From Electric Machinery and Transformers, Third Edition
4.9 The Autotransformer
In the two-winding transformer we have considered thus far, the primary winding is electrically isolated from the secondary winding. The two windings are coupled together magnetically by a common core. Thus, the principle of magnetic induction is responsible for the energy transfer from the primary to the secondary.
When the two windings of a transformer are interconnected electrically, it is called an autotransformer. An autotransformer may have a single continuous winding that is common to both the primary and the secondary. Alternatively, we can connect two or more distinct coils wound on the same magnetic core to form an autotransformer. The principle of operation is the same in either case. The direct electrical connection between the windings ensures that a part of the energy is transferred from the primary to the secondary by conduction. The magnetic coupling between the windings guarantees that some of the energy is also delivered by induction.
Autotransformers may be used for almost all applications in which we use a two-winding transformer. The only disadvantage in doing so is the loss of electrical isolation between the high- and low-voltage sides of the autotransformer. Listed below are some of the advantages of an autotransformer compared with a two-winding transformer.
It is cheaper in first cost than a conventional two-winding transformer of a similar rating.
It delivers more power than a two-winding transformer of similar physical dimensions.
For a similar power rating, an autotransformer is more efficient than a two-winding transformer.
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