Questions and answers for exam Preparation

MCQs

Total Questions : 395 | Page 4 of 40 pages

Question 31. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

power factor as well as armature current will decrease
power factor as well as armature current will increase
power factor will increase but armature current will decrease
power factor will decrease and armature current will increase

Discuss Question

Answer: Option D. -> power factor will decrease and armature current will increase

Question 32. To limit the operating temperature an electrical machine should have proper

voltage rating
current rating
power factor
speed

Discuss Question

Answer: Option B. -> current rating

Question 33. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on

d.c. excitation only
speed of the motor
load on the motor
both the speed and rotor flux

Discuss Question

Answer: Option A. -> d.c. excitation only

Question 34. 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

Discuss Question

Answer: Option B. -> d.c. excitation only

Question 35. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

increase in both armature current and power factor angle
increase in back e.m.f. but decrease in armature current
increase in both armature current and power factor which is lagging
increase in torque angle but decrease in back e.m.f.

Discuss Question

Answer: Option A. -> increase in both armature current and power factor angle

Question 36. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is

Discuss Question

Answer: Option B. -> 3

Question 37. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately

One-third
Twice
Thrice
Six times

Discuss Question

Answer: Option C. -> Thrice

Question 38. 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

Discuss Question

Answer: Option B. -> the direction of instantaneous torque reverses after half cycle

Question 39. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor

will become more
will become less
will remain unchanged
none of the above.

Discuss Question

Answer: Option B. -> will become less

Question 40. The synchronous motor is not inherently self-starting because

the force required to accelerate the rotor to the synchronous speed in an instant is absent
the starting device to accelerate the rotor to near synchronous speed is absent
a rotating magnetic field does not have enough poles
the rotating magnetic field is produced by only 50 Hz frequency currents

Discuss Question

Answer: Option A. -> the force required to accelerate the rotor to the synchronous speed in an instant is absent