Unlike the separately excited generator, there is no current
in the field circuit when the armature is motionless. Since a small amount of
residual magnetism is present in the field poles, a weak residual voltage is
induced in the armature as soon as the armature is rotated. This residual
voltage produces a weak current in the field circuit. If this current is in the
proper direction, an increase in magnetic strength occurs with a corresponding
increase in voltage output. The increased voltage output, in turn, increases
the field current and the field flux which, again, increase the voltage output.
As a result of this action, the output voltage builds up until the increasing
field current saturates the field poles. Once the poles are saturated, the
voltage remains at a constant level, unless the speed of the armature rotation
is changed.
If the direction of armature rotation is reversed, the brush
polarity also is reversed. The residual voltage now produces a field current
which weakens the residual magnetism and the generator voltage fails to build
up. Therefore, a self-excited machine develops its operating voltage for one
direction of armature rotation only. The generator load switch may be closed
when the desired voltage is reached.