How does an externally exicted ac alternator works?

Question

How does an externally exicted ac alternator works?

Answer 1

This is a synchronous machine and the majority of large generators work this way. The rotor is a large electromagnet. The DC power (excitation) is provided from some other source, although this can be another generator mounted on the same shaft. More often with modern electronics, its a static power supply.

The rotor has collector rings and brushes to get the current to the windings.

Answer 2

By having an electromagnet  (not a permanent magnet, otherwise there would be no need for excitation),  which is energised with DC via slip rings  (therefore it always has the same polarity)  spinning inside a coil.

When the excitation current is removed, the alternator stops generating electricity.

Another way would be to feed the armature electromagnet from AC at the same frequency as the alternator is generating, but via a commutator so that the reversals of the AC are cancelled out by the rotation of the armature.