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How Does a Transformer Work?

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

A transformer is the inseparable part of a power system. Proper functioning of transmission and distribution systems is not possible without the transformer. For the stable operation of the power system, the transformer should be available.

The Power Transformer was invented towards the end of nineteenth century. The invention of the transformer led to the development of constant power AC supply systems. Before the invention of the transformer, DC systems were used for the supply of electricity. Installation of the power transformers made the distribution system more flexible and more efficient.

What Is a Transformer?

A transformer is an electrical device used to convert the voltage of one magnitude to voltage of another magnitude without changing the frequency. The voltage is either stepped up or stepped down with out altering the frequency.

The property of induction was discovered in the 1830s by Joseph Henry and Michael Faraday. Ottó Bláthy, Miksa Déri, Károly Zipernowsky designed and used the first transformer in both experimental, and commercian systems. Later on their work was further perfected by Lucien Gaulard, Sebstian Ferranti, and William Stanley perfected the design. Finally Stanley made the transformer cheap to produce, and easy to adjust for final use.

First transformer built by Ottó Bláthy, Miksa Déri, Károly Zipernowsky.

First transformer built by Ottó Bláthy, Miksa Déri, Károly Zipernowsky.

Power transformer


Why are transformers used in the power system??

Transformers are used in the power system in order to step up or step down the voltages. In the transmission end the voltage is stepped up and in the distribution side the voltage is stepped down in order to reduce the power loss (i.e.) copper loss or I2R loss.

The current decreases with increase in voltage. Hence the voltage is stepped up at the transmission end to minimize the transmission losses. At the distribution end the voltage is stepped down to the required voltage in order as per the rating of the required load.

Principle of Operation

Transformers work on the principle of Faraday’s law of electromagnetic induction.

Faraday’s law states that, “Rate of change of flux linkage with respect to time is directly proportional to the induced EMF in a conductor or coil”.

In this picture you can see that the primary and secondary winding are made at different limbs of the core. But in practice they are made on the same limb one over the other to reduce losses.

In this picture you can see that the primary and secondary winding are made at different limbs of the core. But in practice they are made on the same limb one over the other to reduce losses.

Basic Working of Transformers

The basic transformer consists of two types of coils, namely:

  1. Primary coil
  2. Secondary coil

Primary coil

The coil to which the supply is given is called as the primary coil.

Secondary coil

The coil from which the supply is taken is called as the secondary coil.

Based on the required output voltage the number if turns in the primary coil and the secondary coil are varied.

The processes occurring inside the transformer can be grouped into two:

  1. Magnetic flux is produced in a coil when ever there is a change in current flowing through the coil.
  2. Similarly change in magnetic flux linked with the coil induces EMF in the coil.

The first process occurs in the windings of the transformer. When the ac supply is given to the primary winding alternating flux is produced in the coil

The second process occurs in the secondary winding of the transformer. The flux alternating flux produced in the transformer links the coils in the secondary winding and hence emf is induced in the secondary winding.

Whenever an ac supply is given to the primary coil, flux is produced in the coil. These flux links with the secondary winding thereby inducing emf in the secondary coil. The flow of flux through the magnetic core is shown by doted lines. This is the very basic working of the transformer.

The voltage produced in the secondary coil depends mainly on the turns ratio of the transformer.

There relationship between the number of turns and the voltage is given by the following equations.

N1/N2 = V1/V2 = I2/I1


N1= number of turns in the primary coil of the transformer.

N2= number of turns in the secondary coil of the transformer.

V1= voltage in the primary coil of the transformer.

V2= voltage in the secondary coil of the transformer.

I1= current through the primary coil of the transformer.

I2= current through the secondary coil of the transformer.

Basic Parts

Any transformer consists of the following three basic parts in it.

  1. Primary coil
  2. Secondary coil
  3. Magnetic core

1. Primary coil.

The primary coil is the coil to which the source is connected. It may be the high voltage side or low voltage side of the transformer. An alternating flux is produced in the primary coil.

2. Secondary coil

The output is taken from the secondary coil. The alternating flux produced in the primary coil passes through the core and links with there coil and hence emf is induced in this coil.

3. Magnetic core

The flux produced in the primary passes through this magnetic core. It is made up of laminated soft iron core. It provides support to the coil and also provides a low reluctance path for the flux.

Components of a Transformer

  1. Core
  2. Windings
  3. Transformer oil
  4. Tap changer
  5. Conservator
  6. Breather
  7. Cooling tubes
  8. Buchholz Relay
  9. Explosion vent

Classification of Transformers


Based on application

Step up transformer


Step down transformer

Based on Construction

Core type transformers


Shell type transformers

Based on the number of phases.

Single phase


Three phase

Based on the method of cooling

Self-air–cooled (Dry type)


Air-blast–cooled (Dry type)


Oil-immersed, combination self-cooled and air-blast


Oil-immersed, water-cooled


Oil-immersed, forced-oil–cooled


Oil-immersed, combination self-cooled and water-cooled

Equivalent circuit of transformer


Phasor diagram


Why transformers are rated in KVA?

It is a commonly asked question. The reason behind this is: the losses occurring in transformers depends only on the current and voltage. The power factor has no effect over copper loss (depends on current) or the iron loss (depends on voltage). Hence it is rated in KVA / MVA.

Losses in Transformers

Transformer is the most efficient electrical machine. Since the transformer has no moving parts, its efficiency is much higher than that of rotating machines. The various losses in a transformer are enumerated as follows:

1. Core loss

2. Copper loss

3. Load (stray) loss

4. Dielectric loss

When the core of the transformer undergoes cyclic magnetization power losses occur in it. The core losses comprises of two components:

  • Hysteresis loss
  • Eddy current loss

When the magnetic core flux varies in a magnetic core with respect to time, voltage is induced in all possible paths enclosing the flux. This will result in the production of circulating currents in the transformer core. These currents are known as eddy currents. These eddy currents leads to power loss called Eddy current loss. Copper loss occurs in the winding of the transformer due to the resistance of the coil.

The History of the Transformer

Discovery of the principle of electromagnetic induction paved way for the invention of transfomer. Here is a short time line of development of transformer.

  • 1831 - Michael Faraday and Joseph Henry discovered the process of electromagnetic induction between two coils.
  • 1836 - Rev. Nicholas Callan of Maynooth College, Ireland invented was the induction coil, which was the first type of transformer.
  • 1876- Pavel Yablochkov, a Russian engineer invented a lighting system based on a set of induction coils.
  • 1878- The Ganz factory, Budapest, Hungary, began manufacturing equipment for electric lighting based on of induction coils.
  • 1881 - Charles F. Brush develops his own design of transformer.
  • 1884- Ottó Bláthy and Károly Zipernowsky suggested the use of closed-cores and shunt connections.
  • 1884 - Lucien Gaulard's transformer system (a series system) was used in the first large exposition of AC power in Turin, Italy.
  • 1885 - George Westinghouse orders a Siemens alternator (AC generator) and a transformer from Gaulard and Gibbs. Stanley began experimenting with this system.
  • 1885 – William Stanley modifies the design by Gaulard and Gibbs. He makes the transformer more practical by using induction coils with single cores of soft iron and adjustable gaps to regulate the EMF present in the secondary winding.
  • 1886 - William Stanley made the first demonstration of distribution system using step and step down transformers.
  • 1889 - Mikhail Dolivo-Dobrovolsky, a Russian-born engineer developed the first three-phase transformer at the Allgemeine Elektricitäts-Gesellschaft, Germany.
  • 1891- Nikola Tesla, a Serbian American inventor, invented the Tesla coil for generating very high voltages at high frequency.
  • 1891 – Three phase transformer was built by Siemens and Halske Company.
  • 1895 - William Stanley built a three phase Air cooled transformer.
  • Today - Transformers are improved by increasing efficiency as well as capacity and reducing size and cost.

Try to answer!

For each question, choose the best answer. The answer key is below.

  1. What is the principle behind the working of transformer?
    • Faraday's Law of electromagnetic induction
    • Lenz Law
    • Biot–Savart law
  2. Transformer works on:
    • AC
    • DC

Answer Key

  1. Faraday's Law of electromagnetic induction
  2. AC
  • NEXT>>>Basic Parts of a Transformer
    Various components of a power transformer can be easily understood from this article. The working of those components is also explained briefly.

Transformer FAQ


Rakesh on December 03, 2019:

Appreciate the contribution Osbert.

Good to go through the writings.

Best Regards

Pat on November 14, 2018:

Thank you for your help sir.I would like to know which side of transformer is wound first.primary or secondary in a 220/110volts transformer.I want to wind one.

Kim taehyung on September 09, 2018:


JSV on August 19, 2018:

Thank you for your article, Sir.

OSBERT JOEL C (author) from CHENNAI on April 01, 2018:

Thank you

darshika on April 01, 2018:

veryy good notes thank you

haji achuu on March 14, 2018:


it was a nice article

manish on July 15, 2017:

very excellent sir....thanku

Jovz on June 05, 2017:

it helps me a lot, now I understand more about he working of the transformer. Can you please give me an article regarding the Transformer maintenance please. Thank you so much author.

Irfan Mohammed on May 12, 2017:

Very good and necessary things to learn... Thanks

Dr Irshad on November 04, 2015:

Nic page. Thanks

Laksh S. on June 15, 2015:

now i know about working ang functions of transformers .....

OSBERT JOEL C (author) from CHENNAI on November 18, 2014:

Thank you Olagsinquito.

ologsinquito from USA on November 18, 2014:

This is so above my non-existent knowledge of electrical engineering, but it looks as if you did a really good job with it. I like all the details and the illustrations.

Reshma.D.Benedict on January 31, 2014:

learned a lot about transformers. Its also very easy to understand

OSBERT JOEL C (author) from CHENNAI on December 04, 2013:

Thank you Kelly..

CJ Kelly from the PNW on November 13, 2013:

I learned a ton from this. Thanks very much. We all really need to have knowledge about what makes our modern, everyday world work. This encapsulates it perfectly. Good job. Voted up.