An autotransformer is an electrical transformer with only one winding.
This single coil has one or more extra taps or electrical connections in various positions along the winding. Each tap corresponds to a different voltage so that effectively a portion of the same inductor acts as part of both the primary and secondary winding. 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 and current to be applied to the output. This is an important safety consideration when deciding to use an autotransfomer in a given application. However, 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 480 V supplies to operate on the local 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).
In some rural applications, power companies distribute deliberately-incorrect voltage, for example, starting at a slightly-overvoltage 254V (instead of 240) and 127V (instead of 120) in order to compensate for losses over long distribution lines. An autotransformer can be used to provide a slight boost (or step-up) to correct an undervoltage condition or buck (step-down) to correct an overvoltage condition. Some autotransformers are 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.
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 are not used directly on DC.
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