Polyphase Induction Motors

61. Stepless speed control of induction motor is possible by which of the following methods?

1. e.m.f. injection in rotor eueuit
2. Changing the number of poles
3. Cascade operation
4. None of the above

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62. Temperature of the stator winding of a three phase induction motor is obtained by

1. resistance rise method
2. thermometer method
3. embedded temperature method
4. all above methods

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63. The 'cogging' of an induction motor can be avoided by

1. proper ventilation
2. using DOL starter
3. auto-transformer starter
4. having number of rotor slots more or less than the number of stator slots (not equal)

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64. The 'crawling' in an induction motor is caused by

1. high loads
2. low voltage supply
3. improper design of machine
4. harmonics developed in the motor

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65. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type

1. star connected only
2. delta connected only
3. (a) and (b) both
4. none of the above

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66. The circle diagram for an induction motor cannot be used to determine

1. efficiency
2. power factor
3. frequency
4. output

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67. The complete circle diagram of induetion motor can be drawn with the help of data found from

1. noload test
2. blocked rotor test
3. stator resistance test
4. all of the above

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68. The crawling in the induction motor is caused by

1. low voltage supply
2. high loads
3. harmonics develped in the motor
4. improper design of the machine

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69. The frame of an induction motor is usually made of

1. silicon steel
2. cast iron
3. aluminium
4. bronze

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70. The good power factor of an induction motor can be achieved if the average flux density in the air gap is

1. absent
2. small
3. large
4. infinity

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