TOP 100+ TRANSFORMER Interview Questions & Answers

Posted On:February 6, 2019, Posted By: Latest Interview Questions, Views: 3178, Rating :

Best TRANSFORMER Interview Questions and Answers

Dear Readers, Welcome to Transformer Interview Questions and Answers have been designed specially to get you acquainted with the nature of questions you may encounter during your Job interview for the subject of Transformer. These Transformer Questions are very important for campus placement test and job interviews. As per my experience good interviewers hardly plan to ask any particular questions during your Job interview and these model questions are asked in the online technical test and interview of many IT & Non IT Industries.

1. What is a Transformer?

A: A transformer is a static device which can transfer power from one circuit to another at same frequency. 

Interview Questions on TRANSFORMER

2. How Does a Transformer Work?

A:  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.


3. Can DC be applied to transformers ?


i) 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.

ii) 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. 


4. What are the various types of Transformers?

Based on supply

1.Single phase transformer 

2.Three phase transformer 

Based on winding

1.Auto transformer (single winding)

2.Two winding transformer 

3. Three winding transformer 

4. Six winding transformer 

Based on construction 

1.  Core Type transformer 

2.  Shell Type transformer 

Based on the service

1.  Distribution transformer 

2.  Power transformer 

Based on measurement

1.  Current transformer

2.  potential transformer 

Based on cooling

1.  Dry type Transformer

2. Oil immersed type transformer 

Based on function

1. Step up transformer 

2.Step down transformer

3.Isolation transformer 


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


12. What are power transformer?

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? 

1.  The distribution transformer will have low iron loss and higher value of copper loss 

2.  The capacity of transformers will be up to 500 KVA 

3.  The transformers will have plain walled tanks or provided with cooling tubes or radiators. 

4.  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? 

(i)  To reduce the space effectively. 

(ii) To obtain reduced length of mean turn of the windings. 

(iii) 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?

A: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 coreplate 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 is the EMF equation of transformer?

Erms = 4.44 f F maxT  volt


30. What is voltage transformation ratio or turn ratio?

It is expressed as ratio of the primary and secondary voltages of transformer.


It is expressed as ratio of the primary and secondary turns of transformer.

E1/ E2= T1/T2 = K


31. What is transformation ratio for step up, step down and isolation  transformer?

K>1 for step up transformer

K<1 for step down transformer

K=1 for isolation transformer


32. What are the properties of ideal transformer?

1.There should be no losses

2.The winding should have zero resistance

3.The leakage flux should be zero

4. The permeability of core should be so high that the negligible current is required to  establish the flux in it.


33. 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. 


34. Does the transformer draw any current when secondary is open ? Why ?

Yes,it (primary) will draw the current from the main supply in order to magnetize the core and to supply iron and copper losses on no load . There will not be any current in the secondary since secondary is open


35. What are two components of no load current in transformer?

i) IM (Magnetizing/reactive /wattless) current. It magnetizes core

ii) Iw (Coreloss/ active/wattful ) current. It supplies hysteresis and eddy current loss and negligible  I² R loss. 


36. How much is the no load current of transformer?

No load curent of transformer is 3 to 5% of rated current in transformer.


37. Give equivalent resistance, reactance and impedance referred to primary circuit.

Req = R1 +R2' =  R1+a² R2'

Xeq = X1 +X2' =  X1+a² X2'

Zeq = Z1 +Z2' =  Z1+a² Z2'

where a=T1/T2


38. Give equivalent resistance, reactance and impedance referred to secondary circuit.

Req = R2 +R1' =  R2+ R1'/a² 

Xeq = X2 +X1' =  X2+ X1'/a² 

Zeq = Z2 + Z1' =  Z2+ Z1'/a² 

where a=T1/T2


39. Why to calculate equivalent impedance referred to primary or secondary side?

Equivalent impedance is essential to be calculated because the electrical power transformer is an electric power system equipment, for estimating different parameters of electric power system which may be required to calculate total internal impedance of an electric power transformer, viewing from primary side or secondary side as per requirement. This calculation requires equivalent circuit of transformer referred to primary or equivalent circuit of transformer referred to secondary sides respectively.


40. What is Meant by Regulation in a Transformer?

Voltage regulation in transformers is the difference between the no load voltage and the full load voltage. 

This is usually expressed in terms of percentage. For example: A transformer delivers 100 volts at no load and the voltage drops to 95_volts at full load, the regulation would be 5%.

dry type distribution transformers generally have regulation from 2% to 4%, depending on the size and the application for which they are used.The  change  in  secondary  terminal  voltage from  no  load  to  full  load expressed as a percentage of no load or full load voltage istermed as regulation.

 %regulation =E2-V2/E2 *100

%regulation = voltage drop in transformer/ no load voltage of transformer

V2>E2 for leading p.f load

V2<E2 for lagging p.f load


41. 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.


42. What is the condition for maximum voltage regulation?

 It occurs when load is inductive and power factor is lagging.


43. What are the factors affecting voltage regulation?

Load current

Equivalent resistance 

Equivalent reactance 

power factor


44. 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 emfs, 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.


45. How to minimize Eddy current loss?

By using less thickness of laminations

By using less value of Magnetic flux density


46. 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.


47. How to minimize Hysteresis loss?

By using good magnetic material.

By using less value of Magnetic flux density


48. 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


49. why transformers are rated in kVA ? 

A: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. 


50. Define efficiency of the transformer? 

Transformer efficiency ?= (output power/input power) x 100


51. What is the range of efficiency of transformers? 

A:The efficiency of the transformer will be in the range of 94% to 99%. Among the available electrical machines the transformer has the highest efficiency 


52. Why the range of efficiency in transformers higher than those of other electrical machines?

Transformers operate at higher efficiency when compared to other electrical machines.This is due to the absence of mechanical losses which is due to the absence of moving parts


53. What is the condition for maximum efficiency of transformer?

When Iron losses is equal to copper losses.


54. Define all day efficiency of a transformer?

A: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.


55. What are the necessary tests to determine the efficiency, voltage regulation, and temperature rise of winding & insulation of transformer?

1.Direct loading test

2.Open circuit test

3. Short circuit test

4. Sumpner's or back to back test


56. What is determined from open circuit test?

Iron loss


57. 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.


58. 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.


59. Why the open circuit test on a transformer is conducted at rated voltage?

A: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.


60. What is determined from short circuit test?

Copper loss


61. What is determined from Sumpner's test?

Efficiency as well as temperature rise of winding.


62. Why Sumpner's test is carried out?

Loading a transformer is a very difficult thing to test. Say the transformer is rated at 300 MVA, now how to find a 300MVA load to test your transformer? Also the load will waste all of its energy to heat. Sumpner's test allows to fully load a transformer without actually having that load. The energy lost is only equal to the losses in the transformers, nothing which the transformer cooling system cannot handle. In Sumpner's test two transformers are connected back to back means,primaries of two transformers connected in parallel and secondaries  side connected in series. 


63. 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.


64. 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


65. What will happen if transformer in parallel connection have opposite polarity?

Connecting transformers with wrong polarity can result in circulating currents or short circuits


66. 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.


67. 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.


68. 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.


69. What are the typical uses of auto transformer ? 

i) As a booster to compensate the voltage drop for better regulation

ii)As induction motor starters.

iii) In locomotive

iv) As furnace transformer  


70. 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 


71. Why is an auto-transformer not used as a distribution transformer? 

The autotransformer cannot provide isolation between HV and LV side. Due to open circuit in the common portion, the voltage on the load side may shoot up to dangerously high voltage causing damage to equipment. This unexpected rise in the voltage on LV side is potentially dangerous. Hence the autotransformer cannot be used as distribution transformer


72. What are the factors affecting the selection of Maximum flux density?

1. Net core weight: Higher the flux density lesser net weight of core.

2.Saving in copper:Higher the flux density lesser mean turn of windings.

3.Overall economy: Higher the flux density lesser net weight of core and windings. Hence cheaper

4.Net weight: Due to above reasons weight is also reduced.

5. High magnetizing current due to higher flux density

6. High Flux density results in high losses

7. High Flux density results in high losses and hence efficiency decreases

8.Hence high temperature rise and noise


73. What is window space factor?

It is the ratio of copper area in the window to the total window area.


74. 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. 


75. What are the advantages of three single phase transformer over three phase transformer?

1. Continuity of supply

2. Less Installation of cost

3. Easy transportation

4.Stand by function

5.Unbalanced load supply


76. What are the advantages of three phase transformer over three single phase transformer?

1.Saving in iron material

2. Small size

3. Less transformer oil

4. Economical

5. Higher efficiency


77. What are the various types of three phase transformer connections?

 The most common types of transformer connections are, 

i. Star-Star (Y-Y)

ii. Delta-Delta(?-?) 

iii. Star-Delta (Y-?) 

iv. Delta-Star (?-Y) 

v. Open Delta (V-V)

vi. Scott Connection (T-T)


78. Which type of connection is used in distribution transformer?

Distribution transformers are connected in delta-star. This arrangement requires 3 conductors in the high voltage side and 4 in the low voltage side, as well as it provides the star point conductor as a neutral point. This can serve single phase as well as three phase loads.


79. Why are breathers used in transformers ? 

Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil.

 Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases.  

Also to avoid sledging of oil i.e. decomposition of oil. 


80. 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. 


81. What is the colour of silica gel in dry stage and when saturated with moisture in a breather?


Dry stage – Deep blue 

Saturated stage – Whitish pink


82. What is the function of transformer oil in a transformer ? 

Transformer oil provides , (i)good insulation and (ii)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.


83. Mention the properties of oil used in transformers. 

The following are the desirable properties of transformer oil: 

-->It should be free from moisture

--> It should have high dielectric strength

--> It should have thermally stability and higher thermal conductivity 

-->It should be contaminated by temperature rise.


84. 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. 


85. 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. 


86. 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..


87. Where is buchholz relay located?

It is located between transformer tank and conservator.


88. In buchholz relay how the top and bottom mercury switches are connected?

Top mercury switch is connected for alarm and bottom switch for trip.


89. What is the purpose of providing Taps in transformer and where these are provided?

In order to attain the required voltage, taps are provided, normally at high voltages side(low current).


90. 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. 


91. Why the tapings are placed on HV side?

1.A fine voltage regulation is possible with hV side as it has large number of turns.

2.LV side carries large current

3. HV side is placed outside so its feasible

4. In case of step down transformer at light loads the HV side voltage increases, it is required to decrease this voltage by adjusting tappings on HV . With large no. of turns the flux and flux density decreases. Hence core losses decreases results in higher efficiency.


92. List some methods of cooling of transformers. 

A: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. 


93. 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. 


94.  How the heat dissipates in a transformer? 

The heat dissipation of a transformer occurs by convection, conduction and radiation. 


95. Why cooling tubes are provided? 

Cooling tubes are provided to increase the heat dissipating area of the tank. 


96.  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. 


97.  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. 


98. How does change in frequency affect the operation of a given transformer?

With a change in frequency, iron and copper loss, regulation, efficiency & heating varies so the operation of transformer is highly affected.


99. What is the Difference Between "Insulating", "Isolating", and "Shielded Winding" Transformers?

Insulating and Isolating transformers are identical. These terms are used to describe the isolation of the primary and secondary windings, or insulation between the two. A shielded winding transformer, on the other hand, is designed with a metallic shield between the primary and secondary windings, providing a safety factor by grounding, thus preventing accidental contact between windings under faulty conditions. All two, three and four winding transformers are of the insulating or isolating types. Only autotransformers, which are a type whose primary and secondary are connected to each other electrically, are not of the insulating or isolating variety.


100.  Can Transformers be Operated at Voltages other than Nameplate Voltages?

 In some cases, transformers can be operated at voltages below the nameplate rated voltage. In NO case should a transformer be operated at a voltage in excess of its name- plate rating unless taps are provided for this purpose. When operating below the rated voltage the KVA capacity is reduced correspondingly. For example, if a 480 volt primary trans¬former with a 240 volt secondary is operated at 240 volts, the secondary voltage is reduced to 120 volts and if the trans¬former were originally rated 10 KVA, the reduced rating would be 5 KVA, or in direct proportion to the applied voltage.