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Parts of a Power Transformer

Electrical and Automation Engineer . Specialized in LV Switchgear Design and process automation

Components of a transformer

Components of a transformer

What Is a Transformer?

A transformer is an electrical device that transfers electrical energy from one circuit to another by electromagnetic induction (also called transformer action). It is used to step up or step down AC voltage.

Basic Parts of a Transformer

These are the basic components of a transformer:

  1. Laminated core
  2. Windings
  3. Insulating materials
  4. Transformer oil
  5. Tap changer
  6. Oil Conservator
  7. Breather
  8. Cooling tubes
  9. Buchholz Relay
  10. Explosion vent

Of the above, laminated soft iron core, windings and insulating material are the primary parts and are present in all transformers, whereas the rest can be seen only in transformers having a capacity of more than 100 kVA.




The core acts as a support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends on factors such as voltage, current, and frequency. The diameter of the transformer core is directly proportional to copper loss and is inversely proportional to iron loss. If the diameter of the core is decreased, the weight of the steel in the core is reduced, which leads to less core loss of the transformer and the copper loss increase. When the diameter of the core is increased, the opposite occurs.

Why Are Windings Made of Copper?

  • Copper has high conductivity. This minimizes losses as well as the amount of copper needed for the winding (volume & weight of winding).
  • Copper has high ductility. This means it is easy to bend conductors into tight windings around the transformer's core, thus minimizing the amount of copper needed as well as the overall volume of the winding.


Two sets of winding are made over the transformer core and are insulated from each other. Winding consists of several turns of copper conductors bundled together and connected in series.

Winding can be classified in two different ways:

  1. Based on the input and output supply
  2. Based on the voltage range

Within the input/output supply classification, winding is further categorized:

  1. Primary winding – These are the winding to which the input voltage is applied.
  2. Secondary winding – These are the winding to which the output voltage is applied.

Within the voltage range classification, winding is further categorized:

  1. High voltage winding – It is made of copper conductor. The number of turns made shall be the multiple of the number of turns in the low voltage winding. The conductor used will be thinner than that of the low voltage winding.
  2. Low voltage winding – It consists of a fewer number of turns than the high voltage winding. It is made of thick copper conductors. This is because the current in the low voltage winding is higher than that of high voltage winding.

Input supply to the transformers can be applied from either low voltage (LV) or high voltage (HV) winding based on the requirement.

Insulating Materials

Insulating paper and cardboard are used in transformers to isolate primary and secondary winding from each other and from the transformer core.

Transformer oil is another insulating material. Transformer oil performs two important functions: in addition to insulating function, it can also cool the core and coil assembly. The transformer's core and winding must be completely immersed in the oil. Normally, hydrocarbon mineral oils are used as transformer oil. Oil contamination is a serious problem because contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.

Parts of the transformer.

Parts of the transformer.


The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer. The conservator tank is vented to the atmosphere at the top, and the normal oil level is approximately in the middle of the conservator to allow the oil to expand and contract as the temperature varies. The conservator is connected to the main tank inside the transformer, which is completely filled with transformer oil through a pipeline.




The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator. Pressure changes are balanced by a flow of atmospheric air in and out of the conservator, which is how moisture can enter the system.

If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free.

The transformer's breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by the silica crystals. The breather acts like an air filter for the transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe.

Tap Changer

Tap Changer

Tap Changer

The output voltage of transformers varies according to their input voltage and the load. During loaded conditions, the voltage on the output terminal decreases, whereas during off-load conditions, the output voltage increases. In order to balance the voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. In an on-load tap changer, the tapping can be changed without isolating the transformer from the supply. In an off-load tap changer, it is done after disconnecting the transformer. Automatic tap changers are also available.

Cooling Tubes

Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises, the hot oil naturally rises to the top, and the cold oil sinks downward. Thus the oil naturally circulates through the tubes. In forced circulation, an external pump is used to circulate the oil.

Buchholz Relay

The Buchholz Relay is a protective device container housed over the connecting pipe from the main tank to the conservator tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that is operated by the gases emitted during the decomposition of transformer oil during internal faults. It helps in sensing and protecting the transformer from internal faults.

Explosion Vent

The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained above the level of the conservatory tank.

More About Transformers

I have written a series of articles to help the reader understand power transformers. I've listed two here, and if you are interested in finding more, you can find them by clicking on my author profile at the top of this article.

How Does a Transformer Work – Basic working principles of the transformer.

Transformer FAQ

  • Transformer FAQ | Interview Questions
    The transformer is an electric device used to alter the voltage levels of an AC electric circuit. List of the most important FAQs in the electric transformer.


reny diamond on August 26, 2020:


Ajjaiah on July 09, 2020:

How to find out oil,, good conditions

Edet S. Clement on June 02, 2020:

It's nice, I really love it.

But my question is, what are the types of armour cables use in installation of 11kva, 33kva, 132kv & 330kv

ope Shina on May 23, 2020:

I love this

Nicholas mase on April 04, 2020:

What of tx structure from line

daniel on January 03, 2020:

center tap transformer?

Is taped from Lv or Hv ?

Thankes For the abuot ppt analysing !!

Premprakash singh on December 30, 2019:


Good morning.

peter sanga on December 27, 2019:

how to differentiate primary and secondary winding

clifforn momanyi on November 19, 2019:

thanks for making as to understand how the transformer works

Farhat munir on November 09, 2019:

If u have one pic top of transformer oltc oil conection partsname and function

Lakshman on November 06, 2019:

Excellent information sir

Venky on September 14, 2019:

Your know what is transformer and how is it works. But I don't know the capacity of transformer

PRIYANSHU KUMAR SINGH on September 02, 2019:

Sir if Diameter of Transformer is directly proportional to copper loss then shouldn't it be decreasing with decrease in Dia instead of increasing as you have mentioned

Amitabha Manna on August 17, 2019:

Very Vital Part Of Electrical Enginearing.

I Am Now study Of Junior Enginearing

ANKUR YADAV on August 06, 2019:



Eric Oduwah on July 11, 2019:

Please for transformers are they base on the principles of mutual inductance OR faraday's law.

SK myni on June 25, 2019:

Sir pls tell me which transformer is best current transformer or potential transformer

Akhilesh Kumar on June 25, 2019:

sir, your notes is more well

Rajat Rajendra Thorve on June 22, 2019:


babu lal jatiya on June 13, 2019:


Anil Patel on June 05, 2019:

Which material use when LV busbar joint time some luddi type put on LV Busbar and jonit and bresing. I forgot that material name.some one say asbestos shit .anybody help the find name.

Phel on May 29, 2019:

Thanks sir

dhilleswara rao on May 24, 2019:


elisha sanga on May 14, 2019:

Thanks sir

Mukesh Poddar on April 23, 2019:


Khan abutaha on April 07, 2019:

Thanks sir

CK.Lakshminarayana on April 03, 2019:

Exclent topic for Engineering students ,

santhakumar on March 26, 2019:

such a nice article thank you so much

Narendra Singh rathore on February 07, 2019:

Electricity is very important dont work do work depend all people

Amzad on February 01, 2019:

please explain about the drain coke.

Philip Tsetsekple on January 16, 2019:

thanks for helping me understand my reseach

ZAKIR HUSSAIN on January 07, 2019:


Shrimant on December 21, 2018:

What is the core tansformer

Sowmya on November 26, 2018:

Hello..! It is nice to learn n helpful.. But i would like to know much more in deep regarding the transformer action & briefly about the conversion process which differs from theoretical to practical.. So i wanted to learn little more for my practice.. Thankyou..

Nanaji on November 21, 2018:

Hi sir good evening parts of transformer nice sir

Ajjaih on November 16, 2018:

Thank u sir

endyboy on October 29, 2018:

it was an awesome experience in this website. i pray for further upgrading, thanks

Jwala nayak on October 21, 2018:

Very well explanation

Saumya on October 21, 2018:

Very nicely explained but there should be some diagrams also for our convinence

Saumya on October 21, 2018:

Very well explained

Dinesh Kumar on August 29, 2018:

Dg and transformer rating from low to high explain and DG rating depends upon what factor in company and industries,home , residential complex etc ..

JSV on August 19, 2018:

Thanks for your article, Sir.

MAAN Shahzada on August 15, 2018:

Thanks sir for me help

ishrat gull on August 06, 2018:

thanks, sir

Nahabwe Bruno on August 03, 2018:

Thanks for that helpful information

Shweta Pandit on July 30, 2018:

Nice explanation...

My question is what are the various insulating materials used in trasformer and for what purpose?

Prakash.b on July 14, 2018:

Super notes

Mahendra Patil on July 04, 2018:

Send to HT to LT voltage and current formula.

salman on June 30, 2018:

nice explanation


Ashish mundele on May 22, 2018:

Very nice explen sr.

Dibya on May 10, 2018:

Please send electrical book

Sundaraperumal on May 10, 2018:


Sai on May 08, 2018:

Give more information about the how the instrument in transformers are made up and some other things which are not in books

Emmanuel Mba Santumah on May 03, 2018:

Thanks a lot, you are really ready to help. Great work.

eldhose k shaju on April 28, 2018:

very much helpful information..thank you for your help...

Parag on April 22, 2018:

Very Nice, simply understood about transformer operation

Solomon on April 18, 2018:

Now i understand better,well done.

pury on April 16, 2018:

well done

Shaik Ali shabbeer on April 16, 2018:

Superb and nice

Visi on April 10, 2018:


prashanth kuamr on March 22, 2018:

Short answer and very nice

Rakesh Kumar on March 16, 2018:

Very useful article and perfectly explained details of transformer. Which is easy to understand.

RUTH AMANKWAH on March 06, 2018:

Well done,

The arrangement and the simplification of the concept is understandable and meaningful for a certain standard.

Nisha on March 02, 2018:

Easy to understand..

U have any u tube channel..

abidoye smuel on February 14, 2018:

this is a very good lecture

Bharamana gouda on February 07, 2018:


Kiran kumar on February 05, 2018:

Very a simple answer in transformer parts

Dinesh on February 01, 2018:

Very use full to get the knowledge about transformer.thanks a lot.

Anthony Opara on January 26, 2018:

Good lecture, I have learnt lot today

Umesh Singh on January 10, 2018:

If we measure voltage between neutral and a phase, we'll get 220V.

But if we measure the voltage between two phases say phase R and phase Y, we'll get nearly 400

(√3 x 220), due to three phase.

tej on January 08, 2018:

three phases are R,Y and B

voltage between R and Y is 220

voltage between Y and B is 220

voltage between R and B is sum of ry and by is 440.

it is ideally practically 220 to 240 volts in single phase

Shaikh F.A on January 04, 2018:

Sir, I want to work in the field of transformer designing

How can I?

sareem deshmukh on December 28, 2017:

In the transformer each phase have 230V a transformer have three phase 230+230+230=690

how does become 440V

Why not come to 690?

NAGARAJ on December 05, 2017:

Thanks u for sharing this post.

nitin on November 14, 2017:


OSBERT JOEL C (author) from CHENNAI on November 02, 2017:

The primary function of the transformer is insulation. Apart from that it also acts as a coolant and arc suppressor also. When the temperature of the oil rises the hot oil naturally rises to the top and the cold oil at the top moves downward. (Natural Cooling) Evaporating point of transformer oil is above 200 deg. celsius. So it is a rare phenomenon.


GAURAV KUMAR JHA on November 01, 2017:


OSBERT JOEL C (author) from CHENNAI on October 30, 2017:

Mr. Sunny, Please read:

Sunny on October 29, 2017:

Plz tell me something about oil function

mahee on October 12, 2017:

Thank you very much for sharing your knowledge like this. it is very helpful to me to enhance my knowledge.

Tirupati Transformers on October 05, 2017:

Thanks for sharing this post and make us the part of this epic post.

Saddam hossain on September 17, 2017:

Can we say explosion vent another name PRD=Pressure Relief Device.

Kishor chandegara on August 18, 2017:

Dear sir

I don't no English language so please all information in Hindi language

arun kumar verma on August 17, 2017:

sir , how can i make my career in transformer field

kuldeep kumar on August 14, 2017:

who many temperature of winding and oil in transformer

PURUSHOTTAM KUMAR on July 18, 2017:

Its topic my favourite topic so I'm very very thankyou

Joginder pal on July 06, 2017:

Please taken to knowledge for transformer

Srinu on July 05, 2017:


sadiq suhail abubakar on July 02, 2017:

Thank you very much sir,my que. Hare is that, what brought the change of windings of tranformer.

vairachelvam on July 01, 2017:

Difference between kVA AND KW


transfprmer losses depents on voltage ang current (not in pf).so that we are mention in KVA


motor losses depnts on load(pf) so that we are mention Kw

SK on June 23, 2017:

It's awesome....

Rj H.S.Meena on June 08, 2017:

Is there were 22 parts in the transformer?????????

Nikunj on June 04, 2017:

Plz u tell me about motor concept

manoranjan on June 01, 2017:

Deiffrsnce between kva and kw

Frank mkony on June 01, 2017:

In like it

SUMAN DEKA on May 31, 2017:

It is nice

Nnamdi Onwuania on May 29, 2017:

A very useful information on transformer. Now is with me on it components and function. Assembly of it is very important. In de sense of avoiding likely problem that can erupt in de transformer when not properly installed.

Firoz khan on May 24, 2017:

it is so helpfull web. thnx sir

tamil on May 24, 2017:

Very usefull sir

fs on May 22, 2017:

Sir,what's the process during transfer power one circuit to another circuit.....????