Description of the different types of power sources
Synchronous machines - Excitement

To supply the DC excitation current for a synchronous machine, a DC voltage source is required, which can be varied over a wide range, since the excitation current of a salient-pole machine varies approximately twofold between no-load and rated operation, and can vary approximately threefold in the case of a machine with a constant air gap.

The study of the excitation system therefore involves, on the one hand, the choice of excitation sources, which is the subject of this paragraph, and, on the other, the means used to vary the excitation current (§