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Electrical Engineering Objective Questions { Induction Motors }

372.  A 3-phase induction motor develops a torque 'T' when driven from a balanced 3-phase supply. The terminal voltage and frequency are halved so that the air-gap flux remains the same while the load torque is kept constant. Then the slip
A. is reduced to half
B. remains same as the previous value
C. speed is reduced to half
D. speed remains the same
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373.  In a three-phase induction motor with increase in load from light to heavy
A. both stator and rotor power factor increase
B. both stator and rotor power factor decrease
C. stator power factor increases while rotor power factor decreases
D. stator power factor decreases while rotor power factor increases
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374.  The following performance characteris tics are attributed to a 3-phase cage induction motor without skew : 1. lower magnetizing current compared to a slip ring motor.2. low efficiency.3. high pl.4. possible cogging and crawling. Which of these statements are correct?
A. 1, 2 and 3
B. 2, 3 and 4
C. 1, 3 and 4
D. 1, 2 and 4
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375.  The power input to an induction motor is 50kW when it is running at 4% slip. If the stator resistance and core losses are neglected, the rotor copper loss is equal to
A. 38 kW
B. 20 kW
C. 4 kW
D. 2 kW
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376. In order to control the speed of a slip ring 3-phase induction motor through injected voltage in its rotor circuit, this voltage and the rotor voltage should essentially be
A. in same phase
B. in quadrature
C. in phase opposition
D. of same frequency
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