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Parts of a Power Transformer

Updated on December 1, 2016
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Osbert Joel is a Design Engineer in a leading LV Switchgear and Control System manufacturer in Oman.

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 most often used to step up or step down voltage levels.

Components of a Transformer

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. Conservator
  7. Breather
  8. Cooling tubes
  9. Buchholz Relay
  10. Explosion vent

Of the above, the first four are found in almost all the transformers, whereas the rest are found only in transformers that are more than 50 KVA.



The core is used to support the windings 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 such factors 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.


There are two windings wound over the transformer core that are insulated from each other. Windings consists of several turns of copper coils bundled together, and each bundle is connected in series to form a winding.

Windings 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, windings are further categorized:

  1. Primary windings - These are the windings to which the input voltage is applied.
  2. Secondary windings - These are the windings to which the output voltage is applied.

Within the voltage range classification, windings are further categorized:

  1. High voltage winding - These are made of copper coil. The number of turns is the multiple of the number of turns in the low voltage windings. The copper coils are thinner than those of the low voltage windings.
  2. Low voltage windings - These have fewer turns than the high voltage windings. It is made of thick copper conductors. This is because the current in the low voltage windings is higher than that of high voltage windings.

Transformers can be supplied from either low voltage (LV) or high voltage (HV) windings based on the requirement.

Insulating Materials

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

Transformer oil is another insulating material. Transformer oil can actually have two functions: in addition to insulating it can also work to cool the core and coil assembly. The transformer's core and windings 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


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

The output voltage may vary according to the 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 Bochholz 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 operates by the gases emitted due to 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.

Classification of Transformers - The various classifications of transformers and how they differ from each other.


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    • profile image

      Ahmed said 3 years ago

      Thanks a lot

    • profile image

      Sudipt Jena 3 years ago

      The above article stands very useful for me for working in power grid and dealing with power transformers. I hope I can get more helps from this site.

      Thanks .

    • goodnews11 profile image

      OSBERT JOEL C 3 years ago from CHENNAI

      I am ready to help you!

    • profile image

      DIVI 2 years ago

      Can you please give me more information about tap changing in transformers

    • profile image

      Ravinaik.B 2 years ago

      why the transformer rated in kva?

    • profile image

      Ravinaik.B 2 years ago

      why the transformer rated in kva?

    • goodnews11 profile image

      OSBERT JOEL C 2 years ago from CHENNAI


      Transformer Losses : the Iron loss and the copper loss are independent of the power factor. These losses depends on the voltage and current only. Power factor has no effect on the losses occurring on the transformers. That's why it is rated in kVA.

      More info:

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      javascript:hpLoadLink('article',3760064, '', '')...

    • profile image

      s.kavitha 2 years ago

      i got excellent information along with very clear pictures

    • profile image

      Sahil 2 years ago

      What is oil temperature indicator nd it's work

    • goodnews11 profile image

      OSBERT JOEL C 2 years ago from CHENNAI


      Oil temperature indicator used to indicate the temperature of oil inside a transformer.

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      arjun 2 years ago

      In the case of current why low voltage windings is higher than that of high voltage windings

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      OSBERT JOEL C 2 years ago from CHENNAI

      @ Arjun

      Function of a transformer is to either step-up or step-down the applied voltage level based on its turns ratio. Current purely depends on the load.

      For an ideal transformer input power = output power

      Power = VI CosØ.

      From this, you can quote that to maintain the power constant, as the voltage is increased the current should be decreased and as voltage is decreased current should increase.

    • profile image

      manoj 2 years ago

      why dc voltage not supply in transfarmor???

    • goodnews11 profile image

      OSBERT JOEL C 2 years ago from CHENNAI


      When DC voltage is connected across the transformer terminals the constant flux will be produced. The current through the windings increases rapidly, overheating and thereby damaging the windings.

    • profile image

      AJAJ ANSARI 24 months ago

      It is very helpful for every students and teacher

    • profile image

      Raj 21 months ago

      wher is tpmo switch is used in the transformer.??

    • profile image

      ashwini bidkar 21 months ago

      It is a presous information provide about transformer operation

    • profile image

      Shareef 20 months ago

      Use full information to me......thanks a lot.

    • profile image

      praveen 15 months ago

      thanks sir

    • profile image

      jitendra gondaliya 12 months ago

      Thanks sir

    • goodnews11 profile image

      OSBERT JOEL C 12 months ago from CHENNAI

      You are Welcome.

    • profile image

      gokul singh 11 months ago

      Sir , the step up transformer used to step up for voltage . And why we not use it to generation of electricity , first we use 1step up . Then2nd .......... .

      I mean we step up220/440 , and 440/11kv and so on...

    • goodnews11 profile image

      OSBERT JOEL C 11 months ago from CHENNAI

      Transformer can stepup voltage alone not power. Input power will be equal to the output power ( neglecting losses).

    • profile image

      shakeel 6 months ago

      why dont we make the core simply from 1 sinlge there any reason to use the cut to length parts like yoke, centre limb or side limb?

      kindly explain

    • profile image

      Habtamu kasaye 5 months ago

      It is really useful. thank you sir!

    • profile image

      Prashant Kotkar 4 months ago


    • profile image

      NILONG 5 weeks ago

      Thank You and God bless!

    • profile image

      Hassan Iqbal 3 weeks ago

      It was helpful. Thanks

    • profile image

      Hassan Iqbal 3 weeks ago

      Information about tap changers is not enough, Can U explain it more precisely? please

    • profile image

      Ashutosh Dubey 11 days ago

      Thank you.....Sir.

    • profile image

      dileep 5 days ago

      thanks ur informaction

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