Constant Current Operation - Lincoln Electric LN-15 WIRE FEEDER SVM166-A Service Manual

B-9

CONSTANT CURRENT OPERATION

( See Figure B.3)
CAUTION
Lincoln Electric does NOT recommend constant
current semiautomatic welding for applications
which need to meet specified weld metal chemical
or mechanical property requirements or weld qual-
ity requirements.
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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 specify limitations on the
current, voltage, heat input and preheat temperature
based on the material to be welded. The intention is to
assure that proper weld material properties will develop.
Welding is sometimes performed using constant current
power sources. The operation can be more convenient
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 out-
put settings.
For constant current operation, the power source is set to
deliver the specified current. The power source regulates
this current regardless of changes in the welding 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.
Constant Current
Power Source
OPERATION
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.
FIGURE B.3
Current
+
Welding Cable
(Electrode)
Current
-
Welding Cable
(Work)
LN-15
LN-15
GUN AND CABLE
Wire
ASSEMBLY
Feeder
WFS
B-9
CTWD