Synchronous Motors Model Test(2nd)

An important advantage of a synchronous motor over wound round induction motor is that

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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

In case one phase of a three-phase synchronous motor is short-circuited the motor will

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run at ²/₃ of synchronous speed
run with excessive vibrations
not start
take less than the rated load

In a 3-phase synchronous motor, the negative phase sequence exists when the motor is

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none of the above
under-loaded
supplied with unbalanced voltage
over-loaded

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

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infinity
zero
two
one

The power factor of a synchronous motor is better than that of induction motor because

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mechanical load on the motor can be adjusted
synchronous motor has large air gap
stator supply is relieved of responsibility of producing magnetic field
synchronous motor runs at synchronous speed

Hunting in a synchronous motor cannot be due to

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windage friction
variable supply voltage
variable frequency
variable load

By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?

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By changing the supply frequency
By interchanging any two phases
By changing the applied voltage
By changing the load

As the load is applied to a synchronous motor, the motor takes more armature current because

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The increased load has to take more current
The back e.m.f. decreases causing an increase in motor current
The rotor by shifting its phase backward causes motor to take more current
The rotor strengthens the rotating field causing more motor current

For power factor correction, synchronous motors operate at

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normal load with zero excitation
normal load with minimum excitation
no-load and under-excited fields
no-load and greatly over-excited fields

In a synchronous motor, the maximum power developed depends on all of the following except

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maximum value of coupling angle
supply voltage
rotor excitation
direction of rotation

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

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twice
less
more
equal

Riunting in a synchronous motor takes place

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Motor is under loaded
When load varies
When power factor is unity
When supply voltage fluctuates

A synchronous motor can be made self starting by providing

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Damper winding on stator
Damper winding on rotor poles
None of the above
Damper winding on stator as well as rotor poles

The maximum speed variation in a 3-phase synchronous motor is

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4 per cent
6 per cent
2. per cent
zero
10 per cent

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

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one second
none of the above
one minute
one hour

A synchronous machine with large air gap has

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a small value of inherent regulation
a higher value of stability limit
a higher synchronizing power which makes the machine less sensitive to load variations
all of the above

An over excited synchronous motor is used for

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Variable speed loads
Low torque loads
Power factor corrections
Fluctuating loads

The size of a synchronous motor decreases with the increase in

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speed
flux density
all of the above
horse power rating

The rotor copper losses, in a synchronous motor, are met by

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motor input
supply lines
d.c. source
armature input

The percentage slip in case of a synchronous motor is

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100%
1%
0.5%
zero

Synchronous Motors Model Test(2nd) : Question 1 of 20