Synchronous Motors Mcqs – (Electrical Engineering) MCQs Latest For FPSC, PPSC, NTS, KPPSC, SPSC & Other Tests
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Under which of the following conditions hunting of synchronous motor is likely to occur?0
A. Periodic variation of load
B. Over-loading for long periods
C. Over-excitation
D. Small and constant load
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 increase
B. power factor as well as armature current will decrease
C. power factor will increase but armature current will decrease
D. power factor will decrease and armature current will increase
As the load is applied to a synchronous motor, the motor takes more armature current because
A. the rotor strengthens the rotating field causing more motor 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 increased load has to take more current
The power developed by a synchronous motor will be maximum when the load angle is____________?
A. 45°
B. zero
C. 90°
D. 120°
A synchronous motor can be started by________?
A. D.C. compound motor
B. pony motor
C. providing damper winding
D. any of the above
A pony motor is basically a_________?
A. small induction motor
B. D.C. shunt motor
C. D.C. series motor
D. double winding A.C./D.C. motor
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 a lower speed
C. it runs as a reluctance motor at the same speed
D. none of the above
A synchronous motor can be used as a synchronous capacitor when it is____________?
A. over-loaded
B. under-loaded
C. under-excited
D. over-excited
Mostly, synchronous motors are of_____________?
A. induction type machines
B. alternator type machines
C. salient pole type machines
D. smooth cylindrical type machines
The working of a synchronous motor is similar to_____________?
A. distribution transformer
B. transmission of mechanical power by shaft
C. gear train arrangement
D. turbine
E. none of the above
In a synchronous motor, the magnitude of stator back e.m.f. & depends on_______________?
A. d.c. excitation only
B. load on the motor
C. speed of the motor
D. both the speed and rotor flux
The back e.m.f. of a synchronous motor depends on______________?
A. load
B. speed
C. load angle
D. all of the above
A synchronous motor can be made self starting by providing?
A. damper winding on stator
B. damper winding on rotor poles
C. damper winding on stator as well as rotor poles
D. none of the above
The shaft of synchronous motor is made of____________?
A. mild steel
B. alnico
C. chrome steel
D. stainless steel
The operating speed of a synchronous motor can be changed to new fixed value by___________?
A. changing the supply voltage
B. changing the load
C. changing frequency
D. using brakes
The maximum power developed in a synchronous motor occurs at a coupling angle of____________?
A. 60°
B. 30°
C. 90°
D. 180°
In which of the following motors the stator and rotor magnetic field rotate at the same speed?
A. Induction motor
B. Synchronous motor
C. Universal motor
D. Reluctance motor
A synchronous motor running with normal excitation adjusts to load increases essentially by increase in______________?
A. power factor
B. armature current
C. back e.m.f.
D. torque angle
The efficiency of a properly designed synchronous motor will usually fall in range___________?
A. 75 to 80%
B. 60 to 70%
C. 85 to 95%
D. 99 to 99.5%
The armature current of the synchronous motor has higher values for_____________?
A. low excitation only
B. high excitation only
C. both A. and (b)
D. none of the above
When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop_________________?
A. synchronous motor torque
B. induction generator torque
C. inductor motor torque
D. d.c. motor toque
E. none of the above
While starting a synchronous motor by induction motor action, field winding is usually_____________?
A. kept open-circuited
B. short-circuited by low resistance
C. connected to D.C. supply
D. none of the above
Synchronous Motors Mcqs
Armature of a synchronous machine is____________?
A. of difficulty of providing high voltage insulation on rotor
B. armature is associated with large power as compared to the field circuits
C. of reducing number of slip rings on the rotor
D. all of the above reasons
The power factor of a synchronous motor is better than that of induction motor because
A. stator supply is relieved of responsibility of producing magnetic field
B. synchronous motor has large air gap
C. synchronous motor runs at synchronous speed
D. mechanical load on the motor can be adjusted
If one-phase of a 3-phase synchronous motor is short-circuited, motor will ______________?
A. will refuse to start
B. will not come upto speed
C. will overheat in spots
D. will fail to pull into step
The maximum speed variation in a 3-phase synchronous motor is____________?
A. 6 per cent
B. 10 per cent
C. 4 per cent
D. 2. per cent
E. zero
In a synchronous motor, the maximum power developed depends on all of the following except:
A. maximum value of coupling angle
B. rotor excitation
C. direction of rotation
D. supply voltage
The effect of increasing the load on a synchronous motor running with normal excitation is to___________?
A. decrease armature current but increase power factor
B. decrease both armature current and power factor
C. increase armature current but decrease power factor
D. increase both its armature current and power factor
In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on___________?
A. both the speed and rotor flux
B. d.c. excitation only
C. load on the motor
D. none of the above
Hunting in a synchronous motor cannot be due to___________?
A. windage friction
B. variable frequency
C. variable load
D. variable supply voltage
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. lagging power factor with over-excitation
B. no change of power factor
C. leading power factor with under-excitation
D. leading power factor with over-excitation
A synchronous motor develops maximum power when load angle is____________?
A. 60°
B. 45°
C. 90°
D. 120°
The armature current of the synchronous motor
A. has large values for high excitation only
B. has large values for low excitation i niy
C. has large values for low and high excitation
D. any of the above
Exciters of synchronous machines are______________?
A. d.c. compound machines
B. d.c. shunt machines
C. d.c. series machines
D. any of the above
The duration of sudden snort-circuit test on a synchronous motor is usually about_____________?
A. one minute
B. one hour
C. one second
D. none of the above
The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as_____________?
A. pull-out torque
B. slip torque
C. breaking torque
D. synchronising torque
Which of the following motors is non-self starting ?
A. Squirrel cage induction motor
B. synchronous motor
C. D.C. series motor
D. Wound round induction motor
The magnitude of field flux in a 3-phase synchronous motor?
A. varies with speed
B. varies with power factor
C. varies with the load
D. remains constant at all loads
The ratio of starting torque to running torque in a synchronous motor is____________?
A. zero
B. two
C. one
D. infinity
The induced e.m.f. in a synchronous motor working on leading power factor will be_____________?
A. more than the supply voltage
B. equal to the supply voltage
C. less than the supply voltage
D. None of these
Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor ?
A. Stator winding to earthed frame
B. Phase to phase winding resistance
C. Rotor winding to earthed shaft
D. All of the above
The speed of a synchronous motor:
A. decreases as the load decreases
B. increases as the load increases
C. always remains constant
D. none of the above
When a 3-phase synchronous generator is supplying a zero power factor lagging load, the armature field affects the main field in the following way
A. cross-magnetises it
B. directly opposes it
C. augments it directly
D. none of the above
If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor__________?
A. remain constant
B. become less
C. become more
D. none of the above
Change of D.C. excitation of a synchronous motor changes:
A. motor speed
B. power factor of power drawn by the motor
C. applied voltage of the motor
D. none of the these
E. all of the these
The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of_____________?
A. two degrees
B. zero degree
C. five degrees
D. ten degrees
An important advantage of a synchronous motor over wound round induction motor is that
A. its power factor may be varied at will
B. its speed may be controlled more easily
C. its speed is independent of supply frequency
D. none of the above
The angle between the rotating stator flux and rotor poles is called _________ angle.
A. obtuse
B. synchronizing
C. torque
D. power factor
Slip-rings in a synchronous motor carry____________?
A. direct current
B. no current
C. alternating current
D. all of the above
A synchronous motor working at leading power factor can be used as____________?
A. noise generator
B. phase advancer
C. voltage booster
D. mechanical synchronizer
The standard full-load power factor ratings for synchronous motors are___________?
A. unity or 0.8 lagging
B. zero or 0.8 leading
C. unity or 0.8 leading
D. unity or zero
The rotor copper losses, in a synchronous motor, are met by____________?
A. d.c. source
B. motor input
C. armature input
D. supply lines
A synchronous motor will always stop when:______________?
A. load in motor varies
B. supply voltage fluctuates
C. excitation winding gets disconnected
D. supply voltage frequency changes
The speed regulation of a synchronous motor is always___________?
A. 0.5%
B. 1%
C. positive
D. zero
. In a synchronous motor which loss varies with load ?
A. Bearing friction loss
B. Windage loss
C. Copper loss
D. Core loss
For V-curves for a synchronous motor the graph is drawn between:_______________?
A. terminal voltage and load factor
B. armature current and power factor
C. power factor and field current
D. field current and armature current
An over-excited synchronous motor takes__________?
A. leading current
B. lagging current
C. both of the above
D. none of the above
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. a rotating magnetic field does not have enough poles
C. the starting device to accelerate the rotor to near synchronous speed is absent
D. the rotating magnetic field is produced by only 50 Hz frequency currents
When V is the applied voltage, then the breakdown torque of a synchronous motor varies as_________?
A. V
B. V2
C. V312
D. 1/V
A three-phase synchronous motor will have_____________?
A. one slip-ring
B. no slip-rings
C. two slip-rings
D. three slip-rings
In case one phase of a three-phase synchronous motor is short-circuited the motor will____________?
A. not start
B. run with excessive vibrations
C. run at 2/3 of synchronous speed
D. take less than the rated load
Synchronous motors are generally not self-starting because:
A. startes cannot be used on these machines
B. the direction of instantaneous torque reverses after half cycle
C. the direction of rotation is not fixed
D. starting winding is not provided on the machines