An autotransformer, also known as a step-down transformer, is an electrical transformer with only one winding. The winding has at least 3 taps where electrical connections are made. Autotransformers have the benefit of being small, light and cheap compared to dual-winding transformers.

Autotransformers are used to step up or step down voltages in the 110V-115V-120V range and 220V-230V-240V range. This allows electrical equipment from the US to be fed from higher European voltage.

Applications

• Can be used as a method of soft starting induction motors

• In UK railway applications

• In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage distribution systems

• Power applications to interconnect systems operating at different voltages

• Special autotransformers are used on rural power distribution lines so that customers further away receive the same average voltage as ones nearer to the source