Question 1

If the field of a synchronous motor is underexcited, the power factor will be
A)zero
B)unity
C)lagging
D)leading

Accepted Answer

When the field of a synchronous motor is underexcited, it behaves like an inductive load, causing the power factor to be 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

Accepted Answer

D

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

Accepted Answer

D

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

Accepted Answer

In a synchronous motor, the armature current can be large for both high and low excitation. For low excitation, the motor operates in a lagging power factor mode, drawing more current to maintain the required power output. For high excitation, it operates in a leading power factor mode, again drawing more current to counteract the excessive magnetic field.

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

Accepted Answer

Windage loss in a synchronous motor is primarily due to the friction of the rotor moving through the air and does not vary significantly with the load. It is more related to the speed of the rotor, which is constant in a synchronous motor. Copper losses, on the other hand, depend on the current flowing through the windings, which varies with load.

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

Accepted Answer

The answer of The size of a synchronous motor decreases...

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

Accepted Answer

Windage losses are related to the friction and air resistance encountered by the rotating parts of the motor, not dissipated by the stator core surface.

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

Accepted Answer

The duration of a sudden short-circuit test on a synchronous motor is usually very short, about one second, to prevent damage to the motor.

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

Accepted Answer

A synchronous machine with a low value of short-circuit ratio (SCR) has a lower stability limit. The short-circuit ratio is an indicator of the machine's ability to handle load changes without losing synchronism. A lower SCR means the machine has less capability to maintain stability under disturbances, leading to a lower stability limit.

Question 10

The construction of a synchronous motor resembles
A)a series motor
B)an induction motor
C)an alternator
D)a rotary converter

Accepted Answer

Synchronous motors have a construction similar to that of alternators (AC generators), especially in terms of their rotor and stator structures.

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

Accepted Answer

The answer of If the field winding of an unloaded...

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

Accepted Answer

Synchronous motors can be used for power factor correction when they operate at no-load and with their fields over-excited. This condition allows them to behave like capacitors, supplying reactive power to the system, thereby improving the power factor.

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

Accepted Answer

Locked-rotor torque is the maximum torque a synchronous motor will develop at rest for any angular position of the rotor, when supplied with rated voltage and frequency. This is a key parameter in the design and operation of synchronous motors, indicating the torque available to overcome initial inertia and start the motor.

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

Accepted Answer

The answer of Exciters of synchronous machines are
A)d.c. shunt machines
B)d.c....

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

Accepted Answer

The coupling angle or load angle of a synchronous motor is defined as the angle between the rotor and the stator poles of opposite polarity. This angle is crucial for the operation of the motor as it determines the torque produced.

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

Accepted Answer

The synchronous motor, when running on no-load and having negligible losses, will draw a very small stator current to supply the core losses and friction and windage losses.

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

Accepted Answer

The armature current of a synchronous motor has large values for both low and high excitation. At low excitation, the motor draws more current to maintain the required power output, and at high excitation, the excess reactive power generated leads to an increase in armature current as well.

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°)

Accepted Answer

The maximum power developed in a synchronous motor is influenced by the rotor excitation, supply voltage, and the maximum value of the coupling angle, which is typically 90 degrees.

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

Accepted Answer

A synchronous motor operating on a leading power factor and not driving a mechanical load is referred to as a synchronous condenser. It is used for power factor correction in electrical power systems.

Question 20

A synchronous motor develops maximum power when load angle is
A)45°
B)60°
C)90°
D)120°

Accepted Answer

A synchronous motor develops maximum power when the load angle is 90°. This is because the power developed by a synchronous motor is directly proportional to the sine of the load angle (δ), and the sine function reaches its maximum value at 90°.

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

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

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

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

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

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

The size of a synchronous motor decreases with the increase in

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

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

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

The construction of a synchronous motor resembles

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

For power factor correction, synchronous motors operate at

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

Exciters of synchronous machines are

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

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

The armature current of the synchronous motor

The maximum power developed in a synchronous motor will depend on

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

A synchronous motor develops maximum power when load angle is