Objective Synchronous Motors Questions and Answers pdf

81.    The maximum speed variation in a 3-phase synchronous motor is
(a) 10 per cent     
(b) 6 per cent
(c) 4 per cent 
(d) 2. per cent
(e) zero
Ans: e

82.    Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
(a)    Phase to phase winding resistance
(b)    Stator winding to earthed frame
(c)    Rotor winding to earthed shaft
(d)    All of the above
Ans: d

83.    Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?
(a)     High field current
(b)     Low short circuit ratio
(c)    High core losses
(d)    Low field current
Ans: d

84.    In a synchronous motor, the maximum power developed depends on all of the following except
(a)    rotor excitation
(b)    maximum value of coupling angle
(c)    direction of rotation
(d)    supply voltage
Ans: c

85.    In a 3-phase synchronous motor, the negative phase sequence exists when the motor is
(a)    supplied with unbalanced voltage
(b)    under-loaded
(c)    over-loaded
(d)     none of the above
Ans: a

86.    In a synchronous motor, damper windings are provided on
(a) stator frame   
(b) rotor shaft
(c) pole faces
(d) none of the above
Ans: c

87.    The induced e.m.f. in a synchronous motor working on leading power factor will be
(a) more than the supply voltage
(b) less than the supply voltage
(c) equal to the supply voltage
Ans: a

88. The effect of  increasing the load on a synchronous motor running with normal excitation is to
(a)     decrease both armature current and power factor
(6)     decrease armature current but increase power factor
(c)    increase armature current but decrease power factor
(d)    increase both its armature current and power factor
Ans: c

89.    The net armature voltage of a synchronous motor is equal to the
(a)    vector sum of Eb and V
(b)    arithmetic sum of Eb and V
(c)    arithmetic difference of Eb and V
(d)    vector difference of Eh and V
Ans: d

90.    The ratio of starting torque to running torque in a synchronous motor is
(a) zero 
(b) one
(c) two 
(d) infinity
Ans: a

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