# Electrical Machines Expected MCQ PDF 11 For Coal India Limited (CIL) MT Exam 2017

1)   Use of an autotransformer is economical when its transformation ratio is
a. Near unity
b. Much greater than unity
c. Much lesser than unity
d. None of these

2)   If an
autotransformer having transformation ratio equal to 0.6 is supplying a
load of 8kw then its power transferred from primary to secondary is
given by

a. 3 kW
b. 3.2 kW
c. 3.4 kW
d. 3.5 kW

3)   In an
autotransformer if the power transferred inductively is equal to the
power conducted through, then transformation ratio is given by

a. 1
b. 0.5
c. 2
d. 0

4)   For a two winding transformer if VA rating is given by (VA)tw and K is the transformation ratio of an autotransformer then the VA rating of the autotransformer is given by
a. (K/(K-1)) * (VA)tw
b. (K/(1-K)) * (VA)tw
c. ((1-K)/K) * (VA)tw
d. ((K-1)/K) * (VA)tw

5)   Whether the given autotransformer is step up or step down, its VA rating is always
a. Greater than the two winding transformer
b. Equal to the two winding transformer
c. lesser than the two winding transformer
d. cannot say

 ANSWER: Greater than the two winding transformer
6)   Compared to the two winding transformer, in an autotransformer the leakage reactance and copper losses is
a. less, more
b. less, less
c. more, more
d. more, less

7)   If a two
winding transformer is converted into an autotransformer by applying
additive polarity and subtractive polarity which results in the
secondary voltages of 1840 and 1810 volts. Then the primary and
secondary voltages of transformer are

a. 1800V, 50V
b. 1810V, 40V
c. 1820V, 30V
d. 1825V, 15V

8)   A star
connected three phase transformer is provided with tertiary delta
connected winding which allows the flow of

a. Second harmonic of exciting current
b. Third harmonic of exciting current
c. Fifth harmonic of exciting current
d. Seventh harmonic of exciting current

 ANSWER: Third harmonic of exciting current
9)   A ferro resonant transformer regulation is also known as
a. Constant current transformer
b. Constant voltage transformer
c. Variable current transformer
d. Variable voltage transformer

10)   In a constant voltage transformer, the primary winding and secondary winding is formed by the
a. capacitor, linear inductor
b. Non linear inductor, capacitor
c. Linear inductor and non linear inductor
d. Capacitor, non linear inductor

 ANSWER: Linear inductor and non linear inductor
11)   A constant voltage transformer is fed with the sinusoidal input voltage. Its output is
a. Sinusoidal
b. Flat topped
c. Saw tooth
d. Zig-zag

12)   The output
voltage of constant voltage transformer contains excessive harmonics
which can be filtered out by using

a. RC filter
b. RL filter
c. LC filter
d. None of these

13)   The constant voltage transformers are most commonly used in
a. Personal computers
b. Arc welding
c. Power distribution
d. Electric cars

14)   In the
operation of constant current transformer, two magnetic fields are
produced. The magnetic field produced in the secondary is

a. In the same direction of magnetic field produced in the primary
b. In the opposite direction of magnetic field produced in the primary
c. Perpendicular to the direction of magnetic field produced in the primary
d. Both (b) and (c)

 ANSWER: In the opposite direction of magnetic field produced in the primary
15)   The constant current transformer are mainly used for
a. Supplying series connected street light
b. Personal computers
c. Arc welding
d. Electric toys

 ANSWER: Supplying series connected street light
16)   The high leakage impedance transformers are suitable for the applications of
a. Arc welding
b. Personal computers
c. Street lights
d. Electric lamp

17)   Small iron
core transformers used in certain frequency range is also called audio
frequency transformer. The audio frequency range is

a. 20 t0 2000 Hz
b. 20 to 20000 Hz
c. 2 to 2000 Hz
d. 200 to 20000 Hz

18)   Pulse transformers are small in size. The leakage inductance and permeability of alloy used is
a. Low, high
b. Low, low
c. High, low
d. High, high

19)   In a three phase transformer, the angle between two consecutive cores is
a. 30 degree
b. 60 degree
c. 120 degree
d. 150 degree

20)   In a three
phase transformer, the current flowing in three primaries produces three
corresponding fluxes. The sum of these three fluxes at any instant is

a. Zero
b. Three times of any individual flux
c. One third of any individual flux
d. None of these

21)   A bank of
three single phase transformer can be used for obtaining the three phase
output. Three magnetic circuits produced in case of a bank of three
single phase transformer and in case of single phase transformer are

d. Both are independent

22)   For the star
– star connection of three phase transformer, the phase angle between
the phase voltages and line voltages on both primary and secondary side
is

a. 0 degree
b. 30 degree
c. 60 degree
d. 120 degree

23)   While
connecting three phase transformer in star-star connection, the
terminals of secondaries are wrongly joined. Then the mutual phase angle
and magnitude of line to line voltages of secondaries will

a. Equal to 120 degree and equal as before
b. Not equal to 120 degree and different
c. Equal to 120 degree and different
d. None of these

 ANSWER: Not equal to 120 degree and different
24)   For the
parallel operation of three phase transformers, which among the
following connection is not applicable?

a. Δ – Δ to Y – Y
b. Y – Δ to Δ – Y
c. Y – Y to Y – Y
d. None of these

25)   Due to
presence of third harmonic component in the star-star connection of
three phase transformer, the frequency of the circuit component becomes

a. Three times of the circuit frequency
b. One third of the circuit frequency
c. Remains same
d. None of these

 ANSWER: Three times of the circuit frequency
26)   In star-star connection of three phase transformer, if VL is the line voltage and IL is the line current then phase voltage and phase current is given by
a. VL / √3, IL
b. VL , IL
c. √3VL , IL / √3
d. VL, IL / √3

27)   The phase shift in a star-star connected three phase transformer is
a. 0 degree
b. 30 degree
c. 60 degree
d. 120 degree

28)   If K is the
transformation ratio, then the secondary phase voltage of delta-delta
connected three phase transformer will be

a. 1 / K times of the primary phase voltage
b. Equal to the primary phase voltage
c. 1 / K ∧ 2 times of the primary phase voltage
d. K times the primary phase voltage

 ANSWER: K times the primary phase voltage
29)   For large low voltage transformers, the most commonly used connections are
a. Star – star connection
b. Delta – delta connection
c. V connection
d. All of these

30)   In case of
delta-star connection of three phase transformer, secondary line voltage
with respect of primary line voltage is at

a. 0 degree
c. 30 degree lagging
d. 60 degree lagging

31)   The most commonly used connections for power systems as a step – up and step – down transformers are
a. Star – delta, star – star
b. Delta – star, star – delta
c. Star – star, delta – delta
d. Star – delta, delta – star

 ANSWER: Star – delta, delta – star
32)   The load carried by V – V connection is
a. 47.7% of the original load
b. 57.7% of the original load
c. 67.7% of the original load
d. 87.7% of the original load

33)   Consider
three transformers connected in delta-delta fashion and supplying their
rated load. Now if one of the transformer is removed then each of the
transformer is given as

a. 1
b. 1.232
c. 1.732
d. 1.872

34)   The average
power factor at which V-V bank is operating is less than that with the

a. 57.7% of the balanced load power factor
b. 66.7% of the balanced load power factor
c. 86.6% of the balanced load power factor
d. None of theses

35)   With a bank
of two single phase transformers connected in V – V fashion supplying a
balanced three phase load with Cosφ as power factor. The power factor of
the two transformers is given by

a. Cosφ , Cosφ
b. Cosφ, Cos(30+φ)
c. Cos(30-φ), Cos(30+φ)
d. Cos(30-φ), Cosφ

36)   In T-T connection, the percentage tapping of main transformer and teaser transformer are
a. 50%, 86.6%
b. 50%, 50%
c. 86.6%, 86.6%
d. 86.6%, 50%

37)   In T-T connection, the ratio of actual capacity to the available capacity is
a. 1
b. 0.928
c. 1.928
d. 0.5

38)   Scott connection is used for the conversion of
a. Single phase to three phase
b. Three phase to single phase
c. Single phase to two phase
d. All of these

 ANSWER: Three phase to two phase
39)   In Scott connection, the voltage across the teaser leads the mains by
a. 30 degree
b. 60 degree
c. 90 degree
d. 120 degree

40)   In a single
phase, full wave bridge circuit and in three phase, delta full wave
bridge circuit, the ripple voltage frequency is always

a. Twice the line frequency, six times the line frequency
b. Both will be twice the line frequency
c. Both will be six times the lines frequency
d. None of these

 ANSWER: Twice the line frequency, six times the line frequency
41)   Most familiar application of zig – zag transformer is as
a. Ground reference on an ungrounded system
b. Converting single phase to two phase
c. Reducing harmonics
d. All of these

 ANSWER: Ground reference on an ungrounded system
42)   In double
delta transformation, a double delta refers to the case where there are
two delta transformations in

a. Parallel
b. Series
c. Both series and parallel
d. Neither series nor parallel

43)   A differential relay comparator used for the protection of three phase transformers has
a. One comparator
b. Two comparator
c. Three comparator
d. Six comparator