Principle of the transformer:

When the two wires are close to each other, and one of them carries fluctuating current, a current will be induced in other wire. This effect of it is termed as electromagnetic induction. All alternating current transformers work according to principle of electromagnetic induction. If the 1st wire carries sine wave alternating current of a certain frequency, then induced current will be sine wave alternating current of the same frequency in the 2nd wire.

Closer the two wires are to each other, greater the induced current, for the given current in the 1st wire. If wires are wound into coils and placed along the common axis, the induced current will be greater than if wires are straight and parallel. More coupling, or efficiency of induced current transfer, can be obtained if the 2 coils are wound one atop the other. The first coil is called as primary winding, and the second coil is called as the Secondary winding. These are spoken of as the primary and secondary simply.

The induced current creates a voltage across secondary. In the step-down transformer, the secondary voltage is less than primary voltage. In the step-up transformer, secondary voltage is greater than primary voltage. The primary voltage can be represented asEpri, and secondary voltage can be represented Esec. Unless otherwise stated, effective (RMS) voltages are always specified.

The windings of the transformer have inductance as they are coils. The inductances of primary and secondary depend on the frequency of operation, and on resistive part of impedance in the circuit. As frequency increases, the required inductance decreases. At the high resistive impedances, more inductance is generally required than at low resistive impedances

Figure--   Magnetic flux between two coils of wire.