Question is ⇒ When the field of a synchronous motor is under-excited, the power factor will be, Options are ⇒ (A) leading, (B) lagging, (C) unity, (D) zero, (E) , Leave your comments or Download question paper.

Question is ⇒ When the field of a synchronous motor is under-excited, the power factor will be, Options are ⇒ (A) leading, (B) lagging, (C) unity, (D) zero, (E) , Leave your comments or Download question paper. Previous question Next question


Q1.	When the field of a synchronous motor is under-excited, the power factor will be
A.	leading [Wrong Answer]
B.	lagging [Correct Answer]
C.	unity [Wrong Answer]
D.	zero [Wrong Answer]
View Answer 3 -2 Explanation:- Answer : B Discuss it below : !! OOPS Login [Click here] is required to post your answer/result Help other students, write article, leave your comments

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⇒ VVhen 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

⇒ A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed
the motor will stop
the motor continue to run in the same direction
the winding of the motor will burn
the motor will run in the reverse direction

⇒ The maximum speed variation in a 3- phase synchronous motor is
10 per cent
6 per cent
4 per cent
2 per cent

⇒ 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

⇒ Which of the following losses is not dissipated by tile stator core surface in a synchronous motor ?
Eddy current losses in the conductors
Iron losses in the stator
Copper losses in the slot portion of the conductors
Windage losses

⇒ 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

⇒ A synchronous motor will always stop when
supply voltage fluctuates
load in motor varies
excitation winding gets disconnected
supply voltage frequency changes

⇒ The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motor running at full load, is of the order of
zero degree
two degrees
five degrees
ten degrees

⇒ A three-phase synchronous motor will have
no slip-rings
one slip-ring
two slip-rings
three slip-rings

⇒ In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
d.c. excitation only
speed of the motor
load on the motor
both the speed and rotor flux

⇒ When V is the supply voltage and R is the rotor resistance per phase, the mechanical power developed by the synchronous motor will be proportional to

V²

R²

⇒ 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

⇒ A synchronous motor develops maximum power when load angle is
45°
60°
90°
120°

⇒ A synchronous motor working at leading power factor can be used as
voltage booster
phase advancer
noise generator
mechanical synchronizer

⇒ 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

⇒ The shaft of synchronous motor is made of
mild steel
chrome steel
alnico
stainless steel

⇒ 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

⇒ 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

⇒ If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
2
3
4
6

⇒ Stability of a synchronous machine
decreases with increase in its excitation
increases with increase in its excitation
remains unaffected with increase in excitation
any of the above

⇒ 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

⇒ Armature of a synchronous machine is kept fixed because
of reducing number of slip rings on the rotor
armature is associated with large power as compared to the field circuits
of difficulty of providing high voltage insulation on rotor
all of the above reasons

⇒ The maximum power developed in a synchronous motor will depend on
the rotor excitation only
the supply voltage only
the rotor excitation and supply voltage both
the rotor excitation, supply voltage and maximum value of coupling angle (90°)

⇒ An over-excited synchronous motor is used for
fluctuating loads
variable speed loads
low torque loads
power factor corrections

⇒ The armature current of the synchronous motor has higher values for
high excitation only
low excitation only
both (a) and (b)
none of the above

⇒ 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.

⇒ In which of the following motors the stator and rotor fields rotate simultaneously ?
D.C. motor
Reluctance motor
Universal motor
Synchronous motor

⇒ 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

⇒ 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

⇒ The speed of the synchronous motor
increases with increase in load
decreases as load increases
varies with power factor
always remains constant