Synchronous motors

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
45°
90°
120°

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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