Viva And Interview Question On Transformer
1.
What Is A Transformer?
A
transformer is a static device which can transfer power from one circuit to
another at same frequency.
2. How Does A Transformer
Work?
Transformer
consists of two coils. If one coil is connected with ac voltage source then it
will produce alternating flux in the core. Most of the flux is linked with
second coil hence mutually induced EMF will produce in the second coil as per
faraday's law of electromagnetic induction.
Based on supply
- Single phase transformer
- Three phase transformer
Based on winding
- Auto transformer (single winding)
- Two winding transformer
- Three winding transformer
- Six winding transformer
Based on construction
- Core Type transformer
- Shell Type transformer
Based on the service
- Distribution transformer
- Power transformer
Based on measurement
- Current transformer
- potential transformer
Based on cooling
- Dry type Transformer
- Oil immersed type transformer
Based on function
- Step up transformer
- Step down transformer
- Isolation transformer
4. Can Dc Be Applied To
Transformers ?
No
- Transformer works on Faraday's law of Electromagnetic Induction for which current in coil must change. If DC is applied current will not change and transformer will not work
- Practically winding resistance is very small. For DC ,inductive reactance is zero and frequency is zero. Therefore impedance is low. Thus winding draws more current which may damage the winding.
5. What Is Single And
Three Phase Transformer?
- The transformer which works on single phase is called single phase transformer.
- The transformer which works on three phase is called three phase transformer.
6. What Is Auto-transformer?
In
these transformer only one winding is used as primary and secondary. Also,
primary and secondary are conductively coupled.
7. What
Is Two Winding Transformer?
Two separate windings one
as primary and other as secondary are used. Both windings are magnetically
coupled.
8. What
Is Three Winding Transformer?
Three windings are used
each work as primary and secondary. Also primary and secondary windings are
conductively couple. They are three phase auto transformer.
9. What
Is Six Winding Transformer?
Three windings in primary
and three windings in secondary are used, They are three phase
transformers. They are connected in either star-star, delta-delta, star delta or
delta star.
10. What
Are The Advantages Of Shell Type Transformer Over Core Type Transformers?
In shell type transformers
the coils are well supported on the all sides and so they can withstand higher
mechanical stresses developed during short circuit conditions. Also the leakage
reactance will be less in shell type transformers.
11. Where
Is Core Type And Shell Type Construction Suitable For A Transformer?
Core type transformers are
popular in High voltage applications like Distribution transformers, Power
transformers, and obviously auto transformers. Where as, Shell type transformers
are popular in Low voltage applications like transformers used in electronic
circuits and power electronic converters etc.
Power transformer is used
for the transmission purpose at heavy load, high voltage greater than 33 KV
& 100% efficiency. It is big in size as compared to distribution
transformer, it used in generating station and Transmission substation.
It is
generally rated above 200MVA.
13. What
Are Distribution Transformer?
The distribution
transformer is used for the distribution of electrical energy at low voltage as
less than 33KV in industrial purpose and 440v-220v in domestic purpose. It
works at low efficiency at 60-70%, small size, easy in installation, having low
magnetic losses & it is not always fully loaded.
14. Why The Efficiency Of Distribution
Transformer Is 60 To 70 % And Not 100%?
Distribution transformer is
designed for maximum efficiency at 60% to 70% load as it normally doesn’t
operate at full load all the time. Its load depends on distribution demand.
Whereas power transformer is designed for maximum efficiency at 100% load as it
always runs at 100% load being near to generating station.
15. What
Are The Salient Features Of Distribution Transformer?
- The distribution transformer will have low iron loss and higher value of copper loss
- The capacity of transformers will be up to 500 KVA
- The transformers will have plain walled tanks or provided with cooling tubes or radiators.
- The leakage reactance and regulation will be low
16. What
Are Current Transformer?
They are protective devices
used to measure very high value of current in power system. The Current
Transformer ( C.T. ), is a type of “instrument transformer” that is designed to
produce an alternating current in its secondary winding which is proportional
to the current being measured in its primary.
17. What
Are Potential Transformer?
They are protective devices
used to measure very high value of voltage in power system.
18. What
Are Dry Type Transformer ?
In this type of
transformer, air is used as coolant. The heat is taken to walls of tank and
dissipated to the surrounding air.
19. What
Is Oil Immersed Type Transformer?
In this type of
transformer, oil is used as coolant. Entire assembly including core and
windings is kept immersed in oil. The developed heat is transferred to the
walls of tank through oil. Finally the heat is transferred to the surrounding
air from tank wall by radiation.
20. What
Are Step Up Transformers?
They step up voltage from
lower value to higher value.
21. What
Are Step Down Transformers?
They step up voltage from
higher value to lower value.
22. What Are Isolation
Transformers?
In this type of transformer
the primary and secondary are same. When it is required to isolate the primary
and secondary circuits these type of transformers are used. It protects the
circuits connected in secondary side when major fault occurs in primary side.
23. Why
Stepped Cores Are Used?
- To reduce the space effectively
- To obtain reduced length of mean turn of the windings
- To reduce I² R loss.
24. What
Is Yoke Section Of Transformers?
The sections of the core
which connect the limbs are called yoke. The yoke is used to provide a closed
path for the flux.
25. What
Is The Purpose Of Laminating The Core In A Transformer?
In order to minimize eddy
current loss.
26.
Explain On The Material Used For Core Construction?
The core
is constructed by sheet steel laminations
assembled to provide a continuous magnetic
path with minimum of air gap
included. The steel used is of high
silicon content sometimes heat treated to produce
a high permeability and a low
hysteresis loss at the usual operating flux
densities. The eddy current loss is
minimized by laminating the core, the laminations being
used from each other by light coat of core-plate varnish or by oxide layer on
the surface. The thickness of lamination varies from 0.35mm for a frequency of
50Hz and 0.5mm for a frequency of 25Hz.
27. Why The Cross-section Of Iron Is Less Than
Total Cross Section Area Of Core?
This is because the core is
laminated & on each lamination the insulations are are used.
28. What
Is Stack Factor?
The ratio of total cross
section of iron to the total cross section of core is called stack factor.
29. What
Are The Properties Of Ideal Transformer?
- There should be no losses
- The winding should have zero resistance
- The leakage flux should be zero
- The permeability of core should be so high that the negligible current is required to establish the flux in it.
30. What
Are The Functions Of No-load Current In A Transformer ?
No-load current produces
flux and supplies iron loss and copper loss on no-load.
31. What
Is The Condition For Zero Voltage Regulation?
Negative sign indicates
zero voltage regulation. It occurs when load is capacitive and power factor is
leading.
32. What
Is The Condition For Maximum Voltage Regulation?
It occurs when load is
inductive and power factor is lagging.
33. What
Are The Factors Affecting Voltage Regulation?
- Load current
- Equivalent resistance
- Equivalent reactance
- power factor
34. What
Is Eddy Current Loss In Transformer?
In transformer, we supply
alternating current in the primary, this alternating current produces
alternating magnetizing flux in the core and as this flux links with secondary
winding, there will be induced voltage in secondary, resulting current to flow
through the load connected with it. Some of the alternating fluxes of
transformer; may also link with other conducting parts like steel core or iron
body of transformer etc. As alternating flux links with these parts of
transformer, there would be a locally induced EMF. Due to these EMF, there
would be currents which will circulate locally at that parts of the
transformer. These circulating current will not contribute in output of the
transformer and dissipated as heat. This type of energy loss is called eddy
current loss of transformer.
35. How
To Minimize Eddy Current Loss?
- By using less thickness of laminations
- By using less value of Magnetic flux density
36. What
Is Hysteresis Loss In Transformer?
The magneto motive force or
mmf applied in the transformer core is alternating. For every cycle due to this
domain reversal, there will be extra work done. For this reason, there will be
a consumption of electrical energy which is known as Hysteresis loss of
transformer.
37. How
To Minimize Hysteresis Loss?
- By using good magnetic material.
- By using less value of Magnetic flux density.
38. Why
Are Iron Losses Considered As Constant Losses In Transformer?
Iron losses depend on
supply frequency and flux density in the core. For all normal operations, the
frequency of flux reversals which is same as supply frequency is constant and
the value of flux density more or less remains constant. Hence iron losses
remain constant under all load conditions. i-e from no-load to full-load.
39. Why
Transformers Are Rated In KVA ?
Copper loss of a
transformer depends on current and iron loss on voltage . Hence total losses
depends on Volt- Ampere and not on the power factor. That is why the rating of
transformers are in kVA and not in kW.
40. What Is The Condition For Maximum
Efficiency Of Transformer?
When Iron losses is equal
to copper losses.
41.
Define All Day Efficiency Of A Transformer?
It is computed on the basis
of energy consumed during a certain period, usually a day of 24 hrs. All day
efficiency=output in kWh/input in kWh for 24 hrs.
42. What
Are The Necessary Tests To Determine The Efficiency, Voltage Regulation, And
Temperature Rise Of Winding & Insulation Of Transformer
- Direct loading test
- Open circuit test
- Short circuit test
- Sumpner's or back to back test
43. What
Is Determined From Open Circuit Test?
Iron loss.
44. Why
OC Test Is Generally Performed On LV Side Of A Transformer?
The high-voltage side is
generally kept open because the current in high-voltage winding is less
compared to that on low-voltage winding. The LV side has higher current so that
maximum no load current can be measured.
45. Why
SC Test Is Generally Performed On HV Side Of A Transformer?
The rated current is less
on HV side. This will also permit to use ammeter and wattmeter of lower current
range.
46. Why
The Open Circuit Test On A Transformer Is Conducted At Rated Voltage?
The open circuit on a
transformer is conducted at a rated voltage because core loss depends upon the
voltage. This open circuit test gives only core loss or iron loss of the
transformer.
47. What Is Determined From Short Circuit
Test?
Copper loss.
48. What
Is Determined From Sumpner's Test?
Efficiency as well as
temperature rise of winding.
49. What
Is The Need For Parallel Operation Of Transformer?
- Non availability of single large transformer to meet the load
- Increased power demand
- To improve reliability
- If many smaller transformer is used one can be used as spare
- Transportation problem for large transformer
50. What
Are The Conditions For Parallel Operation Of Transformer?
- Equal polarity
- Equal turn ratio
- percentage impedance should be same
- Equal X/R ratio
- Equal KVA rating
- Equal phase sequence.
51. What
Will Happen If Transformer In Parallel Connection Have Opposite Polarity?
Connecting transformers
with wrong polarity can result in circulating currents or short circuits.
52. What
Will Happen If Transformer In Parallel Connection Have Different Voltage Ratio?
Two transformers in
Parallel should have the same primary and secondary voltage ratings. Any
error in the voltage ratio would cause heavy circulating currents to flow
between the transformers. This circulating current will result in a
corresponding imbalance in the primary currents, and result in overloading of
one transformer. This circulating current will result in increased copper
losses.
53. What
Will Happen If Transformer In Parallel Connection Have Different Percentage
Impedance Or X/r Ratio ?
A difference in the ratio
of the reactance value to resistance value of the per unit impedance results in
a different phase angle of the currents carried by the two paralleled
transformers; one transformer will be working with a higher power factor and
the other with a lower power factor than that of the combined output. Hence,
the real power will not be proportionally shared by the transformers.
54. What Will Happen If Transformer In
Parallel Connection Have Different Phase Sequence ?
If the phase sequence is in
an incorrect manner, in every cycle each pair of phases will get
short-circuited.
55. What Are The Typical Uses Of Auto
Transformer?
- As a booster to compensate the voltage drop for better regulation
- As induction motor starters.
- In locomotive
- As furnace transformer
56. List
The Merits Of An Autotransformer?
- Require less conductor material
- low cost
- low size
- high VA rating
- high efficiency
- Better voltage regulation
- small amount of no load current
57. What
Is Window Space Factor?
It is the ratio of copper
area in the window to the total window area.
58. What
Is Transformer Bank?
A transformer bank consists
of three independent single phase transformers with their primary and secondary
windings connected either in star or delta.
59. What
Are The Advantages Of Three Single Phase Transformer Over Three Phase
Transformer?
- Continuity of supply
- Less Installation of cost
- Easy transportation
- Stand by function
- Unbalanced load supply
60. What
Are The Advantages Of Three Phase Transformer Over Three Single Phase
Transformer?
- Saving in iron material
- Small size
- Less transformer oil
- Economical
- Higher efficiency
61. Why
Silica Gel Is Used In Breather?
The silica gel is used to
absorb the moisture when the air is drawn from the atmosphere in to the
transformer.
62. What Is The Function Of
Transformer Oil In A Transformer ?
Transformer
oil provides:
- good insulation and
- cooling
Nowadays instead of natural
mineral oil, synthetic oils known as ASKRELS (trade name ) are used. They are
non-inflammable, under an electric arc do not decompose to produce inflammable
gases. PYROCOLOR oil possess high dielectric strength.
63. Why
Transformer Oil Is Used As A Cooling Medium?
When
transformer oil is used as a coolant the heat dissipation by convection is 10
times more than the convection due to air. Hence transformer oil is used as a
cooling medium.
64. What
Is Conservator?
A conservator is a small
cylindrical drum fitted just above the transformer main tank. It is used to
allow the expansion and contraction of oil without contact with surrounding
atmosphere. When conservator is fitted in a transformer, the tank is fully
filled with oil and the conservator is half filled with oil.
65. What
Is Buchholz Relay?
It protects the transformer
from their internal faults like earth faults, winding short circuit, short
circuit between phases, Puncture of bushing etc.
66. Where
Is Buchholz Relay Located?
It is located between
transformer tank and conservator.
67. In
Transformers, Why The Low Voltage Winding Is Placed Near The Core?
The winding & Core are
both made of metals and so an insulation have to be placed in between them, the
thickness of insulation depends on the voltage rating of the winding. In order
to reduce the insulation requirement the low voltage winding place near the
core.
68. List
Some Methods Of Cooling Of Transformers?
Air natural, Air blast, Oil
Natural, Oil natural air forced, Oil natural water forced, Oil forced, Oil
forced air natural, Oil forced air natural, Oil forced water forced.
69. What
Are The Factors To Be Considered For Choosing The Method Of Cooling?
The choice of cooling
method depends on KVA rating of transformer, size, application and the site
conditions where it will be installed.
70. How
The Heat Dissipates In A Transformer?
The heat dissipation of a
transformer occurs by convection, conduction and radiation.
71. Why
Cooling Tubes Are Provided?
Cooling tubes are provided
to increase the heat dissipating area of the tank.
72. How
The Leakage Reactance Of The Transformer Is Reduced?
In transformers the leakage
reactance is reduced by interleaving the high voltage and low voltage
winding.
73. How
The Heat Dissipation Is Improved By Providing The Cooling Tubes?
The cooling tubes will
improve the circulation of oil. The circulation of oil is due to effective
pressure heads produced by columns of oil in tubes. The improvement in cooling
is accounted by taking the specific heat dissipation due to convection as 35%
more than that without tubes.
74. What
Is The Humming Of Transformer?
Humming is a sound, which
is produced due to the vibration of the cores in the transformer. The
vibrations are produced due to the change in polarity of an alternating current
or voltage and by the loose of lamination of the core. Both can be minimized by
tightening the core of the transformer.
75. Can 60 Hz Transformers Be Operated At 50
Hz?
The transformers rated
below 1 KVA can be used on 50 Hz service. Transformers 1 KVA and larger, rated
at 60 Hz, should not be used on 50 Hz service, due to the higher losses and
resultant heat rise. Special designs are required for this service. However,
any 50 Hz transformer will operate on a 60 Hz service.
76. List
Four Applications Of A Transformer?
- It can raise or lower the voltage or current in an AC circuit.
- It can act as an impedance transferring device by increasing or decreasing the value of a capacitor, inductor or resistance in an AC circuit.
- It can isolate two circuits electrically.
- It can be used to prevent DC from passing from one circuit to another
77. Why
Do Transformers Hum?
Transformer noise is caused
by a phenomenon which causes a piece of magnetic sheet steel to extend itself
when magnetized. When the magnetization is taken away, it goes back to its
original condition. This phenomenon is scientifically referred to as
magnetostriction. A transformer is magnetically excited by an alternating
voltage and current so that it becomes extended and contracted twice during a full
cycle of magnetization.
The magnetization of any
given point on the sheet varies, so the extension and contraction is not
uniform. A transformer core is made from many sheets of special steel to reduce
losses and moderate the ensuing heating effect. The extensions and contractions
are taking place erratically all over a sheet. These extensions are
proportionally and therefore not normally visible to the naked eye. However,
they are sufficient to cause a vibration, and consequently noise. Applying
voltage to a transformer produces a magnetic flux, or magnetic lines of force
in the core. The degree of flux determines the amount of magnetostriction and
hence, the noise level.
78. Why
Not Reduce The Noise In The Core By Reducing The Amount Of Flux?
Transformer voltages are
fixed by system requirements. The ratio of these voltages to the number of
turns in the winding determines the amount of magnetization. This ratio of
voltage to turns is determined mainly for economical soundness. Therefore the
amount of flux at the normal voltage is fixed. This also fixes the level of
noise and vibration.
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