Direct Current Power Systems 173
the field coils. The output voltage of generators that use electromagnetic
fields can be controlled with ease.
The direct current used to establish the electromagnetic field is re-
ferred to as the excitation current. When DC excitation current is obtained
from a source separate from the generator, the generator is called a sepa-
rately excited DC generator. This type of generator is shown in Figure 7-12.
Storage batteries are often used to supply the DC excitation current to this
type of generator. The field current is independent of the armature cur-
rent. Therefore, the separately excited generator maintains a very stable
output voltage. Changes in load of the external circuit affect the armature
current, but do not vary the strength of the field. The voltage output of a
separately excited DC generator can be varied by adjusting the current
through the field. A high-wattage rheostat (variable resistance) connected
in series with the field coils will accomplish field control of a separately
excited DC generator.
Separately excited DC generators are used only in certain applications
where precise voltage control is essential. Automatic control processes in
industry often require such precision. However, the cost of a separately
excited DC generator is often prohibitive, and, therefore, other means of
obtaining DC electrical power are usually used.Figure 7-11. Simplified drawing of a permanent-magnet DC generator: (A)
Pictorial diagram, (B) Schematic diagram