Power Supply Controls
Depending on the type of power supply, the type and
number of controls provided for the welder may range
from a tap and a simple rheostat to a multitude of controls
that vary each function. A small autobody type machine
may only have a tap connection for the voltage setting
and a wire feed adjustment for wire speed. Another
machine may be used for making very high quality welds
and would need a number of controls to meet its require-
ments. The various controls that may be located on a
machine, as shown in Figure 3-2, include:
• Open circuit voltage (OCV) range. GMAW
power supplies produce a range of open circuit
voltages to a high near 50 volts. This range of
voltages may be controlled by changing taps,
switches, or levers that are usually located on the
front of the machine. As a general rule, the short-
arc mode of welding requires a setting on the low
end of the OCV scale and the spray-arc mode
requires a setting on the middle to upper end of
the scale. When setting up a welding power
source, the welder actually sets the open circuit
voltage. After the weld is started, the actual arc or
load voltage is established. The OCV may be
established on machines that are equipped with
voltmeters by following these steps:
1. Turn power supply on.
2. Release idler roller pressure on the wire feeder to
prevent wire feeding.
3. Place the voltage range switch in the desired
location. (Check machine manual to select range of
open circuit voltages desired.)
4. Hold the gun away from ground or workpiece and
energize the contactor switch. (When you depress
the switch, the contactor allows current to flow to
the electrode tip and the wire. An arc will be made if
the contact tip or wire touches ground.)
5. Observe the voltmeter and adjust the fine tuning
voltage control to the desired open circuit voltage.
This is the open circuit voltage; arc voltage will be
approximately 2 to 3 volts lower for each 100
amperes.
6. Release the trigger or switch and reset tension on the
wire feeder idler roller.
• Arc voltage adjustment. Also called load
voltage, the arc voltage determines the actual arc
gap established during the welding period. Adjust
the fine voltage control (the same control used to
set the open circuit voltage) to the voltage desired
during welding.
• Slope adjustment. The word slope in GMAW
refers to the slant of the volt-ampere curve and
the operating characteristics of the power supply
under load. In many machines, the slant of the
volt-ampere curve is automatically set as you
change the open circuit voltage. In others, the
slope can be changed by the operator for different
modes of welding. The curve is known as either
flat or steep mode.
When using a steep slope, as shown in Figure 3-3,
there may not be enough current to melt the electrode at
the proper time in the short-arc cycle. If the arc will not
start properly and the wire stubs out on the workpiece,
the slope of the curve has to be decreased (flattened) to
operate properly.
A machine set-up with a rather flat slope, as shown
in Figure 3-3, has too much current available for the
short-arc cycle. As a result, the wire will be blasted off
the end, resulting in considerable spatter. In this case, the
situation requires changing to a steeper slope.
28 Gas Metal Arc Welding Handbook
100% Full Time
60% 6
50% 5
40% 4
30% 3
20% 2
Number of Minutes
Machine Can Be
Operated at Rated
Load in a
10 Minute Period
Duty Cycle
Figure 3-1. Power supply duty cycles limit the number of
minutes that a unit can be operated at rated load.
Figure 3-2. This machine is rated at 300 amperes dc with
adjustable voltage, inductance, and slope.
(Miller Electric Mfg. Co.)
Arc voltage
adjustment
Slope
adjustment
Inductance
adjustment
OCV range
switch