Page 624 - Basic Electrical Engineering

consequence reducing V. This effect of armature flux on the main field flux is
called armature reaction. It may be noted that armature reaction occurs only
when current flows through the armature winding i.e., only when the

generator is loaded. Because of reduction of E due to armature reaction, the
terminal voltage E will further get reduced a little in case of shunt generators.

To reduce the effect of armature reaction, compensating windings and inter
poles are used, which produces the same amount of flux as produced by the

armature current but in opposite direction so as to eliminate the effect of
armature flux on the main field flux. The load characteristics of dc generators

have been shown in Fig. 7.14. The voltage drop from E to V is very small as
compared to E. The shunt generators can be considered as constant voltage
output generators for practical purposes.

In case of series generators, the field current is the same as the load
current. Initially there is some induced EMF due to residual magnetizm of the

field poles. As the generator is loaded, current through the field increases and
E and V increase and later saturation effect takes place. Further loading

increases the armature reaction effect and voltage starts falling.

Figure 7.14 Load characteristics of dc generators: (a) separately excited dc generator; (b) dc shunt
generator; (c) dc series generator