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Electrical Engineering Objective Questions { Synchronous Motors }

1. The d.c. field of a synchronous motor is generally short-circuited during starting period in order that
A. rotor pole flux may not interact with the rotating stator flux
B. voltage and curent induced in the field winding by the rapidly rotating stator flux may aid the damper winding in starting the motor
C. extremely high voltage may not be induced in the field winding by the rotating stator flux
D. unnecessary d.c. power may not be wasted in the field circuit
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2. Which of the following method is used to start a synchronous motor?
A. resistance starter in the armature circuit
B. star-delta starter
C. damper winding
D. damper winding in conjunction with star-delta starter
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3. When running under no-load condition and with normal excitation, armature current la drawn by a synchronous motor
A. leads the back emf Eb by a small angle
B. is large
C. lags the applied voltage V by a small angle
D. lags behind the resultant voltage ER by 90?
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4. When load on a synchronous motor running with normal excitation is increased, armature current drawn by it increases because
A. back emf Eb becomes less than applied voltage V
B. power factor is decreased
C. net resultant voltage ER in armature is increased
D. motor speed is reduced
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5. The effect of increasing load on a synchronous motor running with normal excitation is to
A. increase both its armature current and power factor
B. decrease armature current but increase power factor
C. increase armature, current but decrease power factor
D. decrease both armature current and power factor
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6. When load on a synchronous motor running with fixed excitation is increased three times, its torque angle becomes approximately
A. thrice
B. one third
C. nine times
D. 13 times
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7. The V-curves of a synchronous motor give relation between
A. back emf Eb and Ia for a fixed pl.
B. d.c. field excitation and pl. with fixed applied voltage
C. torque angle and motor load with fixed excitation
D. d.c. field current and armature current for a given load on the motor
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