Synchronous Motor MCQ

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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Answer:

Salient pole rotor 

Explanation:

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

  • A0
  • B1
  • C2
  • D3 or 4
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Answer:

2

Explanation:

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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Answer:

Relative velocity between the stator and the rotor mmfs is zero

Explanation:

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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Answer:

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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Answer:

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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Answer:

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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Answer:

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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Answer:

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

Explanation:

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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Answer:

All the above machine 

Explanation:

10.A 3- phase synchronous motor has

  • AHigh starting torque
  • BNo starting torque 
  • CLow starting current
  • DLow starting torque
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Answer:

No starting torque 

Explanation:

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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Answer:

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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Answer:

Continuously

Explanation:

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

  • ALeads the applied voltage by 900
  • BLags behind the applied voltage by 900
  • CIs in phase will applied voltage
  • DZero
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Answer:

Leads the applied voltage by 900

Explanation:

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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Answer:

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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Answer:

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

Explanation:

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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Answer:

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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Answer:

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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Answer:

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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Answer:

Rotor speed either exceeds or falls below the synchronous speed

Explanation:

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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Answer:

Rotor and stator poles of opposite polarity 

Explanation: