A transformer is an electrical device that transfers electric energy from one coil, or winding to another by electromagnetic induction. The transferred energy may be at a higher or lower voltage. Transformers can operate only on alternating current (AC) or on direct current with a superimposed AC component (pulsting DC).
Transformer are generally classes as step-up or step-down transformer. A step-up trnasformer increases the voltage, a step-down transformer reduces it. At electric generating plants, step up transformers (called generators transformers) increase the generator voltage to a higher voltage, which is then transmitted through power lines over long distances with little loss. In the locally where the electricity will be used, step down transformer makes the final step down in voltage for homes and businesses. Consumers may use additional step down transformers to operate such devices as doorbells and loy electric trains.
The operation of the transformer is based on the principle discoverd in 1830 by Joseph Henry that electrical energy can be transfered efficiently by mutual electromagnetic induction from one winding to another. When the primary winding is energized by an AC source, an alternating magnetic flux is astablished in the transformer core. This flux links the turns of both primary and secondary, thereby inducing voltages in them. Because the same flux cuts both windings, the same voltage is induce in each turn of both windings. The total induced voltage in each winding is therefore proportional to the number of turn in that winding. This proportion is known as the turns ratio, which when multipled by the applied voltage, determines the secondary voltage. The primary is always connected to the source of power, and the secondary winding receives the electrical energy by mutual induction from the primary winding and delivers it to the connected electric load. In some transformer windings, taps are provided so that less than the full number of turns may be used.
Transformers are built in single phase and polyphase units. Most power and distribution systems use three phase lines. A three phase transformer consists of separate insulated windings for the different phases. The windings are wound on a three legged core capable of establishing three magnetic fluxes displaced by 120 in time phase. Transformer may be classed according to the frequency range for which they are designed, such as audio frequency, power frequency, and radio frequency.