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