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Department of Mechatronics, EEE 223 Electrical machines and Drives

Unit -

DC and !C Machines

. "hat is a motor#

A motor is a rotating component that imparts and generates motion. Motors are designed to convert

electrical energy into mechanical energy to perform some physical task or work.

$. "hat is meant %y &enerators#

Electric generators are used to efficiently convert mechanical energy into electrical energy. Magnetism is

the basis of their principles of operation.

'. Define tor(ue.

The twisting force of a motor or gear motor shaft, usually expressed in inch/ounces or inch/pounds.

Torue !Force X Distance

). "hat is MM*# &ive it+s e,pression

MM" is the work done in moving a unit magnetic pole once around the magnetic circuit.

MM"!# $ ampere turns

%here #!no of turns$ !current in amp

.Define manetic field intensity.

$t is the MM" per unit length. Amp turn/metre

&!# $/l where #!no of turns'$!current in amp'l!length in metre

."hat is reluctance#

$t is the property of the magnetic material to oppose the magnetic lines of flux

(!"/" where "!MM" ' "!flux '(!reluctance in amp turns per weber

0."hat is stac1in factor#

$t is the ratio between the net cross sectional area of the core to the cross section occupied by themagnetic material.

2. "hat is lea1ae flu,#

The flux which takes a path which is not intended for it is called leakage flux..

3. 4o5 is the direction of induced e.m.f determined#

)irection of induced e.m.f is determined by using

*i+ right hand gripprng rule and

*ii+ ight hand cork screw rule.

6.7tate Len8+s la5.

Any induced e.m.f will circulate a current in such a direction as to oppose the cause producing it. e! -#d"/dt

."hat is self inductance#

The e.m.f induced in a coil due to change of flux in the same coil is known as self inductance.

Kumaraguru College of Technology, Coimbatore 641 049

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$. &ive the e,pression for maneti8in force.

" ! $ l sin #ewton,

%here

$ is the current carried by the conductor

l is the effective length of the conductor

is the flux density in %b/m0

'."hat are the factors on 5hich hysteresis loss depends#

The hysteresis loss depends on the magnetic flux density , freuency f and the volume of the material 1.

). "hat is core loss# "hat is its sinificance in electric machines#

%hen a magnetic material undergoes cyclic magneti2ation , two kinds of power losses occur on it 3

hysteresis and eddy current loss which together are known as core loss. $t is important in determining

heating, temperature rise , rating and efficiency of transformers, machines and other a.c run magnetic

devices.

."hat is eddy current loss#

%hen a magnetic core carries a time varying flux voltages are induced in all possible paths enclosing

flux. esult is the production of circulating current in core. These induced currents do no useful work areknown as eddy current and have power loss known as eddy current loss.

.4o5 are hysteresis and eddy current losses minimi8ed#

&ysteresis loss can be minimi2ed by selecting materials for core such as silicon steel 4 steel alloys with

low hysteresis coefficient and electrical resistivity.

Eddy current losses are minimi2ed by laminating the core.

0.4o5 5ill you find the direction of emf usin *lemin+s 9iht 4and 9ule#

The thumb, the forefinger and the middle finger of the right hand are held so that these fingers are

mutually perpendicular, then

"orefinger - "ield

Thumb - MotionMiddle finger- $, current

2.4o5 5ill you find the direction of force produced usin *lemin+s Left 4and 9ule#

The thumb , forefinger and middle finger of the left hand are held so that those fingers are mutually

perpendicular then

"orefinger - "ield

Thumb - Motion*due to force+Middle finger - $, current


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3."hich type of d.c armature 5indin re(uires e(uali8er rins#

5ap winding

$6.4o5 are armature 5indins classified %ased on the placement of the coil inside the armature

slots#

(ingle layer winding and

)ouble layer winding

$."hy the armature core in d.c machines is constructed 5ith laminated steel sheets instead of

solid steel steel#

(teel sheets offer low reluctance path for the magnetic field , laminated sheets reduce eddy current loss.

$$. "hy is commutator employed in d.c machines#

6. 7onduct electricity between armature and fixed brushes

0.7onverts alternating emf into unidirectional emf and vice versa

$'. Distinuish %et5een shunt and series field coil constructions.

(hunt field coils are wound with wires of small cross section and have more number of turns. (eries field

coils are wound with wires of larger cross section and have less number of turns.

$). 4o5 can one differentiate %et5een lon shunt compound motor and short shunt compound

motor#

$n a short shunt compound motor the shunt field circuit is shorter i.e. across the armature terminals. $n a

long shunt compound motor the shunt field circuit is connected across the load terminals.

$. Define the term armature reaction in dc machines.

The interaction between the flux set up by the current carrying armature conductors with the main field

flux is defined as armature reaction.

$. "hat are the t5o un5anted effects of armature reaction#

8 7ross magneti2ing effect / )istorting effect8 )emagnetising effect

$0."hy are car%on %rushes preferred for dc machines#

The high contact resistance carbon brushes help the current in the coil undergoing commutation to attain

its full value in the reverse direction at the end of commutation. The carbon brushes also lubricate and

give less wear and tear on commutator surface.

$2. 4o5 does a d.c motor differ from d.c enerator in construction#

9enerators are normally placed in closed room , accessible only to skilled operators. Therefore on

ventilation point of view they may be constructed with large opening in the frame.

Motors on the other hand , have to be installed right in the place of use which may have dust, dampness,

inflammable gases, chemical fumes etc . To protect the motors against these elements , the motor framesare made either partly closed or totally closed or flame proof etc.

$3.4o5 5ill you chane the direction of rotation of a d.c motor#

Either the direction of the main field or the direction of current through the armature conductors is to be

reserved.


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'6. "hat is %ac1 emf in d.c motors#

As the motor armature rotates, the system of conductor come across alternate #orth and (outh pole

magnetic fields causing an emf induced in the conductors. The direction of the emf induced in the

conductors is in the direction opposite to the current .As this emf always opposes the flow of current in

motor operation it is called back emf.

Part : C

6. )escribe the construction and working of three phase induction motor.

0. )raw and explain various load characteristics of )7 (hunt Motor.

:. Explain the speed 3 torue curve of single phase induction motors in detail.

;. )raw and explain the torue-speed characteristic of a three phase induction motor.

() E*+!ain t#e "#ara"teristi"s o, DC series motors)

. )erive the armature torue of a ).7 motor.

Unit - $


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0. Compare s(uirrel cae rotor and slip-rin rotor#

7.no 7(uirrel cae rotor 7lip-rin rotor

6. (imple construction 7onstruction is complicated

0 otor consists of copper bars which are

shorted at the ends with the help of end

rings

otor consists of three phase winding

: (lip ring and brushes are not present (lip ring and brushes are present

; External resistance can not be added External resistance can be added

. Moderate torue &igh starting torue can be obtained

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$. "hy a sinle phase induction motor does not self start#

%hen a single phase supply is fed to the single phase induction motor. $ts stator winding produces a flux

which only alternates along one space axis. $t is not a synchronously revolving field, as in the case of a 0

or :phase stator winding, fed from 0 or : phase supply.

'. "hat is meant %y pluin#

The plugging operation can be achieved by changing the polarity of the motor there by reversing thedirection of rotation of the motor. This can be achieved in ac motors by changing the phase seuence and

in dc motors by changing the polarity.

). &ive some applications of DC motor.

7hunt < driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating

pumps

7eries < electric locomotives, rapid transit systems, trolley cars, cranes and hoists, conveyors

Compound < elevators, air compressors, rolling mills, heavy planners.

. "hat are the different types of electric %ra1in#

)ynamic or heostatic braking,

7ounter current or plugging andegenerative braking

. Compare electrical and mechanical %ra1in.

Mechanical

rakes reuire freuent maintenance

#ot smooth

7an be applied to hold the system at any position

=lectrical

very little maintenance

smooth

cannot produce holding torue.

0. "hen does an induction motor %ehave to run off as a enerator#

%hen the rotor of an induction motor runs faster than the stator field, the slip becomes negative.

egenerative braking occurs and the B.E. of the rotating parts is return back to the supply as electrical

energy and thus the machine generates power.

2. Define synchronous speed.

$t is given by #s ! 60@f / p rpm.

%here #s ! synchronous speed, p ! no. of stator poles, f ! supply freuency in &2

3."hat is meant %y reenerative %ra1in#

egenerative braking occurs when the motor speed exceeds the synchronous speed. $n this case the $M

runs as the induction mCc is converting the mechanical power into electrical power which is deliveredback to the electrical system. This method of braking is known as regenerative braking.

$6. "hat is meant %y dynamic %ra1in#

)ynamic braking of electric motors occurs when the energy stored in the rotating mass is dissipated in an

electrical resistance. This reuires a motor to operate as a gen. to convert the stored energy into electrical.


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$. "hat is critical speed#

$t is the speed that separates continuous conduction from discontinuous conduction mode.

$$. "hich %ra1in is suita%le for reversin the motor#

Dlugging is suitable for reversing the motor.

$'. ;s ;nduction motor runs 5ith synchronous speed or not.

$nduction motor never runs with synchronous speed. $t will stop if it tries to achieve synchronous speed.

$). "hat is the function of a no-voltae release coil provided in a dc motor starter#

As long as the supply voltage is on healthy condition the current through the #1 coil produce enough

magnetic force of attraction and retain the starter handle in the # position against spring force. %hen

the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough

force and the handle will come back to "" position due to spring force automatically. Thus a no-voltageor under voltage protections given to the motor.

$. Mention the 7tarters used to start a DC motor.

Two point (tarterThree point (tarter

"our point (tarter

$. Mention the 7tarters used to start an ;nduction motor.

)..5 (tarter *)irect nline (tarter+

(tar-)elta (tarter

Auto Transformer (tarter

eactance or esistance starter

(tator otor (tarter *otor esistance (tarter+

$0. "hat are the protective devices in a DC/!C motor 7tarter.

ver load elease *.5.+ or #o volt coil&old on 7oil

Thermal elays"uses*(tarting /unning+

ver load relay

$2. &ive the prime purpose of a starter for motors.

when induction motor is switched on to the supply, it takes about to > times full load current at starting.

This starting current may be of such a magnitude as to cause obFectionable voltage drop in the lines. (o

(tarters are necessary

$3."hy motor ta1e heavy current at startin#

%hen : phase supply is given to the stator of an induction motor, magnetic field rotating in space atsynchronous speed is produced. This magnetic field is cut by the rotor conductors, which are short

circuited. This gives to induced current in them.

(ince rotor of an induction motor behaves as a short circuited secondary of a transformer whose primary

is stator winding, heavy rotor current will reuire corresponding heavy stator balancing currents.


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Part : C

6. )raw a neat schematic diagram of a three point starter and explain its working.

0. )raw a neat schematic diagram of a four point starter and explain its working.

:. Explain with neat circuit diagram, the star-delta starter method of starting suirrel cage inductionmotor.

;. Explain the typical control circuits for )7 (eries and (hunt motors.

. Explain the different starting methods of three phase suirrel cage induction motors with neat sketches.

. )raw and explain the push-button operated direct-on line starter for three phase induction motor.

G. )raw and explain the manual auto-transformer starter for three phase induction motor.

6@. *i+ 5ist out the advantages and disadvantages of electrical braking over mechanical braking. *>+ *ii+ )iscuss any one method of electrical braking of )7 Machines. *>+

66. Explain how an induction motor is brought to stop by *i+ Dlugging and *ii+ dynamic braking. *6

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Unit : '

Int'o%u,ton to E"$,t',#" -'&$s

. Define Drive and =lectric Drive.

)riveH A combination of prime mover, transmission euipment and mechanical working load is called a

drive.Electric driveH An Electric )rive can be defined as an electromechanical device for converting electrical

energy to mechanical energy to impart motion to different machines and mechanisms for various kinds of

process control.

$. "hat is meant %y electrical drives#

(ystems employed for motion control are called drives and they employ any of the prime movers such as

diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors for supplying

mathematical energy for motion control. )rives employing electric motion are called electric drives.

'. Classify the type of loads driven %y the motor.

The type of load driven by the motor influences the current drawn and losses of the motor as the slip

various. The normally occurring loads area. 7onstant torue loads.

b. Torue varying proportional to speed.

c. Torue varying preoperational to the suare of the speed.

). List out some e,amples of prime movers.

$.7 Engines, (team engine, Turbine or electric motors.

. List out some advantaes of electric drives.

i. Availability of electric drives over a wide range of power a few watts to mega watts.ii. Ability to provide a wide range of torues over wide range of speeds.

iii. Electric motors are available in a variety of design in order to make them compatible to any type of

load.

. &ive some e,amples of =lectric Drives.

i. )riving fans, ventilators, compressors and pumps.

ii. 5ifting goods by hoists and cranes.

iii. $mparting motion to conveyors in factories, mines and warehouses

iv. unning excavators 4 escalators, electric locomotives trains, cars trolley buses, lifts 4 drum winders

etc.

0. "hat are the types of electric drives#

9roup electric drives *(haft drive+,

$ndividual )rives,

Multi motor electric drives.

2. Classify electric drives %ased on the means of control.

Manual, (emiautomatic, Automatic.

3. "hat is a &roup =lectric Drive >7haft Drive?#

This drive consists of single motor, which drives one or more line shafts supported on bearings.


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The line shaft may be fitted with either pulleys 4 belts or gears, by means of which a group of machinesor mechanisms may be operated.

6. 7pecify the functions of po5er modulator.

Dower modulator performs one or more of the following four functions.

a. Modulates flow of power form the source to the motor in such a manner that motor is imparted speed-

torue characteristics reuired by the load.

b. )uring transient operations, such as starting, braking and speed reversal, it restricts source and motorcurrents within permissible values' excessive current drawn from source may overload it or may cause a

voltage dip.

. Mention the different types of drives.

6+9roup drive

0+$ndividual drive

:+Multimotor drive

$. List the different types of electrical drives.

6+dc drives

0+ac drives

'. "hat are the functions performed %y electric drives#

1arious functions performed by electric drives include the following.

a. )riving fans, ventilators, compressors and pumps etc.

b. 5ifting goods by hoists and cranes

c. $mparting motion to conveyors in factories, mines and warehouses and

d. unning excavators and escalators, electric locomotives, trains, cars, trolley buses, lifts

and drums winders etc.

). "hat are the disadvantaes of electric drives#

The disadvantages of electric drives.

a. Electric drives system is tied only up to the electrified area.

b. The condition arising under the short circuits, leakage from conductors and breakdown ofoverhead conductor may lead to fatal accidents.

c. "ailure in supply for a few minutes may paralyses the whole system.

. "hat are the advantaes of roup drive over individual drive#

The advantages of group drive over individual drive are

a. $nitial costH $nitial cost of group drive is less as compared to that of the individual drive.

b. (euence of operation H 9roup drive system is useful because all the operations are stopped

simultaneously.

c. (pace reuirement H 5ess space is reuired in group drive as compared to individual drive.

d. 5ow maintenance costH $t reuires little maintenance as compared to individual drive.

. "rite short notes on individual electric drives.$n individual drive, each individual machine is driven by a separate motor. This motor also imparts

motion to various other parts of the machine. Examples of such machines are single spindle drilling

machines *Iniversal motor is used+ and lathes. $n a lathe, the motor rotates the spindle, moves the feed

and also with the help of gears, transmits motion to lubricating and cooling pumps. A three phase suirrel

cage induction motor is used as the drive. $n many such applications the electric motor forms an integral

part of the machine.


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0. Mention the parts of electrical drives.

6+Electrical motors and load.

0+Dower modulator

:+(ources

;+7ontrol unit+(ensing unit

2. Mention the applications of electrical drives.

Daper mills

Electric traction

7ement mills

(teel mills

3. Mention the different types of classes of duty.

7ontinuous duty

)iscontinuous duty

(hort time duty$ntermittent duty

$6. "hat are the advantaes and disadvantaes of &roup drive >7haft drive?#

!dvantaes

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