An electrical transformer is a device that changes ac electrical power from one voltage level to ac electric power at another voltage level. The working principle of transformer is based on the action of a magnetic field.
A transformer comprises two coils of wire forming windings that are wrapped around a ferromagnetic core. One of the two windings connected to voltage source while the other one supplies current to the load.
Technically the winding connected to the voltage source is known as primary winding while the winding supply to the load is termed as the secondary winding.