180 Electrical Power Systems Technology
variations in load current. The compound-wound DC generator is used
more extensively than other DC generators, because of its constant volt-
age output and its design flexibility, which allows it to obtain various out-
put characteristics.
Common DC Generator Characteristics
DC generators are used primarily for operation with mobile equip-
ment. In industrial plants and commercial buildings, they are used for
standby power, for battery charging, and for specialized DC operations
such as electroplating. In many situations, rectification systems that con-
vert AC to DC have replaced DC generators, since they are cheaper to op-
erate and maintain.
DC generators supply power to a load circuit by converting the me-
chanical energy of some prime mover, such as a gasoline or diesel engine,
into electrical energy. The prime mover must rotate at a definite speed in
order to produce the desired voltage.
When DC generators operate, a characteristic known as armature re-
action takes place. The current through the armature windings produces a
magnetic flux that reacts with the main field flux, as shown in Figure 7-19.
The result is that a force is created that tends to rotate the armature in the
opposite direction. As load current increases, the increase in armature cur-
rent causes a greater amount of armature reaction to take place. This con-
dition can cause a considerable amount of sparking between the brush-
es and the commutator. However, armature reaction may be reduced by
placing windings called interpoles between the main field windings of the
stator. These windings are connected in series with the armature wind-
ings. Thus, an increase in armature current creates a stronger magnetic
field around the interpoles, which counteracts the main field distortion
created by the armature conductors.
Rating of DC Generator—The output of a DC generator is usually rat-
ed in kilowatts, which is the electrical power capacity of a machine. Other
ratings, which are specified by the manufacturer on the nameplate of the
machine, are current capacity, output voltage, speed, and temperature.
DC generators are made in a wide range of physical sizes, and with vari-
ous electrical characteristics.
DC Generator Applications—DC generator has declined rapidly use
since the development of the low-cost silicon rectifier. However, there are
still certain applications where DC generators are used. These applica-
tions include railroad power systems, synchronous motor-generator units,