Lincoln Electric LN-25 SVM179-B - Manual (Page 21)

OPERATION B-7 B-7
LN-25™ PRO
CONSTANT CURRENT
VS
CONSTANT
VOLTAGE WIRE WELDING
Most semiautomatic welding processes perform better
using constant voltage power sources.
Welding codes usually do not address the power
source selection or specifically, whether the welding
process is to be operated in the constant voltage or
constant current mode. Instead, codes typically spec-
ify limitations on the current, voltage, heat input and
preheat temperature based on the material to be weld-
ed. The intention is to assure that proper weld materi-
al properties will develop.
Welding is sometimes performed using constant cur-
rent power sources. The operation can be more con-
venient because it may allow the use of an existing
stick (SMAW) power source and the power source can
be placed at a distant location without any provision for
adjusting the output settings.
For constant current operation, the power source is set
to deliver the specified current. The power source reg-
ulates this current regardless of changes in the weld-
ing circuit, including cable length, electrode diameter,
wire feed speed, contact tip to work distance, etc.
Changes in the wire feed speed (WFS) or contact tip to
work distance (CTWD) affect the arc voltage when
constant current power sources are used. Lowering
the wire feed speed raises the voltage, raising the wire
feed speed lowers the voltage. Lengthening the con-
tact tip to work distance raises the voltage, shortening
the contact tip to work distance lowers the voltage.
(See Figure B.3)
If the contact tip to work distance is properly main-
tained, a satisfactory operating voltage range may be
achieved, and a sound weld may result. However,
when a welder uses a longer contact tip to work dis-
tance, an arc-sensing wire feeder compensates by
increasing the wire feed speed to regulate the voltage.
Even if the voltage and current remain unchanged, the
increased wire feed speed may result in a deposition
rate well beyond the specified range of the electrode.
Under these conditions, the specified weld metal prop-
erties may not be achieved.
Constant voltage power sources deliver large current
surges to stabilize the arc when the electrode is short-
ed or the arc length is very short. However, a constant
current power source does not provide such a
response to stabilize the arc. It may be difficult to
achieve required weld metal properties, or to achieve
the required quality of welds needed to pass nonde-
structive tests, when such welds are made under con-
stant current operation.
For these reasons, Lincoln Electric does NOT recom-
mend constant current semiautomatic welding for
applications which need to meet specified weld metal
chemical or mechanical property requirements or weld
quality requirements.
Constant Current
Power Source
Wire
Feeder
Current
Current
CTWD
WFS
+
-
FIGURE B-3