Three-phase transformers consist of a three-legged iron assembly with two windings on each leg, a primary winding connecting to the power supply and a secondary winding connecting to the load circuit. Changing current in the primary creates flux, which creates voltage in the secondary winding. Transformers work according to the principle of mutual induction; electric current produces flux, a magnetic field, and a change in flux within a coil of wire induces a voltage across the ends of the wire.
Voltages vary according to the number of coilings of the primary winding in relationship to the number of coilings of the secondary winding. The iron core assembly of the transformer provides a controlled pathway for the magnetic flux flowing through the windings. Three-phase transformers are necessary to raise and lower voltage levels of the three-phase alternating current distributed by power companies to meet the needs of the end-user.
It is possible to configure the winding connections on a three-phase transformer in either a wye configuration, a Y-shaped configuration, or a delta or triangle configuration. Wye connections allow for multiple voltages, while delta connections are single voltage and more reliable.
Transfers of large amounts of power are more efficient through three-phase transformers, which also enable transmission over longer distances. Single-phase transformers typically provide power for residential purposes, such as lighting, heating and air conditioning.