Synchronous motors Archives - Page 4 of 10 - MCQ and Quiz for Exams

31. The rotor copper losses, in a synchronous motor, are met by

d.c. source

armature input

motor input

supply lines

Detailed SolutionThe rotor copper losses, in a synchronous motor, are met by

32. While starting a synchronous motor by induction motor action, field winding is usually

connected to D.C. supply

short-circuited by low resistance

supply B. short-circuited by low resistance C. kept open-circuited

none of the above

Detailed SolutionWhile starting a synchronous motor by induction motor action, field winding is usually

33. The back e.m.f. in the stator of a synchronous motor depends on

number of poles

flux density

rotor speed

rotor excitation

Detailed SolutionThe back e.m.f. in the stator of a synchronous motor depends on

34. A synchronous motor can develop synchronous torque

when under loaded

while over-excited

only at synchronous speed

below or above synchronous speed

Detailed SolutionA synchronous motor can develop synchronous torque

35. The efficiency of a properly designed synchronous motor will usually fall in range

60 to 70%

75 to 80%

85 to 95%

99 to 99.5%

Detailed SolutionThe efficiency of a properly designed synchronous motor will usually fall in range

36. Synchronous motors are generally not self-starting because

the direction of rotation is not fixed

the direction of instantaneous torque reverses after half cycle

startes cannot be used on these machines

starting winding is not provided on the machines

Detailed SolutionSynchronous motors are generally not self-starting because

37. In a 3-phase synchronous motor, the negative phase sequence exists when the motor is

supplied with unbalanced voltage

under-loaded

over-loaded

none of the above

Detailed SolutionIn a 3-phase synchronous motor, the negative phase sequence exists when the motor is

38. 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

augments it directly

directly opposes it

cross-magnetises it

none of the above

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

39. The power developed by a synchronous motor will be maximum when the load angle is

zero

45Â°

90Â°

120Â°

Detailed SolutionThe power developed by a synchronous motor will be maximum when the load angle is

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

voltage rating

current rating

power factor

speed

Detailed SolutionTo limit the operating temperature an electrical machine should have proper

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