Polyphase Induction Motors

11. 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

  1. stall after sometime
  2. stall immediately
  3. continue to run at lower speed without damage
  4. get heated and subsequently get damaged

Correct answer: (D)
get heated and subsequently get damaged

12. A squirrel cage induction motor is not selected when

  1. initial cost is the main consideration
  2. maintenance cost is to be kept low
  3. higher starting torque is the main consideration
  4. all above considerations are involved

Correct answer: (C)
higher starting torque is the main consideration

13. An induction motor can run at synchronous speed when

  1. it is run on load
  2. it is run in reverse direction
  3. it is run on voltage higher than the rated voltage
  4. e.m.f. is injected in the rotor circuit

Correct answer: (D)
e.m.f. is injected in the rotor circuit

14. An induction motor is

  1. self-starting with zero torque
  2. self-starting with high torque
  3. self-starting with low torque
  4. non-self starting

Correct answer: (C)
self-starting with low torque

15. An induction motor is identical to

  1. D.C. compound motor
  2. D.C. series motor
  3. synchronous motor
  4. asynchronous motor

Correct answer: (D)
asynchronous motor

16. An induction motor with 1000 r.p.m. speed will have

  1. 8 poles
  2. 6 poles
  3. 4 poles
  4. 2 poles

Correct answer: (B)
6 poles

17. As load on an induction motor goes on increasing

  1. its power factor goes on decreasing
  2. its power factor remains constant
  3. its power factor goes on increasing even after full load
  4. its power factor goes on increasing up to full load and then it falls again

Correct answer: (D)
its power factor goes on increasing up to full load and then it falls again

18. Blocked rotor test on induction motors is used to find out

  1. leakage reactance
  2. power factor on short circuit
  3. short-circuit current under rated voltage
  4. all of the above

Correct answer: (D)
all of the above

19. DOL starting of induction motors is usually restricted to

  1. low horsepower motors
  2. variable speed motors
  3. high horsepower motors
  4. high speed motors

Correct answer: (A)
low horsepower motors

20. efficiency of an induction motor can be expected to be nearly

  1. 60 to 90%
  2. 80 to 90%
  3. 95 to 98%
  4. 99%

Correct answer: (B)
80 to 90%

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