An autotransformer (sometimes called "autoformer") [ Paul Horowitz and Winfield Hill, "The Art of Electronics Second Edition", Cambridge University Press, Cambridge MA, 1989, ISBN 0-521-37095-7, page 58 ] is an electrical transformer with only one winding. The winding has at least three electrical connection points called taps. The voltage source and the load are each connected to two taps. One tap at the end of the winding is a "common" connection to both circuits (source and load). Each tap corresponds to a different source or load voltage. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding.


The ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting at the middle and bottom of the autotransformer will halve the voltage. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.


A failure of the insulation or the windings of an autotransformer can result in full input voltage applied to the output. This is an important safety consideration when deciding to use an autotransfomer in a given application.

Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1 - beyond that range a two-winding transformer is usually more economical.

In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of [ground fault] currents. A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.

A special form of autotransformer called a "zig zag" is used to provide grounding (earthing) on three-phase systems that otherwise have no connection to ground (earth). A zig-zag transformer provides a path for current that is common to all three phases (so-called "zero sequence" current).

Like multiple-winding transformers, autotransformers operate on time-varying magnetic fields and so cannot be used directly on DC.


Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130 and 200-250).

On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.

In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.

Variable autotransformers

As with two-winding transformers, autotransformers may be equipped with many taps and automatic switchgear to allow them to act as automatic voltage regulators, to maintain a steady voltage at the customers' service during a wide range of load conditions.

By exposing part of the winding coils and making the secondary connection through a sliding brush, an almost continuously variable turns ratio can be obtained, allowing for very smooth control of voltage. Applicable only for relatively low voltage designs, this device is known as a variable AC transformer, or commonly by the trade name of "Variac".

From 1934 to 2002, "Variac" was a U.S. trademark of General Radio for a variable autotransformer intended to conveniently vary the output voltage for a steady AC input voltage. In 2004, Instrument Service Equipment applied for the "Variac" trademark for the same type of product. The term has acquired the status of a genericized trademark and is often used to describe similar variable autotransformers made by other makers.


*Terrell Croft and Wilford Summers (ed), "American Electricians' Handbook, Eleventh Edition", McGraw Hill, New York (1987) ISBN 0-07013932-6

*Donald G. Fink and H. Wayne Beaty, "Standard Handbook for Electrical Engineers, Eleventh Edition",McGraw-Hill, New York, 1978, ISBN 0-07020974-X

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  • Autotransformer — Au to*trans*form er, n. [Auto + transformer.] (Elec.) A transformer in which part of the primary winding is used as a secondary winding, or vice versa; called also a {compensator} or {balancing coil}. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • autotransformer — [ôt΄ō trans΄fôrm′ər] n. Elec. a transformer with at least part of the windings common to both the primary and the secondary circuits …   English World dictionary

  • autotransformer — autotransformatorius statusas T sritis automatika atitikmenys: angl. auto type transformer; autotransformer vok. Autotrafo, m; Autotransformator, m; Spartrafo, m; Spartransformator, m; Sparumspanner, m rus. автотрансформатор, m pranc.… …   Automatikos terminų žodynas

  • autotransformer — autotransformatorius statusas T sritis Standartizacija ir metrologija apibrėžtis Elektrinis transformatorius, kurio žemesniosios įtampos apvija dažniausiai yra aukštesniosios įtampos apvijos dalis. atitikmenys: angl. autotransformer vok.… …   Penkiakalbis aiškinamasis metrologijos terminų žodynas

  • autotransformer — autotransformatorius statusas T sritis fizika atitikmenys: angl. autoformer; autotransformer vok. Autotransformator, m; Spartransformator, m rus. автотрансформатор, m pranc. autotransformateur, m …   Fizikos terminų žodynas

  • autotransformer starter — paleidimo autotransformatorius statusas T sritis automatika atitikmenys: angl. autotransformer starter vok. Anlaßspartransformator, m; Anlaßtransformator in Sparschaltung, m rus. пусковой автотрансформатор, m pranc. autotransformateur de… …   Automatikos terminų žodynas

  • autotransformer — noun Date: 1895 a transformer in which the primary and secondary coils have part or all of their turns in common …   New Collegiate Dictionary

  • autotransformer — /aw toh trans fawr meuhr/, n. Elect. a transformer having a single coil that serves as both a primary coil and a secondary coil. [1890 95; AUTO 1 + TRANSFORMER] * * * …   Universalium

  • autotransformer — noun a transformer with a single winding, output being taken from taps …   Wiktionary

  • autotransformer — au·to·trans·form·er .ȯt ō tran(t)s fȯr mər n a transformer in which the primary and secondary coils have part or all of their turns in common …   Medical dictionary

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