Stock of autotransformers, Toronto warehouse.

Autotransformer is an electrical transformer in which there is one winding, a portion of which is common to both the primary and the secondary circuits. An autotransformer uses common windings and offer no interference or disturbance isolation.
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.

Typical autotransformer circuit.

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 autotransformer 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 autotransformer provides no isolation of the primary and secondary circuits.
Autotransformers are used in small sizes for intermittent-duty starting of motors called as motor starter. For this use the motor is connected for a short time to the common winding voltage.