Auto Transformers
Introduction to Auto Transformers

An auto transformer—also known as an auto step-down or auto regulating transformer—is a transformer with only one winding. In an auto transformer, parts of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where the connections are made, but typically multiple taps are connected to a tap changer for voltage regulation.

Because an auto transformer uses a single winding with “double duty,” it is often smaller, lighter, and more cost-effective than a conventional dual-winding transformer. However, it does not provide electrical isolation between the primary and secondary circuits. Additional advantages of auto transformers include lower leakage reactance, reduced losses, lower excitation current, and an increased MVA rating for a given size and mass.

Typical applications include:

• Serving as a step-down transformer ahead of a furnace transformer

• Operating as a regulating transformer

• Being integrated into the same tank as a furnace transformer

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