Instructions / Assembly

25
GMAW
www.lincolnelectric.com
•
Advanced process power sources dedicated to Surface
T
ension Transfer™ and GMAW-P. They range in output from
225 - 655 amps, and all of these systems require three-phase
input power. For example see Figure 21.
•
Engine driven power sources that range from 200 - 600 amps
of output. See page 31 for an example of a portable engine
driven GMAW system.
FIGURE 21: PowerWave
®
455M/STT GMAW System with
Power Feed 10M
Typically, constant current applications were confined to large
diameter/large weld puddle aluminum GMAW applications or
large diameter/large weld puddle carbon steel applications.
Constant voltage power source designs provide a specific arc
voltage for a given pre-selected wire feed speed. The volt-amp
curve, or slope, is comparatively flat. As the CTWD increases
with these types of power sources, there is a decrease in the
welding current. As the CTWD decreases there is an increase
in the welding current. The arc in this case becomes a series
circuit, and the CTWD provides resistance to current. In either
scenario, the voltage remains the same and the arc length
remains the same, see Figure 24 on page 26.
FIGURE 22: CC (Constant Current) Power Source
Volt-Amp Characteristics
Operating point
Current, A
∆ V
∆ A
Voltage, V
Operating point
Current, A
∆ V
∆ A
Voltage, V
FIGURE 23: CV (Constant Voltage) Power Source
Volt-Amp Characteristics
The Power Source
Power sources incorporate output characteristics designed to
optimize the arc performance for a given welding process. For
GMAW, the output characteristics fall into two main categories:
• constant current
• constant voltage
Each of these two terms references the volt-ampere characteris-
tics of the power source, and in each case, the volt-ampere
relationship references the slope of the output. See Figures 22
and 23 for comparison output curves of CC (constant current)
and CV (constant voltage).
Dedicated constant current power sources were more widely
used in the early days of GMAW than they are today, but they
see continued use in the welding of aluminum. The design uses
a drooping output curve, see Figure 22. In constant current, the
CTWD (contact tip to work distance) determines the arc length.
As the CTWD increases the arc length increases, and as the
CTWD decreases the arc length decreases. This presented a
problem for semiautomatic welding because it is difficult to
maintain the same CTWD. To compensate for this problem an
arc voltage controlled wire feeder was designed to compensate
for c
hanges in arc length. In this scenario, as the CTWD
decreases, the wire feed speed would increase; and as the
CTWD increases, the wire feed speed would decrease.