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CONSTANT CURRENT WIRE WELDING
(See Figure B.3)
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 speci- fy 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 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 provi- sion 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 diam- eter, 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, short- ening the contact tip to work distance lowers the volt- age.
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
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.
FIGURE B-3
Constant Current
Power Source
Current
+
Current
-
Wire
Feeder
CTWD
WFS