May 9, 2014
Q: What is the difference between constant current and constant voltage?
A: I’m often called on to explain this topic when I teach. Electric arc welding has two main variables: current and voltage. Welding machines will supply both, but they are capable of consistently maintaining only one of those variables, while the other is maintained by some other means.
Direct-current (DC) power supplies can be constant-voltage (CV) or constant-current (CC). CV equipment, typically used for semiautomatic and wire-fed processes like gas metal arc welding (GMAW) or flux-cored arc welding (FCAW), provides a consistent preset voltage. Presetting the weld voltage on a CV power supply will maintain a constant arc length, because arc length is directly related to weld voltage.
So what determines the current draw? On CV equipment the wire feed rate, wire diameter, and electrode stick-out draw current from the machine. Set at 28 V, your GMAW or FCAW equipment will set an arc length that will remain constant. As you increase wire feed speed or wire diameter, you will increase your welding current. By increasing current, you increase penetration. On the other hand, maintaining that wire feed speed and increasing electrical stick-out introduces resistance into the electrode and reduces current, thereby reducing penetration.
To avoid burn-through, you will increase stick-out, which reduces current and penetration. You can easily reduce weld current 25 to 50 amps simply by increasing stick-out.
CC equipment, typically used for manual welding processes such as shielded metal arc welding (SMAW), provides a consistent preset current. A CC power supply with a preset current will maintain that amperage setting.
So what determines the voltage? Remember what I said previously about voltage being directly related to arc length? You regulate voltage manually during SMAW or GTAW. By moving that stick or tungsten electrode closer to or farther from the work (generally speaking, less is best).
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