Autotransformer

An autotransformer is a type of electrical transformer in which the primary and secondary circuits share a common winding. Unlike a conventional two-winding transformer, it uses a single continuous winding with a tap that provides the secondary voltage. The input and output voltages are determined by the turns ratio between the common portion and the tapped portion, so the device can be used to step voltage up or down.

Construction and operation: The winding consists of a base portion that is common to both circuits and a series portion extending to a terminal. The input is applied across either the entire winding or a section, and the output is taken from the appropriate portion of the winding depending on the configuration. Energy transfer occurs partly through magnetic coupling between the sections and partly conductively through the common winding. The transformation ratio is set by the relative turns of the common portion and the total winding, allowing small or large voltage changes with a compact design.

Advantages and limitations: Autotransformers are typically smaller, lighter, and cheaper than equivalent two-winding transformers for modest voltage changes, since less conductor and core material are required. They can offer high efficiency and low leakage. However, because the input and output share windings, there is no galvanic isolation between them, which can present safety concerns. Voltage regulation and fault behavior differ from isolated transformers, and insulation must be designed to handle the full input voltage appearing on the common winding.

Applications: They are used for voltage regulation and bus voltage adjustments in power distribution, for starting large induction motors, and in equipment requiring buck-boost voltage adjustment with limited change from the supply. They are common where a compact, economical solution with adjustable voltage is acceptable and isolation is not required.