The Autotransformer

The Autotransformer

Unlike the previous voltage transformer which has two electrically isolated windings called: the primary and the secondary, an Autotransformer has only one single voltage winding which is common to both sides. This single winding is “tapped” at various points along its length to provide a percentage of the primary voltage supply across its secondary load. Then the autotransformer has the usual magnetic core but only has one winding, which is common to both the primary and secondary circuits.

Therefore in an autotransformer the primary and secondary windings are linked together both electrically and magnetically. The main advantage of this type of transformer design is that it can be made a lot cheaper for the same VA rating, but the biggest disadvantage of an autotransformer is that it does not have the primary/secondary winding isolation of a conventional double wound transformer.

The section of winding designated as the primary part of the winding is connected to the AC power source with the secondary being part of this primary winding. An autotransformer can also be used to step the supply voltage up or down by reversing the connections. If the primary is the total winding and is connected to a supply, and the secondary circuit is connected across only a portion of the winding, then the secondary voltage is “stepped-down” as shown.

Autotransformer Design

When the primary current IP is flowing through the single winding in the direction of the arrow as shown, the secondary current, IS, flows in the opposite direction. Therefore, in the portion of the winding that generates the secondary voltage, VS the current flowing out of the winding is the difference of IP and IS.

The Autotransformer can also be constructed with more than one single tapping point. Auto-transformers can be used to provide different voltage points along its winding or increase its supply voltage with respect to its supply voltage VP as shown.

Autotransformer with Multiple Tapping Points

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