Synchronous Motors Mcqs – (Electrical Engineering) MCQs Latest For FPSC, PPSC, NTS, KPPSC, SPSC & Other Tests
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A synchronous motor can develop synchronous torque:
A. while over-excited
B. when under loaded
C. only at synchronous speed
D. below or above synchronous speed
The maximum power developed in a synchronous motor will depend on__________?
A. the supply voltage only
B. the rotor excitation only
C. the rotor excitation and supply volt-age both
D. the rotor excitation, supply voltage and maximum value of coupling angle (90°)
E. none of the above
In a synchronous motor, the breakdown torque is__________?
A. inversely proportional to applied voltage
B. directly proportional to applied voltage
C. directly proportional to the square of the applied voltage
D. none of the above
A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as_________?
A. condenser
B. static condenser
C. synchronous condenser
D. none of the above
If the synchronous motor, properly synchronised to the supply is running on no load and is having negligible loss then_____________?
A. the stator current will be zero
B. the stator current will be very high
C. the stator current will be very small
D. the back e.m.f. will be more than the supply voltage
E. none of the above
The coupling angle or load angle of synchronous motor is defined as the angle between the__________?
A. rotor and the stator poles of the same polarity
B. rotor and the stator poles of opposite polarity
C. rotor and stator teeth
D. none of the above
The maximum torque which a synchronous motor will develop at rest for any angular position of the rotor, at rated stator supply voltage and frequency, is known as__________?
A. locked-rotor torque
B. pull up torque
C. synchronous torque
D. reluctance torque
If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will__________?
A. stop
B. run as induction motor
C. burn with dense smoke
D. function as static condenser
A synchronous machine with low value of short-circuit ratio has___________?
A. lower stability limit
B. good speed regulation
C. high stability limit
D. good voltage regulation
E. none of the above
The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as___________?
A. pull-out torque
B. pull-in torque
C. pull-up torque
D. none of the above
The construction of a synchronous motor resembles____________?
A. an induction motor
B. a series motor
C. an alternator
D. a rotary converter
For power factor correction, synchronous motors operate at____________?
A. no-load and greatly over-excited fields
B. normal load with minimum excitation
C. no-load and under-excited fields
D. normal load with zero excitation
Which of the following losses is not dissipated by the stator core surface in a synchronous motor?
A. Eddy current losses in the conductors
B. Copper losses in the slot portion of the conductors
C. Iron losses in the stator
D. Windage losses
E. None of the above
The size of a synchronous motor decreases with the increase in___________?
A. flux density
B. speed
C. horse power rating
D. all of the above
Which of the following losses, in a synchronous motor, does not vary with load?
A. Windage loss
B. Copper losses
C. Both of the above
D. None of the above
In a synchronous motor, the armature current has large values for____________?
A. low excitation only
B. high excitation only
C. both high and low excitation
D. none of the above
The back e.m.f. in the stator of a synchronous motor depends on____________?
A. flux density
B. number of poles
C. rotor speed
D. rotor excitation
E. none of the above
If the field of a synchronous motor is underexcited, the power factor will be_____________?
A. unity
B. zero
C. lagging
D. leading
In a synchronous motor, the damping winding is generally used to______________?
A. reduce the eddy currents
B. reduce noise level
C. provide starting torque only
D. prevent hunting and provide the starting torque
E. none of the above
In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then
A. the torque generated is maximum
B. the motor is said to be fully loaded
C. the excitation is said to be zero per cent
D. the excitation is said to be hundred per cent
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. 750 r.p.m.
B. 1500 r.p.m.
C. 3000 r.p.m.
D. none of the above
In a synchronous motor, V-curves represent relation between____________?
A. armature current and field current
B. field current and speed
C. power factor and speed
D. field current and power factor
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 changing the applied voltage
C. By interchanging any two phases
D. By changing the load
The torque angle, in a synchronous motor, is the angle between_______________?
A. magnetising current and back e.m.f.
B. the supply voltage and the back e.m.f.
C. the rotating stator flux and rotor poles
D. none of the above
A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
A. the winding of the motor will burn
B. the motor continue to run in the same direction
C. the motor will stop
D. the motor will run in the reverse direction
E. none of the above
The net armature voltage of a synchronous motor is equal to the_______________?
A. arithmetic sum of Eb and V
B. vector sum of Eb and V
C. arithmetic difference of Eb and V
D. vector difference of Eh and V
In a synchronous motor, damper windings are provided on____________?
A. rotor shaft
B. stator frame
C. pole faces
D. none of the above
In a 3-phase synchronous motor, the negative phase sequence exists when the motor is_______________?
A. supplied with unbalanced voltage
B. over-loaded
C. under-loaded
D. none of the above
Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current?
A. Low short circuit ratio
B. High field current
C. High core losses
D. Low field current
A rotory converter can also be run as a___________?
A. d.c. series motor
B. d.c. shunt motor
C. d.c. compound motor
D. induction motor
E. synchronous motor
In which of the following motors the stator and rotor fields rotate simultaneously?
A. Reluctance motor
B. D.C. motor
C. Universal motor
D. Synchronous motor
E. Induction motor
If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
A. it continues to run at the same speed
B. the motor stops
C. it runs at a slower speed
D. it runs at a very high speed
A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
A. power factor is decreased
B. speed of the motor is reduced
C. Eb (back e.m.f.) becomes less than V (applied voltage)
D. Er (net resultant voltage) in armature is increased
E. none of the above
If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor
A. will remain unchanged
B. will become less
C. will become more
D. none of the above.
Stability of a synchronous machine:
A. remains unaffected with increase in excitation
B. increases with increase in its excitation
C. decreases with increase in its excitation
D. any of the above
If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to____________?
A. increase in both armature current and power factor angle
B. increase in torque angle but decrease in back e.m.f.
C. increase in both armature current and power factor which is lagging
D. increase in back e.m.f. but decrease in armature current
While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerating torque is produced by_____________?
A. induction motor torque in damper winding
B. induction motor torque in field winding
C. eddy current and hysteresis torque in pole faces
D. reHetance motor torque due to saliency of the rotor
E. all of the above methods
A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will___________?
A. go down
B. remain same
C. improve
D. none of the above
Which of the following motors will be used in electric clocks?
A. D.C. series motor
B. D.C. shunt motor
C. A.C. induction motor
D. A.C. synchronous motor
Power factor of a synchronous motor is unity when_____________?
A. the armature current is zero
B. the armature current is minimum
C. the armature current is maximum
D. none of the above
Which of the following methods is used to start a synchronous motor?
A. Star-delta starter
B. Damper winding
C. Damper winding in conjunction with star-delta starter
D. Resistance starter in the armature circuit
In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately
A. twice
B. one-third
C. thrice
D. six times
E. nine times
A synchronous machine with large air gap has______________?
A. a small value of inherent regulation
B. a higher value of stability limit
C. a higher synchronizing power which makes the machine less sensitive to load variations
D. all of the above
To limit the operating temperature an electrical machine should have proper____________?
A. power factor
B. current rating
C. voltage rating
D. speed
When the voltage applied to a synchronous motor is increased, which of the following will reduce?
A. Pull in torque
B. Stator flux
C. Both A. and (b)
D. None of the above
An over excited synchronous motor is used for____________?
A. variable speed loads
B. fluctuating loads
C. low torque loads
D. power factor corrections
Slip rings are usually made of__________?
A. silver or gold
B. brass or steel
C. carbon or graphite
D. copper or aluminium
A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because?
A. mechanical load on the rotor remains constant
B. stator supply is not required to produce magnetic field
C. synchronous motor has no slip
D. synchronous motor has large airgap
Synchronous motors are_____________?
A. not-self starting
B. essentially self-starting
C. self-starting
D. none of the above
Synchronsizing power of a synchronous machine is_____________?
A. equal to the synchronous reactance
B. inversely proportional to the synchronous reactance
A. direcly proportional to the synchronous reactance
D. none of the above
When the stator windings are connected in such a fashion that the number of poles are made half, the speed of the rotor of a synchronous motor___________?
A. decreases to half the original value
B. remains same as the original value
C. tends to becomes zero
D. increases to two times the original value
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. equal
B. less
C. more
D. twice
Riunting in a synchronous motor takes place___________?
A. when power factor is unity
B. when load varies
C. when supply voltage fluctuates
D. motor is under loaded
The percentage slip in case of a synchronous motor is____________?
A. 100%
B. 1%
C. 0.5%
D. zero
When the field of a synchronous motor is under-excited, the power factor will be____________?
A. unity
B. lagging
C. leading
D. zero
The oscillations in a synchronous motor can be damped out by_______________?
A. running the motor on leading power factors
B. maintaining constant excitation
C. providing damper bars in the rotor pole faces
D. oscillations cannot be damped
A synchronous motor can operate at____________?
A. leading power factor only
B. lagging power factor only
C. unity power factor only
D. lagging, leading and unity power factors
If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is__________?
A. 4
B. 3
C. 2
D. 6
The minimum armature current of the synchronous motor corresponds to operation at______________?
A. 0.707 power factor lagging
B. unity power factor
C. zero power factor leading
D. 0.707 power factor leading
Synchronous motor always runs at______________?
A. the synchronous speed
B. more than synchronous speed
C. less than synchronous speed
D. none of the above