Practice Synchronous Motors Multiple Choice Questions Quiz-2 - MCQGeeks.com

Q.1

In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

load on the motor
d.c. excitation only
both the speed and rotor flux
none of the above

Q.2

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

One-third
Twice
Thrice
Six times

Q.3

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

will become more
will become less
will remain unchanged
none of the above.

Q.4

The coupling angle or load angle of synchronous motor is defined as the angle between the

rotor and stator teeth
rotor and the stator poles of opposite polarity
rotor and the stator poles of the same polarity
none of the above

Q.5

Slip-rings in a synchronous motor carry

direct current
alternating current
no current
all of the above

Q.6

An over-excited synchronous motor takes

leading current
lagging current
both (A) and (B)
none of the above

Q.7

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

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

Q.8

Synchronous motors are generally not self-starting because

the direction of rotation is not fixed
the direction of instantaneous torque reverses after half cycle
startes cannot be used on these machines
starting winding is not provided on the machines

Q.9

The synchronous motor is not inherently self-starting because

the force required to accelerate the rotor to the synchronous speed in an instant is absent
the starting device to accelerate the rotor to near synchronous speed is absent
a rotating magnetic field does not have enough poles
the rotating magnetic field is produced by only 50 Hz frequency currents

Q.10

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

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

Q.11

A synchronous machine with large air gap has

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

Q.12

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

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

Q.13

The percentage slip in case of a synchronous motor is

1%
100%
0.5%
zero

Q.14

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

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

Q.15

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

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

Q.16

Hunting in a synchronous motor cannot be due to

windage friction
variable load
variable frequency
variable supply voltage

Q.17

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

zero
one
two
infinity

Q.18

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

more
less
equal
twice

Q.19

An over excited synchronous motor is used for

Fluctuating loads
Variable speed loads
Low torque loads
Power factor corrections

Q.20

Riunting in a synchronous motor takes place

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

Q.21

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

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

Q.22

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

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

Q.23

Mostly, synchronous motors are of

alternator type machines
induction type machines
salient pole type machines
smooth cylindrical type machines

Q.24

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

lower stability limit
high stability limit
good speed regulation
good voltage regulation

Q.25

In a synchronous motor it the back e.m.f. generated in the armature at noload is approximately equal to the applied voltage, then

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

Q.26

A synchronous motor develops maximum power when load angle is

45°
60°
90°
120°

Q.27

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

one hour
one minute
one second
none of the above

Q.28

A synchronous motor can be made self starting by providing

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

Q.29

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

d.c. source
armature input
motor input
supply lines

Q.30

For power factor correction, synchronous motors operate at

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

0 h : 0 m : 1 s