A transformer works on the principle of electromagnetic induction between two coils or coupled circuits. According to this principle, an emf is induced in a coil if it links a changing flux. In core type transformers,half of the LV(and HV) winding is on one limb and the other half is on the second limb. In shell type transformer, the LV and HV windings are sandwiched.however for simplifying the drawing and analysis of both these types of transformers, the primary winding is connected to an alternating voltage source,therefore an alternatting current starts flowing through turns. the alternating mmf(NI) sets up alternating flux which is confined to the high permeability iron path. the alternating flux induces voltage in the primary and the secondary. If the load is connected across the secondary,a load current starts flowing.

In addition of the secondary winding,there may be a third (or tertiary) winding on the same iron core. The emf induced in the secondary or tertiary winding is usually referred to as the emf due to transformer action. Thus the transformer action requires the existence of alternating flux linking the various winding on a common magnetic core.

A transformer having primary and secondary winding is called a two winding transformer whereas a transformer having primary, secondary and tertiary winding is known as a three winding transformer. As stated before, primary is connected to source whereas the secondary and tertiary feed the load.