What is "autotransformer"

Autotransformer \Au`to*trans*form"er\, n. [Auto- + transformer.] (Elec.) A transformer in which part of the primary winding is used as a secondary winding, or vice versa; -- called also a compensator or balancing coil.

n. a transformer with a single winding, output being taken from taps

An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. The "auto" (Greek for "self") prefix refers to the single coil acting alone and not to any kind of automatic mechanism. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected.

The winding has at least three taps where electrical connections are made. Since part of the winding does "double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass.

Autotransformers are often used to step up or step down voltages in the 110-115-120 V range and voltages in the 220-230-240 V range - for example, providing 110 V or 120 V (with taps) from 230 V input, allowing equipment designed for 100 or 120 V to be used with a 230 V supply. This allows US electrical equipment to be fed from the higher European voltage. Autotransformers can also be used to supply European 230 V appliances from a 100 or 120 V supply in countries outside Europe. In all cases the supply and the autotransformer must be correctly rated to supply the required power.