MCQ on synchronous motor 102-120

102. Synchronous motors are generally not self-starting because
(a) the direction of rotation is not fixed
(b) the direction of instantaneous torque reverses after half cycle✔
(c) startes cannot be used on these machines
(d) starting winding is not provided on the machines

103. In case one phase of a three-phase synchronous motor is short-circuited the motor will
(a) not start✔
(b) run at 2/3 of synchronous speed
(c) run with excessive vibrations
(d) take less than the rated load

103. A pony motor is basically a
(a) small induction motor✔
(b) D.C. series motor
(c) D.C. shunt motor
(d) double winding A.C./D.C. motor

104. A synchronous motor can develop synchronous torque
(a) when under loaded
(b) while over-excited
(c) only at synchronous speed✔
(d) below or above synchronous speed

105. A synchronous motor can be started by
(a) pony motor
(b) D.C. compound motor
(c) providing damper winding
(d) any of the above✔

106. A three-phase synchronous motor will have
(a) no slip-rings
(b) one slip-ring
(c) two slip-rings✔
(d) three slip-rings

107. 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

108. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
(a) the motor stops✔
(b) it runs as a reluctance motor at the same speed
(c) it runs as a reluctance motor at a lower speed
(d) none of the above

109. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as
(a) V✔
(b) V312
(c) V2
(d) 1/V

110. The power developed by a synchronous motor will be maximum when the load angle is
(a) zero
(b) 45°
(c) 90°✔
(d) 120°

111. 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✔

112. 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 and armature current will increase✔

113. Mostly, synchronous motors are of
(a) alternator type machines
(6) induction type machines
(c) salient pole type machines✔
(d) smooth cylindrical type machines

114. The synchronous motor is not inherently self-starting because
(a) the force required to accelerate the rotor to the synchronous speed in an instant is absent✔
(b) the starting device to accelerate the rotor to near synchronous speed is absent
(c) a rotating magnetic field does not have enough poles
(d) the rotating magnetic field is produced by only 50 Hz frequency currents

115. As the load is applied to a 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 casuing more motor current

116. Synchronous motor always runs at
(a) the synchronous speed✔
(b) less than synchronous speed
(c) more than synchronous speed
(d) none of the above

117. An over-excited synchronous motor takes
(a) leading current✔
(b) lagging current
(c) both (a) and (b)
(d) none of the above

118. The working of a synchronous motor is similar to
(a) gear train arrangement
(b) transmission of mechancial power by shaft✔
(c) distribution transformer
(d) turbine
(e) none of the above

119. 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

120. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
(a) d.c. excitation only✔
(b) speed of the motor
(c) load on the motor
(d) both the speed and rotor flux

21. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
(a) 2
(b) 3✔
(c) 4
(d) 6