Transformer - Introduction

Transformer is a static electrical device which transforms voltage/current and transfers power with out change in frequency. The basic principle of transformer is the electromagnetic induction.

Basic Principle:

Basically a transformer is a pair of coils wound on a ferromagnetic core as shown in figure. The reason for using ferromagnetic core is that it results in high values of magnetic flux linkages. The vertical portions of the core are referred to as limbs and the top and bottom portions are the yokes. The winding which is connected to the supply is called primary    and    the winding connected to the load is called secondary.

When an alternating voltage is applied across the primary winding, it results in a current and produces an alternating flux Q in the core. This flux links with the secondary winding also and induces a voltage in it.

Since the flux linkages are the same for both the windings, the e.m.f induced per turn will also be the same. If N₁and N₂ are the number of turns in the primary and secondary windings respectively, then emf induced in primary, V₁ = N₁x emf per turn and emf induced in secondary, V₂ = N₂x emf per turn. The ratio V₂/V₁ is called transformation ratio and is usually denoted by K. Since the ampere turns in the-primary and the secondary are the same,

N₁I₁ = N₂I₂

If it is assumed that there is no loss in the transformer, then

V_lI_l=V₂I₂

The above relations can be summarized as,

K = N₁/N₂ = V_l/V₂ = I₂/I_l

Ideal transformer:

A transformer having no losses in it is called an ideal transformer. An ideal transformer has no leakage reactance (all the flux produced by the primary winding links with the secondary), no I²R loss in the winding and no core losses. In practice, it is impossible to realize such a transformer.

For ideal transformer

K = E₁/E₂ = V_l/V₂ = I₂/I_l

A small percentage of flux produced by both the windings does not link with the other winding. This flux is called leakage flux. The primary leakage flux induces a voltage in the primary winding alone and the effect of this is like having a reactance in series with the winding. This reactance is called the primary leakage reactance. Similarly there is a secondary leakage reactance due to the leakage flux in the secondary winding.