Lincoln Electric LN-25X Operator's Manual page 25

LN-25X™
constant cUrrent operation
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 quality requirements.
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 output 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 contact tip to work distance raises the voltage,
shortening the contact tip to work distance lowers the voltage.
If the contact tip to work distance is properly maintained, a satis-
factory operating voltage range may be achieved, and a sound
weld may result. However, when a welder uses a longer contact
tip to work distance, 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 properties may not be achieved.
Constant Current
Power Source
Constant voltage power sources deliver large current surges to
stabilize the arc when the electrode is shorted 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 nondestructive tests, when such
welds are made under constant current operation.
For these reasons, 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 quality requirements.
FIGURE B.5
Current
+
Welding Cable
(Electrode)
Current
-
Welding Cable
(Work)
B-9
GUN AND CABLE
Wire
ASSEMBLY
Feeder
WFS
OPERATION
CTWD