120 TOP Objective Synchronous Motors Questions and Answers pdf

91.    In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(a)    load on the motor
(b)    d.c. excitation only
(c)    both the speed and rotor flux
(d)    none of the above
Ans: b

92.    A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
(a)     the motor will stop
(b)     the motor continue to run in the same direction
(c)    the winding of the motor will burn
(d)    the motor will run in the reverse direction
(e)    none of the above
Ans: b

93.    The magnitude of field flux in a 3-phase synchronous motor
(a)     remains constant at all loads
(b)     varies with speed
(c)    varies with the load
(d)    varies with power factor
Ans: a

94.    The torque angle, in a synchronous motor, is the angle between
(a)    the supply voltage and the back e.m.f.
(b)    magnetising current and back e.m.f.
(c)    the rotating stator flux and rotor poles
(d)    none of the above
Ans: c

95.    Hunting in a synchronous motor cannot be due to
(a)    windage friction
(b)    variable load
(c)    variable frequency
(d)    variable supply voltage
Ans: a

96.    By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?
(a)    By changing the supply frequency
(b)    By interchanging any two phases
(c)    By changing the applied voltage
(d)    By changing the load.
Ans: a

97.    In a synchronous motor, V-curves represent relation between
(a)     armature current and field current
(b)     power factor and speed
(c)    field current and speed
(d)    field current and power factor
Ans: a

98.    In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be
(a)     3000 r.p.m.    
(b)     1500 r.p.m.
(c)    750 r.p.m.
(d)    none of the above
Ans: b

99.    A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have
(a)     no change of power factor
(b)     lagging power factor with over-excitation
(c)    leading power factor with under-excitation
(d)    leading power factor with over-excitation
Ans: d

100.   Which of the following motors is non-self starting ?
(a)     D.C. series motor
(b)     synchronous motor
(c)    Squirrel cage induction motor
(d)    Wound round induction motor
Ans: b

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