Working Principle of Transformer

A transformer is a static piece of equipment used either for raising or lowering the voltage of an AC supply with a corresponding decrease or increase in current. It essentially consists of two windings on a common laminated magnetic core.

Primary Winding – AC source is connected to this winding.
Secondary Winding – Load is connected to this winding.

The alternating voltage V₁whose magnitude is to be changed is applied to the primary. Depending upon the number of turns of the primary (N₁) and secondary (N₂), an alternating e.m.f. E₂ is induced in the secondary.

This induced e.m.f. E₂ in the secondary causes a secondary current I₂. Consequently, terminal voltage V₂ will appear across the load.

If V₂ > V₁, it is called a step-up transformer.
If V₂ < V₁, it is called a step-down transformer.

When an alternating voltage V₁ is applied to the primary, an alternating flux φ is set up in the core. This alternating flux links both the windings and induces e.m.f.s E₁ and E₂ in them according to Faraday’s laws of electromagnetic induction. The e.m.f. E₁ is termed as primary e.m.f. and e.m.f. E₂ is termed as secondary e.m.f.

E₁ = –N₁ (dφ/dt)

E₂ = –N₂ (dφ/dt)

∴ E₂/E₁ = N₂/N₁

Note that magnitudes of E₂ and E₁ depend upon the number of turns on the secondary and primary respectively.

If N₂ > N₁, then E₂> E₁ (or V₂ > V₁) and we get a step-up transformer.

On the other hand, if N2 < N1, then E₂ < E₁ (or V₂ < V₁) and we get a step-down transformer.

If the load is connected across the secondary winding, the secondary e.m.f. E₂ will cause a current I₂ to flow through the load. Thus, a transformer enables us to transfer a.c. power from one circuit to another with a change in voltage level.

The following points may be noted carefully:

The transformer action is based on the laws of electromagnetic induction.
There is no electrical connection between the primary and secondary. The a.c. power is transferred from primary to secondary through the magnetic flux.
There is no change in frequency i.e., output power has the same frequency as the input power.

The losses that occur in a transformer are:

core losses – eddy current and hysteresis losses
copper losses—in the resistance of the windings

In practice, these losses are very small so that output power is nearly equal to the input primary power. In other words, a transformer has very high efficiency.