MCQ On Synchronous Machines: Electrical Engineering MCQ

Questions and Answers related to Electrical Engineering. MCQ On Synchronous Machines and their working.

 

MCQ On Synchronous Machines

 

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. Iron losses in the stator
C. Copper losses in the slot portion of the conductors
D. Windage losses

View Answer

D. Windage losses

 

A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as ?

A. static condenser
B. condenser
C. synchronous condenser
D. none of the above

View Answer

C. synchronous condenser

 

Which of the following losses, in a synchronous motor, does not vary with load ?

A. Windage loss
B. Copper losses
C. Any of the above
D. None of the above

View Answer

A. Windage loss

 

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 small
C. the stator current will be very high
D. the back e.m.f. will be more than the supply voltage

View Answer

A. the stator current will be zero

 

In a synchronous motor, the armature current has large values for ?

A. high excitation only
B. low excitation only
C. both high and low excitation
D. none of the above

View Answer

C. both high and low excitation

 

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. synchronous torque
C. pull up torque
D. reluctance torque

View Answer

A. locked-rotor 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. function as static condenser
D. burn with dense smoke

View Answer

A. stop

 

A synchronous machine with low value of short-circuit ratio has ?

A. lower stability limit
B. high stability limit
C. good speed regulation
D. good voltage regulation

View Answer

A. lower stability limit

 

In a synchronous motor, the damping winding is generally used to ?

A. reduce noise level
B. reduce the eddy currents
C. provide starting torque only
D. prevent hunting and provide the starting torque

View Answer

D. prevent hunting and provide the starting torque

 

MCQ On Synchronous Machines

 

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-up torque
B. pull-in torque
C. pull-out torque
D. none of the above

View Answer

B. pull-in torque

 

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 motor is said to be fully loaded
B. the torque generated is maximum
C. the excitation is said to be zero per cent
D. the excitation is said to be hundred per cent

View Answer

D. the excitation is said to be hundred per cent

 

In which of the following motors the stator and rotor fields rotate simultaneously ?

A. D.C. motor
B. Reluctance motor
C. Universal motor
D. Synchronous motor

View Answer

D. Synchronous motor

 

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

View Answer

B. 1500 r.p.m.

 

If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then ?

A. it runs at a slower speed
B. the motor stops
C. it continues to run at the same speed
D. it runs at a very high speed

View Answer

B. the motor stops

 

A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because ?

A. speed of the motor is reduced
B. power factor is decreased
C. Eb (back e.m.f.) becomes less than V (applied voltage)
D. Er (net resultant voltage) in armature is increased

View Answer

D. Er (net resultant voltage) in armature is increased

 

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

View Answer

A. By changing the supply frequency

 

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 become more
B. will become less
C. will remain unchanged
D. none of the above

View Answer

B. will become less

 

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

View Answer

C. the rotating stator flux and rotor poles

 

MCQ On Synchronous Machines

 

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

View Answer

B. the motor continue to run in the same direction

 

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

View Answer

D. vector difference of Eh and V

 

In a synchronous motor, damper windings are provided on ?

A. stator frame
B. rotor shaft
C. pole faces
D. none of the above

View Answer

C. pole faces

 

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 back e.m.f. but decrease in armature current
C. increase in both armature current and power factor which is lagging
D. increase in torque angle but decrease in back e.m.f.

View Answer

A. increase in both armature current and power factor angle

 

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

View Answer

A. supplied with unbalanced voltage

 

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 field winding
B. induction motor torque in damper 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

View Answer

E. all of the above methods

 

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

View Answer

D. Low field current

 

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. remain same
B. go down
C. improve
D. none of the above

View Answer

C. improve

 

Which of the following motors will be used in electric clocks ?

A. D.C. shunt motor
B. D.C. series motor
C. A.C. induction motor
D. A.C. synchronous motor

View Answer

D. A.C. synchronous motor

 

MCQ On Synchronous Machines

 

A rotory converter can also be run as a ?

A. d.c. shunt motor
B. d.c. series motor
C. d.c. compound motor
D. induction motor
E. synchronous motor

View Answer

E. synchronous motor

 

Power factor of a synchronous motor is unity when ?

A. the armature current is maximum
B. the armature current is minimum
C. the armature current is zero
D. none of the above

View Answer

B. the armature current is minimum

 

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. remains same as the original value
B. decreases to half the original value
C. tends to becomes zero
D. increases to two times the original value

View Answer

D. increases to two times the original value

 

Which of the following methods is used to start a synchronous motor ?

A. Damper winding
B. Star-delta starter
C. Damper winding in conjunction with star-delta starter
D. Resistance starter in the armature circuit

View Answer

C. Damper winding in conjunction with star-delta starter

 

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. more
B. less
C. equal
D. twice

View Answer

B. less

 

In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately ?

A. one-third
B. twice
C. thrice
D. six times

View Answer

C. thrice

 

Riunting in a synchronous motor takes place ?

A. when supply voltage fluctuates
B. when load varies
C. when power factor is unity
D. motor is under loaded

View Answer

B. when load varies

 

To limit the operating temperature an electrical machine should have proper ?

A. voltage rating
B. current rating
C. power factor
D. speed

View Answer

B. current rating

 

When the voltage applied to a synchronous motor is increased, which of the following will reduce ?

A. Stator flux
B. Pull in torque
C. Both A. and (b)
D. None of the above

View Answer

D. None of the above

 

MCQ On Synchronous Machines

 

When the field of a synchronous motor is under-excited, the power factor will be ?

A. leading
B. lagging
C. unity
D. zero

View Answer

B. lagging

 

An over excited synchronous motor is used for ?

A. fluctuating loads
B. variable speed loads
C. low torque loads
D. power factor corrections

View Answer

D. power factor corrections

 

A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because ?

A. synchronous motor has no slip
B. stator supply is not required to produce magnetic field
C. mechanical load on the rotor remains constant
D. synchronous motor has large airgap

View Answer

B. stator supply is not required to produce magnetic field

 

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

View Answer

B. 3

 

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

View Answer

B. unity power factor

 

Synchronsizing power of a synchronous machine is ?

A. direcly proportional to the synchronous reactance
B. inversely proportional to the synchronous reactance
A. equal to the synchronous reactance
D. none of the above

View Answer

B. inversely proportional to the synchronous reactance

 

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

View Answer

A. Periodic variation of load

 

The operating speed of a synchronous motor can be changed to new fixed value by ?

A. changing the load
B. changing the supply voltage
C. changing frequency
D. using brakes

View Answer

C. changing frequency

 

The maximum power developed in a synchronous motor occurs at a coupling angle of ?

A. 30°
B. 60°
C. 90°
D. 180°

View Answer

C. 90°

 

MCQ On Synchronous Machines

 

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

View Answer

A. the motor stops

 

In which of the following motors the stator and rotor magnetic field rotate at the same speed ?

A. Universal motor
B. Synchronous motor
C. Induction motor
D. Reluctance motor

View Answer

B. Synchronous motor

 

A synchronous motor running with normal excitation adjusts to load increases essentially by increase in ?

A. back e.m.f.
B. armature current
C. power factor
D. torque angle

View Answer

B. armature current

 

The efficiency of a properly designed synchronous motor will usually fall in range ?

A. 60 to 70%
B. 75 to 80%
C. 85 to 95%
D. 99 to 99.5%

View Answer

C. 85 to 95%

 

The working of a synchronous motor is similar to ?

A. gear train arrangement
B. transmission of mechanical power by shaft
C. distribution transformer
D. turbine

View Answer

B. transmission of mechanical power by shaft

 

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

View Answer

D. leading power factor with over-excitation

 

When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop ?

A. inductor motor torque
B. induction generator torque
C. synchronous motor torque
D. D.C motor toque

View Answer

B. induction generator torque

 

A synchronous motor develops maximum power when load angle is ?

A. 45°
B. 60°
C. 90°
D. 120°

View Answer

C. 90°

 

While starting a synchronous motor by induction motor action, field winding is usually ?

A. connected to D.C. supply
B. short-circuited by low resistance
C. kept open-circuited
D. none of the above

View Answer

B. short-circuited by low resistance

 

MCQ On Synchronous Machines

 

Armature of a synchronous machine is ?

A. of reducing number of slip rings on the rotor
B. armature is associated with large power as compared to the field circuits
C. of difficulty of providing high voltage insulation on rotor
D. all of the above reasons

View Answer

D. all of the above reasons

 

Exciters of synchronous machines are ?

A. d.c. series machines
B. d.c. shunt machines
C. d.c. compound machines
D. any of the above

View Answer

B. d.c. shunt machines

 

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. mechanical load on the motor can be adjusted
C. synchronous motor runs at synchronous speed
D. synchronous motor has large air gap

View Answer

A. stator supply is relieved of responsibility of producing magnetic field

 

The duration of sudden snort-circuit test on a synchronous motor is usually about ?

A. one hour
B. one minute
C. one second
D. none of the above

View Answer

C. one second

 

The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as ?

A. slip torque
B. pull-out torque
C. breaking torque
D. synchronising torque

View Answer

D. synchronising torque

 

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

View Answer

B. synchronous motor

 

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

View Answer

C. direction of rotation

 

The effect of increasing the load on a synchronous motor running with normal excitation is to ?

A. decrease both armature current and power factor
B. decrease armature current but increase power factor
C. increase armature current but decrease power factor
D. increase both its armature current and power factor

View Answer

C. increase armature current but decrease power factor

 

The ratio of starting torque to running torque in a synchronous motor is ?

A. zero
B. one
C. two
D. infinity

View Answer

A. zero

 

MCQ On Synchronous Machines

 

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

View Answer

B. d.c. excitation only

 

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
D. None of these

View Answer

A. more than the supply voltage

 

Hunting in a synchronous motor cannot be due to ?

A. windage friction
B. variable load
C. variable frequency
D. variable supply voltage

View Answer

A. windage friction

 

The standard full-load power factor ratings for synchronous motors are ?

A. zero or 0.8 leading
B. unity or 0.8 lagging
C. unity or 0.8 leading
D. unity or zero

View Answer

C. unity or 0.8 leading

 

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

View Answer

D. All of the above

 

A synchronous motor will always stop when ?

A. supply voltage fluctuates
B. load in motor varies
C. excitation winding gets disconnected
D. supply voltage frequency changes

View Answer

C. excitation winding gets disconnected

 

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. augments it directly
B. directly opposes it
C. cross-magnetises it
D. none of the above

View Answer

B. directly opposes it

 

The speed regulation of a synchronous motor is always ?

A. 1%
B. 0.5%
C. positive
D. zero

View Answer

D. zero

 

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. become more
B. become less
C. remain constant
D. none of the above

View Answer

B. become less

 

MCQ On Synchronous Machines

 

Change of D.C. excitation of a synchronous motor changes ?

A. applied voltage of the motor
B. power factor of power drawn by the motor
C. motor speed
D. none of the these

View Answer

B. power factor of power drawn by the motor

 

In a synchronous motor which loss varies with load ?

A. Windage loss
B. Bearing friction loss
C. Copper loss
D. Core loss

View Answer

C. Copper loss

 

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. zero degree
B. two degrees
C. five degrees
D. ten degrees

View Answer

C. five 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 is independent of supply frequency
C. its speed may be controlled more easily
D. none of the above

View Answer

A. its power factor may be varied at will

 

The angle between the rotating stator flux and rotor poles is called _________ angle ?

A. synchronizing
B. obtuse
C. torque
D. power factor

View Answer

C. torque

 

Slip-rings in a synchronous motor carry ?

A. direct current
B. alternating current
C. no current
D. all of the above

View Answer

A. direct current

 

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

View Answer

A. V

 

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