Electrical Machines Expected MCQ PDF 9 For Coal India Limited (CIL) MT Exam 2017

1)   An over excited synchronous motor operating on no load condition is called as

a. Synchronous capacitor
b. Synchronous condenser
c. Both (a) and (b)
d. None of these

ANSWER: Both (a) and (b)

2)   The synchronous motor as a synchronous condenser is used to

a. Improve power factor
b. Reduce hunting
c. Both (a) and (b)
d. None of these

ANSWER: Improve power factor

3)   In a step-up transformer when the voltage delivered is higher than that received, the current delivered is

a. Higher than that received
b. Lower than that received
c. Equal to that received
d. Equal to half of that received

ANSWER: Lower than that received

4)   What happens if dc supply is given to the transformer?

a. Transformer will work more efficiently
b. It will work less efficiently
c. It will work same as with ac supply
d. Not work at all

ANSWER: Not work at all

5)   For a transformer, permeability and hysteresis loss should be

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

ANSWER: High, low

6)   During starting, the magnetic field produced in a transformer is

a. 1.5 times the maximum
b. Equal to the maximum
c. 0.5 times of the maximum
d. Equal to zero

ANSWER: Equal to zero

7)   For transformers ratings are always given in kVA, not in kW because

a. It is conventional
b. Power factor of load is not known
c. For higher efficiency
d. To make calculation simple

ANSWER: Power factor of load is not known

8)   In a 5 kV /
400V, 75 kVA single phase transformer, the current flowing in the
primary winding of transformer is 10A. what will be the current flowing
in the secondary winding?

a. 100A
b. 120A
c. 125A
d. 130A

ANSWER: 125A

9)   The hysteresis loss in a transformer is minimized by using

a. Laminated sheets of steel
b. Special grade of heat treated grain oriented silicon steel
c. Bracing of transformer assemblies
d. Proper design of winding

ANSWER: Special grade of heat treated grain oriented silicon steel

10)   Laminations
and varnish are used to reduce the eddy current losses. The laminations
and varnish tends to present a

a. High resistance path to eddy current
b. Low resistance path to eddy current
c. Low reluctance magnetic path
d. High reluctance magnetic path

ANSWER: High resistance path to eddy current

11)   In a transformer sludging is principally due to

a. Moisture present in oil
b. Dust particles present inside the tank
c. Decomposition of oil
d. Vaporization of oil

ANSWER: Decomposition of oil

12)   Breather present in a transformer is used to

a. Absorb atmospheric moisture
b. Prevent insulation failure
c. Cooling
d. All of these

ANSWER: Absorb atmospheric moisture

13)   The globules of water present in oil may result directly in a breakdown. This can be prevented by

a. Proper insulation
b. Isolation of oil from air by using an inert gas
c. Heating the tank
d. None of these

ANSWER: Isolation of oil from air by using an inert gas

14)   In a
transformer tank, grounding is given to protect it from lightening
stoke. Grounding lug is usually installed near

a. Bottom of the tank
b. Top of the tank
c. In the middle of the tank
d. Anywhere on the tank

ANSWER: Bottom of the tank

15)   For the calculation of induced emf in a transformer the flux used in the equation is in its

a. RMS value
b. Average value
c. Maximum value
d. Double of its maximum value

ANSWER: Maximum value

16)   If the
primary winding is excited by a purely sinusoidal alternating voltage,
than the nature of the flux produced is

a. Square wave
b. Rectangular wave
c. Saw tooth wave
d. Sinusoidal wave

ANSWER: Sinusoidal wave

17)   The average emf induced per turn in a transformer is given by

a. 1.11f Φm
b. 0.5f Φm
c. 4f Φm
d. 8f Φm

ANSWER: 4f Φm

18)   A transformer is said to be isolation transformer if its primary to secondary ratio is

a. Equal to one
b. More than one
c. Less than one
d. Equal to 0.5

ANSWER: Equal to one

19)   A 5kV /
250V, 50Hz single phase core type transformer with average flux in the
core is 0.05wb. The number of turns in the primary winding is

a. 273
b. 278
c. 280
d. 287

ANSWER: 287

20)   If V1 is the primary applied voltage and E1 is the primary induced emf. For an ideal transformer

a. V1 > E1
b. V1 = E1
c. V1 < E1
d. V1 = E1 / 2

ANSWER: V1 = E1

21)   For an ideal transformer winding resistances and leakage flux should be

a. Least, maximum
b. Both should be maximum
c. Both should be zero
d. Maximum, least

ANSWER: Both should be zero

22)   When a
resistance is to be transferred from primary to secondary side of
transformer with the turns ratio as k, it must be

a. Multiplied by k ∧ 2
b. Multiplied by k
c. Divided by k
d. Divided by k ∧ 2

ANSWER: Multiplied by k ∧ 2

23)   If the power
factor of load is near unity and leading, the full load voltage
regulation of a power transformer will be

a. Zero
b. One
c. 0.5
d. 0.707

ANSWER: Zero

24)   If the
applied voltage of a certain transformer is increased by 50% keeping the
frequency constant, the maximum flux density will become

a. Remains same
b. 1.5 times the original value
c. Double the original value
d. Half of the original value

ANSWER: 1.5 times the original value

25)   A 10 kVA,
2000 / 100v transformer has R1=1.5 ohm, R2=0.005 ohm, X1=2.5 ohm and
X2=0.08 ohm. The equivalent resistance referred to primary is

a. 3 ohm
b. 3.25 ohm
c. 3.5 ohm
d. 3.8 ohm

ANSWER: 3.5 ohm

26)   A
transformer when connected to a 230V, 50Hz supply, under no load draws
a current of 4A at a power factor of 0.2 lagging. The magnetizing
current (Im) and core loss (Pc) is equal to

a. 3.919A, 184W
b. 1.84A, 391.9W
c. 39.19A, 184W
d. 3A, 180W

ANSWER: 3.919A, 184W

27)   If the
supply voltage is kept at constant value and the supply frequency is
increased. The magnetizing and core loss component

a. Both will decrease
b. Magnetizing component will increase and core loss will decrease
c. Magnetizing component will decrease and core loss will increase
d. Both will increase

ANSWER: Both will decrease

28)   For a transformer, no load primary current ( I0 ) has two components, magnetizing component and active components. The magnetizing component is given by

a. I0 cos Φ0
b. I0 sin Φ0
c. I0 cot Φ0
d. I0 tan Φ0

ANSWER: I0 sin Φ0

29)   For a
certain transformer if E1 is the emf across primary winding, E2 is the
emf across secondary winding and K is the ratio of secondary to primary
turns. The value of secondary emf referred to the primary side is

a. E2 / K
b. E2 / K ∧ 2
c. K * E2
d. K ∧ 2 * E2

ANSWER: E2 / K

30)   In transformers for maximum voltage regulation to occur, the power factor and magnitude is

a. Lagging, Zpu
b. Leading, Zpu
c. Unity, Zpu
d. None of these

ANSWER: Lagging, Zpu

31)   If the
voltage supply of a single phase transformer is doubled and the supply
frequency is halved, then the eddy current loss will become

a. remains same
b. double of the original
c. four times of original
d. half of the original

ANSWER: four times of original

32)   For a single phase no load transformer, which among the following losses will be minimum?

a. hysteresis losses
b. eddy current losses
c. copper losses
d. mechanical losses

ANSWER: copper losses

33)   In the design of transformer, higher flux density is used. Due to which

a. weight per kVA increases
b. weight per kW increases
c. weight per kW decreases
d. weight per kVA decreases

ANSWER: weight per kVA decreases

34)   If the
applied voltage is increased by 10% and the applied frequency is
increased by 20% of a single phase transformer, the percentage change in
eddy current losses will be

a. 19%
b. 20%
c. 21%
d. 40%

ANSWER: 21%

35)   A 600kVA
transformer has core losses of 1w at 25 Hz and 2.5w at 50 Hz (neglecting
resistances and assuming flux density to be constant). The power
required at 60 Hz is

a. 3 watt
b. 3.5 watt
c. 3.24 watt
d. 3.54 watt

ANSWER: 3.24 watt

36)   What happens
to the hysteresis losses and eddy current losses, if the frequency of a
transformer is increased keeping the supply voltage constant?

a. Both will decrease
b. Hysteresis losses will increase and eddy current losses will decrease
c. Hysteresis losses will remain same and eddy current losses will decrease
d. Hysteresis losses will decrease and eddy current losses will remain same

ANSWER: Hysteresis losses will decrease and eddy current losses will remain same

37)   For maximum efficiency in a transformer

a. Core losses = 2 * copper losses
b. Core losses = copper losses / 2
c. Core losses = copper losses
d. Core losses = copper losses ∧ 2

ANSWER: Core losses = copper losses

38)   If the power factor of a transformer increases, then its efficiency will

a. Increases
b. Decreases
c. Remains constant
d. Not related to each other

ANSWER: Increases

39)   When a transformer is supplied by full load at 0.8 p.f. lagging its copper losses is given by Pcu. if it is supplied by half load then copper losses will be

a. Pcu
b. 2Pcu
c. Pcu / 2
d. Pcu / 4

ANSWER: Pcu / 4

40)   A 600 kVA
transformer has iron losses of 400 kW and copper losses of 500 kW. Its
kVA rating for maximum efficiency is given by

a. 537 kVA
b. 548 kVA
c. 555 kVA
d. 585 kVA

ANSWER: 537 kVA

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Electrical Machines Expected MCQ PDF 10 For Coal India Limited (CIL) MT Exam 2017
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