A transformer is a power converter that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer’s coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer’s core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying EMF, or “voltage”, in the secondary winding. This effect is called inductive coupling.
If a load is connected to the secondary winding, current will flow in this winding, and electrical energy will be transferred from the primary circuit through the transformer to the load. In an ideal transformer, the induced voltage in the secondary winding (Vs ) is in proportion to the primary voltage (Vp ) and is given by the ratio of the number of turns in the secondary (Ns) to the number of turns in the primary (Np) as follows:
By appropriate selection of the ratio of turns, a transformer thus enables an AC voltage to be “stepped up” by making Ns greater than Np , or “stepped down” by making Ns less than Np. The windings are coils wound around a ferromagnetic core. Air-core transformers are a notable exception to this.
Transformers range in size from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge units weighing hundreds of tons used in power stations, or to interconnect portions of power grids. All operate on the same basic principles, although the range of designs is wide. While new technologies have eliminated the need for transformers in some electronic circuits, transformers are still found in nearly all electronic devices designed for household voltage. Transformers are essential for high-voltage electric power transmission, which makes long-distance transmission economically practical.