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.
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