Question 1

The angle between the rotating stator flux and rotor poles is called _____ angle.

Accepted Answer

A)  torque
B)  obtuse
C)  synchronizing
D)  power factor

Question 2

If the field circuit of an unloaded salientpole synchronous motor gets suddenly open- circuited, then

Accepted Answer

A)  it runs at a slower speed
B)  the motor stops
C)  it continues to run at the same speed
D)  it runs at a very high speed

Question 3

Which of the following motors will be used in electric clocks ?

Accepted Answer

A)  D.C. shunt motor
B)  D.C. series motor
C)  A.C. induction motor
D)  A.C. synchronous motor

Question 4

A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

Accepted Answer

A)  speed of the motor is reduced
B)  power factor is decreased
C)  Eb (back e.m.f.)  becomes less than V (applied voltage) 
D)  Er (net resultant voltage)  in armature is increased

Question 5

Armature of a synchronous machine is

Accepted Answer

A)  of reducing number of slip rings on the rotor
B)  armature is associated with large power as compared to the field circuits
C)  of difficulty of providing high voltage insulation on rotor
D)  all of the above reasons

Question 6

Change of D.C. excitation of a synchronous motor changes

Accepted Answer

A)  applied voltage of the motor
B)  motor speed
C)  power factor of power drawn by the motor
D)  any of the above

Question 7

The power factor of a synchronous motor is better than that of induction motor because

Accepted Answer

A)  stator supply is relieved of responsibility of producing magnetic field
B)  mechanical load on the motor can be adjusted
C)  synchronous motor runs at synchronous speed
D)  synchronous motor has large air gap

Question 8

A synchronous machine with large air gap has

Accepted Answer

A)  a higher value of stability limit
B)  a small value of inherent regulation
C)  a higher synchronizing power which makes the machine less sensitive to load variations
D)  all of the above

Question 9

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

Accepted Answer

A)  one-third
B)  twice
C)  thrice
D)  six times

Question 10

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

Accepted Answer

A)  augments it directly
B)  directly opposes it
C)  cross-magnetises it
D)  none of the above

Question 11

When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop

Accepted Answer

A)  inductor motor torque
B)  induction generator torque
C)  synchronous motor torque
D)  d.c. motor toque

Question 12

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

Accepted Answer

A)  Damper winding
B)  Star-delta starter
C)  Damper winding in conjunction with star-delta starter
D)  Resistance starter in the armature circuit C

Question 13

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

Accepted Answer

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

Question 14

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

Accepted Answer

A)  will become more
B)  will become less
C)  will remain unchanged
D)  none of the above.

Question 15

Slip-rings in a synchronous motor carry

Accepted Answer

A)  direct current
B)  alternating current
C)  no current
D)  all of the above

Question 16

A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will

Accepted Answer

A)  remain same
B)  go down
C)  improve
D)  none of the above

Question 17

Power factor of a synchronous motor is unity when

Accepted Answer

A)  the armature current is maximum
B)  the armature current is minimum
C)  the armature current is zero
D)  none of the above B

Question 18

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

Accepted Answer

A)  connected to D.C. supply
B)  short-circuited by low resistance
C)  kept open-circuited
D)  none of the above

Question 19

While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by

Accepted Answer

A)  induction motor torque in field winding
B)  induction motor torque in damper winding
C)  eddy current and hysteresis torque in pole faces
D)  all of the above methods D

Question 20

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

Accepted Answer

A)  zero degree
B)  two degrees
C)  five degrees
D)  ten degrees

Exam 3: Synchronous Motors and Their Operation

The angle between the rotating stator flux and rotor poles is called _____ angle.

Correct AnswerverifiedShow Answer

If the field circuit of an unloaded salientpole synchronous motor gets suddenly open- circuited, then

Correct AnswerverifiedShow Answer

Which of the following motors will be used in electric clocks ?

Correct AnswerverifiedShow Answer

A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because

Correct AnswerverifiedShow Answer

Armature of a synchronous machine is

Correct AnswerverifiedShow Answer

Change of D.C. excitation of a synchronous motor changes

Correct AnswerverifiedShow Answer

The power factor of a synchronous motor is better than that of induction motor because

Correct AnswerverifiedShow Answer

A synchronous machine with large air gap has

Correct AnswerverifiedShow Answer

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

Correct AnswerverifiedShow Answer

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

Correct AnswerverifiedShow Answer

When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop

Correct AnswerverifiedShow Answer

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

Correct AnswerverifiedC

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

Correct AnswerverifiedShow Answer

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

Correct AnswerverifiedShow Answer

Slip-rings in a synchronous motor carry

Correct AnswerverifiedShow Answer

A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will

Correct AnswerverifiedShow Answer

Power factor of a synchronous motor is unity when

Correct AnswerverifiedB

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

Correct AnswerverifiedShow Answer

While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by

Correct AnswerverifiedD

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

Correct AnswerverifiedShow Answer

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C

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B

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D

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