178 Electrical Power Systems Technology
varies. A large load causes the armature current to decrease. This decrease
is desirable because it provides the generator with a built-in protective
feature, in case of a short circuit.
The self-excited shunt-wound DC generator is used when a constant
output voltage is needed. It may be used to supply excitation current to
a large AC generator or to charge storage batteries. However, in applica-
tions where initial expense is not critical, the compound-wound generator
described next may be more desirable.
Self-excited Compound-wound DC Generator—The compound-wound DC
generator has two sets of field windings. One set is made of low-resistance
windings and is connected in series with the armature circuit. The other set
is made of high-resistance wire and is connected in parallel with the arma-
ture circuit. A compound-wound DC generator is illustrated in Figure 7-17.
As discussed previously, the output voltage of a series-wound DC
generator increases with an increase in load current, while the output volt-
age of a shunt-wound DC generator decreases with an increase in load
current. It is possible to produce a DC compound-wound generator, uti-
lizing both series and shunt windings, that has an almost constant voltage
output under changing loads. A constant voltage output can be obtained
with varying loads, if the series-field windings have the proper character-
istics to set up a sufficient magnetic field to counterbalance the voltage re-
duction caused by the I × R drop in the armature circuit.
A constant output voltage is produced by a flat-compounded DC gen-
erator. The no-load voltage is equal to the rated full-load voltage in this
type of machine, as shown by the output curves in Figure 7-18. If the se-
ries windings produce a stronger field, the generator will possess a series
characteristic. The voltage output will increase with an increase in load.
A compound-wound generator whose full-load voltage is greater than its
no-load voltage is called an overcompounded DC generator. Likewise, if the
shunt windings produce a stronger field, the output will be more charac-
teristic of a shunt generator. Such a generator, whose full-load voltage is
less than its no-load voltage, is called an undercompounded DC generator.
Compound-wound DC generators can be constructed so that the se-
ries and shunt fields either aid or oppose one another. If the magnetic po-
larities of adjacent fields are the same, the magnetic fields aid each other
and are said to be cumulatively wound. Opposing polarities of adjacent coils
produce a differentially wound machine. For almost all applications of com-
pound-wound machines, the cumulative-wound method is used. A gen-
erator wound in this way maintains a fairly constant voltage output with