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

How it is built
Advantages
Limitations

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.

Worked example

A 100 MVA, 132/66 kV three-phase autotransformer has co-ratio 0.5, so the equivalent self rating is 50 MVA. LV line current is 874.8 A and the series and common winding currents are each 437.4 A. The sizing page computes this from throughput and the two line voltages.

References & standards

IEC 60076-1
IEEE C57.12.00
IEEE C57.12.10

Equivalent rating 3D model Book series Academy

Related manufacturers

Central Moloney · CORE Transformers · Daelim Transformer (US) · Delta Star · Hammond Power Solutions (Baraboo) · HD Hyundai Power Transformers USA

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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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