PWM Control for Plasma Cutters

I may be going out on a limb here (how many EE's in the world have designed plasma cutters?), but does anyone have an idea on how the
feedback loop works in a switch mode plasma cutter? Is it direct PWM from the power adjustment pot, or voltage/current mode with the pot adjusting the bias? The no load voltage floats to the peak voltage when there is no arc, and to complicate matters, there's a 10 amp pilot current that needs to be controlled from a microprocessor (or current sensor). If anyone has full schematics of the control section of a switch mode plasma cutter to email, I'd really appreciate it.
BTW, it's for a hobby/educational project of mine, to eventually build a smps plasma cutter.
Thanks.
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I would think that the latter is the case, as the resistance of the load (plasma arc) varies over time due to various factors like distance of torch to work, etc.

Yes.
I am building a SCR controlled plasma cutter actually. I already have this power supply working as a tig welding machine and arbitrary DC power supply, and use it for that, and am slowly working on also making it do plasma at higher voltage. Its transformer can be rewired to produce high voltage. Which I do with contactors. I use a microcontroller programmed in BASIC.
I think that it is a lot easier to work with SCRs, for me (esp. since I got a nice SCR controller already), than with IGBTs or MOSFETs for inverting/PWM.
I would post your questions to sci.electronics.design and rec.crafts.metalworking. Here we have welding experts, and some electronics experts from whom I learned, but not as many electronics gurus as in two newsgroups that I named.
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Ignoramus25686 wrote:

How critical is regulation? Plasma cutting seems fairly tolerant to variations. I've seen one lincoln (procut-25, http://content.lincolnelectric.com/pdfs/products/navigator/im/IM665.pdf pg 26) without an output smoothing cap. It had a shunt on the secondary side, which is something I've almost never seen on a SMPS. Maybe there's a PLC accepting the input from the shunt and the pot to allow the pilot current/nonlinear power control, then using a DAC for direct PWM of the controller. You wouldn't happen to have any upclose pictures of a plasma cutter control board?

I've seen your site, and it's definately a project I would like to do some day. Good luck with that, would be interesting to see just how complicated these $5k+ welder are.

What about turnoff? Forced commutation sounds difficult and slow, but then again there's a company (Arcon) that exclusively makes SCR based inverters. I bet the sound of 10 kHz gets annoying after a while...The dual forward topology means the fets will never be exposed to anything more than Vin, so snubbers are virtually eliminated (although diode recovery delay allows some excess voltage on turnoff). Gate drive can become tricky, but there's plenty of literature available online from both professionals and tesla coilers (sstc's).

Already crossposted to sci.electronics.design, but will post to rec.crafts.metalworking.
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A cap does not regulate current... Am I missing anything here?

Um, I have my own control board, that's not what you are interested in, right?
My immediate plan is to connect my torch to a source of dry air and get a pilot arc going with maybe 10a of current.
If I get to this point, I will have to do a few things such as:
1) attach gas quick couplers to the former water valve on my welder (this valve is not presently used, I use a separate torch cooler that I built). I iwll use this valve as a gas valve for plasma cutting.
2) Add two relays, one for turning off the pilot arc, and one for activating the air valve, to the control board
3) Write BASIC code for plasma cutting (I have some code now, but it is all wrong). By the way, my source code is GPLed and is at
http://igor.chudov.com/projects/Welding/11-New-Rectifier/source.txt
4) I have a plastic carrying case about 20x14 inches, I want to make it into a holder for the torch, air/electricity adapters etc, so that I can use it to store the torch when not used. Then when I NEED TO CUT something, I would open up the case, connect quick connects to air/water/main power/control and start cutting.

Yeah. My welder is not exactly cheap in construction, I just use surplus components so that the project is inexpensive, but doing what I do on a production basis would be expensive. E.g, I have a big high frequency arc starter module, big transformer, a lot of contactors, IGBTs for the inverter part (not yet plugged in), SCRs, SCR firing board, etc. All of that would cost a lot new, but cost little on a used/surplus/ebay basis.
I basically want to build something that would cost over $10k, like a powerful multiprocess tig/mig/arc welder and plasma cutter and DC power source in general and power supply with square wave. Out of the above, I have a tig/arc welder and arbitrary power supply mode basically working. (have not yet tried tig pulsing yet) Unlike some other people, I have pictures of sample welds.

No, I think that I mis-stated what I was doing (or did not state it clearly).
I have a SCR/transformer based DC power supply. (works ok). It is controlled by COTS SCR firing system by PCTI. The transformer is a 60 Hz transformer, a lot of iron.
For the welding square wave inverter (to make square wave AC for welding aluminum) I built a IGBT fired inverter.

Plus there are some good gate drives sold on the market (like Semikron boards)
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20 years ago, as an exam project, I build a 150 A welding machine; That thing used a simple two-transistor (with cascode MOSFET/BIPOLAR switches because all devices were crap then ;-) forward inverter running at fixed 48% duty-cycle and used a saturable inductor on the secondary side to control the current; the control was just an analog PI controller. Nothing complex at all (except the transformer, switches, nasty voltages and currents, decoupling caps going open e.t.c e.t.c).
Took about 4 months to do, at eight-to-twelve hour per day. And I had a decent lab and equipment. .
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On Tue, 25 Jul 2006 17:14:30 +0200, Frithiof Andreas Jensen

That's cool. Could you actually weld anything with it?
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Frithiof Andreas Jensen wrote:

Was this using a flux reseting winding? I bet transients would be easier with cross coupling diodes, which were probably far away 20 years ago.

Wow. What was the majority of the work? I was imagining a complexity similar to http://www.stevehv.4hv.org/12kwPFC.htm
And do you have any pictures/schematics? Igor and I would definately be interested, and I bet a few more people here.
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I think that perhaps we ought to start a power electronics club or something. I am definitely very interested. I am not an electronics designer, I just learned what I thought I needed. I am a computer programmer.
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The converter was similar to a full bridge except with the switches for one cycle replaced with diodes so the flux reset is via the supply rail and those diodes. The half-bridge just works here because the supply was 380 V, 3 phase.
The main problem was that the "pulse-capacitors" burned - i.e. lost the capacitance - because the cascode switches were easily fast enough to exceed the di/dt rating of them. It took a bit of experiment to get decent caps. A second problem was that we used MOSFET's and got to learn something about ESD the hard way - ESD was all new then (here, anyway).

Pretty similar actually. The transformer and filter inductor took ages to design because we had to come up with a sound design method and there was not much information to go by (There were lots of how-to books but one has to show off some analytic capacity to pass university. ;-).
Then there is all the little things like the thermal ratings of filter capacitors and such - we did not even do anything clever like simulations, just simple energy-conservation calculations but all takes time and verifying by measuring takes more time, then one does a better model where there is pain e.t.c.
We also had to rebuild the switches a couple a times ...
But, 4-6 months is not unrealistic starting from scratch, even now that I know much more I.M.O.

I will see if I can dig the report out of the attic - it is there *somewhere* amongst the rubbish.
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