Most Synchronous Motors

101. The standard full-load power factor ratings for synchronous motors are

1. zero or 0.8 leading
2. unity or 0.8 lagging
3. unity or 0.8 leading
4. unity or zero

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102. The synchronous motor is not inherently self-starting because

1. the force required to accelerate the rotor to the synchronous speed in an instant is absent
2. the starting device to accelerate the rotor to near synchronous speed is absent
3. a rotating magnetic field does not have enough poles
4. the rotating magnetic field is produced by only 50 Hz frequency currents

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103. The torque angle, in a synchronous motor, is the angle between

1. the supply voltage and the back e.m.f.
2. magnetising current and back e.m.f.
3. the rotating stator flux and rotor poles
4. none of the above

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104. The working of a synchronous motor is similar to

1. gear train arrangement
2. transmission of mechancial power by shaft
3. distribution transformer
4. turbine

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105. To limit the operating temperature an electrical machine should have proper

1. voltage rating
2. current rating
3. power factor
4. speed

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106. Under which of the following conditions hunting of synchronous motor is likely to occur ?

1. Periodic variation of load
2. Over-excitation
3. Over-loading for long periods
4. Small and constant load

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107. When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way

1. augments it directly
2. directly opposes it
3. cross-magnetises it
4. none of the above

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108. When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is

1. more
2. less
3. equal
4. twice

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109. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

1. the motor stops
2. it runs as a reluctance motor at the same speed
3. it runs as a reluctance motor at a lower speed
4. none of the above

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110. When the field of a synchronous motor is under-excited, the power factor will be

1. leading
2. lagging
3. unity
4. zero

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