You guys who know more about welding, 'splain me this:
How come I couldn't use an existing high-frequency/DC TIG welder as a supply for plasma cutting?
My welder will dial down in amperage to the range most plasma torches expect, and the HF side will allow auto-strike of the arc.
So, what's the difference in a 'real' plasma cutter?
LLoyd
The arc voltage of a plasma cutter is higher than the arc voltage of a welder.
Best Regards Tom.
There is a good 10X difference in output voltage between a TIG welder (~30V) and a plasma cutter (~300V), HF for pilot arc starting has largely been replaced with a "blowback" system (contact start internal to the torch essentially), and the control of the entire process is a lot different.
"azotic" fired this volley in news:k1o556$fua$1 @speranza.aioe.org:
Arc voltage at constant current is a function of the arc gap. The SW-300 is a constant-current source, and provides high frequency to start the arc.
Lloyd
"azotic" fired this volley in news:k1o556$fua$1 @speranza.aioe.org:
I didn't mean that last to sound like I didn't agree with you. It's just that the arc gap is quite small in most torches I've seen, and would be POSSIBLE to have an arc at 30-50V, if you can just get it struck first.
A welder certainly would not work on a blow-back start torch, but I wonder if it would work on an old-style one with high frequency to initate the arc.
In my last... _Once the arc has started_, the voltage is a function of gap, given constant current.
Lloyd
I believe you need a constant voltage source to generate and sustain a plasma. The pressure and volume of the gas flowing thru the plasma torch is going to determine the voltage needed to establish and maintane a true plasma. Also the gas conductivity is a factor ( air, nitrogen, argon, etc.) in plasma formation. I attempted what you are thinking about doing back in in the 1980's and it did not work with a conventional welder even with the HF set on all the time. I now own an old airco plasma cutter 100A, its old (1970's?) but very servicable with descrete components. The dc power supply that runs the plasma torch produces about 200vdc. I did some reasearch back in the 1980's studying schematics i aquired from several manufactures of plasma cutters and they all ran between 200 to 400vdc to the plasma torch depending on how thick a material they would cut. As i recall the thicker the material the higher the voltage required. Hope this helps.
Best Regards Tom.
I do not have a plasma cutter so treat this as opinion not fact.
But the arc voltage is also a function of pressure. See Pachen curves. So the arc voltage would be higher if the arc gap under pressure.
Dan
" snipped-for-privacy@krl.org" fired this volley in news:ea9ef946- snipped-for-privacy@b8g2000yqh.googlegroups.com:
Dan, I don't think the 30psi-or-so necessary would elevate the voltage that much... have to look at the tables for dielectric constant of air at different pressures to make sure.
I also checked; my Miller will go to 100V, so it might work for thin materials (Almost everything I cut is 3/8" or less)
LLoyd
30 psig is three times the pressure. So the Mean Free Path would be 1/3 as long. So the arc voltage would be three times as high. Unless my memory has failed me.
=20 Dan
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