Page 620 - Basic Electrical Engineering

the field winding. Such a generator where the field winding is supplied from
a separate dc source for its excitation is called a separately excited dc
generator.

EMF E will be induced in the armature when it is rotated by a prime mover
and the field windings are excited from a separate dc source.

Figure 7.11 (a) A dc generator with field and armature winding terminals brought out; (b) the field
windings of the generator are excited from a dc supply source

Figure 7.12 (a) Shunt generator; (b) series generator

Now, let as consider what would happen when the armature is rotated by a
prime mover without exciting the field windings, i.e., when field winding

current, I = 0.
f
Even when the field windings are not excited, there is some residual

magnetizm in the field poles due to their earlier excitation.