What is an Auxiliary wound excitation system on an AC alternator? Why is it useful? How does it work?

What is an Auxiliary wound excitation system on an AC alternator?

An auxiliary wound excitation system on an AC alternator works by having a separate second winding embedded into the main stator winding. This winding is used to independently power the AVR.

Why is it useful? Why is it better than shunt / self-excited?

In a standard AC alternator with shunt excitation, the power for the AVR is taken from the main generator terminals. Therefore when loads are applied to an AC Alternator, the terminal voltage dips and the power source of the AVR is reduced. This leads to slower voltage response, larger transient voltage dips and the possibility that the voltage will not recover to its pre-set level.

A Diagram from Stamford showing an auxiliary winding alternator.

Why other alternatives are better than the auxiliary winding?

An alternative to the auxiliary winding is a PMG system, which is generally more expensive, but not expensive in comparison to the cost of the total generator. The PMG system also provides the advantage that no residual magnetism is required in order to start the excitation process at start up.