Autotransformers offer the advantage of a reduction in size, weight and cost compared to an equivalent isolation transformer.

In an Autotransformer the primary and secondary share a common winding. By lowering the required current-carrying capacity in the part of the winding that is common to both primary and secondary circuits, the required core area and conductor size are reduced. As the voltage transformation ratio approaches one the advantage improves. Autotransformers can be very effective when isolation between the primary and secondary is NOT required.

Autotransformers are available in single and three phase and in any core geometry or material.