INDUCTION MOTORS 10

1.Which of the following motors uses a commutator
a) universal motor
b) split phase motor
c) shaded pole motor
d) none of the above
ans. a

2. If a dc series motor is connected to an ac supply
a) the motor will not start
b) the motor will start but then come to stop
c) the motor will start and run but will have poor performance
d) None of these
ans. c

3. If a 3-phase , 4-pole, 50 Hz induction motor runs at a speed of 1440 rpm then the slip is
a) 0.03
b) 0.10
c) 0.04
d) 0.05
ans. c

4. A starting torque of 100 Nm is developed by an auto-transformer starter with a tapping of 40%. If the tapping of auto transformer starter is at 80% , then the starting torque would be
a) 200 Nm
b) 50 Nm
c) 400 Nm
d) 25 Nm
ans. c

5. The complete circle diagram of a 3-phase induction motor can be drawn with the help of
a) running light test alone
b) both running light and blocked rotor tests
c) running light , blocked rotor and stator resistance tests
d) blocked rotor test alone
ans. c

6. The power factor of a delta connected 3-phase , 50 Kw induction motor is 0.4 when delivering 20 % of its rated load. If its stator is reconnected in star , then its pf
a) remains unchanged
b) is improved
c) is worsened
d) none of these
ans. b

7. A 3-phase induction motor runs at 980 rpm at no load. If its squirrel cage rotor is replaced by solid cylinder, then its no load speed would be
a) zero
b) 980 rpm
c) below 980 rpm
d) 490 rpm
ans. c

8. An induction motor in which the stator has the squirrel cage winding is
a) 3-phase squirrel cage motor
b) 1-phase induction motor
c) 1-phase ceiling fan
d) 1-phase table fan
ans. c

9. For satisfactory performance of a 3-phase 420 V , 60 Hz induction motor , the supply voltage at 50 Hz should be equal to
a) 420 V
b) 380 V
c) 350 V
d) 400 V
ans. c

10. A 3-phase wound rotor induction motor with star-connected rotor, runs clockwise. If the phase sequence of the rotor terminals is changed, then the motor would
a) not run
b) run clockwise
c) run anti -clockwise
d) none of these
ans. b

INDUCTION MOTORS 9

1. Rotor slot of squirrel cage induction motor are skewed slightly, so as to
a) increase the mechanical strength of rotor
b) make the rotor construction simplex
c) eliminate locking tendency of the rotor and to reduce the noise
d) save the amount of copper required
ans. c

2. In a 3-phase wound rotor induction motor, 3-phase balanced supply is given to the rotor and stator winding is short circuited. The rotor would
a) not run
b) run in the direction of the field
c) run against the direction of rotating field
d) run at half synchronous speed
ans. c

3. The rotor of an induction motor can't run at synchronous speed, if it did so then
a) rotor emf would be zero
b) rotor current would be zero
c) rotor torque would be zero
d) all of these
ans. d

4. A 3-phase induction motor is running at half full load . If the fuse in one of the phases gets burnt, the motor would
a) come to standstill
b) continue running at the same speed
c) continue running at slightly reduced speed
d) continue running at an increased speed
ans. c

5. A 3-phase , 50 Hz , 4-pole squirrel cage induction motor has its stator rewound for 6 poles without any alternations in the rotor. The motor would now run at a speed
a) less than 1000 rpm
b) less than 1500 rpm
c) less than 3000 rpm
d) zero rpm
ans. a

6. The stator of a 3-phase , 4-pole , slip ring induction motor is fed from 50 Hz source and its rotor from 30 Hz source. The motor will run at
a) 1500 rpm
b) 900 rpm
c) zero rpm
d) 2400 rpm
ans. d

7. The capacitor start motor uses
a) paper capacitor
b) electrolytic capacitor
c) air capacitor
d) none of the above
ans. b

8. The value of capacitor in a capacitor start motor controls the
a) starting value
b) speed of the motor
c) efficiency
d) none of the above
ans. a

9. Under no load conditions, the speed of a universal motor is limited by
a) supply frequency
b) supply voltage
c) windage and friction
d) none of the above
ans. c

10. Which of the following motors does not use a centrifugal switch
a) split phase motor
b) capacitor start capacitor run motor
c) shaded pole motor
d) none of the above
ans. c

INDUCTION MOTORS 8

1. In an induction motor, the value of average flux density in air gap should be small

a) to get good pf

b) to get poor pf

c) to achieve good efficiency

d) for minimum cost

ans. a

2. An increase in the value of air gap flux density in an induction motor

a) increase iron-loss

b) increase efficiency

c) decrease efficiency

d) both a and c are correct

ans. d

3. The pulsation losses and noise can be reduced in an induction motor by using 

a) a large number of narrow slots

b) less number of narrow slots

c) large number of semi slots

d) none of these 

ans. a

4. With increase in length of the air gap in an induction motor, the pulsation losses becomes

a) more 

b) less

c) uncharged

d) none of these 

ans. b

5.  The large number of slots in an induction motor has

a) high over load capacity

b) low over load capacity

c) no effect no overload capacity

d) none of these 

ans. a

6.A short air gap in an induction motor is necessary to achieve

a) to increased overload capacity

b) to reduce pulsation loss

c) to reduce the noise

d) a good pf

ans. d

7. The large value of air gap length in an induction motor results in 

a) to increased overload capacity

b) to reduce pulsation loss

c) providing better cooling

d) reducing the noise

ans. a

8. In an induction motor , the motor fails to start when number of rotor slots is equal to number of stator slots this is known as

a) crawling

b) cogging

c) synchronous cusps

d) none of these

ans. b

9. The simplest way to eliminate the harmonic induction torques is

a) integral slot winding

b) skewing 

c) chording

d) none of these

ans. c

10. The motor noise, vibrations, cogging and synchronous cusps can be reduced by

a) chording

b) integral slot winding

c) skewing

d) increasing the air gap length

ans. c

INDUCTION MOTORS 7

1. Imbalance in shaft of the induction motor occurs due to
a) air gap is not uniform
b) overheating of the winding
c) rugged construction
d) slip rings.
ans. a

2. The noise and tooth pulsation losses can be minimized by using
a) large number of open slots in stator
b) small number of open slots in stator
c) large number of narrow slots in stator
d) small number of narrow slots in stator
ans. c

3. Dispersion coefficient σ is the ratio of
a) magnetizing current to supply voltage
b) magnetizing current to ideal short circuit current
c) open circuit voltage to short circuit for the same excitation
d) none of these
ans. b

4. If two phases open accidentally of 3-phase supply of a running 3-phase induction motor, the motor will
a) stop
b) run at high speed
c) continue to run depending on load
d) none of these
ans. c

5. If 3-phase induction motor is switched on with one phase disconnected, it
a) is likely to burn out
b) will run at very low speed
c) will not run at all
d) none of these
ans. a

6. In squirrel-cage induction motor, if the rotor is open, the rotor
a) will not run
b) will make noise
c) will run at very slow speed
d) will run at very high speed
ans. a

7. If the input voltage and the frequency of supply is halved, to the 3-phase induction motor
a) the maximum torque is halved
b) the maximum torque remains the same
c) the air-gap flux is halved
d) none of these
ans. a

8. An induction motor crawls at lower speed. This is due to
a) skewing
b) harmonic induction torques
c) high secondary reactance
d) low secondary reactance
ans. b

9. Semi-closed stator slot and not open slot is used in induction motor
a) to increase the pull out torque
b) to decrease the slot reactance
c) to decrease magnetizing current
d) both a and b are correct
ans. c

10. In an induction motor, skew of the rotor bar reduces
a) noise
b) vibration
c) synchronous cusps
d) all of the these
ans. d

INDUCTION MOTORS 6

1. In an induction motor, the ratio of torque exerted at 4% slip to maximum torque, when slip at maximum torque is 12% will be
a) 3/5
b) 5/3
c) 1/3
d) none of these
ans. a

2. In an induction motor the rotor resistance is equal to stand-still reactance then the maximum torque is
a) less than starting torque
b) equal to starting torque
c) more than starting torque
d) none of these
ans. b

3. The complete circle diagram of an induction motor can be drawn with the help of data found from
a) no-load test
b) blocked rotor test
c) a and b
d) a, b and stator resistance test
ans. d

4. The power scale of circle diagram of an induction motor can be found from
a) no-load test only
b) short-circuit test only
c) stator resistance test
d) none of these
ans. b

5. Which of the following is not determined by circle diagram?
a) output
b) efficiency
c) power factor
d) frequency
ans. d

6. The torque of an induction motor in synchronous watt is equal to
a) rotor output
b) stator output
c) stator input
d) watts at rotor speeds
ans. b

7. The shape of torque/slip curve of an induction motor is
a) hyperbola
b) parabola
c) straight line
d) rectangular parabola
ans. d

8. When the frequency of the rotor of an induction motor is small it can be measured by
a) galvanometer
b) dc moving coil millivoltmeter
c) dc moving coil ammeter
d) ac voltmeter
ans. b

9. The stator frame in an induction motor is used
a) as a return path for the flux
b) to hold the armature stamping/stator
c) to protect the whole machine
d) to ventilate the armature
ans. a

10. The speed of a 3-phase cage rotor induction motor depends on
a) frequency of the supply only
b) number of poles only
c) number of poles and frequency of the supply
d) input voltage
ans. c 

INDUCTION MOTORS 5

1. Which of the following motors is used most frequently ?
a) DC shunt motor
b) AC induction motor
c) 3-phase commutator motor
d) 3-phase induction motor
ans. d

2. What is the advantage of the slip ring induction motor over the squirrel cage induction motor?
a) It is suitable for higher speeds
b) Its efficiency is higher
c) Its power factor is higher
d) It can be started with the help of rotor resistances
ans. d

3. The starting torque of the slip ring induction motor can be increased by
a) adding resistance to the rotor
b) adding resistance to the stator
c) adding resistance to both
d) none of these
ans.

4. What will happen to rotor copper losses of the frequency of the input to an induction motor increases?
a) Increases
b) Decreases
c) Remains same
d) None of these
ans. a

5. In an induction motor, if the phase sequence of applied voltage is changed
a) no back emf will be developed in rotor
b) no torque will be developed by rotor
c) speed will drop to zero
d) direction of rotation of motor will change
ans. d

6. If a rotor of induction motor could be theoretically rotated at synchronous speed, the voltage that would be generated in the rotor is
a) zero
b) equal to applied voltage
c) more than applied voltage
d) none of these
ans. a

7. In an induction motor, if the rotor is locked, then the rotor frequency of the induction motor will be
a) more than the supply frequency
b) zero
c) equal to the supply frequency
d) less than the supply frequency
ans. c

8. When measuring the no load input to an induction motor, using two wattmeter method, it is necessary to subtract wattmeter readings , because the  no load power factor is
a) lagging
b) less than 0.5
c) unity
d) zero
ans. b

9. The purpose of skewing of rotor slots in induction motor is
a) to reduce the magnetic hum of motor
b) to increase the distribution factor
c) to reduce the locking tendency of rotor
d) to increase the breadth factor
ans. c

10. If the speed is 460 rpm then the number of poles of a 50 Hz induction motor are
a) 4
b) 8
c) 12
d) 18
ans. c

INDUCTION MOTORS 4

1. In which of the following motors, external resistance can be added to start the motor?
a) a squirrel cage induction motor
b) a slip ring induction motor
c) a wound synchronous motor
d) a salient pole synchronous motor
ans. b

2. which of the following lubricants is the shaft of the motor lubricated?
a) Graphite
b) Grease
c) Silicon oil
d) Mineral oil
ans. b

3. An induction motor has a rated speed of 715 rpm. How many poles has its rotating magnetic field?
a) 2 poles
b) 4 poles
c) 6 poles
d) none of these
ans. d

4. The rated speed of an induction motor is 1410 rpm. What is meant by the statement " the slip s=1"?
a) The speed of the motor is higher than the synchronous speed
b) The motor runs at a synchronous speed
c) The motor runs at its rated speed
d) The rotor is stationary
ans. d

5. The injected emf in the rotor of an induction motor must have
a) the same frequency as the slip frequency
b) the same phase as the motor emf
c) a high value for satisfactory speed control
d) none of these
ans. a

6. The field of an induction motor rotor rotates with reference to stator at
a) synchronous speed
b) slip speed
c) rotor speed
d) very low speed
ans. a

7. The reactance per phase as compared to the resistance per phase of and induction motor is
a) quite high
b) slightly large
c) almost same
d) very small
ans. a

8. Which of the following is most economical method for starting single-phase motor?
a) Capacitor-start method
b) Split phase method
c) Induction start method
d) Resistance start method
ans. a

9. Which of the following motors is most suitable for signalling devices and timer
a) DC series motor
b) DC shunt motor
c) two phase induction motor
d) reluctance motor
ans. d

10. Which of the following consumers draws inductive reactive power ?
a) an electrical heater
b) a filament lamp
c) an electrolysis system which draws the current via transformers and rectifiers
d) a 3-phase induction motor
ans. d

INDUCTION MOTORS 3

1. The number of slip rings in a 3-phase wound motor induction motor is
a) 3
b) 4
c) 6
d) 12
ans. a

2. The core of the rotor of a 3-phase induction motor
a) is always laminated
b) is solid
c) is sometimes laminated
d) none of these
ans. a

3. Under running condition the rotor circuit of a 3-phase induction motor is
a) always closed
b) always open
c) sometimes open and sometimes closed
d) none of these
ans. a

4. Under blocked rotor conditions, a 3-phase induction motor is similar to a transformer
a) with secondary open-circuited
b) with secondary under-loaded
c) with secondary short circuited
d) none of these
ans. c

5. Which of the following motors is used most frequently?
a) dc shunt motor
b) ac induction motor
c) 3-phase commutator rotor
d) 3-phase induction motor
ans. d

6. Which motor is widely used?
a) A squirrel cage induction motor
b) A slip-ring induction motor
c) A wound rotor synchronous motor
d) A salient pole synchronous motor
ans. a

7. An induction motor has a rated speed of 725 rpm. How many poles has its rotating magnetic field?
a) 2 poles
b) 4 poles
c) 6 poles
d) 8 poles
ans. d

8. Synchronous speed is defined as
a) The speed of the rotor of an induction motor
b) The speed of a synchronous motor
c) The speed of an induction motor at no-load
d) The natural speed at which a magnetic field rotates
ans. d

9. The direction of rotation of a 3-phase induction motor  is reversed by
a) interchanging the connection of any two phases
b) interchanging the connection of all the 3 phases
c) rewinding the stator
d) adding a capacitor in any phase
ans. a

10. Which of the following is not a part of a squirrel cage induction motor?
a) Rotor
b) Stator
c) Carbon brushes
d) Shaft
ans. c

INDUCTION MOTORS 2

1. If a motor of lower speed is purchased in place of high speed motor, its cost will be
a) same
b) more
c) less
d) slightly less
ans. b

2. If a 3-phase induction motor runs at 25% supply than the rated voltage , the motor will
a) not start
b) start at lower speed
c) run at normal speed
d) get heated and damaged
ans. d

3. If a 3-phase induction motor runs on 2-phase supply it will
a) run at slow speed
b) run with jerks
c) likely to burn
d) blow off the fuses
ans. c

4. The speed of the squirrel cage induction motor can be controlled by
a) changing supply frequency
b) changing number of poles
c) reducing supply voltage
d) all of these
ans. d

5. The speed of the slip ring induction motor can be controlled by
a) changing supply frequency
b) changing number of poles
c) changing the resistance of the rotor winding
d) all of these
ans. d

6. The speed of the squirrel cage induction motor can be controlled from
a) stator side
b) rotor side
c) stator and rotor sides both
d) supply voltage side
ans. a

7. The speed of an induction motor is
a) always less than the synchronous speed
b) can be equal to or less than the synchronous speed
c) always more than the synchronous speed
d) none of these
ans. a

8. The speed of the rotor field of an induction motor
a) is equal to synchronous speed
b) is equal to motor speed
c) is equal to zero
d) none of these
ans. a

9. Majority of the induction motors have
a) totally enclosed frame
b) drip proof type frame
c) totally enclosed fan closed frame
d) none of these
ans. c

10. The squirrel cage rotor at at 4-pole induction motor
a) can be used for only 4 pole induction motor
b) can be used for motors having 2,4 or 8 poles only
c) can be used for motor with any number of poles
d) none of these
ans. c

INDUCTION MOTORS

1. In a 3-phase induction motor, the current is produced in the rotor conductors by

a) giving ac supply 

b) giving dc supply 

c) induction effect

d) pulsating dc supply 

ans. c

2. When 3-phase supply is given to the stator of the motor , a

a) revolving field is set up

b) pulsating field is set up

c) revolving field at synchronous speed is set up

d) revolving field at the rotor speed is set up

ans. c

3. If any two phases of the 3-phase supply are interchanged , the motor will

a) run in the same direction

b) stop running

c) run in reverse direction

d) draw high current

ans. c

4. Different types of a 3-phase induction motors are 

a) squirrel cage

b) slip-ring

c) commutator

d) all of these

ans.d

5. The rotor of a 3-phase induction motor rotates in the same direction as the direction of revolving field because of 

a) Faraday's laws of electro-magnetic induction 

b) Lenz's law

c) Fleming's right hand rule

d) none of these

ans. b

6. The rotor of a 3-phase induction motor always runs at 

a) synchronous speed

b) less than synchronous speed

c) more than synchronous speed

d) none of these

ans. b

7. The synchronous speed of the revolving field depends upon 

a) number of poles

b) supply frequency

c) flux

d) a and b are correct

ans. d

8. The frequency of induced emf in the rotor circuit is 

a) maximum at stand still

b) zero at stand still

c) maximum at synchronous speed

d) none of these 

ans. a

9. If one of the 3-phases of a 3-phase induction motor is disconnected while running at light load, it will

a) stop running

b) continue to run at same speed

c) run at low speed

d) run at high speed

ans. b

10. If the rotor terminals of a 3-phase slip ring induction motor are not short-circuited and the supply is given to the stator , the motor will

a) not start

b) start running

c) run at high speed

d) run at low speed

ans. a