Autotransformer

The "auto" (Greek for "self") prefix refers to the single coil acting alone.

In contrast, an ordinary transformer has separate primary and secondary windings that are not connected by an electrically conductive path between them.

Since part of the winding does "double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical isolation between primary and secondary circuits.

[2] An example of an application of an autotransformer is one style of traveler's voltage converter, that allows 230-volt devices to be used on 120-volt supply circuits, or the reverse.

It is a transformer in which the primary and secondary coils have part of their turns in common.

The portion of the winding shared by both the primary and secondary is the common section.

[3] Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns.

In an autotransformer, part of the output current flows directly from the input to the output (through the series section), and only part is transferred inductively (through the common section), allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding.

One end of the winding is usually connected in common to both the voltage source and the electrical load.

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 isolation of the windings of an autotransformer can result in full input voltage applied to the output.

Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in nearly full input voltage being applied to the output).

[5] 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.

A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.

The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power.

They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100–130 V and 200–250 V).

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 railway applications, it is common to power the trains at 25 kV AC.

To increase the distance between electricity Grid feeder points, they can be arranged to supply a split-phase 25–0–25 kV feed with the third wire (opposite phase) out of reach of the train's overhead collector pantograph.

At frequent (about 10 km) intervals, an autotransformer links the contact wire to rail and to the second (antiphase) supply conductor.

A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.

Max Korndorfer assigned his patent to the General Electric Company.

One basic method to reduce the starting current is with a reduced voltage autotransformer with taps at 50%, 65% and 80% of the applied line voltage; once the motor is started the autotransformer is switched out of circuit.

By exposing part of the winding coils and making the secondary connection through a sliding brush, a continuously variable turns ratio can be obtained, allowing for very smooth control of output voltage.

The voltage can be smoothly varied between turns as the brush has a relatively high resistance (compared with a metal contact) and the actual output voltage is a function of the relative area of brush in contact with adjacent windings.

The manual type is applicable only for relatively low voltage and is known as a variable AC transformer (often referred to by the trademark name Variac).

In 2004, Instrument Service Equipment applied for and obtained the Variac trademark for the same type of product.

[8] The term variac has become a genericised trademark, being used to refer to a variable autotransformer.