Most Synchronous Motors

31. For power factor correction, synchronous motors operate at

1. no-load and greatly over-excited fields
2. no-load and under-excited fields
3. normal load with minimum excitation
4. normal load with zero excitation

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32. For V-curves for a synchronous motor the graph is drawn between

1. field current and armature current
2. terminal voltage and load factor
3. power factor and field current
4. armature current and power factor

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33. he armature current of the synchronous motor

1. has large values for low excitation i niy
2. has large values for high excitation only
3. has large values for low and high excitation
4. any of the above

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34. Hunting in a synchronous motor cannot be due to

1. windage friction
2. variable load
3. variable frequency
4. variable supply voltage

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35. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

1. increase in both armature current and power factor angle
2. increase in back e.m.f. but decrease in armature current
3. increase in both armature current and power factor which is lagging
4. increase in torque angle but decrease in back e.m.f.

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36. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor

1. will become more
2. will become less
3. will remain unchanged
4. none of the above.

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37. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor

1. become more
2. become less
3. remain constant
4. none of the above

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38. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is

1. 2
2. 3
3. 4
4. 6

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39. If one-phase of a 3-phase synchronous motor is short-circuited, motor

1. will refuse to start
2. will overheat in spots
3. will not come upto speed
4. will fail to pull into step

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40. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then

1. it runs at a slower speed
2. the motor stops
3. it continues to run at the same speed
4. it runs at a very high speed

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