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Electrical Engineering
Synchronous Motors
Quiz 1
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Q.1
When the voltage applied to a synchronous motor is increased, which of the following will reduce ?
Stator flux
Pull in torque
Both (A) and (B)
None of the above
Q.2
When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected
the motor stops
it runs as a reluctance motor at the same speed
it runs as a reluctance motor at a lower speed
none of the above
Q.3
The effect of increasing the load on a synchronous motor running with normal excitation is to
decrease both armature current and power factor
decrease armature current but increase power factor
increase armature current but decrease power factor
increase both its armature current and power factor
Q.4
Synchronous motor always runs at
the synchronous speed
less than synchronous speed
more than synchronous speed
none of the above
Q.5
A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because
synchronous motor has no slip
stator supply is not required to produce magnetic field
mechanical load on the rotor remains constant
synchronous motor has large airgap
Q.6
Under which of the following conditions hunting of synchronous motor is likely to occur ?
Periodic variation of load
Over-excitation
Over-loading for long periods
Small and constant load
Q.7
A synchronous motor will always stop when
supply voltage fluctuates
load in motor varies
excitation winding gets disconnected
supply voltage frequency changes
Q.8
The power developed by a synchronous motor will be maximum when the load angle is
zero
45°
90°
120°
Q.9
In a synchronous motor, V-curves represent relation between
armature current and field current
power factor and speed
field current and speed
field current and power factor
Q.10
A synchronous motor can operate at
lagging power factor only
leading power factor only
unity power factor only
lagging, leading and unity power factors
Q.11
The armature current of the synchronous motor
has large values for low excitation i niy
has large values for high excitation only
has large values for low and high excitation
any of the above
Q.12
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
become more
become less
remain constant
none of the above
Q.13
The torque angle, in a synchronous motor, is the angle between
the supply voltage and the back e.m.f.
magnetising current and back e.m.f.
the rotating stator flux and rotor poles
none of the above
Q.14
A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
Speed of the motor is reduced
Power factor is decreased
Eb (back e.m.f.) becomes less than V (applied voltage)
Er (net resultant voltage) in armature is increased
Q.15
Which of the following motors will be used in electric clocks ?
D.C. shunt motor
D.C. series motor
A.C. induction motor
A.C. synchronous motor
Q.16
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
augments it directly
directly opposes it
cross-magnetises it
none of the above
Q.17
A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased
power factor as well as armature current will decrease
power factor as well as armature current will increase
power factor will increase but armature current will decrease
power factor will decrease and armature current will increase
Q.18
To limit the operating temperature an electrical machine should have proper
voltage rating
current rating
power factor
speed
Q.19
If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
2
3
4
6
Q.20
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
locked-rotor torque
synchronous torque
pull up torque
reluctance torque
Q.21
In a synchronous motor, the armature current has large values for
high excitation only
low excitation only
both high and low excitation
none of the above
Q.22
The maximum value of torque that a synchronous motor can develop without losing its synchronism, is known as
slip torque
pull-out torque
breaking torque
synchronising torque
Q.23
Exciters of synchronous machines are
d.c. shunt machines
d.c. series machines
d.c. compound machines
any of the above
Q.24
A synchronous motor running with normal excitation adjusts to load increases essentially by increase in
back e.m.f.
armature current
power factor
torque angle
Q.25
The oscillations in a synchronous motor can be damped out by
maintaining constant excitation
running the motor on leading power factors
providing damper bars in the rotor pole faces
oscillations cannot be damped
Q.26
A synchronous motor can be used as a synchronous capacitor when it is
under-loaded
over-loaded
under-excited
over-excited
Q.27
If the field winding of an unloaded salient pole synchronous motor is open circuited, the motor will
stop
run as induction motor
function as static condenser
burn with dense smoke
Q.28
In a synchronous motor, damper windings are provided on
stator frame
rotor shaft
pole faces
none of the above
Q.29
In a synchronous motor, the damping winding is generally used to
prevent hunting and provide the starting torque
reduce the eddy currents
provide starting torque only
reduce noise level
Q.30
In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
d.c. excitation only
speed of the motor
load on the motor
both the speed and rotor flux
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