Interview Questions.

Question 1. What Is A Transformer?

Answer :

A transformer is a static device that could switch electricity from one circuit to another at identical frequency.

Question 2. How Does A Transformer Work?

Answer :

Transformer consists of two coils.If one coil is hooked up with ac voltage supply then it will produce alternating flux within the middle. Most of the flux is related with 2nd coil hence at the same time induced emf will produce within the 2d coil as according to faraday's law of electromagnetic induction.

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Question 3. Can Dc Be Applied To Transformers ?

Answer :

No

Transformer works on Faraday's law of Electromagnetic Induction for which present day in coil should exchange. If DC is applied contemporary will not change and transformer will now not paintings.
Practically winding resistance is very small. For DC ,inductive reactance is zero and frequency is zero. Therefore impedance is low. Thus winding draws more cutting-edge which may also damage the winding.
Question four. What Are The Various Types Of Transformers?

Answer :

Based on supply

Single phase transformer 
Three segment transformer 
Based on winding

Auto transformer (single winding)
Two winding transformer 
Three winding transformer 
Six winding transformer 
Based on creation 

Core Type transformer 
Shell Type transformer 
Based at the carrier

Distribution transformer 
Power transformer 
Based on size

Current transformer
capability transformer 
Based on cooling

Dry type Transformer
Oil immersed type transformer 
Based on function

Step up transformer 
Step down transformer
Isolation transformer
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Question five. What Is Single And Three Phase Transformer?

Answer :

The transformer which matches on unmarried phase is known as single phase transformer.
The transformer which matches on 3 segment is called 3 phase transformer.

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Question 6. What Is Auto-transformer?

Answer :

In those transformer simplest one winding is used as number one and secondary. Also, number one and secondary are conductively coupled.

Question 7. What Is Two Winding Transformer?

Answer :

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

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Question eight. What Is Three Winding Transformer?

Answer :

Three windings are used each paintings as number one and secondary. Also number one and secondary windings are conductively couple. They are three phase car transformer.

Question nine. What Is Six Winding Transformer?

Answer :

Three windings in number one and three windings in secondary are used, They are 3 section transformers.They are connected in both megastar-superstar, delta-delta, celebrity delta or delta star.

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

Answer :

In shell kind transformers the coils are well supported at the all aspects and with a view to withstand better mechanical stresses developed all through brief circuit situations. Also the leakage reactance can 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 packages like Distribution transformers, Power transformers, and manifestly auto transformers. Where as, Shell type transformers are popular in Low voltage programs like transformers used in digital circuits and electricity digital converters etc

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Question 12. What Are Power Transformer?

Answer :

Power transformer is used for the transmission reason at heavy load, high voltage greater than 33 KV & a hundred% performance. It is huge in length compared to distribution transformer, it utilized in generating station and Transmission substation. It is generally rated above 200MVA.

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Question 13. What Are Distribution Transformer?

Answer :

The distribution transformer is used for the distribution of electrical energy at low voltage as much less than 33KV in business cause and 440v-220v in home purpose. It works at low efficiency at 60-70%, small size, easy in set up, having low magnetic losses & it isn't constantly completely loaded.

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

Answer :

Distribution transformer is designed for maximum efficiency at 60% to 70% load because it typically doesn’t operate at full load all of the time. Its load depends on distribution call for. Whereas energy transformer is designed for optimum efficiency at a hundred% load because it constantly runs at 100% load being near to producing station.

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

Answer :

The distribution transformer could have low iron loss and higher fee of copper loss 
The potential of transformers may be as much as 500 KVA 
The transformers will have undeniable walled tanks or provided with cooling tubes or radiators. 
The leakage reactance and regulation will be low. 
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Question sixteen. What Are Current Transformer?

Answer :

They are protective devices used to degree very high price of contemporary in  electricity system. The Current Transformer ( C.T. ), is a form of “device transformer” this is designed to produce an alternating modern-day in its secondary winding that is proportional to the modern-day being measured in its number one.

Question 17. What Are Potential Transformer?

Answer :

They are defensive devices used to degree very excessive price of voltage in  power device.

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Question 18. What Are Dry Type Transformer ?

Answer :

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

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Question 19. What Is Oil Immersed Type Transformer?

Answer :

In this sort of transformer, oil is used as coolant. Entire assembly which includes center and windings is stored immersed in oil. The advanced heat is transferred to the partitions of tank thru oil. Finally the heat is transferred to the encircling air from tank wall by radiation.

Question 20. What Are Step Up Transformers?

Answer :

They step up voltage from decrease fee to better cost.

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Question 21. What Are Step Down Transformers?

Answer :

They step up voltage from higher price to decrease fee.

Question 22. What Are Isolation Transformers?

Answer :

In this kind of transformer the number one and secondary are identical. When it is required to isolate the number one and secondary circuits these form of transformers are used. It protects the circuits linked in secondary side whilst fundamental fault takes place in number one facet.

Question 23. Why Stepped Cores Are Used?

Answer :

To lessen the distance efficaciously. 
To gain reduced period of imply turn of the windings. 
To lessen  I² R loss. 
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Question 24. What Is Yoke Section Of Transformers?

Answer :

The sections of the core which connect the limbs are referred to as yoke. The yoke is used to offer a closed path for the flux.

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

Answer :

In order to reduce eddy current loss.

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

Answer :

The  center  is  constructed  by way of  sheet  metallic  laminations  assembled  to  provide  a  continuous magnetic  course  with  minimal  of  air  hole  blanketed.  The  steel  used  is  of  high  silicon content on occasion  heat  handled  to  produce  a  high  permeability  and  a  low  hysteresis  loss at  the  regular operating  flux  densities.  The  eddy  modern  loss  is  minimized  via laminating  the  middle,  the laminations being used from each different via light coat of coreplate varnish or by way of oxide layer on the floor. The thickness of lamination varies from zero.35mm for a frequency of 50Hz and zero.5mm for a frequency of 25Hz.

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

Answer :

This is due to the fact the core is laminated & on each lamination the insulations are are used.

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Question 28. What Is Stack Factor?

Answer :

The ratio of general cross segment of iron to the overall move segment of middle is known as stack factor.

Question 29. What Are The Properties Of Ideal Transformer?

Answer :

There need to be no losses
The winding ought to have zero resistance
The leakage flux have to be 0
The permeability of middle must be so excessive that the negligible modern is required to  set up the flux in it.
Question 30. What Are The Functions Of No-load Current In A Transformer ?

Answer :

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

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

Answer :

Negative sign suggests 0 voltage law. It takes place whilst load is capacitive and electricity thing is leading.

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

Answer :

It takes place whilst load is inductive and strength thing is lagging.

Question 33. What Are The Factors Affecting Voltage Regulation?

Answer :

Load current
Equivalent resistance 
Equivalent reactance 
strength factor
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Question 34. What Is Eddy Current Loss In Transformer?

Answer :

In transformer, we supply alternating modern in the primary, this alternating cutting-edge produces alternating magnetizing flux inside the center and as this flux links with secondary winding, there may be prompted voltage in secondary, resulting present day to flow through the weight connected with it. Some of the alternating fluxes of transformer; may additionally hyperlink with different undertaking components like metal center or iron frame of transformer etc. As alternating flux hyperlinks with these components of transformer, there might be a locally induced emf. Due to those emfs, there would be currents so that you can flow into locally at that elements of the transformer. These circulating current will not make contributions in output of the transformer and dissipated as warmness. This sort of electricity loss is called eddy current lack of transformer.

Question 35. How To Minimize Eddy Current Loss?

Answer :

By the usage of less thickness of laminations
By the usage of much less fee of Magnetic flux density

Question 36. What Is Hysteresis Loss In Transformer?

Answer :

The magneto purpose pressure or mmf carried out inside the transformer core is alternating. For every cycle due to this domain reversal, there will be more work performed. For this cause, there can be a consumption of electrical energy that is called Hysteresis lack of transformer.

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Question 37. How To Minimize Hysteresis Loss?

Answer :

By using desirable magnetic cloth.
By using much less cost of Magnetic flux density.

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

Answer :

Iron losses rely upon deliver frequency and flux density in the center. For all everyday operations, the frequency of flux reversals that's identical as supply frequency is consistent and the value of flux density greater or less stays steady. Hence iron losses remain regular underneath all load situations. I-e from no-load to complete-load.

Question 39. Why Transformers Are Rated In Kva ?

Answer :

Copper loss of a transformer depends on modern-day and iron loss on voltage . Hence general losses relies upon on Volt- Ampere and no longer at the power component. 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 identical to copper losses.

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Question 41. Define All Day Efficiency Of A Transformer?

Answer :

It is computed on the basis of power ate up at some stage in a certain length, generally an afternoon of 24 hrs. All day performance=output in kWh/input in kWh for twenty-four hrs.

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

Answer :

Direct loading test
Open circuit test
Short circuit check
Sumpner's or again to again test
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Question 43. What Is Determined From Open Circuit Test?

Answer :

Iron loss.

Question 44. Why Oc Test Is Generally Performed On Lv Side Of A Transformer?

Answer :

The high-voltage aspect is usually kept open because the modern in high-voltage winding is less as compared to that on low-voltage winding.The LV side has higher current so that maximum no load modern can be measured.

Question forty five. Why Sc Test Is Generally Performed On Hv Side Of A Transformer?

Answer :

The rated cutting-edge is less on HV facet. This will also permit to apply ammeter and wattmeter of decrease present day variety.

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

Answer :

The open circuit on a transformer is carried out at a rated voltage due to the fact center loss relies upon upon the voltage. This open circuit take a look at offers best center 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 Sumpner's Test?

Answer :

Efficiency in addition to temperature rise of winding.

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

Answer :

Non availability of single massive transformer to satisfy the burden
Increased electricity call for
To improve reliability
If many smaller transformer is used one may be used as spare
Transportation problem for massive transformer.
Question 50. What Are The Conditions For Parallel Operation Of Transformer?

Answer :

Equal polarity
Equal flip ratio
percentage impedance need to be same
Equal X/R ratio
Equal KVA rating
Equal section collection.
Question 51. What Will Happen If Transformer In Parallel Connection Have Opposite Polarity?

Answer :

Connecting transformers with incorrect polarity can bring about circulating currents or brief circuits.

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

Answer :

Two transformers in Parallel should have the identical primary and secondary voltage ratings.  Any error within the voltage ratio might motive heavy circulating currents to flow between the transformers. This circulating modern-day will result in a corresponding imbalance inside the primary currents, and result in overloading of one transformer.  This circulating modern-day will result in elevated copper losses.

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

Answer :

A difference within the ratio of the reactance value to resistance value of the in line with unit impedance consequences in a unique phase attitude of the currents carried by way of the 2 paralleled transformers; one transformer will be working with a better power factor and the opposite with a decrease strength thing than that of the combined output. Hence, the real energy will now not be proportionally shared with the aid of the transformers.

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

Answer :

If the section sequence is in an incorrect way, in each cycle every pair of levels will get short-circuited.

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

Answer :

As a booster to compensate the voltage drop for higher law
As induction motor starters.
In locomotive
As furnace transformer 
Question 56. List The Merits Of An Autotransformer?

Answer :

Require less conductor material
low value
low size
high VA rating
high performance
Better voltage law
small amount of no load modern 
Question 57. What Is Window Space Factor?

Answer :

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

Question 58. What Is Transformer Bank?

Answer :

A transformer financial institution consists of 3 unbiased unmarried segment transformers with their number one and secondary windings linked both in star or delta.

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

Answer :

Continuity of deliver
Less Installation of cost
Easy transportation
Stand by way of characteristic
Unbalanced load supply
Question 60. What Are The Advantages Of Three Phase Transformer Over Three Single Phase Transformer?

Answer :

Saving in iron material
Small length
Less transformer oil
Economical
Higher performance
Question sixty one. Why Silica Gel Is Used In Breather?

Answer :

The silica gel is used to soak up the moisture when the air is drawn from the surroundings in to the transformer.

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

Answer :

Transformer oil presents:

exact insulation and
cooling .
Nowadays in preference to natural mineral oil, synthetic oils referred to as ASKRELS (trade name ) are used. They are non-inflammable, below an electric arc do no longer decompose to supply inflammable gases. PYROCOLOR oil possess excessive dielectric electricity.

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

Answer :

When transformer oil is used as a coolant the warmth dissipation by using 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 equipped simply above the transformer most important tank. It is used to permit the enlargement and contraction of oil without contact with surrounding ecosystem. When conservator is equipped in a transformer, the tank is absolutely full of oil and the conservator is half of filled with oil.

Question sixty five. What Is Buchholz Relay?

Answer :

It protects the transformer from their inner faults like earth faults, winding quick circuit, brief circuit between phases, Puncture of bushing and so on.

Question sixty six. Where Is Buchholz Relay Located?

Answer :

It is placed between transformer tank and conservator.

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

Answer :

The winding & Core are both fabricated from metals and so an insulation have to be placed in among them, the thickness of insulation relies upon at the voltage rating of the winding. In order to lessen the insulation requirement the low voltage winding location close to the middle. 

Question 68. List Some Methods Of Cooling Of Transformers?

Answer :

Air herbal, Air blast, Oil Natural, Oil natural air pressured, Oil natural water forced, Oil forced, Oil pressured air natural, Oil pressured air natural, Oil pressured water forced. 

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

Answer :

The desire of cooling technique depends on KVA score of transformer, size, utility and the site conditions in which it will be established. 

Question 70. How The Heat Dissipates In A Transformer?

Answer :

The warmness dissipation of a transformer occurs with the aid of convection, conduction and radiation.

Question seventy one. Why Cooling Tubes Are Provided?

Answer :

Cooling tubes are provided to boom the warmth dissipating area of the tank. 

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

Answer :

In transformers the leakage reactance is decreased via interleaving the high voltage and coffee voltage winding. 

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

Answer :

The cooling tubes will improve the movement of oil. The circulate of oil is because of powerful strain heads produced by means of columns of oil in tubes. The development in cooling is accounted by taking the precise warmness dissipation due to convection as 35% more than that without tubes.

Question 74. What Is The Humming Of Transformer?

Answer :

Humming is a sound, that's produced due to the vibration of the cores within the transformer. The vibrations are produced due to the change in polarity of an alternating present day or voltage and with the aid of the unfastened of lamination of the center. Both can be minimised by means of tightening the core of the transformer.

Question seventy five. Can 60 Hz Transformers Be Operated At 50 Hz?

Answer :

The transformers rated underneath 1 KVA can be used on 50 Hz carrier. Transformers 1 KVA and large, rated at 60 Hz, ought to now not be used on 50 Hz provider, due to the higher losses and resultant heat upward thrust. Special designs are required for this carrier. However, any 50 Hz transformer will function on a 60 Hz service.

Question seventy six. List Four Applications Of A Transformer?

Answer :

It can boost or decrease the voltage or contemporary in an AC circuit.
It can act as an impedance shifting device with the aid of growing or reducing the price of a capacitor, inductor or resistance in an AC circuit.
It can isolate two circuits electrically.
It may be used to save you DC from passing from one circuit to every other
Question seventy seven. Why Do Transformers Hum?

Answer :

Transformer noise is resulting from a phenomenon which reasons a chunk of magnetic sheet metal to increase itself when magnetized. When the magnetization is taken away, it is going lower back to its original condition. This phenomenon is scientifically called magnetostriction. A transformer is magnetically excited by way of an alternating voltage and contemporary in order that it turns into extended and shrunk two times at some point of a complete cycle of magnetization.

The magnetization of any given point on the sheet varies, so the extension and contraction isn't uniform. A transformer middle is crafted from many sheets of special metal to reduce losses and moderate the ensuing heating impact. The extensions and contractions are taking region unevenly throughout a sheet. These extensions are proportionally and therefore no longer generally seen to the naked eye. However, they may be enough to cause a vibration, and consequently noise. Applying voltage to a transformer produces a magnetic flux, or magnetic lines of pressure inside the core. The degree of flux determines the amount of magnetostriction and for this reason, the noise level.

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

Answer :

Transformer voltages are fixed by way of system requirements. The ratio of these voltages to the wide variety of turns within the winding determines the amount of magnetization. This ratio of voltage to turns is determined particularly for cost-effective soundness. Therefore the amount of flux at the normal voltage is constant. This additionally fixes the extent of noise and vibration.