Question 1

In which of the following motors the stator and rotor fields rotate simultaneously ?
A)D.C. motor
B)Reluctance motor
C)Universal motor
D)Synchronous motor

Accepted Answer

In a synchronous motor, both the stator and rotor fields rotate at the same speed, hence they rotate simultaneously. This is because the rotor speed is synchronized with the frequency of the supply current.

Question 2

The speed of a synchronous motor
A)increases as the load increases
B)decreases as the load decreases
C)always remains constant
D)none of the above

Accepted Answer

The speed of a synchronous motor always remains constant regardless of the load, because it operates at a constant speed determined by the supply frequency and the number of poles in the motor.

Question 3

A rotory converter can also be run as a
A)d.c. shunt motor
B)d.c. series motor
C)d.c. compound motor
D)synchronous motor

Accepted Answer

D

Question 4

The maximum speed variation in a 3-phase synchronous motor is
A)10 per cent
B)6 per cent
C)4 per cent
D)zero

Accepted Answer

The speed of a 3-phase synchronous motor is constant at a given frequency, regardless of the load, which means the speed variation is zero. This is because the speed is determined by the supply frequency and the number of poles in the motor, according to the formula $N_s = \frac{120f}{P}$, where $N_s$ is the synchronous speed, $f$ is the supply frequency, and $P$ is the number of poles.

Question 5

Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
A)Phase to phase winding resistance
B)Stator winding to earthed frame
C)Rotor winding to earthed shaft
D)All of the above

Accepted Answer

The insulation resistance test on a synchronous motor is designed to measure the resistance between windings and between windings and the earthed parts of the motor, which includes phase to phase winding resistance, stator winding to earthed frame, and rotor winding to earthed shaft.

Question 6

Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?
A)High field current
B)Low short circuit ratio
C)High core losses
D)Low field current

Accepted Answer

A synchronous motor fails to pull into synchronism if the field current is too low because it cannot develop sufficient electromagnetic torque to overcome the load and reach synchronous speed.

Question 7

In a synchronous motor, the maximum power developed depends on all of the following except
A)rotor excitation
B)maximum value of coupling angle
C)direction of rotation
D)supply voltage

Accepted Answer

The maximum power developed in a synchronous motor is not influenced by the direction of rotation. It depends on rotor excitation, the maximum value of the coupling angle, and the supply voltage.

Question 8

In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
A)supplied with unbalanced voltage
B)under-loaded
C)over-loaded
D)none of the above

Accepted Answer

Negative phase sequence occurs in a 3-phase synchronous motor when it is supplied with unbalanced voltage, leading to unequal phase voltages and potential issues with motor operation.

Question 9

In a synchronous motor, damper windings are provided on
A)stator frame
B)rotor shaft
C)pole faces
D)none of the above

Accepted Answer

Damper windings are provided on the pole faces of a synchronous motor to provide starting torque and to reduce oscillations or hunting when the motor is subjected to load variations.

Question 10

The induced e.m.f. in a synchronous motor working on leading power factor will be
A)more than the supply voltage
B)less than the supply voltage
C)equal to the supply voltage
D)none

Accepted Answer

When a synchronous motor operates at a leading power factor, the induced e.m.f. is more than the supply voltage due to the demagnetizing effect of the leading current, which requires a higher induced e.m.f. to balance the supply voltage.

Question 11

The effect of increasing the load on a synchronous motor running with normal excitation is to
A)decrease both armature current and power factor
B)decrease armature current but increase power factor
C)increase armature current but decrease power factor
D)increase both its armature current and power factor

Accepted Answer

Increasing the load on a synchronous motor causes the armature current to increase in order to supply the additional power required by the load. The power factor decreases because the motor draws more reactive power to maintain the magnetic field against the increased load.

Question 12

The net armature voltage of a synchronous motor is equal to the
A)vector sum of Eb and V
B)arithmetic sum of Eb and V
C)arithmetic difference of Eb and V
D)vector difference of Eh and V

Accepted Answer

The net armature voltage of a synchronous motor is equal to the vector difference of the generated voltage in the armature (Eh) and the terminal voltage (V).

Question 13

The ratio of starting torque to running torque in a synchronous motor is
A)zero
B)one
C)two
D)infinity

Accepted Answer

Synchronous motors do not have starting torque; they require an auxiliary means or an external force to bring them to synchronous speed before they can start running on their own.

Question 14

In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
A)load on the motor
B)d.c. excitation only
C)both the speed and rotor flux
D)none of the above

Accepted Answer

The magnitude of stator back e.m.f. (Eb) in a synchronous motor primarily depends on the d.c. excitation of the motor, as it directly influences the magnetic field strength and thus the induced e.m.f.

Question 15

A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
A)the motor will stop
B)the motor continue to run in the same direction
C)the winding of the motor will burn
D)the motor will run in the reverse direction

Accepted Answer

Reversing the field current in a synchronous motor does not change the direction of rotation. The motor will continue to run in the same direction because its direction is determined by the sequence of the 3-phase power supply, not the polarity of the field current.

Question 16

The magnitude of field flux in a 3-phase synchronous motor
A)remains constant at all loads
B)varies with speed
C)varies with the load
D)varies with power factor

Accepted Answer

The magnitude of field flux in a 3-phase synchronous motor remains constant at all loads because it is determined by the DC excitation to the field windings, which does not change with load, speed, or power factor.

Question 17

The torque angle, in a synchronous motor, is the angle between
A)the supply voltage and the back e.m.f.
B)magnetising current and back e.m.f.
C)the rotating stator flux and rotor poles
D)none of the above

Accepted Answer

The torque angle in a synchronous motor is the angle between the rotating stator flux and the rotor poles. This angle is crucial for the operation of the motor, as it determines the torque produced by the motor.

Question 18

Hunting in a synchronous motor cannot be due to
A)windage friction
B)variable load
C)variable frequency
D)variable supply voltage

Accepted Answer

Hunting in a synchronous motor is caused by disturbances in its steady operation, such as changes in load, frequency, or supply voltage. Windage friction, which is the resistance encountered by the rotating parts due to air, does not cause hunting but is a constant factor affecting the motor's efficiency and heat generation.

Question 19

By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?
A)By changing the supply frequency
B)By interchanging any two phases
C)By changing the applied voltage
D)By changing the load.

Accepted Answer

The speed of a synchronous motor is determined by the supply frequency and the number of poles in the motor, according to the formula $Speed = \frac{Frequency}{Number\ of\ Poles}$. Changing the supply frequency is the only method listed that can change the motor's speed to a new fixed value. Interchanging phases, changing the applied voltage, or changing the load does not affect the speed of a synchronous motor.

Question 20

In a synchronous motor, V-curves represent relation between
A)armature current and field current
B)power factor and speed
C)field current and speed
D)field current and power factor

Accepted Answer

V-curves in a synchronous motor illustrate the relationship between the armature current and the field current at a constant load and speed. As the field current varies, the armature current first decreases, reaches a minimum value (at the point of unity power factor), and then increases, forming a V-shape.

Quiz 2: Synchronous Motor: Part B

In which of the following motors the stator and rotor fields rotate simultaneously ?

The speed of a synchronous motor

A rotory converter can also be run as a

The maximum speed variation in a 3-phase synchronous motor is

Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?

Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?

In a synchronous motor, the maximum power developed depends on all of the following except

In a 3-phase synchronous motor, the negative phase sequence exists when the motor is

In a synchronous motor, damper windings are provided on

The induced e.m.f. in a synchronous motor working on leading power factor will be

The effect of increasing the load on a synchronous motor running with normal excitation is to

The net armature voltage of a synchronous motor is equal to the

The ratio of starting torque to running torque in a synchronous motor is

In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed

The magnitude of field flux in a 3-phase synchronous motor

The torque angle, in a synchronous motor, is the angle between

Hunting in a synchronous motor cannot be due to

By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?

In a synchronous motor, V-curves represent relation between