SOLUTION

An autotransformer is a type of transformer that uses a single tapped winding rather than the two separate and electrically isolated windings used by mutual transformers. Because autotransformers don’t have separate windings, unlike mutual transformers there is no electrical isolation between the primary and secondary circuits.

• Therefore in autotransformer, the primary is electrically connected to the secondary, as well as magnetically coupled to it.
• The alternating current applied between the input points will induce a flow of magnetic flux around the core.
• This magnetic flux will link with all the turns forming the coil, inducing a voltage into each turn of the winding.
• Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns.
• In an autotransformer, part of the current flows directly from the input to the output, and only that part is transferred.

Where electrical isolation between the primary and secondary windings is unimportant, the use of an autotransformer has a number of advantages over a mutual transformer.

• With only one winding, the volume of copper required in its manufacture can be lower than for a mutual transformer.
• There are no secondary copper losses, so autotransformers are more efficient than a corresponding mutual transformer.