Direct Current Power Systems 175
If the load circuit is disconnected, no current flows through the gen-
erator. However, the field coils retain a small amount of magnetism known
as esidual magnetism. r Because of residual magnetism, a current will begin
to flow through the generator and the load circuit when a load circuit is
connected. The current will continue to increase, thus causing an increase
in the magnetic strength of the field. The output voltage rises proportion-
ally with increases in current flow through the load and generator circuits.
The output curve of a series-wound DC generator is shown in Figure 7-14.
The peak of the curve is reached when magnetic saturation of the field oc-
curs. Magnetic saturation prohibits any increase in output voltage. After
saturation is reached, any increase in load current causes a rapid decline
in the output voltage due to circuit losses.
The output voltage of a self-excited series-wound DC generator varies
appreciably with changes in load current. However, beyond the peak of
its output curve, the load current remains fairly stable, even with large
variations in voltage. There are specific applications, such as arc welding,Figure 7-13. Simplified illustration of a self-excited, series-wound DC generator:
(A) Pictorial diagram, (8) Schematic diagram