Lincoln Electric BIG RED 500 Service Manual page 54

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CONSTANT CURRENT OPERATION WITH
A LINCOLN ELECTRIC WIRE FEEDER
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 oper-
ated in the constant voltage or constant current mode.
Instead, codes typically specify limitations on the cur-
rent, voltage, heat input and preheat temperature
based on the material to be welded. The intention is to
assure that proper weld material properties will devel-
op. 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 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. If
the contact tip to work distance is properly maintained,
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 shorted or the arc length is very
short.
OPERATION
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.
BIG RED ® 500/600
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