What is the purpose of excitation in generators?

Excitation provides the DC current to create the magnetic field in the generator rotor (field winding). Without excitation, no voltage would be generated. In synchronous generators, excitation level controls terminal voltage and reactive power output. Self-excited generators use part of output through rotating rectifiers or slip rings. Separately-excited generators use an independent DC source. Proper excitation control is essential for voltage regulation and parallel operation.