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Electrical Engineering
Synchronous Motors
Quiz 3
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Q.1
The power factor of a synchronous motor is better than that of induction motor because
stator supply is relieved of responsibility of producing magnetic field
mechanical load on the motor can be adjusted
synchronous motor runs at synchronous speed
synchronous motor has large air gap
Q.2
An important advantage of a synchronous motor over wound round induction motor is that
its power factor may be varied at will
its speed is independent of supply frequency
its speed may be controlled more easily
none of the above
Q.3
In a synchronous motor which loss varies with load ?
Windage loss
Bearing friction loss
Copper loss
Core loss
Q.4
The magnitude of field flux in a 3-phase synchronous motor
remains constant at all loads
varies with speed
varies with the load
varies with power factor
Q.5
The efficiency of a properly designed synchronous motor will usually fall in range
60 to 70%
75 to 80%
85 to 95%
99 to 99.5%
Q.6
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
no change of power factor
lagging power factor with over-excitation
leading power factor with under-excitation
leading power factor with over-excitation
Q.7
The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as
pull-up torque
pull-in torque
pull-out torque
none of the above
Q.8
The shaft of synchronous motor is made of
mild steel
chrome steel
alnico
stainless steel
Q.9
A synchronous motor can develop synchronous torque
when under loaded
while over-excited
only at synchronous speed
below or above synchronous speed
Q.10
While starting a salient pole synchronous motor by induction motor action and connecting field discharge resistance across field, starting and accelerting torque is produced by
induction motor torque in field winding
induction motor torque in damper winding
eddy current and hysteresis torque in pole faces
reHetance motor torque due to saliency of the rotor
all of the above methods
Q.11
If one-phase of a 3-phase synchronous motor is short-circuited, motor
will refuse to start
will overheat in spots
will not come upto speed
will fail to pull into step
Q.12
The size of a synchronous motor decreases with the increase in
flux density
horse power rating
speed
all of the above
Q.13
In case one phase of a three-phase synchronous motor is short-circuited the motor will
not start
run at
2
/
3
of synchronous speed
run with excessive vibrations
take less than the rated load
Q.14
Change of D.C. excitation of a synchronous motor changes
Applied voltage of the motor
Motor speed
Power factor of power drawn by the motor
Any of the above
Q.15
When the field of a synchronous motor is under-excited, the power factor will be
leading
lagging
unity
zero
Q.16
The induced e.m.f. in a synchronous motor working on leading power factor will be
more than the supply voltage
less than the supply voltage
equal to the supply voltage
none of the above
Q.17
The net armature voltage of a synchronous motor is equal to the
vector sum of Eb and V
arithmetic sum of Eb and V
arithmetic difference of Eb and V
vector difference of Eh and V
Q.18
A three-phase synchronous motor will have
no slip-rings
one slip-ring
two slip-rings
three slip-rings
Q.19
The operating speed of a synchronous motor can be changed to new fixed value by
changing the load
changing the supply voltage
changing frequency
using brakes
Q.20
Synchronous motors are
Not-self starting
Self-starting
Essentially self-starting
None of the above
Q.21
The standard full-load power factor ratings for synchronous motors are
zero or 0.8 leading
unity or 0.8 lagging
unity or 0.8 leading
unity or zero
Q.22
If the field of a synchronous motor is underexcited, the power factor will be
zero
unity
lagging
leading
Q.23
The back e.m.f. of a synchronous motor depends on
speed
load
load angle
all of the above
Q.24
The back e.m.f. in the stator of a synchronous motor depends on
number of poles
flux density
rotor speed
rotor excitation
Q.25
The minimum armature current of the synchronous motor corresponds to operation at
zero power factor leading
unity power factor
0.707 power factor lagging
0.707 power factor leading
Q.26
If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
it runs at a slower speed
the motor stops
it continues to run at the same speed
it runs at a very high speed
Q.27
A synchronous motor can be started by
pony motor
D.C. compound motor
providing damper winding
any of the above
Q.28
If the synchronous motor, properly synchronised to the supply is running on no load and is having negligible loss then
the stator current will be zero
the stator current will be very small
the stator current will be very high
the back e.m.f. will be more than the supply voltage
Q.29
The speed regulation of a synchronous motor is always
1%
0.5%
positive
zero
Q.30
The construction of a synchronous motor resembles
a series motor
an induction motor
an alternator
a rotary converter
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