Deck 3: Synchronous Motors and Their Operation

Question 1

Slip-rings in a synchronous motor carry

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

Answer 1

direct current

Question 2

A synchronous machine with large air gap has

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

Answer 2

all of the above

Question 3

The armature current of the synchronous motor has higher values for

A)high excitation only
B)low excitation only
C)both (a) and (b)
D)none of the above

Answer 3

both (a) and (b)

Question 4

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

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

Question 5

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

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

Question 6

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

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

Question 7

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

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

Question 8

An important advantage of a synchronous motor over wound round induction motor is that

A)its power factor may be varied at will
B)its speed is independent of supply frequency
C)its speed may be controlled more easily
D)none of the above

Question 9

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

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

Question 10

Power factor of a synchronous motor is unity when

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

Question 11

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

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

Question 12

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

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

Question 13

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

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

Question 14

If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor

A)become more
B)become less
C)remain constant
D)none of the above

Question 15

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

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

Question 16

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

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

Question 17

Armature of a synchronous machine is

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 18

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

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 19

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

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

Question 20

Stability of a synchronous machine

A)decreases with increase in its excitation
B)increases with increase in its excitation
C)remains unaffected with increase in excitation
D)any of the above

Question 21

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

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 22

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

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

Question 23

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

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 24

If one-phase of a 3-phase synchronous motor is short-circuited, motor

A)will refuse to start
B)will overheat in spots
C)will not come upto speed
D)will fail to pull into step

Question 25

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

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