Excitation of alternating-current generators

In order to induce an electromotive force in electrical machinery some sort of excitation must always be provided. In direct-current machines an e.m.f. is induced in the armature conductors by their motion across a stationary magnetic field. This is sometimes also the case with alternators, although it is more usual that the field is revolving, so that the magnetic flux travels past the armature conductors, which are stationary.