Auto Transformer A special form of transformer having one winding, a portion of which is common to both the primary and the secondary circuits. The current in the high-voltage circuit flows through the series and common windings (see illustration).

The current in the low-voltage circuit flows through the common winding and adds vectorially to the current in the high-voltage circuit to give the common winding current. Thus, an electrical connection exists between high-voltage and low-voltage windings.

Because of this sharing of parts of the winding, an auto-transformer having the same kilovolt-ampere (kVA) output rating is generally smaller in weight and dimensions than a two-winding transformer. One possible disadvantage of auto-transformers is that the windings are not insulated from each other and that the auto-transformer provides no isolation of the primary and secondary circuits.