Sunday, September 16, 2012

Synchronous Motor Objective Questions - Part 9

Synchronous Motor Objective Questions with Answers:

[1] In a synchronous motor, damper winding is provided to
A. Stabilize rotor motion
B. Suppress rotor oscillations
C. Develop necessary starting torque
D. Both B and C

[2] Synchronous capacitor is
A. An ordinary static capacitor bank
B. An over excited synchronous motor driving mechanical load
C. An over excited synchronous motor running without mechanical load
D. None of the above

[3] A synchronous machine is called as doubly excited machine because
A. It can be over excited
B. It has two sets of rotor poles
C. Both its rotor and stator are excited
D. It needs twice the normal exciting current

[4]If the field of a synchronous motor is under excited, the power factor will be
A. Lagging
B. Leading
C. Unity
D. More than unity

[5] The direction of rotation of a synchronous motor can be reversed by reversing
A. Current to the field winding
B. Supply phase sequence
C. Polarity of rotor poles
D. None of the above

[6]A synchronous motor connected to infinite busbars has at constant full-load, 100% excitation and unity pf. On changing the excitation only, the armature current will have
A. Leading pf with under-excitation
B. Leading pf with over excitation
C. Lagging pf with over excitation
D. No change of pf

[7] The maximum value of torque angle a  in a synchronous motor is.......degrees electrical
A. 45
B. 90
C. Between 45 and 90
D. Below 60

[8] The angle between the synchronous rotating stator flux and rotor poles of a synchronous motor is 
A. Synchronizing angle
B. Torque angle
C. Power factor angle
D. Slip angle

[9] In a synchronous machine when the rotor speed becomes more than the synchronous speed during hunting, the damping bars develop
A. Synchronous motor torque
B. DC motor torque
C. Induction motor torque
D. Induction generator torque

[10] When load on a synchronous motor is increased its armature current is increased provided it is
A. Normally excited
B. Over excited
C. Under exciter
D. All of the above

8:14 PM


  1. q-10 wrongly answered
    option B correct ( see V curve )

    1. All conditions excitation is the correct Ans. Because as load changes armature current also changes they are directly proportional to each other under constant excitation whatever may be the power factor

    2. No its 4 is correct because armature current wil increase for all excitation if load increases

  2. Q-10 actually it is under all conditions excitation is the right Ans because he didn't mention here power factor and even whatever be the power factor load is proportional to armature current under constant excitation...