Induction Motor Questions

1. The frame of an induction motor is made of

carbon
(a) closed grained cast iron
(b) aluminium
(c) stainless steel

2. Slip rings for induction motors are made of

(a) phosphor bronze
(b) aluminium
(c) carbon
(d) cobalt steel

3 . If ‘s’ is the slip and ‘f’ is the supply frequency, the frequency of rotor current is given by

(a) sf
(b) s²f
(c) sf/2
(d) f/s

4 . Type of bearing used for 25 H.P. motor is

(a) roller bearing
(b) bush bearing
(c) needle bearing
(d) ball bearing

5 . When an
induction motor is switched on the rotor frequency is

(a) zero
(b) 60 Hz
(c) same as slip frequency
(d) same as supply frequency

6 . In an induction motor, the rotor reactance per phase is proportional to

(a) s
(b) 1/s
(c) 1/s²
(d) s²

7. The shaft, on which the rotor of an induction motor is mounted is made of

(a) high speed steel
(b) chrome vanadium steel
(c) cast iron
(d) mild steel

8. Which of the following type of bearing is generally used to support the rotor of an induction motor ?

(a) Ball bearing
(b) Needle bearing
(c) Plummer block
(d) Bush bearing

9. The stator of 5 H.P. induction motor is provided with

(a) open slots with tapered teeth
(b) closed slots with parallel teeth
(c) semi-closed slots with parallel teeth
(d) open slots with parallel teeth

10. A wound rotor induction motor is usually not selected when

(a) variable speed operation is desired
(b) cost is the main consideration
(c) high starting torque is the main consideration
(d) external voltage is to be fed into rotor

11. Which of the following features of induction motor helps in preventing cogging of motor?

(a) Skewed slots
(b) High slip
(c) Use of better insulating materials
(d) Large number of poles

12.Synchronous wattage of induction motor means

(a) stator input in watts
(b) rotor input in watts
(c) combined stator and rotor input in watts
(d) shaft output in watts

13. If stator voltage of a squirrel cage induction motor is reduced to one-half its normal value, its starting current is reduced to…………. percent of its full voltage value.

(a) 75
(b) 60
(c) 50
(d) 15

14. If stator voltage of a squirrel cage induction motor is reduced to 50 percent of its rated value, torque developed is reduced by ………… percent of its full load value.

(a) 90
(b) 75
(c) 50
(d) 25

15. In a 3-phase induction motor which of the following statements regarding frequency of induced rotor e.m.f. is incorrect ?

(a) Its speed varies inversely as slip.
(b) It is zero at synchronous speed.
(c) It is directly dependent on slip.
(d) It is maximum at standstill.

16. The stator frame in an induction motor is used

(a) to protect the whole machine
(b) to ventilate the armature
(c) as a return path for the flux
(d) to hold the armature stamping/ stator

17. The noise and tooth pulsation losses can be minimized by using

(a) large number of narrow slots in stator
(b) large number of open slots In stator
(c) small number of narrow slots in stator
(d) number of open small slots in stator

18. The value of average flux density in air gap in an induction motor should be small

(a) to achieve good efficiency
(b) to get poor power factor
(c) to get good power factor
(d) for minimum cost

19. An induction motor with large number of slots has

(a) low over-load capacity
(b) high over-load capacity
(c) either of the above
(d) none of the above

20. In an induction motor the pulsation losses and noise can be reduced by using

(a) large number of semi-open slots
(b) large number of narrow slots
(c) none of the above

21. Whenever any polyphase induction motor is loaded

(a) induced e.m.f. decreases and frequency increases
(b) induced e.m.f. in the rotor remains constant
(c) induced em.f. in the rotor increases and its frequency also increases
(d) induced e.m.f in the rotor increases and its frequency falls

22. Which of the following motors requires the most complicated speed control arrangement?

(a) D.C. shunt motor
(b) Rotor supplied three phase commutator
(c) Stator supplied three phase commutator
(d) Three-phase squirrel cage induction motor

23. A 3-phase induction motor is running at a load of the rated torque. What happens when one of the outer mains is interrupted while the motor is running?

(a) Motor keeps on running and the current drawn does not change.
(b) Motor keeps on running but draws more current.
(c) The motor stops immediately.
(d) The motor stops after a few seconds.

24. What Is the function of putting resistance in parallel to one phase of 3-phase induction motor ?

(a) To attain a higher starting torque.
(b) To achieve a smooth starting torque.
(c) To reduce the starting current to a very low value.
(d) To attain a higher maximum torque

25. What is the disadvantage of the speed control of slip-ring induction motor with the help of resistances in the rotor circuit ?

(a) By using this method the speed can be easily controlled.
(b) This method is associated with high losses.
(c) With reductions in torque the speed decreases considerably.
(d) None of the above.

26. The reactance per phase as compared to the resistance per phase of an induction motor is

(a) very small
(b) quite high
(c) almost same
(d) slightly large

27. What is the disadvantage of starting an induction motor with a star-delta starter?

(a) The starting torque is one third of the torque in case of the delta connection.
(b) During starting high losses are produced.
(c) The starting torque increases and the motor runs with jerks.
(d) None of the above.

28. The induction motors which are provided with open slots have

(a) more break down torque
(b) less break down torque
(c) leading power factor
(d) higher efficiency

29. For a 50 H.P. motor, which type of rotor will be preferred ?

(a) Die cast aluminum rotor.
(b) Wound rotor.
(c) Squirrel cage rotor using rectangular copper bars.
(d) Squirrel cage rotor using round copper bars.

30. If a three phase squirrel cage induction motor runs slow, which of the following could not be the reason for the same ?

(a) Shorted stator coils
(b) Low voltage
(c) Overload
(d) High frequency

1. Polyphase Induction Motor MCQ
2. Induction Motor Questions
3. Induction Motor Question Bank
4. Induction Motor MCQ Questions
5. Induction Motor MCQ PDF
6. Squirrel Cage Induction Motor MCQ
7. Induction Motor Objective Questions
8. Induction Motor Objective Type Questions
9. Speed Control of Induction Motor MCQ

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