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

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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

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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

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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

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15. An induction motor is identical to

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

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

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

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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

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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

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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

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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%

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