Autotransformators

An autotransformer is a type of electrical transformer in which a single winding acts as both the primary and secondary circuits. A portion of the winding is common to both, and taps along the winding provide the required output voltages. The input voltage is typically applied across the full winding, while the output is taken from a tap to the end of the winding. The voltage ratio is determined by the turns between the tap and the full winding.

Because the same winding serves both sides, autotransformers transfer part of the power conductively through the common winding and part inductively through mutual coupling. This arrangement generally yields higher efficiency and a smaller, cheaper device for modest voltage changes compared with two-winding transformers, since less copper and core material are required for the same load.

However, the lack of isolation between primary and secondary is a major characteristic. The input and output are electrically connected, which poses shock hazards and makes autotransformers unsuitable for safety-critical isolation applications. They also exhibit voltage regulation that depends on load, and insulation and thermal limits must be respected for the tapped windings.

Applications commonly include voltage regulation on distribution networks, small step-up or step-down adjustments where isolation is not required, and motor-starting schemes that benefit from reduced voltage. Design considerations cover selecting appropriate tap positions, managing impedance and losses, ensuring adequate insulation for the exposed winding, and complying with applicable electrical safety standards.