5 Armature reaction - nptel - Indian Institute of Technology Madras

Electrical Machines I Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao
Indian Institute of Technology Madras
Terminal voltage
0
Open circuit
characteristic
A
Q
S
R B
P
External
characteristic
Load Current
PS=PQ-PR
Armature
characteristic
Figure 33: External characteristics of a Series Generator
open circuited.In a series generator load current being the field current of the machine the
self excitation characteristic or one and the same. This is shown in Fig. 33
5.2.5 Self excitation of compound generators
Most of the compound machines are basically shunt machines with the series wind-
ing doing the act of strengthening/weakening the field on load, depending up on the con-
nections. In cumulatively compounded machines the mmf of the two fields aid each other
and in a differentially compounded machine they oppose each other. Due to the presence of
the shunt winding, the self excitation can proceed as in a shunt machine. A small difference
exists however depending up on the way the shunt winding is connected to the armature. It
can be a short shunt connection or a long shunt connection. In long shunt connection the
shunt field current passes through the series winding also. But it does not affect the process
of self excitation as the mmf contribution from the series field is negligible.
74