# Electrical Machines Expected MCQ PDF 7 For Coal India Limited (CIL) MT Exam 2017

1)   Consider a
purely inductive load connected to the alternator having zero lagging
power factor. In this case the effect of armature reaction will be

a. Demagnetizing effect
b. Cross magnetizing effect
c. Both (a) and (b)
d. None of these

2)   When a purely
power factor then the armature flux and main flux will be

a. In the same direction
b. In the opposite direction
c. Main flux will lead armature flux by 90 degree
d. Main flux will lag armature flux by 90 degree

3)   The open
circuit voltage and short circuit current of a 3 phase, star connected
alternator are 1050 V and 250 A respectively. Its field current is 12 A.
Then the synchronous impedance of alternator will be

a. 1.34 ohm
b. 1.69 ohm
c. 2.42 ohm
d. 2.85 ohm

4)   A 6 kVA, 220 v
3 phase, star connected synchronous generator has a winding resistance
of 0.15 ohm per phase and synchronous reactance of 6.1 ohm per phase.
The voltage regulation of alternator at a load power factor of 0.8
lagging will be

a. 58.10 %
b. 68.43 %
c. 69.02 %
d. 72.81 %

5)   If two
mechanically coupled alternators deliver power at 50 Hz and 60 Hz
respectively, then the highest speed of alternators will be

a. 1200 rpm
b. 1500 rpm
c. 600 rpm
d. 300 rpm

6)   If the
armature resistance and synchronous reactance of a 2 MVA, 11 kV, 3 phase
star connected alternator are 2 ohm and 20 ohm per phase respectively.
The value of full load generated voltage per phase at unity power factor
will be

a. 6888.45 V
b. 6899.65 V
c. 6999.45 V
d. 7111.45 V

7)   As the value of load power factor of an alternator changes, the value of armature reaction reactance

a. Remains constant
b. Changes
c. None of these

8)   The given figure shows the load characteristics of an alternator. Which among the following is correct?

a. A – Leading p.f., B – unity p.f., C – lagging p.f.
b. A – Lagging p.f., B – unity p.f., C – leading p.f.
c. A – Leading p.f., B – lagging p.f., C – unity p.f.
d. A – Unity p.f., B – leading p.f., C – lagging p.f.

 ANSWER: A – Leading p.f., B – unity p.f., C – lagging p.f.

9)   In an alternator, the voltage regulation will be positive when the power factor of the load is

b. Unity
c. Lagging
d. Both ( b ) & ( c )
e. None of these

 ANSWER: Both ( b ) & ( c )

10)   On unity power factor, the terminal voltage of an alternator is

a. Always less than induced emf
b. Always greater than induced emf
c. Same as induced emf
d. None of these

 ANSWER: Always less than induced emf

11)   The percentage voltage regulation of an alternator is given by

a. {(Eph – Vph/ Eph) *100}
b. {(Vph – Eph / Eph)*100}
c. {(Eph – Vph / V ph)*100}
d. {(Vph – Eph / V ph)*100}

 ANSWER: {(Eph – Vph / V ph)*100}

12)   Which method for finding percentage regulation in synchronous machines is called pessimistic method?

a. E.M.F. method
b. M.M.F. method
c. Z.P.F. method

13)   For high capacity alternators, which method is not suitable for finding percentage regulation?

b. Synchronous impedance method
c. Ampere-turns method
d. Potier triangle method

14)   For finding voltage regulation of an alternator, the method which gives most accurate result is

a. E.M.F method
b. M.M.F. method
c. Z.P.F method
d. ASA modification of M.M.F. method

15)   Short circuit ratio of an alternator is given by

a. If for rated short circuit current / I f for rated short circuit voltage
b. If for rated short circuit Voltage / If for rated short circuit current
c. If for rated open circuit Voltage / If for rated short circuit current
d. If for rated short circuit current / If for rated open circuit voltage

 ANSWER: If for rated open circuit Voltage / If for rated short circuit current

16)   Low value of SCR in an alternator indicates

a. High stability limit
b. Low stability limit
c. None of these

17)   If the value
of SCR in an alternator is low then the air gap and voltage drop across
synchronous reactance will be

a. Low, high
b. High, low
c. Low, low
d. High, high

18)   In Potier’s
triangle method, to determine armature leakage reactance and armature
reaction mmf separately, the tests performed are

a. Open circuit test and short circuit test
b. Open circuit test and zero power factor test
c. Short circuit test and zero power factor test
d. Open circuit test, short circuit test and zero power factor test

 ANSWER: Open circuit test and zero power factor test

19)   The synchronous generator is loaded for obtaining zero power characteristics by using

a. D.C. motor
c. Synchronous motor
d. All of these

20)   The direct method for finding the voltage regulation of an alternator is/are

a. E.M.F method
b. M.M.F. method
c. Z.P.F method
d. None of these

21)   What happens if a stationary alternator is connected to live bus bar?

a. It can result in short circuit
b. A large emf will be induced by alternator
c. Both (a) and (b)
d. None of these

 ANSWER: It can result in short circuit

22)   To have
effective synchronization without any interruption, the necessary
conditions which must be satisfied are

a. The terminal voltage and frequency of the incoming machine must be same as that of bus bar voltage and frequency
b. With respect to the external load, the phase of alternator voltage must be identical with that of the bus bar voltage
c. Both (a) and (b)
d. None of these

23)   In lamps
dark method for synchronization of single phase alternator, if the
frequency of two alternators is exactly same as well as their voltages
are in exact phase opposition then the resultant voltage will be

a. Sum of the voltages of two alternators
b. Equal to the voltage of alternators
c. Zero
d. None of these

24)   If the
frequencies of the two alternators in lamp dark method for
synchronization of alternators are unequal, then the two lamps will

a. Glow together
b. Become alternately bright and dark
c. Not glow at all
d. None of the above

 ANSWER: Become alternately bright and dark

25)   For synchronization of single phase alternators which method gives the more accurate result?

a. Lamps dark method
b. Lamps bright method
c. Both (a) and (b)
d. None of these

26)   While
synchronization of three phase alternators by lamps bright and dark
method, if the lamps pair become dark and bright simultaneously, it
indicates

a. Incorrect phase sequence
b. That the frequencies of alternators are different
c. That voltage of the alternators are not in phase opposition
d. All of these

27)   For synchronization of alternators, the most suitable method is

a. Lamps method
b. Voltmeter
c. Synchroscope
d. None of the above

28)   What does
the pointer of synchroscope indicate when it is rotating in
anticlockwise direction and when it is rotating in clockwise direction?

a. Incoming machine is running slow, incoming machine is running faster
b. Incoming machine is running faster, incoming machine is running slow
c. Pointer rotates only in clockwise direction
d. all of the above

 ANSWER: Incoming machine is running slow, incoming machine is running faster

29)   The electrical power output in case of synchronous machine is

a. Directly proportional to the power angle
b. Inversely proportional to the power angle
c. Directly proportional to the square of power angle
d. None of these

 ANSWER: Directly proportional to the power angle

30)   The electrical power output of an alternator will be maximum, if the power angle is equal to

a. 0 degree
b. 45 degree
c. 90 degree
d. 180 degree

31)   The synchronous generator supplies leading power factor and lagging power factor respectively when it is

a. Over excited, under excited
b. Over excited, over excited
c. Under excited, under excited
d. Under excited, over excited

32)   Blondel’s two reaction theory which gives the method of analysis of disturbing effects caused by

a. Salient pole construction
b. Non salient pole construction
c. Both (a) and (b)
d. None of these

33)   According to
Blondel’s two reaction theory the armature mmf is divided into
components, direct axis component and quadrature axis component. The
component acting along quadrature axis is

a. Magnetizing
b. Demagnetizing
c. Cross magnetizing
d. None of these

34)   According to Blondel’s two reaction theory, the armature mmf component acting along direct axis can be

a. Demagnetizing
b. Magnetizing
c. Cross magnetizing
d. Either (a) or (b)

35)   The reluctance offered to the mmf wave is lowest when

a. It is aligned with the field pole axis
b. It is oriented at 90 degree to the field pole axis
c. Both (a) and (b)
d. None of these

 ANSWER: It is aligned with the field pole axis

36)   In Blondel’s two reaction theory, if Ia is armature current per phase, Ra is armature resistance, Xd is direct axis synchronous reactance, Xq is quadrature axis synchronous reactance and V is the terminal voltage, then the voltage in phase with direct axis will be

a. V + IaRa + Ia Xd
b. V + IaRa + IaXq
c. V + IaXd + IaXq
d. V + IaRa + IaXd + IaXq

 ANSWER: V + IaRa + Ia Xd

37)   Depending
upon the reluctances offered along the direct axis and quadrature axis,
the armature reaction flux will

b. Lags behind the armature current
c. In phase with the armature current
d. None of these

 ANSWER: Lags behind the armature current

38)   The method used to determine Xd and Xq, the direct and quadrature axis reactance is called

a. Reactance test
b. Blondel’s two reaction theory
c. Slip test
d. All of the above

39)   In a salient pole synchronous generator if Ea is the internal induced emf with Id and Iq be the direct axis and quadrature axis component of armature current, then

a. Iq is in phase with Ea
b. Id is in phase with Ea
c. Id is at 90 degree to Ea
d. Both (a) & (c)
e. None of these

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