Vedic Pivot: Transformer Interview Question & Answer

Question 1. What Is a Transformer?

Answer :

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

Question 2. How Does A Transformer Work?

Answer:

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.

Question 3. Can Dc Be Applied To Transformers?

Answer:

o 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.

o 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.

Question 4. What are The Various Types Of Transformers ?

Answer:

Based on supply

o Single phase transformer

o Three phase transformer

Based on winding

o Auto transformer (single winding)

o Two winding transformer

o Three winding transformer

o Six winding transformer

Based on construction

o Core Type transformer

o Shell Type transformer

Based on the service

o Distribution transformer

o Power transformer

Based on measurement

o Current transformer

o potential transformer

Based on cooling

o Dry type Transformer

o Oil immersed type transformer

Based on function

o Step up transformer

o Step down transformer

o Isolation transformer

Question 5. What Is Single And Three Phase Transformer?

Answer :

The transformer which works on single phase is called single phase transformer.
The transformer which works on three phase is called three phase transformer.

Question 6. What Is Auto-transformer?

Answer :

In these transformer only one winding is used as primary and secondary. Also, primary and secondary are conductively coupled.

Question 7. What Is Two Winding Transformer?

Answer :

Two separate windings one as primary and other as secondary are used. Both windings are magnetically coupled.

Question 8. What Is Three Winding Transformer?

Answer :

Three windings are used each work as primary and secondary. Also primary and secondary windings are conductively couple. They are three phase auto transformer.

Question 9. What Is Six Winding Transformer?

Answer :

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.

Question 10. What Are The Advantages Of Shell Type Transformer Over Core Type Transformers?

Answer :

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.

Question 11. Where Is Core Type And Shell Type Construction Suitable For A Transformer?

Answer :

Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. Whereas, Shell type transformers are popular in Low voltage applications like transformers used in electronic circuits and power electronic converters etc.

Question 12. What Are Power Transformer?

Answer :

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.

Question 13. What Are Distribution Transformer?

Answer :

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.

Question 14. Why The Efficiency Of Distribution Transformer Is 60 To 70 % And Not 100%?

Answer :

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.

Question 15. What Are The Salient Features Of Distribution Transformer?

Answer:

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

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

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

3. The leakage reactance and regulation will be low.

Question 16. What Are Current Transformer?

Answer :

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.

Question 17. What Are Potential Transformer?

Answer :

4. They are protective devices used to measure very high value of voltage in power system.

Question 18. What Are Dry Type Transformer ?

Answer :

In this type of transformer, air is used as coolant.The heat is taken to walls of tank and dissipated to the surrounding air.

Question 19. What Is Oil Immersed Type Transformer?

Answer :

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.

Question 20. What Are Step Up Transformers?

Answer :

They step up voltage from lower value to higher value.

Question 21. What Are Step Down Transformers?

Answer :

They step up voltage from higher value to lower value.

Question 22. What Are Isolation Transformers?

Answer :

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.

Question 23. Why Stepped Cores Are Used?

Answer :

o To reduce the space effectively.

o To obtain reduced length of mean turn of the winding.

o To reduce I² R loss.

Question 24. What Is Yoke Section Of Transformers?

Answer :

The sections of the core which connect the limbs are called yoke. The yoke is used to provide a closed path for the flux.

Question 25. What Is The Purpose Of Laminating The Core In A Transformer?

Answer :

In order to minimize eddy current loss.

Question 26. Explain On The Material Used For Core Construction?

Answer :

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.

Question 27. Why The Cross-section Of Iron Is Less Than Total Cross Section Area Of Core?

Answer :

This is because the core is laminated & on each lamination the insulation are are used.

Question 28. What Is Stack Factor?

Answer :

The ratio of total cross section of iron to the total cross section of core is called stack factor.

Question 29. What Are The Properties Of Ideal Transformer?

Answer :

o There should be no losses

o The winding should have zero resistance

o The leakage flux should be zero

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

Question 30. What Are The Functions Of No-load Current In A Transformer ?

Answer :

No-load current produces flux and supplies iron loss and copper loss on no-load.

Question 31. What Is The Condition For Zero Voltage Regulation?

Answer :

Negative sign indicates zero voltage regulation. It occurs when load is capacitive and power factor is leading.

Question 32. What Is The Condition For Maximum Voltage Regulation?

Answer :

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

Question 33. What Are The Factors Affecting Voltage Regulation?

Answer :

o Load current

o Equivalent resistance

o Equivalent reactance

o power factor

Question 34. What Is Eddy Current Loss In Transformer?

Answer :

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.

Question 35. How To Minimize Eddy Current Loss?

Answer :

By using less thickness of laminations
By using less value of Magnetic flux density

Question 36. What Is Hysteresis Loss In Transformer?

Answer :

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.

Question 37. How To Minimize Hysteresis Loss?

Answer :

By using good magnetic material.
By using less value of Magnetic flux density.

Question 38. Why Are Iron Losses Considered As Constant Losses In Transformer?

Answer :

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.

Question 39. Why Transformers Are Rated In Kva ?

Answer :

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.

Question 40. What Is The Condition For Maximum Efficiency Of Transformer?

Answer :

When Iron losses is equal to copper losses.

Question 41. Define All Day Efficiency Of A Transformer?

Answer :

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.

Question 42. What Are The Necessary Tests To Determine The Efficiency, Voltage Regulation, And Temperature Rise Of Winding & Insulation Of Transformer?

Answer :

o Direct loading test

o Open circuit test

o Short circuit test

o Sumner’s or back to back test

Question 44. Why OC Test Is Generally Performed On LV Side Of A Transformer?

Answer :

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.

Question 45. Why SC Test Is Generally Performed On HV Side Of A Transformer?

Answer :

The rated current is less on HV side. This will also permit to use ammeter and wattmeter of lower current range.

Question 46. Why The Open Circuit Test On A Transformer Is Conducted At Rated Voltage?

Answer :

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.

Question 47. What Is Determined From Short Circuit Test?

Answer :

Copper loss.

Question 48. What Is Determined From Sumner’s Test?

Answer :

Efficiency as well as temperature rise of winding.

Question 49. What Is The Need For Parallel Operation Of Transformer?

Answer :

o Non availability of single large transformer to meet the load

o Increased power demand

o To improve reliability

o If many smaller transformer is used one can be used as spare

o Transportation problem for large transformer.

Question 50. What Are The Conditions For Parallel Operation Of Transformer?

Answer :

o Equal polarity

o Equal turn ratio

o percentage impedance should be same

o Equal X/R ratio

o Equal KVA rating

o Equal phase sequence.

Question 51. What Will Happen If Transformer In Parallel Connection Have Opposite Polarity?

Answer :

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

Question 52. What Will Happen If Transformer In Parallel Connection Have Different Voltage Ratio?

Answer :

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.

Question 53. What Will Happen If Transformer In Parallel Connection Have Different Percentage Impedance Or X/r Ratio ?

Answer :

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.

Question 54. What Will Happen If Transformer In Parallel Connection Have Different Phase Sequence ?

Answer :

If the phase sequence is in an incorrect manner, in every cycle each pair of phases will get short-circuited.

Question 55. What Are The Typical Uses Of Auto Transformer?

Answer :

o As a booster to compensate the voltage drop for better regulation

o As induction motor starters.

o In locomotive

o As furnace transformer

Question 56. List The Merits Of An Auto-transformer?

Answer :

o Require less conductor material

o low cost

o low size

o high VA rating

o high efficiency

o Better voltage regulation

o small amount of no load current

Question 57. What Is Window Space Factor?

Answer :

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

Question 58. What Is Transformer Bank?

Answer :

A transformer bank consists of three independent single phase transformers with their primary and secondary wingdings connected either in star or delta.

Question 59. What Are The Advantages Of Three Single Phase Transformer Over Three Phase Transformer?

Answer :

o Continuity of supply

o Less Installation of cost

o Easy transportation

o Stand by function

o Unbalanced load supply

Question 60. What Are The Advantages Of Three Phase Transformer Over Three Single Phase Transformer?

Answer :

o Saving in iron material

o Small size

o Less transformer oil

o Economical

o Higher efficiency

Question 61. Why Silica Gel Is Used In Breather?

Answer :

The silica gel is used to absorb the moisture when the air is drawn from the atmosphere in to the transformer.

Question 62. What Is The Function Of Transformer Oil In A Transformer ?

Answer :

Transformer oil provides:

o good insulation and

o 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.

Question 63. Why Transformer Oil Is Used As A Cooling Medium?

Answer :

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.

Question 64. What Is Conservator?

Answer :

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.

Question 65. What Is Buchholz Relay?

Answer :

It protects the transformer from their internal faults like earth faults, winding short circuit, short circuit between phases, Puncture of bushing etc.

Question 66. Where Is Buchholz Relay Located?

Answer :

It is located between transformer tank and conservator.

Question 67. In Transformers, Why The Low Voltage Winding Is Placed Near The Core?

Answer :

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.

Question 68. List Some Methods Of Cooling Of Transformers?

Answer :

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.

Question 69. What Are The Factors To Be Considered For Choosing The Method Of Cooling?

Answer :

The choice of cooling method depends on KVA rating of transformer, size, application and the site conditions where it will be installed.

Question 70. How The Heat Dissipates In A Transformer?

Answer :

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

Question 71. Why Cooling Tubes Are Provided?

Answer :

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

Question 72. How The Leakage Reactance Of The Transformer Is Reduced?

Answer :

In transformers the leakage reactance is reduced by interleaving the high voltage and low voltage winding.

Question 73. How The Heat Dissipation Is Improved By Providing The Cooling Tubes?

Answer :

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.

Question 74. What Is The Humming Of Transformer?

Answer :

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 minimised by tightening the core of the transformer.

Question 75. Can 60 Hz Transformers Be Operated At 50 Hz?

Answer :

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.

Question 76. List Four Applications Of A Transformer?

Answer :

o It can raise or lower the voltage or current in an AC circuit.

o It can act as an impedance transferring device by increasing or decreasing the value of a capacitor, inductor or resistance in an AC circuit.

o It can isolate two circuits electrically.

o It can be used to prevent DC from passing from one circuit to another

Question 77. Why Do Transformers Hum?

Answer :

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.

Question 78. Why Not Reduce The Noise In The Core By Reducing The Amount Of Flux?

Answer :

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.

Question 78 What is earthing transformer & where is it used ?

Ans : An earthing transformer is a transformer which is intended primarily for the purpose of providing a neutral point for grounding purposes. It may be a two winding unit with a delta connected primary and a star connected secondary or a single winding three phase auto-transformer with windings in interconnected star or “Zig-Zag”. When the necessity arises for earthing the neutral of one part or another of on interconnected transmission or distribution network at a place where no natural neutral point is available, an earthing transformer is used for that purpose. The neutral point of the earthing transformer is connected to earth directly or through a current limiting impedance which the terminals are connected to the three phase lines.

Question 43. What Is Determined From Open Circuit Test?

Answer :

Iron loss.