 # Synchronous Motor MCQ Question and Answer ### 1.Synchronous motors generally have

• ASalient pole rotor
• BSmooth cylindrical rotor
• CEither salient pole or smooth cylindrical rotor
• DNone of the above
Correct Answer : (A)

Salient pole rotor

Explanation:

Synchronous motors generally have Salient pole rotor.

### 2.Number of slip rings in a 3-phase synchronous motor will be

• A0
• B1
• C2
• D3 or 4
Correct Answer : (C)

2

Explanation:

Number of slip rings in a 3-phase synchronous motor will be 2

### 3.An ideal synchronous motor has no starting torque because the

• ARotor is made up of salient poles
• BRelative velocity between the stator and the rotor mmfs is zero
• CRelative velocity between the stator and rotor mmfs is not zero
• DRotor winding is highly reactive
Correct Answer : (B)

Relative velocity between the stator and the rotor mmfs is zero

Explanation:

An ideal synchronous motor has no starting torque because the Relative velocity between the stator and the rotor mmfs is zero.

### 4.In a synchronous motor

• AThe rotor mmf and stator mmf are stationary with respect to each other
• BRotor mmf rotates slightly faster in comparison to stator mmf
• CStator mmf rotates slightly faster than rotor mmf
• DNone of the above
Correct Answer : (A)

The rotor mmf and stator mmf are stationary with respect to each other

Explanation:

### 5.A synchronous machine is revolving armature and stationary field type Under steady running condition the air gap field

• ARotates ate synchronous speed with respect to stator
• BRotates at synchronous speed in the direction of rotation of rotor
• CRemains stationary with respect to stator
• DRemains stationary with respect to rotor
Correct Answer : (C)

Remains stationary with respect to stator

Explanation:

### 6.In a synchronous machine, if the armature field axis is ahead of the field flux axis in the direction of rotation the machine operating is

• ASynchronous motor
• BSynchronous generator
• CAsynchronous motor
• DAsynchronous generator
Correct Answer : (A)

Synchronous motor

Explanation:

### 7.In a 3-phase synchronous motor

• ARemains constant at all loads
• BVaries with load
• CVaries with speed
• DVaries with power factor
Correct Answer : (A)

Remains constant at all loads

Explanation:

### 8.In a 3-phase synchronous motor

• AThe filed mmf leads the air gap flux leads the armature mmf
• BThe armature mmf leads the air gpap flux and air gap flux leads the field mmf
• CThe armature mmf leads the air gap flux and air gap flux lags behind the field mmf
• DNone of the above
Correct Answer : (B)

The armature mmf leads the air gpap flux and air gap flux leads the field mmf

Explanation:

In a 3-phase synchronous motor the armature mmf leads the air gpap flux and air gap flux leads the field mmf.

### 9.The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for

• AA dc machines
• BAn induction machine
• CA synchronous machine
• DAll the above machine
Correct Answer : (D)

All the above machine

Explanation:

The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for dc machines, induction machine and synchronous machine.

### 10.A 3- phase synchronous motor has

• AHigh starting torque
• BNo starting torque
• CLow starting current
• DLow starting torque
Correct Answer : (B)

No starting torque

Explanation:

A 3- phase synchronous motor has No starting torque.

### 11.A 10 pole, 25Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor. What is the number of poles for the synchronous motor?

• A48
• B12
• C24
• D16
Correct Answer : (C)

24

Explanation:

### 12.A 3- phase synchronous motor needs dc supply for excitation

• AContinuously
• BAt the starting instant only
• COf stator
• DNone of these
Correct Answer : (A)

Continuously

Explanation:

A 3- phase synchronous motor needs dc supply for excitation Continuously.

### 13.The armature current of a synchronous motor on no load

• ALeads the applied voltage by 90 degree.
• BLags behind the applied voltage by 90 degree
• CIs in phase will applied voltage
• DZero
Correct Answer : (A)

Leads the applied voltage by 90 degree.

Explanation:

The armature current of a synchronous motor on no load Leads the applied voltage by 90 degree.

### 14.As the load is increased on a synchronous motor, its speed

• BIncreases
• CRemains constant and additional load is supplied by shift in relative position no of the rotor with respect to stator rotating magnetic field
• DRemains constant for some time and then falls abruptly
Correct Answer : (C)

Remains constant and additional load is supplied by shift in relative position no of the rotor with respect to stator rotating magnetic field

Explanation:

### 15.The synchronous motor meets increase in load by taking more armature current as

• AThe rotor pole falls back relative to the stator pole causing an increase in motor current
• BThe back emf decreases causing an increase in motor current
• CRotating field is strengthened causing an increase in motor current
• DNone of the above
Correct Answer : (A)

The rotor pole falls back relative to the stator pole causing an increase in motor current

Explanation:

The synchronous motor meets increase in load by taking more armature current as the rotor pole falls back relative to the stator pole causing an increase in motor current.

### 16.A synchronous motor is operating with normal excitation. With the increase in load the armature current drawn from the supply main increases due to

• AIncrease in back emf
• BFall in motor speed
• CIncrease in resultant voltage across the armature
• DIncrease in power factor
Correct Answer : (C)

Increase in resultant voltage across the armature

Explanation:

### 17.The resultant voltage acting across the armature circuit of a synchronous motor (Er) is the _____of induced emf in the armature circuit (Eb) and supply voltage V

• AArithmetic sum
• BArithmetic difference
• CPhasor difference
• DPhasor sum
Correct Answer : (C)

Phasor difference

Explanation:

### 18.The phase of emf induced in the armature of a synchronous motor (Eb) depends upon

• ARotor speed
• CBoth load and speed
• DNone of the above
Correct Answer : (B)

Explanation:

### 19.In a synchronous motor, the synchronizing power come into action when

• ARotor speed either exceeds or falls below the synchronous speed
• BRotor speed is equal to synchronous speeds
• CRotor speed falls below the synchronous speed
• DRotor speed exceeds the synchronous speed
Correct Answer : (A)

Rotor speed either exceeds or falls below the synchronous speed

Explanation:

In a synchronous motor, the synchronizing power come into action when Rotor speed either exceeds or falls below the synchronous speed.

### 20.The coupling angle or load angle of a synchronous motor is defined as the space angle between the

• ARotor and stator poles of the same polarity
• BRotor and stator poles of opposite polarity
• CRotor and stator teeth
• DNone of the above
Correct Answer : (B)

Rotor and stator poles of opposite polarity

Explanation:

The coupling angle or load angle of a synchronous motor is defined as the space angle between the Rotor and stator poles of opposite polarity.