Electrical Machines Expected MCQ PDF 8 For Coal India Limited (CIL) MT Exam 2017

1)   In a salient pole synchronous generator direct axis reactance is

a. Greater than quadrature axis reactance
b. Less than quadrature axis reactance
c. Equal to quadrature axis reactance
d. None of the above

ANSWER: Greater than quadrature axis reactance
2)   The reluctance power of a salient pole alternator is given by

a. V2[Xd – Xq / 2XdXq] sin2 δ
b. V2[Xq – Xd / 2Xd Xq] sin2 δ
c. V2[Xd – Xq / XdXq] sin δ
d. V[Xd – Xq / 2Xd Xq] sin2 δ

ANSWER: V2[Xd – Xq / 2XdXq] sin2 δ

3)   In case of salient pole alternators, maximum output power can be obtained when power angle (d) is

a. Less than 90 degree
b. Equal to 90 degree
c. Greater than 90 degree
d. None of the above

ANSWER: Less than 90 degree

4)   Two identical alternators having impedances Z1 and Z2 are connected in parallel produces E1 and E2
as their induced emf’s. if no load is connected to the alternators then
a circulating current will flow in the circuit. This current is given
by

a. E1 + E2 / Z1 – Z2
b. E1 – E2 / Z1 + Z2
c. E2 – E1 / Z1 + Z2
d. E1 + E2 / Z1 + Z2

ANSWER: E1 – E2 / Z1 + Z2

5)   If a 3 phase
alternator is short circuited at its terminal, then the initial value of
short circuit current will be limited by

a. Synchronous reactance
b. Transient reactance
c. Sub transient reactance
d. All of the above

ANSWER: Sub transient reactance

6)   The change in excitation of the two alternators operating in parallel causes

a. Only the kVAR sharing of two alternators without disturbing kW sharing of the two machines
b. Only the kW sharing of two alternators without disturbing kVAR sharing of the two machines
c. Both the kVAR sharing and kW sharing of two alternators
d. None of the above

ANSWER: Only the kVAR sharing of two alternators without disturbing kW sharing of the two machines

7)   In case of hydro generators, the driving torque can be changed by controlling the

a. Gate opening
b. Throttle opening
c. Both (a) and (b)
d. None of these

ANSWER: Gate opening

8)   The increase in mechanical torque in case of alternator will

a. Increase the armature current and reduce the power factor
b. Increase the armature current and improve the power factor
c. Decrease the armature current and improve the power factor
d. Decrease the armature current and reduce the power factor

ANSWER: Increase the armature current and improve the power factor

9)   When an alternator is connected to infinite bus bar, change in excitation for alternator will

a. Change the terminal voltage and power factor both
b. Affect only terminal voltage and power factor remains unaffected
c. Affect only power factor and terminal voltage remains unaffected
d. Neither terminal voltage will get affected nor power factor

ANSWER: Affect only power factor and terminal voltage remains unaffected

10)   The plot of
armature current verses excitation is known as V curves for synchronous
generator. Determine the correct combination of load.

a. A – full load, B – half load, C – no load

b. A – full load, B – no load, C – half load
c. A – no load, B – half load, C – full load
d. A – half load, B – full load, C – no load

ANSWER: A – full load, B – half load, C – no load

11)   If a three
phase winding is displaced in space by 120 degree in a synchronous motor
then the magnitude of resultant flux when it is rotated by Θ = 0 degree
and Θ = 60 degree is

a. 0, 1.5 times the magnitude of maximum flux
b. 1.5 times the magnitude of maximum flux, 0
c. 0.866 times the magnitude of maximum flux, 1.5 times the magnitude of maximum flux
d. 1.5 times the magnitude of maximum flux, 1.5 times the magnitude of maximum flux

ANSWER: 1.5 times the magnitude of maximum flux, 1.5 times the magnitude of maximum flux

12)   For a half cycle of fluxes in a synchronous motor, the resultant flux has rotated through

a. 0 degree
b. 90 degree
c. 180 degree
d. 360 degree

ANSWER: 180 degree

13)   For a two pole, 50 Hz, 3 phase synchronous motor the speed of rotating magnetic field is

a. 1000 rpm
b. 2000 rpm
c. 3000 rpm
d. 3500 rpm

ANSWER: 3000 rpm

14)   A
synchronous motor draws 2 MVA at a power factor of 0.8 leading. If the
efficiency developed in the motor is 90% then the power developed is
equal to

a. 1212 kW
b. 1440 kW
c. 1676 kW
d. 1767 kW

ANSWER: 1440 kW

15)   Synchronous motor works on the principle of

a. Mutual induction
b. Electromagnetic induction
c. Magnetic locking
d. None of these

ANSWER: Magnetic locking

16)   Synchronous motors are

a. Self starting
b. Not self starting
c. Both (a) and (b)
d. None of these

ANSWER: Not self starting

17)   Can synchronous motor rotate at any speed other than the synchronous speed?

a. Yes
b. No

ANSWER: No

18)   The method used for starting synchronous motor is/are

a. Using pony motors
b. Using damper windings
c. As a slip ring induction motor
d. All of the above

ANSWER: All of the above

19)   A
synchronous motor is started by using damper winding. When the motor
runs at synchronous speed then the induced emf in the damper winding
will be

a. Zero
b. Maximum
c. Half of the terminal voltage
d. None of these

ANSWER: Zero

20)   With increase in load, the speed of synchronous motor

a. Increases
b. Decreases
c. Remains constant
d. None of the above

ANSWER: Remains constant

21)   Back emf in case of synchronous motor depends on the

a. Excitation given to the field winding
b. Speed
c. Both (a) and (b)
d. None of the above

ANSWER: Excitation given to the field winding

22)   If the load on a synchronous motor increases then its

a. Current drawn by motor increases and load angle decreases
b. Current drawn by motor decreases and load angle remains constant
c. Current drawn by motor remains constant and load angle increases
d. Current drawn by motor increases and load angle also increases

ANSWER: Current drawn by motor increases and load angle also increases

23)   In a synchronous motor the torque produced will be maximum when load angle is equal to

a. 0° electrical
b. 90° electrical
c. 180° electrical
d. 360° electrical

ANSWER: 90° electrical

24)   Practically in a synchronous motor, the pull-out torque will occur when the torque angle is nearly equal to

a. 30° electrical
b. 45 ° electrical
c. 60 ° electrical
d. 75 ° electrical

ANSWER: 75 ° electrical

25)   For a
synchronous motor, as the load angle reaches up to 90 ° electrical i.e.
half a pole pitch, then the magnetic locking between the stator and
rotor

a. No longer exist
b. Will be strongest
c. Will be weakest
d. None of these

ANSWER: No longer exist

26)   The impedance angle or internal machine angle (°) of a synchronous motor is equal to

a. tan-1Xs / Ra
b. tan-1 Rs / Xa
c. tan-1 Xs / Za
d. tan-1 Zs / Ra

ANSWER: tan-1Xs / Ra

27)   When the excitation of a synchronous motor is changed keeping the load constant then the

a. Power factor will change
b. Ia cos ∅ remains constant
c. Both (a) and (b)
d. None of these

ANSWER: Both (a) and (b)

28)   Under
excitation occurs when the magnitude of induced emf of synchronous motor
is less than the applied voltage. During under excitation the current
drawn by the motor and power factor

a. Increases, decreases and will be lagging in nature
b. Decreases, increases and will be leading in nature
c. Increases, increases and will be leading in nature
d. Decreases, decreases and will be lagging in nature

ANSWER: Increases, decreases and will be lagging in nature

29)   If the field of the synchronous motor is overexcited then its power factor will be

a. Lagging
b. Leading
c. Unity
d. Zero

ANSWER: Leading

30)   For critical excitation the power factor of the synchronous motor will be equal to

a. 0
b. 1
c. 0.5 MΩ
d. None of the above

ANSWER: 1

31)   If graph of
armature current drawn by synchronous motor is against field current is
plotted then the resulted graph is known as

a. V-curves
b. Inverted V-curves
c. Both (a) and (b)
d. None of these

ANSWER: V-curves

32)   The plot of power factor against the field current of synchronous motor is known as

a. V-curves
b. Inverted V-curves
c. Both (a) and (b)
d. None of these

ANSWER: Inverted V-curves

33)   The
expression for the mechanical power developed in terms of the load angle
δ and the internal machine angle ∅, for constant voltageVph and constant excitation Eb is

a. [{(EbVph) / Zs} * cos(θ – δ)] – [{(E2b) / Zs} * cos θ]
b. [{(EbV ph) / Zs} * cos(δ – θ)] – [{(E2b) / Zs}* cos θ]
c. [{(E2b) / Zs} * cos θ] – [{(Eb Vph) / Zs} * cos(δ – θ)]
d. [{(E2b) / Zs} * cos θ] – [({Eb Vph) / Zs} * cos(θ – δ)]

ANSWER: [{(EbVph) / Zs} * cos(θ – δ)] – [{(E2b) / Zs} * cos θ]

34)   The value of
load angle δ and the internal machine angle ∅ for maximum power
developed in synchronous motor is equal to

a. 0°electrical, 0° electrical
b. 0° electrical, 90° electrical
c. 90° electrical, 0° electrical
d. 90° electrical, 90° electrical

ANSWER: 90° electrical, 90° electrical

35)   A
synchronous motor is running at a load angle of 25 degree at rated
frequency with negligible armature resistance. Now, if the supply
frequency is increased by 15% keeping the other parameters constant,
then the new load angle will be equal to

a. 26.89 degree
b. 29.07 degree
c. 32.05 degree
d. 38.20 degree

ANSWER: 29.07 degree

36)   Oscillations
of the rotor about its new equilibrium position, due to sudden
application or removal of load in synchronous motor is called

a. Hunting
b. Galloping
c. Swinging
d. Both (a) and (c)
e. None of the above

ANSWER: Both (a) and (c)

37)   Hunting in a synchronous motor results in

a. Loss of synchronism
b. Mechanical and electrical stresses on motor
c. Large current drawn by motor
d. all of the above
e. None of the above

ANSWER: all of the above

38)   In a
synchronous motor, the phenomenon of hunting caused by the sudden
application or removal of load is prevented by

a. Using damper winding
b. Using a capacitor
c. Using an inductor
d. All of the above
e. None of the above

ANSWER: Using damper winding

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GENERAL KNOWLEDGE QUIZ-26
Electrical Machines Expected MCQ PDF 7 For Coal India Limited (CIL) MT Exam 2017

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