©
COPYRIGHT 1999 THE ESAB GROUP, INC. LESSON
I, PART B 1.8.7
Rectifying
AC to
DC -
Although much welding is accomplished with AC welding
power sources, the majority of industrial
welding is done with machines that produce a direct
current arc. The commercially produced AC power
that operates the welding machine must
then be changed (rectified) to direct current
for the DC arc. This is accom- plished
with a device called a rectifier. Two
types of rectifiers have been used extensively
in welding machines, the old
selenium rectifiers and the more modern
silicon rectifiers, often referred to
as diodes. See Figure 16. 1.8.7.1
The function of a rectifier in the
circuit can best be shown by the use
of the AC sine
wave. With one diode in the circuit, half-wave
rectification takes place as shown in
Figure 17. 1.8.7.2
The negative half-wave is simply cut
off and a pulsating DC is produced. During the
positive half-cycle, current is allowed to flow through the rectifier. During
the negative half-cycle,
the current is blocked. This produces a DC composed of 60 positive pulses
per second.
1.8.7.3 By
using four rectifiers connected in a certain
manner, a bridge rectifier is created, producing full
wave rectification. The bridge rectifier results in
120 positive half-cycles per second,
producing a considerably
smoother direct current than half-wave rectification.
See Figure 18. 1.8.7.4
Three-phase AC can be rectified to
produce an even smoother DC than single-phase
AC. Since three-phase AC power
produces three times
as many half-cycles per second as single- phase
power, a relatively smooth DC voltage results
as shown in Figure 19. SINGLE
PHASE HALF WAVE RECTIFICATION FIGURE
17 FIGURE 16
SILICON RECTIFIER
SELENIUM RECTIFIER
SINGLE PHASE FULL WAVE RECTIFICATION
FIGURE 18 1
CYCLE 3 PHASE
FULL WAVE RECTIFICATION FIGURE
19
