Autotransformers are transformers with a common winding in which primary and secondary are not galvanically/electrically separated. This transformer construction is considerable lighter and smaller than a similar isolating transformer.

Some typical applications are:
- Speed control of ventilation fans
- Soft start of larger 3-phase motors
- Easy transforming of voltage up and down

Autotransformers are not able to eliminate earth faults, all earth fault problems will be propagated through the transformer, resulting in a considerable risk to safety.