Working Principle of a Transformer


A transformer is a static electrical machine which is used for either increasing or decreasing the voltage level of the AC supply with a corresponding decrease or increase in the current at constant frequency.

Parts of a Transformer

A typical transformer essentially consists of following main parts −

  • Magnetic Core
  • Transformer Windings
  • Transformer Body / Tank and Dielectric Oil
  • Oil Conservator Tank
  • Breather

Magnetic Core

The core of the transformer is made up of magnetic materials having high permeability. As the transformer is subjected to the AC supply, thus its core is built up of thin lamination stacked together to reduce the eddy current losses in the core. The windings of the transformer are wound on the core of the transformer. The core of the transformer mainly serves two purposes as,

  • It provides support to the windings.
  • It provides a low reluctance path to the magnetic flux.

Transformer Windings

The transformer consists of two windings viz. the primary winding and the secondary winding. The winding connected to the source of AC supply is called as the primary winding while the winding of the transformer to which the load is connected is known as the secondary winding. The AC voltage V1 whose magnitude is to be changed is applied across the primary winding.

Depending upon the number of turns in the primary and secondary windings, an alternating EMF (E2) being induced in the secondary winding of the transformer. This induced EMF (E2) results a load current I2, hence a terminal V2 would appear across the load.

If V2> V1, the transformer is said to be step-up transformer. On the other hand, if V1 > V2, the transformer is said to be step-down transformer.

Transformer Body / Tank and Dielectric Oil

The transformer tank provides protection to the core and the windings of the transformer. The transformer core and windings arrangement are immersed in the tank containing dielectric oil. The dielectric oil / insulating oil acts as an insulating medium for the core and windings of the transformer and it also absorbs the heat generated, hence it works as cooling medium of the transformer.

Oil Conservator Tank

The main tank of the transformer is connected through a pipe to a small tank, called as conservator tank. The main function of the conservator tank is to keep the transformer tank completely filled with the dielectric oil at all operating conditions. The conservator tank is designed to act as a reservoir for the transformer oil. When the temperature of the transformer is increased, the oil inside the transformer being expended. The conservator tank provides space for this expansion of the dielectric oil.


The breather of the transformer consists of silica gel, which prevents any atmospheric moisture from entering the tank of the transformer with air. When the temperature of the transformer goes down, it will make the dielectric oil to contract and hence the atmospheric air gets inhaled by the transformer. Thus, all the moisture of the air gets absorbed by silica gel of the breather.

Working Principle of Transformer

The working of the transformer is based on the principle of mutual inductance between two coils which are magnetic coupled.

According to the principle of mutual inductance, when an alternating voltage is applied to the primary winding of the transformer, an alternating flux ϕm which is called as the mutual flux is produced in the core. This alternating flux links both the windings magnetically and induces EMFs E1 in the primary winding and E2 in the secondary winding of the transformer according to Faraday’s law of electromagnetic induction. The EMF (E1) is called as primary EMF and the EMF (E2) is known as secondary EMF and being given as,






From the above expression it can be seen that the magnitude of EMFs E1 and E2 depend upon the number of turns in the primary and secondary windings of the transformer respectively, i.e., if N2 > N1, then E2 > E1, thus the transformer will be a step-up transformer and if N2 < N1, then E2 < E1, thus the transformer will be a step-down transformer.

If a load is now connected across the secondary winding, the EMF E2 will cause a load current I2 to flow through the load. Therefore, a transformer enables the transfer of power from one electric circuit to another with a change in voltage level.