7 Alternating
Current Welding
Theoretically,
straight a.c. welding is a combination of DCSP and DCRP welding. This can best
be explained
by showing
the three current waves visually. As shown in Fig. 1-4, half of each complete
a.c. cycle is DCSP;
the other
is DCRP. In order to weld aluminum without chemically cleaning the surface just
prior to welding,
alternating
current is often used. The
reverse polarity portion of the AC wave removes the
oxide layer
(cleaning)
while the straight polarity portion provides the heat
for melting the joint.
However,
at lower currents or when transferring from straight to reverse
polarity, the arc is extinguished
because
the flow of current is obstructed (partially or completely) during the reverse
polarity cycle. This is
called
rectification. During the reverse polarity cycle, the oxides, scale, moisture,
etc. provide a point for the
electric
arc to initiate (cathode). At low potentials (low voltage) on a clean aluminum
plate, the reverse
polarity
cycle could be lost due to this phenomenon of rectification. For example, if
no current at all flowed in
the
reverse-polarity direction, the arc would be lost, and the current wave would
look something like Fig. 1-5.
FIG. 1-4 – A.C. Wave
FIG.
1-5 – Rectified A.C. Wave
