Synchronous Motor MCQ Question and Answer

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 
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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
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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 
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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
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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
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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 
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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
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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 
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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 
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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
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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
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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
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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
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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

  • ADecreases
  • 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
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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 
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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
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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
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Correct Answer : (C)

Phasor difference

Explanation:

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

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

Load

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
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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
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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.