360+ Electrical Machine 1 Solved MCQs

Chapters

Chapter: Transformers
1.

Which of the following does not change in a transformer ?

A. Current
B. Voltage
C. Frequency
D. All of the above
Answer» C. Frequency
2.

In a transformer the energy is conveyed from primary to secondary

A. through cooling coil
B. through air
C. by the flux
D. none of the above
Answer» C. by the flux
3.

A transformer core is laminated to

A. reduce hysteresis loss
B. reduce eddy current losses
C. reduce copper losses
D. reduce all above losses
Answer» B. reduce eddy current losses
4.

The degree of mechanical vibrations produced by the laminations of a transformer depends on

A. tightness of clamping
B. gauge of laminations
C. size of laminations
D. all of the above
Answer» D. all of the above
5.

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
Answer» B. 2 to 5 per cent
6.

The path of a magnetic flux in a transformer should have

A. high resistance
B. high reluctance
C. low resistance
D. low reluctance
Answer» D. low reluctance
7.

No-load on a transformer is carried out to determine

A. copper loss
B. magnetising current
C. magnetising current and loss
D. efficiency of the tr
Answer» A. copper loss
8.

The dielectric strength of transformer oil is expected to be

A. lkV
B. 33 kV
C. 100 kV
D. 330 kV
Answer» B. 33 kV
9.

Sumpner's test is conducted on trans-formers to determine

A. temperature
B. stray losses
C. all-day efficiency
D. none of the above
Answer» A. temperature
10.

The permissible flux density in case of cold rolled grain oriented steel is around

A. 1.7 Wb/m2
B. 2.7 Wb/m2
C. 3.7 Wb/m2
D. 4.7 Wb/m2
Answer» A. 1.7 Wb/m2
11.

The efficiency of a transformer will be maximum when

A. copper losses = hysteresis losses
B. hysteresis losses = eddy current losses
C. eddy current losses = copper losses
D. copper losses = iron losses
Answer» D. copper losses = iron losses
12.

No-load current in a transformer

A. lags behind the voltage by about 75°
B. leads the voltage by about 75°
C. lags behind the voltage by about 15°
D. leads the voltage by about 15°
Answer» A. lags behind the voltage by about 75°
13.

The purpose of providing an iron core in a transformer is to

A. provide support to windings
B. reduce hysteresis loss
C. decrease the reluctance of the magnetic path
D. reduce eddy current losses
Answer» C. decrease the reluctance of the magnetic path
14.

Which of the following is not a part of transformer installation ?

A. Conservator
B. Breather
C. Buchholz relay
D. Exciter
Answer» D. Exciter
15.

While conducting short-circuit test on a transformer the following side is short circuited

A. High voltage side
B. Low voltage side
C. Primary side
D. Secondary side
Answer» B. Low voltage side
16.

In the transformer following winding has got more cross-sectional area

A. Low voltage winding
B. High voltage winding
C. Primary winding
D. Secondary winding
Answer» A. Low voltage winding
17.

A transformer transforms

A. voltage
B. current
C. power
D. frequency
Answer» C. power
18.

A transformer cannot raise or lower the voltage of a D.C. supply because

A. there is no need to change the D.C. voltage
B. a D.C. circuit has more losses
C. Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero
D. none of the above
Answer» C. Faraday's laws of electromagnetic induction are not valid since the rate of change of flux is zero
19.

Primary winding of a transformer

A. is always a low voltage winding
B. is always a high voltage winding
C. could either be a low voltage or high voltage winding
D. none of the above
Answer» C. could either be a low voltage or high voltage winding
20.

Which winding in a transformer has more number of turns ?

A. Low voltage winding
B. High voltage winding
C. Primary winding
D. Secondary winding
Answer» B. High voltage winding
21.

Efficiency of a power transformer is of the order of

A. 100 per cent
B. 98 per cent
C. 50 per cent
D. 25 per cent
Answer» B. 98 per cent
22.

In a given transformer for given applied voltage, losses which remainconstant irrespective of load changes are

A. friction and windage losses
B. copper losses
C. hysteresis and eddy current losses
D. none of the above
Answer» C. hysteresis and eddy current losses
23.

A common method of cooling a power transformer is

A. natural air cooling
B. air blast cooling
C. oil cooling
D. any of the above
Answer» C. oil cooling
24.

The no load current in a transformer lags behind the appliedvoltage by an angle of about

A. 180°
B. 120"
C. 90°
D. 75°
Answer» D. 75°
25.

In a transformer routine efficiency depends upon

A. supply frequency
B. load current
C. power factor of load
D. both (b) and (c)
Answer» D. both (b) and (c)
26.

In the transformer the function of a conservator is to

A. provide fresh air for cooling the transformer
B. supply cooling oil to transformer in time of need
C. protect the transformer from damage when oil expends due to heating
D. none of the above
Answer» C. protect the transformer from damage when oil expends due to heating
27.

Natural oil cooling is used for transformers upto a rating of

A. 3000 kVA
B. 1000 kVA
C. 500 kVA
D. 250 kVA
Answer» A. 3000 kVA
28.

Power transformers are designed to have maximum efficiency at

A. nearly full load
B. 70% full load
C. 50% full load
D. no load
Answer» A. nearly full load
29.

The maximum efficiency of a distribution transformer is

A. at no load
B. at 50% full load
C. at 80% full load
D. at full load
Answer» B. at 50% full load
30.

Transformer breaths in when

A. load on it increases
B. load on it decreases
C. load remains constant
D. none of the above
Answer» B. load on it decreases
31.

No-load current of a transformer has

A. has high magnitude and low power factor
B. has high magnitude and high power factor
C. has small magnitude and high power factor
D. has small magnitude and low powerfactor
Answer» D. has small magnitude and low powerfactor
32.

Spacers are provided between adjacent coils

A. to provide free passage to the cool¬ing oil
B. to insulate the coils from each other
C. both (a) and (b)
D. none of the above
Answer» A. to provide free passage to the cool¬ing oil
33.

Greater the secondary leakage flux

A. less will be the secondary induced e.m.f.
B. less will be the primary induced e.m.f.
C. less will be the primary terminal voltage
D. none of the above
Answer» A. less will be the secondary induced e.m.f.
34.

The purpose of providing iron core in a step-up transformer is

A. to provide coupling between primary and secondary
B. to increase the magnitude of mutual flux
C. to decrease the magnitude of mag-netizing current
D. to provide all above features
Answer» C. to decrease the magnitude of mag-netizing current
35.

The power transformer is a constant

A. voltage device
B. current device
C. power device
D. main flux device
Answer» D. main flux device
36.

Two transformers operating in parallel will share the load depending upon their

A. leakage reactance
B. per unit impedance
C. efficiencies
D. ratings
Answer» B. per unit impedance
37.

If R2 is the resistance of secondary winding of the transformer and K is the transformation ratio then the equivalent secondary resistance referred to primary will be

A. R2/VK
B. R2IK2
C. R22!K2
D. R22/K
Answer» B. R2IK2
38.

What will happen if the transformers working in parallel are not connected with regard to polarity ?

A. The power factor of the two trans-formers will be different from the power factor of common load
B. Incorrect polarity will result in dead short circuit
C. The transformers will not share load in proportion to their kVA ratings
D. none of the above
Answer» B. Incorrect polarity will result in dead short circuit
39.

Ifthe percentage impedances of the two transformers working in parallel aredifferent, then

A. transformers will be overheated
B. power factors of both the trans-formers will be same
C. parallel operation will be not possible
D. parallel operation will still be possible, but the power factors at which the two tr
Answer» A. transformers will be overheated
40.

In a transformer the tappings are generally provided on

A. primary side
B. secondary side
C. low voltage side
D. high voltage side
Answer» C. low voltage side
41.

The use of higher flux density in the transformer design

A. reduces weight per kVA
B. reduces iron losses
C. reduces copper losses
D. increases part load efficiency
Answer» A. reduces weight per kVA
42.

The chemical used in breather for transformer should have the quality of

A. ionizing air
B. absorbing moisture
C. cleansing the transformer oil
D. cooling the tr
Answer» A. ionizing air
43.

The chemical used in breather is

A. asbestos fibre
B. silica sand
C. sodium chloride
D. silica gel
Answer» D. silica gel
44.

The transformer ratings are usually expressed in terms of

A. volts
B. amperes
C. kW
D. kVA
Answer» D. kVA
45.

The noise resulting from vibrations of laminations set by magnetic forces, is termedas

A. magnetostrication
B. boo
C. hum
D. zoom
Answer» C. hum
46.

Hysteresis loss in a transformer varies as CBmax = maximum flux density)

A. Bmax
B. Bmax1-6
C. Bmax1-83
D. B max
Answer» B. Bmax1-6
47.

Material used for construction of transformer core is usually

A. wood
B. copper
C. aluminium
D. silicon steel
Answer» D. silicon steel
48.

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
Answer» A. 0.4 mm to 0.5 mm
49.

The function of conservator in a transformer is

A. to project against'internal fault
B. to reduce copper as well as core losses
C. to cool the transformer oil
D. to take care of the exp
Answer» A. to project against'internal fault
50.

The highest voltage for transmitting electrical power in India is

A. 33 kV.
B. 66 kV
C. 132 kV
D. 400 kV
Answer» D. 400 kV
51.

In a transformer the resistance between its primary and secondary is

A. zero
B. 1 ohm
C. 1000 ohms
D. infinite
Answer» D. infinite
52.

A transformer oil must be free from

A. sludge
B. odour
C. gases
D. moisture
Answer» D. moisture
53.

A Buchholz relay can be installed on

A. auto-transformers
B. air-cooled transformers
C. welding transformers
D. oil cooled tr
Answer» A. auto-transformers
54.

Gas is usually not liberated due to dissociation of transformer oil unless the oil temperature exceeds

A. 50°C
B. 80°C
C. 100°C
D. 150°C
Answer» D. 150°C
55.

The main reason for generation of harmonics in a transformer could be

A. fluctuating load
B. poor insulation
C. mechanical vibrations
D. saturation of core
Answer» D. saturation of core
56.

Distribution transformers are generally designed for maximum efficiency around

A. 90% load
B. zero load
C. 25% load
D. 50% load
Answer» D. 50% load
57.

Which of the following property is not necessarily desirable in the material for transformer core ?

A. Mechanical strength
B. Low hysteresis loss
C. High thermal conductivity
D. High permeability
Answer» C. High thermal conductivity
58.

Star/star transformers work satisfactorily when

A. load is unbalanced only
B. load is balanced only
C. on balanced as well as unbalanced loads
D. none of the above
Answer» B. load is balanced only
59.

Delta/star transformer works 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
Answer» C. on balanced as well as unbalanced loads
60.

Buchholz's relay gives warning and protection against

A. electrical fault inside the transformer itself
B. electrical fault outside the transformer in outgoing feeder
C. for both outside and inside faults
D. none of the above
Answer» A. electrical fault inside the transformer itself
61.

The magnetising current of a transformer is usually small because it has

A. small air gap
B. large leakage flux
C. laminated silicon steel core
D. fewer rotating parts
Answer» A. small air gap
62.

Which of the following does not change in an ordinary transformer ?

A. Frequency
B. Voltage
C. Current
D. Any of the above
Answer» A. Frequency
63.

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 mechanicalstrength
Answer» C. High thermal conductivity
64.

The leakage flux in a transformer depends upon

A. load current
B. load current and voltage
C. load current, voltage and frequency
D. load current, voltage, frequency and powerfactor
Answer» A. load current
65.

The path of the magnetic flux in transformer should have

A. high reluctance
B. low reactance
C. high resistance
D. low resistance
Answer» B. low reactance
66.

Noise level test in a transformer is a

A. special test
B. routine test
C. type test
D. none of the above
Answer» C. type test
67.

Which of the foJIowing is not a routine test on transformers ?

A. Core insulation voltage test
B. Impedance test
C. Radio interference test
D. Polarity test
Answer» C. Radio interference test
68.

A transformer can have zero voltage regulation at

A. leading power factor
B. lagging powerfactor
C. unity power factor
D. zero power factor
Answer» A. leading power factor
69.

Helical coils can be used on

A. low voltage side of high kVA trans¬formers
B. high frequency transformers
C. high voltage side of small capacity transformers
D. high voltage side of high kVA rating tr
Answer» A. low voltage side of high kVA trans¬formers
70.

Harmonics in transformer result in

A. increased core losses
B. increased I2R losses
C. magnetic interference with communication circuits
D. all of the above
Answer» D. all of the above
71.

The core used in high frequency transformer is usually

A. copper core
B. cost iron core
C. air core
D. mild steel core
Answer» C. air core
72.

The full-load copper loss of a trans¬former is 1600 W. At half-load, the copper loss will be

A. 6400 W
B. 1600 W
C. 800 W
D. 400 W
Answer» D. 400 W
73.

The value of flux involved m the e.m.f. equation of a transformer is

A. average value
B. r.m.s. value
C. maximum value
D. instantaneous value
Answer» C. maximum value
74.

Silicon steel used in laminations mainly reduces

A. hysteresis loss
B. eddy current losses
C. copper losses
D. all of the above
Answer» A. hysteresis loss
75.

Which winding of the transformer has less cross-sectional area ?

A. Primary winding
B. Secondary winding
C. Low voltage winding
D. High voltage winding
Answer» D. High voltage winding
76.

Power transformers are generally designed to have maximum efficiency around

A. no-load
B. half-load
C. near full-load
D. 10% overload
Answer» C. near full-load
77.

Which of the following is the main advantage of an auto-transformer over a two winding transformer ?

A. Hysteresis losses are reduced
B. Saving in winding material
C. Copper losses are negligible
D. Eddy losses are totally eliminated
Answer» B. Saving in winding material
78.

During short circuit test iron losses are negligible because

A. the current on secondary side is negligible
B. the voltage on secondary side does not vary
C. the voltage applied on primary side is low
D. full-load current is not supplied to thetr
Answer» A. the current on secondary side is negligible
79.

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. power factor of one of the trans¬formers is leading while that of the otherlagging
C. transformers having higher copper losses will have negligible core losses
D. loading of the tr
Answer» A. short-circuiting of the secondaries
80.

The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer

A. Conservator
B. Breather
C. Bushings
D. Buchholz relay
Answer» A. Conservator
81.

An ideal transformer is one which has

A. no losses and magnetic leakage
B. interleaved primary and secondary windings
C. a common core for its primary and secondary windings
D. core of stainless steel and winding of pure copper metal
Answer» A. no losses and magnetic leakage
82.

When a given transformer is run at its rated voltage but reduced frequency, its

A. flux density remains unaffected
B. iron losses are reduced
C. core flux density is reduced
D. core flux density is increased
Answer» D. core flux density is increased
83.

In an actual transformer the iron loss remains practically constant from noload to fullload because

A. value of transformation ratio remains constant
B. permeability of transformer core remains constant
C. core flux remains practically constant
D. primary voltage remains constant
Answer» C. core flux remains practically constant
84.

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
Answer» A. copper loss = iron loss
85.

If the supply frequency to the transformer is increased,"the iron loss will

A. not change
B. decrease
C. increase
D. any of the above
Answer» C. increase
86.

Negative voltage regulation is indicative that the load is

A. capacitive only
B. inductive only
C. inductive or resistive
D. none of the above
Answer» A. capacitive only
87.

Iron loss of a transformer can be measured by

A. low power factor wattmeter
B. unity power factor wattmeter
C. frequency meter
D. any type of wattmeter
Answer» A. low power factor wattmeter
88.

When secondary of a current transformer is open-circuited its iron core will be

A. hot because of heavy iron losses taking place in it due to high flux density
B. hot because primary will carry heavy current
C. cool as there is no secondary current
D. none of above will happen
Answer» A. hot because of heavy iron losses taking place in it due to high flux density
89.

The transformer laminations are insulated from each other by

A. mica strip
B. thin coat of varnish
C. paper
D. any of the above
Answer» B. thin coat of varnish
90.

Which type of winding is used in 3phase shell-type transformer ?

A. Circular type
B. Sandwich type
C. Cylindrical type
D. Rectangular type
Answer» B. Sandwich type
91.

During open circuit test of a transformer

A. primary is supplied rated voltage
B. primary is supplied full-load current
C. primary is supplied current at reduced voltage
D. primary is supplied rated kVA
Answer» A. primary is supplied rated voltage
92.

Open circuit test on transformers is conducted to determine

A. hysteresis losses
B. copper losses
C. core losses
D. eddy current losses
Answer» C. core losses
93.

Short circuit test on transformers is conducted to determine

A. hysteresis losses
B. copper losses
C. core losses
D. eddy current losses
Answer» B. copper losses
94.

For the parallel operation of single phase transformers it is necessary that they should have

A. same efficiency
B. same polarity
C. same kVA rating
D. same number of turns on the secondary side.
Answer» B. same polarity
95.

The transformer oil should have volatility and viscosity.

A. low,low
B. high,high
C. low,high
D. high,low
Answer» A. low,low
96.

The function of breather in a transformer is

A. to provide oxygen inside the tank
B. to cool the coils during reduced load
C. to cool the transformer oil
D. to arrest flow of moisture when outside air enters thetr
Answer» B. to cool the coils during reduced load
97.

The secondary winding of which of the following transformers is always kept closed?

A. Step-up transformer
B. Step-down transformer
C. Potential transformer
D. Current tr
Answer» A. Step-up transformer
98.

The size of a transformer core will depend on

A. frequency
B. area of the core
C. flux density of the core material
D. (a) and (b) both
Answer» D. (a) and (b) both
99.

N atural air coo ling is generally restricted for transformers up to

A. 1.5 MVA
B. 5 MVA
C. 15 MVA
D. 50 MVA
Answer» A. 1.5 MVA
100.

A shell-type transformer has

A. high eddy current losses
B. reduced magnetic leakage
C. negligibly hysteresis losses
D. none of the above
Answer» B. reduced magnetic leakage
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