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EEE : Transformers - Quiz(MCQ)
1 .
What is Transformer?
A)
Transformers are used only for low alternating voltage
B)
Transformer is a device used to convert alternating current to direct current
C)
Transformer is a device used to convert low alternating voltage to a high alternating voltage
D)
Transformer is a device used to convert low alternating current to a high alternating current

Correct Answer :   Transformer is a device used to convert low alternating voltage to a high alternating voltage

Explanation : A Transformer is a device used to convert low alternating voltage to a high alternating voltage and vice versa. Transformers are based on the phenomena of mutual induction. A transformer consists of a soft iron coil with two coils wound around it which are not connected to one another.

2 .
What is the function of a transformer?
A)
Transformer converts DC to AC voltages
B)
Transformer converts AC to DC voltages
C)
Transformer is used to step down or up the DC voltages and currents
D)
Transformer is used to step down or up the AC voltages and currents

Correct Answer :   Transformer is used to step down or up the AC voltages and currents

Explanation : A Transformer does not work on DC and operates only on AC, therefore it Step up or Step down the level of AC Voltage or Current, by keeping frequency of the supply unaltered on the secondary side.

3 .
The purpose of providing an iron core in a transformer is to
A)
reduce hysteresis loss
B)
reduce eddy current losses
C)
provide support to windings
D)
decrease the reluctance of the magnetic path

Correct Answer :   decrease the reluctance of the magnetic path

4 .
A good voltage regulation of a transformer means
A)
output voltage fluctuation with power factor is least
B)
output voltage fluctuation from no load to full load is least
C)
difference between primary and secondary voltage is least
D)
difference between primary and secondary voltage is maximum

Correct Answer :   output voltage fluctuation from no load to full load is least

5 .
The primary winding of a transformer has a 120 V ac supply. What is the value of secondary voltage if the turn ratio is 10?
A)
12 V
B)
120 V
C)
1200 V
D)
12000 V

Correct Answer :   1200 V

Explanation : N2/N1 = 10

V1 = 120

As we know,

V2/V1 = N2/N1

V2/120 = 10

V2 = 120*10

V2 = 1200 V

6 .
Power transformers are designed to have maximum efficiency at
A)
No-load
B)
60 % load
C)
80 % load
D)
Full load

Correct Answer :   Full load

Explanation : Power transformers are usually used for transmission purposes as a step-up device. Power transformers are not directly linked to the consumer side; therefore, it causes very less load fluctuation. So, the power transformer can operate on full load.

7 .
If the supply frequency (f) of a transformer decreases, the effect of frequency on the transformer's secondary output voltage?
A)
Remain the same
B)
Increases
C)
Decreases
D)
All of the above

Correct Answer :   Remain the same

Explanation : The transformer is a static electromagnetic device that transforms the voltage from one side of its coil to the other side of the coil without a change in frequency. As we know, the working principle of transformer is based on Mutual induction, which happens at a constant frequency.

8 .
What is the working principle of a Transformer?
A)
Transformer works on the principle of ampere law
B)
Transformer works on the principle of coulomb law
C)
Transformer works on the principle of self induction
D)
Transformer works on the principle of mutual induction

Correct Answer :   Transformer works on the principle of mutual induction

Explanation : A transformer is an electrical device used to vary the input voltage. Transformer works on the principle of mutual induction.

9 .
What is the current transformer?
A)
transformer used with an D.C. voltmeter
B)
transformer used with an A.C. ammeter
C)
transformer used with an A.C. voltmeter
D)
transformer used with an D.C. ammeter

Correct Answer :   transformer used with an A.C. ammeter

Explanation : A transformer used to extend the range of an A.C. ammeter is known as a current transformer. A current transformer is also abbreviated as C.T.

10 .
If the percentage impedances of the two transformers working in parallel are different, then
A)
transformers will be overheated
B)
parallel operation will be not possible
C)
power factors of both the transformers will be same
D)
parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load

Correct Answer :   parallel operation will still be possible, but the power factors at which the two transformers operate will be different from the power factor of the common load

11 .
An ideal transformer will have maximum efficiency at a load such that
A)
copper loss = iron loss
B)
copper loss < iron loss
C)
copper loss > iron loss
D)
None of the above

Correct Answer :   copper loss = iron loss

12 .
A Buchholz relay can be installed on
A)
auto-transformers
B)
welding transformers
C)
oil cooled transformers
D)
air-cooled transformers

Correct Answer :   oil cooled transformers

13 .
Transformer ratings are given in __________
A)
HP
B)
kW
C)
kVAR
D)
kVA

Correct Answer :   kVA

Explanation : There are two types of losses in a transformer, Copper Losses and Iron Losses or Core Losses or Insulation Losses. Copper losses (I2R) depends on current passing through transformer winding while Iron losses or Core Losses or Insulation Losses depends on Voltage. That’s why the rating of Transformer is in kVA.

14 .
Current transformers are ______________
A)
parallel connected normal transformers
B)
series connected type of instrument transformers
C)
parallel connected type of instrument transformers
D)
series-parallel connected type of instrument transformers

Correct Answer :   series connected type of instrument transformers

Explanation : Current transformer (CT) is a series connected type of instrument transformer. They are designed to present negligible load to the supply which is being measured and also have an accurate current ratio and phase relationship to enable accurate secondary connected metering.

15 .
The full-load copper loss of a transformer is 1600 W. At half-load, the copper loss will be
A)
400 W
B)
800 W
C)
1600 W
D)
6400 W

Correct Answer :   400 W

Explanation : Copper losses are defined as I2*R losses many times, as they are directly proportional to the square of current flowing through them. Thus, copper losses will reduce if load is reduced that too in square proportion.

16 .
The size of a transformer core will depend on
A)
frequency
B)
area of the core
C)
flux density of the core material
D)
Both (A) and (B)

Correct Answer :   Both (A) and (B)

17 .
During the open circuit test of a transformer
A)
The secondary is supplied rated KVA.
B)
Primary is supplied with no-load current.
C)
Primary is supplied rated voltage.
D)
Primary is supplied current at high voltage.

Correct Answer :   Primary is supplied rated voltage.

Explanation : Open circuit test is performed to find the core loss in the transformer. In this test, the transformer's secondary winding is kept open-circuited, and a wattmeter is connected to the primary side. An ammeter (Used to measure the current) is connected in series with the transformer's primary winding, and rated voltage is applied at the primary side.

If normal voltage is applied at the primary side, flux is induced, and iron loss will occur. Hence the iron loss is maximum at rated voltage, and it is measured by using the wattmeter.

18 .
Lamination of the transformer core is made up of
A)
Iron
B)
Steel
C)
Silicon steel
D)
Aluminum

Correct Answer :   Silicon steel

Explanation : A high permeable material made up of thin silicon steel laminations are used for lamination of transformer core and other electrical devices for the given reasons:

High resistance

High permeability

Minimum hysteresis loss.

19 .
A power system has 3 synchronous generators. The turbine-control characteristics equivalent to the generators are X1 =60(60 - f), X2 = 120(70 - f), X3 = 140(80 - f) Where, f denotes the system frequency in Hz, and X1, X2, X3 are the power outputs of the turbines in MW. Considering the generators and transmission network to be lossless, the system frequency for a load of 800 MW is
A)
80 Hz
B)
70 Hz
C)
65 Hz
D)
50 Hz

Correct Answer :   70 Hz

Explaination :

X1 = 60(60-f)

X2 = 120(70 - f)

X3 = 140(80 - f)

X1+X2+X3 = 800

60(60-f) + 120(70 - f) + 140(80 - f) = 800

3600 - 60f + 8400-120f + 11200 - 140f = 800

22400 = 320f

f = 70 Hz

20 .
A transformer has a primary coil with 1600 loops and a secondary coil with 1200 loops. If the current in the primary coil is 6 Ampere, then what is the current in the secondary coil of a transformer.
A)
58 Ampere
B)
68 Ampere
C)
78 Ampere
D)
98 Ampere

Correct Answer :   98 Ampere

Explaination :

Primary coil (Np) = 1600 loops

Secondary coil (Ns) = 1200 loops

The current in the primary coil (Ip) = 6 Ampere

Is =?

As we know,

Is/Ip = Np/Ns

The current in the secondary coil is

Is/6 = 1600/1200

Is/6 = 1.33

Is = 1.33 * 6

Is = 7.98 Ampere

21 .
Which of the following is not a part of transformer installation?
A)
Exciter
B)
Breather
C)
Conservator
D)
Buchholz relay

Correct Answer :   Exciter

Explanation : Conservator, breather, Buchholz relay are the parts which are much important in transformer construction in order to maintain temperature of the transformer and to work transformer with good efficiency.

22 .
Transformer core is generally made of ______
A)
Cannot be determined
B)
Single block of core material
C)
Can be made with any of the above method
D)
By stacking large number of sheets together

Correct Answer :   By stacking large number of sheets together

Explanation : Transformer core experiences eddy current losses when transformer is in the operations. In order to reduce eddy current losses, it is advisable to use large number of sheets laminated from each other are stick together than using one single block.

23 .
The purpose of the transformer core is to provide __________
A)
Low reluctance path
B)
High inductive path
C)
High capacitive path
D)
High reluctance path

Correct Answer :   Low reluctance path

Explanation : The purpose of a transformer core is to provide a low-reluctance path for the magnetic flux linking primary and secondary windings. In doing so, the core experiences iron losses due to hysteresis and eddy currents flowing within it which, in turn, show themselves as heating of the core material.

24 .
A transformer cannot raise or lower the voltage of a D.C. supply because
A)
A D.C. circuit has more losses
B)
There is no need to change the D.C. voltage
C)
Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero
D)
None of the above

Correct Answer :   Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero

25 .
Which of the following loss in a transformer is zero even at full load?
A)
Core loss
B)
Friction loss
C)
Hysteresis loss
D)
Eddy current loss

Correct Answer :   Friction loss

26 .
Part of the transformer which is most subject to damage from overheating is
A)
iron core
B)
frame or case
C)
copper winding
D)
winding insulation

Correct Answer :   winding insulation

27 .
The no-load current drawn by transformer is usually what per cent of the full-load current?
A)
0.2 to 0.5 per cent
B)
2 to 5 per cent
C)
12 to 15 per cent
D)
20 to 30 per cent

Correct Answer :   2 to 5 per cent

28 .
The chemical used in breather for transformer should have the quality of
A)
ionizing air
B)
absorbing moisture
C)
cooling the transformer oil
D)
cleansing the transformer oil

Correct Answer :   absorbing moisture

29 .
The short circuit test in the transformer is performed on
A)
High voltage side
B)
light voltage side
C)
Both (A) and (B)
D)
Either (A) or (B)

Correct Answer :   High voltage side

Explanation : The short circuit test in the transformer is always carried on the high voltage side mainly due to the power supply limitations used to perform the transformer's short circuit test.

The primary purpose of performing a short circuit test in a transformer is to calculate the transformer's positive sequence impedance (Z). A short circuit test is also performed to calculate the copper loss.

30 .
The secondary voltage is 440 Volt, and primary voltage is 220 Volt, then a comparison of the secondary coil and primary coil is
A)
2/1
B)
4/1
C)
5/2
D)
7/5

Correct Answer :   2/1

Explanation : Secondary voltage (Vs) = 440 Volt

Primary voltage (Vp) = 220 Volt

Then,

Ns/Np

As we know,

Vs/Vp = Ns/Np

440/220 = Ns/Np

44/22 = Ns/Np

2/1 = Ns/Np

31 .
Which test determines the efficiency of two identical transformers under load conditions?
A)
Full - load test
B)
Low voltage test
C)
Back-to-Back (Sumpner's Test)
D)
Open circuit test.

Correct Answer :   Back-to-Back (Sumpner's Test)

Explanation : In the back-to-back test or Sumpner's test on a transformer, two identical transformers are connected in such a way that one transformer is kept onto another transformer. The primary side of the two identical transformers is connected in parallel across the supply voltage. In contrast, the secondary side of the transformer is connected in series in such a way that emf's are in series opposition. It is a method for determining the efficiency, voltage regulation and heading under loaded conditions of the transformer.

32 .
A transformer has 6 windings in its primary core and 3 in its secondary coil. If the primary voltage is 440 V, find the secondary voltage.
A)
155 V
B)
165 V
C)
175 V
D)
185 V

Correct Answer :   165 V

Explanation : As we know, electric power is the same in both the coils of the transformer, so that the given relationship must be true

N1/N2 = V1/V2 = I2/I1

In the given question, we only need the number of turns (n) and voltage (V)

N1/N2 = V1/V2

Rearranging the above equation, we get,

V2 = V1N2/N1

Given,

V1 = 440, N1 = 6 Turns, N2 = 3 turns

V2 = V1N2/N1 = (440 V) (3 turns)/ (8 turns)

= 165 V

33 .
Oil is provided in an oil-filled transformer for
A)
Cooling
B)
Insulation
C)
Lubrication
D)
Both cooling and Insulation

Correct Answer :   Both cooling and Insulation

Explanation : Transformers have broad applications for various industries; mainly, distribution transformers are filled with oil. The two-primary function of the transformer oil is given below

Coolant : As we know, the transformer's coils are made up of copper and carrying a very high current, so it becomes hot in a very short time. The transformer oil is a good conductor of electricity and heat, so it reduces the coil's temperature.

Insulator : The transformer oil has high dielectric strength; it means it can withstand very high voltage; this is the only reason it acts as an insulator in the transformer

34 .
Transformers are generally designed for _________
A)
short-time use
B)
one-time use
C)
off-site problem solving
D)
on-site problem solving

Correct Answer :   on-site problem solving

Explanation : Every transformer is designed for use it for multiple years, thus transformers are designed to handle the problems on site itself because it not only saves time but also makes repairing work easy.

35 .
What is the thickness of laminations used in a transformer?
A)
0.1 mm to 0.5 mm
B)
4 mm to 5 mm
C)
14 mm to 15 mm
D)
25 mm to 40 mm

Correct Answer :   0.1 mm to 0.5 mm

Explanation : Laminations are made to reduce the eddy currents and is made of thin strips. Generally, the steel transformer lamination range for 50 Hz varies from 0.25mm to 0.5mm, if it is a 60 Hz transformer then it ranges from 0.17–0.27mm.

36 .
Which type of flux does transformer action need?
A)
Alternating electric flux
B)
Alternating magnetic flux
C)
Constant magnetic flux
D)
Increasing magnetic flux

Correct Answer :   Alternating magnetic flux

Explanation : The energy transfer in a transformer, is from one winding to another, entirely through magnetic medium it is known as transformer action. Therefore, transformer action requires an alternating or time varying magnetic flux in order to transfer power from primary side to secondary side. Since induced emf in the winding is due to flux linkage.

37 .
Primary winding of a transformer _______
A)
Cannot be determined
B)
Is always a low voltage winding
C)
Is always a high voltage winding
D)
Could either be a low voltage or high voltage winding

Correct Answer :   Is always a high voltage winding

Explanation : Primary winding used in a transformer, can be at higher or lower voltage potential, depending on the number of turns with secondary winding. For step up and step-down transformers primary winding will be at lower and higher potential respectively.

38 .
What is the function of a breather in a transformer?
A)
To cool the transformer oil
B)
To provide oxygen inside the tank
C)
To cool the coils during reduced load
D)
To arrest flow of moisture when outside air enters the transformer

Correct Answer :   To arrest flow of moisture when outside air enters the transformer

Explanation : Most of the power generation stations use silica gel breathers fitted to conservator of oil filled transformers. The most used purpose of these silica gel breathers is to arrest the moisture when the outside air is sucked in by the transformer during the breathing process.

39 .
An ideal transformer will have maximum efficiency at a load such that ______
A)
cannot be determined
B)
copper loss = iron loss
C)
copper loss > iron loss
D)
copper loss < iron loss

Correct Answer :   copper loss = iron loss

Explaination : Maximum efficiency of a transformer is defined at that value when copper losses become completely equal to the iron losses. In all other cases the efficiency will be lower than the maximum value.

40 .
Which transformer insulation material is best compare to Kraft paper?
A)
Oil
B)
Cotton
C)
Asbestos
D)
Low grade pressboard

Correct Answer :   Oil

Explanation : Newly developed oil-filled transformers have capabilities much greater than those transformers which used Kraft paper as dielectric material. Also, electrical properties of Kraft paper depend on physical and chemical properties of paper.

41 .
For a transformer with primary turns 400, secondary turns 100, if 20A current is flowing through primary, we will get _________
A)
5A at secondary
B)
40A at secondary
C)
80A at secondary
D)
800A at secondary

Correct Answer :   80A at secondary

Explanation : Current in the primary of the transformer will get modified in the transformer secondary, according to the number of turns, in inverse proportion. Thus, turns are modified with 1/4 times the primary, we’ll get 4 times higher current at secondary.

42 .
The function of conservator in a transformer is
A)
to cool the transformer oil
B)
to project against internal fault
C)
to reduce copper as well as core losses
D)
to take care of the expansion and contraction of transformer oil due to variation of temper-ature of sur-roundings

Correct Answer :   to take care of the expansion and contraction of transformer oil due to variation of temper-ature of sur-roundings

43 .
Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in
A)
short-circuiting of the secondaries
B)
loading of the transformers not in proportion to their kVA ratings
C)
transformers having higher copper losses will have negligible core losses
D)
power factor of one of the transformers is leading while that of the other lagging

Correct Answer :   loading of the transformers not in proportion to their kVA ratings

44 .
The value of flux involved m the e.m.f. equation of a transformer is
A)
r.m.s. value
B)
maximum value
C)
average value
D)
instantaneous value

Correct Answer :   maximum value

45 .
In a given transformer for given applied voltage, losses which remain constant irrespective of load changes are
A)
copper losses
B)
friction and windage losses
C)
hysteresis and eddy current losses
D)
None of the above

Correct Answer :   hysteresis and eddy current losses

46 .
The changes in volume of transformer cooling oil due to variation of atmospheric tem-perature during day and night is taken care of by which part of transformer
A)
Conservator
B)
Breather
C)
Bushings
D)
Buchholz relay

Correct Answer :   Conservator

47 .
Star/star transformers work satisfactorily when
A)
load is balanced only
B)
load is unbalanced only
C)
on balanced as well as unbalanced loads
D)
None of the above

Correct Answer :   load is balanced only

48 .
Delta/star transformer works satisfactorily when
A)
load is balanced only
B)
load is unbalanced only
C)
Can Not Say
D)
on balanced as well as unbalanced loads

Correct Answer :   on balanced as well as unbalanced loads

49 .
The noise of the transformer primarily due to
A)
Cooling fan
B)
Mechanical vibration
C)
Magnetostriction in an iron core
D)
All of the above

Correct Answer :   Magnetostriction in an iron core

Explanation : The noise of the transformer, also called humming, is mainly due to two reasons

Magnetostriction : Magnetostriction refers to the expansion and contraction of the laminations (iron core) due to the magnetic effect of the current flowing the coil of the transformer. Magnetostriction may be controlled, but it can't be totally removed depending on the transformer's design.

Stray magnetic fields : Generally, the steel used in the transformer core and wire used in the transformer coil vibrating at 50 Hz to 60 Hz frequency because of the interaction of the magnetic fields.

50 .
If a transformer is connected to a direct current supply, what happened?
A)
No effect
B)
Damage the transformer
C)
Operate with low frequency
D)
Operate with high frequency

Correct Answer :   Damage the transformer

Explanation : As we know, a transformer works on the principle of mutual induction, where you need a dynamic magnetic field to generate an induced emf in the secondary winding. In direct current supply, the change in frequency with respect to time is null, which means the dc supply can't produce a dynamic magnetic field; hence mutual induction can't possible on direct current supply since the primary winding of the transformer has a very low value of resistance so it can't oppose the excessive flow of current; therefore, the current having high value can damage the windings.

51 .
Cruciform shape is used in the transformer core to
A)
Reduce core loss
B)
Reduce core reluctance
C)
Reduce copper winding
D)
All of the above

Correct Answer :   All of the above

Explanation : Cruciform shape is used in transformer core to reduce core loss, copper, in the winding, reduce core reluctance, and reduction of iron and copper losses. In a transformer, eddy current losses in the iron core are directly proportional to the core's thickness. To reduce eddy current loss, we usually use thin sheets of the iron core to reduce skin effect.

52 .
Which of the following does not change in an ordinary transformer?
A)
Voltage
B)
Current
C)
Frequency
D)
All of the above

Correct Answer :   Frequency

Explanation : The transformer is a static (no moving parts) device, so the rate of magnetic flux change is constant for a certain operation. The moving flux in the transformer core is cut by the coils whose frequency is controlled by the primary supply. Since the coils or stationary conductors are static, the frequency does not change.

53 .
A 25 KVA transformer is constructed to a turns ratio of N1/N2 = 10. The impedance of primary winding is 3+j5 ohms and of secondary winding is 0.5+j0.8 ohms. What will be the impedance of transformer when referred to primary?
A)
3.5 + 5.8j ohms
B)
53j + 85 ohms
C)
53 + 85j ohms
D)
Can’t be calculated

Correct Answer :   53 + 85j ohms

Explaination : Given turns ratio is 10. Thus, secondary resistance when referred to the primary is equal to k2*Z2, so net impedance on primary is equal to primary impedance + k^2*Z2 . Substituting all the terms we get net impedance = 53 + 85j ohms.

54 .
Power transformers other than distribution transformers are generally designed to have maximum efficiency around ______
A)
Near full-load
B)
Half-load
C)
10% overload
D)
No-load

Correct Answer :   Near full-load

Explanation : Similar to normal transformers power transformers are also designed to get maximum efficiency at load which is near to the full load of a transformer specified. Only in the case distribution transformer maximum efficiency is achieved at 60% of full load.

55 .
For a power transformer operating at full load it draws voltage and current equal to 200 V and 100 A respectively at 0.8 pf. Iron and copper losses are equal to 120 kW and 300kW. What is efficiency?
A)
86.44%
B)
96.44%
C)
97.44%
D)
99.12%

Correct Answer :   97.44%

Explaination : Power output= VI cosθ= 200*100*0.8 = 16000 W (Independent of lag and lead). While total losses are equal to iron loss+ k2*copper losses =120+ 300= 420 W. Efficiency is equal to 1- 420/(16000+420)= 97.44%.

56 .
Voltage regulation of transformer is given by __________
A)
V2-E2/E2
B)
V2-E2/V2
C)
E2-V2/V2
D)
E2-V2/E2

Correct Answer :   E2-V2/E2

Explaination : Voltage regulation is defined as change in the voltage or rise in voltage when transformer is load is thrown off. Thus, it is the difference of the no load voltage with the full load voltage divide by full load voltage to get % increase.

57 .
In a transformer the resistance between its primary and secondary should be
A)
Zero
B)
1 ohm
C)
1000 ohms
D)
Infinity

Correct Answer :   Infinity

58 .
Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds
A)
150°C
B)
100°C
C)
80°C
D)
50°C

Correct Answer :   150°C

59 .
For given applied voltage, with the increase in frequency of the applied voltage
A)
eddy current loss will increase
B)
eddy current loss will decrease
C)
eddy current loss will remain unchanged
D)
None of the above

Correct Answer :   eddy current loss will remain unchanged

60 .
In an actual transformer the iron loss remains practically constant from noload to fullload because
A)
primary voltage remains constant
B)
core flux remains practically constant
C)
value of transformation ratio remains constant
D)
permeability of transformer core remains constant

Correct Answer :   core flux remains practically constant

61 .
The dielectric strength of transformer oil is expected to be
A)
3 kV
B)
33 kV
C)
100 kV
D)
330 kV

Correct Answer :   33 kV

Explanation : The dielectric strength refers to the withstand value; it means the electrical stress which oil can withstand safely. Usually, we monitored in transformed oil is BDV (breakdown voltage), i.e., the maximum electrical stress on which the transformer oil breaks down.

62 .
In the transformer, the function of a conservator is to
A)
It provides air for cooling the transformer.
B)
It provided colling oil to transformer when transformer needed.
C)
Both (A) and (B)
D)
Protect the transformer form damage when oil expands due to heating.

Correct Answer :   Protect the transformer form damage when oil expands due to heating.

Explanation : In the transformer, the conservatory provides free space for the expansion of the oil. When the transformer is on full load, the oil heats up and expands. When the load is reduced from the transformer, oil temperature shrinks and oil contracts. Here, the air space in the conservatory enables breathing when oil expands or contracts. It is possible to manufacture a transformer without a conservator, but in that particular case, a tank with a gas cushion is required at the top of the transformer.

63 .
The efficiency of a transformer is maximum when
A)
Iron loss = Copper loss.
B)
Eddy current loss = iron loss
C)
Hysteresis loss = copper loss.
D)
Eddy current loss = hysteresis loss

Correct Answer :   Iron loss = Copper loss.

Explanation : The Efficiency of the transformer refers to the ratio of useful power output to the input power, the two being measured in the same unit. Its unit is either in Watts (W) or Kilowatt (KW). Transformer efficiency is denoted by È . Therefore, the efficiency of the transformer is maximum when iron loss = copper loss.

64 .
Which winding in a transformer has a greater number of turns
A)
Low voltage winding
B)
Secondary winding
C)
High voltage winding
D)
Constant voltage winding

Correct Answer :   High voltage winding

Explanation : In the transformer, the high voltage winding always has a greater number of turns, as a voltage is directly proportional to the number of turns.

65 .
No-load current in the transformer is _______
A)
Steps
B)
Sinusoidal
C)
Straight DC
D)
Sinusoidal distorted

Correct Answer :   Sinusoidal distorted

Explanation : The no-load current in a transformer is non-sinusoidal. The basic cause for this phenomenon, which lies in hysteresis and saturation non-linearities of the core material, will now be investigated; this can only be accomplished graphically.

66 .
Which of the following is the main advantage of an auto-transformer over a two-winding transformer?
A)
Copper losses are negligible
B)
Saving in winding material
C)
Hysteresis losses are reduced
D)
Eddy losses are totally eliminated

Correct Answer :   Saving in winding material

Explanation : Auto transformer is a special type of transformer which has primary and secondary winding both located on same winding. Thus, winding material required for a transformer is very less in the case of autotransformer.

67 .
For a 20kVA transformer with a turn ratio of 0.4 what amount of total power is transferred inductively?
A)
8kVA
B)
10kVA
C)
12kVA
D)
50kVA

Correct Answer :   12kVA

Explanation : For an auto transformer power is transferred partially inductively and partially conductively. Thus, out of total power, power transferred inductively is given by (1-k)*total power= 0.6*20= 12kVA.

68 .
Which of the following is the major requirement for the transformers used for electronic purposes?
A)
Perfect DC isolation
B)
Maximum efficiency
C)
Constant amplitude voltage gain
D)
Perfect DC isolation, maximum efficiency and constant voltage gain

Correct Answer :   Constant amplitude voltage gain

Explanation : An important requirement of these transformers is that the amplitude voltage gain (ratio of output/input voltage amplitude) should remain almost constant over the range of frequencies (bandwidth) of the signal.

69 .
Which type of winding is used in 3-phase shell-type transformer?
A)
Sandwich type
B)
Circular type
C)
Cylindrical type
D)
Rectangular type

Correct Answer :   Sandwich type

Explanation : In core type of the transformer, winding is done by normal method, while in the shell type transformer, winding is sandwiched between corresponding shells of core material. Hence, it is sandwich type.

70 .
Why shell type 3-phase transformer is used in large power transforming applications?
A)
Due to other reasons
B)
Can be made with less height
C)
Can be made with more height
D)
More height and less height flexibility

Correct Answer :   Can be made with less height

Explanation : The shell-type five-limb type three-phase transformer construction is heavier and more expensive to build than the core-type. Five-limb cores are generally used for very large power transformers as they can be made with reduced height.

71 .
Which of the following is not a routine test for transformers?
A)
Polarity test
B)
Impedance test
C)
Core insulation voltage test
D)
Radio interference test

Correct Answer :   Radio interference test

Explanation : Impedance test is done on a transformer in order to check net impedance offered by a transformer circuit at rated supply. Polarity test is also done before SC and OC test on transformer. Core insulation is also checked before installations in such tests.

72 .
In case of a double cage induction motor, the inner cage has
A)
low inductance and low resistance
B)
low inductance and high resistance
C)
high inductance arid low resistance
D)
high inductance and high resistance

Correct Answer :   high inductance arid low resistance

73 .
If the supply frequency to the transformer is increased, the iron loss will
A)
increase
B)
decrease
C)
not change
D)
any of the above

Correct Answer :   increase

74 .
The term 'cogging' is associated with
A)
series motors
B)
induction motors
C)
compound generators
D)
three phase transformers

Correct Answer :   induction motors

75 .
Minimum voltage regulation occurs when the power factor of the load is
A)
leading
B)
unity
C)
lagging
D)
Zero

Correct Answer :   leading

76 .
The highest voltage for transmitting electrical power in India is
A)
66 kV
B)
132 kV
C)
400 kV
D)
765 kV

Correct Answer :   765 kV

Explanation : In India, the highest voltage used for transmitting electrical power is 765 kV.

This ultra-high voltage is employed to efficiently transmit electricity over extremely long distances while minimizing losses. Higher voltage levels result in lower current flow through the transmission lines, which in turn reduces resistive losses.

This is a critical factor in power transmission, allowing for efficient and cost-effective distribution of electricity across the country. Options A, B, C, and D have lower voltage ratings and are generally used for sub-transmission and distribution purposes.

77 .
In a transformer, tappings are usually provided.
A)
Low voltage side
B)
High voltage side
C)
Primary side
D)
Both (A) and (B)

Correct Answer :   Both (A) and (B)

Explanation : The tapings can be placed either on high voltage (HV) or low voltage (LV) windings.

Usually, the tapings are provided on high voltage (HV) winding due to the following reasons-

Good voltage regulation is possible with high voltage winding as it carries a large number of turns.

The low voltage winding of the transformer carries more current. So, if tapings are provided on the low voltage side, there are difficulties experienced in the interruption of high currents, making it unworkable. Hence, it is convenient to provide tapings on high voltage (HV) winding.

78 .
The purpose of a breather in a transformer to
A)
Filter the transformer oil
B)
Provide cold air in the transformer
C)
Absorb moisture of air during breathing
D)
None of the above

Correct Answer :   Absorb moisture of air during breathing

Explanation : As the name suggests, "Breather" helps out the transformer to breathe, i.e., it assists in allowing the flow of air from the internal part of the transformer to the atmosphere and allowing the fresh air to come inside from the atmosphere. Generally, the breather comprises silica gel, which absorbs the moisture form air.

79 .
The necessary condition for the parallel operation of two single-phase transformers is that they must have the same.
A)
Polarity
B)
Turn Ratio
C)
KVA Rating
D)
Both (A) and (B)

Correct Answer :   Both (A) and (B)

Explanation : Parallel operation of single-phase transformer is done to increase the load capacity and reliability of the supply. It is done by adding another transformer in parallel simultaneously, given the continuous supply and frequent maintenance can be done evenly.

Some conditions must be fulfilled before the parallel operation of a transformer is given below.

The ratio of primary and secondary voltage ratings must be identical. It means the same transformation ratio for the transformer.

Both transformers must have the same polarity.

The X/R ratio must be the same.

80 .
The primary purpose of performing a short circuit test in a transformer is to measure its
A)
Iron loss
B)
Copper loss
C)
Core loss
D)
Insulation loss

Correct Answer :   Copper loss

Explanation : The short circuit test is performed to find the copper loss in the transformer. It is done by shorting the low voltage terminal and applying a low voltage across the high voltage terminal (because the current in the high voltage terminal will be less and simple to handle) and connect a wattmeter to measure the power dissipated in the low voltage terminal. The wattmeter reflects the full load copper loss.

81 .
The flashpoint of transformer oil must be greater than
A)
100 degree
B)
125 degree
C)
140 degree
D)
160 degree

Correct Answer :   140 degree

Explanation : Usually, transformer oil has excellent insulating properties so that it is capable of withstanding high temperature. The Flashpoint of a transformer refers to the lowest temperature at which transformer oil gives vapor.

Its primary function is to determine the self-ignition temperature of oil for safety reasons.

The flashpoint of the transformer is directly proportional to the safe operation of the transformer.

It means the higher the flash point value safer the operation. If the flash point value is lower than the specified value, the risk of fire in the transformer increases.

82 .
What will happen if the secondary of a current transformer is open-circuited?
A)
depends on other parameters
B)
hot because of heavy iron losses
C)
cool as there is no secondary current
D)
hot because primary will carry heavy current

Correct Answer :   hot because of heavy iron losses

Explanation : If secondary of current transformer is made open-circuited the transformer temperature will rise to higher value because of heavy iron losses taking place in the circuit due to high flux density.

83 .
Welding transformers work on principle that weld is actually ___________
A)
short circuit
B)
open circuit
C)
circuit with finite resistance
D)
circuit with finite reactance

Correct Answer :   short circuit

Explanation : The increase in primary or secondary current of welding transformer increases the reactance voltage drop across the respective windings, which is essential to limit the welding current as the weld is practically a short circuit.

84 .
Insulation of HVDC transformers should be ______
A)
wet always
B)
dry always
C)
can be dry or wet
D)
None of the above

Correct Answer :   dry always

Explanation : Because of the effect of moisture on the resistivity of insulation material, it is necessary to obtain and maintain a high level of dryness in the insulation of HVDC transformers. This is equally important in service as it is in the factory at the time of testing.

85 .
Core flux in the transformer is ________
A)
triangular
B)
sinusoidal
C)
square wave
D)
flat-topped

Correct Answer :   flat-topped

Explanation : The supply voltage provides the input current to the transformer primary, here primary current is only sinusoidal magnetizing current. Thus, this sinusoidal magnetic current will produce core flux, flat-topped.

86 .
For a transformer given of 100 kVA, 220/6000-V transformer, short circuit test is performed. What current rating is needed?
A)
30A
B)
40A
C)
60A
D)
445A

Correct Answer :   445A

Explanation : For a given transformer here, test is performed on low voltage side, thus we need the value of current on the low voltage side, by dividing the reactive power by the rated voltage value, i.e. 200*1000/100 = 445A.

87 .
For a transformer with primary turns 100, secondary turns 400, if 200 V is applied at primary we will get _____
A)
3200 V at secondary
B)
1600 V at secondary
C)
800 V at secondary
D)
80 V at secondary

Correct Answer :   800 V at secondary

Explanation : Voltage in the primary of the transformer will get modified in the transformer secondary, according to the number of turns. Thus, turns are modified with 4 times the primary, we’ll get 4 times higher voltage at secondary.

88 .
The secondary winding of which of the following transformers is always kept closed?
A)
Current transformer
B)
Potential transformer
C)
Step-up transformer
D)
Step-down transformer

Correct Answer :   Current transformer

89 .
If a transformer is switched on to a voltage more than the rated voltage
A)
its power factor will increase
B)
its power factor will deteriorate
C)
its power factor will be zero
D)
its power factor will remain unaffected

Correct Answer :   its power factor will deteriorate

90 .
The transformer oil should have _____ volatility and _____ viscosity.
A)
low,low
B)
high,high
C)
low,high
D)
high,low

Correct Answer :   low,low

91 .
Which of the given test determine the iron loss of the transformer?
A)
Back-to-back test
B)
Short circuit test
C)
Both (A) and (B)
D)
Open circuit test

Correct Answer :   Open circuit test

Explanation : An open circuit test is also known as a no-load test. It is used to find the iron loss at the rated applied voltage to the primary side. Iron loss of the transformer is not varying with a change in load. The iron loss also helps to determine the voltage regulation and efficiency of the transformer. Open circuit test of a transformer is carried out on the low voltage side by keeping the high voltage side open.

92 .
The frictional loss in a transformer is
A)
0 %
B)
10 %
C)
50 %
D)
More than 50 %

Correct Answer :   0 %

Explanation : As we know, the transformer is a static device based on the principle of mutual induction with no rotating part, so there is no frictional loss.

93 .
Which of the given winding of the transformer has less cross-sectional area?
A)
Primary winding
B)
Low voltage winding
C)
High voltage winding
D)
Secondary winding

Correct Answer :   High voltage winding

Explanation : High voltage winding of a transformer has the less cross-sectional area. The net current flowing through the winding calculates the cross-sectional area. It means if the current high current flows through the winding, a high cross-sectional area will be obtained since the high voltage winding of a transformer has a low current, so it has a less cross-sectional area.

94 .
The secondary winding of which of the given transformer is always kept closed?
A)
Power transformer
B)
Voltage transformer
C)
Current transformer
D)
Step-up transformer

Correct Answer :   Current transformer

Explanation : The secondary winding of the current transformer is always kept closed because if it open-circuited, the current transformer would develop an extremely high voltage across the secondary terminals. It may lead to damage to the transformer insulation as well as arcing across the terminal.

95 .
Which of the given losses changes with the load in the transformer?
A)
Iron loss
B)
Copper loss
C)
Core loss
D)
None of the above

Correct Answer :   Copper loss

Explanation : As we know, the transformer has primarily two types of losses Iron loss and copper loss. Iron loss is always constant, whereas copper losses are varying with a load.

Iron loss = Hysteresis loss + Eddy current loss

Both hysteresis loss and eddy current loss do not depend upon load current, which means iron losses are constant with varying load.

Copper loss = Conduction losses in conductor winding, which are directly proportional to the current square.

96 .
A transformer transforms
A)
Voltage
B)
Current
C)
Frequency
D)
Current and voltage

Correct Answer :   Current and voltage

Explanation : The transformer is a static electromechanical device that works on the principle of mutual induction. Transformer transforms:

Voltage : nU2 = Nu1, where U2 being the secondary side and U1 the primary side.

Current : I2 = I1/n, so when voltage is increased, the current is decreased.

97 .
A transformer has negative voltage regulation, only if its load power factor is
A)
Leading
B)
Unity
C)
Lagging
D)
None of the above

Correct Answer :   Leading

Explanation :

First, we need to understand the term "Voltage regulation" in brief:

Voltage Regulation:

When the secondary terminal of the transformer carried the full load, and the secondary voltage varies even though the primary supply voltage is held constant at rated value.

% Voltage Regulation = [ V2(NL) - V2 (FL)] *100 /[V2(NL)]

If the rated voltage is applied to the primary winding of the transformer, the secondary voltage changes with the load current and power factor, even the primary voltage is kept constant. These changes in voltage of windings are known as voltage regulation.

Voltage regulation depends on the voltage drop in the transformer's impedance, load current and load power factor.

98 .
In an autotransformer, the primary and secondary are ________ Coupled.
A)
Magnetically coupled
B)
Electrically coupled
C)
Both magnetically and electrically coupled.
D)
None of the above

Correct Answer :   Both magnetically and electrically coupled.

Explanation : Auto-transformers is a transformer with one common winding for primary and secondary, wound on a common core.

If you have a winding with a minimum of one tapping, you have an autotransformer. One extreme terminal in common for input and output. Two terminals are used for input, usually supply voltage, while the third terminal gives output with a common terminal. It is even possible to have more than one output.

In Auto Transformer, one single winding is used for primary winding and secondary winding, which means the primary and secondary windings have the common single winding. Therefore, the primary is electrically connected to the secondary and magnetically coupled to it.

Such an autotransformer saves on the cost of copper, and losses are less; flux linkage is better since there is just one winding. It is also smaller in size compared with a normal transformer.

99 .
Crushed rocks are provided in the substation to
A)
To provide insulation
B)
To avoid growing plants and weeds.
C)
To avoid fire accidents during leakage of transformer oil.
D)
All of the above

Correct Answer :   All of the above

Explanation : Stones or crushed rock are provided in the substation to provide an extra layer of high resistance between the soil and a person walking or standing over there. It provides a safety layer that reduces the step and touch voltages that occur during ground faults, which can happen both inside and outside the substation.

All substations are designed with a ground grid. Copper conductors are arranged in a grid pattern and laid in trenches dug into and buried in the soil when the substation is constructed. If the crushed rock were not used, more copper would be needed in the ground grid to achieve tolerable step and touch voltages during ground faults.

100 .
The maximum load that a power transformer can carry is limited by its
A)
Copper loss
B)
Voltage ratio
C)
Temperature rise
D)
Dielectric strength of the coil

Correct Answer :   Voltage ratio

Explanation : The voltage ratio of a transformer refers to the ratio of the RMS terminal voltage of a high voltage winding to the RMS terminal voltage of a low voltage winding in a specified load condition. The total power a transformer can deliver depends primarily on the magnetic properties of the transformer and the volume of its core. The power of a transformer is directly proportional to the volume and frequency of the transformer.

101 .
The degree of mechanical vibrations produced by the laminations of a transformer depends on
A)
size of laminations
B)
gauge of laminations
C)
tightness of clamping
D)
All of the above

Correct Answer :   All of the above

102 .
The thickness of laminations used in a transformer is usually
A)
0.4 mm to 0.5 mm
B)
4 mm to 5 mm
C)
14 mm to 15 mm
D)
25 mm to 40 mm

Correct Answer :   0.4 mm to 0.5 mm

103 .
The core used in high frequency transformer is usually
A)
copper core
B)
air core
C)
cost iron core
D)
mild steel core

Correct Answer :   air core

104 .
The efficiency of two identical transformers under load conditions can be determined by
A)
open circuit test
B)
short-circuit test
C)
back-to-back test
D)
Any of the above

Correct Answer :   back-to-back test

105 .
For a transformer, operating at constant load current, maximum efficiency will occur at
A)
unity power factor
B)
zero power factor
C)
0.8 lagging power factor
D)
0.8 leading power factor

Correct Answer :   unity power factor

106 .
The transformer draws the current when its secondary winding is open.
A)
Full load primary current
B)
Full load secondary current
C)
No-load secondary current
D)
No-load primary current

Correct Answer :   No-load primary current

Explanation : The transformer draws the no-load primary current when its secondary winding is open.

107 .
Buck-boost transformers are used when
A)
The supply voltage matches the voltage required by the load.
B)
The supply current matches the voltage required by the load.
C)
The supply voltage does not match the voltage required by the load.
D)
The supply current does not match the voltage required by the load.

Correct Answer :   The supply voltage does not match the voltage required by the load.

Explanation : Buck-boost transformers are used when the supply voltage does not match the voltage required by the load.

108 .
During short circuit test of the power Input to a transformer comprises predominately.
A)
Iron loss
B)
Copper loss
C)
Hysteresis loss
D)
Eddy current loss

Correct Answer :   Copper loss

Explanation : Short circuit test is conducted on the HV side of the transformer, where the LV side of the transformer is short-circuited. Afterward, a wattmeter is connected to the primary side of the transformer as well as an ammeter is connected in series with the primary winding of the transformer.

The total applied voltage for full load current in short circuit test on primary HV side of the transformer is quite low as compared to the rated voltage. With the help of this test, we get the total copper loss to happen in the transformer during full load condition.

109 .
When the supply frequency of a transformer increases, the secondary output voltage of the transformer
A)
Constant
B)
Increases
C)
Decreases
D)
None of the above

Correct Answer :   Constant

Explanation : As we know, the transformer is a static electrotechnical device that step-up or step-down voltage form one side of its coil to another side at a constant frequency. The transformer is based on the principle of mutual induction, which happens at a constant frequency, so the frequency doesn't change.

110 .
Open circuit test in a transformer is used to determine
A)
Iron loss
B)
Core loss
C)
Total loss
D)
Copper loss

Correct Answer :   Core loss

Explanation : Open circuit test in a transformer is used to determine the core loss, i.e., eddy current loss and hysteresis loss. The core loss of a transformer is independent of load; it depends upon rated voltage.

111 .
In a transformer, the resistance between its primary and secondary is
A)
Zero
B)
100 ohm
C)
1000 ohm
D)
Infinite

Correct Answer :   Infinite

Explanation : The primary and secondary windings of Transformers are magnetically coupled and electrically isolated; ideally, the resistance between the two windings should be infinity, but practically the insulation resistance is in the order of megaohms.

112 .
The direction of magnetic flux in a transformer must have
A)
no reluctance
B)
Low reluctance
C)
Low resistance
D)
High Impedance

Correct Answer :   Low reluctance

Explanation : Reluctance refers to analogous resistance. As we know, resistance opposes the flow of electric current in an electric circuit that same work does by a reluctance in a magnetic circuit.

Reluctance opposes the flow of magnetic flux in the magnetic circuit. It stores magnetic energy in the magnetic circuit. If the reluctance is low, less the opposite of the magnetic flux in the circuit; therefore, more flux can flow through the transformer core.

113 .
Which of the following part is not associated with the transformer?
A)
Exciter
B)
Breather
C)
Conservator
D)
Buchholz relay

Correct Answer :   Exciter

Explanation : Exciter is usually used for starting the motor. It is a small generator mounted on the shaft of the main generator, which produces DC power for the main generator.

As we know, the transformer is a static device, so there is no moving part as in the motor.

114 .
Helical coils can be used on
A)
high frequency transformers
B)
low voltage side of high kVA transformers
C)
high voltage side of small capacity transformers
D)
high voltage side of high kVA rating transformers

Correct Answer :   low voltage side of high kVA transformers

115 .
Greater the secondary leakage flux
A)
less will be the primary induced e.m.f.
B)
less will be the primary terminal voltage
C)
less will be the secondary induced e.m.f.
D)
None of the above

Correct Answer :   less will be the secondary induced e.m.f.

116 .
A transformer core is laminated to
A)
reduce copper losses
B)
reduce hysteresis loss
C)
reduce all above losses
D)
reduce eddy current losses

Correct Answer :   reduce eddy current losses

117 .
In the transformer following winding has got more cross-sectional area
A)
Primary winding
B)
Secondary winding
C)
Low voltage winding
D)
High voltage winding

Correct Answer :   Low voltage winding

118 .
The transformer laminations are insulated from each other by
A)
thin coat of varnish
B)
paper
C)
mica strip
D)
Any of the above

Correct Answer :   thin coat of varnish

119 .
Buchholz's relay gives warning and protection against
A)
for both outside and inside faults
B)
electrical fault inside the transformer itself
C)
electrical fault outside the transformer in outgoing feeder
D)
None of the above

Correct Answer :   electrical fault inside the transformer itself

120 .
If the transformer is working on, no-load is switched on to a specific source of voltage. It will draw a current
A)
Same as the dynamic magnetizing current.
B)
Same as the steady-state magnetizing current.
C)
Once the steady-state magnetizing current provided to the core has huge residual flux.
D)
Mostly, the steady-state magnetizing current relying upon the initial state of the residual flux in the transformer core.

Correct Answer :   Mostly, the steady-state magnetizing current relying upon the initial state of the residual flux in the transformer core.

Explanation : When the transformer is in no-load condition, the secondary winding of the transformer has no load associated with it, and the transformer draws the zero current form the primary side of the winding.

Practically whenever the transformer is in no-load condition, a small amount of current is drawn form the primary side of the winding to set up the required magnetic flux in the core of the transformer.

121 .
Two transformers operating in parallel will share the load depending upon their
A)
ratings
B)
efficiencies
C)
per unit impedance
D)
leakage reactance

Correct Answer :   per unit impedance

122 .
When a given transformer is run at its rated voltage but reduced frequency, its
A)
iron losses are reduced
B)
core flux density is increased
C)
core flux density is reduced
D)
flux density remains unaffected

Correct Answer :   core flux density is increased

123 .
Which of the following does not change in a transformer?
A)
Frequency
B)
Voltage
C)
Current
D)
All of the above

Correct Answer :   Frequency

124 .
The starting torque of a squirrel-cage induction motor is
A)
low
B)
negligible
C)
same as full-load torque
D)
slightly more than full-load torque

Correct Answer :   low

125 .
A transformer can have zero voltage regulation at
A)
zero power factor
B)
unity power factor
C)
lagging power factor
D)
leading power factor

Correct Answer :   leading power factor

126 .
The secondary of a current transformer is always short-circuited under operating conditions because it
A)
is safe to human beings
B)
protects the primary circuit
C)
avoids core saturation and high voltage induction
D)
None of the above

Correct Answer :   avoids core saturation and high voltage induction

127 .
The 'cogging' of an induction motor can be avoided by
A)
using DOL starter
B)
proper ventilation
C)
auto-transformer starter
D)
having number of rotor slots more or less than the number of stator slots (not equal)

Correct Answer :   having number of rotor slots more or less than the number of stator slots (not equal)

128 .
In a transformer the energy is conveyed from primary to secondary
A)
through air
B)
by the flux
C)
through cooling coil
D)
None of the above

Correct Answer :   by the flux

129 .
The voltage transformation ratio of a transformer is equal to the ratio of
A)
secondary induced e.m.f. to primary induced e.m.f.
B)
primary turns to secondary turns
C)
secondary current to primary current
D)
secondary terminal voltage to primary applied voltage

Correct Answer :   secondary induced e.m.f. to primary induced e.m.f.

130 .
Iron loss of a transformer can be measured by
A)
frequency meter
B)
low power factor wattmeter
C)
unity power factor wattmeter
D)
any type of wattmeter

Correct Answer :   low power factor wattmeter

131 .
Efficiency of a power transformer is of the order of
A)
25 per cent
B)
50 per cent
C)
98 per cent
D)
100 per cent

Correct Answer :   98 per cent

132 .
Which of the following property is not necessarily desirable in the material for transformer core?
A)
Low hysteresis loss
B)
Mechanical strength
C)
High permeability
D)
High thermal conductivity

Correct Answer :   High thermal conductivity

133 .
Which of the following properties is not necessarily desirable for the material for transformer core?
A)
Low hysteresis loss
B)
High permeability
C)
High thermal conductivity
D)
Adequate mechanical strength

Correct Answer :   High thermal conductivity

134 .
While conducting short-circuit test on a transformer the following side is short circuited
A)
Primary side
B)
Low voltage side
C)
Secondary side
D)
High voltage side

Correct Answer :   Low voltage side

135 .
During short circuit test iron losses are negligible because
A)
the voltage applied on primary side is low
B)
the current on secondary side is negligible
C)
the voltage on secondary side does not vary
D)
full-load current is not supplied to the transformer

Correct Answer :   the voltage applied on primary side is low

136 .
If a transformer is continuously operated the maximum temperature rise will occur in
A)
core
B)
tank
C)
windings
D)
None of the above

Correct Answer :   windings

137 .
The primary coil of a transformer is connected to a 60 V ac source. The secondary coil is connected to a 330 Ω load. The turns ratio is 3:1. What is the secondary voltage?
A)
2 V
B)
18 V
C)
20 V
D)
180 V

Correct Answer :   20 V

138 .
Natural air cooling is generally restricted for transformers up to
A)
50 MVA
B)
15 MVA
C)
5 MVA
D)
1.5 MVA

Correct Answer :   1.5 MVA

139 .
Reduction in core losses and increase in permeability are obtained with transformer employing
A)
core built-up of laminations of hot rolled sheet
B)
core built-up of laminations of cold rolled grain oriented steel
C)
Either (A) or (B)
D)
None of the above

Correct Answer :   core built-up of laminations of cold rolled grain oriented steel

140 .
Harmonics in transformer result in
A)
Increased core losses
B)
Increased I2R losses
C)
Magnetic interference with communication circuits
D)
All of the above

Correct Answer :   All of the above

141 .
A transformer has a 1:6 turns ratio and a secondary coil load resistance of 470 Ω. The load resistance as seen by the source is
A)
1.3 Ω
B)
7.8 Ω
C)
13 Ω
D)
78 Ω

Correct Answer :   13 Ω

142 .
No-load current in a transformer
A)
leads the voltage by about 15°
B)
leads the voltage by about 75°
C)
lags behind the voltage by about 15°
D)
lags behind the voltage by about 75°

Correct Answer :   lags behind the voltage by about 75°

143 .
The purpose of providing iron core in a step-up transformer is
A)
To provide all above features
B)
To increase the magnitude of mutual flux
C)
To decrease the magnitude of magnetizing current
D)
To provide coupling between primary and secondary

Correct Answer :   To decrease the magnitude of magnetizing current

144 .
When the equivalent circuit diagram of doouble squirrelcage induction motor is constructed the two cages can be considered
A)
in series
B)
in parallel
C)
in series-parallel
D)
in parallel with stator

Correct Answer :   in parallel

145 .
During open circuit test of a transformer
A)
primary is supplied rated voltage
B)
primary is supplied full-load current
C)
primary is supplied rated kVA
D)
primary is supplied current at reduced voltage

Correct Answer :   primary is supplied rated voltage

146 .
What will happen if the transformers working in parallel are not connected with regard to polarity?
A)
Incorrect polarity will result in dead short circuit
B)
The transformers will not share load in proportion to their kVA ratings
C)
The power factor of the two transformers will be different from the power factor of common load
D)
None of the above

Correct Answer :   Incorrect polarity will result in dead short circuit

147 .
Material used for construction of transformer core is usually
A)
wood
B)
silicon steel
C)
copper
D)
aluminium

Correct Answer :   silicon steel

148 .
Natural oil cooling is used for transformers up to a rating of
A)
250 kVA
B)
500 kVA
C)
1000 kVA
D)
3000 kVA

Correct Answer :   3000 kVA

149 .
A no-load test is performed on a transformer to determine
A)
efficiency
B)
copper loss
C)
magnetising current
D)
magnetising current and loss

Correct Answer :   magnetising current and loss

150 .
Which of the following parts of a transformer is visible from outside?
A)
Core
B)
Bushings
C)
Primary winding
D)
Secondary winding

Correct Answer :   Bushings