(Reference:Chapman-4th/5th edition:Article:2.9)
What is an Autotransformer?
A normal transformer will have two windings which are physically separated but magnetically
coupled together with the help of a magnetic core. As they are separately been isolated, they are
called as primary winding which receives the voltage from the source and secondary winding
which transfers to the output load. But the transformer in which there will be only one winding
which is common to both primary and secondary is called Autotransformer. The term Auto
here refers to that the voltage input variations will be automatically can be improved or can be
reduced utilizing the single winding. Auto Transformers are used in applications where there is
no requirement for electrical insulation between input and output windings. These are popular for
industrial automation and marine applications.
Mainly, on some occasions it is desirable to change voltage levels by only a small amount. For
example, it may be necessary to increase a voltage from 110 to 120 V or from 13.2 to 13.8 kV. In
such circumstances, it is wasteful and excessively expensive to wind a transformer with two full
windings, each rated at about the same voltage. As special-purpose transformer, autotransformer
is used instead.
Figure 1: Auto transformer
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