent magnet are equipped withpole shoes which have curved faces to distribute theflux evenly over the rotating element. In the centerof the space between the magnet poles can be seena round soft iron core which aids in concentratingand distributing the magnetic field over the spacein which the coil moves. The needle is attached tothe rotating or moving element so it will swing.across the scale when the coil is rotated. This typeof construction is known as the D'Arsonval, be-cause it was first developed by a Frenchman namedD'Arsonval. The D'Arsonval or moving coil type
146
Direct Current
meter is one of the most accurate and commonlyused types of D. C. Meters.
Fig. 2 shows a separate view of the moving coilwith the needle attached. You will also note thesmall coil -spring on each end of the moving coil.This coil is usually wound with very fine wire on alight -weight aluminum frame, the shaft of which isthen set in jeweled pivots made of first -grade sap-phires. These pivots make it possible for the coilto move with an extremely small amount of energywhich makes the instrument very sensitive and ac-curate.
3. OPERATING PRINCIPLES OF D. C.VOLTMETERS AND AMMETERS
The operating principles of meters of this typeare very similar to those of a D. C. motor. When asmall amount of current is sent through the turns ofthe moving coil, it sets up around this coil a fluxwhich reacts with the flux of the permanent magnetfield and exerts torque to turn the moving coilagainst the action of the fine coil springs. The coilsprings tend to hold the needle, or pointer, innormal or zero position, usually at the left side ofthe scale.
These springs are often called counter torquesprings as they tend to oppose the torque or turn-ing effort of the moving coil. These two springsare wound in opposite directions to compensatefor temperature changes. When an increase oftemperature increases the tension of one spring, itreduces the tension on the other one and thus keepsthe meter more accurate under varying tempera-tures.
The greater the current passed through the mov-ing coil, the stronger will be its flux ; and the re-action between this flux and that of the permanentmagnet will tend to move the needle across the
147
D. C. Meters
scale, until the magnetic torce is balanced by theforce of the springs.
HOW TO MAKE SPECIAL MEASUREMENTS
Portable dynamic -balancer permits bal. Electrode -pressure gage measures forceancing of rotors in their own or other between electrodes of resistance -weldingbearings, at speeds from 600 to 5000 machines. Range: 0 to 4500 lb. Easy torpm. Simple to operate, and readings can use. Also used for checking springs inbe taken quickly. compression, weighing, etc.
Torque meter measures torque, in pound -feet, transmitted by a calibrated shaftwithout absorbing power. It measurestwist in the shaft by electric strain gages.Ranges to 10,000 lb -ft.
Strain gage measures static strains causedby steady compressive, tensile, or bend-ingrapidly
loads,changingo r dynamic strainsds,sc;asbracausedt yn
in machines or airplanes.
The amount of voltage applied to the coil will de-termine the amount of current flow through it. Sothe distance that the needle is moved across thescale will be an indication of the amount of voltageor current in the circuit to which the meter is at-tached.
The same type of meter element can be used foreither a voltmeter or ammeter, according to the
148
Direct Current
manner in which the instrument is connected to thecircuit to be measured.
The permanent magnets used with good -grademeters are made of the best quality of steel, and are'usually aged before they are used in the meters.This aging process leaves them with a certainamount of magnetic strength, which they will retainfor very long periods without noticeable weakening.
The pole shoes are made of good -grade soft iron
HOW TO MAKE SPECIAL MEASUREMENTS
Vimanon-velocity meter for measuringvibration velocity and displacement. Fre-quency range is 10 to 4000 vibrationsper second, and velocity range 1 to40,000 mils per second.
Film -thickness gage measures thicknessof nonmagnetic materials --such as en-amel, lacquers, glass, mica-when placedon steel. Standard scale range is 0 to 0.1inch.
VISCOSIlltetltr measures viscosity of Electric tachometer is used to indicateliquids such as paint, varnish, lacquer, rpm. Can also be calibrated in "leet perjapan, and oil. Time required is approx minute" of circumference. Indicating30 seconds for one measurement. Its meter can be mounted away from therange is 10 to 1200 centipoisea. machine to facilitate reading.
to provide a low reluctance path for the flux of thepermanent magnets. An additional stationary core
149
D. C. Meters
of soft iron is often placed within the rotating coil,to provide a better magnetic path between the poleshoes, and to more evenly distribute the flux.
Fig. 2. An excellent view of the movable coil, pointer and springof a D. C. meter.
4. DAMPING OF METER NEEDLES ORPOINTERS
As the aluminum coil -frame is rotated throughthe flux of a meter of this type, small eddy currentsare induced in the frame. These tend to set up adamping effect which slows or retards the rapidmovement of the coil and needle, making the instru-ment more stable and preventing the needle fromvibrating back and forth with small fluctuations inthe voltage or current.
Some instruments have a light -weight air -vaneattached to the needle, to provide a further dampingeffect and to prevent the needle from strikingagainst the case at the end of the scale when sud-den increases occur in the voltage or current of thecircuit.
Small rubber cushions, or "stops," mounted onlight wire springs are usually provided on eachside of the needle, to limit its travel and preventit from striking against the case. These stops canbe seen in Fig. 1.
150
Direct Current
Meter scales are usually printed in black on awhite cardboard background, and are located di-rectly behind the pointers, as shown in Fig. 4.
Fig. 3. A very convenient portable combination ammeter and triplerange voltmeter. Circuits of different voltages can be tested byconnecting to proper marked terminals.
To obtain very accurate readings, some instru-ments have a mirror strip parallel to the scale anddirectly behind the pointer. In reading a meter ofthis type, one should stand in such a position thatthe pointer covers its own reflection on the mirror.This eliminates viewing the meter from an angleand perhaps reading the voltage or current at ascale line which is not directly under the pointer.
The instrument shown in Fig. 4 is one for switch-board use and is designed to be mounted flushwith the surface of the board by setting the casein an opening cut in the switchboard panel. Thismeter is provided with a marker, or additionalblack needle with a round head, which can be setin any desired position on the scale by turning thebutton on the front of the case. This makes iteasy to tell when the voltage of the generator orcircuit has reached normal value, as the movingneedle would then be directly over the marker.
151
D. C. Meters
5. CARE AND ADJUSTMENT OF METERSBecause of the delicate construction of the mov-
ing coils and the manner in which they are mountedin jeweled bearings, electric meters- should be verycarefully handled when they are being moved
Fig. 4. Switchboard type voltmeter for mounting flush with the sur-face of the board. Note the stationary index pointer or marker.to indicate when full voltage is reached by the movable pointer.
about; because, if they are dropped or severelyjarred it may damage the mechanism and causetheir readings to be inaccurate. Jarring also tendsto weaken the permanent magnets. Meters shouldnot be mounted where they are subject to severevibration or mechanical shocks.
On many meters adjustments are provided bymeans of which the tension on the coil spring canbe regulated by a small screw, thereby correctingany slight inaccuracies in the meter reading. Pivot
152
Direct Current
screws should be kept tight enough to prevent toomuch end play of the shaft and coil, but never tightenough to keep the coil from moving freely.
6. VOLTMETERS
When meter elements of the type just describedare used for voltmeters, the moving coil is connectedin parallel, or across the positive and negative wiresof the circuit on which the voltage is to be meas-ured.
It is difficult to wind a sufficient number of turnson the moving coil to have high enough resistanceto stand the full line voltage on ordinary powerand light circuits. For this reason, special resist-ance coils are connected in series with the movingcoil element and the meter terminals, as shown inFig. 5.
Fig. 5. This diagram shows the parts also the connections for aD. C. voltmeter.
These resistance coils limit the current flowthrough the meter to a very small fraction of anampere, and thereby allow the meter element to
153
D. C. Meters
be constructed of light weight and as delicatelybalanced as required for accuracy. Voltmeter re-sistance coils can be located eithe inside the caseor outside. Portable instruments usually have themlocated within the case, while with switchboardinstruments the resistance coils are sometimesmounted on the back of the switchboard behindthe instrument.
By changing the number of these coils in series,or by changing their size and resistance, we canoften adapt the same meter element for use oncircuits of different voltages. When a meter ischanged in this manner to operate on a different
Fig. 6. The above view shows a D. C. voltmeter of a slightly differenttype, with the case removed to show the resistance coils whichare connected in series with the movable element.
voltage, a different scale will probably also be re-quired.
Fig. 6 shows a view of the inside of a voltmeterin which are mounted four resistance coils that are
154
Direct Current
connected in series with a meter element.Fig. 7 shows two types of external voltmeter re-
sistance coils that can be used for mounting onthe rear of the boards with voltmeters for switch-board use. With these resistance coils in serieswith the voltmeter element, it requires only a fewmilli -volts across the terminals of the moving coil
Fig. 7. External resistors for use with voltmeters and wattmeters.Resistors of this type are to be mounted outside the meter case,and usually on the rear of the switchboard.
itself to send through it enough current to operatethe meter. Therefore, when the instrument is usedwithout the resistance coils it can be connecteddirectly to very low voltage circuits of one voltor less, and used as a milli -volt meter.
Whether it is used with or without the resistancecoils, the strength of the flux of the moving coil
155
D. C. Meters
and the amount of movement of the needle willdepend entirely upon the voltage applied, becausethe current through the coil is directly proportionalto this voltage.
Any tye of voltmeter, whether for portable orswitchboard use, should always be connected acrossthe circuit, as shown in Fig. 8 at "A."
7. AMMETERS AND AMMETER SHUNTS
The construction and parts of an ordinary D.C.ammeter are the same as those of the voltmeter.When the instrument is used as an ammeter, theterminals of the moving coil are connected in paral-lel with an ammeter shunt, and this shunt is con-nected in series with the one side of Ale circuit tobe measured, as shown in Fig. 8-B.
The ammeter shunt is simply a piece of lowresistance metal, the resistance of which has a fixedrelation to that of the ammeter coil. The loadcurrent in flowing through this shunt causes avoltage drop of just a few milli -volts and this isthe voltage applied to the terminals of the ammetercoil.
In other words, the meter element simply meas-ures the milli -volt drop across the shunt ; but, asthis drop is always proportional to the currentflowing through the shunt, the meter can be madeso that the load in amperes can be read directlyfrom the meter scale.
This principle can be explained by anothermethod, as follows : We know that electric currentwill always divide through any number of parallelpaths which it is given. As the ammeter shunt isconnected in parallel with the instrument coil andis of much lower resistance than this coil, thegreater part of the load current passes through theshunt, and only a very small fraction of the currentflows through the meter coil.
The use of a shunt in this manner eliminates thenecessity of constructing meter coils large enough
156
Direct Current
to carry the load current. This would be practicallyimpossible on meters of this style for heavy dutycircuits. Shunts also make possible the use of thesame type of moving coil element for either am-meters or voltmeters.
Ammeter shunts for portable instruments areusually mounted inside the instrument case ; andfor switchboard instruments on heavy power cir-cuits, the shunt is usually mounted on the rear ofthe switchboard.
To obtain accurate readings on the meters, am-meter shunts should be made of material the re-sistance of which will riot change materially withordinary changes in temperature, as the shunt maybecome heated to a certain extent by the flow ofthe load current through it. The material commonlyused for these shunts is an alloy of copper, manga-nese, and nickel, and is called "manganin." This
Fig. 8. This diagram shows the proper methods of connecting volt-meters and ammeters to electric circuits. Note carefully the man-ner of connecting voltmeters in parallel with the line and am-meters or their shunts in series with the line.
alloy has a temperature co -efficient of almost zero ;in other words, its resistance doesn't vary any ap-preciable amount with changes in its temperature.Manganin is used also because it doesn't develop
157
D. C. Meters
thermo-electric currents from its contact with thecopper terminals at its ends.
Ammeter shunts for use with D.C. ammeters aremade in sizes up to several thousand amperescapacity. Fig. 9 shows several sizes and typesof these shunts. Note the manner in which thestrips of alloy are assembled in parallel betweenthe bus connector stubs. This allows air circulationthrough the shunt to cool it.
8. CONNECTION OF AMMETERS A.NDSHUNTS
Remember that ammeter shunts or ammetersmust always be connected in series with the line
Fig. 9. The above photo shows several sizes and types of ammetershunts which are generally used with ammeters where heavy loadsare to be measured.
and never in parallel. The resistance of shunts isvery low and if they were connected in parallelacross positive and negative wires of a circuit, itwould produce a severe short circuit, which onheavy circuits would be dangerous to the personconnecting the meter and would at least blow the
158
Direct Current
fuse and kick out circuit breakers. ft would alsoprobably burn out the meter or destroy the shunt.
rig. 10. Portable meters of the above type are very convenient andnecessary devices for the practical electrician to use in testingvarious machines and circuits.
Fig. 10. shows a common type of portable meter,such as is used in testing various electrical ma-chines and circuits. The protective case and con-venient carrying handle make these instrumentsvery handy for use on the job. Voltmeters, am-meters, and wattmeters of this type are very essen-tial in any plant where a large number of electricmachines are to be maintained.
Testing the voltage and current of motors ofdifferent sizes will often disclose an overload ordefective condition in time to prevent a completeburnout or serious damage to the machine windings.
Some portable instruments have two separate
159
Meters
elements in the case and two separate scales, onefor a voltmeter and one for an ammeter. Portableinstruments of this type are very convenient fortests, but extreme care must he used to be sure toconnect the voltmeter terminals in parallel and theammeter terminals in series with any circuit to betested.
Fig. 11. Switchboard type ammeter for surface mounting. This me erdoes not require any large opening to be cut in the switchboardpanel.
D. C. meters must be connected to the line withthe proper polarity, and their terminals are usuallymarked "positive" and "negative," as shown in Fig.11. If meters of this type are connected to theline with wrong polarity, the needle will tend tomove backwards and will be forced against the stopwire or case at the zero end of the scale.
The meter shown in Fig. 11 is another type ofswitchboard meter for surface mounting. Thisinstrument doesn't require cutting any opening in
160
Direct Current
the switchboard panel, since the meter is mountedflat against the front surface of the panel.
Fig. 13. This photo shows a high grade portable combination volt-meter and ammeter combined in one convenient carrying case.Such instruments are very handy for maintenance electricians.
These meters are often mounted on a hingedbracket at the end of the switchboard so that theycan be seen from any point along the board.
9. CHANGING METERS FOR HIGHER ORLOWER READINGS
In certain cases an electrician may not have suit-able meters for testing all of the various circuitvoltages and current loads in the plant, and insuch cases it is often very convenient to know howto change the meters on hand to indicate voltagesor currents other than those for which they weredesigned. This can be quite easily done by chang-ing the resistors on voltmeters, or the shunts usedwith ammeters.
In recent years instrument manufacturers havebegun to standardize on the construction of essen-tial parts of meters. This not only reduces original
161
D. C. Meters
costs and makes it easier to secure repair parts, butit also makes certain meters more flexible or adap-table to a wider range of service. For example,many voltmeters and ammeters are now made witha standard moving coil having a resistance of 2Y2ohms and designed to give full scale deflection ofthe pointer with a current of 20 milliamperes, or.020 ampere. According to Ohms law formulaI X R = E, or .0201 X 2.5R = .050E, or 50 milli-volts drop, or pressure applied to force full scalecurrent through this coil. A coil having 2Y2 Ohmsfor a 50 millivolt reading would be on a basis of50 OHMS PER VOLT. As one volt or 1,000m. v. 50 m. v. - 20, and 20 X 2.5=5b.
Now if we wish to use this 50 m. v. meter tomeasure 100 m. v. or double the present voltagerating we should simply double the resistance ofthe meter circuit or add 272 ohms more resistancein series with the 272 ohm moving coil. Then2Y2 2Y2 = 5 ohms total resistance, which whenconnected across a 100 m. v. circuit would draw.100 -4- 5 or .020 ampere and again give full scaledeflection. If we now re -mark or recalibrate thescale for 100 m. v. we have doubled the range of themeter.
You can readily see that if the 272 ohm coil wereconnected on a circuit of double its rated voltagewithout increasing the resistance the coil wouldreceive double current and be burned out. There-fore in changing voltmeter resistances to adapt themeter for correct readings of higher voltage valueswe simply use the following formula to determinethe correct resistors to use in series with the metercoil :-Desired voltage range total resistance
- for meter cir-full scale meter coil current cuit.
162
Direct Current
Then by subtracting the resistance of the movingcoil from this total resistance we cart determinethe amount of extra resistance to use in series forthe higher readings. For example, suppose we wishto adapt this same meter element for a full scalereading of 150 volts, and for safe use on a 150 voltcircuit.
150 EThen
.020 I- 7500 R. total resistance.
Then 7500 - 2.5 = 7497.5 ohms of additionalresistance to be used.
Fig. 14. This diagram shoWs a meter designed for reading twodifferent voltages.
If we wish to use the same meter for dual serv-ice or 150 and 300 volt circuits we can arrange an-other resistor of 14997.5 ohms as shown in Fig. 14.
163
D. C. Meters
Then by connecting the wires of the circuits tobe tested to the proper terminals or resistors wecan measure either voltage. Some multiple rangemeters have these extra resistors located inside thecase and connected to proper terminals. Thesemeters may also have the scale marked for 3 ormore voltage ranges as in Fig. 3.
Line -
Fig. 15. This figure shows how the ammeter shunt is connected inparallel with the meter coil.
The same changes can be applied to ammetersto adapt them for other ranges by changing theresistance of the shunts which are used with thissame standard meter element and 2%2 ohm movingcoil. Using only the meter coil without any shuntthe instrument's capacity and full scale readingwould be only 20 milliamperes. If we wish to changeit to measure current up to 100 M. A. or 5 timesits former rating, we would place in parallel withthe moving coil a shunt having a resistance .1/4th
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Direct Current
that of the coil or, 2.5 4 = .625 Ohms resistancefor the shunt.
With this shunt connected in parallel with 'themeter as shown in Fig. 15, the current will dividein inverse proportion to the resistance of the twoparallel paths, and 4/5 of the current or 80 M. A.will pass through the shunt, while 1/5 of the cur-rent or 20 M. A. will pass through the meter coil.
When making 'such changes for scale readingsof 2 amperes or less, we should determine the shuntresistance according to the desired division of cur-rent between the meter coil and the shunt, as wehave just done in the foregoing problem. This isdue to the fact that in order to obtain readingswhich are. accurate at least within one per centon such small current loads we must consider theamount of current which flows through the meterelement. However for changes over 2 amperesthe following simple formula can be used to de-termine the shunt resistance :voltage rating of meter coil
resistance of shunt.desired current capacityThen if we want to change this same type of
meter with the 50 millivolt coil to measure currents.050
up to 10 amperes at full scale reading -= .00510
ohm shunt to be used in paralled with the meterelement. Note that the shunt resistance is 1/500of ttie meter coil resistance, and the meter coil cur-rent of .020 is 1/500 of the new full scale current of10 amperes.
If we desire to change this type of meter to read.050
200 amperes, then =7-- .00025 ohm shunt.200
Meters which use these standard coils of 2%2ohms resistance for 20 M. A. current at 50 ohms
165
D. C. Meters
per volt are guaranteed by the manufacturers foraccuracy within one per cent. This is accurateenough for all ordinary shop tests. When a higherdegree of accuracy is required for laboratory mea-surements, etc., meters with higher resistance mov-ing elements are used. The more resistance pervolt which is used in the meter coil, the higherthe degree of accuracy will be.
10. INDICATING WATTMETERS
Wattmeters, as previously mentioned, are usedfor measuring the power of circuits in watts. Asthis power is proportional to both the voltage andamperage of the circuit, wattmeters 'use two coils,one of which is known as the voltage or potentialelement, and the other as the current element.
The potential element is connected across theline, similarly to a voltmeter coil ; while the currentelement is connected in series with one side of theline, similarly to an ammeter coil.
A diagram of the internal wiring and the con-nections of a wattmeter is shown in Fig. 16.
The potential coil is the movable element andis wound with very fine wire and connected in serieswith resistance coils, similarly to those used in thevoltmeter. As this coil is connected across the line,the strength of its flux will always be proportionalto the line voltage.
The current element is stationary and consistsof a few turns of larger wire. As this coil is con-nected in series with the line, its strength will beproportional to the load and the current which isflowing. This current element supplies the fieldand takes the place of the permanent magnet usedin voltmeters and ammeters.
As the turning effort, or torque, exerted on the
166
Direct Current
movable coil is the result of reaction between itsflux and the flux of the current element, the pointermovement will always be proportional to the
CurrentElement
SupplyLine
PotentialElement
Resistance.
Load
Fig. 16. This diagram shows the potential and current coils of a watt-meter. Note the manner in which each of these elements are con-nected in the circuit. The movable coil is shown in a sectionalview so you can observe the direction of current through its turnsand note how the flux of this movable coil will react with thatof the current coils and cause the pointer to move.
product of these two fields and will, therefore, readthe power in watts directly from the scale.
The coils of these instruments are not wound oniron cores but are wound on non-magnetic spoolsor in some cases the wires are stiff enough to holdtheir own shape in the coils. Wattmeters of thisdesign, can be used on either D.C. or A.C., as theywill read correctly on A.C. circuits if the reactancesof both the moving and stationary coils are equal.
Wattmeters are designed for different amountsof voltage and current and should never be usedon circuits with a greater amount of power in wattsthan they are rated for, nor circuits with highervoltage or heavier currents than the instrumentsare designed for.
The terminals for the potential and current ele-ments can be distinguished by their size, as those
167
D. C. Meters
of the current element are usually much larger thanthose of the potential element. Extreme care should
Fig. 17. This view shows the current coils, resistor coils, and generalconstruction of a common type wattmeter.
be used never to connect these in the wrong rela-tion to the circuit, because if the current coil isconnected across the line, a short circuit will re-sult.
Fig. 17 shows the internal construction of a D.C.wattmeter. In this view the current coils, consist-ing of a few turns of heavy wire, can be plainlyseen. The potential coil cannot be seen, however,as it is inside of the current coil.
168
Direct Current
EXAMINATION QUESTIONS
1. Name three important parts of an ordinaryD.C. voltmeter or ammeter.
2. Describe two methods by which the meterneedle is protected against striking the case whensudden increases of voltage or current take placethrough the instrument.
3. Why are mirrors used in certain meters?4. How does the resistance of a voltmeter in-
cluding the resistance coils compare with the re-sistance of an ammeter including the shunt?
5. How are voltmeters and ammeters connectedwith relation to the line and load?
6. a. Are voltmeter resistances connected inseries or parallel with the meter element?
b. Are ammeter shunts connected in series orparallel with the meter element?
7. On a certain voltmeter with a coil *resistanceof 2.5 ohms, and a full scale current of .020 am-peres, what resistance must be connected in serieswith the coil to adapt the meter for use in mea-suring voltages up to 220 volts?
8. Give the formula to be used in figuring theshunt resistance on meters of over 2 amperesrating.
9. How many coils are used in Wattmeters?Name them.
10. How are Wattmeter coils connected withrelation to the line?
169
D. C. Meters
170
Direct Current
WATTHOUR METERS
The common type of meter used in homes, fac-tories, and power plants for measuring in kilowatthours the total amount of power used during anycertain period, is known as a watthour meter.
These meters have a current and potential ele-ment somewhat similar to those in the indicatingwattmeter. The potential element, however, isallowed to revolve continuously, like the armatureof a D.C. motor, as long as there is any load on thecircuit or line to which the meter is attached.
This element is not limited to a fraction of aturn by the coil springs, as in the case of indicatingmeters, but is mounted on a vertical shaft set injeweled bearings and is free to revolve completelyaround, with the application of a very small torque.
This rotating element is connected to a seriesof gears which operate the hands or pointers onthe clock -like dials of these instruments. The cur-rent element consists of a few turns of large wireand is connected in series with the line, or in paral-lel with an ammeter shunt which is connected inseries with the line. This stationary current coilprovides a magnetic 'field similar to that of a D.C.motor, and in which the potential coil or. armatureelement rotates.
1. PRINCIPLES OF WATTHOUR METERSThe potential element, being connected across the
positive and negative leads of the line, is always ex-cited and has a very small current flowing throughit as long as it is connected to the circuit. Thiscoil, usually has additional resistance coils placedin series with it, to limit the current flow to a verysmall value. Therefore, it doesn't waste any appre-ciable amount of current by being permanentlyconnected across the line.
As long as no load current is flowing throughthe line and the current element of the meter, thereis no field flux for the flux of the potential coil toreact with, and so it doesn't turn. As soon as loadis applied to the line and current starts to flow
171
Watthour Meters
through the stationary coils, it sets up a field whichreacts with that of the potential coil, causing thelatter to start to turn.
The greater the load of current, the stronger will
Fig. 1. The above photo shows several potential or armature coils ofwatthour meters and illustrates the manner in which they arewound.
be this field and the faster will be the rotation ofthe potential element or armature. This will causethe pointers on the dials to revolve faster and totalup power more rapidly. The longer the load isleft on the circuit, the farther these pointers willbe revolved and the greater will be the total powerreading.
2. CONSTRUCTION OF POTENTIAL ANDCURRENT COILS
Fig. 1 shows three views of the armature orpotential element of a watthour meter, both partlywound and completely wound. The coils of finewire are wound on a drum or hollow ball of lightweight non-magnetic material and are held in placeby a coating of insulating compound. You willnote that they are wound similarly to the coils ofa simple D.C. motor armature. The leads of thecoils are brought up to a very small commutatorlocated on the top end of the shaft at the right.
Fig. 2 'shows both the current coils and poten-tial coil of a watthour meter. The current coilsare wound of heavy copper strip and are eachdivided into two sections. They are mounted close
172
Direct Current
to the potential or rotating element, which can beseen just inside of them. You will note at the topof this figure the very small metal brushes mountedon wire springs and in contact with the small com-mutator to which the leads of the potential elementare attached. Directly above this commutator isthe small wormgear which drives the series of smallgears that operate the dials. The brushes of themeter are connected in series with the proper re-sistance coils and then across the line wires, andthey complete the. circuit through the potential ele-ment, or armature, of the meter.
Fig. 2. This view shows the potential and current coils and also thecommutator and brushes of a watthour meter.
These brushes are commonly made of silver orsome very good conducting material, in order toprevent resistance and voltage drop at the brushcontact with the commutator.
3. DAMPING DISK AND MAGNETS
The speed at which the armature of the watthourmeter will rotate depends upon the voltage appliedto the potential element and the current flowing
173
Watthour Meters
through the current element. Because of the veryslow speed at which this armature revolves, itsspeed is not regulated by counter-E.M.F., as arma-tures of direct current motors are.
In order to prevent over -speeding and to makethe driving torque remain proportional to the powerapplied, the motor armature must have some damp-ing or retarding effect to oppose the torque exertedby the magnetic fields. This counter -torque is ob-tained by mounting a thin aluminum disk upon thelower end of the armature shaft, and allowing itto rotate in the field of one or more permanentmagnets of the horse -shoe type. This disk and thedamping magnets can be seen in the lower partof the meter, shown in Fig. 3 with the cover re-moved.
As the aluminum disk is rotated, it cuts throughthe lines of force from the magnet poles and thisgenerates eddy currents in the disk. The reactionbetween the flux of these eddy currents and thatof the magnets tends to oppose rotation, just asplacing a load upon a generator will producecounter -torque and require effort from the primemover to turn it.
The induced eddy currents will be proportionalto the speed of rotation of the disk and, as the fluxof the permanent magnets is constant, the counter-torque exerted by the disk will be proportional tothe product of the flux from these eddy currentsand that from the permanent magnets.
When the load on the meter is increased, thespeed of its armature increases, until the counter -torque developed by the disk just balances thetorque exerted by the armature. In this manner,the armature speed is maintained proportional towhatever load is applied to the meter, causing thepointers on the dials to read the correct power inkilowatt hours.
This type of meter is often referred to as a watt-hour meter, but the gears and speed of most of themare so adjusted and of the proper ratio so that thereadings will be in kilowatt hours, instead of watt-hours.4. ADJUSTING DAMPING EFFECT
The amount of damping effect produced by the
174
Direct Current
disk can be adjusted by moving the poles of thepermanent magnets in or out along the disk. Ifthe poles are moved closer to the outer edge ofthe disk where it will cut their flux at higher speed,a greater amount of eddy current will be inducedand cause a greater damping effect, and if the mag-net poles are moved closer to the shaft where thedisk is traveling at lower speed the induced eddycurrents will be less, and the damping effect willbe reduced.
Fig. 3. Ccmplete view of a KW -hour meter with th' cover removedclearly showing the dials, current and potential coils, compensatingcoil, damping disk, and drag magnets.
5. COMPENSATING COILNo matter how carefully the armature of a meter
of this type may be mounted, there is always aslight amount of friction to offer resistance to its
175
Watthour Meters
rotation. Some of the energy produced by themeter coils will be required to overcome this fric-tion and the friction of the gears on the dials.
In order to make a meter register accurately onlight loads, this friction should be compensated for.This is done by means of a coil consisting of manyturns of fine wire, connected in series with thearmature or voltage coil of the meter. This com-pensating coil is mounted on an adjustable bracketin a position where its flux will react with that ofthe potential and current coils.
This coil can be seen in front of the current coilsand armature of the meter shown in Fig. 3. Byhaving this coil adjustable, it can be moved closerto or farther from the meter coils and its effect canbe accurately adjusted so it will just compensatefor the friction, and no more.
Sometimes these coils have a number of tapsprovided at various sections of the winding andalso a small switch to shift the connections to in-clude more or less of the turns of the coil. Thisalso provides an adjustment of the amount of torquethe coil will exert to overcome friction.
Fig. 4 shows the coils and connections of a D.C.kilowatt-hour meter. You will note in this figurethat the friction compensating coil is connected inseries with the armature and resistance coil, andthis group are connected across the positive andnegative line wires.
Current coils are connected in series with oneside of the line so they will carry the full loadcurrent. The terminals of a watthour meter of thistype are usually marked for the line and load con-nections, and these connections must, of course, beproperly made so that the meter will run in theright direction.
6. WATTHOUR CONSTANT AND TIMEELEMENT
A given amount of power in watts must passthrough a watthour meter to produce one revolu-tion of the armature and disk. For example, itmay require a flow of energy representing 6 watt -
176
Direct Current
hours, or the equivalent of b watts for one hour,to produce one revolution of the meter armature.This amount would be termed the watthour con-stant of the meter.
Knowing the number of watts per revolution, itonly remains to get the total number of revolutionsduring a certain period of time, in order to knowor measure the total amount of energy passedthrough the meter during that time. As each revo-lution of the armature is transmitted to the gearswhich operate the pointers on the dials, the totalpower in kilowatt hours can be read directly fromthese dials.
Fig. 4. This diagram shows the coils and circuits of a KW -hour meterand the manner in which they are connected to the line and load.
The operation of the gears and dials or register-ing mechanism is very simple. The worm -gear onthe upper end of the armature shaft is meshed withthe teeth of a gear which is the first of a row orchain of gears all coupled together. This gear hasattached to it a small pinion which meshes withthe teeth of the next gear and drives it at 1/10 thespeed of the first one. This second gear, in turn,drives the third gear 1/10 as fast as it runs, and thethird drives a fourth, the speed of which is againreduced to ten times lower than the third one.
177
Watthour Meters
Referring to Fig. 3, when the pointer on theright has made one complete revolution, the pointeron the next dial to the left will have travelled justone division or one -tenth of a revolution.
When the first pointer has made ten revolutions,the second one will have completed one revolution,and the third pointer will have moved one point.When the first pointer completes 100 revolucions,the second will have completed 10; and the thirdwill have completed one revolution.
In this manner the first dial will have to make1000 revolutions to cause the left-hand dial to com-plete one revolution.
Fig. S. The above sketches, A, B, C, shows the dials of a kilowatt-hour meter in three different positions. If you will practice read-ing each set of these dials with the instructions here given, youwill be able to easily and accurately read any KW -hour meter.
7. READING WATTHOUR METERSBy noting the figures at which the pointers stand,
in order from left to right, we can read the kilo-watt hours indicated by the meter. Some metersused on larger power circuits are adjusted so thattheir dials and pointers don't show the amount of
178
Direct Current
power directly, but provide a reading which mustbe multiplied by some certain figure, such as 10,20, or 50, to obtain the correct total reading. Thisfigure is called a constant or multiplier, and itshould be used whenever reading a meter of thistype. This constant, or multiplier, is usuallymarked beneath the dials of the meter.
When reading kilowatt-hour meters, we shouldalways read the last number which has been passedby the pointer on any dial. Some care is requiredin doing this until one has had enough practice todo it automatically. If each dial is not carefullyobserved, mistakes will be made ; because each ad-jacent pointer revolves in the opposite direction tothe last, as can be seen by the numbers marked onthe dials shown in Fig. 5-A.
When the pointer is almost directly over oneof the numbers, there may be a question as towhether the pointer has actually passed this . num-ber or is still approaching it. This should alwaysbe determined by referring to the next dial to theright to see whether or not its pointer has com-pleted its revolution. If it has completed the revo-lution or passed' zero on its dial, the pointer to theleft should be read as having passed its number.
If the pointer to the right has not completed itslast revolution, the one next to the left should notbe read as having passed its number, even thoughit may appear to be beyond the number.
If the readings are carefully checked in this man,ner there is very little chance of mistakes.
On the second dial from the left in Fig. 5-A,the pointer revolves in a clockwise direction, andit might easily appear that it has passed the No. 2.By checking with the dial next to the right, how-ever, we find that this pointer, which revolvescounter -clockwise, has not quite completed its revo-lution or passed zero. Therefore, the dial at theleft should still be read as No. 1. The correct read-ing for a meter with the pointers in the positionshown in Fig. 5-A would be 3194 kilowatt hours.
179
Watthour Meters
The reading for the pointers in Fig. 5-B shouldbe 4510 kilowatt hours. Here again the pointeron dial No. 3 appears to be on figure No. 1 ; and,by checking with dial No. 4, we find that its pointeris on zero or has just completed a revolution; soit is correct to read dial No. 3 as No. 1.
The reading for the set of dials in Fig. 5-Cshould be 7692. The pointer on dial No. 2 in thiscase appears to have passed No. 7; but, by checkingwith dial No. 3 tp the right, we find its pointer hasnot quite completed its revolution ; therefore, thedial to the left should he read as No. 6.
Fig. 6. Common type of recording voltmeter used for keeping anhourly and daily record of the voltages on the system to whichit is attached.
8. "CREEPING"The armature of a watthour meter will some-
times he found to be rotating slowly, even whenall load is disconnected from the circuits. This iscommonly called creeping of the meter. It maybe caused by a high resistance ground or a short
180
Direct Current
on the line. The resistance of such a ground orshort may not be low enough to cause the fuse toblow, and yet there may be a small amount ofcurrent flowing through it at all times.
If the load wires are entirely disconnected fromthe meter and the disk is still creeping, it may bedue to the effects of stray magnetic fields fromlarge conductors which are located near, the meterand carrying heavy currents, or it may be causedby the fields from large electrical machines locatednear by.
For this reason, watthour meters, or for thatmatter any other electric meters, should not belocated within a few feet of large machines, unlessthey are magnetically shielded, and they should bekept at least a few inches away from large conduc-tors carrying heavy currents.
Large bus bars or cables carrying currents ofseveral hundred or several thousand amperes setup quite strong magnetic fields around them fordistances of several feet, and very strong fields afew inches away from them.
Sometimes a very small load such as a bell trans-former or electric clock may cause the meter torotate very slowly, but this is actual load and notcreeping.
Vibration of the buiding or panel to which themeter is attached may sometimes be the cause ofcreeping. In some cases this may be stopped byproper adjustment of the compensating coil; or asmall iron clip can be placed on the edge of thealuminum disk, if the clip does not rub the dampingmagnets as the disk revolves.
When this iron clip comes under the poles of thepermanent magnets, their attraction for the ironwill stop the disk and prevent it from creeping. Aslong as this clip doesn't touch the permanent mag-nets, it will not interfere with the accuracy of themeter ; because its retarding effect when leaving thepols of the magnets is balanced by its acceleratingeffect when approaching the poles.
9. TESTING KILOWATT-HOUR METERS
Kilowatt-hour meters can be tested for accuracy,
181
Watthour Meters
or calibrated, by comparison with standard portabletest instruments.
A known load consisting of a resistance box canbe connected to the load terminals of the meterwhen all other load is off. Then, by counting therevolutions per min. of the disk and comparing thisnumber with the revolutions made by the disk ofa "rotating standard" test instrument, the accuracyof the meter can be determined.
When no standard load box or test instrument isavailable, a test can be conveniently made with aknown load of several lamps or some device ofwhich the wattage is known.
For this test the following formula should beused:
WHK X 3600 X R
W= seconds
In which :WHK = the watt-hour constant marked on the
. meter disk.3600 = number of seconds in an hour.R = any chosen number of revolutions of the
disk.W = known load in watts which is connected to
the meter.For example, suppose we wish to test a meter
which has a constant of .6, marked on its disk. Wecan connect a new 200 -watt lamp, or two 100 -wattlamps across the load terminals of the meter, afterall other load has been disconnected. At the instantthe lamp load is connected, start counting the revo-lutions of the meter and observe accurately theamount of time it requires to make a certain num-ber of revolutions. Let us say it is 5 "revolutions.
Then, according to the formula, the time requiredfor the disk to make these 5 revolutions should be :
.6 X 3600 X 5, or 54 seconds
200If it actually requires longer than this, the meter
is running too slow. If the time required to make
182
L
Direct Current
the 5 revolutions is less than 54 sec., the meter isrunning too fast.
Remember where to find this formula for futurereference, as it may often be very convenient touse.
10. RECORDING INSTRUMENTSIn power plants or substations where large
amounts of power are generated and handled, it isoften very important to keep accurate? records ofthe voltage, current, and power on principal circuitsat all hours of the day and night.
Records of this kind will show any unusual varia-tions in load or voltage and they are often themeans of effecting great savings and improvementsin the operation of power plants and industrial elec-tric machinery.
It is usually not practical for an operator or elec-
Fig. 7. Paper disk or chart from a direct acting recording meter.The irregular black line shows the voltage curve traced by thepointer and pen throughout each hour of the day and night.
trician to keep constant watch of meters to obtaina record of their readings hourly or more often.
183
Recording Instruments
Recording meters which will mark a continuousrecord of their readings on a paper chart or diskcan be used for this purpose.
11. DIRECT -ACTING RECORDING METERS
One of the simplest types of recording instru-ments uses the ordinary meter element and has acase quite similar to that used for D.C. voltmetersor ammeters, and has a small ink cup and pen at-tached to the end of the needle or pointer. Thispen rests lightly on a paper disk which is rotatedonce around every 24 hrs. by a clock -work mechan-ism inside the meter. See Fig. 6.
As the disk slowly revolves, the pointer pen traceson it a line which shows the movements of thepointer and the variations in voltage or current,whichever the instrument is used to measure.
The paper disks have on them circular lines whichrepresent the voltage or current scale. By the posi-tion of the ink line on this scale the voltage or am-perage at any point can be determined. Around theouter edge of the disk is marked the time in hours,so the readings for any period 'of the day can bequickly determined. Fig. 7 shows a disk from ameter of this type.
Recording meters of the type just described arecalled Direct -Acting instruments. One of the dis-advantages of meters of this type is that the frictionof the pen on the paper chart does not allow thepointer and pen to move freely enough to make.themeter very sensitive or accurate on small variationsin the voltage or current. They also require fre-quent winding, replacing of charts, and refilling ofthe pen, but they are low in cost and very satis-factory for certain requirements.
12. RELAY TYPE RECORDING METERS
Another type of recording instument in verycommon use is the Relay Type, which operates onthe electro-dynamometer, or Kelvin balance, prin-ciple.
The Kelvin balance consists of a set of stationary
184
Direct Current
coils and a set of movable coils. These coils can beseen at the top of the instrument shown in Fig. 8,which is a relay -type recording meter.
The thin moving coils are shown balanced be-tween the larger stationary coils, and are equippedwith a torsion spring which tends to oppose theirmovements in either direction.
Any change of voltage or current in these coilschanges the repulsion or attraction between the
Fig. 8. This photo shows a complete recording instrument of therelay type with the cover removed. Note the stationary coils andbalance coils at the top, and the roll for carrying the paper chartbeneath the pointer.
fields of the moving and stationary elements, andwill force the coils of the moving element up ordown. This moving element then operates a set ofrelay contacts which close a circuit to the solenoidsor small operating motor which moves the pen.
The instrument shown in Fig. 8 uses a motorfor the operation of the pen and pointer. The motor,
185
Recording Instruments
which can be seen- above the chart roll, revolves aworm shaft which moves the pen. The movementof the pen also readjusts the counter-torque springon the movable coil so that it is balanced properlyfor the new position of the pen. This causes the
Fig. 9. This view shows the recording instrument which was shownin Fig. 8, with the paper chart in place. The glass ink cup andpen can be dimly attached to the lower part of the pointer.
balance coils to open the relay contacts and stopthe motor; so the pen will remain in this positionuntil another change of the voltage or currentOccurs.
The "clock" mechanism which. drives the paperchart in this type of instrument is electrically woundand' therefore does not require frequent attention.
Fig. 9 shows a recording instrument of this, type,with the chart roll in place. This paper chart iscontinuous throughout the roll. So, as the rolltravels and the pen moves sidewise across it, a con-tinuous record of the voltage or power is kept.When the end of one roll is reached, a new one canbe inserted.
Fig. 10 shows the connection's for a recordingmeter of the type just described. Terminals 1 and
186
Direct Current
2 are for the motor circuit, and 3 and 4 are forthe control circuit.
13. LOAD DEMAND INDICATORSPower and lighting loads which are of a steady
or constant nature and do not vary greatly through-out the day are most desirable to power companies.Loads which have high "peaks" in proportion tothe average hourly load, require the operation andmaintenance of generating equipment which is suffi-cient for these peak periods, and may be either idleor lightly loaded at other periods. This tends toreduce the operating efficiency and economy in thepower plant, and power companies will often give acustomer lower rates per KW hr, on his power ifhis peak load is not over a certain percentage higherthan his average load.
MoviN9 Contact Stationary coils
Imoy,N, colts
Sepias fff
Resistance 1/11..1 11(.117111
D111Clock
Resistance
ClockMotor
1717711 III( [Till
Cut outSwitch
Control
0 -Fig. 10. This diagram shows the coils and winding of a recording meter
such as shown in Figs. 8 and 9
To determine the maximum load, or peak, for anyperiod during the day or week, Maximum DemandIndicators are used. They are sometimes called"max. meters".
187
Demand Indicators
One type of demand indicator is the Wrightmaximum ampere -demand indicator, which operateson the thermal or heat expansion principle.
Fig. 11. This sketch illustrates the principle of a common type maxi-mum demand meter which operates by expansion of the air in thebulb "A", when current is passed through the coil around this bulb.
This instrument consists of a specially shapedsealed glass tube, as shown in Fig. 11. In this tubeis sealed a certain amount of colored liquid, usuallysulphuric acid, and a certain amount of air.
A resistance coil of platinoid metal is woundaround the bulb as shown at "A" in the figure.This coil is connected in series with the line andload, or in parallel with an ammeter shunt. Whencurrent passes through the coil it causes it to be-come slightly heated and this heat expands the airin the bulb "A".
This expansion increases the air pressure andforces more of the liquid over into the right-handpart of the tube. If the liquid is forced high enoughin this tube, some of it will run over into the smallIndex Tube, "C".
188
Direct Current
As the heat developed in the resistance coil isproportional to the square of the current passingthrough it, the index tube "C" can be graduated orequipped with a graduated scale behind it ; so themaximum current in amperes can be read from theheight of the liquid in this tube.
A momentary increase in load will not registeron an indicator of this type, because it requires alittle time for the heat in the coil to expand the airinside the tube. This is a desirable feature, as itusually is not desired to measure peak loads thatlast only an instant.
A load increase which lasts for 30 minutes willregister the full amount, or 100%, of the increase.
After a reading, this type of instrument can bereset by tilting the tube and allowing the liquid toflow back into tube "B".
Small, inverted, glass funnels are fastened insidethe bottom of each side of the tube, to prevent thepassage of air from one side of the tube to the other.These are called traps. When the tube is tilted toreset the indicator, these traps remain covered withliquid and prevent air from passing through.
Recording wattmeters or ammeters also serve asmaximum -demand meters, as they show all loadvariations.
Another type of maximum -demand meter uses acombination of a wattmeter element and pointerand a watthour meter time element, to allow thewatt -meter pointer to register only over certaintime periods.
Some demand meters use a thermostatic strip tomove the pointer as the strip is expanded andwarped by the heat of the load current.
14. WHEATSTONE BRIDGEThis instrument is a very convenient device for
measuring the resistance of electric circuits or de-vices, by comparison with standard resistances ofknown value.
189
Wheatstone Bridge
You have already learned that electric currentwill tend to follow the path of lowest resistance, andwill divide through parallel paths in inverse propor-tion to their resistance.
10 E
5 R
A
5R SR
at
411111111
Fig. 13. The sketch at "A" shows the manner in which current willdivide in inverse proportion to the resistance of two parallel cir-cuits. At "B" is shown the manner in which current will flowthrough a galvanometer placed between four resistances, one ofwhich is of a different value than the rest.
For example, suppose we have one resistance coilof 5 ohms and one of 10 ohms connected in parallel,as shown in Fig. 13-A. If we apply 10 volts to theend terminals, 2 amperes will flow through the5 -ohm coil and 1 ampere through the 10 -ohm coil.
Now let us connect a group of four coils 'as shownin Fig. 13-B. Here we have two coils of 5 ohmseach in series on one path, and a 5 -ohm coil and a3 -ohm coil in series on the other path.
If we now connect a sensitive galvanometeracross the center of the paths between the coils, asshown, it will indicate a flow of current from the
190
Direct Current
upper path to the lower when voltage is applied tothe terminals of the group.
Tracing from the positive terminal to the centerof the group, the resistance of each path is equal,but from this point on to the negative terminal thelower path or coil "X" has the lowest resistance.For this reason, some of the current tends to flowdown through the galvanometer wire to the lowercoil or easier path.
Fig. 14. Resistance box of a common Wheatstone bridge. Note theplugs which are used for varying the amount of resistance in thecircuit.
If we changed the coil "D" to one of 3 ohms,both sides of the circuit would again be balancedand no current would flow through the galvan-ometer.
On this same principle, if the resistance of coil"X" is not known, we can determine it by varyingthe resistance of coil "D" in known amounts untilthe galvanometer indicates zero, or a balanced cir-cuit. We would then know .that the resistance ofcoil "X" is equal to whatever amount of resistancewe have in coil "D" to secure the balance.
15. OPERATION AND CIRCUIT OFWHEATSTONE BRIDGE
The Wheatsone Bridge operates on the same
191
Wheatstone Bridge
general principle first described. It consist of a boxof resistance coils with convenient plugs for cut-ting coils of various resistance in and out of thebalancing circuits. Fig. 14 shows the resistance boxof a bridge of this type.
Fig. 15. This diagram shows the connections and principle of aWheatstone bridge or resistance balancer. Note how the splitmetal sockets can be used to short out various resistance coilswhen a metal plug is inserted in these sockets. Study this dia-gram carefully while referring to the explanations on these pages.
Some bridges have knobs and dial switches in-stead of plugs for switching the resistance units ;
and some have the galvanometer built in the topof the box, and the dry cells inside.
Fig. 15 shows a diagram of a common type ofbridge and the methods by which the coils can beleft in the various circuits or shorted out by insert-ing metal plugs in the round hales between metalblocks, attached WI the ends of each resistancecoil.
The coil or line of which the resistance is to bemeasured is connected at X. The circuits A, B, andC are called Bridge Arms. A and B are called RatioArms, or balance arms; and C is called the Rheo-stat Arm.
Arms A and B usually have the same number ofresistor units of similar values in ohms. Arm Chas a slumber of resistors of different values.
When the unknown resistance, X, has been con-nected in and' the bridge arms so balanced that thegalvanometer shows no reading when the button
192
. Direct Current
is pressed, the resistance of X in ohms can be de-termined by the use of the following formula :
AX = X C,
B
In which :X = resistance in ohms of device under test.A = known resistance in ratio arm A.B = known resistance in ratio arm B.C = known resistance in rheostat arm C.
Fig. 16. The above photo shows a "Megger", or device used for meas-uring the resistance of insulation and high resistance circuits. Thisinstrument contains its own D. C. generator as well as meterelement.
The Wheatstone bridge is a very convenient de-vice for testing the resistance of coils or windingsof electrical equipment ; of lines, cables and circuits ;and of the insulation on various wires or devices.
There are a number of types of bridges for re-sistance measurement, most of which are supplied
. 193
Meggers
with a connection chart and instructions for opera-tion. So, with a knowledge of their general princi-ples as covered here, you should be able to use andoperate any ordinary bridge.
16. "MEGGER"
Another testing instrument frequently used bythe practical electrician for testing the resistanceof insulation on electrical machinery is known as aMegger. This name comes from the fact that thisinstrument is commonly used to measure resistancesof millions of ohms ; and a million ohms is calledone meg-ohm.
The megger consists of a small hand -operatedD.C. generator and one or more meter elements,mounted in a portable box, as shown in Fig. 16.When the crank is turned, the D.C. generator willproduce from 100 to 1000 volts D.C., according tothe speed at which the generator is rotated and thenumber of turns in its winding.
Normal operating voltage is usually from 300 to500 volts, and is marked on the meter scale. Some
Fig. 17. Simple circuit showing the connection and principles of a"Megger". Note the arrangement of the D. C. generator arma-ture and meter element at opposite ends of the magnet poles andthe connections of this device to the line or that test terminals.
of these instruments .have a voltmeter to show thegenerator voltage, and an ohm -meter to indicate theinsulation resistance of the device under test.
194
Direct Current
The terminals of the instrument can be connectedto one terminal of a machine winding and to themachine frame. Then, when the crank is rotatedthe insulation resistance in meg-ohms can be readdirectly from the scale.
Fig. 17 shows the internal connections of a meg-'ger and the terminals for connections to the equip-ment to be tested. As the insulation of electricalmachines or lines become aged, or in some caseswhere it has been oil or water -soaked, its resistancein ohms is considerably reduced. Therefore, theresistance test with the megger is a good indicationof the condition or quality of the insulation.
Periodic megger tests of electrical equipment andrecords of the insulation resistance will often showup approaching trouble before the insulation breaksdown completely and burns out the equipment.
Either the Wheatsdne bridge or the megger canbe used to determine the approximate location ofgrounds or faults in cables and long lines, by meas-uring the resistance from the end of the line to thefault, through the cable and its sheath or the earth.Then, by comparing this resistance with the knownresistance total of the line or with its resistanceper foot or per 1000 ft., the distance to the faultcan easily be calculated.
17. METERS ESSENTIAL IN ELECTRICALWORK
A number of simple and practical tests of resist-ance can also be made with voltmeters and am-meters, and the use of ohms law formulas. Byapplying voltage of a known value to any deviceand accurately measuring the current flow set upby this voltage, we can readily calculate the re-sistance of the circuit or device by the simple .
formula :E
R=I
While on the subject of meters, it will be wellto mention that very sensitive relays are often made
195
Meggers
from regular meter elements, using a short arma-ture or moving contact in place of the regularpointer or needle. Fig. 18 shows a relay of thistype. In this figure you can see the short contactneedle attached to the moving coil, and the adjust-able contacts on each side of this needle.
By proper adjustment of the contacts of relaysof this type, they can be made to close or open cir-cuits when the voltage or current values rise aboveor fall below any certain values.
Fig. 18. Very sensitive relays such as shown above are commonlymade with the same principle elements used in voltmeters orammeters. Relays of this type can be used to open or close variouscircuits at any set voltage or current values.
Keep well in mind the importance of ordinaryelectric meters in the work of any practical, up-to-date electrician, and remember that great savings
196
Direct Current
in power or equipment can often be made by theproper use of electrical meters and instruments.
For testing the efficiency of machines, checkingoperations in power plants, inspe.lion of electricalequipment, and for trouble shooting and fault loca-tion, electrical meters of the proper types are ofenormous value.
The trained practical man should never overlookan opportunity to effect a saving or improve opera-tion by the sele,tion and use of the proper meters.
197
Meggers
EXAMINATION QUESTIONS
1. For what purpose are watthour meters used ?
2. What method is used to prevent the Watt-meter motor armature from overspeeding?
3. Name two troubles which may cause "creep-ing" in a Watthour meter.
4. Give the formula for use in testing a kilowatt-hour meter with a known load.
5. How many seconds would be required for thedisk on a watt-hour meter with a constant of .6to make 10 revolutions with a connected load of 300watts?
6. For what purpose are recording instrumentsused ?
7. Name two types of recording instruments.
8. a. On what principle does the Wright maxi-mum demand meter operate?
b. How much time is required before thismeter will record 100% of the increase in load?
9. For what purpose is the wheatstone bridgeused ?-
10. What would be the resistance at X as shownby Fig. 15 when the resistance A is 10 R, B 15 R,and C 6 R, and the galvanometer reading is zero?
198
Direct Current
DIRECT CURRENT MOTORS
An electric motor, as you have already learned,is a device for converting electrical energy intomechanical energy, or to perform just the oppositefunction to that of a generator. Motors supplymechanical energy to drive various machines andequipment by means of belts, gears, and direct shaftconnections.
When electricity from the line is supplied toterminals of the motor, it develops mechanical forcewhich tends to rotate its armature and any equip-ment which may be attached to its shaft.
B
OPERATIONThe D.C. motor operates on the first law of mag-
netism which states that like poles repel and unlikepoles attract. Current flowing through the field coilsproduces the field poles, and current through thearmature coils develops armature poles midway be-tween the field poles. Attraction and repulsion be-tween these two sets of poles produces rotation.
In order that the attractive and repulsive forcesbetween the armature poles and the field poles bemaintained, the armature poles must remain sta-tionary in space. This condition is made possibleby the action of the commutator and brushes for,as the armature moves forward a different set -ofcommutator segments come under the brushes andcurrent is consequently fed to the armature at thesame point in space at all times ; therefore, the ar-mature poles remain stationary in space eventhough the armature itself rotates.
It is this magnetic action between the armature
199
Motors
poles and the held poles that develops the twistingforce at the motor shaft that enables it to revolvethe loads mechanically connected to it. This twist-ing effort is called torque. A motor capable of exert-ing a relatively high twisting effort per amperewhen it is starting is said to have a relatively highstarting torque; one that generates a low twistingeffort per ampere when starting is said to have arelatively low starting torque.
The torque per ampere that any given motor willdevelop depends upon the type of motor : series,shunt, or compound, and how well the -machine isdesigned. In general, the starting torque per am-pere will be greatest for the series motor, somewhatless for the compound motor, and least of all for theshunt motor.
ROTATIONBy reversing the direction of current flow through
the fields or through the armature, the field polesor the armature poles will be reversed, and the di-rection of rotation changed. Compare A with B andC with D.
200
Direct Current
ARMATURE POLESDiagrams E and F show a 4 pole motor. Note that
the number of armature poles always equals thenumber of field poles, and that the armature polesare located midway between the field poles. Fromthe above it is obvious that a 2 pole armature willnot work in a 4 pole field. Note also that when thedirection of armature current flow is reversed, allpoles are reversed.
TORQUEAs previously stated, torque is defined as twisting
effort. When a mechanic places a wrench on a nutand pulls, he applies torque to the nut, and theamount of torque applied will depend upon thelength of the wrench and the magnitude of the pullemployed. If the wrench is one foot long and thepull applied 50 pounds, then the torque applied tothe nut is 50 pounds -feet. If the nut does not turn
TWISTING EFFORT OR TORQUE
APPLIE0 To NUT IS 50 Los.FT.
under these conditions, more torque is necessary.The mechanic may increase the torque appliedeither by pulling harder on the foot -long wrench, orby using the same pull on a longer wrench. In the
201
Motor Speeds
first case he is increasing the force used on thewrench ; in the second case he is increasing the lev-erage with the same force ; in either case, the netresult would be an increase in the torque on the nut.It should be noted here that applying torque to amovable body (the nut in this instance) does notnecessarily mean that the body will move. Motionwill take place only if the torque applied to pro-duce motion is greater than the resisting torquewhich opposes the motion. In the case of the nut,the resisting torque is practically all due to friction,and motion will not take place until the appliedtorque is great enough' to .overcome this frictionalopposition.
From the above it is evident that the twisting ef-fort applied to any body depends upon two things ;the force applied to the lever; and the length ofthe lever used. The handle on a grindstone, forexample, represents a lever. If this handle is onefoot distant from the shaft of the grindstone, thenthis arrangement is equivalent to a lever one footlong. Assuming a force of 20 pounds be applied tothis handle, then a torque of 20 pounds times onefoot or 20 pounds -feet is tending to rotate thegrindstone. If the force applied to the handle israised, the applied torque will increase in pro-portion. Therefore it is seen that torque is pro-portional to the product of the applied force (inpounds) and the length of the lever (in feet). Con-sequently, torque is found by merely multiplyingthese two values together and expressing the resultin pounds -feet. For example, if a motor, equippedwith a pulley 2 feet in diameter is found to exert apull at the surface of the pulley of. 100 pounds thenthe torque developed equals 100 x 1 or 100 pounds -feet. It will be noted that a pulley 2 feet in diameterhas its surface or rim only one foot from the shaft;consequently the leverage is only one foot and thetorque 100 pounds -feet.
From the above it is evident that the revolving ofany rotatable body requires application of a twist-ing effort or driving torque. In the case of the D.C.motor this torque is produced by the magnetic ac-tion between armature poles and the field poles.
202
Direct Current
If the armature is 2 feet in diameter and the mag-netic force applied at the armature surface is 30pounds, then the torque that the shaft of the motorcould evert would be 30 zc 1 or 30 pounds -feet. Inthis way the 'magnetic attraction and repulsion be-tween the armature poles and the field poles is con-verted in mechanical torque at the motor shaft.
For a rotatable body, such as the armature of amotor, to be put into motion, it is essential that,the magnetic driving torque developed by it begreater than the mechanical resisting torque ap-plied to the shaft. Once the armature is in motionit will accelerate until the driving torque and theresisting torque beComes equal; when this condi-tion is established, the speed will become constant.
If the driving torque is assumed to be constantand the resisting torque increases, the motor speedwill decrease steadily until it stops. If the resistingtorque is less than the driving torque, the motor willcontinue to accelerate until the driving torques andresisting torques become equal. If the operatingconditions are such that the resisting torque cannever equal the driving torque, the motor will ac-celerate to destruction.
It will later be shown that the electric motortends to be self regulating with regard to the ad-justment of the driving torque to the resistingtorque. Whenever the resisting torque resultingfrom the motor load changes, a change in the motorspeed will occur, and this change in speed will bein a direction that will cause the driving torque andthe resisting torque to become equal again.
When considering the action of a given type ofmotor under varying load conditions, application ofthe above information concerning the action of ro-tating bodies represents the logical starting point.If the relation between the resisting and the drivingtorque is understood, the effect of a change in loadon motor speed can readily be determined.
1. TYPES OF D.C. MOTORSDirect current motors can be divided into three
general classes, the same as D.C. generators were,namely, Shunt, Series, and Compound motors.These motors are classified according to their field
203
Motor Type
connections with respect to the armature, in thesame manner in which the generators were classi-fied.
Compound motors can be convected either cumu-lative or differential. With generators we find, thatthe shunt, series, and compound types each havedifferent voltage characteristics. With motors, theeffect of these different field connections is to pro-duce different speed and torque characteristics.
Motors are made with various types of frames,known as Open Type, Semi -enclosed, and ClosedType frames.
Fig. 1 shows a modern .D.C. motor with an opentype frame. A frame of this type allows easy accessto the commutator, brushes and parts, for adjust-ment, cleaning and repairs; and also allows goodventilation Open type motors are generally used
Fig. I. This photo shows a modern D.C. motor with an open typeframe. Note the easy access a frame of this construction providesto the commutator, brushes, and field coils.
where they are to be operated in clean places, andwhere there is no danger of employees coming incontact with their live parts ; and no danger of fireor explosions which might be caused by sparks atthe brushes.
Fig. 3 shows a motor with a completely enclosedframe. Motors of this type are often built larger
204
Direct Current
and wound with larger wire, so they do not developas much heat. In some cases they are practicallyair -tight, and have ventilating tubes, attached totheir casings to bring cooling air from another room.
Motors with enclosed frames of this type can beused in places where the air is filled with dust anddirt, or possibly vapors or explosive gases.
Enclosed frame motors should always be usedwhere abrasive dust or metal dust is present in theair, or in mills where wood or grain dust might beexploded by any possible sparks from brushes.
2. MOTOR SPEEDS AND H.P.D.C. motors are always rated in horse power and
range in size from those of a small fraction of onehorse power to those of several thousand horsepower each. The smaller motors are used for driv-ing household appliances, laboratory equipment, andsmall individual shop machines, such as drillpresses, small lathes, etc. Medium-sized motors,ranging from one horse power to several hundredhorse power each, are used for driving machinery infactories and industrial plants, for street railwaysand electrical locomotives, and for elevator ma -
This pump, unit is used to load gasoline on ships. The Louis AllisCompany of Milwaukee pioneered and developed the first types offan cooled explosion proof motors.
chinery. The larger types, ranging from severalhundred to several thousand horse power each, areused principally, in steel mills and on electrically -driven ships.
205
Motor Speed
The horsepower ratings of motors refer to themaximum continuous output they can deliver with-out overheating.
Fig. 3. The above motor has a frame of the enclosed type. Motorsof this type are particularly well suited for operation in placeswhere the air is full of dust or vapor.
The speed at which D.C. motors are designed tooperate depends principally upon their size, becausethe diameter of the armature, as well as the R.P.M.,are what determine the centrifugal forces set up inthe conductors and commutator bars.
Very small motors commonly have speeds from2000 to 4000 R. P. M., while motors of medium oraverage size, ranging from 1 to 25 H.P., usuallyrotate at speeds from 1000 to 2000 R. P. M.
Very large motors operate at much lower speeds,generally ranging from 100 to 500 R. P. M. ; al-though some large steel -mill motors have speeds aslow as 40 R. P. M.
3. MOTOR SPEED REGULATION ANDCONTROL
In referring to the characteristics and operationof electric motors, we frequently use the termsSpeed Regulation and Speed Control. These termshave entirely separate meanings, and their differ-ence is very important.
Speed Regulation refers to changes in speed
206
Direct Current
which are automatically made by the motor itself,as the load *on the machine is varied. Speed regu-lation is largely determined by the construction ofthe motor and its windings and is a ver' importantfactor in the selection of motors for different classesof work. The speed regulation of a motor is usu-ally expressed in percentage and refers to the dif-ference in the speed of the machine at no load andfull load. It can be determined by the followingformula :
Speedregulation
No load R. P. M.-full load R. P. M.
Full load R. P. M.For example, if we have a motor that operates at
1800 R. P. M. when no load is connected to it andslows down to 1720 R. P. M. when it is fully loaded.its speed regulation would be :
1800 - 1720, or .046 ±
1720This would be expressed as 4.6%.Motor speed regulation is entirely automatic and
is performed by the motor itself, as the load varies.The term Speed Control refers to changes which
are made in the motor speed by the use of manualor automatic control devices. These speed controldevices are usually external to the motor and con-sist of some form of variable resistance. They willbe fully explained in the following pages.4. MOTOR RATINGS IN VOLTS
AMPERES, AND H.P.The rating of a D.C. motor in horse power, am-
peres, and volts depends on the same factors in theirdesign as the rating of generators does. The motorratings in horse power are also based on the samefactor of the temperature increase in their windingswhen operated at full rated load.
For example, a 10 H.P. motor is one that whensupplied with the proper voltage for which it is de-signed will drive a 10 H.P. mechanical load con-tinuously without overheating its windings. Thecurrent required by a motor is, of course, propor-tional to the mechanical load in H.P. which it isdriving:
207
Motor Torque
In addition to carrying the load without heatingthe windings, the motor must also be able to carryits full load current without excessive heating orsparking at the brushes and commutator.
Motors are generally designed to carry overloadsof a greater amount and for longer periods of timethan generators are. Most D.C. motors can carry a25% overload for a period of two hours, withoutserious overheating.
We have already learned that D.C. motors aresimilar to D.C. generators in all details of theirmechanical construction. In fact. manufacturersfrequently use the same D.C. machines either asmotors or generators, by merely changing the nameplates on them and making a few minor changes inthe connections of the field windings and settingof the brushes.
= 17;
-== trj,- -
Fig. 4. 4. The above diagram illustrates the manner in which thereaction between the lines of force from the armature and fieldwindings set up the torque or turning effort in a motor.
6. MOTOR TORQUE, SPEED AND H.P.
The torque exerted by such a motor will, ofcourse, depend on the strength of the magnetic fluxfrom the field poles and the strength of the armatureflux. Therefore, the torque exerted by a motor canbe increased by increasing either the field strengthor the armature current, or both.
The horse power or mechanical power output ofa D.C. motor is proportional to the product of itstorque and speed. The higher the speed at which amotor is operated while maintaining the sameamount of torque, the greater will be its horsepower.
208
Direct Current
D.C. motors rated at higher speeds will producethe same horse power with smaller frames andarmatures. The cost of high speed motors is there-fore much less per H.P. A motor frame that israted a 5 H.P. at 900 R. P. M. will deliver 10H.P. at 1800 R.P.M.
7. DIRECTION OF MOTOR ROTATIONThe direction of rotation of a D.C. motor can be
easily determined by the use of Fleming's left-handrule. This rule is similar to the right-hand rulewhich we have learned to use for generators.
Hold the first finger, thumb, and remaining fing-ers of the left hand all at right angles to each other.Let the first finger point in the direction of fluxfrom the field poles, the remaining fingers in thedirection of current through the armature conduc-tors, and the thumb will then indicate the directionof rotation of the armature.
. This rule can be quickly and easily applied todiagrams such as shown in Fig. 4 and can also beused on the actual machines, when the armatureconductors and connections to the commutator can'be seen and the polarity of the field poles is known.
The direction of rotation can also be easily deter-mined with diagrams such as shown in Fig. 4, bysimply remembering that the repelling or crowdingforce on the armature conductor will be on the sidewhere its flux lines join with those of the field flux.
From this study of the direction of rotation ofmotors, you can see that any D.C. motor can bereversed either by reversing the direction of cur-rent through the armature winding or by reversingthe field connections to change the polarity of thefield poles. Refer to Fig. 4, and using the left-hand rule, note the direction in which these conduc-tors would rotate if their current were reversed orthe poles of the motor were reversed.
8. COUNTER -E. M. F. IN MOTORS
You have already learned that a voltage will beinduced or generated in the armature conductors ofa motor whenever the machine is running, and that
209
Counter -E. M. F.
this voltage is called Counter -E. M. F. As the arma-ture of the motor rotates, its conductors will be go-ing through the field flux and so will producecounter -voltage in the same direction as that of thevoltage of a generator rotated in the same directionas the motor. Therefore, this counter -E. M. F. in-duced in a motor is always in a direction opposingthe applied line voltage, but of course is normallynot quite as great as the applied voltage.
The amount of counter -voltage which will be gen-erated depends upon the number of conductors inthe armature, the strength of the motor field, andthe speed at which the machine is operated.
Keep this rule well in mind, because the effectsof counter -voltage are extremely important in theoperation of D.C. motors and control equipment.
In Fig. 5, the direction of the current and voltageapplied to the armature conductors from the line isshown by the solid black symbols in the two arma-ture conductors, and the direction of the counter-voltage generated in these conductors with the po-larity and rotation shown, is indicated by the lightersymbols above the conductors.
Fig. 5. The above sketch illustrates the manner in which thecounter -voltage is generated in the opposite direction to the appliedvoltage in a motor armature.
As the counter -voltage is generated in the op-posite direction to the applied line voltage, we canreadily see how it limits or regulates the currentwhich will flow through the armature and therebyacts co control the motor speed.
The voltage applied to a D.C. motor armaturemust always be equal to the voltage drop in thewinding plus the C. E. M. F., or
Ea = Ra Ia CEa
Ea = Applied voltage210
in which
Direct Current
Ra = Resistance of armatureIa = Current in armature
CEa = Counter voltage of armatureThen, for example, the applied voltage of a certain
110 -volt motor might be used as follows:Ex. Ra Ia CEa
No load : 110 = 1 + 109Full load : 110 = 5 + 105
9. ARMATURE RESISTANCE NECESSARYWHEN STARTING D.C. MOTORS
When the motor armature is idle or at rest nocounter -voltage is produced, and the current whichwould then flow through the armature would be de-termined entirely by its resistance and the voltageapplied ; according to the formula :
I = E .÷ R.The resistance of D.C. motor armatures is very
low, usually less than one ohm. Therefore, exces-sive currents would flow through them if we wereto apply the full line voltage to start the machine.
For this reason, when starting D.C. motors ofany but the very smallest sizes, it is necessary toplace some resistance in series with the armatureto limit the current until the machine comes up tospeed. As the motor increases its speed the counter -voltage becomes higher and higher, until it limitsthe current to such an extent that the motor speedcannot further increase. At this point the differencebetween the counter -voltage and the line -voltagemay be only a few volts, even on motors of quitehigh voltage.
The voltage effective in forcing current throughthe armature of a motor when it is running will bejust that amount of the line voltage which is notneutralized by counter -voltage. In other words, thevalue of the voltage used to force current throughthe armature winding will be line voltage minuscounter -voltage. This is illustrated in Fig. 6, whichshows the amount of the applied voltage which isneutralized by the counter -voltage developed in thearmature. For this illlustration we have used evenand convenient figures, but in actual operation of a
211
Counter -E. M. F.
motor running without load the counter -voltagewould be almost equal to the line voltage.
I ApplledI voltaic.
100 E
yoga qe
Counter
Fig. 6. From the above illustration you will note that the counter -voltage is often nearly as high as the applied voltage. This sketchillustrates the extent to which counter -voltage regulates or limitsthe current flow through a motor armature.
If we assume the resistance of the armature inthis figure to be .2 of an ohm, the current whichwould flow through its winding if full line voltagewere applied would be 100 .2, or 500 amperes.That is, of course, provided that no external re-sistance were used in series with the armature.
If this same armature develops 90 volts C.E.M.F.when rotating at full speed under full load condi-tions, the difference between the applied voltageand the counter -voltage would be 100 - 90 or 10volts. This 10 volts would be available for forcingcurrent through the armature. So, when runningat this speed, the armature current would be10 ÷ .2, or 50 amperes. From this example you cansee what a great effect counter -voltage has uponthe current flow in a motor armature.
10. EFFECT OF COUNTER -VOLTAGE ONMOTOR SPEED
The current required to operate a D.C. motorwhen no load is connected to it is comparativelysmall. Let us say that the armature shown in Fig.4 requires 50 amperes to operate it at full load, andonly 5 amperes to operate it when the load is dis-connected. As the resistance of the armature is only.2 of an ohm, the net voltage applied to run themachine at full speed and at no load would be.2 X 5, or 1 volt. So the counter -E. M. F. during thetime this motor is running idle should be 100- 1,or 99 volts.
212
Direct Current
When the mechanical load is removed from amotor, its armature tends to speed up because thedriving torque is then considerably greater thanthe resisting- torque and, as previously indicated,this state of affairs results in acceleration. How-ever, as the armature speed increases, the C.E.M.F.rises; this results in diminishing the currentthrough the armature and decreasing the drivingtorque. These actions continue until the two torquesbecome equal again. The increase in speed requiredto bring about this equality will depend upon thespeed regulation of the motor in question, and themagnitude of the change in the resisting torque.
11. D.C. MOTOR CHARACTERISTICSIn selecting D.C. motors for any particular work
or applicatibn we must, of course, use a machine ofthe proper horse -power rating to start and carry theload the motor is intended to drive. In addition tothe Horse Power Rating of the motor, the otheressential points to be considered are its StartingTorque and Speed Regulation characteristics. Thesecharacteristics vary widely for shunt, series, andcompound motors, which will be thoroughly ex-plained in the following paragraphs. Make a carefulstudy of this section because it may often be ofgreat advantage to you on a job to be able to selectthe proper motors for different applications.
A battery of DIRECT -CURRENT Splash -Proof Motormidwestern brewer,
12. SHUNT MOTORSThe field winding of a shunt motor is connected
directly across the line or source of current supply,
213
Shunt Motors
in parallel with its armature. This shunt field wind-ing is made up of many turns of small wire and hassufficient ohmic resistance to limit the currentthrough the coils to the safe carrying capacity ofthe conductors they are wound with. As the resist-ance of the shunt field winding is practically con-stant, this current and the strength of the field itsets up will be determined by the line voltage whichis applied to the motor.
Shunt Motor
Fig. 7. Diagram of the connections for a simple shunt motor.
A simple diagram of the connections of the arma-ture and field of a two -pole shunt motor is shown inFig. 7.
13. STARTING TORQUE- OF SHUNTMOTORS
The starting torque of shunt motors is only fairand they cannot start very heavy loads because theirfield strength remains approximately constant aslong as the applied voltage is constant.
While a motor is starting the armature flux isvery dense, because of the heavier currents flowingthrough the armature at this time.
This increased armature flux of course increasesthe motor torque, but it also weakens the field bydistorting it and forcing it to take a path of higherreluctance ; so shunt motors cannot build up as goodstarting torque as series or compound machines do.
As the torque of the motor depends upon its fieldstrength as well as upon the armature current, wecan see that the starting torque of a shunt motorwill not be very good.
214
Direct Current
14. STALLING TORQUE OF SHUNTMOTORS
If a shunt motor is overloaded to too great anextent it will slow down and possibly be stoppedentirely if the overload is great enough. A motor
.1
is
!I
1
i71ts.s
PI11"P
ie
I
II-/:...
10iIiI
I
iiII1
0
&ivil
i
... ----
...-... 1
I
t ------.Di FERE '
i
I
1920
1900
0400
1300
1200
1100
1 00 0
0 10 20 30 40LOAD IN AMPERES
Fig. 8. The above diagram shows the characteristic speed curvesfor several types of D.C. motors. Note carefully the manner inwhich the speed varies with increase of load.
should never be allowed to remain connected to theline when in this stalled condition, or its windingswill be burned out. This is due to the fact thatwhen the armature is stopped it is generating nocounter -voltage, and the applied line voltage willsend a severe overload of current through the lowresistance armature. Fuses or circuit breakersshould be provided to open the line circuit to themotor in a case of this kind.
The ability of a motor to carry overload withoutstalling is often referred to as the Stalling Torqueof the motor.
Shunt motors will carry their full, rated load butshould not he overloaded to any great extent, astheir stalling torque is not very high.
50 BO
215
Shunt Motors
15. SPEED REGULATION OF SHUNTMOTORS
The speed regulation of shunt motors is excellent,as the strength of their field remains practically con-stant, and as long as the proper line voltage is ap7plied they will maintain practically constant speedunder wide variations of the load.
The shunt motors will of course slow down a lit-tle when the load is increased ; but, as soon as thearmature speed is reduced even slightly, this reducesthe counter -voltage generated and immediatelyallows more current to flow through the armature,thereby increasing the torque and maintaining ap-proximately the same speed.
The speed of shunt motors ordinarily should notvary more than three to five per cent from no loadto full load. Fig. 8 shows a set of curves whichillustrate the speed regulation of series, shunt, andcompound motors. Note that the speed of the shuntmotor only falls off very gradually as the load isincreased.16. SPEED CONTROL AND APPLICATIONS
. OF SHUNT MOTORSThe speed of shunt motors can easily be varied
or controlled by inserting a rheostat in series withtheir field. If the field is weakened, the motor speedwill increase, because the reduced counter -voltageallows more current to flow through the- armature.If the field is strengthened, the motor speed willdecrease, because this stronger field allows thenormal counter -voltage to be generated at lowerspeed.
The uses and applications of shunt motors aremany and varied. They may be used on any jobwhere more than full -load torque is not requiredfor starting and where practically constant speed isessential. They are extensively used for pumps,elevators, motor -generator sets, and for the opera-tion of lathes and various machines used in manu-facturing appliances.17. SERIES MOTORS
Series motors have their field coils connected inseries with the armature and the line, as shown in
216
Direct Current
Fig. 9. The windings for the fields of series motorsare made of heavy copper wire or strap copper, andmay consist of anywhere from a few dozen to a fewhundred turns.
Fig. 9. Diagram of the connections for a simple series motor. Thedotted lines show where a shunt can be connected in parallel withthe field coils for varying the speed.
The strength of the field of the series motor de-pends upon the amount of current flowing throughthe armature and series field coils.
As the armature current depends upon themotorload and speed, the field strength of a series motorwill be much greater at heavy loads and low speedsthan at light loads and higher speeds, when thearmature is developing a greater counter -E. M. F.
18. STARTING TORQUEThe armature current is usually at its greatest
value when starting a motor because when thearmature is idle or rotating at low speeds it doesn'tgenerate much counter -E. M. F., and very heavycurrents will flow through the armature until itcomes up to speed. For this reason the startingtorque of series motors is excellent.
The torque of motors of this type varies directlywith the square of the armature current, becauseany increase of current through the armature alsoincreases the field strength, as the two windings arein series.
217
Series Motors
Series motors are capable of starting very heavyloads, and this makes them particularly adaptablefor use on street cars and electric railways, andother special applications where the machinery to bedriven is difficult to start.
This pro d type LOY15 AllisD.C. notes operstiny . merle
dusting shine is typical ofthe dependability of these mo-tors on touth jobs.
19. STALLING TORQUESeries motors also have excellent stalling torque,
because, when they are overloaded, their speed isreduced and less counter -voltage will be generatedin the armature. This allows more current to flowboth through the armature and field coils, greatly in-creasing the flux around the armature conductorsand from the field poles.
It is almost impossible to stall a series motor withany reasonable load, because the slower the speedbecomes, the more current will flow through thearmature and field of the motor, and the greater itstorque becomes.
Of course, it is possible to burn out a series mo-tor by overloading it in this manner, if the over-load is left on it too long.20. SPEED REGULATION
The speed regulation of series motors is verypoor, because their speed varies inversely with theload applied. Any increase of load actually strength -
218
Direct Current
ens the field flux of the series motor. This causes ahigher counter -voltage to be generated and momen-tarily reduces the armature current, until the speedof the motor drops enough lower to bring thecounter -voltage back to normal or less than normal,to allow the increased current flow required forthe additional load.
If some of the load is removed from the seriesmotor, this decreases the flow of current and weak-ens its field. The weaker field develops lesscounter -voltage and momentarily allows more cur-rent to flow, until the speed is increased enough tobuild the counter -voltage up again somewhat abovenormal value.
Thus, series motors will operate at very highspeeds when the load is light and they will over -speed if the load is entirely disconnected. For thisreason series motors should never be operatedwithout load, or the speed will increase to a pointwhere centrifugal force may throw the armatureapart.
Series motors should always be attached to theirload by gears or direct shaft connection, and neverby belts, because if the belt should break or slip offthe pulleys, the motor might dangerously overspeedbefore it could be stopped.
In Fig. 8, the speed curve for a series motor isshown, and you will note how rapidly the speeddecreases with any increase of load.
There are certain applications for motors, how-ever, where the decrease of speed with increase ofload is very desirable.
21. SPEED CONTROLThe speed of a series motor can be controlled or
varied at will by the use of resistance in series withthe motor. Increasing this resistance will reducethe voltage applied to the armature and series field,thereby momentarily reducing the current flow andthe torque, until the machine reduces its speed andcounter -E. M. F., to a point where the counter -E.M. F., and effective voltage again balance the re-duced applied voltage.
This is one of the methods used to vary the speedof electric street cars, by cutting resistance in or out
219
Series Motors
of the motor circuit with the drum controller.When the resistance in series with the machine isvaried the voltage across the armature is variedaccordinglY, and the armature slows down or speedsup correspondingly until the counter -E. M. F. andeffective voltage again equal the applied voltage.
The speed of series motors can also be varied byconnecting a shunt in parallel with their field coils,as shown by the dotted lines in Fig. 9. This shuntmerely passes a certain amount of the armature cur-rent around the field winding, and thereby weakensthe field strength and increases the speed of themotor. These shunts can not be used to decreasethe motor speed below normal.
22. USES OF SERIES MOTORSThe uses and applications of series motors are
somewhat limited because of their wide variation inspeed when the load is varied, and their tendencyto overspeed when the load is removed. Seriesmotors are not adapted to driving machinery orequipment which places a variable load on the motorand require practically constant speed.
Series motors are used principally for electriccranes, hoists, and railway service, and are wellsuited to this work because of their high torque de-
220
Direct Current
veloped at low speeds. They are particularly welladapted to electrical traction work because of theirsplendid starting torque, which enables them tostart heavy cars quickly and also climb hills withheavy loads. Their speed characteristic is also anadvantage in this case, because it is possible toobtain high speeds with cars operated by this typeof motor when the cars are running on the levelor with light loads.23. COMPOUND MOTORS
Compound D. C. motors.. have some of the charac-teristics of both the shunt and series motors, as theyhave both a shunt and series field winding on eachpole. The shunt field of the ordinary machine ismade up of many turns of small wire and is con-nected in parallel with the armature and line, asshown in Fig. 10. The strength of the shunt fieldflux will therefore be proportional to the appliedline voltage and will be practically constant as longas this voltage is not varied.
The series field winding is usually made of veryheavy copper wires or strap copper and may varyfrom a few turns to 100 turns or more per pole. Thiswinding is connected in series with the armature,as shown in Fig. 10, and carries the full load cur-rent which passes through the armature.
The strength of the series field will therefore beproportional to the load applied to the motor. Theshunt field, however, is the one that always deter-mines the polarity of the machine under ordinaryconditions and, therefore, it is called the main fieldwinding.
Compound motors can be connected either cumu-lative or differential, by simply reversing the con-nections of their series field windings. The connec-tions shown in Fig. 10 are for a cumulativecompound motor, and most D.C. motors are under-stood to be connected in this manner, unless theyare marked or designated as differential -compound.
With the series field connected for cumulative -compound operation, the current flows throughthese coils in the same direction that it does throughthe shunt coils, and therefore aids in setting up astronger field when there is any load on the motor.
221
Compound Motors
24. STARTING AND STALLING TORQUECumulative -compound motors have a very much
better starting -torque than shunt motors, becausethe heavier armature currents which flow duringstarting also pass through the series field andgreatly strengthen its flux, thereby increasing themotor torque.
Motors of this type can be used for starting very
CUMULATIVE COMPOUND MOTOR
LINE
11111[410111111111
Fig. 10. Diagram of connections for a cumulative compound motor.Note that the series field winding is connected so it will aid theshunt field winding in providing a strong field.
heavy loads, or machinery that is difficult to startand bring up to speed.
The stalling torque of cumulative compound mo-tors is also quite high, because any increase of loadon the machine will increase its armature currentand the current through the series field. This in-creases the flux of the field poles, which in turn in-creases the motor torque and enables it 'to carry theadditional load at slightly reduced speed.
Such motors can be allowed to carry reasonableoverloads of 15 to 25 per cent. as long as they don'toverheat enough to damage their insulation.25. SPEED REGULATION AND APPLICA-
TIONSThe speed regulation of cumulative -compound
motors can be considered as fair. Their speed willvary inversely with the load, because any increaseof load also increases the field flux due to the actionof the series winding ; and when the field flux is in-creased, the armature speed must decrease, in order
222
Direct Current
to lower the counter -E. M. F. sufficiently to allowenough current to flow to carry the load. Thestronger the field of any motor, the lower will bethe speed at which it can generate the normalcounter -E. M. F.
Fig. 11. Differential compound motor connections. Note that theseries field is connected so it will oppose and weaken the effectof the shunt field.
Compound motors are used extensively to drivepower shears, the rolls of steel mills, and in factoriesand industrial plants for running machines whichrequire good starting and stalling torque and don'trequire very close speed regulation.26. DIFFERENTIAL COMPOUND MOTORS
When compound motors are connected for differ-ential operation their characteristics change con-siderably from those of cumulative machines.
A differential compound motor has its series fieldso connected that the current will flow through itin the opposite direction to that of the current inthe shunt field windings, as shown in Fig. 11. Thistends to weaken the field flux whenever any load isbeing carried by the motor.
The shunt field winding is the main winding andunder ordinary conditions it determines the polarityof the field poles. Occasionally, however, whenthese motors are started up rather suddenly and
223
Compound Motors
under heavy load, the current flow through the dif-ferential series winding becomes very strong; anddue to its strong flux and the inductive effectwhich it has on the shunt field coils during thetime this flux is building up around the series wind-ing, it may overcome the shunt field flux and re-verse the polarity of the field poles. This will causethe motor to start up in the wrong direction.
To avoid this, the series field of a differentialmotor should be short-circuited when starting.This can be done by the use of a single -pole knifeswitch of the proper size, connected across theseries field terminals, as shown in Fig. 11.
Cutaway 'wk. of G -E Ty,. CD'1000 Sark,- dripptool, direcwwcurrent poke, whopin, wintilatingsystem which P &signed for
rewwwwwkle wok.. nil Popppkes vou a motor of km height,
wipthked wwwight, midn
wstingper hope size tha IkreolowePPwible.
27. STARTING TORQUE AND STALLINGTORQUE
The starting torque of an ordinary differential -compound motor is very poor, even poorer than thatof a shunt motor. This is due to the effect of theheavy starting currents flowing through this fieldand weakening the flux of the shunt field to suchan extent that the motor has very poor startingtorque. Motors of this type are usually startedwithout any load connected to them.
A reversing switch can be used to reverse thepolarity of the differential field and make the motoroperate cumulative during starting, and therebyimprove the starting torque of this motor.
Differential motors will not carry overload with-out stalling. In fact they will usually only carryabout 75% of the full rated load of a shunt motorof the same size. Whenever the load on such amachine is increased, the series field current is in-creased and, because it flows in the opposite direc-tion of that in the shunt winding, it tends to neu-
224
Direct Current
tralize and weaken tne total field flux and alsoweaken the load -pulling torque.28. SPEED REGULATION AND APPLICA-
TIONS
Differential -compound motors have excellentspeed regulation up to a certain amount of load. Asthe load is slightly increased, the motor tends toslow down, but the increased current through thedifferential series field immediately weakens theshunt field flux and thereby causes the counter -E. M. F. in the armature to be reduced.
This allows more current flow through the arma-ture and maintains the speed at normal value. Withjust the proper number of turns on a differentialseries field, the tendency of the motor to slow downwith increased load and the tendency to speed upwith weakened field can be so balanced that theywill neutralize each other, and the speed will remainalmost perfectly constant if the load change is nottoo great.
Note the speed -curve shown for this type of mo-tor in Fig. 8.
Differential -compound motors are not used veryextensively, because of their very poor startingtorque ; but they have certain applications wherevery little starting torque is required and goodspeed regulation is essential. The operation of tex-tile mill machinery is a good example of this appli-cation.
A convenient, practical method for determiningwhether a compound motor has its series field con-nected differential or cumulative is to operate themotor and note its speed. Then reverse the seriesfield connection and again note the speed. Which-ever connection gives the most speed is the differ-ential connection of the series field winding.
The table on characteristics and applications ofD.C. motors provides some useful information ontorque, speed regulation and application of the dif-ferent types of motors. Note that the change inspeed from no-load to full -load is least for theshunt motor and greatest for the series motor. Inother words, the shunt motor has the, best speedregulation.
225
Cha
ract
eris
tics*
and
App
licat
ions
of D
-c
Mot
ors,
1 to
300
Hp
I%
0.A
pplic
atio
nsS
tart
ing
Dut
y
Max
imum
Mom
enta
ryR
unni
ngT
orqu
e
Spe
edR
egul
atio
nS
peed
Con
trol
t
Shu
nt-
wou
nd,
cons
tant
-sp
eed
Med
ium
star
ting
torq
ue. V
arie
s w
ithvo
ltage
sup
plie
d to
arm
atur
e,an
dis
limite
d by
sta
rtin
gre
sist
or to
I25.
to 2
00pe
rce
ntfu
ll -lo
adto
rque
125
to .0
0 pe
r ce
ntLi
mite
d by
com
-m
utat
ion
8 to
12
per
cent
Bas
ic s
peed
to 2
00pe
rce
ntba
sic
spee
dby
field
cont
rol
Driv
es w
here
sta
rtin
g re
quire
men
ts a
re n
otse
vere
. Use
con
stan
t -sp
eed
or a
djus
tabl
e -
Shu
nt-
wou
nd,
adju
st-
able
-sp
eed
10 to
20
per
cent
,in
crea
ses
with
wea
kfie
lds
Bas
ic s
peed
to 6
00pe
r ce
nt b
asic
spe
ed(lo
wer
for
som
era
tings
)by
field
cont
rol
spee
d, d
epen
ding
on
spee
d re
quire
d. C
en-
trifu
gal p
umps
, fan
s, b
low
ers,
con
veyo
rs,
elev
ator
s, w
ood-
and
met
al -
wor
king
ma-
. crim
es
Cor
n-po
und-
wou
nd,
cons
tant
spee
d
Hea
vy s
tart
ing
torq
ue. L
imite
d by
star
ting
resi
stor
to13
0 to
260
per
cen
tof
full
-load
torq
ue
130
to 2
60 p
er c
ent.
Lim
ited
by c
om-
mut
atio
n
Sta
ndar
d co
mpo
und-
ing
25 p
er c
ent.
De-
pend
s on
am
ount
of
serie
s w
indi
ng
Bas
ic s
peed
to 1
25pe
rce
ntba
sic
spee
d by
fiel
d co
n-tr
ol
Driv
es r
equi
ring
high
sta
rtin
g to
rque
and
fairl
yco
nsta
ntsp
eed.
Pul
satin
glo
ads.
She
ars,
ben
ding
rol
ls, p
lung
er p
umps
, con
-ve
yor
crus
hers
, etc
.
Ser
ies-
wou
nd,
vary
ing-
spee
d
Ver
y he
avy
star
ting
torq
ue.
Lim
ited
to30
0 pe
r ce
nt to
350
per
cent
-ful
l-loa
dto
rque
300
to 3
50 p
erce
nt.
Lim
ited
by c
orn-
mut
atio
n
Ver
y hi
gh.
Infin
iteno
-load
spe
edF
rom
zer
o to
max
-im
um s
peed
,de
-pe
ndin
g on
con
trol
and
load
Driv
es w
here
ver
y hi
gh s
tart
ing
torq
ue is
requ
ired
and
spee
d ca
n be
reg
ulat
ed. C
rane
s,ho
ists
, gat
es, b
ridge
s, c
ar d
umpe
rs, e
tc.
Tab
le s
how
s av
erag
e va
lues
for
stan
dard
mot
ors.
t Min
imum
spe
ed b
elow
bas
ic s
peed
by
arm
atur
e co
ntro
l lim
ited
by h
eatin
g.
Direct Current
The maximum overload that a motor can carryfor a brief interval of time is limited by sparkingat the brushes. This is indicated by the statementson maximum momentary torque given in the thirdcolumn of the table. Note that due to sparking diffi-culties, the series motor cannot be allowed to carrymore than 350 per cent of the normal full loadtorque. If the load demanded it, the series motorcould actually develop a much higher torque thanindicated ; however, such a high value of torquewould result in severe flashing at the commutator.
The last column shows that each motor is suitedto a particular application or series of applicationsand some of the more frequently encountered onesare listed. This brings out the fact that no motor issuitable for all types of drives, and it indicates theneed for analyzing the drive requirements beforeselecting a motor for any given application.
The variation in speed obtainable by field controlon the ordinary D.C. motor will not, in the averagecase, exceed 4 to 1 due to the sparking difficultiesexperienced with very weak fields. Although therange may be increased by inserting resistance inseries with the armature, this can be done only atthe expense of efficiency and speed regulation.
With constant voltage applied to the field, thespeed of a D.C. motor varies directly with the arma-ture voltage ; therefore, such a motor may be step-lessly varied from zero to maximum operatingspeed by increasing the voltage applied to its ar-mature. The sketch shows the arrangement of ma-chines and the connections used in the Ward Leon-ard type of variable voltage control designed tochange speed and reverse rotation. The constantspeed D.C. generator (B) is usually driven by anA.C. motor (A) and its voltage is controlled bymeans of rheostat R. Note that the fields of bothgenerator (B) and driving motor (C) are energizedfrom a separate D.C. supply or by an auxiliary ex-citer driven off the generator shaft. Thus thestrength of the motor field is held constant, whilethe generator field may be varied widely by rheo-stat R.
With the set in operation generator (B) is drivenat a constant speed by prime mover A. Voltage
227
N N
VA
R/A
BLE
VO
LTA
GE
CO
NT
RO
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Ui z
GE
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RA
TO
R F
IELD
RH
EO
ST
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.
--11
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EV
ER
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WIT
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.
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TO
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IELD
RH
EO
ST
AT
.
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ING
MO
TO
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k=>
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OT
OR
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KD -*
--vE
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RA
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.M
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FIE
LD
C>
Direct Current
from B is applied to the D.C. motor (C) which isconnected to the machine to be driven. By propermanipulation of rheostat R and field reversingswitch S the D.C. motor may be gradually started,brought up to and held at any speed, or reversed.As all of these changes may be accomplished with-out breaking lines to the main motor, the controlmechanism is small, relatively inexpensive, and lesslikely to give trouble than the equipments designedfor heavier currents.
The advantages of this system lie in the flexibilityof the control, the complete elimination of resistorlosses, the relatively great range over which thespeed can be varied, the excellent speed regulationon each setting, and the fact that changing thearmature voltage does not diminish the maximumtorque which the motor is capable of exerting sincethe field flux is constant.
By means of the arrangement shown, speedranges of 20 to 1-as, compared to 4 to 1 for shuntfield control-may be secured. Speeds above therated normal full load speed may be obtained byinserting resistance in the motor shunt field. Thisrepresents a modification of the variable voltagecontrol method which was originally designed forthe operation of constant torque loads up to therated normal full load speed.
As three machines are usually required, this typeof speed control finds application only where greatvariations in speed and unusually smooth control aredesired. Steel mill rolls, electric shovels, passengerelevators, machine tools, turntables, large ventilat-ing fans and similar quipments represent the typeof machinery to which this method of speed controlhas been applied.29. BRAKE HORSE -POWER TEST FOR
MOTORSOccasionally it may be desirable to make an
actual test of the horse -power .output of a motor, inorder to determine its condition or efficiency. Thiscan be done by arranging a brake or clamp to ap-ply load to the pulley of the motor and therebymeasure the pull in pounds or the torque exertedby the motor.
229
Prony Brake
This method is known as the Prony Brake Test..Fig. 12 shows the equipment and method of its
use for making this test. The brake can be madeof wood blocks cut to shape to fit the pulley and
Fig. 12. The above diagram illustrates the method of making abrake-h.u. test on a motor.
fitted with bolts and wing nuts so the grip or ten-sion of the blocks on the pulley can be adjusted.When making a number of these tests, it is also agood plan to line the curved faces of the block withordinary brake lining such as used on automobiles.This makes it possible to apply a smoother brakingeffect without generating too much heat due to thefriction.
An arm or bar, of either wood or metal, can beattached to the brake blocks as shown in the figure,and fitted with a bolt or screw eyes for attachingthe scales to the end of the bar. A spring scale,such as shown in Fig. 12, can be used, or the bolton the underside of the arm can be allowed to reston the top of a platform scale.
The brake arm should preferably be of some evenlength, such as 2 ft. or 3 ft., in order to simplify thehorse -power calculation. The arm length is mea-sured from the center of the shaft to the point atwhich the scale is attached.
With'a device of this kind, load can be graduallyapplied to the motor by tightening the brake shoesor clamps until the motor is fully loaded.
An ammeter can be used in series with one of theline leads to the motor to determine when the ma -
230
Direct Current
chine has been loaded to its rated current capacity.In case an ammeter is used, a voltmeter should alsobe used, to see that the proper line voltage is ap-plied to the motor at the time of the test. A watt-meter can be used instead of the voltmeter and d.m-meter if desired.
When the brake has been adjusted so that themotor is drawing its full rated load in watts, thepound pull on the scale should be noted and thespeed of the motor in revolutions per minute shouldbe carefully checked.
The adjustment on the brake should be main-tained to keep the motor pulling the same amounton the scales and drawing the same load in wattsduring the time the speed is being checked.
The motor speed can easily be checked by meansof a speed counter or tachometer applied to the endof its shaft while running. A watch with a second-hand should be used for gauging the time accur-ately.
30. HORSE POWER CALCULATIONThe horse power of a motor is proportional to the
product of its torque and speed. Therefore, whenwe know the length of the lever arm in feet, thepull in lbs. on the scales, and the speed of themotor in R. P. M., we can easily determine thehorse -power output by the following simpleformula : 2 X at X R.P.M. XPXL
h.p. =In which : 33,000
h. p. = the horse power developed by the mo-tor
= 3.1416, or the ratio between the diam-eter and circumference of a circle.(2 X a = 6.28)
R. P. M. = Speed of the motor in revolutions perminute.
P = Lbs. pull on the scale.L = Length of lever arm in feet.
33,000 = Number of foot-pounds required perminute for one h.p.
As an example, suppose we have made a test ona motor using a brake arm two ft. in length, andhave found that when the motor is fully loaded ac -
231
Efficiency
cording to the electrical instruments, it applies 9lbs. pull on the end of the arm and revolves at aspeed of 1500 R. P. M. Then, according to ourformula :
6.28 X 1500 x 9 X 2h. p. = or 5.1 h. p.
33,000
31. EFFICIENCY TESTSThe efficiency of a motor is, of course, an im-
portant item, especially where a large number ofmotors are being chosen for continuous operation ofcertain equipment. The higher the efficiency of anymotor, the greater the h. p. it will produce from agiven amount of electrical energy in watts, and theless power will be wasted in losses within themachine.
These losses are partly mechanical, such as bear-ing friction and "windage" due to the armature re-volving through the air at high speed. They are alsopartly electrical, such as losses in the armature andfield windings due to resistance and to a certainamount of energy being transformed into heat, andthe slight magnetic losses due to hysteresis andeddy currents.
The efficiency of D.C. motors may vary from 50%
EFFICENCY Or 230 VOLT COMPOUND D.C. MOTORS
SIZEP.IN 14.
AT A LOAD AT % LOAD AT FULL LOAD
5 7.3 % 78 % 807.
10 797. 82.5% 857.
25 847. 87 % 87.5%
S0 8.57. 87.57. 88.57.
200 877. 897. 91 .5 %
Fig. 13. This table gives the approximate efficiencies of various sizedD.C. motors at various percentages of load.
or less for the very small fractional horse powermachines up to 90% for the larger ones, and evenhigher than this for extremely large motors.
The efficiency of ordinary motors from 5 to 50h. p. will usually range between 75 and 90 per cent;
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Direct Current
so, when the efficiency of a machine is not known, agood average figure to use is 80% or 85%.
As a general rule, the larger the motor, the higherwill be its efficiency. Fig. 13 shows a table inwhich are given the efficiences of several sizes ofmotors, from 3 to 200 h. p. You will also note byexamining this table that the efficiency of any mo-tor is better at full or nearly full load. Therefore,it does not pay to operate motors lightly loadedwhenever it can be avoided by the selection and useof motors of the proper size. In many cases, mo-tors which are larger than necessary have been in-stalled to operate certain machines, because thesemachines require considerable starting torque. In acase of this kind, the selection of a different typemotor with a better starting torque can often effectconsiderable power saving.32. EFFICIENCY CALCULATION
The efficiency of any motor can be found by di-viding its output in watts by the input in watts.This is stated by the following formula :
Fig. 14. The above diagram shows the method of connecting awattmeter or voltmeter and ammeter, to determine the KW orh. p. input to a motor.
e =W 0
W I
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Efficiency
In which :e = the efficiency of the motor in per cent.
W 0 = watts outputW I = watts input
The output and input can both be determined inhorse power or kilowatts, if preferred, and used inthe same manner in the formula.
When we have made a test of the horse power ofa motor by the Prony brake method and have mea-sured the electrical power input either with a watt-meter or a voltmeter and ammeter, it is. then aneasy matter to determine the efficiency of the ma-chine with the formula just given.
For example, suppose we have tested a machinand found its full load output to by 35.5 h. p. Dur-ing this test the wattmeter connected to the motorleads indicated that it was consuming 31,150 watts.To obtain the output in watts, we multiply 35.5 by746, as there are 746 watts in each h. p., and wefind that the output is 26,483 watts.
Then, according to the formula, the efficiency ofthis motor will be found as follows :
26,483e = , or 85 + % efficiency
31,150Fig. 14 shows the method of connecting a watt-
meter to the terminals of a motor for determiningthe input or energy consumed. At "B" in this samefigure are shown the proper connections for a volt-meter and ammeter used to determine the input ofthe motor. Watts equal volts multiplied by am-peres.
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Direct Current
EXAMINATION QUESTIONS
1. What determines the speed of any D.C. mo-tor?
2. What is the difference between the term"Speed Control" and "Speed Regulation" whenused in connection with D.C. motors?
3. How does the horsepower output of a highspeed motor compare to the H.P. output of a lowspeed motor of the same size?
4. Give Fleming's rule for determining the di-rection of rotation for a D.C. motor?
5. How is the field winding of a shunt motorconnected in relation to the armature?
6. Is there any danger in operating a series mo-tor without a load? Why?
7. Draw a simple sketch of a two pole cumula-tive compound motor.
8. Which type motor would you select for eachof the following uses?
a. Power shears? b. lathe? c. cranes?
9. Give the formula for determining the brakehorsepower of a motor.
10. What would be the efficiency of a motor op-erating on 1800 W and developing 2 horsepower?
235
Efficiency
\° 64",
A motor almost big enough to live in. This gigantic and inspectedmotor is being tested by the two workmen in upper left cornerbefore being shipped.
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Direct Current
D. C. MOTOR STARTERS
AND CONTROLS
There are two general types of D. C. motor con-trol equipment. 'One of these is used for startingduty only, and the other can be used both for start-ing and for controlling or regulating the speed ofthe motors while running.
Motors of H. P. or less can be started by con-necting them directly across the line, as their arma-tures are so small and light in weight that theycome up to full speed almost instantly. Therefore,the heavy rush of starting current does not lastlong enough to overheat their windings.
Medium-sized and larger D. C. motors shouldnever be connected directly across the full line volt-age to start them, as their heavier armaturesrequire more time to speed up and develop thenecessary counter -voltage to protect them from ex-cessive starting current.
If these armatures are connected directly acrossfull line voltage when they are at rest, the rush ofstarting current through them is likely to be morethan 10 times full -load current. This excessive cur-rent will overheat the winding, and also possiblydamage the insulation of the coils by the powerfulmagnetic field it sets up and the mechanical forcesthe coils exert on the slots in trying to practicallyjerk the armature up tc full speed.
So, for this reason, a starting resistance shouldalways be connected in series with the armature ofa D. C. motor when starting it, and left in the circuituntil the motor armature has reached full speed andhas built up its own protective counter-voltage.
When the current flows through this resistance, itcauses sufficient voltage drop so that only aboutone-fourth of the line voltage is applied to the arma-ture. Fig. 1 shows the method of connecting thestarting resistance in series with the motor arma-ture.
These starting resistances are usually arranged so
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Starters and Controllers
they can be gradually cut out of the armature circuitas the motor comes up to speed, and when full speedis reached the resistance is all cut out.
ShuntField
e4110
4.4.0
410.
SeriesField
Fig. 1. This simple sketch shows the manner in which a resistanceis used in series with the armature when starting a D. C. motor.
The starting current for D. C. motors should belimited to about 1% to 2% times full -load current.It is therefore necessary that starting rheostats havethe proper resistance value and current capacity forthe motors with which they are used.
1. TIME ALLOWED FOR STARTINGMOTORS
The period of time for which the starter resist-ance should be left in series with the motor whenstarting, depends upon the size of the motor and thenature of the load attached to it. A motor connectedto a heavy load, of course, requires more time tocome up to full speed, and the larger the armatureof a motor, the more time is required for it to reachfull speed.
Usually from 15 to 30 seconds will be required onordinary motors. This rule, however, cannot bestrictly followed, as the time allowed for starting amotor must be largely a matter of observation and
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Direct Current
good judgment on the part of the operator. One canreadily tell by the sound of the motor when it hasreached full speed.
While starting and operating various motors youwill gain considerable practice in judging the timerequired for different motors. Always watch andlisten to the motor. closely when starting it up, andnever leave the resistance in the circuit any longerthan necessary, or it is likely to become damagedby overheating.
1.7.qeocehlpr=Wel 14 thIN
2. MOTOR STARTING RHEOSTATS
Starting resistances or Rheostats, as they arecalled, should never be used to regulate the speedof a motor after it is running. Starting rheostatsare designed to carry the armature current onlyfor a very short period and should then he cut outof the circuit. If they are used for speed regula-tion and left in the circuit for longer periods, theyare very likely to become overheated to a pointwhere the resistance metal will burn in two and re-sult in an open circuit in the rheostat.
Armature starting resistances for small machinesare usually made up of iron wire, or wire consistingof an alloy of nickle and iron. This resistance Wireis wound on an insulating base, or form of asbestos
239
Starters and Controllers
or slate. The turns of the coil are so spaced thatthey don't short together.
Taps are made at various points along the coiland are connected to segments or stationary con-tacts which are mounted on the face -plate of thestarter. A lever arm with a sliding contact is thenused to cut out the resistance gradually as themotor comes up to speed.
3. SPEED CONTROL RHEOSTATSSpeed -regulating resistance can be used for start-
ing motors and also for controlling their speed overindefinite periods. Rheostats for this use are madeof larger and longer resistance material and aredesigned to carry the armature current for longperiods without damage from overheating.
Speed -regulating resistances are in some casesmade of heavy iron wire, but for medium and largersized motors are generally made of cast iron grids,or grids consisting of an alloy of nickle and iron.The nickle alloy is generally preferred in the betterclass controls.
4. METHODS OF CONTROLLING THESPEED OF D. C. MOTORS
The speed of shunt or compound motors mayeasily be controlled by the use of a rheostat inseries with the shunt field, as shown in Fig. 2.By varying the resistance of the field rheostat, wecan vary the current through the field of the motor.If the field is weakened, the counter -E. M. F. gener-ated in the armature is momentarily reduced andmore current is allowed to flow through the ma-chine.
This will cause the machine to speed up untilthe counter -voltage produced in this weaker fieldis again normal. If the motor field is strengthened,the counter -voltage developed in the armature willbe increased, and this will cause the current flow-ing through the machine to be reduced, allowingthe speed to decrease until the counter -voltagedeveloped in this stronger field is again normal.
It is possible to vary the speed of a motor onlyabove its normal speed by the use of shunt fieldrheostats.
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Direct Current
. The torque of a motor armature will vary in-versely with the speed when the field is weakenedin the manner just described. The output in h. p.,however, will remain approximately the same, as theh. p. is proportional to the product of the speed andtorque.
Fig. 2. This diagram shows the armature starting resistance and alsoa shunt field rheostat for varying the speed of the motor.
,For example, if a certain motor normally rotatesat 1000 R. P. M. and develops 10 lbs. torque at theend of a brake arm, the product of this speed andtorque is 1000 X 10, or 10,000.
Now, if we were to increase the speed of thismotor to 2000 R. P. M., or double its normal speed,the torque, which varies inversely with the speed,will be reduced to 5 lbs.,. or one-half its formervalue. In this case, the speed times the torqueequals 2000 X 5, or 10,000 as before.
Motors that do not have interpoles. should notordinarily be operated at speeds -greater than 65 to70 per cent above their normal speed ratings. Onmotors that have interpoles, if is possible to obtainspeed variations as great as 6 to 1 ratio.
Field control is a very economical means of speedvariation for D. C. motors, since the output of themotor in horse power remains practically unchanged
241
Starters and Controllers
and the power lost in the field rheostat is ver5rsmall.
The power lost due to heating in any resistanceis equal to the square of the current multiplied bythe resistance, or I2 X R = W.
For example, let us assume that the resistanceof the field rheostat shown in Fig. 2 is 100 ohms,and that the field current required by this motor is2 amperes ; then the power lost in the field rheostatwould be 22 X 100, or 400 watts.
5. SPEED CONTROL BY USE OF ARMA-TURE RESISTANCE
The speed of shunt, series, and compound motorscan also be regulated or varied by means of arheostat in series with the armature, as shown inFig. 3. An armature resistance used in this man-ner merely produces a voltage drop as the machinecurrent flows through it, and thus it varies the volt-age applied to the armature.
When this method of speed control is used, thestrength of the shunt field of the motor is notvaried, as it is connected directly across the line soit is not affected by the armature resistance. Ob-serve this method of connection. in Fig. 3. .
When the voltage applied to the armature is de-creased by cutting in the resistance of the armaturerheostat, this will decrease the armature currentand the speed of the motor. Since the torque ofany motor varies with the product of the armaturecurrent and field flux, any change of this armaturecurrent produces a corresponding change in thetorque and speed developed by the machine. Whenthe machine slows down to a speed at which itscounter E. M. F.; and effective armature voltageagain balance the applied voltage, the current andtorque will again be the same as before changingthe speed.
6. SPEED CONTROL BY FIELD RESIST-ANCE MOST ECONOMICAL GENERALLY
Speed control by means of armature resistance isvery wasteful of power because of the very heavy
242
Direct Current
armature current which must be passed through therheostat, and the losses due to heat and I R drop inthe rheostat.
Fig. 3. Rheostats for speed regulating duty use heavier resistanceunits to stand the continued current load without overheating.
If the armature shown in Fig. 3 requires 50 am-peres for full load operation and the speed regu-lating rheostat has .5 of an ohm resistance, then theenergy lost due to heat in the rheostat will be12 X R, or 502 X .5, which equals 1250 watts.
If the field resistance were used for speed controlof this motor, the losses would be much less. Wewill assume the field current to be 2 amperes, andthe field rheostat resistance 100 ohms. Then theloss with this form of speed control would only be22 X 100, or 400 watts.
The speed regulation of the motor which is con-trolled by armature resistance is very poor whenthe machine is operated below normal speed, whilethe speed regulation of a motor controlled by thefield rheostat is very good, because the armaturein this case is always operated at the same voltage.
Shunt field rheostats for ordinary motors aresmall compact devices, because they don't need tocarry a great amount of current or to have a largeheat radiating surface.
243
Starters and Controllers
Armature rheostats are much larger and usuallymade of heavy cast iron grids, or, in the case ofsome of the latest type controls, they are made fromalloy of manganese and copper.
From the foregoing material, it is easy to see thatthe use of the field rheostat provides a much moreeconomical method of motor speed control than theuse of armature resistance, and it should thereforebe used whenever possible.
The three principal advantages of field controlover armature control are as follows :
1. The horse power output remains practicallyunchanged with field control but decreasesconsiderably with armature control.
2. Power lost in the field rheostat is much lowerthan in armature rheostats, which must carrythe heavier armature current.
3. The speed regulation of a motor which is con-trolled by field rheostats is much better thanthat of a machine controlled by armature re-sistance.
Resistance should never be cut in to both arma-ture and field circuits at the same time on anymotor, because resistance in the armature circuittends to reduce the speed, while resistance in thefield circuit tends to increase speed. So each onewould tend to defeat the purpose of the other.
Both armature and field control are often usedtogether on the same motor, however, cutting outresistance from the armature circuit to bring thespeed from zero up to normal, and cutting in resist-ance in the field circuit to raise the speed abovenormal.
7. D. C. MOTOR CONTROLLERS
There are many types of D.C. motor starters andspeed controllers, but the general principles ofpractically all of them are very much the same.Their function is usually to place resistance inseries with the motor armature when the machineis started, and gradually cut out this resistance asthe machine comes up to speed.
Some controllers also make a slight variation inthe resistance in the shunt field circuit at the same
244
Direct Current
time the armature resistance is cut out. Some typesof controls have reversing switches or contacts inaddition to the rheostat element, so they can beused for starting and reversing of motors.
The operation of controllers may be eitherManual or Automatic. In the manual types thelever arm or sliding contact which cuts out the re-sistance is operated by hand ; while, in automatictypes, the movement of the sliding contact orswitches which cut out the resistance is ac-complished by means of electro-magnets or sole-noids, which may be operated by a small push but-ton switch located either at the controller or somedistance from it. Because of this feature, certaincontrollers are known as Automatic Remote -Controldevices.
The design of the various controllers depends ineach case upon the size of the motors they are tooperate and upon the class of duty they are to per-form.
8. CONSTRUCTION FEATURESCommon small motor controls consist of a box
or panel on which are mounted the stationary con-tacts and sliding contact oi controller arm ; andusually some form of latch or holding magnet toheld the arm in running position, and frequentlysome form of line switch or, possibly, reversingswitch.
On some of the smaller type controllers thesecontacts, coils, and switches are on the outside ofthe box or on what is called a "face plate," made ofslate or insulating material.
Controllers, used for small motors frequently havethe resistance coils mounted inside the box, di-rectly behind the face plate. In such cases the boxis usually of well -ventilated construction, to allowthe heat to escape.
On larger controllers, the resistance coils or gridsare frequently located in a separate box or on apanel, and have copper leads run from the contactson the panel to the resistance element.
Modern automatic types of controls frequentlyhave their entire assembly of magnets, switches, and
245
Starters and Controllers
contacts enclosed in a metal safety cabinet.Regardless of the type or application of the con-
troller, you should be able to easily understand theircircuits and principles, with the knowledge youalready have of electrical circuits, electro-magnets,switches and rheostats.
9. THREE AND FOUR POINT STARTERSSome of the most simple and common types of
controls used with shunt and compound motors arecalled 3 -point and 4 -point controllers. The names3 -point and 4 -point are derived from the number ofconnections or terminals on the face plate of these
Fig. 4. This diagram shows the connections for a simple 3 -pointD. C. motor starter. Trace the circuit carefully with the accom-panying instructions.
controllers. The 3 -point control is usually arrangedfor starting duty only, but in some cases it may alsobe' used for speed control, if it is properly designed.
Fig. 4 shows the wiring and electrical connec-tions of a simple 3 -point starter. In this diagramall parts and connections are in plain view and thepath of the armature current is marked with solidblack arrows, while the field circuit is shown by thedotted arrows. Trace this circuit but thorotighly
246
Direct Current
and become familiar with the principles and opera-tion of this fundamental type of starter.
To operate a controller of this type and start themotor, the first step will be to close the line switchto apply the line voltage to the controller and motor.
You will note that one side of the line connectsdirectly to the motor and that the controller is in-serted in the other line wire, so that its resistancewill affect both the armature and field circuits dur-ing starting.
The first step after closing the line switch is tomove the lever arm "H" to the first point or con-tact attached to the left end of the controller re-sistance. Current will then start to flow from theopposite line wire, through the controller arm, andthrough the entire resistance to the motor armatureand series field, and then back to the negative lint.wire, as shown by the solid arrows.
Another circuit can also be traced from the leverarm when it is in contact with point No. 1, as thecurrent divides at this point and a small amountflows through the holding magnet "M", thenthrough the shunt field winding and back to thenegative line wire, as shown by the dotted arrows.
As soon as the motor starts to turn, the controllerarm can be moved slowly across the contacts inorder, 1, 2, 3, etc. This cuts out the resistance fromthe armature circuit step by step as the armaturedevelops speed and begins to generate counter -voltage.
When the last contact point is reached, all re-sistance has been cut out of the armature circuit,and the leVer arm will be held in this running po-sition by the holding magnet "M", which is in se-ries with the shunt field circuit.10. "NO VOLTAGE" and "NO FIELD"
RELEASE COILThe reason for connecting this holding magnet
in series with the motor field is to provide what isknown as "no field" protection.
We have learned that a motor with a very weakfield is likely to overspeed dangerously. This would
247
Starters and Controllers
probably be the case if an open circuit should oc-cur in the shunt field coils or connections of a motorof this type, when it is not loaded.
However, with the holding magnet, "M", con-nected in series with the shunt field, if any breakoccurs in this circuit the magnet, `:1\4", will be de -energized and allow the controller arm to be thrownback to the "off" position by means of a spring.This will stop the motor before it has a chance tooverspeed.
This holding magnet also acts as a "no voltage"release, so that if the voltage or power suppliedat the line should fail, the starter arm will be re-leased and return to normal position and thus stopthe motor.
If this protection were not provided and thecontroller arm were left in running position, themotor might be burned out or injured when thepower came back on the line, because there wouldthen be no resistance in series with the motor arma-ture.
This holding often referred to as ano -field or no -voltage release coil, and provides thisvery important protection to the motor, in additionto serving its function of holding the starter armin place.
11. ALL RESISTANCE OUT OF FIELDCIRCUIT DURING STARTING
You will note by tracing the circuit when thestarter arm is in the running. position, that the fieldcurrent will then have to pass back through theentire starter resistance, through coil "M", and theshunt, field. We find, therefore, that as the con-tcoller cuts the resistance out of the armature cir-cuit, it places the same resistance in the shunt fieldcircuit. The advantage of this is that it providesmaximum strength of the shunt field during startingof the motor, when it is naturally desired to providethe best possible starting torque.
As the motor comes up to speed, the shunt fieldstrength can be reduced to normal by causing itscurrent to flow through the starter resistance.
248
Direct Current
The value of the starter resistance in ohms isvery low and it therefore doesn't affect the shuntfield as much as it does the armature, because thevery- small current required by the shunt fielddoesn't create much voltage drop when flowingthrough this resistance.
12. STOPPING A MOTOR
To stop the motor, we should always open theline switch, which will interrupt the current flowthrough the armature and field, and also allow thecontroller arm to fall back to starting position.
Never attempt to stop a motor by pulling thecontroller arm back across the contacts while theline switch is closed.
This would cause severe arcing and damage tothe controller contacts, which should always be keptsmooth and in good copdition.
Fig. 5. Wiring diagram of a 4 -point starter for speed regulating.Observe the connections and operating principle carefully.
13. STARTER TERMINALS ANDCONNECTIONS
You will note in Fig. 4 that the terminals on thestarter are marked L, A, and F, to indicate the con -
249
Starters and Controllers
nections for the line, armature, and field. Thismakes it a very simple matter to connect up con-trollers of this type to the line and motor.
The principal point to keep in mind is that oneline wire should connect directly to the motor, be-ing attached to both the shunt field and armature, orseries field leads. The other side of the line shouldconnect to the line terminal on the controller, andthe remaining armature and field leads of the motorshould connect respectively to the armature andfield terminals on the controller. These terminalsare usually marked on the controller or on the blueprint supplied with it by the manufacturers.
54 -inch hot -strip mill. General view of runout table from finishing mill.Each 12 -inch table roller driven by G -E shunt-wound d -c motor,type MD -441, 4 HP, 750 RPM, 250 Volts. Protective cabinets,one per each pair of motors, mounted at table. Installed in 1939.
14. SPEED REGULATING CONTROLLERS
Fig. 5 shows a 4 -point controller of the typewhich can be used both for starting and speed regu-
250
Direct Current
lation of D. C., motors. The -resistance element ofthis controller is made of heavier grids of iron ornickle alloy, and is designed to carry the full arma-ture current of the motor for indefinite periods.
The principal differences between this controllerand the one shown in Fig. 4 are the larger re-sistance element, the use of 4 terminal points in-stead of 3, and the arrangement of the holding mag-net, "M". With this speed -regulating controller,the lever arm and holding magnet are mechanicallyarranged so that the arm can be held in any positionbetween No. 1 and 6 on the resistance contacts.
This allows the arm to be set for any desiredspeed of the motor. In this case, both line wiresare connected to the controller terminals marked"L-1" and "L-2". The reason for connecting thenegative line wire to the controller at "L-2" is
Fig. 6. Photo of a simple 3 -point starter enclosed in a metalsafety box.
merely to complete the circuit of the holding coil"M", directly across the line.
The small resistance "R" is placed in series withthe magnet coil to keep it from overheating.
The armature path of current in Fig. 5 is shownby the large solid arrows, the shunt field current bythe dotted arrows, and the current through theholding coil "M", by the small solid arrows. Traceeach of these circuits out very carefully to be sureyou thoroughly understand the operation of thiscontroller.
Fig. 6 shows two views of a simple motor starterof the 3 -point type. The view on the left shows
251
Starters and Controllers
the starter completely enclosed in the safety boxwith just the handle projecting from the front cover.When the cover is closed this handle connects withthe sliding contact arm inside the box. The viewon the right shows this arm as well as the station-ary contacts and holding magnet.
Where small, low priced starters of the type justdescribed are used, fuses are generally used withthem to provide overload protection for the motors.Sometimes these fuses as well as the line switch areenclosed in the same box with the starter, as shownin Fig. 7.
Fig. 7. Speed regulating controller with fuses and line switch enclosedin a controller box.
The switch in this case is also operated by asafety handle on the outside of the box.
Fig. 8 shows three forms of resistance elementssuch as are dommonly used with motor starters.In the lower view, the resistance wire is wound oninsulating forms of heat -resisting material, and thencoated over with a plaster -like substance of thesame nature. Note how a number of these coils canbe mounted on a rack and spaced to allow ventila-tion. We can then connect several such units orcoils in series or parallel, as desired, to obtain theproper resistance with convenient standard units.
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Direct Current
The view on the upper right shows a heavy-dutyresistor made in the form of grids. These grids areclamped together with bolts as shown, and arespaced with washers of porcelain or some other in-sulating and heat resisting material.
Resistance coils are frequently wound on tubularshapes or forms, and mounted in the starter box,as shown at the upper left in Fig. 8. The copperwires or leads shown attached to these coils are
Fig. 8. Several styles of resistance units commonly used with motorstarters and speed controls.
used for connecting them to the stationary segmentsor contacts on the starter plate.
15. CARBON PILE STARTERSIn some classes of work, such as the operation of
textile mill machinery and certain other equipment,it is desirable to have very gradual application ofthe .starting torque of the motor when the machinesare first put in motion.
To accomplish this, it would, of course, be neces-sary to start the motor with extremely high resist-ance in the armature circuit, so that the startingcurrent could be limited to only a very small frac-
253
Starters and Controllers
tion of the load current. For this purpose, somestarters are made with resistance elements consist-ing of small carbon disks stacked in tubes of non-comt ustible material with an insulating lining, asshown in the left-hand view in Fig. 10.
Fig. 9. A simple type of D. C. motor starter and speed control.Note the extra sets of contacts for the field resistance used invarying the speed.
As, long as these carbon disks are left loose in thecolumn or tube, the resistance through them is veryhigh, because of the loose contact between each diskand the next. If pressure is gradually applied to theends of this column by means of a lever and spring,this tightens the contacts between the disk and verygradually reduces the resistance through the pile.
One or more of these tubular piles or resistanceelements can be arranged in a starter as shown inthe right-hand view of Fig. 10; so that pressure canbe smoothly applied to them by means of the levershown in this view. Starters of this type are knownas carbon pile starters, and they afford a means ofstarting motors more gradually and smoothly thanwith practically any other device on the market.16. SMOOTH STARTING OF MOTORS
WITH CARBON STARTERSWhen using a starter of this type, there is prar-
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Direct Current
tically no sudden increase in the starting currentthrough the motor, as there is when the lever armof the "step by step" starter is shifted from one con-tact to the next.
In addition to the pressure -applying device in
Fig. 10. Carbon -pile rheostat for starting D. C. motors very gradually.On the left is shown one of the carbon resistance elements usedwith such starters.
starters of this type, there must also be some formof switch or contactor to short circuit the carbonpiles entirely out of the armature circuit after fullpressure has been applied and the machine is up tospeed. The reason for using this short-circuiting
255
Starters and Controllers
switch is that the resistance of the carbon pile is stilltoo high to leave in the motor circuit, even when thedisks are under maximum pressure ; and they wouldtend to overheat if left in the circuit too long.
Tubes with larger disks are provided, however, foruse with speed -regulating controllers, and these canbe left in the circuit while the motor is running.
Two or more of these carbon pile tubes can beconnected in series or parallel to obtain the propercurrent -carrying capacity of different controllers. Ifthe disks become worn or damaged at any time, theycan easily be replaced by removing tht. tubes fromthe controller and replacing with complete newtubes ; or the end plug can be removed from any tubeand the disks taken out, so that one or more of thosewhich may be damaged or cracked can be replaced.
Carbon pile controllers are also made in automatictypes as well as those for manual operation.
Motor controllers are made in various h. p. rat-ings, and when purchasing or installing them, careshould be used to see that they are of the propersize to carry the current for the motor which they
elements or burning the contacts.
17. CIRCUIT OF A CARBON CONTROLLERFig. 11 shows the circuit of a simple, manual -type,
carbon pile, motor starter. In this diagram the pathof the armature current is shown by the solid ar-rows and can be traced from the positive line wirethrough the armature of the starter to contact 1.From this point, the armature current flows throughthe lower wire to the bottom of the carbon pile, upthrough the carbon disks, out at the top through aflexible lead, on through the armature and seriesfield, and back through the negative line wire.
As soon as the starter arm makes contact with 1,field current can also flow, as shown by the dottedarrows from the positive line, through the starterarm ; and from contact 1 the current flows upthrough the curved brass strip, through the holdingcoil, "M" ; through the shunt field ; and then. backthrough the negative line wire.
As the starter arm is moved slowly upward, it ap-plies more and more pressure to the carbon disks by
256
Direct Current
means of the hook and spring shown in thefigure. When the starter arm reaches contact 2, fullpressure has been applied to the carbon disks ; andthe arm, upon touching contact 2, short-circuits thecarbon pile out of the armature circuit.
The current then flows from the starter arm tocontact 2, out at the armature terminal "A"through the motor, and then to the negative linewire.
18. AUTOMATIC STARTERSAs previously mentioned, a great number of mo-
tor starters and controllers are equipped with solen-oids or electro-magnets which operate the switcheson arms which cut out the starting resistance as themotor comes up to speed. This type of construc-tion eliminates manual operation of the controllerand reduces liability of damage to motors and con-trollers by improper use when controllers are oper-ated manually by careless operators.
If a manual starter is operated too rapidly and allof the resistance is cut out before the motor comesup to speed, or if the starter is operated too slowlythus leaving the armature resistance in the circuittoo long, it is likely to damage both the controllerand the motor.
Automatic controllers which are operated by sole-noids or electro-magnets usually have a time con-trol device, in the form of a dash -pot attached to thesolenoid or starter arm. By the proper adjustmentof the dash -pot, the controller can be set so that itwill start the motor in the same period of time ateach operation.
Other controllers have the time period which theyare left in the circuit regulated by the armature cur-rent of the motor so that the resistance cannot all becut out of the circuit until the starting current hasbeen sufficiently reduced by the increased speed ofthe motor.
19. REMOTE CONTROLAnother great advantage of magnetically operated
controllers is that they can be controlled or operatedfrom a distance by means of push-button switches
257
Starters and Controllers
which close the circuit to the operating solenoids ormagnets.
For example, a motor located in one room or on acertain floor of a building can be controlled fromany other room or floor of the building. Elevatorcontrols are a good example of the use of remote
Fig. 11. Wiring diagram for a carbon -pile motor starter. Trace thiscircuit carefully.
control equipment. Elevator motors are usually lo-cated on the top floor of the building and are con-trolled by a switch in the car of the elevator whichmerely operates the circuits of the magnets or sole-noids on controllers located near the motors.
Remote control devices can be used to improvethe safety of Operation of many types of machinerydriven by electric motors. Push buttons for stop-ping and starting the motor which drives a machinecan be located at several convenient places aroundthe machine so that they are always within reach ofthe operator in case he should become caught in anypart of the running machinery.
Automatic and remote types of controllers are, ofcourse, more expensive to install, but they will usu-ally save considerably more than the difference intheir first cost, by increasing the life of the motorand control equipment, -and by reducing repair billswhich are caused by careless operation of manualstarters.
258
Direct Current
There are many types of automatic starters onthe market and in use, but their general principlesare very much the same; so you should have nodifficulty in understanding or installing any of thecommon types, if .you will make a thorough studyof the principles covered in the following pages.20. OPERATION OF AUTOMATIC
CONTROLERSFig. 12 shows a diagram of an automatic starter
which uses a solenoid coil at "S" to draw up an ironcore or plunger and at the same time raise the con-tact bar "B", which in this case takes the place ofthe lever arm used on the previously described con-trollers.
This controller is arranged for remote control bymeans of the stop and start push buttons shown atthe upper right-hand corner. When the "start" but-ton is closed, it. completes a circuit through thesolenoid coil, as shown by the small dotted arrows.
ARM. RES.A
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Fig. 12. This diagram shows the wiring for an automatic starter ofthe solenoid type. Trace each circuit until you thoroughly under-stand the operation of this controller. ,
Trace the circuit of this controller in Fig. 12 verycarefully while reading the following explanation.
259
Starters and Controllers
Assuming the line switch to be closed when thestart button is pressed, current will flow as shownby the small dotted arrows from the positive linewire, through the solenoid coil and contacts "B",which are closed at this time; leaying the controllerat terminal 1 and passing through the closed cir-cuit stop switch, through the starting switch, andback to terminal 3.
From this point, it passes through a wire insidethe controller to terminal L-2 and back to the nega-tive line wire, thus completing the solenoid circuit.This energizes the solenoid coil and causes it to liftits plunger and raise the upper main contact bar4111
This bar is prevented from rising too rapidly by adash -pot attached to the solenoid plunger. Thedash -pot consists of a cylinder in which are encloseda piston and a quantity of oil. As the piston rises itpresses the oil before it and retards the motion' ofthe plunger, allowing it to move upward only asfast as the oil can escape around the edges of thepiston, or through a by-pass tube which is some-times arranged at the side of the cylinders.
As soon as the solenoid plunger starts to rise, itallows the spring contact "A" to close and completethe "holding" circuit through the solenoid, withoutthe aid of a. start button.
The start button then can be released and currentwill continue to flow through the solenoid, as shownby the small solid arrows, causing it to continue todraw up the plunger.
As the plunger moves up a little further, the cop-per contact bar "B" touches the contact finger orspring 1, which connects to the first step of thearmature resistance. This allows current to flow asshown by the large solid arrows, from the positiveline wire and line terminal "L-1", through the flexi-ble connection to bar "B", contact spring No. 1, thenthrough the full armature resistance to armatureterminal "A", series field, motor armature, back toterminal "L-2", and the negative line wire.
A circuit can also be traced through the shuntfield of the motor, as shown by the large dottedarrows.
260
Direct Current
Motor shaft supportedby its own Marines. imdependent of the gearassembly.
As the solenoid continues to draw the plungerslowly upward, the bar "B" next makes contactwith springs 2, 3, 4 in succession, thus short cir-cuiting and cutting out the armature resistance onestep at a time.
When the bar touches contact spring 4, the cur-rent will flow directly from the bar to terminal "A",and through the motor armature, without passingthrough any of the starter resistance.21. ECONOMY COIL
The small auxiliary switch shown at "B" at thelower right of the solenoid in Fig. 12 is for thepurpose of cutting a protective resistance in serieswith the solenoid coil, after the plunger has beenraised to the top of its stroke. When the plungerreaches this point it will lift the arm of this switch,causing the contacts to open.
alignmentc
because of rabbet fit ofgear assembly with mo-tor frame.
Lorge oil well, providedwith drain plug, afford*lmple lubrication. Oil fillerocated on top of nit.
HelicI planetgears are quiet inoperation.
Oil -tight cover plate hes rabbetfit to provide proper al.g..rn,Felt -Packed -V- seal is providedat Maft to rm., lubricous,
Low -.peed .shaft,,ln ..... withplanetbearings.
Helical -type Pr.v.k ct'n.tinuo tooth contact, This CO,tributeus s to smooth operation.
Output shaft and head of planetcage-a single steel forging.
Planet gears arc rigidly mountedon their shafts, which are eupponedby two widely spaced ball bearings.In the smaller gear sixes, wheretM combination of horsepowerend torque gives low Maring prasemem sleeve bearings ate used.
Internal. helical, semieteel ring
CM...1i .low eri typical el "1000 %risk" antle-redsetlen wits, 1.16.4-1611 wand. 500 to 155
The current required to hold the plunger in posi-tion once it is up is much less than the current re-quired to start it and pull it up. This smaller hold-ing current will flow through the economy resist-ance, .instead of through the contacts at "B", as itdid while starting.
Cutting in this economy resistance not only savescurrent but prevents the solenoid coil from becom-ing overheated when it holds the controller in op-eration for long periods. The economy resistance
261
Starters and Controllers
will usually reduce the current flow through thesolenoid coil to one-half or less than one-half itsvalue during starting.
22. STOPPING THE MOTORTo stop the motor with a controller of this type,
it is only necessary to press the stop button. Thisbreaks the holding circuit through the solenoid coiland allows the plunger to fall. The plunger is per-mitted to fall rapidly by means of a flap valve, whichallows the oil to escape rapidly when the pistonmoves in a downward direction. When the plungerreaches the bottom of its stroke, it trips open theswitch "A" in the holding circuit ; so it will then benecessary to close the starting switch to energizethe solenoid once more. The motor can also bestopped by opening the line switch.
Fig. 13 shows the front view of a solenoid -typestarter very similar to the one just described. Thespring contact -fingers which cut out the armatureresistance are slightly different on this starter thanon the bar and spring type illustrated in the dia-
Fig. 14. Several types of push-button stations used with automaticremote controllers.
gram, but their electrical principle is identically thesame.23. DASH -POTS FOR TIME DELAY ON
CONTROLLERSFig. 15 illustrates the principle of the dash -pot
timing device used with many automatic starters.When the plunger rod "R" is drawn up by the sole -
262
Direct Current
noid, the piston on the lower end of this rod liftsthe oil by the suction of the piston and forces it
Fig. 15. The above sketch illustrates the principle of an oil dash -potused as a time control on motor starters.
through the needle valve "V'', and around into thelower part of the cylinder.
The speed with which the plunger will rise can,therefore, be adjusted by means of the screw of theneedle valve, which will allow the oil to pass moreor less rapidly through this opening.
During the period that the piston is liftingagainst the oil, the disk "D" holds tightly againstthe openings or ports at "P" in the piston. \V henthe line switch is opened or the stop button ispressed, allowing the plunger to fall, the pressure onthe under side of piston forces the disk "D" toopen die ports at "13," and allows the plunger to fallvery rapidly.
This dash -pot time -delay device should be care-fully adjusted, according to the load on the motorand the time required for the motor to acceleratethis load to full speed.
263
Starters and Controllers
EXAMINATION QUESTIONS
1. Why are motor starters necessary on D. C.motors larger than M. H. P.?
2. What is the average amount of time requiredto bring a medium sized motor up to full speed?
3. How many Watts power loss would there bein a field rheostat of 125 ohms, if the field current is2 amperes?
4. Will the motor speed increase or decrease whenthe shunt field resistance is increased? Why?
5. Give three advantages of field resistancespeed control over armature resistance speed con-trol.
6. Why is the holding magnet coil as shown byFig. 4 connected in series with the motor field?
7. Which type of starter would you select for usein cases where a motor must he started very slowly?
8. How are the switch arms operated on an auto-matic starter?
9. What advantage is there in using remote con-trol starters?
10. What two advantages are there in using theeconomy coil resistance?
264
Direct Current
MAGNETIC STARTERSThe term magnetic starter is commonly used to
apply to starters on which the operation dependsalmost entirely on relays, although they may haveeither a solenoid or ari electro-magnet for overloadprotection.
Controllers of this type have a number of separatecontactors, each operated by its own electromag-net. These contactors and their circuits are so ar-ranged that they operate in succession, and thusgradually short out resistance from the motor ar-mature circuit.
Controls of this type are used very extensivelyon large industrial motors, steel mill motors, eleva-tor motors, etc.
On medium-sized motors, the controller mechan-ism and contactors are ,often assembled inside themetal box or cabinet. For very large motors thecontactors and magnets are usually assembled ona panel similar to a switchboard, and the resistancegrids or elements are generally located at the rearof this special rackabove.
Fig. 10 shows a diagram of a magnetic controller.This controller operates as follows :
After closing the line switch, either of the startbuttons at the remote control stations can bepressed to close a circuit through the remote con-trol relay "A," as shown by the small dotted ar-rows.
This relay magnet then attracts its double arma-ture and closes. contacts 1 and 2. Contactor 2 com-pletes a holding circuit through relay "A" in serieswith the stop switches of the. remote control sta-tions. This circuit is shown by the small solidarrows.
The same contactor, No. 2, also completes a cir-cuit through relay "B," as shown by the smallcurved arrows.
The current for this relay passes through thelower portion of the armature resistance and doesn'tclose contactor 4 immediately. Current for coil "B"is limited by the voltage drop in the armatureresistance.
265
Magnetic Starters
Contactor 1, which was operated by relay "A,"closes a circuit through the overload release coil,"0. L.," to the motor terminal "M," as shown bythe large dotted arrows. At this point the currentdivides and passes through both the armature andfield circuits in parallel, and through the controllerback to the negative line wire.
The armature current shown by the solid black
Fig. 1. This photo shows.. the manner in which the magnetic con-tactors of industrial controls of the larger type are often mountedon an open panel.
arrows returns through the terminal "A-1" on thecontroller, through the winding of relay "F" andarmature starting resistance in parallel. This cur-rent divides through the relay winding and arma-ture resistance in proportion to the resistance ofeach path. As the relay winding is of much higherresistance than the armature resistance unit, mostof the current will pass through the armature re-sistance and back to the line. However, enough
266
Direct Current
current flows through the winding of relay "F" tocause it to become energized and close contactor3, which short circuits the field rheostat, "F R,"cutting this resistance out of the shunt field circuitof the motor.
The many motions on this traveling -head milling machineare controlled, from one position, by oil -proof push-buttonstations mounted on the machine within convenient reachof the operator. While the magnetic control is located at theback, in out-of-the-way space, it is nevertheless convenientfor inspection and maintenance.
The armature resistance used with this controllerperforms the same function as with any other type,
267
Magnetic Starters
namely that of causing a voltage drop and reducingthe current flow through the motor armature duringstarting.
When contactor 3 is closed, the shunt field of themotor is connected directly across full line voltage,thus allowing the shunt field to receive full strengthcurrent and produce the good torque necessary forstarting.1. TIME OF STARTING DEPENDS ON
STARTING CURRENTWe recall that relay "B" didn't energize when the
circuit through its coil was first closed because it isin series with about one-third of the armature re-sistance. Therefore, as long as the heavy startingcurrent is flowing through this armature resistanceand causing considerable voltage drop, part of thatvoltage drop being in series with the coil of relay"B," limits its current and prevents it from becom-ing strong enough to close its armature.
As the motor comes up to speed and developscounter -E. M. F., thereby reducing the startingcurrent through the armature resistance, this willalso reduce the voltage drop through that section ofthe resistance which is in series with coil "B." Thisallows the current through coil "B" to increaseslightly and causes it to close contactor 4. Whenthis contactor closes it places a short circuit on thecoil of relay "F," which can be traced from X toX-1, and X-2 to X-3. This shorts the current aroundthe coil of relay "F" and causes it to de -energizeand release contactor 3.
When this contactor opens, it releases the shortcircuit on the field rheostat, "F. R." and places thisresistance back in series with the shunt field of themotor. This allows the motor speed to make itsfinal increase for the starting operation, and alsoallows the field rheostat to be used for regulatingthe speed of the motor.
Contactor 4 is adjustable and can be set to pullin on any desired voltage within the range of thiscontroller. By adjusting the screws to allow therelay armature to normally rest farther away fromthe core, it will require a higher voltage to operatethis contactor.
268
Direct Current
This means that the motor will have to reach alittle higher speed, develop more counter -E. M. F.,and further reduce the starting current flowing
Fig. 2. Photo of the control panels for a group of elevator motors.Note the contactors on the face of the panel and the resistancegrids located above.
through the armature resistance, and thereby re-duce the voltage drop, allowing a higher voltage tobe aDDlied to the coil of relay "B" before it willoperate.
When this relay does operate, it short-circuits thearmature resistance completely out of the motorcircuit by providing a path of copper around theresistance from X-1 to X-2. So we find that thetime delay on this relay and controller dependsupon the reduction of the starting current throughthe motor armature and the armature resistanceof the controller.
Therefore, if the motor is more heavily loaded atone time of starting than at another and requires
269
Fig.
3. T
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elev
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on
thes
e pa
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are
ope
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rem
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rom
the
elev
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car
.
Direct Current
longer to come up to- full speed and develop theproper counter -voltage, this controller will auto-matically leave the armature resistance in seriesthat much longer. For this reason it is a very prac-tical and dependable type of control.
After the motor is up to full speed and the con-troller starting operation completed, the armaturecurrent will then be flowing through the circuitsas shown by the square dots.
The full -voltage magnetic starter used on this metal saw is controlledfrom a push-button station located at the operator's hand. Thecomplete metal enclosure protects the switch itself from metal chips,and the operator from live parts.
2. OVERLOAD PROTECTIONIn tracing this circuit you will find that the ar-
271
Magnetic Starters
mature current passes continuously through thecoil of the overload relay "0. L." as long as themotor is in operation.
The purpose of this overload relay, which is in-cluded with many controllers of this type, is to pro-tect the motor from overload, both during startingand while the motor is running at full speed.
The coil of this 'relay is in series with the motorarmature and therefore consists of a very few turnsof heavy conductor capable of carrying the full ar-mature current for indefinite periods.
If an overload is placed on the motor, thereby in-creasing its armature current, the increased currentwill increase the strength of the coil of the over-load -relay solenoid.
This will cause it to draw up the plunger "P"slowly against the action of the oil in the dash -pot"T." This dash -pot can be so adjusted that it willrequire more or less time for the plunger to com-plete its upward stroke, and so that an overloadwhich only lasts for an instant does not raise theplunger far enough to stop the motor.
The dash -pot is often called an inverse time limitdevice, because the time required to draw up theplunger is inversely proportional to the current oramount of overload on the motor. A severe over-load increases the strength of the coil to such anextent that the plunger will come up very quickly.
If the overload remains on the motor, the plungerwill be drawn up completely until it strikes theoverload -trip -contact, "0. L. T." This opens thecircuit of the relay coil "A," allowing it to releaseboth its armatures and contactors 1 and 2.
When contactor 1 is opened, it disconnects themotor from the line, and 2 breaks the holding cir-cuit of coil "A," requiring it to be closed again, bymeans of the start buttons, after the overload on themotor is removed.
3. "BLOW-OUT" COIL
The magnetic blow-out coil, "B. 0." is for thepurpose of providing a strong magnetic flux forextinguishing the arc drawn at contactor 1 whenthe motor circuit is broken at this point.
272
Direct Current
The action of this blow-out coil is purely mag-netic. The few turns of which it consist are woundon a small iron core, which has its poles placed oneither side of the contacts where the circuit will bebroken. This provides a powerful magnetic field atthe exact point where an arc would be formed whenthe circuit is broken by this contactor.
Ers,%.14111
Fig. 4. The above sketch illustrates the principle by which the mag-netic blow out coil extinguishes arcs on controller contacts.
As the arc is in itself a conductor of electrical cur-rent and has a magnetic field set up around it, thisfield will be reacted upon by the flux of the blow-outcoil and cause the arc to become distorted orstretched so that it is quickly broken or extin-guished. ,This prevents the arc from lasting longenough to overheat arid burn the contacts to anygreat extent.
Regardless of the extent of the overload, the mag-netic blow-out coil is very effective, because theentire load current of the motor flows through itsturns and its strength is therefore proportional tothe current to be interrupted at any time.
Fig. 4 illustrates the principle and action of thisblow-out coil on an arc drawn between two contactswhich are located between the poles of the magnet.
In the view at the left, the solid lines between thecontacts "A". and "B" represent the arc and the cur-rent flowing through it, while the dotted lines be-tween the magnet poles represent the strong fluxwhich is set up by them.
In the view at the right, the circle and dot repre-sent and end view of the arc, and the direction ofthe flux around the arc is shown by the three ar-
273
Drum Controllers
rows. The dotted lines show the magnetic fluxfrom the poles of the blow-out magnet.
By noting the direction of this flux and thataround the arc, we find that the lines of force willtend to be distorted as shown, and will stretch thearc out of its normal path in the direction shown bythe dotted arrow.
The circuit of a controller such as shown in Fig.10 may at first seem rather complicated, but youwill find after carefully tracing through each 'partof it several times, that its operation is exceedinglysimple. It is by tracing such circuits as these, thatyou will be able to fully understand the operationof controls of this type and become competent intesting their circuits to locate any troubles whichmay develop in them.
This diagram and the explanation given in theaccompanying paragraphs are, therefore, well worththorough and careful study.
The controller shown in Fig: 10 uses a field rheo-stat for controlling the speed of the motor. Thisrheostat can he adjusted, or set at various points,
hand.4. DRUM CONTROLLERS
Drum controllers are very extensively used in theoperation of D.C. motors where it is required to beable to start, stop, reverse, and vary the speed of themotors. The name Drum controller comes from theshape of this device, and the manner in which thecontacts or segments are mounted on a shaft ordrum. This cylindrical arrangement of the contactsis made in order that they may be rotated part of aturn in either direction and brought into connectionwith one or more sets of stationary contacts.
Drum controllers are usually manually operatedand can be provided with almost any number anddesired arrangement of contacts. Drum controls areextensively used for controlling the motors used onstreet cars and electric trains, cranes, hoists andmachine -tool equipment, where it is necessary to beable to reverse and vary the speed of the motors.5. OPERATION OF SIMPLE DRUM
CONTROLFig. 6 shows a very simple form of drum con-
274
Direct Current
trol and illustrates the manner in which the mov-able drum contacts can be used to short out the ar-mature resistance step by step from the motor cir-
Fig. 6. Simple drum controller showing the method in which thecontacts and segments cut out the armature resistance whenstarting the- motor.
cuit. When the drum shown in this figure is rotatedthe first step and brings the movable segments "A"and "B" into connection with the stationary con-tacts, current will start to flow through the entireset of resistance coils, through segments "B," andthe jumper which connects it to `IA," through seg-ments "A" to contact 1; then through the motorarmature and back to the negative side of the line.
When the drum is rotated another step to theleft, segment "C" touches contact 3, and as "C" isconnected to "B" by the jumper, this short circuitsthe resistance between contacts 3 and 2.
Rotating the drum two more steps will short outthe remaining two sections of resistance in the sameorder. Thus a simple drum -control can be used togradually cut out the resistance as the motor comesup to full speed.
By making the resistance elements large enoughto carry the motor current continuously, and thedrum contacts and segments of heavy copper so
275
Drum Controllers
they can stand the arcing and wear caused by open-ing and closing the motor armature circuit, thistype of drum controller can be used for speed -regu-lating duty as well as for starting.
The motor used with this controller in Fig. 6 isa straight series motor similar to the type used onstreet cars and traction equipment.
PRINCIPLES OF DRUM TYPECONTROLLERS.
Drum controllers are extensively used to handleheavy loads such as, Street cars, hoist and craneoperation. The name is derived from the fact thatthe movable contacts are mounted on a cylindricalmetal drum as shown in sketch below. The movablepart of the controller consists of an iron drum withcopper contacts mounted the surface. These con-tacts complete a connection with stationary con-tacts when the drum is rotated. A fibre collar sur-rounding the shaft insulates the movable drumfrom the shaft and controller handle.
DRUM CONSTRUCTION.
INSULATION
I -INSULATION. 2 -SHAFT.
3 AND 4 -CIRCULAR CONTACTING
SHOES.
Al AND A2 - STATIONARY COPPERCONTACTS.
SERIES FIELD.
This type of controller, relatively simple and rug-ged in construction, may be designed to perform avariety of control functions not readily obtainable
276
Direct Current
in motor controllers of other types; moreover, thedurable and simple design lessens maintenancetroubles.
POSITION.OFF
Li3 4 5
RI
R2R3R4Rs
CHART FOR OFF POSITION
Figure A shows a drum type controller designedfor starting a simple series type motor. As the drumsegments, shown by the heavy black lines, moveto the right, a connection is made from the line wireL1, through to R1 and then through the startingresistor and motor to the other line wire L2. As thedrum is revolved, positions number 2, 3, 4, and 5are passed through, thereby cutting out the seriesresistance and connecting the motor directly acrossthe line.
4 3 t I 5 4 3 2
9I I
0
- 4 3
B' 4 C'DRUM FIXEDSEGMENTS CONTACTS.
Analysis of the controller action may be made onone of two methods; either the contacts on themovable drum may be visualized as moving fromleft to right as the drum is moved forward; or thestationary contacts-shown by the small round cir-cles - may be regarded as moving right to left.Either method of analysis may be used. Diagram A,
277
Drum Controllers
B, and C show the one method, while sketches Al,B', and Cl show the other.
This sketch shows how connection is made be-tween the movable segments on the drum and thestationary contacts on the controller as the drum ismoved through the different positions.
The CONTROL SEQUENCE CHART shownin diagrams A, B, and C depicts a simple graphicalscheme for indicating the order or sequence in whichcontact is made between the various segments onthe drum and the stationary contacts. Compare thechart with its corresponding controller position andnote how the different connections are indicated.
The connection chart shows what happens in themotor circuit as the controller is moved throughpositions 1, 2, 3, 4, and 5.
POSITIONOFF I 2 3 4 5
LiRIReR3RRs
CHART FOR FIRST POSITION.
This shows the controller in the first position.Note how these connections are shown on the se-quence chart. In No. 1 position the drum segmentsare making contact with stationary contacts L1 andR1. Therefore a dot is placed opposite each of thesecontacts to indicate this. An understanding of se-quence charts is very helpful in analyzing the con-troller action.
This sketch shows the controller in the No. 2position. The movable drum segments are nowtouching stationary contacts L, and R1 and R2. Thisis indicated by three dots under position 2 in thesequence chart Note that the resistor connected be-tween stationary contacts R1 and R2 is now short-circuited.
278
Direct Current
The connection chart indicates the change inelectrical connections as the controller is movedthrough the different positions. Note how the re -
POSITION.OFF I 2 3 4 5
LiRI
R2R3_R4R5
CHART FOR SECOND POSITION.
RESISTORS. MOTOR.
IL. n_i ID 0 0 p -o-16-6-6-N-0i 2o 0-1) 0 \ -0
v130 600 \-0a_
4 00P-0
5 ° ° (i- \ '°--1.-1CONNECTION CHART.
sistors are short-circuited or bridged as the con-troller is carried through the various positions.
The drum type controller designed for reversingduty is provided with the extra drum segments andstationary contacts necessary to reverse the arma-ture connections. As the controller handle is movedfrom the OFF position, current is passed thru thearmature in one direction ; moving from OFF in theopposite direction reverses the armature current andthe motor rotation.
Comparison of diagrams A, B, and C will showhow the simple controller shown in A is convertedto the reversible type shown in C.
279
Drum Controllers
Figure A shows the simple controller. In B extradrum segments and stationary contacts have beenadded to permit reversal of the armature connec-tions. Fig. C shows that an extra set of drum seg-ments similar to those shown in B have been in-
APOSITIONS.5 3
MOROLLDRUM SE5-MENTS
STAY!
CONTACTSErc.
stalled on the opposite side of the drum therebyproducing a reversing type drum controller.
CONTROL SEQUENCE TABLE.CON-
TACTS.FORWARD. -P-- OFF -- - REVERSE.
5 4 3 2 I I 2 3 4 ELiRI
Re
R3
R4RsAiA2Si
The Control Sequence Table shows the order inwhich contact is established between the variousdrum segments and stationary contacts as the con -
280
Direct Current
troller is moved through the different positions,while the chart below indicates the electrical con-nections made on each position.
CONNECTION CHART.
RESISTORS. MOTOR.
iI=
a.
a
0 `--0o-i j\ArjArj\4r-1 /-0 11-i 61/0
10--W\A~Ar------ 00 0'--0
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6. REVERSING ROTATION OF MOTORSWe have learned that, in order to reverse a D.C.
motor, it is necessary to reverse either the field orthe armature current, but not both. Some controllersare connected to reverse the field of a motor, whileothers reverse the armature. On ordinary shuntmotors the field is usually reversed, but with com-pound motors it is necessary to reverse both theshunt and series field if this method of reversingthe motor is used.
So, for motors of this type, it is common practiceto reverse the armature and leave both the shuntand series fields remain the same polarity. To re-verse the armature leads will require only half asmany extra contacts on the controller as would berequired to reverse both the series and shunt fieldleads.
281
Drum Controllers
When the direction of rotation of a compoundmotor is changed by reversing the field, both theseries and shunt field leads should always be re-versed ; because if only one of these fields were re-versed the motor would be changed from cumula-tive to differential compound.
Fig. 7 shows the manner in which. a simpledouble -pole, double -throw, knife switch can be usedto reverse a shunt motor by reversing the connec-tion betwen the field and the line.
Fig. 7. The above sketch shows the manner in which a motor canbe reversed by reversing its field with a double -pole, double -throwknife switch.
When switch blades are closed to the left.; currentwill flow through the shunt field in the directionshown by the arrows. If the switch is thrown tothe right, the current will flow through the field inthe opposite direction, as can readily be seen bytracing the circuit through the crossed wires be-tween the stationary clips. This same switchingmethod can, of course, be used to reverse the con-nections of the armature to the line, if desired. Thisreversing switch effect can be built into a drum con-troller by the proper arrangement of its contacts.
7. REVERSING DRUM SWITCHESFig. 8 shows a simple reversing drum -control
used for reversing the direction of current throughthe armature only. This diagram doesn't show thestarting resistance or contacts, but merely illus-trates the principle or method by which several ofthe contacts on a drum controller can be used fora reversing switch.
The drum control shown in Fig. 8 has one setof stationary contacts-A. B, C, and D, and two sets
282
Direct Current
of moving segments or contacts, Nos. 1 to 8. Thesetwo sets of moving contacts are mounted on thesame drum and both revolve at the same"time, butin diagrams of this sort these parts are shown in aflat view in order to more easily trace the circuit.
Fig. 8. Drum control with the contacts arranged to reverse thedirection of current through the armature and thereby reversethe motor.
If the drum contacts are moved to the left, themovable contacts 1, 2, 3, and 4, will be brought intoconnection with the stationary contacts A, B, C,and D. The current flow through the armature canthen be traced by the solid arrows, from the positiveline wire to stationary contact "A," movable seg-ment 1, through the jumper to movable segment2, stationary contact "B," through the armature ina right-hand direction, then to stationary contact"D," movable segment 4, through the jumper tomovable segment 3, stationary contact "C" ; andback to the negative line wire.
If the drum contacts are moved to the right, themovable segments 5, 6, 7, and 8 will be brought,into connection with the stationary contacts, andthe armature current will then flow as shown bythe dotted arrows. The field of the motor is leftthe same polarity and only the armature circuit isreversed. If the field and armature or a motor wereboth reversed at the same time, the direction ofrotation would still remain the same.8. REVERSING DRUM CONTROLLERS
Fig. 9 shows the circuit of a drum controller
283
Drum Controllers
which is used for starting a D.C. motor, as wellas for reversing duty. This controller has two setsof stationary contacts and two sets of movable seg-ments. The diagram also shows the armature re-sistance used for starting the motor and while itis being brought up to speed. The contacts andparts of this drum are also laid out in a flat viewin the diagram.
Fig. 9. Wiring diagram of a drum controller for starting and re-versing shunt motors.
The two sets of movable segments are arrangedon opposite sides of the drum, as' are the stationarycontacts. This is illustrated by the small sketchin the lower left-hand corner, which shows froma top view the position of the contacts at the timesegments 1 to 5 are approaching the stationary con-tacts, "P" to "U."
Now, suppose we move the drum of this con-troller so it will shift both sets of movable segmentsto the left in the flat diagram. The first step of thismovement will bring movable segments 1 and 2into connection with. stationary -contacts "Q" and"R," and will also bring segments "V" and "W"into connection with stationary contacts "A" and
284
Direct Current
"L." A circuit can then be traced, as shown bythe solid arrows, from the positive line wire tostationary contact "U," through all of the armatureresistance to stationary contact "R," movable seg-ment .2, through the jumper to movable segment 1,stationary contact Q to stationary contact A-1 ; thenthrough the motor armature, back to stationarycontact "A," movable segments "V" and "W," sta-tionary contact "L," and back to the negative sideof the line.
As we advance the controller still farther in thissame direction, the successive steps will bring mov-able segments 3, 4 and 5 into connection with sta-tionary contacts "S," "T," and "U," thus gradually
285
Drum Controllers
shorting out the armature resistance step by step.When the controller has been moved as far as it
will go in this direction and all armature resistancehas been cut out, the circuit is from the positive linewire to stationary contact "U," movable segment 5,through the jumpers and segments to movable seg-ment 1, stationary contact Q and on through thearmature and back to the negative side of the line.
You will note that the movable segments 1 and 2,and "V" and "W" are all of sufficient length to re-main in connection with the stationary contacts asthey slide around and allow the step by step move-ment which brings the larger segments into connec-tion with their stationary contacts
To reverse the motor, we will now move the con-troller in the opposite direction, which will bringthe movable segments 1 to 5 clear around on theopposite side to the position shown by the dottedsegments, 1 to 5; and the movable segments "V" to"Z" will be brought into connection with stationarycontacts "P" to "U."
Before attempting to trace the circuit, get well inmind the position of these movable segments in thisnew location. Another reference to the circularsketch in the lower corner of the figure will helpyou to see the manner in which the movable seg-ments are brought up on the opposite side of thestationary contacts as the drum is revolved in theopposite direction.
We now find that, on the first step of the drummovements, the movable segments "V" and "W"will be brought into connection with the stationarycontacts "P" and "R," and the movable segments1 and 2 (dotted) will be brought into connectionwith stationary contacts "A-1" and "L."
We can now trace a circuit through the armature,as shown by the dotted arrows, from the positiveline to stationary contact "U," through the full ar-mature resistance to stationary contact "R," themovable segments "W" and "V," stationary con-tacts "P" and "A"; then through the armature inthe opposite direction to what it formerly flowed,and back to stationary contact A-1; then throughmovable segments 1 and 2 to negative line terminal
286
Direct Current
As the controller is advanced step by step, in thisdirection, the movable segments "X," "Y," "Z" willcut out the armature resistance as the machinecomes up to speed. In this position the movablesegments 3, 4, and 5 will be idle.
Fig. 11. The above photo shows the mechanical construction andarrangement of parts of a modern drum control. Note the flashbarriers on the inner cover which is shown opened to the left.
The shunt field connections of this motor are leftthe same, so its polarity will remain the same atall times. Trace this diagram carefully until you areable to trace the circuit very readily in either direc-tion or position of the control. A good knowledgeof the principles and circuits of these controllerswill be of great help to you in the selection of theproper controller for various applications in thefield, and also in locating troubles which may occurin controllers of this type or the resistance attachedto them.
287
Drum Controllers
9. CONSTRUCTION OF DRUM CONTROLSFig. 11 shows a photo of a drum controller with
the cover removed so that all of the parts can bequite clearly seen. The movable segments are madeof copper and are attached to the shaft or drum,which is operated by the crank or handle.
The stationary contacts are in the form of fingerswith flat springs to hold them in good contact withthe segments when they are passed under thesefingers. You will note that the copper shoes of therotating segments and the individual fingers or sta-tionary contacts are both removable, so they caneasily be replaced when they are worn or burnedby arcing which occurs during the operation of thecontroller. '
These contacts should always be kept in goodcondition in order to assure the proper operationof the motor to which the controller is attached.At the left of the stationary contact fingers, can beseen a row of blow-out coils all of which are inseries with their respective contacts and circuits.These blow-out coils, as previously mentioned, arefor the purpose of extinguishing the arc drawnwhen the circuits are broken at the contacts.
The inner hinged cover, which is shown swungout to the left, is simply an assembly of boards orbarriers made of fireproof material. When thisgroup of barriers is swung into place, one of themcomes between each stationary contact and thenext. The purpose of these barriers is to preventa flash -over or short-circuit between adjacent con-tacts.
This particular drum controller has a separateset of reversing contacts mounted in the top of thecase and operated by a separate small handle, whichis shown at the right of the main crank.
In addition to the function of starting, reversing,and varying the speed of motors, some controllersare also equipped with extra contacts for short cir-cuiting the armature through a resistance, in orderto provide what is known as dynamic braking.
This form of braking, which is frequently used tostop large motors, operates on the principle of agenerator, using the counter -voltage generated inthe armature to force a current load through thedynamic brake resistance. This method provides a
288
Direct Current
very effective and smooth braking, and will be ex-plained more fully in later paragraphs.
10. DRUM CONTROL FOR REVERSINGAND SPEED REGULATING
Fig. 12 shows a diagram of a drum control whichis arranged for starting, reversing, speed -regulating.and dynamic braking duty. This controller has two
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Fig. 12. This diagram shows the wiring for a drum controller usedfor starting, reversing, and varyinv the speed of a compound motor.
sets of heavy-duty segments and contacts for thearmature circuit, and in the upper section are twosets of smaller contacts which are used in the shuntfield circuit, and are short circuited by two largeangular segments.
The shunt field resistance, or rheostat, is showndivided into two sections in the diagram; but youwill note that the taps made to this resistance runconsecutively from No. 1 to 18, and the resistanceitself can be located all in one group and have theseparate leads brought to the two rows of contacts
289
Drum Controllers
as shown. One of the heavy-duty resistances is usedfor the armature during starting, and the other isused for the dynamic braking.
When this controller is operated in either direc-tion, the first step will close the armature circuitthrough the armature resistance, and energize thefield at the same time, thus starting the motor. Asthe controller is advanced step by step, the arma-ture resistance will first be cut out and then resis-tance will be cut in to the shunt field circuit, caus-ing the motor to speed up as much more as desired.
When the controller is in idle position, as shownin the diagram, the motor armature is short cir-cuited by the small movable segments which arenow resting on the contacts "D" and "D-1." Thesecontacts are the ones used for the operation of thedynamic brake circuit.
11. FORWARD POSITION, STARTING
The long movable segment "1-A" is continuousaround the drum and always makes contact with"L-1." When moved to the leftone step, the movable segment "1-B" connects withthe stationary terminal "A-2" in the center of thediagram, and the segments "C" and "D" connectwith stationary contacts "A-1" and "R-1" at theleft of the diagram.
This completes a circuit through the motor arma-ture and full armature resistance, as shown by thelarger solid arrows. In tracing this circuit, remem-ber that this first step of movement of the con-troller will remove the short segments from con-tacts "D" and "D-1," thus breaking the short cir-cuit on the armature.
The armature circuit which has just been closedcan be traced from the positive line wire to "L-1," tothe left through segment "1-A," through the jump-ers to segment "1-C," then to terminal "A -I," andthrough the armature in a right-hand direction, onto terminal "A-2," segment "1-B," through thejumpers and segments 3 and 2-A of the right-handgroup, through jumpers and segments 2 and 1-Dof the left group, then to contact "R-1," throughall of the armature resistance, to the contact whichis marked "R-3" and "L-2" ; then through the series
290
Direct Current
field winding and back to the negative side of theline.
This first step or movement of the controller alsocauses the approaching tip of the large angular seg-ment "1-E" to connect to contact "F-1" and closea circuit directly through the shunt field of themotor, without any resistance in series. This circuitcan be traced from the positive .side of the line tocontact "L-1," segment "1-A," up through jumperto segment "1-E," contact "F-1," and then throughthe shunt field winding and back to the line.
When the controller is advanced another step,segment 2 of the left group connects with contact"R-2," and cuts out the upper section of the arma-ture resistance. When the controller is moved stillanother step, segment 3 of the right-hand groupconnects with- the contact which is marked "R-3"and "L-2," and cuts out the entire armature resis-tance.
By checking the circuit again with the controllerin this position, you will find that a circuit can betraced from the line through the controller and themotor armature. back to the line, without passingthrough the armature resistance.
The dotted lines which run vertically through thecontroller and are numbered 1, 2, 3 at their topends, show which segments make connection withthe stationary contacts on the first, second, andthird steps of either the forward or reverse rotationof the controller.
For example, the dotted lines No. 1 in both for-ward gro"ns touch only the segments which willconnect with the stationary contacts on the firstjoint of the controller. The dotted lines No. 2 runthrough the segments which will connect with thestationary contacts on the second step of the con-troller, etc. The dotted lines in the columns marked"reverse" show which segments make contact inorder when the controller is moved in the reversedirection.
So far we have moved the controller only threesteps to the left or in the forward direction, and wefind that this has cut out all of the armature resis-tance and brought the motor up to approximatelynormal speed.
291
Drum Controllers12. SPEED CONTROL
During these three steps or movements, the largeangular segment "1-E" has been moving acrosscontacts 1 to 9 of the shunt field resistance. Thesecontacts are all shorted together by the segment"1-E," but this makes no difference, because they arenot in the field circuit after the first step of thecontroller.
When the controller is moved the fourth step tothe left, the lower end of segment "1-E" will havepassed clear across the stationary contacts, and itslower right edge will begin to leave these contactsin the order -1, 2, 3, etc. This begins to cut in re-sistance in series with the shunt field winding ofthe motor, thus increasing the speed of the machineas much as desired.
During the time that segment "1-E" has beenbreaking away from contacts 1 to 9, the segment"1-F' has been moving across contacts 10 to 18;and after the upper right-hand corner, or segment"1-E," has cut in the last step of the resistance from1 to 9, the segment "1-F" starts to cut in the resis-tance in the steps from 10 to 18. This gives a widerange of speed variation by means of the shunt fieldcontroller. The shunt field circuit is traced with thesmall solid arrows through the controller for thefirst position only.13. REVERSE POSITION
To reverse the motor, the controller will be re-turned to neutral or "off" position, and then ad-vanced step by step in a right-hand direction.- Asthe controller advances the first, step in this direc-tion, the right-hand ends of the movable segmentswill make connections with the groups of stationarycontacts opposite to which they were connectedbefore.
This means that segments "1-B" and 3 will havepassed around the drum and will approach contacts"A-1" and "R-1" from the left, as shown by the dot-ted segments "1-B" and 3; and segments "1-C" and"1-D" will approach contacts "A-2" and "R-3" frcmthe left.
With the controller in the first step of this re-versed position, current can be traced through the
292
Direct Current
armature by 'the dotted arrows, and we find it isin the reverse direction to what it formerly flowedthrough the motor armature.
This circuit is traced from the positive line wireto "L-1," segment "1-A," jumpers and segment"1-C" to contact "A-2," and then through the arma-ture to the left, to contact "A-1," segments "1-B"and 3, contact "R-1," through the full armaturestarting resistance to contact "L-2" ; then throughthe series field in the same direction as before andback to the negative line wire.
In tracing this circuit, we find that the directionof current through the armature has been revertedbut that it remains the same through the series fieldwinding. It is necessary to maintain the polarityof the series field the same in either direction ofrotation, in order to keep the motor operating as acumulative compound machine.
If the controller is advanced in the reverse di-rection, the additional steps will cut out the arma-ture resistance and begin to insert resistance inthe shunt field circuit, the same as it did in theformer direction
REVERSIBLE DRUM TYPE CONTROLLER(D.C.)
This controller is designed to provide forward orreverse operation, the rotation of the motor beingdetermined by the movement of the controller han-dle. When the controller is moved to the OFF posi-tion, terminals X and X1 complete the dynamicbrake circuit. Notice that the armature starting re-sistors are in series with the armature when brak-ing to protect the armature against excessive cur-rent.
The basic connections effected by the controllerare shown in the connection chart below. The OFFsection shows the circuit arrangement for dynamicbraking, while the remaining sketches indicate howthe connections for forward and reverse operationare obtained.
It is important to note that when the armaturecircuit is reversed for opposite rotation, that thearmature and the interpole winding are treated as
293
Drum Controllers
REVERSIBLE DRUM TYPE CONTROLLER. (D.C) I
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one unit, the current through both being reversedat the same time. Check the step by step operationof this controller and fill in sequence chart.
294
Direct Current
DYNAMIC BRAKINGAs the magnetic reactions between the armature
poles and the field poles may be used to rotate thearmature of a D.C. motor, it is evident that thissame force might be used to stop the machine, pro-vided a reversal of polarity of either the armaturepoles or the field poles can be brought about whilethe machine is operating. Such braking action iscalled dynamic braking.
Fig. A shows the rotation and polarity ofthe motor when connected as in C. Fig. B indicatesthe operation when connected as shown in D. Themachine is still turning clockwise due to its momen-tum, but the armature poles have reversed and themagnetic action opposes rotation.
Figure A shows polarity and connections on anordinary shunt motor ; note the polarity of the ar-mature poles and the field poles is such as to pro-duce clockwise rotation. Observe also the directionof current flow through armature and field as shownin the connection diagram. Current is flowing downthrough the armature and down through the field.
In Figure B the motor has been disconnectedfrom the line and a resistance has been connectedacross the armature. As the armature, due to its
295
Drum Controllersmomentum, continues to rotate, the voltage gen-erated in it causes the armature current to reverse;this reverses the polarity of the armature poles asshown in B. This action opposes rotation and tendsto bring the motor to a standstill. The armaturevoltage that produces this reversed armature cur-rent is the sanie voltage that is called a countere.m.f. when the machine is operating as a motor.What actually happens is that when the switch isthrown down, the machine is changed from a motorto a generator and the energy of momentum in therotating armature is changed to electrical energythat is dissipated in the external resistor.
Figures E and F show the usual braking arrange-ment on a shunt motor. Full strength field is main-tained at all times. Note the reversal of armaturepolarity shown on the right.
The disadvantage of the arrangement shown inFig. C and D is that the field poles are weakenedas the machine slows down ; consequently the brak-ing action diminishes as the speed falls. To over-come this defect, the connection scheme used at Eand F is employed. In this case, the field remainspermanently connected to the line but the armatureis connected to a resistance as before ; since the fieldis maintained at full strength, the dynamic brakingaction is considerably greater than with the pre-vious arrangement. Note that in both connectingschemes, the direction of current flow through the
296
Direct Current
armature is changed, but the direction of field cur-rent is unaltered.
E DOEFigures G and H show that connecting a resistor
across a series motor will not produce dynamicbraking. Reversal of armature current demagnetizesthe field ; result is no polarity and no braking action.
Figures J and K show the connections for dy-namic braking on a series motor. Note that thearmature connections have been reversed in K to
297
Drum Controllers
produce the generator action essential to dynamicbraking.
In the series motor dynamic braking cannot beobtained by merely connecting a resistor across themotor as shown in G and H. This is because re-versal of the armature current results in a changein direction of the field current also ; as a result, thearmature current demagnetizes the field and thegenerator action responsible for the braking is notobtainable. To overcome this, arrangement is madeto reverse the armature connections at the sametime the braking resistor is connected across themotor as shown in J and K. Note- in K that thefield current is the same direction as shown in J,but that the direction of current flow through thearmature has reversed; consequently the generatoraction is obtained and braking results. In some ap-plications the same effect is produced by connectingthe series field across the supply line (with a suit-able resistor in series to limit the current to a safevalue) and then connecting a resistance across thearmature in the manner shown.
Figures L and M show an arrangement some-times used on series motors to ensure full strengthfield during braking. In L the machine is operatingas a motor; in M the series field has been connectedacross the line with a resistor in series and the ar-mature connected to a resistor. This arrangementprovides better braking action on series machines.
298
Direct Current
When the controller shown in Fig. 12 is broughtback to neutral or off position, we find that thesbort movable segments directly above the longsegment "1-A" will be brought to rest on contacts"D" and "D-1," thus short-circuiting the motor ar-mature through the dynamic braking resistance,contact "D," segment "1-A," contact "D-1," and thecommutating field.
When the current is shut off from the motor ar-mature by bringing the controller to the "off" po-sition, if the motor is a large one or if the loadattached to it has considerable momentum, themotor and machine or car which it is driving, willtend to keep on moving or coasting for some timebefore coming to a complete stop.
If we leave the shunt field excited during thisperiod, the motor armature will continue to gener-ate its counter -voltage as long as it is turning.Then, if we short circuit the armature through thedynamic brake resistance, this counter -voltage willforce a heavy load of current to flow and the coast-ing motor armature will act as a generator.
We know that it requires pbwer to drive a gen-erator armature ; so, when this short or load isplaced on the motor armature, the energy of itsmomentum is quickly absorbed by the generatoraction, thus bringing the armature to a smooth,quick stop.
The field circuit is left complete when the control-ler is in the off position and as long as the lineswitch remains closed. This circuit can be tracedfrom the positive line wire to contact "L-1," seg-ment "1-A," segment "1-E," and the jumper at "X,"segment "F D," contact 9 and through one-halfof the shunt field resistance, then through the shuntfield back to the negative line wire.
This half of the shunt field rheostat is left in .series with the field for dynamic braking so thatthe motor armature will not generate too high acounter -voltage.
The effect of dynamic braking and the period oftime in which the motor armature can be stopped
299
Drum Controllers
by this method will depend upon the strength Offield excitation which is left on the motor when thecontroller is placed in the off position, and uponthe amount of resistance used for dynamic braking.
Fig. 14 shows a simplified connection diagram fora shunt motor equipped with dynamic braking. Thisdiagram shows only the controlling contacts whichwould be used for dynamic braking alone. The solidarrows show the normal direction of current flowthrough the armature when it is operating from theline voltage, and with contacts "L-1" and "L-2"closed. During this time the contacts "D-1" and"D-2" are, of course, open.
When this motor is stopped, line contacts "L-1"and "L-2" are opened and contacts "D-1" and "D-2"are closed. The counter -voltage in the armaturethen sends current through it in the reverse direc-tion, as shown by the dotted arrows.
Fig. 14. Diagram showing connections and contacts used for switchinga dynamic brake resistance across the armature of a D.C. motorwhen the line is disconnected.
The shunt field is connected across the line inseries with its resistance and, as long as it remainsexcited, the current in the reverse direction willtend to reverse the rotation of the armature. Asthe motor armature slows down, the counter-voltage generated becomes less and less, and the
300
Direct Current
effect of dynamic braking is reduced.When the motor armature reaches a complete
stop, the voltage on its conductors, of course, ceasesto be generated. This results in a sort of cushioningeffect and provides one of the smoothest forms ofbraking which can be used on D.C. motors.
15. REGENERATIVE BRAKING
if the armature of a D.C. motor connected to thepower line be driven above its normal speed bythe mechanical load attached to it, the c.e.m.f. ofthe motor armature will increase until it is greaterthan the applied voltage; when this occurs, themotor becomes a generator and feeds ,power backinto the line. As soon_ as the armature starts todeliver power to the line, an opposition to rotationis set up, and this retarding action may be used toprevent the mechanical load from driving the motorat excessive speeds. This action is called regenera-tive braking.
This type of braking is frequently used on electrictrains, street cars, and on electric cranes and hoists.When an electric car or train, for example, startsdown a grade and attempts to rotate its armaturerapidly, the motor armature will generate a highercounter -voltage than the applied line voltage.
This will actually force current back into the line,as though this machine were operating in parallelwith the power -plant generators.
Regenerative braking effects great savings inpower in this manner and in some cases may supplyfrom 10 to 35 per cent of the energy required byall trains on the system.
Dynamic braking on electric railway applicationsalso saves an enormous amount of wear on brakeshoes and air -brake equipment, and a great amountof wear and tear in cases where it is used forcranes, hoists, etc.
Neither dynamic braking nor regenerative brak-ing is effective when the machine is at a stop orpractically stopped. Therefore, it is necessary tohave either mechanical or magnetic brakes to holdthe motor armature stationary if there is some loadwhich tends to revolve it, such as the load on a
301
Drum Controllers
crane or elevator motor, or the tendency of a trainto run down a grade.
HOIST CONTROLDue to its ability to develop a high starting
torque, the series motor is widely used for hoistsand cranes. The relatively high torque per amperewhiCh this motor can develop makes possible thelifting of heavy loads with a relatively small motor;moreover, the speed characteristic of the seriestypes is such as to produce rapid acceleration onlight load, and this is desirable in crane and hoistapplications.
As the control requirements on a crane or hoistare somewhat different from those associated withordinary power drives, the controlling equipmentis a little different from that ordinarily encountered.Fig. A shows the connections made by a simplehoisting controller set in the first hoisting position.
B1
MAGNET
COIL.
B2
COMPRESSION
SPRINGS.This shows the construction of a magnetically operated mechanical brake.
When the magnet coil is energized, the magnet cores move togetherand release the brake. With the coil deenergized the compressionsprings set the brake.
Contacts W and Q and P are closed; all the othersare open. Under these conditions current will flowfrom the positive line through closed contact W,through the series resistors R1 and R2 and R3,through the brake coil B1 -B2, through the seriesfield S1- S2, and through the armature A1- A2, andthrough contact Q back to the negative line. Note
302
Direct Current
that resistor R4 is connected in parallel with thearmature; this is done to produce a very low orcreeping speed on the first controller position.
Li + L14 --
HOISTING, 15 POSITION.
OPEN 4CONTACT.
HOISTING, POSITION.
CLOSED+ CONTACT.
In the second hoisting position (Fig. B) thecontactor P is opened and the armature shuntingresistor R4 is taken out of the circuit. The next threesteps on the controller short circuit R1 and R2 andR3 by closing contactors T and U and V in thatorder; the motor is then directly across the line,raising the load at maximum speed.
In the first lowering position, the contacts areclosed as indicated in I. ig. C. Note that current cannow flow from the upper positive lines in two par-allel paths ; one of these is through contactors T andU and V, the brake coil, the series field, contactorP and resistor R4 to the negative line; the other isthrough contactor X, resistor R5, the armature A2 -A1, and the resistor R4 to the negative line. Since
303
Drum Controllers
with this connection the armature is in parallel withthe field, the machine is now acting as a shuntmotor.
LOWCWING, ILL POSITION. OFF POSITION.
Connecting the motor as shown in C has twoprincipal advantages. In the first place, if the loadon the crane is zero, the weight of the crane hookmay be insufficient to overcome the friction andinertia of the hoisting equipment; consequently themotor has to be connected in a manner that willdrive the hook down at a reasonable speed and theconnections shown will accomplish this. Further-more, if the load is heavy and tends to overhaul themotor-that is, rotate the motor armature abovenormal speed, the machine will act as a generatorand deliver electrical power to the line. In this way,
-regenerative braking is obtained to prevent the loadfrom running away. Speed in the downward direc-tion can be increased by moving the controller andopening contactors T and U and V in order therebyinserting resistance in the field circuit.
304
Direct Current
Figure D shows the connections made when thecontroller handle is moved to the OFF or STOPposition. Contactor Y is closed and resistor R5 con-nected across the armature; this connection pro-duces dynamic braking if the machine is in motion.Note that the brake coil B1 - B2 is not energizedwhich means that the magnetically -controlled me-chanical brake is set. If the mechanical brake shouldfail, the load cannot run away because the connec-tions are such as to then cause the motor to act asa generator with resistor R5 as a load. The opposi-tion to rotation provided by this generator actionacts as a brake and thus prevents excessive down-ward speeds in the case of failure of the mechanicalbrake.
Although there are many types of hoist control,the general principles involved will be found to beas above indicated. Some hoist motors may be pro-vided with a shunt as well as a series field, for ex-ample, and some controllers may be designed fora wider speed range; nevertheless, the hoist motoris basically a series type, and the control functionsare usually designed to include operation as a seriesmotor when hoisting, operation as a shunt motorwhen lowering, dynamic and regenerative braking,and the use of a magnetically operated mechanicalbrake that sets automatically in case of powerfailure.
305
Drum Controllers
EXAMINATION QUESTIONS
1. Briefly describe a magnetic starter.
3. Briefly explain the operating principle of the"Blow-out coil."
3. Are "Drum Controllers" usually manuallyoperated, or automatically operated?
4. Name three kinds of service for which Drumcontrolled motors are extensively used.
5. What connection changes are necessary inorder to reverse the direction of rotation on a D.C.shunt motor?
6. Copy Fig. 7 and trace the circuits throughwhich current will flow when the switch is closedin the right hand position.
7. How is "Dynamic Braking" accomplished ona D.C. motor?
8. Would you consider "Dynamic Braking" de-sirable or undesirable when smooth reasonablyquick stopping is necessary?
9. Name two advantages of "Regenerative Brak-ing."
10. Is Dynamic or. Regenerative Braking effec-tive when the motor is standing still? Why?
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Direct Current
CARBON BRUSHES
The brushes play a very important part in theoperation of any D. C. motor or generator, and arewell worth a little special attention and study inthis section.
Fig. 144. Above are shown several carbon brushes of common types,such as used on D. C. motors and generators.
The purpose of the brushes, as we already know,is to provide a sliding contact with the commutatorand to convey the current from a generator arma-ture to the line, or from the line to a motor arma-ture, as the case may be. We should also keepin mind that the type of brush used can have agreat effect on the wear on commutators and inproducing good or bad commutation.
307
Carbon Brushes
It should not be assumed, just because any brushwill earry current, that any piece of carbon or anytype of brush will do for the replacement of wornbrushes on a D. C. generator or motor. This istoo often done by untrained maintenance men, andit frequently results in poor commutation and some-times serious damage to commutators and ma-chines.
Many different grades of brushes are made foruse on machines of various voltages and commu-tator speeds and with different current loads.
In order to avoid sparking, heating, and possibledamage to commutators, it is very important whenreplacing worn brushes to select the same type ofbrushes or brush materials as those which are re-moved.
In special cases it is necessary to use only thebrushes made by the manufacturers of certain mo-tors or generators for those particular types ofmachines. Or, in difficult cases of brush or com-mutator trouble, it may be necessary to have aspecialist from a brush manufacturing companydetermine exactly the type of brush needed. Butin the great majority of cases you can replacebrushes very satisfactorily by applying the princi-ples and instructions given in the following para-graphs.185. BRUSH REQUIREMENTS
A good brush should be of low enough resistancelengthwise and of great enough cross-sectional areato carry the load current of the machine withoutexcessive heating. The brush should also be ofhigh enough resistance at the face or contact withthe commutator to keep down excessive currentsdue to shorting the armature coils during commu-tation. In addition, the brush should have justenough abrasive property to keep the surface ofthe commutator bright and the mica worn down,but not enough to cut or wear the commutator sur-face unnecessarily fast.
Figs. 144 and 145 show several carbon brushes
308
Direct Current
of different shapes, with the "pigtail" connectionsused for carrying the current to the brush -holderstuds.186. BRUSH MATERIALS
The most commonly used brushes are made ofpowdered carbon and graphite, mixed with tarrypitch for a binder, and molded under high pressureinto the shapes desired. This material can bemolded into brushes of a certain size, or into blocksof a standard size from which the brushes can becut.
Fig. 145. Two carbon brushes of slightly different shapes, such asused with reaction type brush holders.
The molded material is then baked at high temperatures to give it the proper strength and hard-ness and to bake out the pitch and volatile matter.
Carbon is a very good brush material because itis of low enough resistance to carry the load cur-rents without too great losses, and yet its resistanceis high enough to limit the short circuit currentsbetween commutator bars to a fairly low value.Carbon also possesses sufficient abrasiveness tokeep commutator mica cut down as the commutatorwears.
Graphite mixed with carbon in the brushes pro-vides a sort of lubricant to reduce friction with the
309
Carbon Brushes
commutator surface. It also provides a brush oflower contact resistance and lower general resist-ance, and one with greater current capacity.
Powdered copper is sometimes added or mixedwith graphite to produce brushes of very high cur-rent capacity and very low resistance. Thesebrushes are used on low -voltage machines such asautomobile starting motors, electro-plating genera-tors, etc. They often contain from 30 to 80 percent of copper, and such brushes will carry from 75to 200 amperes per sq. in.
Lamp -black is added to some brushes to increasetheir resistance for special brushes on high -voltagemachines.
187. COMMON BRUSH MATERIAL. BRUSHRESISTANCE
A very common grade of carbon -graphite brushis made of 60% coke carbon and 40% graphite,and is known as the "utility grade". Brush materialof this grade can be purchased in standard blocks4" wide and 9" long, and in various thicknesses.
Brushes for repairs and replacement can thenbe cut from these blocks. They should always becut so that the thickness of the block forms thethickness of the brush, as the resistance per inchthrough these blocks is higher from side to sidethan it is from end to end or edge to edge. Thisis due to the manner in which the brush materialis molded and the way the molding pressure isapplied, so that it forms a sort of layer effect or"grain" in the carbon particles.
This higher "cross resistance" or lateral resist-ance is a decided advantage if the brushes areproperly cut to utilize it, as it helps to reduce short-circuit currents between commutator bars whenthey are shorted by the end of the brush.
Fig. 146 shows how brush measurements shouldbe taken and illustrates why it is an advantageto have the highest resistance through the thicknessof the brush and in the circuit between the shortedcommutator bars.
310
Direct Current
The resistance of ordinary carbon -graphite brushesusually ranges .001 to .002 ohms per cubic inch,and these brushes can be allowed to carry from30 to 50 amperes per sq. in. of brush contact area.
These brushes can be used on ordinary 110, 220,and 440 -volt D.C. motors ; and on small, medium,and large sized generators which have either flushor undercut mica, and commutator surface speedsof not over 4000 feet per min.188. HARDER BRUSHES FOR SEVERE
SERVICEThese utility grade carbon -graphite brushes can
be obtained in a harder grade, produced by special
T ThicknessW. WidthL- Len5th
Fig. 14i. This sketch illustrates the method of taking measurementfor the length, width, and thickness of carbon brushes.
processing, and suitable for use on machines whichget more severe service and require slightly moreabrasiveness. These harder brushes are used forsteel mill motors, crane motors, elevator motors,mine and mine locomotive motors, etc.
Brushes with a higher percentage of carbon canbe used where necessary to cut down high mica,and on machines up to 500 volts and with commu-tator speeds not over 2500 feet per minute. Thistype of brush is usually not allowed to carry over35 amperes' per square inch, and is generally usedon machines under 10 h. p. in size.189. GRAPHITE USED TO INCREASE
CURRENT CAPACITYBrushes of higher graphite content are used
311
Carbon Brushes
where high mica is not encountered, and for heaviercurrent capacity. Such brushes are generally usedonly on machines which have the mica undercut ;
and are particularly adapted for use on older typesof generators and motors. exhaust fans, vacuumcleaners, washing machines, and drill motors.
Brushes with the higher percentage of graphitedo not wear or cut the commutators much, butthey usually provide a highly -polished surface onboth the commutator and brush face. After aperiod of operation with these brushes the surfaceof the commutator will usually take on a sort ofbrown or chocolate -colored glaze which is verydesirable for long wear and good commutation.190. SPECIAL BRUSHES
Some brushes are made of practically puregraphite and have very low contact resistance andhigh current -carrying capacity. Brushes of thisnature will carry from 60 to 75 amperes per squareinch and they can be used very satisfactorily onmachines of 110 and 220 volts, or on high-speedslip rings with speeds even as high as 10,000 feetper minute.
The greater amount of graphite offers the neces-sary lubrication properties to keep down frictionat this high speed.
Another type of brush consisting of graphite andlamp -black, and known as the electro-graphiticbrush, is made for use with high -voltage machineswhich have very high commutator speeds. Thesebrushes have very high contact resistance, whichpromotes good commutation. They can be used tocarry up to 35 amperes per square inch and oncommutators with surface speeds of 3000 to 5000feet per minute.
These brushes are made in several grades, ac-cording to their hardness ; the harder ones are welladapted for use on high-speed fan motors, vacuumcleaners, drill motors with soft mica. D. C. genera-tors, industrial motors, and the D. C. side of rotaryconverters. They are also used for street railway
312
Direct Current
motors and automobile generators. and those of aspecial grade are used for high-speed turbine -drivengenerators and high-speed converters.
191. BRUSH PRESSURE OR TENSIONIt is very important to keep the springs of brush
holders or brush hammers properly adjusted sothey will apply an even amount of pressure on allbrushes. If the pressure is higher on one brushthan on another, the brush with the higher pressuremakes the best contact to the commutator surfaceand will carry more than its share of the current.This will probably cause that brush to become over-heated.
To remedy this, the spring tension should beincreased on the brushes which are operating cool,until they carry their share of the load.
100:22Ring Terminals
Spade Terminals
YET
marror.e
Fla,9 Terminals
=11le.r.......,....i.
r.
Fig. 147. Above are shown several types of terminals for brushshunts or leads.
Brush pressure should usually be from I% to 3lbs. per square inch of brush contact surface. Thisbrush tension can be tested and adjusted by theuse of a small spring scale attached to the end ofthe brush spring or hammer, directly over the topof the brush. Then adjust the brush holder springuntil it requires the right amount of pull on thescale to lift the spring or hammer from the headof the brush. One can usually tell merely by lifting
313
Carbon Brushes
the brushes by hand, whether or not there are somebrushes with very light tension and others with tooheavy tension or pressure.
Motors used on street cars, trucks, and movingvehicles, or in places where they are subject tosevere vibration, will usually require a higherbrush -pressure to keep the brushes well seated. Onsuch motors the pressure required may even rangeas high as 4 lbs. per square inch.192. BRUSH LEADS OR SHUNTS
All brushes should be provided with flexible cop-per leads, which are often called "pigtails" or brushshunts. These leads should be securely connectedto the brushes and also to the terminal screws orbolts on the brush holders, and their purpose isto provide a low -resistance path to carry the cur-rent from the brush to the holder studs.
fre2224:1Cemrsammirmeasi canuassug:C
IMMOMMINESIIIIMEei ignnireigrareerafrair
IIMMISMOV firofreereagelire!wiliCII
Malt 4ICFig. 14$. Brush shunts can be obtained with threaded plugs on their
ends for securely attaching them to the brushes.
If these brush shunts or leads become loose orbroken, the current will then have to flow fromthe brush through the holder or brush hammersand springs. This will often cause arcing that willdamage the brush and holder and, in many cases,
314
Direct Current
will overheat the springs so that they becomesoftened and weakened and don't apply the propertension on the brushes.
The brushes shown in Figs. 144 and 145 areequipped with leads or brush shunts of this typeand Fig. 147 shows a number of the types of copperterminals or clips that are used to attach theseleads to the brush holders by means of terminalscrews or bolts.
When new brushes are cut from standard blocksof brush material, the pigtails can be attached bydrilling a hole in the brush and either screwing theend of the pigtail into threads in this hole or pack-ing the strands of wire in the hole with a specialcontact cement.
Fig. 148 shows a number of brush shunts or leadswith threaded plugs attached to them. These leadscan be purchased in different sizes already equippedwith threaded end plugs.
Fig. 148 illustrates the method of preparing abrush and inserting the threaded ends of the brushshunts. The top view shows a bar of brush stockfrom which the brushes may be cut, and in thecenter is shown the manner of drilling a hole inthe corner of the brush.
Carbon graphite brushes are soft enough to bedrilled easily with an ordinary metal drill, and theyare then tapped with a hand tap, as shown in theleft view in Fig. 148. The threaded plug on theend of the copper lead can then be screwed intothis hole in the brush by means of pliers, as shownin the lower right-hand view of the same figure.
Brush leads of this type save considerable timein preparing new brushes, and insure a good lowresistance connection to the brush. These leadscan be unscrewed and removed from worn brushes,and used over a number of times.193. CEMENT FOR. ATTACHING LEADS
TO BRUSHESWhen brush leads with threaded tips are not
available, a special compound or cement can be
315
Carbon Brushes
made by mixing powdered bronze and mercury.the bronze powder should first be soaked in muri-atic or hydrochloric acid, to thoroughly clean it.
The acid should then be washed from the powderwith lukewarm water. The bronze powder can thenbe mixed with mercury to form a thick paste. Thispaste is tamped solidly around the copper strandsof the ,brush lead in the hole in the carbon brush,and will very soon harden and make a secure con-nection of low resistance.
Fig. 148-A. These views illustrate the method of preparing a carbonbrush to attach the threaded end of the brush lead or pigtail.
Care must be used not to make this cement toothick or it may harden before it can be tampedin place. It is usually advisable to mix only a verysmall quantity of the paste at one time, becauseit may require a little experimenting to get it justthe right consistency.194. DUPLICATING AND ORDERING
BRUSHESWorn or broken brushes should always be
promptly replaced, before they cause severe spark-ing and damage to the commutator. Always re -
316
Direct Current
place brushes with others of the same grade ofmaterial if possible. The new brushes should alsobe carefully cut to the same size, so they will spanjust the same width on the commutator bar andwill not fit too tight or too loose in the holders.
If it is necessary in emergencies to replace oneor more brushes with others of a slightly differentgrade, place all those of one grade in the positivebrush holders, and those of the other grade in thenegative holders. If brushes of different grades areplaced in the same set of holders, the current willdivide unequally through them and cause heatingof certain ones, and it may also cause unequal wearon the commutator.
Fig. 149. The above sketches show several methods of cutting brushfaces to fit the commutator surface. Each is explained in theaccompanying paragraphs.
When ordering new brushes for any certain ma-chine, careful measurements should be taken ofthe brush width, thickness, and length. The brushthickness is measured in the direction of travel ofthe commutator or slip rings ; the width is measuredparallel to the armature shaft or commutator bars;and the length is measured perpendicularly to thecommutator or slip ring surface. These measure-ments are shown in Fig. 146. Any other specialmeasurements should also be given, and in somecases it is well to send the old brush as a sample.
The length of brush leads or shunts should alsobe specified when ordering new brushes. They areusually provided in standard lengths of five inches,
317
Carbon Brushes
but can be turnished shorter or longer where re-quired.
The style of terminal or end -clip should also begiven, along with the diameter of the slot or holeby which they are attached to the bolts on thebrush -holder studs.
It is generally advisable to have on hand a fewof the brushes most commonly required for re -
Fig. 150. Brushes made of copper strips or copper "leaf" constructionare often used for low voltage machines which handle very heavycurrents.
placement on any machines you may be maintain-ing. It is also well to have a catalogue of somereliable brush manufacturer, to simplify orderingby giving the number and exact specifications ofthe brushes required.
195. FITTING NEW BRUSHES TO THECOMMUTATOR
New brushes should always be carefully fittedto the surface or curvature of the commutator.This can be done by setting the brush in the holderwith the spring tension applied, and then drawinga piece of sandpaper under the contact surface orface of the brush, as shown in the center view inFig. 149. Never use emery cloth for fitting brushes,
318
Direct Current
as the electrical conducting of the emery particlestends to short circuit the commutator bars.
The sandpaper should be laid on the commutatorwith the smooth side next to the bars and therough or sanded side against the face of the brush.Then, with the brush held against the paper bythe brush spring, draw the paper back and forthuntil the face of the brush is cut to the same shapeas the commutator surface.
Be sure to hold the ends of the sandpaper downalong the commutator surface so these ends willnot cut the edges of the brush up away from thecommutator bars.
On small machines where it is difficult to usesandpaper in the manner just described, .a brush -seater stone can be used. These stones consist offine sand pressed in block or stick form with acement binder.
The brush seater is held against the surface ofthe commutator in front of the brush, as shown onthe left in Fig. 149, and as the commutator revolvesit wears off sharp particles of sand and carries themunder the brush, thus cutting out the end of thebrush until it fits the commutator.
When fitting a number of brushes, a brush jigsuch as shown on the right in Fig. 149 can beused to save considerable time. This jig can bemade of either metal or wood, and in the form ofa box into which the brush will fit. The open endof the box or jig has its sides cut to the same curveas the commutator surface.
A new brush can then be dropped in this boxand its face cut out to the curve or edge of thebox by means of a file. The bulk of the carboncan be cut out very quickly in this manner, andthe brush can then be set in the holder and givena little final shaping with sandpaper as previouslyexplained.
Graphite brushes should generally be used oniron slip rings on three -wire generators, and metal -graphite brushes on copper rings.
319
Carbon Brushes
On certain very low voltage machines whereheavy currents are handled, "copper leaf" brushesare used. These are made of a number of thinflat strips of hard drawn copper, with the end ofthe group beveled as shown in Fig. 150.
When brushes of too low resistance are used,they will generally cause long, yellow, trailingsparks at the commutator surface behind thebrushes.
Brushes of too high resistance will cause bluesparks and will also cause the brushes to overheat.
If the commutator mica is not being cut downby the brushes and becomes too high, it will causesparking and burned spaces to the rear of the micasegments, on the leading edges of the commutatorbars.
The proper type of brushes and their properfitting, well deserve thorough attention on the partof any electrical maintenance man or power plantoperator; as a great many troubles in motors andgenerators can be prevented or cured by intelligentselection and care of brushes.
320
Direct Current
MAINTENANCE OF D.C. MACHINES
Direct current motors and generators are sosimilar to each other in mechanical construction andelectric operation that many of the same rules forcare and maintenance apply to both.
With the many thousands of these machines inuse in factories, mines, power plants, steel mills,stores, and office buildings, and on railways, theelectrician who can intelligently and efficiently op-erate and maintain them is in great demand.
Most of the repairs and adjustments which haveto be made on D.C. machines are usually on partsthat are easily accessible and which can be easilyhandled with simple tools.
In the majority of cases, the brushes, commutator,and bearings require closer attention and more fre-quent repair than other parts of the machines.These should not, however, require an excessiveamount of attention if the motors or generators areoperating under favorable conditions and are giventhe proper care.
The windings of motors and generators very sel-dom give any trouble, unless the machines are fre-quently overloaded or if the windings are very oldor are subjected to oil and dirt.
1. IMPORTANCE OF CLEANINGOne of the most important rules for the main-
tenance of all electric machines is to keep themclean and well lubricated. If this simple rule isfollowed it will prevent a great many of the com-mon troubles and interruptions to the operation ofthe equipment.
If dust and dirt are allowed to accumulate in thewindings of motors or generators, they clog theventilation spaces and shut off the air which isnecessary for proper cooling of the machine. Alayer of dust is also an excellent insulator of heat,and tends to confine the heat to the windings andprevent its escape to the surrounding air Dust and
321
Maintenance
dirt also absorb and accumulate oil and moisture.For these reasons, the windings of all electric
machines should be kept well cleaned by brushingthem with a duster or cloth and occasionally blow-ing out the dust from the small crevices by meansof a hand -bellows or low pressure compressed air.Never use compressed air of over 40 lbs. pressureper square inch, or air that contains particles of gritor metal or any moisture.
Periodic Cleaning Prevents Dust and Dirtfrom Getting into the Motor Windings
Sometimes it is necessary to wash off an accumu-lation of oily or greasy dirt from the windings ofmachines. This can be done with a cloth and.gasoline. If the windings are well impregnated withinsulating compound, the gasoline will not penetratedeeply into them, but if it is allowed to soak intothe windings to any extent they should be thor-
322
Direct Current
oughly dried before the machine is again connectedto a line or operated.
A mixture of from Y4. to 72 of carbon -tetra -chlor-ide with gasoline reduces the danger of fire or ex-plosion when using it as a cleaning solution.
2. EXCESSIVE OIL VERY DETRIMENTALTO ELECTRIC MACHINES
Oil is very detrimental and damaging to the in-sulation of machine windings and should never beallowed to get on them. Once a winding becomesthoroughly oil -soaked, it will probably have to berewound.
In some cases, if the oil has not penetrated toodeeply, it may be possible to wash it out with gaso-line and then thoroughly dry out the gasoline be-fore the winding is put back in service.
When oiling the bearings of a motor or genera-tor, extreme care should be used not to fill the oil-cups or wells too full and cause oil to run over onto the commutator or windings of the machine.
It is practically impossible to secure good com-mutation if the commutator of a motor or genera-tor is covered with dirt and oil. This will causethe faces of the brushes to become glazed andpacked with dirt and will in many cases cause con-siderable sparking.
Dirt and oil will form a high -resistance film onthe surface of the commutator, which will tend toinsulate the brushes and prevent them from makinggood contact.
Oil is also very damaging to the cement used inthe mica segments of commutators.
If any oil accidentally gets on the surface of acommutator, it should be wiped off immediatelywith a cloth and a small amount of kerosene orgasoline and carbon -tetra -chloride. Gasolineshould not be used around a running machine be-cause of the danger of igniting it by a spark fromthe brushes.
323
Maintenance
Faulty Bearings Cause Most Motor Failures
All motors depend upon a mechanical assem-bly of some sort for the transformation of elec-trical 'energy into mechanical energy or work.The bearings are a very important link in thismechanical assembly. Bearing troubles prob-ably cause more motor shut -downs, delay, andexpense than any other cause. This is not sur-prising when it is remembered that bearings areoaten affected by poor foundations, misalign-ment, vibration, thrust from couplings, dirt,too much or too little lubrication, and wronglubricant.
Proper Lubrication a Basic Requirement
* The first requirement of successful bearingoperation is lubrication. This entails more thanjust an adequate supply of lubricant; the lubri-cant, the bearing design, and its condition mustbe correct.
The maximum safe temperature for bearingsunder normal operating conditions is consideredto be 40 degrees C. (72 degrees F.) above thesurrounding room air. At this temperature, abearing feels comfortably warm to the hand.Assuming proper mechanical condition of thebearing, whether sleeve or ball, and the bearingassembly, and also assuming that proper lubri-cants are used, temperatures above a 40 -degreeC. rise call for immediate investigation.
Lubrication of Sleeve Bearings
In a sleeve bearing, the oil adheres to the shaftand is dragged along by the rotation of the shaftso as to form a wedge-shaped film between theshaft and the bearing. This film of oil carriesthe load and prevents metal -to -metal contact. Solong as this film is established and maintained,
324
Direct Current
there is no metal -to -metal contact in the bearingwhile the shaft is rotating, and, therefore, noperceptible bearing wear.
Oiling the Motor Regularly is Essential toLong Bearing and Shaft Life, but Too MuchOil May Get into the Motor Windings and
Cause Deterioration of the Insulation
Two outstanding considerations govern themaintenance of sleeve bearings. The first is toinsure the existence of the oil film, once rotationhas begun. Use the right oil. The second is tominimize the destructive effects of metal -to -metal contact when the film is lost, either byaccident or during the starting period. Use theright babbitt.
For sleeve bearings, proper maintenance keepsthe oil well filled to the proper level and the oil -
325
Maintenance
rings turning freely. New oil should be addedonly when the motor is at rest to prevent over-filling of the reservoir. Oiling of bearings ismore often overdone than Underdone. Sleevebearings that require frequent refilling, so thatoil leaks on the stator windings, should be re-placed with sealed sleeve type brackets andbearings.
It is well known that oil -rings generally carryfar more oil than is necessary for proper lubri-
After Eleven Years of Severe and ContinuousService, a Bearing Wear of Less than 0.0015Inch-Thanks to Proper Lubrication and
Intelligent Maintenance
cation of the bearing. When running, most ofthe oil is carried on the outside diameter of therings by centrifugal force, This excess oil causessplashing and spray inside of the bearing hous-ing. Air currents passing through the housingpick this spray up and deposit it on the motorwindings. Sealing the bearing against the entry
326
Direct Current
of air currents is, therefore, necessary, and isaccomplished in all modern designs by close tol-erances, felt seals, and air bypasses to offset theblower action of rotating parts.
The purpose of the felt washer in these seals,often misunderstood, is to keep out air and dirt.In itself, it has little value in preventing leakageof oil, once it becomes oil -soaked. Bearing wear,which results in radial movement of the shaft,spoils the effectiveness of the bearing seals. Re-place all felt seals at the same time when sleevebearings are being replaced. These felt sealsshould be ordered from the manufacturer of themotor. If it is necessary to make them in anemergency, use high-grade felt not less than 1/4inch thick before compression. Make the insidediameter of the washer the same as that of theshaft or slightly less. Cut the felt true, withedges at right angles to its facing surfaces. Oilleakage is generally aggravated by high temper-atures. Keep bearings as cool as is practicable,and use an oil that does not foam easily.
Recommended Maintenance Practice
Keep motor off line when not needed. See..ummeary wear of bruelies commutator. andMariana traces lubrication.
Do oot leave field circuit excited ante. motorhas hem especially d.elaoed for this type duty.
Check temperature of shunt fields with thermometer toma that temperature does oot exceed 90 degrees C.When field must be excited. mutton maintenance mento he sore field circuit is opened before working on themotor.
Keep motor clear of metal dust or cuttings thatcat be dram Into IrtodInge sad pole piece.
Magnetic attraction will draw metal parts into air MPend damage windings.
Resseembling of motor. Be pate main proper air gape in motor by checkingbore of poto lo taco, before dismantling. Reassemble. re-placing poles and liners in their original posilloo.
Note wearing parte and parte frequently re-placed to determine anticipated repairs.
Cerra . proper storwroom stock of replacement parte.Make surrey of staridard repair parts to avoid duplica-tion of pert, to be carried.
Factors in Ball -Bearing Lubrication
Ball bearings are being widely used in motors,particularly in the totally enclosed and fan-cooled types. In a sleeve bearing, as just ex-plained, the shaft, when in motion, is separatedfrom the bearing by the oil film. In a ball bear-ing, a series of steel balls act as the separating
327
Maintenance
medium, both when the motor is stationary andwhen it is running.
To keep the steel balls uniformly distributedaround the bearing, a cage or retainer is used,each ball having its own pocket. The balls haverolling contact with the raceway, but slidingcontact with the surface of the retainer. Thismeans that lubrication is necessary.
Most ball bearings used in horizontal motorsare grease -lubricated, although some supplied
Maintenance Checking Cheer for Alternating- and Direct -Current Motors
TnaSI. Cam. Whet to De
Mee Searing' Boot or elvInla Oatftcesalre belt pullPalley too ear anyPulley diameter too .coalMisalignment
Straighten or replace shaft_Decrease hell tension.
Move pulley closer to hearing.(i.e larger pulleysCorrect by realignment of drive.
Sleeve Swags In bearing obstructedray amBent or damaged oil -CageOil too beanOil too lightInsullIcleat oil
Too meek end threat
Badly worn bearing
.Rriteve..17;nkr ,:r.r%lentl.,ve...ith bearing1. all clean
Repair or replace oiltie.. recommended lighter oil.Dee recommended heavier oil.Fill reservoir to proper level in wrarlow ping withmotor at restReduce thrust Induced by driven machine or .applyexternal means to carry thrustReplace hearing`
MY Insufficient greaseDeterioration of grease or lubricantcontaminatedExcess lubricant
Heat from hot motor or externalsourceOverloaded bearingBroken hall or rough races
Maintain proper quantity of grease In heartenifl.eromle., oeldnderzadec wagsh bearing (borough!' in
new rase.Rmerl.b.c;bluatflailiVgolfedgreaee Rewind should not be
Protect bearing by reducing motor temperature
Check elignment side thriwt. and end thrustReplace Waring: firet clean housing thorough,
0, y...,.. Stem of overflow plug not tightCracked or broken 001.001 plugPing cover not tight
Remove. racement threads. replace. and tighten.Replace the plug.Rerwiree cork {wk... or if screw type. may betightened.
Be. 0.w4rnvPies.
M., ys, Ventilation Mocked. end windingsfilled with ens dual or lint
Rotor windings clogged
gaafied and toad.ets coated inside
Clean motor will sIlll 10 to 30 Oct.... cooler Dustmay he cement. sawdust, rook dust grain dust. coaldust. etc Dismantle entire motor and clear. allwinch{. and part.Clean. grind. and undereut commuttor Clean andtreat windings with good insulating varnish.Duet will wash with cleaning solvent,
Skew Wel Subject to lopping
Drenched condition
Submerged in good water.
Wipe motor and dry by circulating heated air.rough motor Install drip or canopy type coversover motor for protection.Motor should he covered to retain beat and therotor position shifted frequent,/Dismantle and clean parts. Bake windings in ovenat 105 degrees C. for twenty.fonr hours or until re-Istence to ground le eunicient pint make surecommutator hushing is drained of water
with vertical motors use oil. Follow the adviceof the motor manufacturer in selecting a suit-able grease.. Soda -base soap greases are usuallypreferred on account of their high melting pointand their stability. They mix readily with water,
328
Direct Current
however, and blend to form an emulsion.Carelessness in allowing containers to remain
open often causes trouble from abrasive dirt.Ball bearings that are not in good condition canusually be detected by undue heating or by un-usual noise. Broken or nicked balls cause rapiddestruction of the bearing. They can be detectedby the "clicks."
If the conventional 40 -degree C. rise abovethe surrounding air is exceeded, look for anoverfilled bearing, since the first result of over -greasing is heating, caused by churning of thegrease. The general rule is that the housingshould not be over half full. Clean the old greasefrom the bearing and from the housing once ayear and replace it with new grease. Averageservice is assumed.
After it is dismantled, the bearing should becarefully wrapped in clean cloth or paper to pro-tect it from outside dirt. Remove all old greasefrom the housing, and clean the housing and thebearing either in Stoddard solvent or in carbontetrachloride. This is not an easy operation be-cause particles of grit are insoluble and are re-moved with difficulty. Remove any final residueof the cleaning medium with a light oil beforefilling with new grease. The container withfresh grease must be carefully protected fromdirt. Keep the cover on tight. Use a clean, non-metallic paddle for applying the fresh grease.
Check Air Gap with Four Readings
The correct air gap between the stator andthe rotor in a motor is dependent, first, on prop-er maintenance of the bearings, and second, onproper alignment of the brackets or pedestalswith the frame. Alternating -current motors op-erate with less air gap than direct -current mo-tors, and are, therefore, more critical. Check theair gap with a feeler gage at the established
329
Maintenance
schedule period. Make these checks at the pulleyend, taking four readings on each motor 90 de-grees apart. For motors below 10 H.P., a mini-mum gap of 0.005 inch should be maintained ;for those above 10 H.P., the minimum gap shouldbe 0.010 inch.
Trouble Cometion Chart foe Diner-Conoot Moon.
Trade Case Mat I. De
Pas ft Sun Circuit not completeBrice.. Oct down on commutator:held up by brueb mintageBrushes Mock In holdersarmatureflock. by frt... b....In motor or maln drivePower off
Check to see I/ twitch is open or leads brokenRushee worn oat; need replacement
Remove and sand. clean op brueb boo.Remove brackets and replace hearings or r -
lion old bearings If inspection mak. possibleCheck line connections to starter with light Checkcontacts In elarter.
MM. filtarta.met 5...eaP..... Db...Ws 4 Meddler
Revere* polarity of generator thatmina. POwerShunt and .rlee field. bucking etchothercorrect
Check generating unit for cane of ehangingWarier.Reconnect either the serene or eerie* firm an a to
the polarity. Then connect armstnre leadsfor desired direction of runtion. U. fields ean hetried eerraratM to determine the diuretic. of Nita.Don Individually. and connected so both Onerotation
M.I. Dose NetCome me toRolla Slimed
Overload
Starting resistance not all out
Voltage low
Short circuit In armature winding.
...log heap load with rm., weakfield
Motor oil neutral
Motor cold
Check bearing to see If In Preto.. condi...el.correct IntoccatIon Check driven load tor eicusive1.0 or friction.Check a.rtine to 0. If mechanically and electric-ally in correct conditionMeasure voltage with meter and check with motornameplateIneout commutator for blackened bars and hurriedadlacent barn Inspect windings for burned coilsor wedges.Check falLfield relay end enumnitine of falLneldnetting of the field rheostatCheck for facto, MItIne of bra. rigging or teat
or for true neutral settingIncrea. load on nuror so ae to Increase Ds tem-perature. or add field rheostat to net speed
fig... Rome Taetem
Voltage above rated
Lead too light
Shunt field coil abortedShunt field coil eeeee NedSeries coil receivedSerie@ geld coil abortedNeutral welting shifted off ....al
Teo meth .....r.. 0eM ."..,Part of .heel 1101010 rheoe.t mat libcut isMotor antilatIon restricted. ceasinghot shunt eta
Collet volroe. nr ret recommended ch..* in airfly Dom manufactorerlures. iota or Install ffsed rule... In armcircuitInstall new coilReconnect .11 leads In lover..Reconnect .11 lade In
repareverse
1.1.11 new or aired coltby cb.king factory setting markRemit to for
or testing tor neutrIhleanire voltage amour Bold and check with -
plate min.
Hot field Is high .osele.nce Check cum. forhot field. In order to restore normal skeet deld<arrest
Nene. Cam.. Une.ble speed load regulation
Rommel field .1 sheet or seriesT..trona a commutating pole orStritimiltatIni ImIe Mr rep W. small
Inspect motor to see If off neuivel Check urn.field to determine shorted torus See If eerie. fieldhu a shunt around the earl. circuit that can beremoved.'Poet with compau and reconnect coltCheck with facnory for recommended elung Incan or sir gut.
*wed Seeddriv.beveled.sod
Lord Dom Mee11.. 6 0....
Ideme doTee Semcow/mooed*
Voltage below rated
ov.do.o
Motor °perinea cold
Neutral setting shifted
aremIero hm MaIrtmf roIM fir rmmMolotov. Dare
lieuure coltage and try to correct to rana on mefor nameplateCheck haarlete of motors and the dr., to me If Infirstolaas condition. Check for eaceesie frictionIn dr.*Motor may run 20 per cent Wow dee to itibt load
.II smaller motor. Increase load. or 11 11
partial covers to Increase heelingCheck for factory setting of inns!. rigging or Wetfor true neutral settingRemove armature to repair ehop and pat In fire,Mess condltIon
330
Direct Current
Trouble Correction Chart For Direct -Carr -Mt Motore-(Continued)
Dodds Came What to Do
maim Over-hes or IldeoHee
Overloaded and drawl, 216 per Peat toSO pet cent more current than ratedVoltage above cat.
Inadequately ventilated
Dram eessive current doe tort.sho toll
003. wince m eiittt ..eonArmature rube pole faces doe to di-ctate? rotor. causing friction and ex-cessive current
Reduce load by reducing Speed of gearing In thedr., or load op the drive.Motor rues drive above lated speed. recluiriag es.ceseive horsepower. Reduce voltage to alomPletoratingLocation of motor should be changed. or restrictingmirroundings remov. Covens osed for protection
restrIctIng of ventilating air and ehonkl bemodified or removed Open motors cannot be to.tally enclosed for motinuom oPerettonRepair armature coils or ins.II new coil
Locate grounds and repair or rewind with new tatof coilscheek brookete or 'Medals to teeter rotor anddetermine condition of bearing wear for Mertonreplacement
Het Aroma.. Core hot In one spot, *diesel..honed ponChIng nod high Iron loss
Punching. uniesulatedPundilms have been tamed or and
Y00000machined In the core.
Machined slots
Sometimes full slot metal wedges have Mea medfor balancing These should be remos.d and othermeans of Delete.. Investigated.
No-load running of motor will .how hot core andbleb so -load arroatore current Replace core adrewind armature. If amen., to add and grooves.Rind into core. Check temperature on core subthermometer. not to exceed 90 degrees C.
HetCam...dater
Brush tenolon too high. mimingoverload
Brushes off neutralBrush grade too abradesShorted bare
Hot core and coils that tranemll heatto commu.tor
Inadequate ventilation
Limll pressure tog to I lbe Pio ea. Ito.brush density anti limit to density recommended bythe brush manufacturer.Rent neutral(let rmoremendation from manufacturer.Inv. .gate nom odor mica nd undercolUng endrepairCheck temperature of commu.tor with therm..
ambleme that total temperstua does not exceed. plus 66 degrees C dee. total not to ex.
mod 106 degrees CCheck as for hot motor.
H ee Held. voltage too high
Shorted biros or grounded tamsRedman., of each con oot the same
'Inadequate ventilationColl not large enough to radiate Itsloss wat.geOverhmtlng deteriorates Insulationand shortem life of the motor
Cheat with meter and thermometer and coormtvoltage to nameplate value.Repair or replace with new coil.Check each individual loll for .mom residence toM within IO per tent, and if one mgr Is too low.replace milCheck so for hot motor.Replace all coils if room le aml.ble In motor.
Meter Namemdthtkalmea
Armature out of ',.lance
MisalimmentLoom omcentrk pulleyBelt or chain whiphilsmating of gear and pinloaUnbalance Iv emplIngBout shaftFoundation InadequateMotor loosely mountedMotor feet uneven
Remove and s.tically balanceor balance In dy-er balancing machine.
Realign.
Tighten pulley on shaft or correct eccentric pulley.Adjust belt tension.Recut, reatign, or replace partsReMlance couplingReplace or straighten .all.Stiffen mounting members.Tighten holding down bolts.Add shim. under foot pads to mount each foottight.
Meter Sparke asgreshm or DemMel Commode
Neutral setting not true neutral
Commutator roughCommu.tor eccentrtcMks high. not undercutcommu.ting pole strength too great.causing over compensation. or streogthtoo weak. caustog under compensationShorted commutating pole tunsShorted ormature coils on commutator bareOpen circuited milePoor soldered comettions to comm.tator barsHigh bar or loose bar In commutatorat bleb speed.Brush grade 11011{ en.Brush pressure too lightCurrent density excessiveBrushes stuck holden.Brushes shunt looseBras.. chatter due to dirty dim oncommuttonVitiralloe
Check sod eel on factory set.* or tee for trueneutral.Orlod and roll edge of each bar.Turn and grind commutator.Undercut micaCheck wIth manufacturer for correct champ, in airgap or new coils for the come:m.0nd coils.
Repair coils or Install new coilsRepair ...tare by putting into dest-clme condi.
Some as mans.Resolder with proper alloy or lin solder.
Inspect commutator nut or bolts and retightentre -turn and grind commutator face.See motion on Maintenance of Bradlee in article.
Resurface commutator face and chmk for changein bombes.Ellmlnate reuse of vibration by checking ?mintingmid balance of rotor.
331
Maintenance
3. KEEP BEARINGS WELL LUBRICATED
The bearings of all motors and generators shouldbe kept well oiled but not flooded with oil. The oilin the bearings should be examined frequently tomake sure that it is clean and free from dirt andgrit, and should be changed whenever necessary.
If the oil in a bearing has become exceptionallydirty or mixed with any abrasive dirt, the oil shouldbe drained and the bearing and oil -cup washed outwith kerosene or gasoline. The bearing and cupshould then be refilled with clean fresh oil and whenthe machine is started it should be revolved slowlyat first, to be sure that all the kerosene or gasolineon the bearing surfaces has been replaced by oil be-fore the machine is running at full speed.
Bearings should not be filled from the top whenregular oil openings or .vehts are provided on theside.
Bearings which are equipped with oil rings shouldbe inspected frequently to make sure that the ringsare turning and supplying oil to the shaft. Checkthe temperature of bearings frequently either bymeans of a thermometer, or by feeling of them withthe hand to make sure that they are not operatingmuch above normal temperature.
A great amount of work and trouble and costlyshut -downs of electrical machinery can be preventedby proper attention to lubrication of bearings.
4. WINDING TEMPERATURESThe temperature of machine windings should be
frequently checked to see that they are not oper-ating too hot, that is at temperatur higher than40° C. above that of the surrounding air.
Convenient thermometers can be obtained forthis use and placed in crevices in the winding oragainst the side of the winding with a small wadof putty pressed around the thermometer bulb andagainst the winding.
All terminals and connections on electric ma-chines should be frequently inspected and kept se -
332
Direct Current
curely tightened. This includes those at the line, atthe controller or starting switches, and at thebrushes and field coils.
5. PROTECT MACHINES FROM WATERMoisture or water is always a menace to the in-
sulation and operation of electrical machinery, andmachines should be thoroughly protected to keepall water away from their windings and com-mutators. If a motor or generator is located wherewater from above may drip upon the commutator,it is very likely to cause flash-overs and damage tothe brushes and commutator.
If the windings of a machine become water -soaked or damp, they must be thoroughly dried,either by baking in an oven or by passing low -voltage direct current through the machine to drythem out.
Where a machine is too large to put in an ovenor where no oven is available, the armature can belocked to prevent its rotation and then, by the useof a rheostat, low -voltage direct current can be ap-plied in just the right amount to dry out the wind-ing.
Water should be carefully excluded from oil wellsand bearings, as it is not a good lubricant and itmay cause serious damage if it mixes with the oil.
Motors that operate pumps may often have to beenclosed in a special box or shielding to preventany drip or spray from coming in contact with them.
6. BRUSH ADJUSTMENT AND MICAUNDERCUTTING
Brushes should be frequently inspected to seethat they are seated properly on the commutatorand have the proper spring tension. If the com-mutator mica becomes high it should be corrected,either by using brushes of a type that will keep themica cut down, or by undercutting the mica witha tool for this purpose.
Commutator mica on small machines can be un-dercut by hand with a piece of hack -saw bladeequipped with a handle, as shown in Fig. 1. The
333
INC
OR
RE
CT
SLO
TT
ING
\
CO
RR
EC
T,
SHA
PE O
F.,
SLO
TS
Fig.
1.T
he a
bove
ske
tch
show
s m
etho
ds o
f un
derc
uttin
g m
ica
segm
ents
on
the
com
mut
ator
.M
ica
mus
t be
kept
dow
n ev
enw
ith, o
r be
low
the
surf
ace
of th
e co
mm
utat
or b
ars,
or
itw
ill c
ause
the
brus
hes
to m
ake
poor
con
tact
and
cau
se s
ever
esp
arki
ng.
Direct Current
views on the right in this figure show the correctand incorrect methods of undercutting mica.
Mica should be cut squarely with smooth, easy
To Assure Proper Maintenance and PreventSparking, This Tool Under -cuts the Mica 1 /16 Inchbelow the Surface of the Faster Wearing Copper
Commutator Bars on a Direct -current Motor
strokes of the hack -saw blade held in a verticalposition. The mica should not be cut away toodeeply, or the grooves will tend to accumulate dustand dirt, and cause short circuits between the com-mutator bars.
On small and medium-sized machines, the under-cutting need not to be deeper than from 1/64 to 1/32of an inch. Care should be taken not to scratch orscar the commutator bars while undercutting mica,and one should also be careful not to leave on theedges or corners of the bars any burrs which might
335
Maintenance
1
cause a short circuit between them.A small, three -cornered file can be used for clean-
ing the ends or corners of the mica segments, asshown in the left view in Fig 1, but a file or three -cornered object should not be used for undercuttingmica, as the top of the mica segments must be cutsquarely, as shown in the right-hand view in thefigure.
Proper Method of Cleaning and Surfacing Brushes isto Place Sandpaper under Brush, Flat against theCommutator Surface. Several Passes of Sandpapershould be Enough, Depending on Brush Condition
For undercutting the mica on large machines, aregular motor -driven mica cutter can be used.These machines consist of a small rotary saw,driven by a motor with a flexible shaft and equippedwith handles for guiding the saw blade in the micaslots.
336
Direct Current
7. RESURFACING AND TRUING OF COM-MUTATORS
If the surface of the commutator becomes roughand pitted it can be cleaned with sandpaper. Smallspots of dirt or very lightly burned spots may be re-moved by holding sandpaper against the commuta-tor while the machine is running.
If the commutator requires much sand -paperingit should be done with a block, with a curved sur-face to fit the commutator, to hold the sandpaper ina manner that will tend to smooth out hollowspots or high spots on the bars, and bring the com-mutator back to a true round shape.
wr
Fig. 2. Commutator stones of the above types are used for dressing orgrinding down the burned and rough spots on commutator surfaces.
Several strips of sandpaper can be folded over thecurved end of a block of this type and fastened inplace with clamps or tacks. As each strip becomesworn it can be removed, exposing the next strip,etc.
Special commutator stones can be obtained fordressing or re -surfacing commutators. These stonesconsist of a block of grinding or abrasive materialequipped with handles for convenient applicationto the commutator surface. Several stones of this
337
Maintenance
' type are shown in Fig. 2. These can be obtainedin different sizes and degrees of hardness for usewith machines having commutators of different di-ameters and surface speeds.
If a commutator has become badly pitted orburned or out of round, it may be necessary to re-move the armature from the machine and turn thecommutator down in a lathe, as shown in Fig. 4.When truing a commutator in a lathe one shouldnever remove any more copper than is absolutelynecessary, because even a very light cut with a lathetool will remove more copper from the bars thanseveral years of ordinary wear would destroy.
The armature should be carefully centered to runtrue in the lathe, and the tool set to remove onlya very thin coating of copper, no thicker than athin piece of paper. If this first cutting doesn't re-move the flat spots, another cut can be made.
Commutators should never be turned down ex-cept as a last resort or when they are badly out ofround.
Special tools for turning large commutators rightin the machine are available.
Motors and generators should always have secureand firm foundations and should be anchored so thatthey don't vibrate while running. If the machineis allowed to vibrate, it may cause serious damageto the bearings and possibly also damage the com-mutator, shaft, or windings.
8. CARE OF CONTROLLERS
All switches, circuit breakers and controllersused in connection with motors and generatorsshould be kept in good condition, because if theyare allowed to become defective, they may causedamage to the machines by frequent interruptions
in the current supply, or by causing voltage dropand lower voltage than the machine is supposed tooperate on, or by failure to protect the motor incase of overload.
All contact shoes or fingers on starting and con-trol equipment should be kept in good condition andsecurely tightened. Bolts, nuts, screws, and/ ter-minals should also be kept tight and clean.
338
Direct Current
Sliding contacts or make -and -break contacts ofcontrollers should be kept properly lubricated toprevent excessive wear. A good grade of vaselineserves very well for this purpose, as it will remainwhere applied on the contacts and will not run orspread over the equipment.
Fig. 3.-The above photo illustrates the method of testing the contactpressure or tension of controller contacts. The scale can be usedin the same manner for adjusting tension on brushes.
Resistance elements of starting and control equip -men should be kept in good condition. In case ofopen circuits in resistance units, it may be necessaryto temporarily bridge this open section of the re-sistance with a shunt or jumper, in order to keepthe machine in operation ; but the defective resist-ance unit should be replaced with a new one asquickly as possible to prevent overloading the ma-chine when starting, due to having insufficient re-sistance in the armature circuit.
Dash pots and time element devices should bekept properly adjusted to allow the proper time forstarting of motors.
9. CARE OF OVERLOAD PROTECTIVEDEVICES
Fuses and overload devices on control equipmentor anywhere in the circuit to electrical machinesshould be kept in good condition, and should be of
339
Maintenance
the proper size and adjustment to protect the ma-chines from current overloads. Fuses should neverbe replaced with others of larger current ratingsthan the machines are supposed to carry.
Overload trip coils on circuit breakers should bekept properly adjusted to trip at any current abovethe normal percentage of overload which the ma-chine is allowed to carry.
If breakers or fuses open frequently in the circuit,it is an indication of some overload or fault on themachines, and the trouble should be located andremedied, instead of setting the circuit breakers forheavier currents or using larger fuses.
10. LIST OF COMMON TROUBLESIn the following lists are given a number of the
more common troubles of D.C. machines and thesymptoms which indicate these troubles:
MOTORSMOTOR FAILS TO START
1. Fuse out, causing an open circuit2. Brushes not making proper contact3. Line switch open4. Bearings "seized" due to lack of oil5. Motor overloaded. This will usually blow
the fuse6. Open field circuit at the terminal block or in
the starting box"No voltage" release magnet burned out
7. Open armature or line connections. either atthe motor or controller
8. Grounded winding, frequently blows the fuse9. Brushes not set on neutral point
10. Armature wedged. Remove the woodenwedges from air gap of new machines
11. Dirty commutator or brush faces12. High mica insulation on commutator pre-
venting brush contact13. Field coils short-circuited or grounded. Will
usually cause excessive armature currentsand blow the fuses
340
Direct Current
14. Reversed field connections. Test for polaritywith a pocket compass
15. Low voltageY6. Pulley, gear, or coupling, may be tight
against the bearing17. Bent shaft, causing armature to stick on pole
faces18. Bad13 worn bearings, allowing armature to
rub19. Burned out armature.
11. MOTOR STARTS TOO QUICKLY1. Starting box resistance too low for the motor2. Starting box resistance short-circuited3. Insufficient time allowed for starting4. Line voltage too high5. Series motor without enough load for the
starting resistance used with it6. Too much resistance in field circuit.
12. MOTOR ROTATION REVERSED1. Reversed field connections2. Brush connections reversed or brushes in
wrong position3. Compound motor connected differential and
starts in reverse direction from the seriesfield. Speed will be high and torque verylow
4. No field. Residual magnetism 'may start themotor in reverse direction on very lightloads only. Motor will not start under heavyload
5. Wrong field connection in starting box.Armature resistance may be in series withthe field.
13. SLOW STARTING OF MOTORS ANDWEAK POWER
1. Low voltage2. Resistance of starting box too high3. Brushes off neutral, and will cause bad
sparking4. Motor overloaded5. Heavy flywheel on driven machines6. Weak field due to resistance in its circuit
341
Maintenance
7. Dirty or loose connections8. Dirty or loose brushes9. Brushes improperly spaced on commutator
10. Armature defects, shorts, grounds or opens11. Wet armature or commutator.
14. MOTOR BUCKING OR JERKING1. Overloaded motor2. Reversed interpole polarity3. Loose field connections which alternately
open and close the field circuit and cause themotor to run jerkily
4. Wet or shorted field coils5. Defects or loose connections in starting box.
15. MOTOR OVERSPEEDS1. Open field circuit, may cause dangerously
high speed2. Shorted or grounded field coils3. Load suddenly reduced on compound motor
using field control4. Brushes off neutral5. Shorted or grounded armature conductors6. Line voltage too high7. Series motor overspeeds on light loads or no
load.
16. SPARKING AT BRUSHES1. Brushes or commutator dirty2. Rough or burned commutator3. High or low bars in commutator4. Commutator out of round5. Commutator segments shorted by carbon or
copper dust in the mica slots, or by solderbridged across the bars.
6. High mica7. Brushes off neutral8. Wrong type of brushes9. Brushes poorly fitted
10. Brushes stuck in holders11. Poor or unequal brush tension
342
Direct Current
12. Weak field, due to short circuits or groundsin the coils
13. Reversed field coils14. Opens or shorts in armature winding. Opens
usually cause long blue sparks and shortsare generally indicated by yellow or reddishsparks. " The location of the defective coilswill usually be indicated by burned bars towhich they are connected
15. Oil grease or water on the commutator16. Unequal air gaps due to worn bearings17. Unbalanced armature winding18. Bent shaft which causes brushes to chatter19. Poor foundation, permitting vibration of the
machine.
17. OVERHEATING OF MACHINES1. Overloading will cause heat on both motors
and generators due to excessive currentpassing through their windings and brushes
2. Excessive brush friction and brush tensiontoo great
3. Brushes too high resistance4. Brushes off neutral5. Damp windings6. Excessive sparking at commutator, which
may cause enough heat to melt the solderand loosen the armature connections
7. Opens or shorts in armature winding8. Hot field coils caused by high voltage or
short circuits in the coilsField shunts loose or disconnected9.
10. Windings shorted by oil -soaked insulation11. Hot field poles may be due to poor design
causing eddy currents in the pole shoes. Un-equal air gaps may cause field poles closestto the armature to heat
12. Hot bearings due to poor lubrication. Maybe caused by poor oil, stuck oil rings, orclogged oil wicks. Also caused by poor shaftalignment or excessive belt tension
13. Armature out of center with field poles, due
343
Maintenance
to worn bearings. Causes excessive currentsin parts of the armature winding and eddy
currents in the field poles. Bearings shouldbe repaired immediately
14. Clogged ventilating ducts15. Loose connections between armature coils
and commutator bars16. Weak field, not allowing sufficient counter-
E.M.F. to be generated to keep the arma-ture current normal
17 Heat transfer through direct shaft connec-tions from air compressors, steam enginesor other machinery.
Normal operating' temperatures of D.C. motorsshould not exceed 40° C. above the surroundingroom temperature when operated at full load, or55° C. at 25% overload for two-hour 'periods. Ifthe machines are operated at temperatues abovethese values for any length of time, the insulationof the windings will become damaged and even-tually destroyed. (Safe operating temperatureabout 140 to 150 degrees F.)18. UNUSUAL NOISES
1. Belt slapping due to a loose, waving belt2. Belt squealing due to belt slipping on the
pulley, caused by loose belt or overloads3. Brush squealing due to excessive spring ten-
sion, hard brushes, or dry commutator sur-face. Application of a good commutatorcompound will usually stop the squealingdue to a dry unlubricated commutator
4. Knocking or clanking may be caused by aloose pulley, excessive end play in the shaft,a loose key on the armature spider, or aloose bearing cap
5. Chattering vibration, caused by poor brushadjustment and loose brushes, hard brushes,or commutator out of round
6. Heavy vibration due to unbalanced arma-tures, bent shaft, or loose foundations.
19. GENERATOR TROUBLES.FAILURE TO BUILD UP VOLTAGE
1. Residual field lost or neutralized2. Reversed field
344
Direct Current
3. Poor brush contact or dirty commutator4. Open field circuit due to loose connections
or broken wires5. Field rheostat open or of too high resistance6. Series field reversed so it opposes the shunt
field7. Shunts disconnected or improperly con-
nected8. Wet or shorted field coils9. Too heavy load on a shunt generator
10. Residual magnetism reversed by flux fromnearby generators.
20. POOR VOLTAGE REGULATION
1. Loose field shunts or connections2. Poor regulation of engine speed3. Belt slipping (if generator is belt driven)4. Brushes off neutral5. Improper resistance of field rheostat, or
loose connections at this rheostat6. Series field shunts not properly adjusted7. Overheated field coils8. Loose or grounded field wires between gen-
erator and switchboard9. Armature out of center
10. Brushes improperly spaced11. Weak field caused by short circuits or
grounds in the field windings12. Shorts, opens, or grounds in the armature
coils13. Excessive and frequent variations in load14. Improper compounding.
21. GENERATORS WILL NOT OPERATEIN PARALLEL
1. Poor speed regulation on prime mover,caused by improper governor adjustment
2. Open equalizer connections3. Incorrect field shunts, open or loose field
connections, or weak fields4. Defective field rheostat5. Wet field coils6. Improper adjustment of series fields for
compounding effects
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Maintenance
7. Extreme difference in size, causing thesmaller machine to be more responsive toload changes than the larger machine
8. Belt slipping, on belt -driven generators9. Variations in steam pressure, on generators
driven by steam engines10. Defective voltmeter, causing operator to
make wrong adjustment.
22. SYSTEMATIC TESTINGThe preceding lists of common troubles and their
symptoms are given to serve as a general guideor reminder of the possible causes of trouble inD.C. machines. They do not cover every possibletrouble or defect, but intelligent application of theprinciples covered throughout these sections onD.C. equipment and careful systematic testing,should enable you to locate any of the troubleslisted or any others.
Keep well in mind the advice previously givento the effect that even the troubles most difficult tolocate can always be found by methodically andsystematically testing circuits and equipment.
Let us remind you once more that any defector trouble in electrical equipment or circuits can befound, and that someone is going to find it. Itwill be to your credit to be able to locate any andall troubles, and the best way to gain experience'arid confidence is to undertake willingly every trou-ble -shooting problem you can find. Go about itcooly and intelligently, use your knowledge of theprinciples of electricity and electrical equipmentand circuits, and in this manner you will save agreat deal of time and many mistakes.
You will also be surprised to find out how verysimple some of the apparently baffling electricaltroubles are, to the trained man who knows how totest and locatethem.
23. TEST EQUIPMENT FOR LOCATINGFAULTS
Some of the more common devices used for trou-ble shooting and testing are as follows : -
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Direct Current
1. Test lamp and leads2. Magneto tester3. Battery and buzzer tester4. Voltmeter (portable type)5. Ammeter (portable type)6. Thermometer7. Speed indicator8. Wheatstone bridge
TROUBLE CAUU BUNDY
liSflOtSoarIdoo 1
Motor is not getting power.
Mot. ..darted with weakor no held.
Motor torque insuffictent todrive load.
Line voltage too low.
Br..er ahead of neutral.Overload.
Weak field_
Line voltage too high.Bras.. back of neutral_
Commutator in bad con.dition.Eccentric a rough co.mutator.Excessive vibration.
Broken or sloth.. act figbrah.holder spring.Brushes too Han.Machine overloaded.short camps in armature.
Excessive clearnce of brushholders.Incorrect of brushes.Incorrect braangle hes He theare.,High miccIncorrect brush -spring pr.mrfe.
Check voltage at the motor terminals: atio fuses.clip., and overload relay.
If adjustable -speed motor, check rheostat for correctsetting. If correct. check condition of rheatat.Check field coils r open winding.Cheek wiring for loose or broken connection.Check it, voltage with nmeplate rating. Uselarger motor o.r one with suitable eke rrrrrr istic tomatch load.
Check and remove any came reeistance in supplyline. connection or control.Pet bra on neutrI.Check to see that load does not tatted allowableload on mato,Check kr resistance in shunt .held COCUito.Check for grounds.Correct high -voltage condition.Set brushes on neutral.
Clan and asset braha.
Grind and true commutator. ala and mica.
Balance armature.Chat bashes to make are they ride freely in the
Replace spring, and adjust pressure to manufac-turer, recommendaionsReplace brushes.Reduce load, Of ;tall Iv motor.Check commutator. and remove any snetidlicparticles between segments Check for short be....an adjacent commutator risers.Test for internal thorn amature and repair.
Adjust holders.
Adjust to the correct angle.Get manufacturer.. recommendations.
Undercut lam..Adjust to correct value.
0010.0.osesIon., Moo..
gam, ol Pa next .. bar sheel wally inane. the presence of lane or high -ante (OfonoOilo... .comma Pe ann . coil and tii
9. Megger.Every maintenance electrician's kit should in-
clude a test lamp and a battery and buzzer test -outfit. These are very inexpensive and can easilybe made up in a few minutes' time. It is a goodidea to use two sockets and bulbs in series, for atest lamp which can be used either on 220 or 110-
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Maintenance
volt circuits. The two lamps will burn at fullbrilliancy when connected at 220 volts, and at one-half brilliancy on 110 -volt circuits
24. USE OF TEST LAMPS, BUZZERS ANDMAGNETS
Test lamps of this type can be used for locatingopen circuits, short circuits, and grounds on themachines themselves or the wires leading to them.They are also very convenient for testing to locateblown fuses and to determine whether or not thereis any voltage or the proper voltage supplied tothe terminals of the machines.
The battery and buzzer test -outfit can be madeof one or two dry cells taped together, with the
suzzut)r/N
Fig. 5. A simple test set consisting of dry cells and busier is avery effective device for trouble shooting and locating of faults inelectric circuits and machines.
buzzer taped securely to them. This unit shouldthen be supplied with flexible test leads several feetlong. The dry cells and buzzer can be located ina portable box if desired.
A simple test outfit of this kind can be used forlocating grounds, opens, and short circuits on ma-chines or circuits that are not alive.
The magneto test -outfit is very effective for lo-cating high -resistance short circuits or grounds.These hand -driven magnetos generate voltage suf-ficiently high to break down the resistance at thepoint of the fault or defect, while a battery test setor test lamp used with ordinary line voltage might.not show the fault.
348
Direct Current
When installing any new circuits to generators,motors, or controllers, the wiring should be thor-oughly tested for grounds, shorts, and opens beforeconnecting the machines.
An ordinary A.C. test magneto will ring through20,000 to 40,000 ohms resistance. The use of mag-netos above 50,000 ohms is not advised becausethey will ring through the insulation of conductorson long circuits.
In some cases an A.C. magneto will cause itsbell to ring when the terminals are attached to thewindings of very large machines, due to capacityor condenser effect between the windings and theframe of the machine. In such cases the ringingof the magneto doesn't necessarily indicate defec-tive insulation.25. USE OF PORTABLE VOLTMETERS
AND AMMETERSVoltmeters are very essential in plants having
a great number of electrial machines and circuits.Voltmeters should be used for measuring linevoltages or voltage drop on various circuits, todetermine whether or not the proper voltage issupplied to the equipment.
It is very important that D.C. motors be operatedat their proper rated voltage and not at voltages10% or more below this, which sometimes resultsfrom overloaded line circuits and excessive voltagedrop.
Low reading voltmeters are very satisfactory testdevices for locating faults in armatures and fieldcoils, as well as commutator defects. They canalso be used for testing voltage drop in controllercoils and resistors and to locate defective coils inthis manner.
Ammeters can be used to measure the currentthrough any circuit or machine and to determinewhether wires or machine windings are properlyloaded or overloaded.
One or more ammeters should always be avail-able in plants where numerous electrical machinesare to be operated and maintained.
349
Maintenance
26. THERMOMETERSThermometers should be used to determine the
temperature at which various machines are oper-
ro
AC=C
E
uA
.2 a
03
V.
.1!
15.00
Y
orgi
AGE
00
(U
ated, and especially if a machine is known to beoperating somewhat overloaded. On machines thatare not overloaded, if the temperatures rise abovethe rated temperature increase for a normal load,the cause should be determined and remedied atonce.
350
Direct Current
By checking the temperatures at different pointson a machine .or its windings, the exact locationof the fault or trouble can frequently be found bynoting the points of higher temperature. Somethermometers for this use are marked with thecentigrade scale, while others are marked with theFahrenheit scale.
A convenient rule for converting the temperaturein either scale to the other is as follows :
Temperature C. = 5/9 X (Temperature F - 32)Temperature F. = (9/5 X Temperature C.) ± 32
27. SPEED INDICATORS
Speed indicators or revolution counters are com-monly used to determine the speed of various ma-chines. If machines are overloaded or thought tobe operating at low voltage, it is often necessaryto test their speed.
In other cases, checking the speed of machinesmay assist in locating certain faults within themachine or its own windings. In many industrialplants and factories it is very important that themotors driving production machines be kept oper-ating at their proper rated speed, in order not todelay the production of the article being manufac-tured.
With the ordinary low-priced revolution countersor speed indicators, a watch with a second-handcan be used to check the time during which therevolutions are counted, and to get the speed inR.P.M.
Where a large number of machines are to betested frequently, a higher -priced speed indicatorknown as a "tachometer" may be used. This devicewhen placed against the shaft of any revolvingmachine indicates the speed in R.P.M. instantly.
28. IMPORTANCE OF RESISTANCETESTS ON INSULATION
Since the quality of the insulating materials usedon any electrical machine deteriorates with age,due to the action of moisture, dirt, oil, acids, etc.,
351
Maintenance
it is necessary to periodically test the electrical re-sistance of the insulation so that weaknesses maybe detected and corrected before they result incomplete failure.
Commet To Wises PLAT'OR SHAM
RECTIIIIN FILTERUNIT USED To STE
or NOE.A.C.ANDC IT To 500VOLTS D.C7)
IIT, T.Ts Ts Ts
The above sketch shows how an insulation resistance test may bemade by using a rectifier and filter combination for obtainingthe D.C. voltage needed for this type of check. Note that theconnections are such as to read the current leakage through orover the insulation from the conductor to the frame of themachine.
Insulation resistance tests are usually made byapplying 500 volts D.C. between the winding of themachine and the frame; the current which thispressure forces through or over the insulation tothe frame is measured by a sensitive instrument,the scale of which is usually calibrated to read inmegohms. The 500 volts D.C. may he developed bya hand -operated generator as in the megger, or itmay be supplied from an A.C. source by a rectifier-filter combination as shown above.
The readings obtained on any given machine willvary greatly with the temperature of the insulation,a 10 degree Centigrade rise in temperature reduc-ing the insulation resistance as much as 50%. Thedampness of the location, and the amount of oil,dust, or dirt on the winding, will also materiallyaffect the readings. Wherever possible, the testshould be made when the insulation is at the max-imum operating temperature, 167 degrees F., (75degrees C.). The minimum safe insulation resistanceat maximum operating temperature should not be
352
Direct Current
lower than one megohm for equipment having avoltage rating below 1000 volts.
To make the test, connect the rectifier unit to110 volts A.C., set the control switch on the meterto the one mil position, set switch in D.C. position,make the connections shown above, and read theinsulation resistance on the top scale of the dial.Usually a general test is made between one lead ofthe machine and the frame, and if this proves to betoo low, the windings are tested individually. Soafter the general test, test the armature, shunt field,series field, and brush holders separately. To dothis, take the brushes from the holders, disconnectthe windings from each other, and test the insula-tion resistance of each. In this manner, the faultyelement can quickly he found. This same procedureis used on A.C. equipment also. If such readingsare taken at regular intervals and the values re-corded, a close check may be kept on the conditionof the insulation resistance of all electrical equip-ment, and apparatus may be removed from serviceand reconditioned before breakdown occurs. If amegger is employed, the 500 volt potential is ob-tained by cranking the generator inside the instru-ment, the method of testing is the same as givenabove.
In medium-sized and larger plants, instrumentsof this type will very soon save much more thantheir original cost.
Electric instruments are usually furnished by theemployers or plant owners. although in some casesthe maintenance man and electrician can well affordto own one or more low-priced portable instru-ments for the great convenience and aid they givein his work.
Whether these instruments are supplied by theemployer or owned by the electrician, they shouldalways be handled with proper care and intelli-gence.
Most meters are delicate devices and they shouldnot be carelessly handled or banged around. Ex-treme caution should always be used not to connectammeters across a line or in circuits with greaterloads than the ammeter is designed for. The samewarning applies to connecting voltmeters and watt -
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Maintenance
meters, which should never be connected to circuitsof higher voltage than the instrument is made for.29. COMMON TOOLS FOR MAINTENANCE
WORK
A few of the more common tools used by theelectrical maintenance man are as follows :
1. Knife2. Pliers (side cutting)3. Gas pliers4. Screw drivers5. Adjustable wrenches6. Pipe wrenches7 Machinists hammer8. Center punch9. Cold chisels
10. Soldering iron11. Blow torch12. Tin snips13. Bearing scrapers14. Speed indicator15. Air -gap gauge16. Files ; flat, round, and three -cornered17. Hack saw18. Breast drill.This list covers the more essential tools for ordi-
nary jobs. Various other tools can be added forcertain things, according to the class of work andequipment to be handled. A few good pointers inthe selection and use of these tools are given inthe following paragraphs.
An electrician's knife should be a good substan-tial one, with one sharp blade that can be used forthe removal of insulation from conductors, and onegeneral utility blade for miscellaneous cutting,scraping, etc.
The most common and handy size of pliers isthe 7 -inch length, and if only one pair is used thisshould be the size. If one wishes to carry or tohave on hand two or more sizes, the 6 -inch and8 -inch sizes should also be included.
354
Direct Current
Cheap pliers never save any money, and onlygoqd pliers with strong jaws and good cuttingblades should be purchased. Pliers larger than9 -inches are seldom used, except for the handlingof very heavy wires, and cables. Good pliers aremade of the best grade of tempered steel and shouldnever be held in the flame of a torch or allowedto become overheated in any way. Pliers shouldnot be used to cut hard steel bolts or spikes.
The gas pliers are very convenient for holdingcable lugs when heating them to melt solder andapply to cable ends, and for other general usessuch as gripping bolts, nuts, and small parts. An8 or 10 -inch size is usually most convenient.
You should have at least three or four sizes ofscrew drivers and sometimes more. It is well tohave one short and one long screw driver, bothwith points to fit a No. 7 wood screw ; one shortand one long driver to fit a No. 10 wood screw ;and at least one large screw driver for No. 14 toNo. 16 screws.
Never use a screw driver for a crow bar or chisel,as such abuse will only bend their bits or split thehandles and render them unfit for the purpose forwhich they were intended.
If screw drivers become dull they can be care -full reground on the flat side of an emery wheel.Never grind them to a sharp point, as it tends tomake them slip out of the slots in screws.
Adjustable wrenches should be of the 6 -inch, 8 -inch and 10 -inch sizes, and these will handle allexcept the very heavy work. These tools are usedfor tightening bolts and nuts on motors, controllers,and all kinds of electrical equipment; and both fortaking apart and reassembling motors and machinesto be repaired.
When using an adjustable wrench, always tightenthe jaws securely on the nut before applying anypull on the handle, as this will avoid slipping andinjury to the operator as well as "rounding" of thecorners on nuts or bolt heads.
Never use a wrench upside down or backward,and don't hammer the handles, as it will only spring
355
1
Maintenance
the jaws and spoil the wrench. Wrenthes are madewith handles long enough to apply by a steady pullall the pressure their jaws will stand.
Pipe wrenches should be used for loosening stub-born or worn nuts on which the adjustable wrenchslips, and also for making BX or conduit connec-tions. One 10 -inch and one 12 -inch pipe wrenchwill usually be sufficient for ordinary repair work.
A good hack saw is indispensible for cuttingbolts, BX, conduit, and heavy cable. Either anadjustable or a rigid frame may be used, and sev-eral good sharp blades should always be on handfor this saw.
When using a hack saw, the object to be cutshould be securely held in a vise or, clamp. If theobject is allowed to wobble or twist it will crackthe teeth out of the saw blade.
A machinists hammer of one lb., one and a halflbs., or two lbs. weight will usually be found mostconvenient.
Center punches are very handy for markingplaces for drilling holes in metal, or for markingthe endplates of motors or machines before they areremoved, so you can be sure of getting them re-placed properly.
A small breast drill or Yankee drill with a dozenor more short drills will be found very convenientin making many time -saving repairs.
Several sizes of cold chisels are needed for cut-ting bolts, screws, metal strips, etc., on which thehack saw cannot be conveniently used.
Tin snips are very convenient for cutting stripsof hard insulation, such as fibre, or for cuttingshims of thin metal for lining up bearings or ma-chine bases. They can also be used for cutting shimsto place under field poles when adjusting air gapson the poles of motors or generators.
A set of bearing scrapers, such as used on auto-motive work are usually very convenient. These areto be used for scraping sleeve -bearings to fit theshafts of motors or generators.
An air -gap gauge consist of a group of thin356
Direct Current
metal feeler gauges that can be used for determin-ing the air gap between the armature core andvarious field -pole faces.
It is quite important to keep the armature cen-tered in the machine, in order to secure best opera-tion, and when bearings become worn and allowthe armature to drop below the center, an air -gapgauge can be used to re -center the armature or de-termine which poles it is closest to.
One or more pieces of hack saw blade can beeasily fitted with file handles and used for under-cutting mica on small and medium-sized machines,as was explained in a previous article.
Flat files are very convenient for resurfacing andlressing the fa/es of contacts on controllers, andhundreds of other uses which are not necessary tomention, as most everyone knows the common usesfor a file.
Where most of the work to be done is withinreach of electrical circuits of the proper voltage, anelectric soldering -iron is generally most convenient.Where electric supply of the proper voltage is notavailable or where very heavy soldering is to bedone, a blow torch is essential. One or moresoldering coppers can then be used, by heating themin the flame of the blow torch.
30. OPERATION AND CARE OF BLOWTORCHES
At this point it will be well to give a few generalhints on the use of gasoline blow -torches.
A torch of one quart size is usually most con-venient for ordinary work. To fill the torch, un-screw the cap in the bottom and pour the gasolinein the opening with a funnel. If any gasoline isspilled on the bottom of the torch it can be run in-side by gently rocking the torch back and forthuntil most of its runs into the opening.
After filling, replace the cap, making sure thatthe composition washer is in place, to seal the
357
Maintenance
torch air -tight and prevent leakage of gasoline.Tighten the screw cap securely and pump a smallamount of air into the tank. Six to ten strokes ofthe pump is sufficient for starting a torch. Thenhold the hand or some object over the torch nozzle,tipping the torch back slightly, and open the needlevalve a small amount. The gasoline which is al-lowed to escape will then drain into the small ves-sel or cup under the torch. The cup should benearly full before the valve is closed.
This gasoline should then be carefully ignitedwith a match and allowed to burn away almostcompletely before opening the valve again.
This flame heats the torch nozzle and gas genera-tor so the liquid gasoline will be turned into vaporas it escape. This is necessary for proper opera-tion and to secure the full heat of the flame.
When the torch is well -heated, open the needlevalve and adjust it until the flame is a sort of bluecolor with a slight pink tinge.
If the torch is operated in a breeze or wind, turnthe torch so the flame points against the breeze.This will tend to confine the heat of the flame Whereit will do the most good and keep the torch hotenough to operate.
When through using the blow torch, it should beextinguished by closing the needle valve ; never byblowing or smothering the flame. After extinguish-ing the torch, let it stand a few minutes ; then openthe needle valve until a hissing sound is heard.This relieves the pressure in the tank and the needlevalve can then be closed gently and the torch putaway until it is to be used again.
Never use a pliers on the needle valve or youmay damage the soft metal seat of the valve. Neveruse one blow torch to heat another, or it may re-sult in an explosion and dangerout burns.
These few general hints on the types of tools andthe methods of their use are intended simply to aidthose who have never used tools of this kind to be-come properly acquainted with them.
Thoughtfulness, pride, and care in your work,and the application of a little mechanical ability
358
Direct Current
along with practice, are all that are required tomake most anyone proficient in the use of thesetools and in ordinary electric maintenance work.
Always do all repair work neatly and thoroughly.You will find that in the long run it saves time andtrouble. Take a reasonable pride in all electricalmachinery and equipment which you may be op-erating or maintaining, and also in your knowledgeof the proper operation and care of this equipment.
Conscientious and intelligent application of theknowledge you can gain from this lesson and pre-ceeding lessons and experiments should enable youto qualify in the operation or maintenance of prac-tically any Direct Current equipment.
MAINTENANCE SCHEDULEGetting the most out of motors depends on carefulselection and proper installation as well as on -schedule inspections, lubrication and cleaning.
Work stoppages caused by motor failures at crit-ical moments can be forestalled to a large degreeby proper preventive measures.
Although not actually maintenance, the correctselection and application of a motor involve a greatmany factors affecting the installation, operation,and subsequent servicing of the motor. Here aresome genral rules on selection :
1. Check motor rating against load require-ments.
The motor should be the proper type as wellas size. Overloading causes overheating andoverheating means shortened motor life. Under -loading wastes current, lowers power factor-anitem in many rate schedules.
2. Be sure voltage and frequency are correctfot circuit.
Variations in voltage of more than 10% plusor minus and frequency variations of more than5% above or below nameplate rating should beavoided. Voltages below normal are particularlyobjectionable.
3. Select a high-speed motor-if possible.Where application permits, a high speed motor
means smaller size with less cost and higheroperating efficiency.
359
Maintenance
More specific instructions for matching the motorto .the power supply and the driven machine maybe obtained from any motor manufacturer.
After the correct motor has been selected, itmust be properly installed in order to deliver themaximum efficiency and require the minimum ofservicing. The following installation pointers willget the motor off to a good start:
Install the motor so that it is accessible for inspections and repairs.Align it properly with the driven load.
1. Mount in a dry, clean, accessible place.Motors must be protected from dirt, moisture,
dripping oil, fumes-either by proper location orby using an enclosed type. Good ventilation andaccessibility are essential.
2. Match control to motor rating and load.Control equipment in correct size and type
will protect the motor against overloads in start-ing, stopping, and acceleration.
3. Check Connections.Terminal lugs should be tightly soldered or
bolted to prevent loosening from vibration. Trythe motor for correct rotation-then tape leadswith rubber and friction tape.
360
Direct Current
4. Make certain of proper alignment.Align motor with shaft of machine to be driven
and fasten securely to a rigid base or foundation.Sprung shafts, overload, burned -out bearings arcsome consequences of misalignment.Inspection, lubrication and servicing of each mo-
tor should be systematic. Frequency and degree ofthoroughness will depend on the amount of opera-tion, nature of service, and environment underwhich the motor operates. The following schedulecovering both a -c and d -c motors is based on av-erage conditions insofar as duty and dirt are con-cerned:
Keep motors free from dust and dirt by frequent wiping.
Every Week1. Check oil level in bearings and see that oil
rings are moving freely.2. Examine fuses and control equipment. Wipe
off control housings.3. Check commutator and brushes for possible
adjustment.4. Start motor and see that it is brought up to
speed in normal time.
361
Maintenance
Every 6 Months1. Blow dirt out of motor with dry compressed
air at not over 50 lb pressure.Wipe commutator and brushes.2. Check grease in anti -friction bearings.Avoid excessive grease pressure as it causes
overheating.
Reprinted by JAMES G. BIDDLE CO. fromINDUSTRY and POWER,
3. Examine brushes and brush holders.Replace brushes more than half worn.Clean holders and check brush pressure and
position.4. Check operating speed and listen for signs
of bearing wear.5. Inspect condition of control apparatus.Dress or renew pitted contactor tips and see
that fuses are in order so that motor will notsingle phase. Tighten connections.
6. Examine commutator and commutatorclamping ring. Clean with cloth dampened withcarbon tetrachloride.
7. Check motor foot bolts, end -shield bolts, pul-ley, coupling gear, and journal set screws.
8. See that all motor covers and guards arein place and securely fastened.
Once a Year1. Clean out and renew grease in bearing hous-
ings. Clean sleeve bearing housings and renew oil.2. Test insulation with an insulation resistance
tester. If reading is less than 1 megohm per 1000volts; winding should be baked out until readingis satisfactory.
3. Clean out undercut slots in commutator.If commutator is spot worn have it trued up ina lathe. Undercut high mica separators.
4. Check current input and compare with nor-mal to prevent underload or overload.
5. Inspect armature bands and connection ofarmature coils and commutator.
6. Check rotor clearance at each end with a
362
Direct Current
feeler gage. Keep record of clearance as a checkon bearing wear. For less than 10 HP the min-imum gap permissible is .005 inch. For more than10 HP gap should not be less than .010 inch.
7. Clean out magnetic dirt that may be hang-ing on poles.
insulation resistance tester being used to determine condition ofmotor -generator insulation by indicating insulation's resistanceto leakage currents.
Every 2 Years1. Dismantle the motor and clean.2. See that windings are tight.3. Replace loose wedges and loose bands.4. Dry out motor in an oven at 115° C. max-
imum temperature. Dip in varnish and bake, 2to 4 times depending on operating conditions.
5. Remove bracket and wash out bearing hous-ing, using hot kerosene oil and compressed air,if available.
363
Maintenance
An up-to-date record card should be kept for eachmotor showing periodic test values of clearance andinsulation, all repair work and its cost. A visualrecord should also be maintained to show the sched-ule of lubricating, cleaning and inspection. Mis-applications, poor drive engineering and excessiveoperating costs will be quickly disclosed with ac-curate records. Several excellent record systemshave been developed and printed, so that it is notnecessary to devise and prepare individual systems.
An adequate stock of spare motor parts and sparemotors should be maintained for emergency repairsand replacements. Bearings of different sizes, sparearmatures for d -c motors, brushes and other partsshould be on hand in different sizes and properlyclassified for easy identification. Auxiliary motorsin racks and large motors on skids should be readyfor immediate installation at any point in the plant.
The importance of an adequate motor mainte-nance program cannot be over -emphasized. Such aprogram will do much toward maintaining peakoperating efficiency in any plant.
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Direct Current
EXAMINATION QUESTIONS
1. Which part of a D.C. motor or generatorusually requires more attention than any other partof the machine?
2. Give one of the most important rules to fol-low in the maintenance of electric machines.
3. Is excessive oil detrimental to any part of aD. C. motor or generator? If so which parts?
4. How may the winding of a machine be driedout when there is no oven available or when themachine is too large for the oven?
5. If the mica between commutator segmentsbecomes high, how may it be corrected?
6. Name at least five troubles which may causeexcessive sparking at the brushes?
7. Name at least three troubles which may causea D.C. motor to overspeed?
8. Give at least three reasons why a D.C. gen-erator may fail to build up voltage?
9. ' What would be the temperature Fahrenheitwhen a centigrade thermometer registers thirty?
10. What instruments are recommended for usein measuring insulation resistance?
365
INDEXA
Ammeters, 156-160Ammeters, connections for, 158
principles of, 147
BBearings, 23Breakers, circuit, 126
instrument switches, 134circuit-care & maintenance, 132
shunt trip coils, 131trip coils overload release, 129operation, 127
Braking, dynamic, 294-301regenerative, 301
Brushes, 103-116, 18holders, 19requirements, 308, 103fitting new to 0%1 commu-
tators, 113cement for attaching leads, 111leads & shunts, 314, 109pressure & tension, 108use of graphite in, 107special types of, 107resistance, 105photos of. 104materials for, 309, 103experiments. 115carbon types, 307-320
Bus bars, 135-136connections of, 137
CCarbon, brushes. 307-320Charts, trouble shooting, 347Circuit
magnetic 4 pole, 30Coil
no voitage-no field releasetype. 247
blow out. 272Commutation, 85-90
process of, 85Commutators, 17
fitting new brushes to, 113importance of proper brush
setting, 87speed regulating, 250shifting brushes with varying
load on machines, 88self induction in coils shorted
by brushes, 86use of commutating poles to
prevent sparking, 90Compound, generators, 47Control, speed by use of armature
resistance, 242Controllers, drum, control
sequence chart, 280drum, construction, 288
forward position starting, 290operation of, 274-293principles of, 275
dash pot & time delay, 262circuit & carbon type, 256reversible drum types, 293reversing drum types. 282-284reversing rotation of motors, 281
Direct Current power, advan-tages, 1-8
armatures for, 16construction of generators, 13electro-magnets, 8field poles, 14generator ratings, 9
speeds. 11operating temperatures, 10
Dynamic, braking, 294-301
EElectro Magnets, 8Elevator Motor, 4Excitation, field, 31
FFlemings, right hand rule, 28Frames, field generator, 14
GGenerators, armatures, 16
resistance & I R loss, 39reaction, 36
building up voltage, 33brushes for, 18bearings in, 23compound types, 47commutators for, 17construction of, 13cumulative & differential
compound types, 49calculation of Hp for prime
movers, 57correcting wrong polarity, 65differential compound types, 52equalizer switches, 69field frames, 14field poles, 14failure to build up voltage, 34flat compound types, 50inspection procedure before
starting, 60instrument connections with
parallel types, 71magnetic circuits. 29operating principles, 27neutral plane, 35over compound types, 50operation of. 57-83photos of, 8-5-59paralleling of, 63
rule for paralleling, 63speeds of, 11self excited, 32shunt-characteristics of, 41-42series, 44starting, 61starting, paralleling & adjust-
ing loads in, 72testing, adjusting & compound-
ing of, 67-68types of drives for, 12voltage drop in brushes &
lines, 40voltage adjustment, 36voltage characteristics of series
types, 45
HHoists, control of, 302
lowering chart, 302Holders, brush-reaction
types, 22
IIndicators, demand type, 188Interpoles, 90-100
adjusting by changing airgap, 96
adjustment of brushes, 94commutation on motors, 97number of, 100polarity for generator, 92position neutral plane in
motors, 98reversing rotation, 100
Load, chart, 53Locomotive, 8
L
MMagnetic, starters, 265-272
Maintenance, 321-365care of overload-protective
devices, 339care of controllers, 338checking chart DC & AC
motors. 328faulty bearings, 824importance of cleaning, 321mica undercutting. 334resurfacing & truing commu-
tators, 837recommended practices, 327schedule, 359-365trouble shooting, 340-846
(a) motor won't start(b) starts too quickly(c) rotation reversed(d) bucking or jerking(e) overspeeds(f) sparking at brushes(g) generator troubles(h) poor voltage regulation(i) generators will not
operate in parallel(j) systematic testing
trouble correction chart, 330-331tools needed for, 354use of test lamps, buzzers &
magnets. 348Megger°, 193-195Meters, 145-198
vibration-velocity type, 149viscosimeter, 149electric tachometer, 149film thickness gage. 149damping of needles. 150combination portable ammeter
& voltmeter, 151switchboard type voltmeter, 152types of, 145operating principles ammeters
& voltmeters, 147how to make special measure-
ments, 148
Meters-Cont'd.changing for higher or lower
readings, 161watthour types, 171-185
Methods, speed DC motors, 240Motors, 199-235
armature poles, 201armature resistance necessary
when starting. 211brake Hp test for, 229characteristics, 318counter EMF, 209compound differentials, 223compound, speed regulation &
applications, 222compound types of, 221elevator. 4effect of counter EMF on
motor speed, 212direction of rotation, 209efficiency calculations, 233-234efficiency tests, 232Hp calculations, 231operation of, 199rotation of, 200ratings in ampere Hp, 207speeds & Hp. 205shunt - speed regulation, 216
stalling torque, 215starting torque, 214types of, 218
speed regulation & control, 206series-starting torque, 217series types, 216-218series-speed control, 219stopping of motors, 249types of DC motors, 203torque, 201
PParalleling, generators, 68Poles, field, 14Prony brake, 280-281
QQuestions
carbon brushes, 116generators, 25, 55maintenance, 865meters, 198, 169motors, 235starters & controllers, 306-264switchboards, 1438 wire systems, 88-148motor starting, 289speed control, 240
RRing, rocker type, 21
SSeries, generator, 44Shunt, generator diagram, 48
series field, 46ammeter, 156
Switches, care & operationfor, 125
construction of, 128types of, 117mounting & current ratings 124
Switchboards, 117-128bench types diagram, 119frames for, 120generator or feeder panels, 118
Switchboards-Cont'd.knife switches. 122layout & circuits, ISAlocations of meters & switch -
gear, 141pipe frames & their advan-
tages, 121wiring of, 140
Starters. 237-272automatic. 257carbon types, 254carbon pile types, 253motor starting rheostats, 239magnetic types. 265-272overload protection. 271remote control. 2573 & 4 point types of, 246time allowance, 238
T3 wire systems
direction & amount of currentin neutral wire. 74
balancing of unequal loads, 81effects of shunts & series fields
of balancer generators. 79motor generator & balancer
sets, 78
3 wire systems-Cont'd.unbalanced load, 77principles of balanced coil. 76balancing. 75
Torqueseries motor. 217
stalling, 218
VVoltmeters. 153-154
principles, 147
wWatthour, meters. 171-185
damping disk & magnets, 173indicating types, 166
WattmetersLoad demand indicators. 187recording instrument relay
type, 183"creeping". 180reading of, 178-180constant & time element, .176compensating coil, 175adjusting damning disk. 174
Wheatstone Bridge. 190-194operation & circuit, 191
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