Field Excitation

The magnetic fields in DC generators are most generally given through electromagnets. Current have to flow by the electromagnet conductors to generate a magnetic field.  Within order for a DC generator  to  operate  properly,  the  magnetic  field  must  always  be  in  the  same  direction. Thus, the current by the field winding must be direct current.  That current is called as the field excitation current and could be supplied to the field winding in one of two ways.  It could come from a separate DC source external to the generator example for a separately excited generator or it could come directly from the output of the generator, in that case it is known as a self-excited generator.
In a self-excited generator, the field winding is linked directly to the generator output.  The field might be linked in series along with the output, in parallel along with the outcome, or a combination of the two.

Separate excitation needs an external source, like as a battery or another DC source.  It is commonly more expensive than a self-excited generator.Separately excited generators are, thus, used only where self-excitation is not satisfactory.  They will be used in cases where the generator must respond rapidly to an external control source or where the generated voltage has to be varied over a huge range in during normal operations.