Page 622 - Basic Electrical Engineering

Fig. 7.13 (a) shows a separately excited dc generator. The generator armature
is rotated at a constant speed by a prime mover. When no field current is
there, a small amount of EMF will be induced due to residual magnetism. By

increasing the field current, I gradually, we will get the no-load or open-
f
circuit characteristic as shown in Fig. 7.13 (a).

Figure 7.13 No-load or open-circuit characteristic (OCC) of dc generators: (a) separately excited
generator; (b) shunt generator

In the case of a shunt generator, the voltage gets built up due to residual

magnetism as has been shown in Fig. 7.13 (b). The induced voltage will be
the value at which the OCC and field resistance line cross each other. By

adjusting the value of the field-circuit resistance, i.e., by adding an extra
resistance in the field circuit, the value of E can be adjusted. The speed of the
prime mover is assumed constant. A self-excited dc shunt generator will fail

to build up its voltage if there is no residual magnetism in the field poles and
if the value of the field resistance in higher than the critical field resistance.