190 Electrical Power Systems Technology
The resulting DC output voltage of the three-phase, full-wave rectifier
circuit is shown in Figure 7-25B. The operation of the circuit is similar to
that of a single-phase bridge rectifier in many respects. At any single in-
stant of time during the three-phase AC input cycle, the anode voltage
of one of the diodes is more positive than that of all the others, while the
cathode voltage of another diode is more negative than that of all the oth-
ers. These two diodes will then form the conduction path for that time
period. This conducting action is similar to a bridge rectifier, since two di-
odes conduct during a time interval. Each rectifier in this circuit conducts
during one-third of an AC cycle (120°). Peak positive DC output voltage
occurs during every 60° of the three-phase AC input.
Rotary Converters
Another method that has been used to convert AC to DC is the use
of a rotary converter. Rotating AC-to-DC converters are seldom used today.
However, a motor-driven generator unit, such as shown in Figure 7-26
can be used to convert DC to AC. This system is called an invertor. When
operated as a converter to produce DC, the machine is run off an AC line.
The AC is transferred to the machine windings through slip rings and con-
verted to DC by a split-ring commutator located on the same shaft. The
amount of DC voltage output is determined by the magnitude of the AC
voltage applied to the machine. Converters may be designed as two units,
with motor and generator shafts coupled together, or as one unit housing
both the motor and generator.
Figure 7-24. Three-phase half-wave rectification using a transformer