If it is an arc welder it is constant current. MIG uses constant voltage and is a completely different design.
There is no easy conversion. There are voltage sensing wire feeders that will work but they only work well at higher currents (usually 120A and above.)
Hello Shenion and thanks for the reply. This is very interesting. I can't help wondering why mig uses constant voltage? I have also heard there are welders that operate on constant power. Can you explain the purpose of these different ways of delivering heat to the weld?
Thanks and best regards. CIJ
MIG requires constant voltage (CV) to work properly. On constant current (CC), when the wire contacts the weld pool and a drop of metal is pulled off, it will melt back all the way to the tip like a fuse burning. With constant voltage, as the arc length increases, the current drops, so it only melts back a little bit until the current drops below the melting point. This is a self regulating arc length.
With CV mode, if you increase the distance between the torch and work, the current drops slightly. The arc will change duty cycle to accommodate (percentage of time the wire is touching the weld pool.)
My Airco (CC Arc/TIG welder) came with a MIG wire feeder. I have tried it on MIG and it was near impossible to use. If you had the current, wire speed and torch length perfect it worked fine. If any was the slightest bit off, the arc length would start oscillating between the work and torch or it would spatter like crazy. It was very sensitive to the distance between the torch and work.
A voltage sensing wire feeder will help, but is limited to about 100A minimum on 0.8mm wire. Once the arc starts, the current needs to drop below 20A for the wire to stop melting back. With CC mode, if you set the current low enough to not melt back, the next time the wire hits the weld pool, it will not start a new arc, the entire wire just melts.
I have read that a MIG current at 100A will vary between about 10A and 400A. It has very short high-current pulses followed by low current once the arc starts. I have measured this in my tests.
As a side note, I made a board for my Airco that varied to current to maintain a constant voltage (the Airco allows external electronic control.) It worked much better down to about 100A. Below that, the minimum current of 20A caused instability.
Constant power is designed to keep the heat constant. With CC mode, as the arc length increases, the voltage increases but the current stays near the same, so the power (heat) actually increases (hence the instability.) With constant voltage the current drops with arc length and the power decreases. Constant power mode adds additional stability to CV mode; it will not vary the amount of heat with torch length. It is used on MIG vrzing of high alloy steels that must be kept below a critical temperature to prevent hardening or annealing of the metal.
constant current vs constant voltage for MIG operation:
Hi again Shenion: I have read and understood your concise explanation. Thanks much for that.
So, with the wide current swing of CV the lower amperage units are very limited for thicker metal? I don't plan to weld anything thicker than 6mm, in that case what current capability do you think would be required for a duty cycle of 60%.
I ask this because, I see manufacturers quoting very optimistic performance that is hard to believe.
For smaller MIGs, the metal thickness tends to be a bit inflated and also for gasless wire.
Here it is common to say 100 amps per 0.1 inch. So 6mm (0.236") is 230 amps or so. I find it is just a ball park.
A 180-220A MIG would probably be fine for your needs. There are many in this range. The cheap ones will have low duty cycles and small wire feeders. at 180A you may need to do a bit of prep to get full penetration welds.
My take on duty cycles is you want to work in the 60% range or better. Suppose, most of your work is 130A (1/8") and the machine operates at 60%. For DIY use, you spend time setting up the next weld so it is hard to get over a 40% actual usage. So, the machine will stay cool.
If you were welding the 6mm all day, then a 220A machine would be working at its limit and might not last as long.
For MIG, the voltage range sets the current range (also set by wire size.) More voltage settings increase costs as it needs more taps on the transformer. So, cheaper ones may not have lower voltage (current) settings as this requires more windings.
Duty cycle has to do with the gauge of the wire used. Copper is better as it conducts more. Aluminum is cheaper but causes more heat losses an usually a lower duty cycle.
Note aluminum windings is not a bad thing. Most power transmission transformers have aluminum on their output side. These run 24/7 with no problems. The issue is for a welder, to use aluminum they will need to use wire twice as thick to get the same duty cycle. That reduces the cost advantage over copper. Low-end migs will use aluminum windings to save costs at the expense of duty cycle.