What are Autotransformers?

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Transformers having only one winding are called autotransformers. This is the most efficient type of transformer since a portion of the one winding carries the difference between the primary and secondary currents.

An autotransformer has the usual magnetic core like a typical transformer but only one winding, which is common to both the primary and secondary circuits. An autotransformer schematic is shown below:

Autotransformer Schematic
The primary is always the portion of the winding connected to the AC power source. This transformer may be used to step voltage up or down. 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 (as shown in schematic (a) above), the secondary voltage is “stepped-down.”  If only a portion of the winding is the primary and is connected to the supply voltage and the secondary includes all the winding(as shown above in schematic (b) above) then the voltage will be “stepped-up” in proportion to the ratio of the total turns to the number of connected turns in the primary winding.

The main disadvantage of the autotransformer is that the primary and secondary
circuits are electrically connected and, therefore, could not safely be used for stepping down from high voltage to a voltage suitable for plant loads.

Uses of the Autotransformer
Autotransformers find wide application in the following situations:
(a) Where the system supplied contains an identified-grounded conductor that is solidly connected to a similar identified grounded conductor of the system supplying the autotransformer.
(b) Where an induction motor is to be started or controlled
(c) Where a dimming action is required, as in the theatre lighting
(d) Where the autotransformer is to be a part of a ballast for supplying lighting units