CV vs CC welding

[The Soulman]

I've been occasionally welding for a couple of years now, both stick electrode and mig.
What always puzzled me why does stick welding use a constant current and mig a constant voltage transformers?
Both create an arc between work piece and electrode (wire). 

[Doctorandus_P]

Probably neither is very "constant".
Simple stick welders are nothing more than a transformer with an adjustable mechanical shunt which limits the energy transfer from primary to secondary side.

With this shunt type power limiting the output voltage collapses once you go over the adjustable current, but the current is also not very stable, and changes with arc length, drops of metal temporarily shorting etc.

MIG welders often have a bunch of fancy electronics in them which can do all sort of fancy things to your welding current / voltage.
The algorithm is probably dependent on the brand of welder used.

For simplicity, let's assume that the impedance of the arc is lineair with it's length.
If you have a CV regulator, then the energy input would get less with a longer arc.
If you have a CC regulator, then the energy input would increase with a longer arc.

My guess is that if you do something in between, you could keep the power input almost constant with a varying arc length, which makes welding probably a bit easier. But making the energy input dependent on arc lenth gives the welder more control during the weld.

Every now and then you find some reference of stick welding with a few car batteries.
This is sort of Constant Voltage, but with welding currents the voltage from a car battery will drop so it is not really "constant" anymore.

[The Soulman]

Thanks, P

Makes sense, it probably has something to do with the control ability of the arc/weld puddle.

The mig welders I have are old school ones with multiple select able primary tabs with on the secondary side rectifier diodes and a massive coil, I could attempt rigging up a stick weld lead from that and see if that works.

The reason for asking is that I might want to get a small inverter welder (stick/tig) and hookup a separate box to that for mig use
for a more portable setup.
 
And because I must know. 

[MadTux]

Stick/TIG: Welder (i.e. you) has control over arc length. You don't want to have contact between electrode and workpiece except for scratch start. With constant, variable current, you can control the heat input. You can control the welding voltage by holding your torch closer (less voltage) or farther (more voltage) from the workpiece.

MIG/MAG: Welder has no control over arc length, because wire comes out continuously with variable speed. However, you don't want to have contact (short circuit) between wire and workpiece. So you need variable current at fixed voltage. If the wire/workpiece distance gets shorter, there will be an arc with more current, which gives higher wire melting rate. Longer arc needs more volts, so less amps and arc lenght gets shorter again. With fixed current, the operator probably would have to set the wire speed very accurately to get an arc (which probably wouldn't be stabe, because fixed current gives no negative feedback on arc length)

[The Soulman]

Thanks MadTux.

Sounds really plausible, would you think spray-arc mig could benefit from constant current?
i.e. more distance=more voltage=more power (at the same current), and so a better controlled spray?

[langwadt]

Stick/TIG: Welder (i.e. you) has control over arc length. You don't want to have contact between electrode and workpiece except for scratch start. With constant, variable current, you can control the heat input. You can control the welding voltage by holding your torch closer (less voltage) or farther (more voltage) from the workpiece.

MIG/MAG: Welder has no control over arc length, because wire comes out continuously with variable speed. However, you don't want to have contact (short circuit) between wire and workpiece. So you need variable current at fixed voltage. If the wire/workpiece distance gets shorter, there will be an arc with more current, which gives higher wire melting rate. Longer arc needs more volts, so less amps and arc lenght gets shorter again. With fixed current, the operator probably would have to set the wire speed very accurately to get an arc (which probably wouldn't be stabe, because fixed current gives no negative feedback on arc length)

the usual "sounds like frying bacon" MIG welding is short circuit

[The Soulman]

Stick/TIG: Welder (i.e. you) has control over arc length. You don't want to have contact between electrode and workpiece except for scratch start. With constant, variable current, you can control the heat input. You can control the welding voltage by holding your torch closer (less voltage) or farther (more voltage) from the workpiece.

MIG/MAG: Welder has no control over arc length, because wire comes out continuously with variable speed. However, you don't want to have contact (short circuit) between wire and workpiece. So you need variable current at fixed voltage. If the wire/workpiece distance gets shorter, there will be an arc with more current, which gives higher wire melting rate. Longer arc needs more volts, so less amps and arc lenght gets shorter again. With fixed current, the operator probably would have to set the wire speed very accurately to get an arc (which probably wouldn't be stabe, because fixed current gives no negative feedback on arc length)

the usual "sounds like frying bacon" MIG welding is short circuit

Correct, and can we therefore assume that the limited voltage prevents the gun going in to spray-arc "mode" and enables normal welding?