A transformer operates on the principle of mutual inductance, between (and some times more) inductively coupled coils. It consists of two winding in close proximity. The two winding are coupled by magnetic induction. (The is no conductive connection between the winding ). One of the windings call primary is energized by a sinusoidal voltage. The second winding, called secondary feeds the load. The alternating current in the primary winding steps up an alternating flux in the core. The secondary winding is linked by most of this flux and EMFs. are induced in the two windings. The emf induced in the secondary winding. Energy is transformed from the primary circuit to the secondary circuit through the medium of the magnetic field.
In brief, a transformer is a device that:
(a) transfer electric power from one circuit to another, (b) it does so without change of frequency; and (c) it accomplishes this by electromagnetic induction ( or mutual inductance)