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 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 generator
In separately excited generator, a separate external d.c. supply is used to provide exciting current through the field winding.
The d.c. generator produces d.c. voltage. If this generated voltage itself is used to excite the field winding of the same d.c. generator, it is called self excited generator. The d.c. voltage is produced in the armature winding of a d.c. generator, which is used to excite the field winding of the same generator. Depending on how electrically the armature winding is connected to the field winding, the self excited d.c. generators are classified as,
a) Shunt generators b) Series generators c) Compound generators.
In shunt the two windings, field and armature are in parallel while in series type the two windings are in series. In compound type the part of the field winding is in parallel while other part in series with the armature winding.
The compound generators are further classified as long shunt and short shunt compound generators. The overall classification of d.c. generators is shown in the Fig. 1.