variable voltage control systems as applied to electric elevators

Variable Voltage Control Systems as AppliedTo Electric ElevcatorsBY EDGAR M. BOUTON

Assoc. A. I. E. E.Section Engineer, Westinghouse Elec. & Mfg. Co.

Review of the Subject.-Low-speed electric elevators, using practically the same for all loads. The higher rate of accelerationd-c. motors, cam.e into use about 18.90. Later, a-c. motors were and retardation permits higher speeds, and the smoothness, besidesemployed, but on account of the difficulty of speed control could not reducing wear and tear on the machinery, makes riding entirelybe used for the high speeds n.ecessary in tall buildings. Since the comfortable to passengers. The impression offalling which is oftenheight of buildings is dependent upon the elevator system, and in given, under rheostatic control, by a heavily loaded car when descend-many districts only a-c. is available, the need for high-speed equip- ing, is inherently avoided, since the car speed follows the generatorments that can be operated from a-c. is evident. A solution to the voltage which at no time changes suddenly.problem isfounnd in the variable voltage system of control. 3. Speed Control and Regulation.-The speed regulation

Jn this system each elevator motor is supplied by an individual remains flat at as low as one tenth full speed. Consequently it isgenerator driven by a motor operating,from the a-c. or d-c. supply easier to make an accurate landing, fewer false stops are made thevoltage. The generator's voltage, and. hence the elevator motor's car may be "inched" easily and quickly. In stopping, regenerativespeed, are controlled by varying thefield of the generator. braking is set up which brings the car quickly but smoothly to a low

The apparatus consists of: speed before the friction brake is applied. Positive speed control1. An ordinary shunt-wound d-c. elevator motor. enables the limit stops to be made in less time and shorter distances,2. A d-c. generator of special design for the elevator motor. and without over-travel.3. Control panelfor the generator and elevator motor. 4. Efficiency and Economy of Power Consumption.-4. An a-c. or d-c. motor to drive the generator. Less power is required in acceleration and retardation. This5. A starting device for the driving motor saving of power over the rheostatic control is greatest when the number

And if the supply is a-c. of starts and stops is large. Power is returned to the line while6. A direct-connected exciter for the field, brake and control making the limit stops. Power consumption is not increased in

circuits. making small movements of the car or in running at low speed.The control panel (1) makes the proper connections for up and During idle periods standby losses may be eliminated by shutting

down motion of the car; (2) releases, or sets, the brake: (3) controls down the motor generator set.the speed; (4) discharges the generatorfield during retardation, and 5. Maintenance.-Since the switching of major currents ison stopping (5) demagnetizes the generator and (6) opens the circuit eliminated and they are controlled indirectly, the number of arcbetween the armature of the generator and the armature of the rupturing contacts is reduced to a minimum the control as a wholeelevator motor. is simpler, less ad(justments are necessary and maintenance costsFrom tests the following conclusions in favor of the variable are lowered.

voltage over the rheostatic control system are drawn: 6. Safety.-Inherent safety features make higher speeds1. Speed.-High-s peed installations are nowv possible for any possible. Limit stops are made accurately and positively; A

commercial a-c. voltage andfrequency. second independent means is provided for stopping the car in2. Acceleration and Retardation.-The rate of acceleration emergencies. On failure of power a dynamic braking circuit closes

increases gradually to about half speed, then decreases uniformly and a field is maintained, on the elevator motor, making certainuntil futll speed is reached. The time and power required for the stopping of the car. An overspeed contact on the motor generatoracceleration are less than with rheostatic control. The time remains set opens the safety circuit independently of the speed governor.

T HE application of electric power to elevators dates is known as variable voltage. It is this system thatback to 1890. The early motors were direct- will be described in this paper. The development ofcurrent machines and were applied to relatively this system has also greatly improved the operation of

low-speed elevators. Later a-c. motors came into use, all classes of elevators to which it has been applied andboth phase-wound and cage-wound secondaries being because of its superior operation and high economy hasused. Higher-speed elevators were developed as build- frequently been used on d-c. power lines.ing heights increased. The modern sky scraper would During the last few years the cost of buildings andnot be practical except through the use of high-speed the ground upon which they are built has increased soelevators. The d-c. motor and its controller were rapidly that effective use must be made of every avail-perfected for high-speed work largely because of the able square foot of floor space. The space occupieddifficulty in controlling the speed of the a-c. motor. by the elevators brings in no direct income and shouldUntil quite recently high-speed elevators invariably be reduced to a minimum. Higher car speed andrequired the use of d-c. power. reduction of lost time due to better control offer con-

In the last three or four years a great deal of engi- siderable help in solving this problem. Variable volt-neering effort has been put into the problem of develop- age control has been used very successfully for elevatorsing a high-speed a-c. elevator equipment. These running at speeds as high as 700 feet per minute. Theefforts have been very fruitful and a-c. power can now system with further refinements in some details willbe applied to elevators at as high a speed as can d-c. no doubt be used for still higher speeds in the future.One of the a-c. systems that has recently been perfected I. REQUIREMENTS OF ELEVATOR SERVICE

Presented at the Midwinter Convention o; the A. I. E. E., It is not the purpose of this paper to discuss all the

Philadelphia, Pa., February4-8,1924.factoors that go to make up elevator service. Elevator199

200 BOUTON: VARIABLE VOLTAGE CONTROL SYSTEMS Transactions A. I. E. E.

service is, however, so affected by the kind of electrical must the safety of passengers be jeopardized in orderequipment installed that in discussing any particular to save time or reduce the first cost of installation.control system for elevator work, it will be necessary Any control system having features which increase theto consider its fundamental requirements. The re- safety with which passengers can be handled is worthyquirements of elevator service that are affected by the of serious consideration from that standpoint alone.electrical equipment and particularly the control may II. DESCRIPTION OF THE SYSTEMbe enumerated as follows: The variable voltage system consists of five main

1. Elevator Speeds. The speed at which cars are run elements, a d-c. motor to drive the elevator, a d-c.affects the service, and isvery largely dependent upon the generator for each elevator motor, an a-c. or d-c.electrical equipment used and upon the control that motor to drive the generator, a control panel for thecan be obtained. Low-speed elevators require rela- elevator motor and generator and a starting devicetively simple equipment, while the maximum speed that for the motor driving the generator. If the supply iscan be used is limited by the control systems in use at a-c., a direct-connected exciter is added. The auxiliarythe present time, and increases in speed are probably equipment, such as the car master switch, limit switchesdependent upon further control developments. and magnet brake may be the same as those used for

2. Acceleration and Retardation. The rate of ac-celeration and retardation has a very marked effect Generatorupon elevator service, not only in so far as comfort to Field Resistorthe passengers is concerned, but it also has a directbearing upon the speed at which cars can be run. It D %; lis mostly dependent upon the control system and the idealcontrol system is one which will accelerate andretard the Downcar at the highest possible rate, and still operate so Upsmoothly as to subject the passengers to no discomfort.

3. Speed Control and Regulation. Elevator speeds F Sin the past have increased directly as methods of in- Generator ILL3Sar2ter Fieldcreasing the speed range over which motors could becontrolled have developed. No matter how high a car Motorspeed is used, the speed from which the landing can be 2ATrm.Ji - Shf.made will remain more or less fixed. The regulation otor AtorArm.of the equipment affects elevator service because if the Series Fieldcars slow down too much under load the service is FIG. 1slowed up at the very time when maximum service is Elementary diagram showing principal connections of variable voltage

control system. The generator is driven by an a-c. motor and the set hasdemanded. a direct-connected exciter.

4. Efficiency and Power Consumption. Electrical other well known systems of control. Fig. 1 is a sche-elevator equipment at the present time has reached a matic diagram showing how the apparatus is connectedhigh stage of development and the matter of economy together.in operation probably receives more attention than in 1. Elevator Motor. The elevator motor requires noalmost any other industry. Electrical elevator equip- special features and is shunt wound which is the ac-ment to give good service must not only handle pas- cepted elevator practise. No speed adjustment bysengers as quickly and smoothly as possible, but it field control is necessary although it is good practisemust also do it in an economical manner. to strengthen the field during the starting period for

5. Upkeep and Maintenance. Upkeep and mainte- applications where high starting torque is required.nance constitutes a considerable item in the cost of A constant-speed motor for a given rating will have aelevator service. It is, therefore, desirable that this lower weight and cost, and better electrical performanceitem be kept as low as possible. It has an even more than the corresponding adjustable-speed motor.important bearing upon the continuity of service. 2. Generator. The generator is one of the mostEquipments that are easy to maintain and which are important units of the system and is designed almostreadily kept in good condition are much less subject entirely from the standpoint of its control functions.to shutdown than those which require constant atten- As shown in the schematic diagram the motor andtion to keep them in operating condition. generator armatures are connected directly together

6. Safetqy. -Although this item appears last in the electrically without the use of any resistors. Thelist it is far from being the least in importance. In fact direction of rotation and the speed at which the elevatorsafety is the first consideration in giving good elevator motor runs depend upon the direction and value of theservice. Any developments in control systems or generator voltage. The field magnet is excited fromapparatus which increase the service must at the same a separate source and the field excitation is governedtime be absolutely safe in operation, and in no case by the controller.

Feb. 1924 BOUTON: VARIABLE VOLTAGE CONTROL SYSTEMS 201

The generator has commutating poles and sufficient pending upon the character of the power supply.commutator capacity to take care of the current peaks Alternating-current motors are cage-wound and de-that must be handled during the accelerating period. signed for low slip and high efficiency. Direct-currentHigh efficiency and good voltage regulation are incor- motors are shunt wound. It is of particular importanceporated as essential features. Fig. 2 shows the per- that the no-load or idling losses be kept as low as pos-formance curves of a 25-kw. machine that are typical sible. The motor generator set whose performanceof what can be obtained. The efficiency reaches a curves are shown in Fig. 2 has a no-load loss of 2.5maximum of 89 per cent and remains high with over- kw. or 1.25 kw. per elevator.loads so that acceleration is accomplished without exces- -_Gov____or_Co_ae__Cl____ e_sive generator losses. D e to Miuadjustmient

550r---r _

ol Net Load 1200 Lbs I

Z 100 0tEfficiency o Motor __ - ~ 0 0 rDeceleraion Curewith otor Field Increased

- 80 - < yo Generat,or 6C=070,rfieyfSet._NetLo5d 1200lbs.

X~- 60 t X ,. X 9 i Z 2u$ /,,>dt|Motor/rz I MLtooWWlLeldin reased J

o4C8X

.H

_ ___ t I!: A:ecleatinC rewLthMoto /,OI 40 80c10 550 ClosE

GovernTi rCureora6CloealsTrcinEeto.Dy200b.

__~~~~~~~~~~cr 55

La.drivingmotN.L. Lossesof SSth Scari terating wit t?20 noFedsZeOals=1 Kiloatt KernrCna z

Fig.3Bshowslaatestdonterates at Baauced Load and a y2kgeneratr d o De eleratio Curve with Motor Fieldincreased

x 0 80 160 240 320 400 481) TIME-EACH DIVISION = ONE SECONDLB., FTJTORQUEOIF MOTOR& LINEAMPERES OF GENERATORS FG

Speed Time Curves for a Gearless Traction Elevator. Duty 2000 lbs.FIG. 2 at 500 F. P. M. The dotted lines show the effect of increasing the motor

Performance curves of a double generator motor generator set with a.n f d

a-p. driving motor. 4. Motor Generator Set Starter. Since the motorFig. 3 shows a test on the rate of building up of a generator sets are started infrequently and always

25-kw. generator designed for variable voltage control start without load a relatively simple automatic starterservice. Full excitation voltage was thrown on the Tests on Gearless Traction Elevatorfield coils in one step. The machine builds up to 85 Duty 2000 lbs-5l0 feet per Minute Full load down

per cent of its final voltage in 1 4 seconds and in 2 '2 Armature Current

seconds is generating full voltage. Curve 2 shows the Generator Voltage -

effect of a short-circuited damper winding on the field Connections to Damper Windingclosed

poles. The damper winding has the effect of increasingthe time constant of the field at the early stage of build-ing up but does not materially affect the total time of

Tests on Gearless Traction ElevatorDuty 2000 lbs.-5O0 feet per MinuteFull load up

Armature Amperes

Generator Voltage

Curs-esshowing the rate of building up of generator voltage. The curves Connections to Damper Winding closedStandin g Field Current

<00

Tests on, Gearless Traction Elevator

Duty 20011 lbs.-500 feet per Minute Balanced Carp

6istance~~~~ ~ECND Aratr Amperesr wndng

Curvesn shoin theratetobicldnupeofti hrceitco h silgahwnrauecretgenerator voltage. ThecuvetonetinntdameiWninngte

field coils can be made in giving the proper shape to the field current.acceleration curve. may be used. For d-c. motors a counter e. In. f.

3. Driving Motor of the Motor Generator Set. The starter with two or three steps of light duty startingdriving motor of the set may be either a-c. or d-c. de- resistance is used. The only special feature required


is a counter e. in. f. relay interlocking with the There is only one contactor carrying armature circuitelevator controller to prevent energizing the generator current. This contactor is included as an additionalfield and brake circuits when the motor generator set safety measure and in normal operation opens only afteris not running or before it has come up to speed. the car has completely stopped and the brake has set.For the a-c. sets a resistance type starter is used. It also opens immediately when the safety devices, suchAlternating-current sets have a direct-connected exciter as the car emergency switch, overspeed governor, etc.,and a switch controlled by the voltage of the exciter is operate. In normal operation no current is rupturedused to short circuit a step of primary resistance when by this contactor. The other contacts on the con-the motor has come up to speed. No interlocking re- troller handle only field, brake and control circuitlays are required since there will be no voltage to currents. When the armature circuit contactor opensenergize the d-c. circuits unless the motor generator set its back contacts close a low-resistance dynamic-

braking circuit for the elevator motor. This contactorB a Balanced Car up with DamperWinding closed has auxiliary contacts through which the brakecoil

Armature Current-=Extra Field -~~~~~~~~~~~~~ Normal Accet Rheotafic Control

Standing Field F aArmature Current Motor Yoltage

FIG. 5Oscillogram showing armature current and current in extra and stand-

ing motor flelds.Rheestatic Control

is running. Reverse phase protection is inherent in the Normal Accelcombination without the use of relays because the FulLoad- donexciter being a self-excited machine will not build upits voltage if the motor generator set is started in the ESotorVotage Eleo. Speedwrong direction.

5. Elev)ator Controller. The elevator controller Rheostatir Controlproper carries the contactors and relays to perform Normal Accel

I :~~~~~~~~~O~~~~~ ~ Bat. Loud -downthe following primary functions:

1. Connect the generator field to the excitationI Eles. Speed

Motor Voltage Motor Current

10 - Prce'toy FluxRheostatic C-ontrol p

V ~~~~~~~~~~~~~~~~~~~~~~NormalAccel,60Dt0 X4f 0 t] Ba1 L- -p Motor Current MotorVoltage

X o _ L _ _ > _ _ _ ] 8 _ _ _ > ~Accelerating characteristics of a gearlooss traction elevator with rheo-

o~~~ T o2rCCCje -t static control.

0 l. 2.n3.0 .0 50 6. 7.08.0current passes so that the brake cannot be released

FIG. 5A unless the armature circuit is closed.Curvres showinb mot.or field flux and torque.

voltage in the proper direction for up or down motion of III CHARACTERISTICS AND OPERATIONthe car. 1. Acceleraltion and Retardation. To obtain maxi-

2. Energize the brake coil. The brake coil is mum service from an elevJator car it should be acceler-opened on both sides and is fed through the same con- ated and retarded at as high a rate as possible with-tactors that energize the generator field. out discomfort to passengers. The uncomfortable

3. Control the car speed by controlling the excita- feeling frequently experienced while riding in high-tion of the generator field. speed elevators is not due to a high rate of acceleration

4. Provide the proper discharge circuits for the but to rapid changes in the rate of acceleration. There-generator field during retardation. fore the rate of acceleration should not change sud-

5. Connect in an extra demagnetizing field on the denly during the accelerating period.generator in stopping. When a fully loaded car is to be accelerated in the

6. Open the connection between the generator and down direction the weight of the unbalanced load aloneelevator motor afterthe car has cometo rest. is sufficient to accelerate the car. Under a system of

7. Control the setting of the brake by means of its rheostatic control, if the motor is now connected to theown self induction and so obtain a smooth stop. line with a resistance in series with its armature, the


voltage impressed on the armature will be reduced only ing a smooth stop. It does not help matters much toby a flow of positive current in the resistance current have the retardation perfectly smooth if the final stopflowing to the motor from the line being regarded as produced by the brake is rough or if the car slides apositive; current returned from the motor, acting as a considerable distance after the brake sets. Electricalgenerator, to the line, as negative. This current braking will reduce the speed of the car to a low valueif it passes through the motor armature would but except under certain load conditions will not bringincrease the accelerating torque to a value which would it to a complete stop. It is, therefore, only necessarybe very uncomfortable to passengers. It is therefore to apply the brake in such a manner that an abruptcustomary to connect resistances in parallel with the change in the rate of retardation is not produced.motor armature so that a voltage drop can be produced The distance that the car will slide when the frictionin the armature series resistance and retard the accelera- brake sets is given by the formula:tion. This is very uneconomical and accomplishes the 2desired result imperfectly so that it is quite a common d = (K, Ml + K2 M2) VTexperience to ride in cars which give one the sensation TB+(Lof falling when descending heavily loaded. See appendix for symbols.The acceleration is considerably affected by the Equation 1 shows that the slide of the car when the

load and there isa considerable difference in the shape of brake sets varies as the square of the speed and inverselythe speed-time curves obtained with different loads. as the net torque exerted by the brake. The net torqueWith full load up the car accelerates too slowly and developed by the brake is the algebraic sum of TLwith full load down the acceleration increases too and TB. The torque of the load TL assists the brakerapidly for comfort. The rate of acceleration increases to stop when the load is positive or is being hoisted.very rapidly when the motor is first connected to the It acts against the brake when the load is being loweredline and when each step of resistance is short-circuited. and increases the slide. The velocity of the car atWith variable voltage control it is possible to obtain the time when electrical braking becomes ineffective,

more uniform accelerating and retarding charac- however, has the greatest effect on the distance the carteristics under widely varying load conditions. The will slide through the brake since the slide varies as thegenerator voltage builds up uniformly, following a fixed square of the velocity. It is evident therefore that forlaw and the car speed follows the generator voltage. minimum slide the car speed should be as low as pos-The torque developed by the motor may be either posi- sible before the friction brake is called upon to stop it.tive or negative depending upon the load, but the speed To avoid a jar when the final stop is made therein all cases depends upon the voltages generated by the should be a smooth transition from the retardationmachines. produced by electrical braking to that developed by the

If the car is to be stopped from high speed in a friction brake. To accomplish this result it is necessaryminimum of time, the maximum rate of retardation to apply the friction brake gradually so that full brakingmust be maintained during nearly the whole retard- torque is not exerted instantly. The brake must being period. When the controller handle is moved to designed and adjusted so as to stop the car smoothlythe low-speed point resistance is connected in series both when stopping from full speed and when stoppingand parallel with the generator field. The field shunt from creeping speed.or discharge resistance can be adjusted if the generator When stopping from full speed the brake should notfield has the proper time constant, so as to give the car set until the car has been slowed down to a low speedexactly the right rate of retardation. As the field by electrical braking. When stopping from low speeddies down the generator voltage falls below that of the the brake should set in a very short time after the con-elevator motor and regenerative braking brings the car trol handle is thrown to the off position but the torqueto a low speed. The generator voltage falls with should build up gradually so as not to produce a jar.absolute smoothness and there are no steps as in the The most practical means of controlling the setting ofcase of rheostatic control. The application of dynamic a d-c. brake is to utilize the self induction of the brakebraking in stepsproducespeaksin the rate of retardation coil in such a way that the flux is maintained in thethat tend to cause the cables to slip so that the average magnet cores and retards or opposes the action of therate of retardation must be kept well below the peaks. brake spring in developing braking torque.The absence of these peaks makes it possible to keep With rheostatic control in which the motor armnaturethe average rate of retardation very high. is disconnected from the line in stopping it is difficult to

Electrical or regenerative braking is used to bring obtain good brake action and very careful adjustmentthe car to a very low speed from which the final stop must be maintained. If the time element of the brakecan be made with a friction brake. The transition is made long enough to make the action smooth withfrom electrical to friction braking is perfectly smooth light loads the slide will be excessive with full load downas a result of the characteristics of the electrical system and loads in the up direction may drop back severaland the method of controlling the friction brake. inches before the brake can take hold and bring the carThe friction brake is a very important factor in mak- to rest. With variable voltage control the armature

204 BOUTON-: VARIABLE VOLTAGE CONTROL SYSTEMS Transactions A. I. E. E.

circuit is not opened immediately in stopping. When The retardation curves have the same generalthe controller handle is moved to the off position the characteristics as the acceleration curves. The timegenerator shunt field is disconnected from the line and required to stop from full speed is very nearly the samethe generator voltage becomes practically zero. Since for all loads. In practise this uniform retardation underthe line between the generator and motor armature is all load conditions makes it possible for car operatorsnot broken, sufficient current will circulate to hold the to gage the stops very accurately in stopping from highcar firmly under control while the brake sets. The speed and so reduce the number of false stops and timecontrol of the flux in the brake core is obtained with lost in making landings. A dotted portion of thesuitable contacts on the control panel and on the brake retardation curves is added to show the effect uponitself. After the brake has set and the car stopped, the retardation of increasing the total field by the additioncontactor in the armature circuit is opened to eliminate of the extra field.circulating currents that might be produced by residual Fig. 6 shows the results of tests similar to those shownmagnetism of the generator. in Figs. 4 and 5 but made on a gearless traction eleva-

Tests were made on a gearless traction elevator rated tor with rheostatic control. The curve of armature2000 lb. at 500 ft. per min., equipped with variable current shows notches as the steps of the startingvoltage control. The motor generator set consisted of resistance are short-circuited. These changes in thean a-e. driving motor, a variable voltage generator, and armature current correspond to changes in motor torquea direct-connected exciter. The tests were made with and in the rate of acceleration. The same notches arethe generator voltage adjusted to give a car speed of apparent as the dynamic braking resistance is short-550 ft. per min. with balanced load. The car was circuited. These rapid changes in the rate of accelera-started by moving the master controller directly to tion and retardation make it difficult to obtain smooththe full-speed position in starting and stopped bymoving directly to the off position. Fig. 4 shows thespeed-time curves obtained on these tests. The curves 8o -- ---were drawn by a graphic meter which recorded the -_ -voltage generated by a magnet driven from the shaft E60C 1-1 1,tt --Kof the elevator motor. L -_ I- _

Fig. 4A shows oscillograph records of armature z 40 - - -I -Icurrent, generator voltage, and current in the standing - - - l - -field of the motor. The motor also has an extra or 220_.starting field which is connected to the line for starting. 2 7This increases the total field during the first part of the 10 120 80 40 0 40 80 120Oaccelerating period, giving a high starting torque. PERCENT OF TORQUEDuring the last part of the accelerating period, the FIG. 7generator field iS overexcited, in order to maintain a Speed torque7e"curves of gearless traction elevator motor at full speedhigh rate of acceleration. The current in the standing and at low speed with variable voltage control.field shows rapid changes in value, but due to its selfinductance and to the mutual inductance between this and comfortable operation. The speed time curves arefield and the extra starting field the resultant change in less uniform than those shown in Fig. 4 and show moreflux is small and takes place quite slowly. Fig. 5 shows variation in time for different loads.the currents in the two motor fields and Fig. 5A shows Experience has proved that variable voltage controlthe resultant flux and motor torque. The action of is smoother in operation than the older system and thatthese two fields is discussed more fully later. passengers can be handled more comfortably andThe torque developed by the motor is proportional to quickly.

the product of the armature current times the field flux. An analysis of the test data shows the followingAs shown by the curves the motor current and torque reasons for these results:increase at a uniform rate during the first half of the 1. The rate of acceleration increases uniformly asaccelerating period and decrease at a uniform rate the car comes up to speed and is not subject to rapidduring the last half. The rate of change of acceleration changes such as are produced by increasing the arma-is proportional to the rate of change of motor torque ture current in steps.and is determined by the tangent or slope of the curve. 2. The time required to reach full speed is reduced.The speed time curves show that the rate of accelera- 3. The accelerating time and retardation time istion increases uniformly as the car starts and reaches a constant for all conditions of load.maximum at approximately one half speed. Above 2. Regulation and Speed Control. The speed athalf speed the rate of acceleration decreases uniformly which elevator cars can be run depends very largelyas the car comes up to speed. The time required to upon having a system of control that will give a positivereach full speed is less than two and one half seconds low speed for making landings which does not fall offand is practically the same for all loads. appreciably under load. Fig. 7 shows a test on an


equipment operating on reduced generator voltage. motor down to 50rev. per min. The current buildsThe speed is reduced to 1/10 of full speed at no-load up quite rapidly in the A field but the current in the Band only falls slightly below this value when hoisting field is reduced by the mutual inductance between thea load. With overhauling load the speed remains low. field coils.This flat regulation at speeds as low as 1/10 of full speed Fig. 9 shows derived curves of the combined amperemakes it possible for the operator to have complete turns of both field coils and the resultant field flux.control of the car under all loads. Curve No. 4 shows that it requires approximately 3½

In elevator work it is entirely practicable to operate seconds for the field flux to reach its final value andthe motor over this speed range, or greater, by controlof the generator field. Fig. 3 shows the speed at whichthe generator fields can be changed. The generator fieldcan be varied over such a wide range that very little fieldcontrol on the motor is necessary. Motor field control,

Zore Une of Current in 'A" Field Zem Uine of 25 Cycle Timing Wave,l........ SI_~.11F I ... -I 111i _rI.......

FIG. 8

Oscillograph records showing the rate of change of field currents of alow-speed elevator motor. ~"''~aw

FIG. 10l°° --

| | | | | & | | | |Photograph of gearless traction elevator motor on which the tests- t4-- -4 - - --4 shown in Fig. 8 were taken.90

73 change the motor speed from 65 to 50 rev. per min.,10O- 751 - - - - ] a In practise an elevator must be retarded and stopped

-r.|1/1 1 1 1 iS 9 in from 112 to 2 seconds so it is quite obvious that the~80 60- - -01L -01 -;/- - - - | | | [C!>full-speed range of this machine cannot be utilized.

-'60451/l 21 1 1 1 1120=10z It is doubtful if a speed range greater than 10 or 15~60 45 .. 120150 zi'O~3C -. i' per cent is practicable for this class of elevator motor.

m 38O8l00o A machine of this type is illustrated in Fig. 10.Armature series and shunt resistance gives inherently>0 20 151' 40 50 o

O C0~~~~~~~~~i23~~~~~0 080- -- .w ~~~~~~~45 LCLx 0 1 2CNX3 xi °°w 0_ m

FIG. 9 E I IOurves derived from the oscillograph tests shown in Fig. 8. The curves z 40-| | | |

show the time required to change the motor speed by fleld control. 0

520L l -if used for any considerable speed range, requires a a: - I-very large motor. With large low-speed machines it is 0l

40I

40I

120

not practicable to use a very great range of motor field PERCENT OF TORQUEcontrol because of the slowness with which the field FIG. 11flux changes. Speed-torque curves of a gearless traction elevator motor with armature

Fig. 8 shows oscillograph records of the field control series and shunt resistance. The curves show the difficulty encounteredin controling thespeed by this method. The curves in Fig. 7 and Fig. 11

of a gearless traction elevator motor. The motor has are for the same motor.two field windings known as the running or B field andthe starting or A field. With both fields energized the poor speed regulation. For gearless traction equip-motor runs at 50 rev. per min. and with the starting ments it has been the practise to use a combination offield disconnected at 65 rev. per min. The curves in armature and motor field control. Fig. 11 shows theFig. 8 shows the current in the running or B field with general shape of speed torque curves of a motor withthe motor at full speed and the current in the starting series and shunt armature resistance. With a motoror A field when it is connected to the line to slow the having the armature resistance shown, when the no-load


speed is reduced to 25 per cent of the full-speed value (c) Running at reduced speed when the cars arethe motor torque is reduced to 70 per cent at stand ahead of their schedule. (With rheostatic control).still. This means that the motor will not have suffi- (d) Making the limit stops.cient torque to hoist full load on this controller point, (e) Long idle periods in which the standby loss inexcept by some automatic adjustment of the armature the motor field and motor generator sets uses up power.resistance. Considerable complication has been intro- (With variable voltage control).duced in the control to make it possible to lift full load The power required to drive the elevator at full speedto the top floor. With overhauling loads it is difficult is usually considerably less than that used up in theto make the speed low enough to obtain accurate stops. items enumerated above. The distribution of thepowerAt best the speed range is quite limited and large cur- losses will depend very greatly upon the system ofrents are drawn from the line when operating on the control used.low-speed points. Fig. 12 shows a typical cycle for a high-speed elevatorThe method of speed control by varying the generator with variable voltage control operating with the average

voltage shows considerably better speed regulation anda greater range than the other two methods. This isclearly shown by comparing the results shown in Fig. f- e a /-7 and Fig. 11. In practise this better control results in Inching

L Stop Running Accration

the following advantages for the variable voltage FIG. 12system. Power cycle of high-speed elevator with variable voltage control tested

1. It is much easier 'for the operator to land accu- with balanced load. The chart was made by a graphic wattmeter.rately at the floors and the number of false stops arereduced.

2. The car may be "inched"' to the floor level very So S un hon AclSlow Speed Running tn Acceleration

accurately and quickly in case the operator does not OMlnstop accurately the first time. FIG. 13

Power cycle of high-speed elevator with variable voltage control tested3. The limit stops can be made in a minimum dis- with load. The chart was made by a graphic wattmeter.

tance and without loss of time or over travel with allloads.

4. Elevators may be run at a higher speed than 51 - _ _ -is possible with rheostatic control. E I __ - _J

3. Efficiency and Power Consumption. The factors __4 -, __ _ _that affect the power consumption of an elevator are -_ --_so many and varied that it is extremely difficult to -predict what it will be for a new installation. The I- // -l __more important factors are as follows:

(a) Load and speed. 2_ _ _(b) Number of stops and starts per mile of travel. _ __(e) The number of miles per hour that the car makes ;IEEII

while in service. -o 50 100 150 200 250 300 350(d) The weight of the moving masses that must be FIG. 14

accelerated at each start. Energy consumption tests of gearless traction elevator with variable(e) Method of operation. voltage control. Duty 2000 pounds at 500 feet per minute.A large number of tests have been made in which the

cars are operated on a fixed schedule with different loads or balanced load. The power input is greatest duringand the power consumption measured. These tests are the accelerating period, and falls off to a very low valueuseful in making comparisons between different equip- as the car comes up to speed. During retardation thements but do not give accurate information for deter- power becomes negative and is returned to the line.mining the power consumption in actual service where The effect of inching the car is shown directly followingthe cycle of operationmaybe quite different. Different the retarding period. The maximum is only 25 peroperators handle the cars in different ways and this will cent of that required during acceleration. This doeshave an effect on the power consumption. In actual not mean that the elevator motor does not developoperation considerable power and time may be wasted sufficient torque to move the car promptly. On thein the following ways: contrary the current flowing in the generator and ele-

(a) Acceleration and retardation for every start vator motor circuit is high enough to develop full-loadand stop. torque or greater but since the generator voltage is

(b) Inching the car to correct for inaccurate stops quite low the actual power input is also low. Fig. 13at the landings. shows a similar cycle but with a loaded car and shows a


period of low-speed running. In this case the power at IV. SAFETY FEATURESlow speed is 13 of that required to run at full speed. Elevators have a number of devices which protectA series of tests were made on a gearless traction the passengers riding in the car. I shall discuss only

elevator having a duty of 2000 pounds at 500 feet per those that are affected by the control system.minute and equipped with variable voltage control. 1. Limit Stops. An elevator control system shouldFig. 14 shows the energy consumption in kilowatthoursper car mile plotted against stops per car mile. With be so designed that the car will be slowed down andbalanced car and 125 stops per mile the energy

finally stopped as it approaches the terminal landings.balancedptca and 125silowatthops per mile. The energy In order to be sure that the car platform will come flush

consumption ish2.2 kinlowatthours pr mle.Twertenefro with the top and bottom landings under all conditionscoelinsumpt sonicdsalhpoetknfm of load, the stop contact on the slow-down device mustthe line. not open until the car has reached the floor level. It

Fig. 15 shows a test made on an elevator having a is preferable that the point of cut-off be a few inchescapacity of 2500 pounds at 600 feet per minute with beyond the floor level so that the operator will not formrheostatic control. Fig. 16 shows a similar test on the the habit of depending entirely upon the automaticsame motor with a smaller sheave and variable voltage device for making the stop. It is necessary then, whencontrol operating at 550 feet per minute. With bal- the car reaches the cut-off point, that it be slowed downanced load and 50 stops per mile the energy consumptioni 2 w h f o u nt to a low enough speed so as not to drift further than thestops per milewatttheurenergy con qumptonis.3.6iow distance provided for over travel. If the car runs bystops per mile the therib voltage ipment and the over-travel limit switch it is not possible, with thehours per mile for the variable voltageequip .ad Iusual connections, to back out and the assistance of an5.2 kilowatt hours for thze rhzeostatic. wh1ie thze two uu

attendant is necessary to put the car into service again.To stop a fully loaded car within the usual over-travel

-12[XIIXlXI4rl2 distance has always been a considerable problemwith rheostatic control because of the extremely poor

=,8 Xl<)o lb. Ne z < < 1 regulation of the motor when operating at reduced speedv) | t tt XX - - -with armature series and shunt resistance. The charac-

o 4 < < e>Lol d | t t 1 teristics of the machine operating under these conditionsrjIJI1IjI]I have already been discussed. Refer to Fig. 11.

0o) 50 100 150 200 250 300 350 A switch is mounted on the car having contactsSTOPS PER MILE actuated by an arm and roller which engages a cam inFIG. 15 the elevator shaft. As the car approaches the top and

Energy consumption tests of gearless traction elevator with rheostatic bottom landings these contacts operate to insert stepscontrol. Duty 2500 pounds at 600 feet perminute. of resistance in the generator field, in the variable voltage

motors are not working under exactly the same condi- system, which reduces the voltage. The car speedtions the slope of the curves gives a fairly good com- reduces with the reduction of voltage due to the regener-parison between the two systems of control. ative braking that takes place until, as the landing isFrom an analysis of the load curves and the test data approached, the car is running at very low speed.

weecan draw the following conclusions: The inherent regulation of the variable voltage1. The variable voltage system requires less power system makes it possible to slow down a fully loaded

for acceleration and retardation because of the ab- carin almostthe same distance as an emptycar. Itissence of rheostatic losses and because of the power re- possible to stop the car at the terminal landings underturned to the line during retardation. all conditions of load within one inch of the same

2. Very little power is taken from the line to make spot. The positive action of the automatic slowdownsmall movements of the car due to the low generator at the limits of travel greatly increases the safetyvoltage. of operation. The automatic slowdown and stop at the

3. Low-speed running does not increase the power terminal landings is one of the elements in the systemconsumption. of safety devices provided for the elevator and it is

4. Power is returned to the line while making the important that it function properly.limit stops. 2. Emergency Stop. When making normal stops

5. The greatestgain ineconomy willbeshowvnwhere with the regular controlling means such as the carthe number of starts and stops is large. master switch or the automatic limit stops, the genera-

6. Long rest periods will cause waste of power if tor field is reduced to zero which sets up regenerativethe motor generator sets are left running. This waste braking in the elevator motor which slows it down. Atcan be eliminated by shutting down the motor generator the same time the friction brake sets which brings theset through a start and stop push button station which car to rest. To provide against failure of the mastermay be located at a point accessible to the starter or switch contacts to function or a failure of the regularmounted in the car. stopping cycle the car is equipped with the usual


emergency switch. This switch opens the armature excitation. In some cases the motors have beencircuit contactor which disconnects the elevator motor equipped with special fields for this purpose. Suchentirely from the generator and connects it to a low- schemes are only partially successful and the principalresistance dynamic braking circuit. The coils to the difficulty is to initiate the functioning of the controlbrake contactors are opened and the brake circuit itself before the car has reached considerable overspeed.is also opened by auxiliary contacts on the armature In special cases the control circuits have been connectedcontactor. This arrangement gives two entirely inde- through auxiliary contacts on the circuit breaker.pendent means of stopping the car and is an advan- Alternating-current squirrel-cage elevator motorstage over the older forms of control in which the emer- that use a separate winding which must be excited fromgency switch forms only an additional means of opening the supply line have no means of obtaining regenerativethe same contactors that disconnect the motor in braking when power fails and the friction brake aloneregular operation. must stop the car.

3. Failure of Power. The failure of the main power Consider next the case of an elevator motor drivensupply on a large system is a comparatively rare through an motor generator set with an a-c. drivingmotoroccurrence, but failure of power from some cause to and having either one or two generators. The motorelevator motors has been a rather frequent source of generator set comprises a small direct-connected, self-trouble. Failure of power to an elevator may be due to excited generator to furnish d-c. excitation to the(1) failure of the main power supply to the building, generator and motor fields, brake and control circuits.(2) the opening of the main circuit breaker in the In case of overhauling load the elevator motor is actingbuilding, (3) opening of the circuit breaker or blowing as a generator, the generator end of the motor generatora fuse in the line feeding the elevator. I shall analyze set is acting as a motor, the motor of the set acts asthe result of a power failure under several conditionsand discuss means for taking care of the condition. F -When the motor is operating under positive load, s 8 ----4

that is when hoisting an unbalanced load, there is no I - ---<---<particular danger from power failure as the car will 6

stop of its own accord in a comparatively short distance. U'|--The condition of balanced or overhauling load presents O -4- -a more serious problem. Let us consider first the caseof overhauling load produced by a fully loaded car 2going down. Consider first the case of a d-c. motor ..controlled by armature series and shunt resistance and %- -i-5 j2-30 50 100 150 200 250 300motor field control. With overhauling load the motor STOPS PER MILEis operating as a generator and returning power to the FIG. 16line and is kept at constant speed by the supply voltage. Energy consumption tests of gearless traction elevator with variable

The contactor magnets on the controller are energized voltage control. Duty 2500 pounds at 550 feet per minute.from the supply voltage. If now the power source iscut off by an opening of the circuit breaker in the line an induction generator and is running above synchron-to the motor, the reservoir into which the motor has ous speed. It should be kept in mind thatthe elevatorbeen delivering its power will have been disconnected motor is generating a voltage higher than its terminaland the motor and car will overspeed. There will be voltage due to the drop in the windings. For the sameno warning to the operator that the power has failed reason the generator, acting now as a motor, is gen-until the car has reached a considerable overspeed, erating a counter voltage lower than its terminal volt-probably high enough to trip the safety clamps under age by the value of its internal drops.the car. The control magnets will be kept energized When the circuit breaker opens the energy that isby the voltage generated in the armature of the elevator being developed by the descending car is expended inmotor so that the dynamic braking circuits will not be accelerating the motor generator set which speeds upset up or the friction brake disconnected until the over- until the generator voltage equals the voltage gener-speed governor driven by the car has opened its con- ated by the elevator motor, the internal drops reduc-tacts. In other words, there is nothing to start the ing to a very low value. The result will be an over-functioning of the control to stop the car until there speed on the set of 15 or 20 per cent above syn-has been an actual overspeeding of the car. After the chronous speed. The motor generator set is equippedcontrol has functioned to stop the car the friction brake with an overspeed device which opens a contact inwill be called upon to make most of the stop since the the safety circuit of the elevator controller that setselectrical braking will be rendered more or less ineffect- the friction brake and sets up the emergency dy-ive due to the dying field of the elevator motor. Con- namic braking circuit. The motor generator set istrol schemes have been developed to connect the field now runningfree at overspeed and due to its inertia runsof the motor to the armature so as to maintain the for several seconds before the direct-connected exciter


loses its voltage. The field of the elevator motor is their appearance, in actual operation. Bearings, oilingconnected directly to the exciter so that a strong field systems and general mechanical design have beenis maintained for dynamic braking long enough to developed to a high degree of perfection.bring the car completely to rest before the exciter Elevator controllers have in recent years reached aloses its voltage. The same cycle is performed when high state of development but there is one inherenthe driving motor of the set is a d-c. machine. The condition that has made the maintenance problem moreonly difference is in the connections of the field of the difficult. I refer now to the rupturingofheavycurrentselevator motor which are such that the counter e. m. f. in starting and stopping the motor and in performingof the driving motor maintains the excitation on the other control functions such as changing speeds. Aelevator motor field. Tests have shown that the car contactor in elevator service is rupturing heavy cur-is stopped promptly and without over travel when a rents continuously and the arc produced when thepower failure occurs while making the limit stops. circuit is opened is bound to burn away the contacts.

4. Overspeed. The cars are equipped with the Considerable improvement has been made in recentusual overspeed governor which performs the usual years in the method of rupturing arcs, such as the usefunctions of first slowing down the car by field control, of arc splitters, and improvements in the design of theopening the safety circuit and finally setting the wedge magnetic blowout. The use of rolling contacts hasclamp safety under the car. The overspeed contact on considerably improved the life of the contacts them-the motor generator set acts as an additional factor of selves.safety as it opens the safety circuit independently of It is quite probable that these details are now prettythe speed governor. It can be set to open its contacts well standardized and that little improvement can bebefore the governor trips and so stop the car entirely expected along this line. The next step is to eliminateby means of the electrical and friction brakes, saving the rupturing of major currents altogether and tothe inconvenience and loss of time in releasing the control them indirectly. This is accomplished bywedge clamp and resetting the governor before the car switching only the field current of the generator, which,can be moved to the nearest landing. as its voltage changes, controls the elevator motor.

The equipment can be so designed that the maximumIV. MAINTENANCE current ruptured by the controller is not more than

The upkeep of equipment is one of the factors that five amperes. From the standpoint of maintenancego to make up the cost of operating electric elevators. this results not only in an increased life of the contactsLarge buildings usually employ someone who is com- themselves but in reduced cost of renewal parts, sincepetent to care for and maintain the elevator equip- the contactors are much smaller in size than if they hadments. Such buildings usually have excellent elevator to handle the motor current directly. The eliminationservice and experience little trouble from shutdowns and of heavy arcs reduces the burning of other parts and thedelays. At the other extreme we find buildings in which chances of damaging flashovers. The acceleration andno regular help is employed for this purpose and ele- retardation is governed by the characteristics of thevator maintenance consists of periodic inspection for generator field and does not depend to a great extentwhich service contracts are often let. on adjustment of relays and contactors so that the con-Maintenance of the electrical equipment consists troller requires very little attention to keep it in good

mainly of inspection, cleaning, making adjustments, operating condition.oiling and installing renewal parts. The first three Another item that has an effect upon the maintenanceitems involve labor costs only, while the last two items of elevators is the wear and tear on machinery incidentalinvolve both labor and material. It should be the to severe service. The life of the cables and the weardesire of every building owner to purchase equipment on gears are both affected by the control equipment.on which he can reduce the last three of these items to a High-speed geared elevators develop backlash in theminimum. gears and this is aggravated by the notching of theThe art of designing rotating machinery has advanced ordinary controller. Sharp peaks in the retardation

to a point where maintenance is merely a matter of torque have a tendency to cause slippage of the cablesroutine. Elevator machinery is usually installed in and reduce their life. Fig. 5 shows the smoothness ofcomparatively clean places so that insulation trouble is acceleration and retardation and the absence of abruptcomparatively rare. The elevator cycle is such that changes in motor torque. This smoothness will resultmachines which are designed to have the proper in longer life and less maintenance cost of the me-operating characteristics seldom reach, in actual chanical equipment.service, the maximum temperature rise which they are .CNLSOgnaranteed to stand. The use of commutating polesV.CNLSOhas made it possible to design high-speed d-c. machines In the foregoing part of this paper I have attemptedthat will commutate heavy-current peaks with practi- to describe the variable voltage system of control ascally no sparking. The present accepted standard of applied to electric elevators and to set forth thecommutation is that the commutators shall improve characteristics and operation obtained. In conclusion


I shall summarize its principal advantages for elevator such as switching operations in the main circuits. In this re-work as compared with older systems. spect, the system described appears to be ideal, the switching

1. Elevators running at the highest speeds may be operations of the main circuit having been reduced to a minimum.1.ivenfromElevatorsrunnling s

at the high estcspees mtay The advantages of having the acceleration controlled by a con-driven from a-c. supply lines of any commercial voltage tinuous change in the generator field flux are also clearly setand frequency. forth in the paper as being smoothness of operation and reduced

2. The acceleration and retardation are perfectly maintenance cost.smooth and uniform. However, while the system is thus seen to possess some very

3. A high rate of acceleration and retardation is marked desirable features, there are some drawbacks. The total3ta whigh prat toaceler on and retardao n machine capacity required is more than three times that requiredobtained which permits the use of high car speeds and for the elevators themselves. This, of course, refers to capacity

increased elevator capacities. and not to physical dimensions, as the motor-generator set is4. Positive speed control under all conditions of load undoubtedly relatively much smaller than the elevator motor,

makes it easy for operators to land the car accurately being operated at much higher speed. But even at best, thewithout loss of time. necessary additional machine capacity is undeniably a seriousdrawback. Furthermore, the losses in the additional machines,5. High economy of power is obtained. especially the standby losses, offset to a considerable extent the

6. A number of inherent safety features make it efficiency gained during acceleration and retardation.particularly suitable for high car speeds. It, therefore, becomes of interest to determine whether or not

7. Maintenance costs are reduced to a minimum it is possible to devise a system which will materially mitigateand the reliability of service increased because of the these drawbacks and at the same time retain the most desirableandlcth ab .o.. features. I believe this is possible, and will briefly describe asimplicity and flexiblity of the system and because no proposed system and submit it for comments and criticism. Thelarge currents are ruptured in operation. system is shown as applied to direct current only, the conversion

being accomplished by other means when the supply is alternating

Appendix current. It will first be described in combination with a three-wire supply line as the connections are then simplified somewhat.

Symbols used in equation (1).M1 = Moment of car counter weights and other

moving parts.M2 = Moment of motor armature.V = Velocity of the car.TB = Torque developed by the friction brake.TL = Torque developed by the load.

K1, K2 = Constant.

Il/li1I1111 Xy eel

Discussion FIG. 1K. L. Hansen: Because of the fact that the inherent speed-

torque characteristic of the induction motor makes it less suitable For the sake of simplicity the diagrams show only the distinguish-than the direct-current motor to applications where masses of ing features, omitting those common to other systems, such asconsiderable inertia have to be accelerated and retarded at fre- the main disconnecting switches, the circuits energizing thequent intervals, the application of alternating-current power to magnet switches, the field circuit of the main motor, etc.certain classes of elevators presented a formidable problem. It Referring to Fig. 1, it will be seen that the system employs anappears that the system of control described in Mr. Bouton's auxiliary motor, with a flywheel to increase the inertia of itsinteresting paper was first thought of in connection with elevators revolving parts. In the standstill position switches S1, S3 andas a solution to this problem. S4 are open and switch S2 is closed. The auxiliary motor is

However, it is evident from the paper that, even when con- running at normal speed between neutral and negative line. Thesidered purely from the standpoint of control, the system must auxiliary motor starter, being of the usual type, has been omittedhave shown some very desirable characteristics in operation, as from the diagram.it has frequently been extended to cases where conversion from To start the elevator S2 is open and S3 or S4 (according toalternating to direct current is not the object, that is to elevators whether the motion is up or down) is closed, thus inserting theon d-c. power lines. main motor in series with the auxiliary motor between neutral

It has long been recognized, but has of late become much more and negative line. Continued movement of the controller handleforcibly impressed upon us, that in motor applications where the weakens the auxiliary motor field, and as the inertia prevents itsstarts and stops are frequent, control of acceleration by means of armature from speeding up rapidly, sufficient current flows toresistors in the armature circuit is extremely wasteful of energy, acceelerate the main motor armature.and there is consequently a general tendency to devise more When the auxiliary motor field has been reduced to zero theeconomical methods of control in suchcases. Application ofthe main motor is running between neutral and negative line aUtsystem described in the paper to elevaJtors where d-c. power is approximately half voltage and speed. Up to this point thea.vailable is obviously in line with the general trend towards auxiliary motor has absorbed energy, its e. mn. f. being in opposi-greater economy. tion to the line voltage and its speed has therefore increased

Another general trend in the evolution of control apparatus is somewhat. Further movement of the controller handle reversesto control transient conditions, such as acceleration and re- the auxiliary motor field and gradually strengthens it. Thetardation of motors by continuous electromagnetic changes auxiliary motor now acts as genera.tor, and by reducing its speed,inherent in the machines, rather thanl by mechanical changes, gives up the energy which was previously stored. Its iaduced


e. m. f. is now in the same direction as the line voltage and there- after the torques due to the brake, bearing friction, bending offore boosting this, so that the voitage impressed on the main cables, and sliding friction have been subtracted. This differencemotor continues to increase. is represented by curve 2 in Fig. 3 shown herewith, and it is theWhen the auxiliary motor field reaches full strength in the true net torque used only for acceleration. The peak of this

opposite direction its speed will have been reduced to approxi- curve comes at the same time at which the maximum rate ofmately normal and its voltage added to the line voltage is approxi- acceleration occurs in Mr. Bouton's Fig. 4 for balanced load, i. .,emately equal to the voltage between the outside main lines. The where the slope of that curve is steepest. Curve 1 shows thevoltage of the main motor, being of approximate equality to the car speed as taken from balanced load curve in Fig. 4. Assumingmain line voltage, switch S1 can be closed, making each machine that this acceleration curve is correct, we derive Curve 2, whichrun independently of the other, the main motor between the is the net accelerating torque, in the following way.outside lines and the auxiliary motor between neutral and Pick out some point, say at 112 seconds, where we may assumepositive line. The switch S1 is so connected and adjusted that that the brake and static friction have reduced to zero, and drawit closes only when the controller handle is in running position a tangent as accurately as possible to the acceleration Curve 1.and the main motor voltage bears a certain ratio to the linevoltage.To stop the elevator the operations are reversed by moving the

controller to standstill position. During retardation theauxiliary motor voltage adjusts itself so that a regenerative eur-rent flows and a breaking torque is produced practically down tostandstill.

It is obvious that the rate of acceleration and retardationfollows a fixed law, precisely as in the system described by Mr.

e o

Bouton, if the controller is moved at once to the full running AlMoorposition in starting and to the off position in stopping. Equally, _______the rate of acceleration may be regulated by varying the resist-ance of a damper winding on the auxiliary motor field. Further- FIG. 2more, the main current adjusts itself to retain approximatelythis rate under widely varying load conditions from largePositive Next take the total motor torque at 1Y seconds and from itto negative or overhauling loads. subtract the running friction torque of 460 lb. as obtained fromThe acceleration and retardation curves and the speed-time a total torque curve (not shown) at some point corresponding to

curves when running at low speed can all be determined by 3 or 4 seconds. The difference will be 2200 lb. which is the netmathematical analysis, but cannot be included in this brief torque for the point on acceleration curve corresponding todiscussion. 1½Y seconds.The current rating of the auxiliary motor depends on the We now have the rate of acceleration, and the torque which

relative amount of time consumed in acceleration and retarda- produces that rate. The unknown quantity is the equivalenttion and full speed running. At most it is equal to the current mass having linear velocity. This we obtain from the simplerating of the main motor, and as it is wound for one-half voltage formula-its capacity is at most one-half that of the main motor. In case T 32of d-c. power line the additional machine capacity is therefore M = X .2less than one-fourth of that required with the control described a X Rin the paper. The losses, especially the standby losses, are, of Where, M = lb. mass having linear velocity.course, correspondingly reduced. T = torque in lb. at 1 ft. radiusEven when the supply is alternating current, the additional

machine capacity and the losses can be materially reduced byusing the auxiliary motor control and a synchronous converter 6000 600for conversion from a-c. to d-c. current. 5000 _ Ca S 500 K

Fig. 2 shows the principal connections when the supply is a L_ 4000 400two-wire d-c. power line. In this case the main motor has two

30003O00armature windings and two commutators connected in series Net Accelerating-Torque zduring acceleration and retardation and in parallel when running z 2000 r e & Stati and Running Friction 200

at full speed. The switches Si and S2 are double-pole. Ss' and 1000 expressed inft. lbs.Torque 100S4' always open and close simultaneously with switches Ss <

and S, respectively. With these modifications the sequence of 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0switching operations is the same as before and the operation Is TIME IN SECONDS

essentially the same as in the case of the three-wire system FIG. 3already discussed.

E. M. Claytor: Referring to Fig. 5A in Mr. Bouton's paper, R = radius of driving sheave in ft.it may appea.r that the maximum rate of acceleration would occur a = rate of acceleration in ft. per sec. per sec.earlier than it actually does because the peak torque is reached and 32.2 = gravity accelera.tion constant.in a4 second after the start, while the maximum rate of accelera- The equivalent mass in this ease wa.s 11,350 lb.tion does not come until one second after zero time. The rate After determining the mass, we rearrange the same formulaof acceleration is proportional to the net torque available for above as follows-accelerating the total equiva.lent mass having lineaJr velocity. M aXRThe curve in Fig. 5A iS somewhat misleading in as much as it T _X 32.2is labeled "Torque of rope sheave" which might be taUken to 3.mean net torque on the ropes for acceleration. As a matter of By substituting known values of M, by previous calculation,fact this curve is the total torque aUt 1 ft. radius (developed by the R, by measurement, and a, by drawn tangents to the graphicmotor armature) divided by the radius of the driving sheave, test curve, we can solve for T which is the net torque Curve 2The net torque is what is left of the torque curve in Fig. 5A in the sketch.


Having determined that component of the total torque curve tion. The maximum acceleration is (7.5 ft./sec.)2 and thewhich is used only for accelerating the balanced car, we subtract maximum rate of change of acceleration is (6.4 feet/sec.)3. Asthat from the total torque curve and we have Curve 3 left. This the operation of the car is very comfortable with the conditionslatter curve represents all friction or anti-torque. We know from shown, higher accelerations can be used than these shown and itother oscillograph tests that the brake reduces its torque to zero is probable that with the variable-voltage system. of control thatin 0.6 second. This means that the static friction is about 2000 a car can be accelerated to 650 to 700 ft./min. in 212 secondsft-lb. at the start and it does not reduce to zero until a point without discomfort to the passengers. In this connection, how-corresponding to 1 2 seconds. This indicates that the driving ever, it should be mentioned that as the rates of acceleration aresheave and shaft have made more than a complete revolution increased other limiting factors appear in addition to the dis-before the static friction becomes zero. comfort felt by the passengers. For example, slipping of theThe dip in the friction Curve 3 cannot be explained except by cables on the driving sheave occurs if the retardation rate is too

considering that oscillations or hunting occurs between accelera- great. This decreases the life of the cables. Also largerting torque and friction torque, or else the graphic meter had equipment is required when these high rates are used, than isenough inertia in moving parts to overshoot the actual speed. required with the lower ones. This point becomes importantFrom the above analysis, we may draw the conclusion that especially when the elevator cab and other parts have a large(a) An appreciable amount of power is wasted in friction inertia.

during acceleration. The above considerations apply equally well to retardation(b) A quick-acting brake on the pick-up is desirable. of the elevator. However, the rates used in retardation can be(c) Anything done to reduce static friction will reduce the somewhat higher than those used in acceleration without causing

current and power peaks of the elevator motor. discomfort. The reason for this is probably that a greater senseW. F. Eames: In an effort to obtain what might be con- of security is felt because the car is slowing down. Although no

sidered a perfect acceleration curve for an elevator, a number of extended experiments have been tried it has been found that, if acurves have been analyzed, and although the results contain person shuts his eyes, much less discomfort is felt in an elevatorconsiderable speculation they are of interest in connection with using high rates of acceleration. If maximum rates are to bethe subject just discussed. It will be necessary first to define demanded in the future, it may become necessary to take stepsperfect acceleration and then to analyze the elements.Any remarks made in connection with acceleration apply in -2(x-2%0-00)56(560-Y) / Rlteof hangeof

600~ ~ ~ ~14--'j6 --Ve'Locity~general equally well to retardation, as retardation may be con- 6001sidered as negative acceleration. Consequently we will con- _ -a4sider only acceleration, and treat retardation as a special case.

IXfPerfect acceleration as applied to elevators might be defined Z , FIG.b4 \

as that which will bring an elevator up to speed in a given time 0/ - o 2 3without discomfort to the passengers. Assuming that the speed / Solid Curve-Actual Velocities SECONDStD 200 DotdCreTce Parabola -- ______of an elevator can be made to follow any curve in going from zero . - - - -speed to full speed, some method must be used to select the best X= o.0056Y - - oone. The problem of comfort to a passenger seems to be tied up 0 -l3 4- L - c.l - A-elhef1 2 3 Accelerationpsychologically in some measure with the idea of falling. That is, SECONDS _ - - -if the motion of the car is such as to suggest falling, a feeling of [FIG.4a, FIG. 4 cidiscomfort is produced. Falling is concerned with a high rate - [ - L - L 1 -of acceleration especially a high rate suddenly applied, which - -- 2 3corresponds to a high rate of change of acceleration. The con- SECONDStinuous curve that represents values between zero and a maxi- FIa. 4mum has necessarily an S shape and most of them can be repre-sented by some combination of the powers of X in the general to prevent the passenger seeing the floors passing. If this is doneequation y = funetion of x. it is likely that the accelerating rates can be raised as high as there

Several curves have been plotted from equations of the form will be any occasion to use.Velocity = K, t2 (Parabolic) M. A. Whiting: For the benefit of those who are not familiar

= K1 t2 - K2 t3 (cubic) with the term, nor with the system which it covers, it may be= K1 t2 - K2 t4 + K3 t6 (cosine) well to explain that the system described by the name "variable

and were all found to lie very close together, and to the eye voltage control" is not an entirely new system first developed toshowed no points where noticeable difference will be felt if an meet elevator conditions. The system thus referred to is, in itselevator were accelerated along any of them. Plotting the first fundamentals, the same system of motor control which is com-and second derivaties the parabolic curve showed the highest monly called generator-voltage control, generator-field controlaccelerating rate and the lowest rate of change of acceleration. or Ward-Leonard control. The comparative antiquity of thisThe cubic equation gave the opposite conditions and the cosine system is shown by the fact that it helped sink the Spanish fleetlay between. off Santiago de Cuba in 1898. Merely as an indication of the

Various observations indicate that the high rate of change extent to which this system is now used it may be mentioned that,of acceleration is more objectionable than a high rate of accelera- over a period of 16 years, one manufacturer has built, or is nowtion. If this is true the parabolic form is the most desirable of building, for steel-mill and mine-hoisting service alone, over 70the three functions. It was also found that the variable-voltage equipments totaling over 110,000 h. p. The correspondingelevator acceleration lies very close to the parabolic form at totals for the other manufacturers covering these items and abalanced car conditions and when lifting full load departs only complete list of other classes of service to which this systemn hasslightly from it. Fig. 4A herewith shows the velocity curve for been applied would also be impressive.the variable voltage elevator rated at 2000 pounds at 550 feet The new development presented in Mr. Bouton's paper is,per minute that has just been discussed by Mr. Bouton. The therefore, not the fundamental system of "variable-voltage" ordotted curve shown in the same figure is a pa.rabolic curve, that generator-voltage control itself but is rather the extensive ap-passes through the zero, mid-point, and full speed vaJlues of the plication of this system to elevators. The problem, or the ac-velocity curve. Figs. 4B and 4c. shows the first and second complishment, should not be belittled, however. Somne of thederivatives, or the acceleration and rate of change of accelera- previous practise in the application of generator voltage control


could be followed but some special problems were presented by thetical gearless elevator plotted to per cent unbalanced load inthe requirements of elevator service, and it has, therefore, been the car. The speed regulation (28 per cent rise from full loadnecessary to devise new practises. hoisting to full load overhauling) is assumed to be the same as thatOne of the principal problems is that of speed regulation over in the author's Fig. 7. The speed curve may be considered to be

the range of loads handled, which is discussed by the author bow-shaped, as in the author's Fig. 7, but I believe that a nearlyunder "Requirements of Elevator Service" and under "Charac- straight inclined line from A to B is equally typical, particularlyteristics and Operation." Attention is called particularly to the if an extra contact on the speed governor is not used to operateauthor's Fig. 7. The speed regulation shown on the low-speed within this speed range. Linie C-D represents the speed at whichcontroller point is much closer than is obtainable with any the overspeed governor will set the safety clamps. It is com-practicable rheostatic machine and provides a correspondingly monly considered that the margin of speed, B-C, between maxi-

mum overhauling speed and the final emergency operation of the9 r governor, should be not less than 15 per cent in terms of speed B.

c - -X -X-X - -x- X X Let us assume also that the elevator rides comfortably and can8 _ E be controlled without difficulty when overhauling at speed B.

Now if a speed as high as B is suitable in all respects when thus7 _ ~=- _.lowering maximum load, a speed as high as B is also equally

suitable when operating at balanced load or when hoisting full6 I_1_2load; in other words, if speed B is right, a speed curve B-E is

< 5 The advantage of a flat speed-regulation curve, as B-E, will begreatest on express elevators during the rush hour going in. If

4 [ _the load goes up at speed A, and the empty car comes down(against the overbalance of the counterweight) at speed F, the

3 average speed up and down will be only 85 per cent of the per-

20 (o 4o 2o l) 2o 4D fo 8o 1 700l100 8060 4020 0 2040O60 80 10 -__ __

OVERHAULING HOISTING -PER CENT UNBALANCED LOAD IN CAR 600

FIG. 5F 500 _ Iz

greater ease of control in making landings. However, let usexamine the author's Fig. 7 more closely. As the results are c 400C llgiven in per cent of motor torque, an interpretation is required. _ _

It is reasonable to assume that 100 per cent positive torque cor- 300responds to the hoisting of an elevator load not exceeding normailcapacity; then when lowering the same elevator load, the motor X 200torque will be not less than 60 per cent negative. The speed at100 per cent load is about 612 rev. per min. a,nd at 60 per cent 100negative load is about 11 2 rev. per min., or about a 75 per centrise in speed from full load "up," to the corresponding load con- 1

120 80 40 0 40 80 120 160700 OVERHAULING HOISTING

AMPERES

6008 1 - - _i_ _IFIG. 7

5001 missible speed B. Allowing about 30 per cent standing time andZ 70 per cent running time for rush-hour express operation, the

400 -X- loss of service per elevator due to the poorer regulation, compareda. t \ | l | [ lwith perfect regulation, is about 11 per cent.u-I 300 t t --< I IIn order to provide the maximum service possible from an

Li | l l l lequipment of a specified maximum overhauling speed, and alsoaw 2001 i l i 1in order to obtain further improvements over previous equip-

ments in ease of handling the elevator, an improvement in100 generator-voltage control has been developed by which regu-

0E tlations are obtained which approach closely the ideal which I1600 1200 800 400 0 400 800 1200 1600 have just described. I intend to prepare a technical paper in

OVERHAULING HOISTING the near future covering this development, but some of the re-L. UNBALANC""ED% LOAD INI CAR

FIG.6~~~~~~~suits are presented herewith.FIG. 6 ~~~~~~~Fig.6 herewith shows results obtained on the first commercialinstallation embodying this development, the elevator being

dition "down." If this speed regulation curve could be made rated 600 ft. per min., 2500 lb. live load and counterbalanced forflat at all loads, this would be found very useful in attaining aJ 40 per cent of the live load (maximum unbalance 1500 lb.). Onstill greater ease of control for making landings. the first or lowest speed point, the speeds at maximum loadOn the full-speed point, the regulation shown by the a.uthor's hoisting and maximum load overhauling are equal at 50 ft. per

Fig. 7 is from 64 rev, per mni. at 100 per cent load to 82 rev, per min., and the prevailing speed over the entire range of overha.ul-min. at 60 per cent overhauling load, or a rise of 28 per cent. ing loads is actually lower, as an aRverage, than over the range ofLet us conlsider further just what such aJ speed regulaJtion at full hoisting loads.speed means in the operation of an elevator. On this sa.me equipment, in Fig. 6, the full speeds at maximum

Fig. 5 herewith shows a speed regulation curve for a hype- load hoisting, at balance and at maximum load overhauling are


equal at 625 ft. per minute, and the overhauling speeds are lower, is not borne out by a careful consideration of all factors. Foron the average, than the hoisting speeds. example, assume a 20-story building having a hatch 7 ft x 6 ft.

Further developments of this system have been made and and the floor space renting for about $3.00 per sq. ft.; theembodied in equipments of more recent manufacture. A ship- elevator to cost $18,000; the actual charges then become on ament of three such equipments was made after only routine yearly basis:factory tests on the individual parts of the equipment; the Rentable floor space occupied by elevator ............ $2520machines were placed in successful commercial service promptly, Interest depreciation, insurance, etc., (15%).2700without difficulty and without personal assistance from head- Operator's salary....... 750quarters. Maintenance, etc.300At the first opportunity, extensive factory tests were made on Spare parts, re-roping, etc...... 350

an equipment in accordance with these further developments. Power bill (20 miles per day, 2.5 kw-hr. per car mile,Numerous proportions and adjustments were studied, and a 300 days per year and 2c. power . 300typical set of adjustments gave results as in Fig. 7 herewith.Since predictions may differ as to the mechanical losses in an Total....... $6920elevator installation beyond the traction-motor armature, the Thus, the power bill is the small part of the total yearly elevatorresults in Fig. 7 are given in amperes, as taken. If the me- bill, approximately 4.0 per cent.chanical efficiency is assumed as 80 per cent, the rated load of the Fig. 11 shows the regulation on a rheostatic-controlled elevatorelevator is represented by 160 amperes hoisting and 102 amperes motor. The regulation between 100 per cent motor torque andoverhauling. Or, on an assumed mechanical efficiency of 90 64 per cent generator torque (this assumes a mechanical effi-per cent, the corresponding loads will be 142 amperes hoisting cieney of 80 per cent) is approximately 63 to 83 rev. per min. orand 115 amperes overhauling. On the former basis, when hoist- 31.8 per cent. Tests on an elevator which has been installeding full load on the first speed point, the speed is 55 ft. per min. about 18 months, show the speed regulation (without centrifugaland when lowering full load, the speed is 67 ft. per min., an speed governor) to be as follows: 100 per cent motor torqueincrease of only 12 ft. per min. at full load overhauling. On the 64.5 r. p. m. Zero torque 62.0 r. p. m., and 64 per cent genera-basis of the higher mechanical efficiency, this regulation is evencloser. If, on this regulation curve a horizontal straight lineis drawn at 65 ft. per min., the speed at nearly all positive loadsis slightly above, and at nearly all negative loads is slightlybelow this average line. For all practical purposes, therefore,the speed regulation on the first speed point is flat over the entirerange of loads.The regulation on the full-speed point for the equipment in

Fig. 7 (assuming 80 per cent mechanical efficiency, which is theless favorable case) is from 615 ft. per min. at full load hoistingto 620 ft. per min. at full load overhauling. The maximum aspeed, 630 ft. per min., occurs at balanced load and various A

heavier loads going up. The significant speed regulation is 2 atherefore 1 per cent, from full load up to full load down. s

After this equipment (as in Fig. 7) is installed, results of its \operation in actual service will be presented in the paper whichI intend to prepare. In that connection, the system of controlwill be described and explained.

J. J. Matson: In Figs. 14 and 16 Mr. Bouton shows results FIG. 8 --TIiE GRAVITATION FIELD IN AN ACCELERATINGof tests to determinie elevator power consumption with various ELEVATOR CARloads and stops per car mile. As the curves are obtained fromtests the power consumption undoubtedly contains the motor- tor torque 68.5 r. p. m. This is regulation of 6.2 per cent.generator losses only during the period of test. Assuming this It is interesting to note that full-speed acceleration was obtainedis true, the results greatly favor the variable-voltage control in about 34 seconds and with perfect comfort to passengers.system. By this, I mean that if the power consumed, the stops Bassett Jones: I want to add something to Mr. Eames'made, the miles traveled and the average elevator load for one discussion of Mr. Bouton's paper. The fact that the mostday were measured and the kilowatt hours per car mile calculated, suitable time-velocity characteristic of a passenger elevatorthe result obtained would be higher than is given in Mr. Bouton's is a reversed parabola may be deduced directly from the me-curves. The reason is apparent if one stops to consider the actual chanics of the passenger's body. This I have already discussedcycle of operation for an elevator which consists of some time in "The Time-Velocity Characteristics of High Speed Passengerrunning, and some time standing (Includes time required for Elevators," General Electric Review, P. 111, February 1924.receiving and discharging passengers). During the standing In Fig. 8 shown herewith the mass, M, represents the mass oftime, which may be as high as 50 per cent of the total, the motor- the passenger's body. The spring, S, represents the elasticgenerator set will surely be running and taking power. This elements of his bodily frame. If the car be standing still orpower would, of course, be shown in the all-dayJ run but when a. traveling at any constant velocity, the force F1 acting on M andtest was run for power consumption, the running-idle losses of the causing a compression of S is the gravitational field of the earth.motor-genera,tor set wvould not be included as the elevator would If. the car accelerates on the up motion or retards on the downbe operated continuously until sufficient readings were taken to motion, a force, F2 = 711 a, is added to F1 and the spring isobtain the power-consumption values. The best wvay to com- further compressed. If the car accelerates on the down motionpare power consumption for variable-volta.ge and rheostatic or retards on the up motion, the force F2 is deducted from F1controlled elevators is on an all-day basis. Even under such and the spring expands.conditions, the va,riable-voltage control shows the lowest power Obviously if this force, F2, is aJpplied or removed suddenly asconsumption, the saving increasing as the stops permileincrease, by constant acceleration, the resulting shock to the passenger'sA great deal of importance has been attached to elevrator body will be uncomfortable. Therefore, it should change from

power consumption by all interested paUrties. In reality this zero to a maximum, and from a maximum to zero in some


gradual manner, and it seems reasonable to suppose that if this If, the ratio of comfort in the two cases is to be as 2:1, then,change be constant, the best conditions will result. also the corresponding values of p are 20 and 14.

This requires that the rate of change in acceleration (or retarda- Assume that Vm = 11 .66 ft. per sec. (700 ft. per min.) thention) be a constant, that is, from (4), and the values of p (20 and 14) given above, tm = 1.53

d a and 1. 3. The latter of these is 19 6 per cent greater than thed P, former.

Of course much of the above is theoretical in the sense that itwhere p may be called the physiological constant. Therefore, is based on an assumed simplicity in the mechanics of theif S be distance, passenger's body that it does possess. Also, it requires a form

d3 S of torque-time characteristic of the hoisting-engine motor that,d t3 =p (1) so far as I know, can only be obtained by special adjustment

and for one particular set of values of Vm and p for anyoneFrom this, the velocity, V, in ft. per see. at any time, t, is motor and control, and for some one value of the load.

V = 1/2 p t2 + Cl t, However, this discussion is intended merely to draw attentionand CV = Vm/tm - 1/4 p ti, to the real problems faced in attempting to attain high velocitiesor V = 1/2 pt2 +(Vmltm- /4p tm)t,where Vm is the maximum velocity in ft. per see. attained in

el/eocl,y- Tometime, tm, given in seconds. CuresAv, p 8.Oft per sec.3d0SEvidently the only possible case is when CurvesLv, p 4.0fft. per sec.'7Curves Cv, p 2.0Oft. Per sec'-?'

V = 1/2 p t2, (2) Distance-71rnegiving (Vm/tm- 1/4 p tm) = 0 (3) CurvesAD, p- .Offt Per sec.3

Curves&', p =4.0ft. perse.Therefore the time-velocity characteristic is a parabola. Also Curves Co', p = 2.f ,er sec.3maximum acceleration reached is -900 36.---- --r------6-

am = p tM/2. liEquation (2) holds between V = 0, t = 0 and V = Vm/2, 2 i/ tLj34t = tm/2. From V = Vm/2, t = tm/2 to V = VM, t = tm the 800X -curve is reversed. A single equation for the entire curve may ___ 4|lbe developed, but is not a practical necessity. r /From (3) it is obvious that 700t.-26l

Vm = 1/4 p tmn, p = 1/4 Vm/tm2, tm = 2 (Vn/p)1/2 (4) _ - 1 1Therefore if any two conditions, Vm, tm or p, are given, the /_remaining condition is fixed. A few such ideal time-velocity 600 2/ --i---characteristics are given in Fig. 9. The whole matter being more ! 1 / --completely discussed in the article mentioned above. r lVrFrom a practical standpoint it is not essential that the para- \-506 -- -pd 7Jt7 -20

bolic form be maintained except during the initial and final '_ l l-parts of the acceleration. Between these, the acceleration may C -| 4y//rbe constant, or nearly so. Probably the time-velocity curve 400 `7-,4/ / --should approximate very closely to a parabola during the first l! _ /I -- K!Iand last 0.5 second of the acceleration period. I

Obviously, for any given value of VUr the smaller is p the -37 t -1- /t- t /longer is ti, and the round trip time for a given traffic will be __ 1-1eincreased. Av Bv / j

Consequently, where comfort is a matter of moment, as in 2 -- -,4tfamily hotels, hospitals and the like, a value of p smaller than -- - - -- -- 6can be properly employed, for instance, in office building equip- / -ment, must be used and the rapidity of service correspondingly - -/< AX CR L- +Xr7 -4sacrificed.-- A 8OCThe next question is, what shall be the relative values of p --. 1

in two such cases? 0o 2.0 T0-4.0 -5.0 6.0* T~~~~~~~~~~~~~imeInM eC0170,t

Having established as above, the manner in which the kineticenergy of motion must be communicated to the passenger's FIG. 9body, it is necessary to put a limit on the total amount thatcan be safely communicated in a given time without over and rapid acceleration in elevators. Definite solutions muststressing the elastic elements of the passenger's body. Given a wait on the necessary physiological tests required to set up thecertain time of acceleration, the impressed kinetic energy values of p.varies as the square of the velocity attained in this time, or as A. A. Gazda: Looking back over the experienlce of the pastp2. Mechanically speaking, the reverse of this case is precisely three yea,rs in the installation and operation of upwards of 150the same as determining the capacity of oil buffers for a given eleva,tors controlled by the variable-voltage system, the one out-retardation time. If the oil buffer is to bring the car to rest stanzding fea,ture is its adaptibility to a broad range of applica-in the same time, irrespective of the velocity, then, if the velocity tions. On one haiid the advantage of smooth control for high-of the loaded car be doubled, the capacity of the oil buffer mnust speed macehines is quite apparent but onl the other hand thebe quadrupled, feature of definite and positive speed points has been utilized not

So, probably, if in two cases p -10 and p = 20 for the same only in slower-speed passenger elevators but also in high-gradevalue of tin, the discomfort of the passenger due to muscular freight installations. For automatic or push-button controlledstress, may be assumed to be as 100 is to 400 in the two cases. elevators particularly, wve ha.ve found it of considerable advan-In the latter case he will expeirience four times the discomfort tage to ha,ve a systemn at hand whereby twvo or more definitehe experiences in the first case. speeds can be set at convenient values and thus insure accurate


stops even und-er changing load conditions. Hitherto, the gers, and with the certainty of making definite stops under allnominal operating speed of automatic elevators has been limited conditions of loading.by the design of induction motors with certain sub-synchronous Those of you who have worked on the traffic problem involvedspeeds and in the case of direct-current machines by the lack of in moving the occupants of a densely populated tall modernsmooth transition from one speed to another. Introducing a office building in or out of the building, over the usual peake ofmotor-generator set (and the variable-voltage system) as out- say 20 to 40 minutes, will readily appreciate the value of anylined in Mr. Bouton's paper has solved both the problem of system of elevator control that offers a practical means of cuttingdefinite speed points and smooth transition. It is only necessary down the time of a round trip or cycle of operation.to locate the desired values on the generator-field rheostat and With modern elevator machinery there is no difficulty en-depend upon the time element of the magnetic circuit to make countered in operating at any speed we may wish, once that uni-the swings smoothly. In accomplishing this, permanent damp- form speed has been reached. The limitation has rather beening windings must be provided. Also final adjustments may imposed by the means available for bringing the car up to speedreadily be made by placing discharge resistors across the shunt or slowing it down to a final stop, quickly and without discomfortfields. If the time constants are too great the elevator operation to the passengers.will be sluggish. On the other hand short time constants thro w In a recent trial on an elevator in regular service equipped withsevere peak loads on the commutators of both the generator and the variable-voltage system of control, and having a nominalthe elevator motor and if these peaks are frequent or of sufficient speed of 600 ft. per min., the car was operated at 850 ft. per mim.intensity the commutators will not stand up. This statement is without the occupants of the car noticing or being aware of themade not to bring out disadvantages of the system but rather as fact that it was not operating at the usual speed. From sucha factor that must not be overlooked. data and experience in the field we are entirely confident inAnother important field of application for this system lies in predicting successful operation of passenger elevators up to a

freight elevators which must be held level with the floor landing speed of 1000 ft. per min.automatically. As shown in Fig. 7 of Mr. Bouton's paper a Laurence D. Jones: Mr. Bouton's paper describes thestable low speed can be maintained throughout the torque range application of the well known system of generator voltage con-and thus insure accurate stops. In such applications the prob- trol to the operation of electric elevators. When this system oflem really lies in changing the motor torque quickly on account control is used with any application where rapid rates of motorof static friction. This has been accomplished by the proper acceleration a-nd retardation are required, these rates are in mostchoice of the generator series field and also by the use of momen- cases dependent upon the rates of building up and dying down oftary contact switches in the main control schemne. Our experi- the generator field.ence has demonstrated that this problem can be solved and there As pointed out by the author, this fact offers one of the prin-are now several installations of this type in successful cipal advantages of applying such a system of control to ele-operation. vator service because it insures very smooth acceleration and

In high-speed elevator work we find that the continued success- retardation.ful operation of the cars depends largely upon the care taken in The inherent rates of change of generator voltage are de-maintaining the original adjustments particularly on the con- pendent on the time constant of the field circuit which, in turn,troller. Our experience with variable voltage installations has depends upon the proportion of resistance and inductance in thedemonstrated that the original control adjustments are of a circuit. It is possible to change the time constant of a field bypermanent character which do not change as contacts wear. changing the proportions of resistance and inductance. TheEven as originally installed by average elevator constructors, the usual method is to increase or decrease the resistance as thesystem is almost fool-proof and we have often been agreeably inductance iS not so easily changed. The resistance may besurprised by the smooth accelerationi and deceleration obtained increased by placing a resistor in the circuit and may be decreasedby inexperienced men. This ease of installation and operation by placing a short-circuited damping winding around the poles.is a factor of considerable importance from the standpoint of the From the usual theory of building up of current in a circuitelevator manufacturers particularly when it is so difficult to find having resistance and inductance, the time required to reach acompetent mechanics. constant value depends upon the proportion of L to R. BearingW. L. Atkinson: Field experienee with the system of control this fact in mind we find it rather difficult to understand how

described by Mr. Bouton has indicated that we have here a means Mr. Bouton obtained the results shown in Fig. 3 of his paperof obtaining operating characteristics in an elevator, not pos- which indicates that the use of a short-circuited damping windingsessed by any other known means of control. While the com- has changed the rate of building up, but has not affected the timeparative economic advantage for any given number of stops per required for the voltage to reach a constant value. It is possiblecar mile can be very readily analyzed and definitely stated, the that the author has used some special means for producing theimportant feature of smoothness of operation through the effect shown. If so, a description of how this was accomplishedaccelerating and decelerating periods, contributed by this method would prove of interest.can only be demonstrated by actual experience with elevators Similar tests which have been made under the same conditionsin the field. as those described in Mr. Bouton's paper show a very materialThe company with which I am connected, has installed and in difference in the time required for voltage to reach constant value,

operation fifty-two elevators equipped with this system of con- with the field damped and undamped. In this case the time wastrol, operating at 400, 500, 600 and 700 ft. per minute, and in 4.4 seconds with the damping winding in use and 2.0 secondsevery installation a distinctive and characteristic smoothness is without this winding. Figs. 10 and 11 shownwith this discussionnoticeable throughout the starting and slowing down periods, a,re oscillograph records of the building up of field current aJndIn the highest developed type of rheostatic control, making use of a.rma,ture voltage of a generator having its field undamped andall the refinements which experience has shown to be of a,dvantage a.lso damped. Fig. 12 is a comparison of the voltage curves forwe cannot attain a smoothness of operation with rheostatic con- the two cases. The genera.l conclusion from these tests is thaJttrol comparable with that quite ordinarily possible with the the use of a damping winding is of value as a means of changingvariable voltage scheme here described, the time constant of the field so a.s to obtain the desired time forWhile we have been limited heretofore, to a maximum speed of accelerating to full speed but has very little effect upon the shape

about 600 ft. per minute, or in exceptional cases to 700 ft., we are of the curve.now enabled by this means, we are quite sulre, to go to speeds From Part III of the paper one is likely to gain the impressionmuchbeyond this limit without occasioninlg discomfort to passen- that the traction motor will have acceleration and retardation


curves of the same shape as the building-up or dying-down considered as alternating-currert elevators. (Indeed were thecurves of the generator voltage and that these curves are inde- variable-voltage generator driven by water or steam power thependent of the load. That this is not the fact is quite evident elevator would be called a hydraulic or steam elevator with theif we compare the shape of the acceleration curves in Fig. 4 with same line of reasoning.)the curves in Fig. 3. The principal reason for the disagreement Fundamentally the only elevator that can be truly called anis that the shape of the speed curves is modified to some extent alternating-current elevator is one in which the elevator machineby voltage drop in the armature circuit. With further reference is driven directly by an alternating-current motor.to Fig. 4, I would like to ask Mr. Bouton why a longer time is Comparing the two systems-variable voltage and therequired for the motor to accelerate a loaded car going down to a straight alternating-current, as defined in the preceding para-

graph-it is seen that the variable voltage as given in the paperconsists of six main elements, while the straight alternatingcurrent consists of but two, the elevator motor and its controller.Although the paper under consideration refers to the alterna-

ting-current elevator equipment, it is noted that none of the testinformation contained therein gives any consideration to analternating-current power supply, but deals only with equipmentrequiring direct current. If the paper leaves the impression thatthe curves apply to equipment having an alternating-currentsupply it is misleading, for the amount of energy returned to theline during slow-down will certainly be somewhat different froman a-c. to d-c. motor-generator set. Another very importantconsideration is the fact that the power factor will be very low

FIG. 10-OSCILLOGRAMI SHOWING BUILDING UP OF GENERATOR during average elevator operating conditions, and idle and slow-FIELD CU[RRENT AND ARMATURE VOLTAGE WITHOUT DAMPING down periods, which will be 90 to 95 per cent of the total time inWINDING ON FIELD service.

Curve A-Armaturc voltage, 230 volts maximum. The paper compares only two types of control-variable-Curve B-Field current. voltage and rheostatic, and these with direct current supplied,Curve 0-Timing wave. (40 cycles.) whereas it is a faet that the standard shunt-control direct-cur-

rent elevator unit, consisting of the two main elements of motorand controller, shows in regular service an energy consumptionstill lower than either of these.The tests given in the paper are interesting, but actually of

little value even for comparison with other types of control.Energy-consumption tests, to be of value, must show what the

particular types of control under comparison are doing in actualservice, where a corresponding number of hours per day, milesper day, stops per mile and kilowatt hours per car mile over anumber of months are recorded. This recorded informationwill represent quite accurately average conditions, and will beof great value to engineers in general.

FIG. 11-OSCILLOGRAM SHOWING BUILDING UP OF GENERATOR The different types of elevator control might more fittingly beFIELD CURRENT AND ARMATURE VOLTAGE WITH DAMPING compared by giving eonsideration to an inereased number of itemsWINDING ON FIELD as follows:

Curve A-Armature voltage, 230 volts maximum. Economy as to first cost.Curve B-Field current. Economy as to maintenanee.Curve C-Timing wave. (40 cycle.) Economy as to energy consumption.

Economy as to space.Safety.

240 _ = Simplicity.200 _ Number of main elements.

VE A Total number of parts./ L/8 -- -Continuity of service.

120 t-A---ISpeed regulation.m80- --------- Accelerating and decelerating characteristics.40 - As engineers we instinctively aim towards ultimate simplicity0 [ l of equipment with maximum safety and refinement of operation0 1 2 3 4 5 combined with a minimum kilowatt-hour consumption for regu-

TIME IN SECONDS lar operating conditions.FIG. 12 It is, therefore, self-evident that a straight alternating-current

elevator with only two main elements, which will give any de-given speed than is required to accelerate a balanced load to the sired car speed with positive control, smooth, quiet, rapid accelera-same speed. tion and deceleration, and a low overall operating cost as well as

E. B. Thurston (by letter): Mr. Bouton's paper leaves the all of the safety requirements, is a goal towards which we, asimpression that an elevator operated by a direct-current motor, electrical engineers, should turn our interest and attention.supplied from a motor-generator set, should be called an alter- E. W. Seeder (by letter): Although the acceleration of annating-current elevator, in case the motive power for the genera- elevator equipped with variable-voltage control is exceedinglytor set be alternating current. It seems this impression should smooth, it is possible to get practically the same results with otherbe corrected, because elevator motors for years have been sup- types of control. In decelerating and stopping, however, theplied from a-c. to d-c. motor-generator sets, and have never been variable-voltage system has a decided advantage because of the


possibility of obtaining a sustained braking effect and a stable thus smoothing out the change in speed from low speedslow speed. The quick acceleration and retardation possible to high.with this type of control will probably lead to the more general In connection with these two-speed elevator motors, as well asuse of high-speed elevators for local service. with any elevator motor, the number of stops per minute and the

It is always difficult to compare data on the power consumption duration of the stops have a great effect upon the heating of theof elevators because so many factors, other than the electrical motor, as does the class of building in which the elevator operates.equipment, affect the result. The exact method of making the For example there are some department stores five or seven storiestest plotted in Fig. 14 is not stated. If the load were placed on in height in which during non-rush periods the car only pauses atthe car and the elevator then started and stopped the required each floor, and, seeing no passengers, the operator proceeds onnumber of times, the test would not show results which would be to the next floor. This constitutes a full stop and start, but withobtained in actual service, but would favor the variable-voltage very little rest period. This is very severe duty on the motorequipment. and is considerably different from that in a tall office buildingAnother factor which will affect power consumption is pro- where the elevators runs three, four, five, or, in some cases, ten,

vision for night service in office buildings. Although it is possible floors without a stop, thus operating the motor a great portionto start the set each time the elevator is required, it is doubtful of its operating time at a high speed and consequently derivingif this practise will be followed and probably one set will be the benefit of its ventilating effect.allowed to run. The question of first cost of an elevator installation is a quite

After acceleration is completed and the motor is running at full important one and the method of drive influences this cost some-speed, the variable-voltage system, due to the losses in the motor- what. These points were not touched upon in this paper. It isgenerator set, is probably the least efficient of any of the accepted also very interesting to know the power consumption of elevatorsmethods of control. per car mile, and the comparative power consumptions ofEven after taking the above factors into consideration, the variable-voltage d-c. elevators and two-speed a-c. elevators, as

variable-voltage system will undoubtedly show a saving on any well as the amounts of maintenance required on these two schemes.installation where a gearless traction machine is justified. This The question of speed of elevators is very interesting, especiallyis particularly true with "green" operators because the power for a tall office building where the ability to handle the peak loadrequired for inching is less. in a space of half an hour two or three times a day is very im-With slower-speed elevators driven by geared machines, it is portant. However, the ability to operate an elevator at such a

more difficult to justify the use of the variable-voltage system. speed that the operator can make the landing at the floor ac-The rheostatic losses are, in general, a much smaller percentage curately without a great amount of juggling up and down, toof the total power, landings are easier to make, and the service which passengers object strenuously, is very important andis less frequent. It follows that the maintenance expense is also should be kept in mind by the elevator designers as well as thesmaller. motor and control designers when laying out elevators for a tall

There is, therefore, a very definite field for the two speed a-c. office building.motor on moderate-speed geared elevators. The acceleration E. M. Boutons Mr. Whiting has shown some very interest-and deceleration obtained with this type of equipment are as ing results obtained with variable voltage control, by payingsatisfactory as obtained on most d-c. installations. Tlhe average particular attention to the details that affect the regulation ofa-c. installation, however, is comparatively noisy, and for this the motor. Unfortunately, he has not described the methodreason the variable-voltage system has been installed in some by which he obtained the results.cases where quiet operation is essential. In general, the regulation of the elevator motor can be im-

In the majority of variable-voltage equipments which have proved by increasing the compounding of the generator or bybeen installed, the car operating switch has been used, not only using control contacts which manipulate the generator or motorto control the contactors on the board, but also to vary the fields in response to changes in motor load or speed. Thegenerator field resistance directly. generator compounding may be accomplished by putting seriesThe generator field resistance is usually placed on the car, and turns upon its own field poles or by putting these series turns

inasmuch as a number of resistance steps are used, there is con- upon an auxiliary generator whieh excites or affects the excita-siderable wiring on the car which is comparatively inaccessible. tion of the main generator. If the generator is compoundedThe writer believes that a more satisfactory arrangement is to so as to have a flat speed torque curve at high speed, it willhave the car-switch-control multi-point relays placed on the probably have a rising curve at slow speed. This is becausecontrol panel. With this arrangement, only one set of resistance the generator fields are more nearly saturated at full speedis required and it can be placed on the control panel rather than and a given number of series turns is more effective at the loweron the car, where space is at a premium. saturation. It is entirely practicable to compound the machineB. M. Jones (by letter): It has been my good fortune to in this way, except that if carried too far it has a tendency to

do some work on a variable-voltage elevator built by Mr. make the equipment unstable.Bouton's company, and it was a pleasure to see the ease with I believe that a little further discussion of Fig. 7 and Fig. 11which the elevator was controlled and the smoothness of its would be helpful. Curve 1 in Fig. 11 shows the inherentoperation. regulation of a gearless machine at full speed without the cor-The writer has also done considerable work on some of the rective effect of any speed governor contacts when connected

early two-speed a-c. elevators using squirrel-cage motors having to a constant potential of 230 volts. The regulation at lowtwo speeds, covering a range of approximately three to one. speed with a.rmature series and shunt resistance will be as shownThis means that the a-c. motor has two primary windings. For by Curve 2.starting the motor it is thrown on the low-speed winding and as The test illustrated in Fig. 7 consisted in taking the samethe car accelerates the motor is automatically transferred to the machine and driving it with a varia.ble volta,ge generator. Thehigh-speed winding through resistance which smooths out the generator compoundingwas so adjusted as to give approximatelyacceleration. The star point of the windings is brought out of thesame regulation at high speed aswas obtained on the constantthe frame of the motor and the resistance connected in these voltage supply. The inherent regulation at low speed is thenleads before the starting connection is made. Magnetic con- shown by Curve 2. A comparison of curves 2 in Fig. 7 and Fig.tactors short circuit this resistance in steps to accelerate the car. 11 shows the advantage that variable voltage control has overWhen switching from high to low speed, the resistance is thrown rheostatic control in making landings when adjusted for thein the high-speed winding before the latter is thrown on the line, same regula.tion aUt full speed.


It was not the purpose of my paper to show the maximum the permeability of the magnetic circuit and upon the amountimprovement over rheostatic control that could be obtained, of magnet leakage between the main winding and the damperbut rather to show that the variable voltage system has certain winding. Now keeping in mind that the field poles are beinginherent characteristics that make it superior to other systems. worked at a lower point on the saturation curve when theIt is entirely possible, as is shown in Mr. Whiting's discussion, field starts to build up than when it has built up to higher values,to make the regulation at full speed as good as is desired. I and also that the increasing magneto force of the main windingwish to state, however, that additional complications are will eause a redistribution of the leakage flux, it is not unreason-involved if extremely close regulation is desired. able to expect a change in the shape of the curve when the damperThe tests shown in Figs. 14 and 16 include the motor-genera- winding is added. For other proportions of magnetic circuit

tor losses only during the period of test. For this reason, in and proportions of windings I would readily believe that differentactual service the all day power consumption would be somewhat results would be obtained, as Mr. Jones states that he obtainedhigher. For example, if the elevator runs 20 mi. in a 10-hr. from his tests. The results I have shown in Fig. 3 were obtainedday, the no load losses will be, (at 1.25 kw. per elevator)- on test with an oscillograph.1.25 X 10 = 12.5 kw-hr. total for the day. This loss must be The speed time curves obtained in Fig. 4 were taken by drivingdistributed over the 20 mi. traveled and will be 12.5 + 20 a magneto by the elevator motor and recording with a graphic= 0.625 (625/1000) kw-hr. per car mile for the idling losses, voltmeter the voltages of the magneto. The voltage of theIt would not be right, however, to a4d all of this to the test magneto was calibrated in terms of car speed with an ordinaryvalue shown in Fig. 14 since time was actually consumed in tachometer. The method is not absolutely accurate and thesemaking the test. It would probably be fair to add 12 of this inaccuracies no doubt account for the discrepancies in the curvesvalue or approximately 0.3 (3/10) kw-hr. per mile to the test that Mr. Jones mentions. I think Mr. Claytor's analysisvalue to obtain the actual power consumption of an elevator of the curves explains why the motor speed does not followoperating on the schedule outlined above. For night service exactly the building up of the generator voltage during the earlythe motor-generator sets car be shut down so that the iosses part of the accelerating period. During the rest of the accelera-do not occur except while the car is running. ting period and during retardation up to the point where the

I wish to point out also that the test results shown in Fig. 15 brake sets, I believe the speed and the generator voltage followfor rheostatic control do not show all the rheostatic losses that each other very closely.will be presert in actual operation. The test does not include Mr. Hansen has described an interesting system whereby heany periods of slow speed running prior to the stops nor does it uses a voltage of rotation in an auxiliary machine to first opposeinclude "inching" the car to make a landing while in actual the impressed line voltage and then add to it to accelerate theservice, these operations occur very frequently. If the motor is main motor to a higher speed. By this means the energy whichcontrolled over a wide range by armature series and shunt in a rheostatic system is dissipated in resistanee is here storedresistance as is the practise with gearless machines, the elevator in a flywheel and then returned as energy used in acceleration.will frequently consume more power while running at low speed It would seem that this system would be quite useful for con-than at high speed. For this reason the test values probably stant speed work where acceleration and stopping make up thefavor the rheostatic system more than the variable voltage principal part of the cycle. It has the one drawback for elevatorsystem when compared with actual service conditions. work, that speeds lower than full speed are not available. If

There seems to be quite a tendency among engineers when the controller handle were left on a slow speed point the maincomparing variable voltage control with other systems to pay motor would not run at this low speed except momentarily,considerable attention to whether or not there will be a saving because the energy input to the auxiliary motor would acceleratein power. As a matter of fact, in the majority of cases the the flywheel and the main motor would accelerate due to theother advantages of variable control that I have described, such reduction in counter e. m. f. of the auxiliary motor. A slowas smoothness of operation, accuracy of co-ntrol, saving in time, speed which is constant and independent of load is one of theetc., are much more important considerations. I think Mr. important requirements of elevator service and this requirementMatson's analysis of the cost of operating elevators brings out is one of the reasons why the variable voltage system has provedquite clearly that the power consumed is a small item in the so successful in this field since it provides not only for smoothcost of operation. acceleration but also for speed control.

L. D. Jones has questioned the results shown in Fig. 3 which The discussion by Mr. Eames and by Mr. Bassett Jonesshow that the rate of building up of the generator fields is illustrate the extent to which elevator engineers have analyzedchanged by the use of a damper winding, but the total time of the passenger himself when providing a conveyance for trans-building up is not materially affected. The inductance of the porting him, in a vertical direction. I have no doubt that in thefield circuit is made up of two factors, the self inductance of future more complete pbysiological as well as psychological teststhe main field winding and the mutual inductance of the short- will be made to determine the effect of acceleration upon thecircuited damper winding. If the inductance were actually passenger, but I believe that a more exact solution of the prob-constant, the field coil would have a true time constant and the lem depends equally as much upon the development of more

field flux would build up according to the well-known logarithmic accurate methods of testing the elevaItor to determine howlaw. However, the inductance is not constant but depends upon closely it fulfills the required conditions.

variable voltage control systems as applied to electric elevators

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