diy plasma cutter

[eloid]

i found this plan of a diy plasma cutter, had a few question, is this what they call high freq start ... or pilot start? i know hf are bad for cnc ? the note memtioned the relay can be removed for the coil starter...the question here is how do pilot start work, is pilot is created in the plasma torch? so it a matter of sourcing that kind of torch?

Does anyone elses have any diy plans?

[Fifty_ohm]

I'll try to answer your questions in order. And if anyone spots an error I've made please chime in.

It looks like the HEI ignition module, coil and spark plug are being used to generate a high voltage pulse to help start the plasma.

HF (high frequency) isn't necessarily bad for cnc but RF (radio frequency) will screw up communications in a hurry. Especially since this is using a spark plug outside of an engine block. It is basically a spark gap transmitter. This thing will have to be in a grounded box and all the cnc components and wiring will have to be grounded also to minimized the problems associated with RF.

What I read on that note about the relay was to disable turning OFF the HF start circuit. Meaning that it would be sparking all the time during operation.

The pilot start circuit adds a high frequency high voltage spark to the torch to initiate generation of a plasma at the tip of the torch. I'm only familiar with one plasma cutter torch but I believe that most (if not all) generate the plasma in basically the same way at the tip. What will be more important will be power handling of the torch you're going to try with this.

This is the first plan I've seen for a plasma cutter but you might check out some of the welding forums to see if anyone has made one. Here is one:

http://www.hobartwelders.com/weldtalk/

Was there any more information posted about this plan that might help explain things?

Good luck and let us know if it works out.

[eloid]

Please ecuse me for some basic set questions...I dont have a plasma unit.. just a newbie....

can someone explain... with off the self units...other than set working height,, , do you set current and how so? please explain typical setup ie for 1/4 steel plate cutting as a example.

[jcc3inc]

Sir,

As I look at the plans, there seems to be NO constant current source; the power supply is not well described. These units typically have a current control as part of the power supply. Also, what is the 7-9 ohms doing in the ground return of the schematic?? If you have a 50 amp plasma current, this will develop a 350 Volt drop!!! Is this supposed to be a substitute for the current control? It will be a 17,500 watt resistor!

Most commercial units use PWM for current control.

There's a WHOLE lot missing from this schematic!

Regards,
Jack C.

[bbutcher85710]

I agree about the current control. I am not sure what the function of the 7-9 ohm resistor would be other than limiting the current in the plasma arc. It appears there is some text that goes with the schematic that may explain it further. It seems reasonable that the resistor would be used to drop the voltage since the arc voltage once established should be much lower than the 310 volts or so the capacitor will charge to open circuit (i.e. peak voltage of 220 volt sine wave). Without some form of current control, the arc would either trip the 220 circuit breaker or destroy the diode bridge rectifiers (or both).

I found a DIY plasma cutter on you-tube (not free plans however) that uses a "water resistor" for current control. The water resistor consists of a water and copper sulfate solution with copper electrodes dipped into the water. I have used similar water resistors in the past, and it is not too difficult to achieve very high power ratings with them. It is another story if you want to keep the resistance constant over time however, since it is very sensitive to temperature and copper sulfate to water ratio. A DC current in a copper sulfate solution is a copper plating technique, so I suspect there will be some issues with copper electrodes eroding or the copper sulfate solution changing concentration, or both.

I suspect a much better method of current control would be to use a big inductor with PWM to control the current, but I have not actually built on yet. That sounds like a good project for the future! It should be a real challenge to get the PWM to work correctly with all the electrical noise from the arc and the high frequency starter.

Bob Butcher

[Adamj12b]

Well, I actually built one of these to see if I could get it to work. Turns out I can!!

The resistor is there just to limit the current through the torch. By what I can tell, this is the simplest and cheapest way to regulate current.

I started with a DC bus I had left over from a 4HP servo pack. This included an SCR bridge rectifier and 2 - 1800uF caps. From here I wired up a 100A contactor for torch current and a solenoid to a momentary push button to control the actual on/off of the torch. For the current limiting resistor, I used a 5500W 240V electric heater element from Home Depot. This is a 10.4 OHM resistor which while cutting, provides 26A to the torch. The element is in a tub of water to provide cooling.

Here is a walkthrough video of all the parts:

YouTube- DIY Plasma Cutter Walkthrough

YouTube- DIY Plasma Cutter Walkthrough

And here is a video of cutting a few different things:

YouTube- DIY Plasma Cutter Test

YouTube- DIY Plasma Cutter Test

Now that I know that can get the torch to work, I will be adding 2 more resistor coils, a closed loop water cooling system, and some better electronics all mounted in a nice enclosure for safety. The torch will be mounted to a 36" square gantry to complete my CNC plasma cutter. I will be starting a thread about it soon.

-Adam

[eloid]

glad to see you got my post working
I have been side tracked working on my cnc
had no time to build it....


FEw questions built as is what max thickness u can cut? what current?

[Adamj12b]

Originally Posted by eloid glad to see you got my post working I have been side tracked working on my cnc had no time to build it.... FEw questions built as is what max thickness u can cut? what current?

Its too bad you havnt had a chance to build it yet. It seems to work great!

Right now I have 1 5500W heater for a 10.4 OHM resistor. This works out to be 26A at the cutting torch during operation. Those readings are taking at 2 different times, so dont be too harsh on me. The 26A was read during operation with a Fluke 337 Amp Clamp. I measured the resistance of the coil when it was cold and not in water. My math shows about 23A practice is 26...

The thickest stuff I tried yesterday was .137" mild steel. I can cut it, but very slowly which tells me 26A isn't quite enough...Not sure though. ive never done this before. lol I can slice through 1/16" aluminum like butter. I cut .055 Stainless steel gas tank strap pretty well. Had trouble making it all the way through, But then realized that the compressor didn't kick on and I was down to 18 PSI!! Im really glad I didn't destroy the consumables in the torch with so little air.

The only problem im having right now, is I cannot get the torch to start with the MSD electronic ignition and coil. It is getting a small arc inside the torch, but I dont think its quite enough. I have been looking at a super high 60,000V coil to replace the old one I have, and Im going to shorten and neaten up the starter circuit to see if that helps too. I think some of the arc is jumping to the ground as the wire I added is not rated to carry that voltage. Well see what happens.

-Adam

[eloid]

Its too bad you havnt had a chance to build it yet. It seems to work great! beenr consummed with my cnc route build
when i got that done i will build one!.... I like to build something that can do .0.75- 1.0 inch easy so keep us updated....


i f you google you may find other post on line about it.

Right now I have 1 5500W heater for a 10.4 OHM resistor. This works out to be 26A at the cutting torch during operation. Those readings are taking at 2 different times, so dont be too harsh on me. The 26A was read during operation with a Fluke 337 Amp Clamp. I measured the resistanceof the coil when it was cold and not in water. My math shows about 23A practice is 26...

The thickest stuff I tried yesterday was .137" mild steel. I can cut it, but very slowly which tells me 26A isn't quite enough...Not sure though. ive never done this before. lol I can slice through 1/16" aluminum like butter. I cut .055 Stainless steel gas tank strap pretty well. Had trouble making it all the way through, But then realized that the compressor didn't kick on and I was down to 18 PSI!! Im really glad I didn't destroy the consumables in the torch with so little air.

The only problem im having right now, is I cannot get the torch to start with the MSD electronic ignition and coil. It is getting a small arc inside the torch, but I dont think its quite enough. I have been looking at a super high 60,000V coil to replace the old one I have, and Im going to shorten and neaten up the starter circuit to see if that helps too. I think some of the arc is jumping to the ground as the wire I added is not rated to carry that voltage. Well see what happens.

[JohnyWard]

Could I get your thoughts on a similar circuit base on a double alternator and 16.5hp lawn tractor setup? The idea is to use 2x140 amp GM units, customs built voltage regulator with about 5 preset setting from 5-45 volts. I expect about 6500W continuous power from this setup. I would like to run it as a tig/plasma and maybe mig if I can get the voltage stable enough.

According to some of the welding machine manufactures websites 6000W should be enough for about 7/8 at 10ipm @ 55ish amps.

The tig should be pretty straight forward at 25ish volts at about 300amps. But for the plasma I was thinking a pulsed DC transformer circuit to get to 150-200volts. I was also thinking the same approach for a HF/HV start function for both. A 555 timer and some bootstrapped high and low side drivers was what I was thinking for the actual components.

[SIGMA7]

well. this is the most traditional way for plasma metal cutter circuit, hv start or pilot start is only hv hf discharge inside the plasma torch by one second or less, when hv hf start the air passing through the torch is ionized making conductive this method is like hid light systems 23,000v for xenon gas ignition after of ignition is reached step down to 70v for keep the arch, plus high power charge from the capacitor bank. this two main elements create the plasma ignition. im electrician and electronic engineer too. the most simplest way to control current is with a load resistor. in commercial plasma cutters systems h bridge is used for control the current rate, but i will to share a secret with you all on cnc zone, there have a way control current and work very well. but you will need an high load induction coil and is with an transformer and hp triac used on hp inverter charger circuit, that triac can manage high current load on ac voltage and you can adjust with a simple circuit and an potentiometer, a separate hf circuit is used for keep the voltage and limiting the current: example with an 80 amp triac working on 150v ac you can get 50v 10 amp and up only for controlling your gate on the right way and the transformer will serve like isolation transformer, the hv hf start circuit can be build from a car spark coil and an very simple circuit using one 555 oscillator. you must build a momentary trigger circuit for shot the ignition momentary spark, i have one build. very simple and cheaper to build, i will post latter because i have to work on the schematics. good luck.

that god!! get to you all the light and the knowledge for understand and make a good project. thanks!!!!!!

[JohnyWard]

I was thinking about doing this project in 2 steps:
1) the mower driven alternator setup and custom voltage regulator. The hope was, that with a modified standard alternator regulator circuit, that I could plumb a pot into the voltage splitter section as well selectable zeners to give a functional range of +/- 95% of the zener diode. If this works it should give me a pretty good tig welder for use with steel. A manual gas valve on the torch should complete the setup.

2) an upgrade to the tig type controller that can be found on the web at telus... several PCB's, a neon sign transformer (and the 555 timer igbt driver specifically for this on a pure DC system), and a handful of IGBTs should create a tig that will do just about anything that I would want it to.

Miscellaneous to these would be a mig function in which a wire feed mechanism would have to be created and the voltage would have to be relatively stable. As well as a the plasma which is the focus of the thread.

I envision 2 hurdles with the plasma cutter. First a 20-30v and 300 amp source would have to transformed to 150v+ and around 50amps under load. And second the HV/HF arc starter.

Because this would be a pure DC based system the way that I was thinking to do both of these was to use the same low side driver approach as above. This would provide the flux necessary for the step up transformer and with a high frequency may maximize the efficacy of the transformer(ie smaller package, less heat) and the piggy back the arc starter transformer to the output line.

So that's what I was thinking. Will it work? Any further thoughts?