Synchronous Motor Quiz | Synchronous Motor Objective Type Questions and Answers

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Questions
21 The standard full-load power factor ratings for synchronous motors are
A Zero or 0.8 leading
B Unity or 0.8 lagging
C Unity or 0.8 leading
D Unity or zero

Answer: Unity or 0.8 leading
22 The angle between the synchronously-roating stator flux and rotor poles of a synchronous motor is called....... angle
A Synchronizing
B Torque
C Power factor
D Slip

Answer: Torque
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23 An electric motor in which both the rotor and stator fields rotates with the same speed is called aa/an ....... motor
A D.C.
B Charge
C Synchronous
D Universal

Answer: Synchronous
24 The maximum power developed in a synchronous motor occurs at a coupling angle of
A 30 degree
B 90 degree
C 60 degree
D 180 degree

Answer: 90 degree
25 The minimum armature current of the synchronous motor corresponds to operation at
A Zero power factor leading
B Unity power factor
C 0.707 power factor lagging
D 0.707 power factor leading

Answer: Unity power factor
26 When load on an over-excited or under-excited synchronous motor is increased, rate of change of its armature current as compared with that of power factor is
A More
B Less
C Equal
D Twice

Answer: Less
27 A synchronous motor connected to infinite bus-bars has at constant full-load, 100% excitation and unity p.f. on changing the excitation only, the armature current will have
A Leading p.f. with under-excitation
B Leading p.f. with over-excitation
C Lagging p.f. with over-excitation
D No change of p.f.

Answer: Leading p.f. with over-excitation
28 The standard full-load power factor ratings for synchronous motors are
A Zero or 0.8 leading
B Unity or 0.8 lagging
C Unity or 0.8 leading
D Unity or zero

Answer: Unity or 0.8 leading
29 As the load is applied to synchronous motor, the motor takes more armature current because
A The increased load has to take more current
B The rotor by shifting its phase backward causes motor to take more current
C The back e.m.f. decreases causing an increase in motor current
D The rotor strengthens the rotating field causing more motor current

Answer: The rotor by shifting its phase backward causes motor to take more current
30 A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased
A Power factor as well as armature current will decrease
B Power factor as well as armature current will increase
C Power factor will increase but armature current will decrease
D Power factor will decrease but armature current will increase

Answer: Power factor will decrease but armature current will increase
31 A synchronous machine with large air gap has
A A higher value of stability limit
B A small value of inherent regulation
C A higher synchronizing power which makes the machine less sensitive to load variations
D All of the above

Answer: All of the above
32 The oscillations in a synchronous motor can be damped out by
A Maintaining constant excitation
B Running the motor on leading power factors
C Providing damper bars in the rotor poles faces
D Oscillations cannot be damped

Answer: Providing damper bars in the rotor poles faces
33 Under which of the following conditions hunting of synchronous motor is likely to occur
A Periodic variation of load
B Over-excitation
C Over-loading for long periods
D Small and constant load

Answer: Periodic variation of load
34 As the load is applied to synchronous motor, the motor takes more armature current because
A The increased load has to take more current
B The rotor by shifting its phase backward causes motor to take more current
C The back e.m.f. decreases causing an increase in motor current
D The rotor strengthens the rotating field causing more motor current

Answer: The rotor by shifting its phase backward causes motor to take more current
35 A synchronous motor can be used as a synchronous capacitor when it is
A Under-loaded
B Over-loaded
C Under-excited
D Over-excited

Answer: Over-excited
36 Hunting in a synchronous motor takes place
A When supply voltage fluctuates
B When load varies
C When power factor is unity
D Motor is under loaded

Answer: When load varies
37 The shaft of synchronous motor is made of
A Mild steel
B Chrome steel
C Alnico
D Stainless steel

Answer: Mild steel
38 The percentage slip in case of a synchronous motor is
A 1%
B 100%
C 0.5%
D Zero

Answer: Zero
39 A synchronous motor working at leading power factor can be used as
A Voltage booster
B Phase advancer
C Noise generator
D Mechanical synchronizer

Answer: Phase advancer
40 A synchronous motor will always stop when
A Supply voltage fluctuates
B Load in motor varies
C Excitation winding gets disconnected
D Supply voltage frequency changes

Answer: Excitation winding gets disconnected

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