subject: Autotransformer [print this page] A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductorsthe transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core, and thus a varying magnetic field through the secondary winding. This varying magnetic field induces a varying electromotive force (EMF) or "voltage" in the secondary winding. This effect is called mutual induction.
A variety of types of electrical transformer are made for different purposes. Despite their design differences, the various types employ the same basic principle as discovered in 1831 by Michael Faraday.
The different types of transformers include power transformers, instrument transformers, pulse transformers, RF transformers, audio transformers, etc.
An autotransformer is an electrical transformer with only one winding. The auto- prefix refers to the single coil rather than any automatic mechanism. In an autotransformer a portion of the same winding acts as part of both the primary and secondary winding.
The winding has at least three taps where electrical connections are made. The voltage source and the electrical load are each both connected to two taps with one tap at the end of the winding being a common connection to both circuits. Each tap along the winding corresponds to a different source or load voltage.
Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 138 kV to 66 kV for transmission. Another application is in industry to adapt machinery built for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100-130 and 200-250).
