Synchronous motors

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

2
3
4
6

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

22. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

load on the motor
d.c. excitation only
both the speed and rotor flux
none of the above

Detailed SolutionIn a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

23. The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of

zero degree
two degrees
five degrees
ten degrees

Detailed SolutionThe mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of

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

leading
lagging
unity
zero

Detailed SolutionWhen the field of a synchronous motor is under-excited, the power factor will be

25. In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then

the motor is said to be fully loaded
the torque generated is maximum
the excitation is said to be zero per cent
the excitation is said to be hundred per cent

Detailed SolutionIn a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then

26. Which of the following methods is used to start a synchronous motor ?

Damper winding
Star-delta starter
Damper winding in conjunction with star-delta starter
Resistance starter in the armature circuit

Detailed SolutionWhich of the following methods is used to start a synchronous motor ?

27. Slip-rings in a synchronous motor carry

direct current
alternating current
no current
all of the above

Detailed SolutionSlip-rings in a synchronous motor carry

28. A synchronous motor develops maximum power when load angle is

45°
60°
90°
120°

Detailed SolutionA synchronous motor develops maximum power when load angle is

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

the motor stops
it runs as a reluctance motor at the same speed
it runs as a reluctance motor at a lower speed
none of the above

Detailed SolutionWhen the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected

30. The minimum armature current of the synchronous motor corresponds to operation at

zero power factor leading
unity power factor
0.707 power factor lagging
0.707 power factor leading

Detailed SolutionThe minimum armature current of the synchronous motor corresponds to operation at

Page 1Page 2Page 3Page 4Page 5
Test 1Test 2