Excitation and Self excitation

An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation
For excitation, DC is used, because DC produces constant rate of magnetic flux.

Modern generators with field coils are usually self-excited, i.e., some of the power output from the rotor is used to power the field coils. The rotor iron retains a degree of residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field induces a weak current in the rotor coils, which in turn creates an initial field current, increasing the field strength, thus increasing the induced current in the rotor, and so on in a feedback process until the machine "builds up" to full voltage