autotransformer
A transformer with a single winding, its output being taken from taps.
autotransformer: one coil does two jobs at once
An autotransformer is a step-up or step-down transformer that uses a single continuous winding instead of two separate primary and secondary coils. Part of the winding serves as the input, part as the output, and they share a common section. This shared winding is what distinguishes it from a conventional two-winding transformer and is why it gets its name: "auto" meaning self-coupled.
The basic design works like this. Current enters at one tap on the winding, exits at another. A second tap pair draws voltage from a different section of the same coil. For a step-down autotransformer, the output is taken from fewer turns than the input. For step-up, the opposite applies. The turns ratio determines the voltage ratio, just as in a conventional transformer. Common ratios run from 2:1 down to 1.2:1, and they are available in both single-phase and three-phase configurations, handling anything from a few hundred watts to several megavatts in industrial duty.
The chief advantage is efficiency. Because part of the power transfers magnetically through the shared core, and part transfers electrically through the common winding itself, an autotransformer needs less copper and iron than a two-winding unit of equivalent power rating. This saves weight, cost, and cooling burden. A 10 kVA conventional transformer might weigh 50 kg; an autotransformer of similar capability might weigh 30 kg. This economy makes autotransformers the standard choice in voltage regulators, soft-starters, and variable-frequency drive input conditioning.
Where autotransformers work and where they don't
Autotransformers excel in applications where input and output share a common ground or where isolation is not required. They are routine in laboratory benches, theatrical lighting dimmer packs, large motor soft-starting circuits, and power factor correction systems. However, they cannot provide galvanic isolation between input and output, making them unsuitable wherever electrical safety isolation is a code requirement or where noise immunity is critical. Also, if the common winding fails, both primary and secondary circuits are compromised simultaneously, unlike a two-winding transformer where a fault in one winding does not necessarily kill the other.
In service, autotransformers are sensitive to imbalanced loading on three-phase systems and to harmonic distortion, especially in presence of nonlinear loads like variable-frequency drives or rectifiers. The shared winding can become a bottleneck if third-harmonic current tries to circulate. Modern autotransformers used in power electronics duty often include delta-connected stabilizing windings or tertiary neutralization circuits to handle these stresses. Maintenance chiefly concerns core temperature, tap contact cleanliness in adjustable models, and insulation condition on the high-voltage end of the winding where dielectric stress is greatest.