Conversion of 2-Winding Transformer into Auto-transformer

Conversion of 2-Winding Transformer into Auto-Transformer

Any two-winding transformer can be converted into an auto-transformer either step-down or step-up. The figure below as shows such a transformer with its polarity markings. Suppose it is a 20-kVA,

You can convert a two winding transformer into an Autotransformer by connecting its winding in series. But while connecting the windings keep the polarity into account because it will change its capacity. suppose you have a 240/120V two winding transformer by connecting these two windings with additive polarity the voltage at the output terminal will be 240+120=360V, then the transformer will be a step-up transformer. Similarly connecting these two windings in subtractive polarity output voltage will be 240-120.

Two winding Transformers to auto-transformer

How does the autotransformer convert into two winding transformers?

An autotransformer is essentially a 2-winding transformer with both primary and secondary windings. As shown by the terminal markings, the primary winding is X connected to terminals 1 and 2. The secondary winding is Y connected to terminals 3 and 4. Both windings share a common core leg in the magnetic circuit so that the sum of the amp-turns on that leg equal zero as in a standard 2-winding. When connected as an auto, terminals 1 and 4 are connected together. The low-voltage side is across terminals 1 and 2 while the high-voltage side is across terminals 2 and 3.

The primary and secondary windings are sometimes referred to as the common (excitation) winding and the series winding respectively. For large power applications, a third winding called the tertiary winding is added for the purposes of harmonic suppression and voltage stabilization. Only by physically adding another winding. If you do this make sure the winding is sufficiently isolated. Do not just wind enamelled wire on top of the existing winding. It needs more insulation than that to be safe.