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polyphase induction motors MCQs

Practise focused polyphase induction motors questions with marked answers for exam preparation and subject revision.

polyphase induction motors Mcqs
1. In case the air gap in an induction motor is increased____________?
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A. speed of motor will increase

B. the magnetising current of the rotor will decrease

C. the windage losses will increase

D. the power factor will decrease
2. The stating torque of the slip ring induction motor can be increased by adding__________?
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A. external inductance to the rotor

B. external resistance to the rotor

C. external capacitance to the rotor

D. both resistance and inductance to rotor

3. The starting torque of a squirrel-cage induction motor is___________?
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A. negligible

B. slightly more than full-load torque

C. same as full-load torque

D. low
4. If any two phases for an induction motor are interchanged_______?
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A. the motor will run in reverse direction

B. the motor will burn

C. the motor will not run

D. the motor will run at reduced speed

5. A change of 4% of supply voltage to an induction motor will produce a change of approximately ________?
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A. 4% in the rotor torque

B. 12% in the rotor torque

C. 8% in the rotor torque

D. 16% in the rotor torque
6. In an induction motor, no-load the slip is generally____________?
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A. less than 1%

B. 1.50%

C. 4%

D. 2%

7. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines?
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A. none of the above

B. universal motor

C. D.C. shunt motor

D. D.C. series motor

8. An induction motor can run at synchronous speed when____________?
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A. it is run on voltage higher than the rated voltage

B. it is run on load

C. it is run in reverse direction

D. e.m.f. is injected in the rotor circuit
9. Which of the following component is usually fabricated out of silicon steel ?
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A. None of the above

B. Statorcore

C. Shaft

D. Bearings

10. The maximum torque in an induction motor depends on___________?
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A. square of supply voltage

B. rotor inductive reactance

C. frequency

D. all of the above
11. The term cogging is associated with___________?
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A. induction motors

B. D.C. series motors

C. compound generators

D. three phase transformers

12. A squirrel cage induction motor is not selected when___________?
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A. all above considerations are involved

B. maintenance cost is to be kept low

C. initial cost is the main consideration

D. higher starting torque is the main consideration
13. Temperature of the stator winding of a three phase induction motor is obtained by___________?
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A. all above methods

B. embedded temperature method

C. resistance rise method

D. thermometer method

14. It is advisable to avoid line-starting of induction motor and use starter because__________?
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A. it will run in reverse direction

B. starting torque is very high

C. motor takes five to seven times its full load current

D. it will pick-up very high speed and may go out of step

15. Slip ring motor is preferred over squirrel cage induction motor where___________?
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A. heavy pull out torque is required

B. load torque is heavy

C. high starting torque is required

D. all of the above

16. In a star-delta starter of an induction motor___________?
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A. applied voltage perl stator phase is 57.7% of the line voltage

B. resistance is inserted in the stator

C. resistance is inserted in the rotor

D. reduced voltage is applied to the stator

17. The crawling in an induction motor is caused by___________?
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A. improper design of machine

B. high loads

C. harmonics developed in the motor

D. low voltage supply

18. A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will____________?
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A. stall after sometime

B. continue to run at lower speed without damage

C. get heated and subsequently get damaged

D. stall immediately

19. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be___________?
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A. Ns

B. (l-s)Ns

C. (Ns-l)s

D. s.N,

20. In three-phase induction motors sometimes copper bars are placed deep in the rotor to___________?
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A. improve starting torque

B. reduce copper losses

C. improve power factor

D. improve efficiency