<P> An Auto - transformer (sometimes called auto - step down transformer) is an electrical transformer with only one winding . The "auto" (Greek for "self") prefix refers to the single coil acting alone and not to any kind of automatic mechanism . In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer . In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected . </P> <P> The winding has at least three taps where electrical connections are made . 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 . Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass . </P>

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