186 Electrical Power Systems Technology
occurs, as shown in Figure 7-22A, diodes D1 and D3 are forward biased,
while diodes D2 and D4 are reverse biased. This biasing condition is due to
the instantaneous charges occurring during the positive alternation. The
conduction path is from the instantaneous negative side of the AC source,
through diode D3, through the load device, through diode D1, and back
to the instantaneous positive side of the AC source.
During the negative alternation of the AC input, diodes D2 and D4 are
forward biased, while diodes 01 and 03 are reverse biased. Conduction oc-
curs, as shown in Figure 7-22B, from the instantaneous negative side of the
source, through diode D2, through the load device, through diode 04, and
back to the instantaneous positive side of the AC source. Since a voltage is
developed across the load device during both half-cycles of the AC input,
a full-wave output is produced, as shown in Figure 7-22, that is similar to
that of the full-wave rectifier discussed previously. For high values of DC
output voltage, the use of a bridge rectifier is desirable, since the diode piv
rating is one-half that of other single-phase rectification methods. A typi-
cal design of a bridge rectifier unit is shown in Figure 7-22.
Three-phase Half-wave Rectification
Most industries are supplied with three-phase AC. It is therefore
beneficial, because of the inherent advantages of three-phase power, to
use ee-phase rectifiers thr to supply DC voltage for industrial use. Single-
phase rectifiers are ordinarily used where low amounts of DC power are
required. To supply larger amounts of DC power for industrial require-
ments, a three-phase rectifier circuit, such as the one shown in Figure 7-
23, could be employed. Three-phase rectifier circuits produce a purer DC
voltage output than single-phase rectifier circuits, thus wasting less AC
power.
Figure 7-23 shows a three-phase half-wave rectifier circuit that does not
use a transformer. Phases A, B, and C of the wye-connected three-phase
source supply voltage to the anodes of diodes D1, D2, and D3. The load
device is connected between the cathodes of the diodes and the neutral
point of the wye-connected source. Maximum conduction occurs through
diode D1, since it is forward biased, when phase A is at its peak positive
value. No conduction occurs through diode D1 during the negative alter-
nation of phase A. The other diodes operate in a similar manner, conduct-
ing during the positive AC input alternation and not conducting during
the associated negative AC alternation. In a sense, this circuit combines
three single-phase, half-wave rectifiers to produce a half-wave DC out-