The magnetic field required for the operation of a d.c. generator is produced by an electromagnet. This electromagnet carries a field winding which produces required magnetic flux when current is passed through it.
Note : The field winding is also called exciting winding and current carried by the field winding is called an exciting current.
Thus supplying current to the field winding is called excitation and the way of supplying the exciting current is called method of excitation.
There are two methods of excitation used for d.c. generators,1. Separate excitation 2. Self excitation
Depending on the method of excitation used, the d.c. generators are classified as,
1. Separately excited generators 2. Self excited generators
In separately excited generators, a separate external d.c. supply is used to provide exciting current through the field winding.
The d.c. generators produces d.c. voltage. If this generated voltage itself is used to excite the filed winding of the same d.c. generator, it is called self excited generator.