Principle of operation of a transformer:

A transformer operates on the principle of mutual induction between two coils. Figure 1 shows the general construction of a transformer. The vertical portions of the steel-ore are termed limbs, and the top and bottom portions are called yokes. The two coils P and S, having N1 and N2 turns, are wound on the limbs. These two winding are electrically unconnected but are linked with one another through a magnetic flux in the core. The coil P is connected to the supply and is therefore called primary; coil S is connected to the load and is termed the secondary.

Basically, two principles are involved in the operation of a transformer. Firstly, an electric current produces a magnetic field and secondly, a changing magnetic field within a coil induces an emf across the ends of the coil. A charging current in the primary circuit creates a changing magnetic field; in turn, this magnetic field induces a voltage in the secondary circuit. Thus, energy is transferred from one circuit to the other.

The figure (b) shows the circuit symbol of a transformer. The thick line denotes the iron core. By having different ratios N1/N2 of the two windings, power at lower or a higher voltage can be obtained. When N2 > N1, the transformer is called a step up transformer; and when N2 < N1, the transformer is called a step down transformer

Equivalent Circuit of the single phase transformer: