Polyphase Induction Motors

31. In an induction motor the relationship between stator slots and rotor slots is that

1. stator slots are equal to rotor slots
2. stator slots are exact multiple of rotor slots
3. stator slots are not exact multiple of rotor slots
4. none of the above

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32. In an induction motor, no-load the slip is generally

1. less than 1%
2. 1.5%
3. 2%
4. 4%

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33. In case of 3-phase induction motors, plugging means

1. pulling the motor directly on line without a starter
2. locking of rotor due to harmonics
3. starting the motor on load which is more than the rated load
4. interchanging two supply phases for quick stopping

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34. In case of a double cage induction motor, the inner cage has

1. high inductance arid low resistance
2. low inductance and high resistance
3. low inductance and low resistance
4. high inductance and high resistance

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35. In case of the induction motors the torque is

1. inversely proportional to (Vslip)
2. directly proportional to (slip)2
3. inversely proportional to slip
4. directly proportional to slip

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36. In case the air gap in an induction motor is increased

1. the magnetising current of the rotor will decrease
2. the power factor will decrease
3. speed of motor will increase
4. the windage losses will increase

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37. In induction motor, percentage slip depends on

1. supply frequency
2. supply voltage
3. copper losses in motor
4. none of the above

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38. In medium sized induction motors, the slip is generally around

1. 0.04%
2. 0.4%
3. 4%
4. 14%

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39. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

1. Ns
2. s.N,
3. (l-s)Ns
4. (Ns-l)s

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40. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

1. reduce windage losses
2. reduce eddy currents
3. reduce accumulation of dirt and dust
4. reduce magnetic hum

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