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.Management Sciences
Category: Synchronous Motors
A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have_____________?
A. no change of power factor
B. lagging power factor with over-excitation
C. leading power factor with under-excitation
D. leading power factor with over-excitation
When V is the applied voltage, then the breakdown torque of a synchronous motor varies as_________?
A. V
B. V312
C. V2
D. 1/V
A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
A. speed of the motor is reduced
B. power factor is decreased
C. Eb (back e.m.f.) becomes less than V (applied voltage)
D. Er (net resultant voltage) in armature is increased
E. none of the above
The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as_____________?
A. slip torque
B. pull-out torque
C. breaking torque
D. synchronising torque
If the field of a synchronous motor is underexcited, the power factor will be_____________?
A. zero
B. unity
C. lagging
D. leading
The speed of a synchronous motor:
A. increases as the load increases
B. decreases as the load decreases
C. always remains constant
D. none of the above
The maximum power developed in a synchronous motor will depend on__________?
A. the rotor excitation only
B. the supply voltage only
C. the rotor excitation and supply volt-age both
D. the rotor excitation, supply voltage and maximum value of coupling angle (90°)
E. none of the above
When the field of a synchronous motor is under-excited, the power factor will be____________?
A. leading
B. lagging
C. unity
D. zero
The torque angle, in a synchronous motor, is the angle between_______________?
A. the supply voltage and the back e.m.f.
B. magnetising current and back e.m.f.
C. the rotating stator flux and rotor poles
D. none of the above
Armature of a synchronous machine is____________?
A. of reducing number of slip rings on the rotor
B. armature is associated with large power as compared to the field circuits
C. of difficulty of providing high voltage insulation on rotor
D. all of the above reasons
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