THC with HV/HF pilot arc??

[Fiero Addiction]

I'm trying to automate a small Thermal Dynamics 35 amp plasma cutter with 12kV pilot arc starter. I've built a THC based on Tom Caudle’s design. I'm having problems with interference on the scaled voltage output during arc starting and pilot arc. I have diodes on the voltage divider input and low-pass filters on the output. I'm getting spikes that make my Fluke meter do strange things. As soon as the the arc transfers to the work, the high voltage starting circuit shuts off and my scaled output voltage stabilizes to normal level.

Any suggestions Tom?

Does the THC300 work on HV/HF machines?

Jon

[Mongkol]

Jon,
I just buy THC300 from Jim Cullins. I will use it with HF plasma cutter same as you at 40 amp.(pilot arc) Jim advise me THC300 is suitable for HF plasma cutter. Pls talk with him.


Mongkol

[Fiero Addiction]

I think I figured it out this morning. It would work fine when I pull the front panel off the plasma machine, but act up when I closed it all up. I had a few inches of wire going from my divider/filter board to a CPC connector on the front panel, and they were against some of the cables inside the machine when I put it together. I shortened the wires and relocated the board and it seems like it will be OK now.
For all the time I've spent playing with this thing, I probably should have just bough a THC300, but what fun is that?

[Mongkol]

Jon,
Do you mean that you can control HF plasma machine at now? What software do you use? Mach3 or not?

Mongkol

[Fiero Addiction]

Yes, I am using Mach 3 and a home-made THC that should work the same way as the Campbell THC300.

[Mongkol]

Hi Jon,
If the circuit is worked and fine with HF plasma machine,Is it possible to share your circuit?

Mongkol

[Fiero Addiction]

Look in Apendix B of the Mach user manual. I started with that schematic to understand how the basic THC system works with Mach. It seems strange to not be able to set the arc voltage until after you start cutting, but I guess that's how the THC300 works. I wanted to be able to pre-set the height before starting the cut, so I made some changes. My meter displays the preset voltage before I start cutting. When it gets an arc good signal from the plasma cutter it displays actual arc volts. I can adjust it anywhere from dragging the tip to well above recommended cutting height. I've only been using it for a couple days, but I'm extremely pleased with how well it works. The panel meter I stole from an old piece of equipment was bad, so I have it hooked up to my Fluke for now. The panel LEDs are up, down, and fault, the extra one hanging out the meter hole is an arc good indicator.

[stirling]

Hi Fiero Addiction - I too am studying the same THC diagrams etc. You mention the "arc good" signal from the plasma unit - I can't see any mention of it on the diags - any tips?

Thanks

[Fiero Addiction]

stirling,

What I refer to as "arc good" is called "TORCH ON" on figure 12.2. I wired the "arc good" or "torch on" relay coil (K2) in parallel with an existing pilot relay inside my palsma machine. The machine's internal logic senses when the arc transfers to the plate, then the "arc good" relay (K2) closes to let Mach know there is a good arc. In this way, Mach can be told not to move until the arc is good. Otherwise you could have a false start, Mach would start moving, the THC would see open circuit voltage and the tip would dive into the sheet to try to reach target voltage. The diagrams do not show the wiring from the "torch on" relay to the parallel port. I ran mine through an optocoupler.

I use the THC300 post processor in sheetcam and it works wonderfully. I have all my inputs/outputs on a single parallel port, so the pin assignments are a bit different than a THC300 install, but after I got it all set up it works just the same.

Edit: Just wanted to add one more note, I have a "torch on" indicator and an "arc good" indicator. The "torch on" LED lights when the machine is triggered. The 'arc good" LED lights when the arc is good. This terminology just makes more sense to me than what is shown on the original diagrams. In reality it doesn't matter what you call them and you will rarely look at them anyway because you can clearly see and hear what's going on, but they will be good diagnostic indicators if something isn't working right.
Jon

[stirling]

Thanks Jon - all makes sense now (famous last words). As far as Mach is concerned - do you use the "dedicated" plasma version? I notice that Mach mill has references to THC etc. Just wondered if it makes any odds. (I currently only use Mach for milling/routing).

Any tips on the "type" rather than the "make" of plasma unit I should be looking for or avoiding - I know this has been mentioned in other threads but I'm still not sure what's what. Everyone just seems to say Hypertherm - there must be others though - nice to have a choice.

[Fiero Addiction]

I use the Plasma version of Mach, I think the only difference is the screen and the way buttons are displayed, functionally I believe they are the same. Using Mach3 Plasma there is a button on the main screen to turn THC on/off. I haven't used Mach mill or turn.

I'm not the right guy to ask about plasma cutters, hopefully someone else can answer your question. I think I will avoid the high frequency start for my next one, I don't have any problems with it interfering with the CNC, but it doesn't start a pilot arc reliably. I get a lot of condensed moisture in the air supply which may be part of the problem.

Jon

[Big John T]

Look in Apendix B of the Mach user manual. I started with that schematic to understand how the basic THC system works with Mach.

Do you have a link to the Mach user manual. I searched but came up empty...

Thanks
John