www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
Questions from transformer 1. Which of the following does not change in a
transformer?
(a) Current
(b) Voltage
(c) Frequency
(d) All of the above
Ans: c
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
Ans: c
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
Ans: b
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
Ans: d
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
Ans: b
6. The path of a magnetic flux in a transformer
should have
(a) high resistance
(b) high reluctance
(c) low resistance
(d) low reluctance
Ans: d
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 transformer
Ans: c
8. The dielectric strength of transformer oil is
expected to be
(a) lkV
(b) 33 kV
(c) 100 kV
(d) 330 kV
Ans: b
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
Ans: a
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
Ans: a
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
Ans: d
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°
Ans: a
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
Ans: c
14. Which of the following is not a part of
transformer installation ?
(a) Conservator
(b) Breather
(c) Buchholz relay
(d) Exciter
Ans: d
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
Ans: b
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
Ans: a
17. A transformer transforms
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
(a) voltage
(b) current
(c) power
(d) frequency
Ans: c
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
Ans: c
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
Ans: c
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
Ans: b
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
Ans: b
22. In a given transformer for given applied
voltage, losses which remain constant irrespective
of load changes are
(a) friction and windage losses
(b) copper losses
(c) hysteresis and eddy current losses
(d) none of the above
Ans: c
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
Ans: c
24. The no load current in a transformer lags behind
the applied voltage by an angle of about
(a) 180°
(b) 120"
(c) 90°
(d) 75°
Ans: d
25. In a transformer routine efficiency depends
upon
(a) supply frequency
(b) load current
(c) power factor of load
(d) both (b) and (c)
Ans: d
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
Ans: c
27. Natural oil cooling is used for transformers up
to a rating of
(a) 3000 kVA
(b) 1000 kVA
(c) 500 kVA
(d) 250 kVA
Ans: a
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
Ans: a
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
Ans: b
30. Transformer breaths in when
(a) load on it increases
(b) load on it decreases
(c) load remains constant
(d) none of the above
Ans: b
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 power factor
Ans: d
32. Spacers are provided between adjacent coils
(a) to provide free passage to the cooling oil
(b) to insulate the coils from each other
(c) both (a) and (b)
(d) none of the above
Ans: a
33. Greater the secondary leakage flux
(a) less will be the secondary induced e.m.f.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
(b) less will be the primary induced e.m.f.
(c) less will be the primary terminal voltage
(d) none of the above
Ans: a
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
Ans: c
35. The power transformer is a constant
(a) voltage device
(b) current device
(c) power device
(d) main flux device
Ans: d
36. Two transformers operating in parallel will
share the load depending upon their
(a) leakage reactance
(b) per unit impedance
(c) efficiencies
(d) ratings
Ans: b
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
Ans: b
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
Ans: b
39. If the percentage impedances of the two
transformers working in parallel are different, then
(a) transformers will be overheated
(b) power factors of both the transformers 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 transformers operate
will be different
from the power factor of the common load
Ans: d
40. In a transformer the tappings are generally
provided on
(a) primary side
(b) secondary side
(c) low voltage side
(d) high voltage side
Ans: c
41. The use of higher flux density in the
transformer design
(a) reduces weight per kVA
(6) reduces iron losses
(c) reduces copper losses
(d) increases part load efficiency
Ans: a
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 transformer oil.
Ans: b
43. The chemical used in breather is
(a) asbestos fiber
(b) silica sand
(c) sodium chloride
(d) silica gel
Ans: d
44. An ideal transformer has infinite values of
primary and secondary inductances. The statement
is
(a) true
(b) false
Ans: b
45. The transformer ratings are usually expressed in
terms of
(a) volts
(b) amperes
(c) kW
(d) kVA
Ans: d
46. The noise resulting from vibrations of
laminations set by magnetic forces, is termed as
(a) magnetostrication
(b) boo
(c) hum
(d) zoom
Ans: c
47. Hysteresis loss in a transformer varies as
CBmax = maximum flux density)
(a) Bmax
(b) Bmax1-6
(C) Bmax1-83
(d) B max
Ans: b
48. Material used for construction of transformer
core is usually
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
(a) wood
(b) copper
(c) aluminium
(d) silicon steel
Ans: d
49. 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
Ans: a
50. 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 expansion and contraction of
transformer oil due to variation of temperature of
sur-roundings
Ans: d
51. The highest voltage for transmitting electrical
power in India is
(a) 33 kV.
(6) 66 kV
(c) 132 kV
(d) 400 kV
Ans: d
52. In a transformer the resistance between its
primary and secondary is
(a) zero
(b) 1 ohm
(c) 1000 ohms
(d) infinite
Ans: d
53. A transformer oil must be free from
(a) sludge
(b) odour
(c) gases
(d) moisture
Ans: d
54. A Buchholz relay can be installed on
(a) auto-transformers
(b) air-cooled transformers
(c) welding transformers
(d) oil cooled transformers
Ans: d
55. 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
Ans: d
56. 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
Ans: d
57. Distribution transformers are generally
designed for maximum efficiency around
(a) 90% load
(b) zero load
(c) 25% load
(d) 50% load
Ans: d
58. Which of the following property is not
necessarily desirable in the material for transformer
core ?
(a) Mechanical strength
(6) Low hysteresis loss
(c) High thermal conductivity
(d) High permeability
Ans: c
59. 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
Ans: b
60. 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
Ans: c
61. 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
Ans: a
62. 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
Ans: a
63. Which of the following does not change in an
ordinary transformer ?
(a) Frequency
(b) Voltage
(c) Current
(d) Any of the above
Ans: a
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
64. 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
Ans: c
65. 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 power
factor
Ans: a
66. The path of the magnetic flux in transformer
should have
(a) high reluctance
(b) low reactance
(c) high resistance
(d) low resistance
Ans: b
67. Noise level test in a transformer is a
(a) special test
(b) routine test
(c) type test
(d) none of the above
Ans: c
68. Which of the following is not a routine test on
transformers ?
(a) Core insulation voltage test
(b) Impedance test
(c) Radio interference test
(d) Polarity test
Ans: c
69. A transformer can have zero voltage regulation
at
(a) leading power factor
(b) lagging power factor
(c) unity power factor
(d) zero power factor
Ans: a
70. Helical coils can be used on
(a) low voltage side of high kVA transformers
(b) high frequency transformers
(c) high voltage side of small capacity transformers
(d) high voltage side of high kVA rating
transformers
Ans: a
71. Harmonics in transformer result in
(a) increased core losses
(b) increased I2R losses
(c) magnetic interference with communication
circuits
(d) all of the above
Ans: d
72. The core used in high frequency transformer is
usually
(a) copper core
(b) cost iron core
(c) air core
(d) mild steel core
Ans: c
73. 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
Ans: d
1.74. 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
Ans: c
L.75. Silicon steel used in laminations mainly
reduces
(a) hysteresis loss
(b) eddy current losses
(c) copper losses
(d) all of the above
Ans: a
76. Which winding of the transformer has less
cross-sectional area ?
(a) Primary winding
(b) Secondary winding
(c) Low voltage winding
(d) High voltage winding
Ans: d
77. Power transformers are generally designed to
have maximum efficiency around
(a) no-load
(b) half-load
(c) near full-load
(d) 10% overload
Ans: c
.78. 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
Ans: b
79. 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
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
(d) full-load current is not supplied to the
transformer
Ans: c
80. 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 transformers is
leading while that of the other lagging
(c) transformers having higher copper losses will
have negligible core losses
(d) loading of the transformers not in proportion to
their kVA ratings
Ans: d
81. 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
Ans: a
82. 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
(e) none of the above
Ans: a
83. 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
Ans: d
84. 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
(c) secondary voltage remains constant
Ans: c
85. 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
Ans: a
86. 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
Ans: c
87. Negative voltage regulation is indicative that
the load is
(a) capacitive only
(b) inductive only
(c) inductive or resistive
(d) none of the above
Ans: a
88. 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
Ans: a
89. 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
Ans: a
90. The transformer laminations are insulated from
each other by
(a) mica strip
(6) thin coat of varnish
(c) paper
(d) any of the above
Ans: b
91. Which type of winding is used in 3phase shell-
type transformer ?
(a) Circular type
(b) Sandwich type
(c) Cylindrical type
(d) Rectangular type
Ans: b
92. 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
Ans: a
93. Open circuit test on transformers is conducted
to determine
(a) hysteresis losses
(b) copper losses
(c) core losses
(d) eddy current losses
Ans: c
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
94. Short circuit test on transformers is conducted
to determine
(a) hysteresis losses
(b) copper losses
(c) core losses
(d) eddy current losses
Ans: b
95. 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.
Ans: b
96. The transformer oil should have _____
volatility and _____ viscosity.
(a) low,low
(b) high,high
(c) low,high
(d) high,low
Ans: a
97. 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 the transformer
Ans: d
98. 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 transformer
Ans: d
99. 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
Ans: d
100. 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
Ans: a
101. A shell-type transformer has
(a) high eddy current losses
(b) reduced magnetic leakage
(c) negligibly hysteresis losses
(d) none of the above
Ans: b
102. A transformer can have regulation closer to
zero
(a) on full-load
(b) on overload
(c) on leading power factor
(d) on zero power factor
Ans: c
103. A transformer transforms
(a) voltage
(b) current
(c) current and voltage
(d) power
Ans: d
104. Which of the following is not the standard
voltage for power supply in India ?
(a) llkV
(b) 33kV
(c) 66 kV
(d) 122 kV
Ans: d
105. Reduction in core losses and increase in
permeability are obtained with transformer
employing
(a) core built-up of laminations of cold rolled grain
oriented steel
(b) core built-up of laminations of hot rolled sheet
(c) either of the above
(d) none of the above
Ans: a
106. In a power or distribution transformer about 10
per cent end turns are heavily insulated
(a) to withstand the high voltage drop due to line
surge produced by the shunting capacitance of the
end turns
(b) to absorb the line surge voltage and save the
winding of transformer from damage
(c) to reflect the line surge and save the winding of
a transformer from damage
(d) none of the above
Ans: a
107. For given applied voltage, with the increase in
frequency of the applied voltage
(a) eddy current loss will decrease
(b) eddy current loss will increase
(c) eddy current loss will remain unchanged
(d) none of the above
Ans: c
108. Losses which occur in rotating electric
machines and do not occur in trans formers are
(a) friction and windage losses
(b) magnetic losses
(c) hysteresis and eddy current losses
(d) copper losses
Ans: a
109. In a given transformer for a given applied
voltage, losses which remain constant irrespective
of load changes are
(a) hysteresis and eddy current losses
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
(b) friction and windage losses
(c) copper losses
(d) none of the above
Ans: a
110. Which of the following statements regarding
an idel single-phase transformer having a turn ratio
of 1 : 2 and
drawing a current of 10 A from 200 V A.C. supply
is incorrect ?
(a) Its secondary current is 5 A
(b) Its secondary voltage is 400 V
(c) Its rating is 2 kVA
(d) Its secondary current is 20 A
(e) It is a step-up transformer
Ans: d
111. The secondary of a current transformer is
always short-circuited under operating conditions
because it
(a) avoids core saturation and high voltage
induction
(b) is safe to human beings
(c) protects the primary circuit
(d) none of the above
Ans: a
112. In a transformer the resistance between its
primary and secondary should be
(a) zero
(b) 10 Q
(c) 1000 Q
(d) infinity
Ans: d
113. A good voltage regulation of a transformer
means
(a) output voltage fluctuation from no load to full
load is least
(b) output voltage fluctuation with power factor is
least
(c) difference between primary and secondary
voltage is least
(d) difference between primary and secondary
voltage is maximum
Ans: a
114. For a transformer, operating at constant load
current, maximum efficiency will occur at
(a) 0.8 leading power factor
(b) 0.8 lagging power factor
(c) zero power factor
(d) unity power factor
Ans: d
115. Which of the following protection is normally
not provided on small distribution transformers ?
(a) Overfluxing protection
(b) Buchholz relay
(c) Overcurrent protection
(d) All of the above
Ans: b
116. Which of the following acts as a protection
against high voltage surges due to lightning and
switching ?
(a) Horn gaps
(b) Thermal overload relays
(c) Breather
(d) Conservator
Ans: a
117. The efficiency of two identical transformers
under load conditions can be determined by
(a) short-circuit test
(b) back-to-back test
(c) open circuit test
(d) any of the above
Ans: b
118. Which of the following insulating materials
can withstand the highest temperature safely ?
(a) Cellulose
(b) Asbestos
(c) Mica
(d) Glass fibre
Ans: c
119. Which of the following parts of a transformer
is visible from outside ?
(a) Bushings
(b) Core
(c) Primary winding
(d) Secondary winding
Ans: a
120. The noise produced by a transformer is termed
as
(a) zoom
(b) hum
(c) ringing
(d) buzz
Ans: b
121. Which of the following loss in a transformer is
zero even at full load ?
(a) Core loss
(b) Friction loss
(c) Eddy current loss
(d) Hysteresis loss
Ans: b
122. Which of the following is the most likely
source of harmonics in a transformer ?
(a) poor insulation
(b) Overload
(c) loose connections
(d) Core saturation
Ans: d
123. If a transformer is continuously operated the
maximum temperature rise will occur in
(a) core
(b) windings
(c) tank
(d) any of the above
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
Ans: b
124. The hum in a transformer is mainly attributed
to
(a) load changes
(b) oil in the transformer
(c) magnetostriction
(d) mechanical vibrations
Ans: c
125. The maximum load that a power transformer
can carry is limited by its
(a) temperature rise
(b) dielectric strength of oil
(c) voltage ratio
(d) copper loss
Ans: c
126. The efficiency of a transformer, under heavy
loads, is comparatively low because
(a) copper loss becomes high in proportion to the
output
(b) iron loss is increased considerably
(c) voltage drop both in primary and secondary
becomes large
(d) secondary output is much less as compared to
primary input
Ans: a
127. An open-circuit test on a transformer is
conducted primarily to measure
(a) insulation resistance
(b) copper loss
(c) core loss
(d) total loss
(e) efficiency
(f) none of the above
Ans: c
128. A no-load test is performed on a transformer
to determine
(a) core loss
(b) copper loss
(c) efficiency
(d) magnetising current
(e) magnetising current and loss
Ans: e
129. The voltage transformation ratio of a
transformer is equal to the ratio of
(a) primary turns to secondary turns
(b) secondary current to primary current
(c) secondary induced e.m.f. to primary induced
e.m.f.
(d) secondary terminal voltage to primary applied
voltage
Ans: c
130. Part of the transformer which is most subject
to damage from overheating is
(a) iron core
(b) copper winding
(c) winding insulation
(d) frame or case
(e) transformer tank
Ans: c
131. If a transformer is switched on to a voltage
more than the rated voltage
(a) its power factor will deteriorate
(b) its power factor will increase
(c) its power factor will remain unaffected
(d) its power factor will be zero
Ans: a
132. Auto-transformer makes effective saving on
copper and copper losses, when its transformation
ratio is
(a) approximately equal to one
(b) less than one
(c) great than one
(d) none of the above
Ans: a
133. Minimum voltage regulation occurs when the
power factor of the load is
(a) unity
(b) lagging
(c) leading
(d) zero
Ans: c
134. In a step-down transformer, there is a change
of 15 A in the load current. This results in change
of supply current of
(a) less than 15 A
(b) more than 15 A
(c) 15 A
(d) none of the above
Ans: a
135. The efficiencies of transformers compared
with that of electric motors of the same power are
(a) about the same
(6) much smaller
General questions
1: What is electric traction?
Answer: Traction means using the electric power
for traction system i.e. for railways, trams, trolleys
etc. electric traction means use of the electricity for
all these. Now a day, magnetic traction is also used
for bullet trains. Basically dc motors are used for
electric traction systems.
2: How can you start-up the 40w tube light with
230v AC/DC without using any choke/Coil?
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
Answer: It's possible by means of Electronic
choke. otherwise it's not possible to ionize the
particles in tube. light, with normal voltage.
3: What is "pu" in electrical engineering?
Answer: Pu stands for per unit and this will be
used in power system single line diagram there it is
like a huge electrical circuit with no of components
(generators, transformers, loads) with different
ratings (in MVA and KV). To bring all the ratings
into common platform we use pu concept in which,
in general largest MVA and KV ratings of the
component is considered as base values, then all
other component ratings will get back into this
basis. Those values are called as pu values.
(p.u=actual value/base value).
4: Operation carried out in Thermal power station?
Answer: The water is obtained in the boiler and the
coal is burnt so that steam is obtained this steam is
allowed to hit the turbine , the turbine which is
coupled with the generator generates the electricity
5:Why link is provided in neutral of an ac circuit
and fuse in phase of ac circuit?
Answer: Link is provided at a Neutral common
point in the circuit from which various connection
are taken for the individual control circuit and so it
is given in a link form to withstand high Amps. But
in the case of Fuse in the Phase of AC circuit it is
designed such that the fuse rating is calculated for
the particular circuit (i.e load) only.So if any
malfunction happen the fuse connected in the
particular control circuit alone will blow off.
6: what is the diff. btwn. Electronic regulator and
ordinary rheostat regulator for fans?
Answer: The difference between the electronic and
ordinary regulator is that in electronic reg. power
losses are less because as we decrease the speed the
electronic reg. give the power needed for that
particular speed but in case of ordinary rheostat
type reg. the power wastage is same for every speed
and no power is saved. In electronic regulator triac
is employed for speed control. By varying the firing
angle speed is controlled but in rheostat control
resistance is decreased by steps to achieve speed
control.
7: How tube light circuit is connected and how it
works?
Answer: A choke is connected in one end of the
tube light and a starter is in series with the circuit.
When supply is provided , the starter will interrupt
the supply cycle of AC. Due to the sudden change
of supply the chock will generate around 1000volts
. This volt will capable of to break the electrons
inside the tube to make electron flow. once the
current passes through the tube the starter circuit
will be out of part. now there is no change of
supply causes choke voltage normalized and act as
minimize the current.
8: whats is MARX CIRCUIT?
Answer: It is used with generators for charging a
number of capacitor in parallel and discharging
them in series.It is used when voltage required for
testing is higher than the available.
9:What is encoder, how it function?
Answer: An encoder is a device used to change a
signal (such as a bit stream) or data into a code. The
code may serve any of a number of purposes such
as compressing information for transmission or
storage, encrypting or adding redundancies to the
input code, or translating from one code to another.
This is usually done by means of a programmed
algorithm,especially if any part is digital, while
most analog encoding is done with analog circuitry.
10:What are the advantages of speed control
using thyristor?
Answer: Advantages :1. Fast Switching
Characteristics than MOSFET, BJT, IGBT 2. Low
cost 3. Higher Accurate.
11: Why Human body feel Electric shock ?? n in an
Electric train during running , We did nt feel any
Shock ? why?
Answer: Unfortunately our body is a pretty good
conductor of electricity, The golden rule is Current
takes the lowest resistant path if you have
insulation to our feet as the circuit is not complete
(wearing rubber footwear which doing some repairs
is advisable as our footwear is a high resistance
path not much current flows through our body).The
electric train is well insulated from its electrical
system.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
12 :what is the principle of motor?
Answer: Whenever a current carrying conductor is
placed in a magnetic field it produce turning or
twisting movement is called as torque.
13: why, when birds sit on transmission lines or
current wires doesn't get shock?
Answer: It’s true that if birds touch the single one
line (phase or neutral) they don't get electrical
shock... if birds touch 2 lines than the circuit is
closed and they get electrical shock.. so if a human
touch single one line(phase) then he doesn't get
shock if he is in the air (not touching - standing on
the ground if he is standing on the ground then
touching the line (phase) he will get a shock
because the ground on what we standing is like line
(ground bed - like neutral)। and in the most of
electric lines the neutral is grounded..so that means
that human who touch the line closes the circuit
between phase and neutral.
14:what is meant by armature reaction?
Answer: The effect of armature flu to main flux is
called armature reaction. The armature flux may
support main flux or opposes main flux.
15: what happen if we give 220 volts dc supply to d
bulb r tube light?
Answer: Bulbs [devices] for AC are designed to
operate such that it offers high impedance to AC
supply. Normally they have low resistance. When
DC supply is applied, due to low resistance, the
current through lamp would be so high that it may
damage the bulb element.
16: Which motor has high Starting Torque and
Staring current DC motor, Induction motor or
Synchronous motor?
Answer:DC Series motor has high starting torque.
We can not start the Induction motor and
Synchronous motors on load, but can not start the
DC series motor without load.
17: what is ACSR cable and where we use it?
Answer: ACSR means Aluminium conductor steel
reinforced, this conductor is used in transmission &
distribution.
18: What is vaccum circuit breaker.define with
cause and where be use it Device?
Answer: A breaker is normally used to break a
circuit. while breaking the circuit, the contact
terminals will be separated. At the time of
separation an air gap is formed in between the
terminals. Due to existing current flow the air in the
gap is ionized and results in the arc. various
mediums are used to quench this arc in respective
CB's. but in VCB the medium is vacuum gas. since
the air in the CB is having vacuum pressure the arc
formation is interrupted. VCB's can be used up to
11kv.
20: What will happen when power factor is leading
in distribution of power?
Answer: If there is high power factor, i.e if the
power factor is close to one:
1.losses in form of heat will be reduced,
2.cable becomes less bulky and easy to carry, and
very
cheap to afford, &
3. it also reduces over heating of transformers.
21: what’s the one main difference between UPS &
inverter? And electrical engineering & electronics
engineering?
Answer: uninterrupted power supply is mainly use
for short time . means according to ups VA it gives
backup. Ups is also two types : on line and offline .
online ups having high volt and amp for long time
backup with high dc voltage. But ups start with 12v
dc with 7 amp. but inverter is start with 12v,24,dc
to 36v dc and 120amp to 180amp battery with long
time backup.
22: What is 2 phase motor?
Answer: A two phase motor is a motor with the
starting winding and the running winding have a
phase split. e.g;AC servo motor. Where the
auxiliary winding and the control winding have a
phase split of 90 degree.
23: Advantages of vvvf drives over non vvvf drives
for EOT cranes?
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
Answer: 1.smooth start and stop.
2. No jerking of load.
3. Exact positioning
4. Better protection for motor.
5. high/low speed selection.
6. Reliability of break shoe.
7. Programmable break control.
8. Easy circuitry
9. Reduction in controls
10. Increases motor life
24:What is the significance of vector grouping in
Power Transformers?
Answer:Every power transformer has a vector
group listed by its manufacturer. Fundamentally it
tells you the information about how the windings
are connected (delta or wye) and the phace
difference betweent the current and voltage. EG.
DYN11 means Delta primary, Wye Secondry and
the current is at 11 o clock reffered to the voltage.
25:Which type of A.C motor is used in the fan
(ceiling fan, exhaust fan, padestal fan, bracket fan
etc) which are find in the houses ?
Answer:Its Single Phase induction motor which
mostly squirrel cage rotor and are capacitor start
capacitor run.
26:Give two basic speed control scheme of DC
shunt motor?
Answer: 1. By using flux control method: in this
method a rheostat is connected across the field
winding to control the field current. So by changing
the current the flux produced by the field winding
can be changed, and since speed is inversely
proportional to flux speed can be controlled
2.armature control method: in this method a
rheostat is connected across armature winding by
varying the resistance the value of resistive
drop(IaRa)can be varied, and since speed is directly
proportional to Eb-IaRa the speed can be
controlled.
27: what is the principle of motor?
Answer: Whenever a current carrying conductor is
placed in an magnetic field it produce turning or
twisting movement is called as torque
28: what is meant by armature reaction?
Answer: The effect of armature flu to main flux is
called armature reaction. The armature flux may
support main flux or opposes main flux.
29:
30:what is the difference between synchronous
generator & asynchronous generator?
Answer:In simple, synchronous generator supply's
both active and reactive power but asynchronous
generator(induction generator) supply's only active
power and observe reactive power for
magnetizing.This type of generators are used in
windmills
31:What is the Polarization index value ? (pi
value)and simple definition of polarization index ?
Answer: Its ratio between insulation
resistance(IR)i.e meggar valuefor 10min
to insulation resistance for 1 min. It ranges from 5-
7 for new motors & normally for motor to be in
good condition it should be Greater than 2.5 .
32: Why syn. generators r used for the production
of electricity?
Answer:synchronous machines have capability to
work on different power factor(or saydifferent
imaginary power varying the field emf. Hence syn.
generators r used for the production of electricity.
33: What is the difference between synchronous
generator & asynchronous generator?
Answer: In simple, synchronous generator supply's
both active and reactive power but asynchronous
generator (induction generator) supply's only active
power and observe reactive power for magnetizing.
This type of generators are used in windmills.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
34:1 ton is equal to how many watts?
Answer:1 ton = 12000 BTU/hr and to convert
BTU/hr to horsepower,
12,000 * 0.0003929 = 4.715 hp therefore 1 ton =
4.715*.746 = 3.5 KW.
35:why syn. generators r used for the production of
electricity?
Answer:synchronous machines have capability to
work on differentpower factor(or saydifferent
imaginary pow varying the field emf. Hence syn.
generators r used for the production of electricity.
36: Enlist types of dc generator?
Answer: D.C.Generators are classified into two
types 1)separatly exicted d.c.generator 2)self
exicted d.c.generator, which is further classified
into;1)series 2)shunt and3)compound(which is
further classified into cumulative and differential).
37:What’sAutomatic Voltage regulator(AVR)?
Answer: AVR is an abbreviation for
Automatic Voltage Regulator.It is important part in
Synchronous Generators, it controls
theoutput voltage of the generator by controlling its
excitation current. Thus it can control the output
Reactive Power of the Generator.
38: What is an exciter and how does it work?
Answer: There are two types of exciters, static
exciter and rotory exciter.purpose of excitor is to
supply the excitation dc voltage to the fixed poles
of generator.Rotory excitor is an additional small
generator mounted on the shaft of main generator.
if it is dc generator, it will supply dc to the rotory
poles through slip ring and brushes( conventional
alternator). if it is an ac excitor, out put of ac
excitor is rectified by rotating diodes and supply dc
to main fixed poles.ac excitor is the ac generator
whose field winding are stationary and armature
rotates. initialvoltage is built up by residual
magnetism.It gives the starting torque to the
generator.
40: Difference between a four point starter and
three point starter?
Answer: The shunt connection in four point stater
is provided separately form the line where as in
three point stater it is connected with line which is
the drawback in three point stater
41: Why use the VCB at High Transmission
System ? Why can't use ACB?
Answer: Actually the thing is vacuum has high arc
queching property compare to air because in VCB
,the die electric strengths equal to 8 times of air .
That y always vaccum used as inHT breaker and air
used as in LT .
42: What is the difference between surge arrestor
and lightning arrestor?
Answer: LA is installed outside and the effect of
lightning is grounded,where as surge arrestor
installed inside panels comprising of resistors
which consumes the energy and nullify the effect of
surge.
43: What happens if i connect a capacitor to a
generator load?
Answer:Connecting a capacitor across a generator
always improves powerfactor,but it will help
depends up on the engine capacity of the
alternator,other wise the alternator will be over
loaded due to the extra watts consumed due to
the improvement on pf. Secondly, don't connect a
capacitor across an alternator while it is picking up
or without any other load.
44: Why the capacitors works on ac only?
Answer: Generally capacitor gives infinite
resistance to dc components(i.e., block the dc
components). it allows the ac components to pass
through.
45: Explain the working principal of the circuit
breaker?
Answer: Circuit Breaker is one which makes or
breaks the circuit.It has two contacts namely fixed
contact & moving contact.under normal condition
the moving contact comes in contact with fixed
contact thereby forming the closed contact for the
flow of current. During abnormal & faulty
conditions(when current exceeds the rated value) an
arc is produced between the fixed & moving
contacts & thereby it forms the open circuitArc is
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
extinguished by the Arc Quenching media like air,
oil, vaccum etc.
46: How many types of colling system it
transformers?
Answer: 1. ONAN (oil natural,air natural)2. ONAF
(oil natural,air forced)3. OFAF (oil forced,air
forced)4. ODWF (oil direct,water forced) 5. OFAN
(oil forced,air forced)
47: What is the function of anti-pumping in circuit
breaker?
Answer: when breaker is close at one time by close
push button,the anti pumping contactor prevent re
close the breaker by close push button after if it
already close.
48: what is stepper motor.what is its uses?
Answer: Stepper motor is the electrical machine
which act upon input pulse applied to it. it is one
type of synchronous motor which runs in steps in
either direction instead of running in complete
cycle.so, in automation parts it is used.
49: How to calculate capacitor bank value to
maintain unity power factor with some suitable
example?
Answer:KVAR= KW(TAN(COS(-1)#e)-
TAN(COS(-1)#d) )
#e= EXISTING P.F.
#d= DESIRED P.F.
50: Tell me in detail about c.t. and p.t.
?(Company:reliance)
Answer:The term C.T means current
transformer,and the term P.T means potential
transformer.In circuit where measurements of high
voltage and high current is involved they are used
there.Particularly when a measuring device like
voltmeter or ammeter is not able to measure such
high value of quantity because of large value of
torque due to such high value it can damage the
measuring device.so, CT and PT are introduced in
the circuits. They work on the same principle of
transformer, which is based on linkage of
electromagneticflux produced by primary with
secondary.They work on the ratio to they are
designed.E.g if CTis of ratio 5000\5A and it has to
measure secondary current of
8000A.then ANS=8000*5\5000=8Aand this result
will be given to ammeter .and after measuring 8A
we can calculate the primary current.same is
the operation of PT but measuring voltage.
51: There are a Transformer and an induction
machine. Those two have the same supply. For
which device the load current will be maximum?
And why?
Answer: The motor has max load current compare
to that of transformer because the motor consumes
real power.. and the transformer is only producing
the working flux and its not consuming.. hence the
load current in the transformer is because of core
loss so it is minimum.
52: what is power factor? whether it should be high
or low? why?
Answer:Power factor should be high in order to get
smooth operation of the system.Low power factor
means losses will be more.it is the ratio of true
power to apperent power. it has to be ideally 1. if it
is too low then cable over heating & equipment
overloading will occur. if it is greater than 1 then
load will act as capacitor and starts feeding the
source and will cause tripping.(if pf is poor ex: 0.17
to meet actual power load has to draw more
current(V constant),result in more lossesif pf is
good ex: 0.95 to meet actual power load has to
draw less current(V constant),result in less losses).
53:What is the difference between Isolator
and Circuit Breaker?
Answer:Isolator is a off load device which is used
for isolating the downstream circuits from upstream
circuits for the reason of any maintenance on
downstream circuits. it is manually operated and
does not contain any solenoid unlike circuit
breaker. it should not be operated while it is having
load. first the load on it must be made zero and then
it can safely operated. its specification only rated
current is given.But circuit breaker is onload
automatic device used for breaking the circuit in
case of abnormal conditions like short circuit,
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
overload etc., it is having three specification 1 is
rated current and 2 is short circuit breaking capacity
and 3 is instantaneous tripping current.
54:what is boucholz relay and the significance of it
in to the transformer?
Answer:Boucholz relay is a device which is used
for the protection of transformer from its internal
faults, it is a gas based relay. whenever any internal
fault occurs in a transformer, the boucholz relay at
once gives a horn for some time, if the transformer
is isolated from the circuit then it stop its sound
itself other wise it trips the circuit by its own
tripping mechanism.
55:What is SF6 Circuit Breaker?
Answer:SF6 is Sulpher hexa Flouride gas.. if this
gas is used as arc quenching medium in a
Circuitbreaker means SF6 CB.
56:what is ferrantic effect?
Answer:Output voltage is greater than the input
voltage or receiving end voltage is greater than the
sending end voltage.
The Ferranti effect is a rise in voltage occurring at
the receiving end of a long transmission line,
relative to the voltage at the sending end, which
occurs when the line is energized but there is a very
light load or the load is disconnected.
This effect is due to the voltage drop across the line
inductance (due to charging current) being in phase
with the sending end voltages. Therefore both
capacitance and inductance are responsible for
producing this phenomenon.[1]
The Ferranti Effect will be more pronounced the
longer the line and the higher the voltage
applied.[2] The relative voltage rise is proportional
to the square of the line length.[3]
Due to high capacitance, the Ferranti effect is much
more pronounced in underground cables, even in
short lengths.
57: what is meant by insulation voltage in cables?
explain it?
Answer:It is the property of a cable by virtue of it
can withstand the applied voltage without rupturing
it is known as insulation level of the cable.
58: Why we do 2 types of earthing on transformer
(ie:)body earthing & neutral earthing , what is
function. i am going to install a 5oo kva
transformer & 380 kva DG set what should the
earthing value?
Answer:The two types of earthing are Familiar as
Equipment earthing and system earthing. In
Equipment earthing: body ( non conducting part)of
the equipment shouldd be earthed to safegaurd the
human beings.system Earthing : In this neutral of
the supply source ( Transformer or Generator)
should be grounded. With this,in case of
unbalanced loading neutral will not be shifted.so
that unbalanced voltages will not arise. We can
protect the equipment also. With size of the
equipment( transformer or alternator)and selection
of relying system earthing will be further classified
into directly earthed,Impedance earthing, resistive
(NGRs) earthing.
59:What is the difference between MCB & MCCB,
Where it can be used?
Answer:MCB is miniature circuit breaker which is
thermal operated and use for short circuit protection
in small current rating circuit. MCCB moulded case
circuit breaker and is thermal operated for over load
current and magnetic operation for instant trip in
short circuit condition.under voltage and under
frequency may be inbuilt. Normally it is used where
normal current is more than 100A.
60:Where should the lighting arrestor be placed in
distribution lines?
Answer:Near distribution transformers and out
going feeders of 11kv and incomming feeder of
33kv and near power transformers in sub-stations.
61:Define IDMT relay?
Answer:It is an inverse definite minimum time
relay.In IDMT relay its operating is inversely
proportional and also a characteristic of minimum
time after which this relay operates.It is inverse in
the sense ,the tripping time will decrease as the
magnitude of fault current increase.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
62:What are the transformer losses?
Answer:TRANSFORMER LOSSES - Transformer
losses have two sources-copper loss and magnetic
loss. Copper losses are caused by the resistance of
the wire (I2R). Magnetic losses are caused by eddy
currents and hysteresis in the core. Copper loss is a
constant after the coil has been wound and
therefore a measurable loss. Hysteresis loss is
constant for a particular voltage and current. Eddy-
current loss, however, is different for each
frequency passed through the transformer.
63:What is meant by regenerative braking?
Answer:When the supply is cutt off for a running
motor, it still continue running due to inertia. In
order to stop it quickly we place a load(resitor)
across the armature winding and the motor should
have maintained continuous field supply. so that
back e.m.f voltage is made to applyacross the
resistor and due to load the motor stops
quickly.This type of breaking is called as
"Regenerative Breaking".
64:Why is the starting current high in a DC motor?
Answer:In DC motors, Voltage equation is V=Eb-
IaRa (V = Terminal voltage,Eb = Back emf in
Motor,Ia = Armature current,Ra = Aramture
resistance).At starting, Eb is zero. Therefore,
V=IaRa, Ia = V/Ra ,where Ra is very less like
0.01ohm.i.e, Ia will become enormously increased.
65:What are the advantages of star-delta starter
with induction motor?
Answer:(1). The main advantage of using the star
delta starter is reduction of current during the
starting of the motor.Starting current is reduced to
3-4 times Of current of Direct online starting.(2).
Hence the starting current is reduced , the voltage
drops during the starting of motor in systems are
reduced.
66:Why Delta Star Transformers are used for
Lighting Loads?
Answer:For lighting loads, neutral conductor is
must and hence the secondary must be star winding.
and this lighting load is always unbalanced in all
three phases. To minimize the current unbalance in
the primary we use delta winding in the primary. So
delta / star transformer is used for lighting loads.
67:Why in a three pin plug the earth pin is thicker
and longer than the other pins?
Answer:It depends upon R=rho l/a where area(a) is
inversely proportional to resistance (R), so if (a)
increases, R decreases & if R is less the leakage
current will take low resistance path so the earth pin
should be thicker. It is longer because the The First
to make the connection and Last to disconnnect
should be earth Pin. This assures Safety for the
person who uses the electrical instrument.
Electrical interview question:Why series motor
cannot be started on no-load?
Answer:Series motor cannot be started without load
because of high starting torque. Series motor are
used in Trains, Crane etc.
68:Why ELCB can't work if N input of ELCB do
not connect to ground?
Answer:ELCB is used to detect earth leakage fault.
Once the phase and neutral are connected in an
ELCB, the current will flow through phase and that
much current will have to return neutral so resultant
current is zero. Once there is a ground fault in the
load side, current from phase will directly pass
through earth and it will not return through neutral
through ELCB. That means once side current is
going and not returning and hence because of
this difference in current ELCB wil trip and it will
safe guard the other circuits from faulty loads. If
the neutral is not grounded, fault current will
definitely high and that full fault current will come
back through ELCB, and there will be
no difference in current.
69:How electrical power is generated by an A.C
Generator?
Answer:For the generation of elect power we need
a prime mover which supplies mechanical power
input to the alternator, can be steam turbines,or
hydro turbines .When poles of the rotor moves
under the armature conductors which are placed on
the stator ,field flux cut the
armature conductor ,therefore voltage is generated
and is of sinusoidal in nature...due to polarity
change of rotor poles(i,e) N-S-N-S.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
70:Why an ac solenoid valve attract the plunger
even though we interchanges the terminal? Will the
poles changes?
Answer:Yes because the poles changes for every
half-cycle of ac voltageso the polarity of
AC voltage is continuously changing for every half
cycle. so, interchanging of terminals in ac system
does not show any difference. That's why the ac
solenoid attract the plunger even though it's
terminals are interchanged.
71:What is derating?, why it is necessary, it is same
for all means for drives, motors,and cables.
Answer:The current currying of cables will change
depending upon the site temperature (location of
site), type of run (it will run through duct, trench,
buried etc.), number of tray, depth of trench,
distance between cables. Considering this condition
actual current currying capacity of cable reduce
than current currying capacity (which given to
cable Catalogue) this is called derating.
72:Why temperature rise is conducted in bus bars
and isolators?
Answer:Bus bars and isolators are rated for
continuous power flow, that means they carry
heavy currents which rises their temperature. so it is
necessary to test this devices for temperature rise.
73:When voltage increases then current also
increases then what is the need of
over voltage relay and over current relay? Can we
measure over voltage and over current by
measuring current only?
Answer:No.We can't sense the over voltage by just
measuring the current only because the current
increases not only for over voltages but also for
under voltage(As most of the loads are non-linear
in nature).So,the over voltage protection & over
current protection are completely different.
Over voltage relay meant for sensing over voltages
& protect the system from insulation break down
and firing. Over current relay meant for sensing any
internal short circuit, over load condition ,earth
fault thereby reducing the system failure & risk of
fire.So, for a better protection of the system.It
should have both over voltage & over current
relay.
74:If one lamp connects between two phases it will
glow or not?
Answer:If the voltage between the two phase is
equal to the lamp voltagethen the lamp will glow.
When the voltage difference is big it will damage
the lamp and when the difference is smaller the
lamp will glow depending on the type of lamp.
75:How do you select a cable size (Cu & Al) for a
particular load?
Answer:At first calculate the electrical current of
the load, after that derate the electrical current
considering derating factor(depending on site
condition and laying of cable) after choose the
cable size from cablecatalog considering derating
electrical current.After that measure the length of
cable required from supply point of load to load
poin. Calculate the voltage drop which will max 3%
(resistance and reactance of cable found from
cable catalog of selecting cable)
if voltage drop>3% then choose next higher size of
cable.
76:What are HRC fuses and where it is used?
Answer:HRC stand for "high rupturing capacity"
fuse and it is used in distribution system for
electrical transformers.
77:Which power plant has high load factor?
Answer:All base load power plants have a high
load factor. If we use high efficiency power plants
to supply the base load,we can reduce the cost of
generation.Hydel power plants have a higher
efficiency than thermal & nuclear power plants.
78:Mention the methods for starting an induction
motor?
Answer:The different methods of starting an
induction motorDOL:direct online starter,Star delta
starter,Auto transformer starter,Resistance
starter,Series reactor starter
79:What is the difference between earth resistance
and earth electrode resistance?
Answer:Only one of the terminals is evident in the
earth resistance. In order to find the second terminal
we should recourse to its definition: Earth
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
Resistance is the resistance existing between the
electrically accessible part of a buried electrode and
another point of the earth, which is far away.
The resistance of the electrode has the following
components:
(A) the resistance of the metal and that of the
connection to it.
(B) the contact resistance of the surrounding earth
to the electrode.
80:What is use of lockout relay in ht voltage?
Answer:A lock-out relay is generally placed in line
before or after the e-stop switch so the power can
be shut off at one central location. This relay is
powered by the same electrical source as the control
power and is operated by a key lock switch. The
relay itself may have up to 24 contact points within
the unit itself. This allows the control power for
multiple machines to be locked out by the turn of a
single key switch.
81:What is the power factor of an alternator at no
load?
Answer:At no load Synchronous Impedance of the
alternator is responsible for creating
angle difference. So it should be zero lagging like
inductor.
82:Why most of analog o/p devices having o/p
range 4 to 20 mA and not 0 to 20 mA ?
Answer:4-20 mA is a standard range used to
indicate measured values for any process. The
reason that 4ma is chosen instead of 0 mA is for fail
safe operation .For example- a pressure instrument
gives output 4mA to indicate 0 psi, up to 20 mA to
indicate 100 psi, or full scale. Due to any problem
in instrument (i.e) broken wire, its output reduces to
0 mA. So if range is 0-20 mA then we can
differentiate whether it is due to broken wire or due
to 0 psi.
83:Two bulbs of 100w and 40w respectively
connected in series across a 230v supply which
bulb will glow bright and why?
Answer:Since two bulbs are in series they will get
equal amount of electrical current but as the supply
voltage is constant across the bulb(P=V^2/R).So
the resistance of 40W bulb is greater and voltage
across 40W is more (V=IR) so 40W bulb will glow
brighter.
84:What is meant by knee point voltage?
Answer:Knee point voltage is calculated for
electrical Current transformers and is very
important factor to choose a CT. It is the voltage at
which a CT gets saturated.(CT-current
transformer).
85:What is reverse power relay?
Answer:Reverse Power flow relay are used in
generating stations's protection. A generating
stations is supposed to fed power to the grid and in
case generating units are off,there is no generation
in the plant then plant may take power from grid.
To stop the flow of power from grid to generator
we use reverse power relay.
86:What will happen if DC supply is given on the
primary of a transformer?
Answer:Mainly transformer has high inductance
and low resistance.In case of DC supply there is no
inductance ,only resistance will act in
theelectrical circuit. So high electrical current will
flow through primary side of the transformer.So for
this reason coil and insulation will burn out.
87:What is the difference between isolators
and electrical circuit breakers? What is bus-bar?
Answer:Isolators are mainly for switching purpose
under normal conditions but they cannot operate in
fault conditions .Actually they used for isolating the
CBs for maintenance. Whereas CB
gets activated under fault conditions according to
the fault detected.Bus bar is nothing but a junction
where the power is getting distributed for
independent loads.
88:What are the advantage of free wheeling diode
in a Full Wave rectifier?
Answer:It reduces the harmonics and it also
reduces sparking and arching across the mechanical
switch so that it reduces the voltage spike seen in a
inductive load
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
89:What are Motor Generator Sets and explain the
different ways the motor generator set can be used
?
Answer:Motor Generator Sets are a combination of
an electrical generator and an engine mounted
together to form a single piece of equipment. Motor
generator set is also referred to as a genset, or more
commonly, a generatorThe motor generator set can
used in the following different ways: 1.Alternating
current (AC) to direct current (DC), 2.DC to AC,
3.DC at one voltage to DC at another voltage, 4.AC
at one frequency to AC at another harmonically-
related frequency
90:What is power quality meter ?
Answer:Power Quality meters are common in
many industrial environment. Small units are now
available for home use as well. They give operators
the ability to monitor the both perturbations on the
power supply, as well as power used within a
building, or by a single machine or appliance. In
some situations, equipment function
and operation is monitored and controlled from a
remote location where communication is via
modem, or high-speed communication lines.So we
can understand the importance of power
measurement through power quality meters.
91:What is the different between digital phase
converter and ordinary phase converter?
Answer:Digital phase converter are a recent
development in phase converter technology that
utilizes proprietary software in a powerful
microprocessor to control solid state power
switching components. This microprocessor, called
a digital signal processor (DSP), monitors the phase
conversion process, continually adjusting the input
and output modules of the converter to maintain
perfectly balanced three-phase power under all load
conditions.
92:Explainthe operation of variablefrequency
transformer?
Answer:A variable frequency transformer is used
to transmit electricity between two asynchronous
alternating current domains. A variable frequency
transformer is a doubly-fed electric machine
resembling a vertical shaft hydroelectric generator
with a three-phase wound rotor, connected by slip
rings to one external ac power circuit. A direct-
current torque motor is mounted on the same shaft.
Changing the direction of torque applied to the
shaft changes the direction of power flow; with no
applied torque, the shaft rotates due to
the difference in frequency between the networks
connected to the rotor and
stator.The variablefrequency transformer behaves
as a continuously adjustable phase-shifting
transformer. It allows control of the power flow
between two networks .
93:What is the main use of rotary phase converter
?
Answer:Rotary phase converter will be converting
single phase power into true balanced 3 phase
power,so it is often called as single phase to three
phase converter .Often the advantages of 3 phase
motors, and other 3 phase equipment, make it
worthwhile to convert single phase to 3 phase so
that small and large consumers need not want to
pay for the extra cost of a 3 phase service but may
still wish to use 3 phase equipment.94Which type
of oil is used as a transformer oil?
Answers:Transformer oil, or insulating oil, is
usually a highly-refined mineral oil that is stable at
high temperatures and has excellent electrical
insulating properties. It is used in oil filled
transformers, some types of high voltage
capacitors, fluorescent lamp ballasts, and some
types of high voltage switches and circuit breakers.
Its functions are to insulate, suppress corona and
arcing, and to serve as a coolant.
Well into the 1970s, polychlorinated biphenyls
(PCB)s were often used as a dielectric fluid since
they are not flammable. They are toxic, and under
incomplete combustion, can form highly toxic
products such as furan. Starting in the early 1970s,
concerns about the toxicity of PCBs have led to
their banning in many countries.
Today, non-toxic, stable silicon-based or
fluoridated hydrocarbons are used, where the added
expense of a fire-resistant liquid offsets additional
building cost for a transformer vault. Combustion-
resistant vegetable oil-based dielectric coolants and
synthetic pentaerythritol tetra fatty acid (C7, C8)
esters are also becoming increasingly common as
alternatives to naphthenic mineral oil. Esters are
non-toxic to aquatic life, readily biodegradable, and
have a lower volatility and higher flash points than
mineral oil.
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
95: If we give 2334 A, 540V on Primary side of
1.125 MVA step up transformer, then what will be
the Secondary Current, If Secondary Voltage=11
KV?
Answers :As we know the Voltage & current
relation for transformer-V1/V2 = I2/I1
We Know, VI= 540 V; V2=11KV or 11000 V; I1=
2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So,I2 = 114.5 Amps
96:what are the points to be consider for
MCB(miniature circuit breaker
selection?Answers:I(L)*1.25=I(MAX) maximum
current. Mcb specification are done on maximum
current flow in circuit.
97:what is the full form of KVAR?
Answers:We know there are three types of power
in Electricals as Active, apparent & reactive. So
KVAR is stand for ``Kilo Volt Amps with Reactive
component.
98:What is excitation?
Answer:Excitation is applying an external voltage
to DC shunt coil in DC motors.
99:In three pin plug 6 Amp. 220v AC rating. why
earth pin diameter is higher than ohter two pin?
what its purpose ?
Answer:Because Current flow in the conductor is
inversely proportional to the conductor diameter.
So if any short circuits occur in the system first
high currents bypassed in the Earthling terminal.(
R=Pl/a area of the conductor increases
resistance value decreases)
100:Different between megger and
contact resistance meter?
Answers:Megger used to measure cable resistance,
conductor continuity, phase identification where as
contact resistance meter used to measure low
resistance like relays ,contactors.
101:When we connect the capacitor bank in series ?
Answers:we connect capacitor bank in series to
improve the voltage profile at the load end in
transmission line there is considerable voltage drop
along the transmission line due toimpedance of the
line. so in order to bring the voltage at the load
terminals within its limits i.e (+ or - %6 )of the
rated terminal voltage the capacitor bank is used in
series
102:What is Diversity factor in electrical
installations?
Answers:Diversity factor is the ratio of the sum of
the individual maximum demands of the various
subdivisions of a system, or part of a system, to the
maximum demand of the whole system, or part of
the system, under consideration. Diversity factor is
usually more than one.
103:Why field rheostat is kept in minimum
position while armature rheostat at maximum
position?
Answers:In motors at the time of starting the
armature resistance is introduced to reduce the high
starting current and the field resistance is kept
minimum to have high starting torque.
104:Why humming sound occurred in h HT
transmission line?
Answers:This sound is coming due to ionization
(breakdown of air into charged particles) of air
around transmission conductor. This effect is called
as Corona effect, and it is considered as power loss.
105:what is rated speed?
Answers:At the time of motor taking normal
current (rated current)the speed of the motor is
called rated speed. It is a speed at which any system
take small current and give maximum efficiency.
106:What is different between grounding and
earthings?
Answers:Grounding means connecting the neutral
point of the load to the ground to carry the residual
current in case of unbalanced conditions through
the neutral to the ground whereas earthing is done
www.jaicj.in [email protected]
Electrical interview questions 2015
Fe
br
ua
ry
7
, 2
01
5
in an electric equipment in order to protect he
equipment in occurence of fault in the system.
107:Why should be the frequency is 50 hz only
why not others like 45, 95 56 or anything , why
should we maintain the frequency constant if so
why it is only 50 ?
Answers:We can have the frequency at any
frequency you like, but than you must also make
your own motors, transformers or any other
equipment you want to use.We maintain the
frequency at 50hz or 60hz cos the world maintains
a standard at 50 /60hz and the equipments are are
made to operate at these frequency.
108:Why transformer rating in KVA instead of
KW?
Any device is rated depending on losses of the
machine. In a transformer the losses are iron losses
which depends on voltage and copper losses which
depends on current so there are no losses which
depend on power factor hence transformer is rated
in KVA.
There are 2 losses in transformer. One is copper
loss which depends on current and the other is iron
loss which depends on voltage. These two factors
are not affected by the power factor. This is why
transformers are rated in KVA and not KW. Single
phase KVA = Amps x Volts/1000. Single phase
KW = Amps x Volts x pf/1000. 3 phase KVA =
Amps x Volts x 1.73/1000. 3 phase KW = Amps x
Volts x 1.73 x pf/1000
yes there are two type of losses in transformer is
that ohmic losses and core losses . ohmic losses due
to transformer current and core losses due to
transformer voltage. these losses are due to
transformer voltage and current. and these losses
not depends on load ie kw that's the way its rating
in kva..