What Is an Autotransformer and How Is It Different?

Published 27 Aug 2026Updated 27 Aug 20265 min readBeginnerApplications
Quick answer — An autotransformer has a single winding used for both the primary and secondary, so part of the winding is common to both sides and energy is transferred partly by conduction and partly by induction. It is smaller, cheaper and lower-loss than a two-winding transformer of the same rating, but provides no voltage isolation between the primary and secondary.
Contents
What is an autotransformer and how does it differ from an isolation transformer?

How it is built

The one winding is tapped: the source connects across the full winding and the load to a tap, so the ratio is set by the tap position. There is no separate LV winding.

Advantages

For a modest ratio change (e.g. 11 kV to 6.6 kV, or a 10% voltage adjustment), the autotransformer uses far less copper and core material, is lighter and has lower losses.

Limitations

There is no galvanic isolation, the short-circuit current can be much higher, and the winding must be insulated for the highest voltage. Two-winding transformers are used where isolation and low fault currents are needed.

References & standards

Related

Book seriesAcademy

Related manufacturers

Prolec GE (GE Vernova) · Prolec GE Waukesha · Hitachi Energy USA · Siemens Energy · Virginia Transformer Corp · Pennsylvania Transformer Technology

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Technical content is prepared from engineering references, standards frameworks, manufacturer documentation and industry sources. Contractual or compliance decisions should refer to current official standards. TransformerPath Technical Editorial.
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