Deck 5: Synchronous Motor: Power Factor, Back EMF, Torque, Losses

Question 1

If the field of a synchronous motor is underexcited, the power factor will be

A)zero
B)unity
C)lagging
D)leading

Answer 1

lagging

Question 2

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

A)number of poles
B)flux density
C)rotor speed
D)rotor excitation

Answer 2

rotor excitation

Question 3

The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as

A)slip torque
B)pull-out torque
C)breaking torque
D)synchronising torque

Answer 3

synchronising torque

Question 4

In a synchronous motor, the armature current has large values for

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

Question 5

Which of the following losses, in a synchronous motor, does not vary with load?

A)Windage loss
B)Copper losses
C)Any of the above
D)None of the above

Question 6

The size of a synchronous motor decreases with the increase in

A)flux density
B)horse power rating
C)speed
D)all of the above

Question 7

Which of the following losses is not dissipated by the stator core surface in a synchronous motor ?

A)Eddy current losses in the conductors
B)Iron losses in the stator
C)Copper losses in the slot portion of the conductors
D)Windage losses

Question 8

The duration of sudden snort-circuit test on a synchronous motor is usually about

A)one hour
B)one minute
C)one second
D)none of the above

Question 9

A synchronous machine with low value of short-circuit ratio has

A)lower stability limit
B)high stability limit
C)good speed regulation
D)good voltage regulation

Question 10

The construction of a synchronous motor resembles

A)a series motor
B)an induction motor
C)an alternator
D)a rotary converter

Question 11

If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will

A)stop
B)run as induction motor
C)function as static condenser
D)burn with dense smoke

Question 12

For power factor correction, synchronous motors operate at

A)no-load and greatly over-excited fields
B)no-load and under-excited fields
C)normal load with minimum excitation
D)normal load with zero excitation

Question 13

The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as

A)locked-rotor torque
B)synchronous torque
C)pull up torque
D)reluctance torque

Question 14

Exciters of synchronous machines are

A)d.c. shunt machines
B)d.c. series machines
C)d.c. compound machines
D)any of the above

Question 15

The coupling angle or load angle of synchronous motor is defined as the angle between the

A)rotor and stator teeth
B)rotor and the stator poles of opposite polarity
C)rotor and the stator poles of the same polarity
D)none of the above

Question 16

If the synchronous motor, properly synchronised to the supply is running on no load and is having negligible loss then

A)the stator current will be zero
B)the stator current will be very small
C)the stator current will be very high
D)the back e.m.f. will be more than the supply voltage

Question 17

The armature current of the synchronous motor

A)has large values for low excitation i niy
B)has large values for high excitation only
C)has large values for low and high excitation
D)any of the above

Question 18

The maximum power developed in a synchronous motor will depend on

A)the rotor excitation only
B)the supply voltage only
C)the rotor excitation and supply volt-age both
D)the rotor excitation, supply voltage and maximum value of coupling angle (90°)

Question 19

A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as

A)static condenser
B)condenser
C)synchronous condenser
D)none of the above

Question 20

A synchronous motor develops maximum power when load angle is

A)45°
B)60°
C)90°
D)120°

Question 21

In a synchronous motor, the breakdown torque is

A)directly proportional to applied voltage
B)directly proportional to the square of the applied voltage
C)inversely proportional to applied voltage
D)none of the above