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.
A cap does not regulate current... Am I missing anything here?
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?
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.
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.
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.
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...
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.
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).
Plus there are some good gate drives sold on the market (like Semikron
boards)
i
