[JC22]

hello everyone and happy new year,
I 've built an CNC plasma cutter 2 axis (X and Y) driven by a hobbycncpro 4 axis board.
My problem is the following : each time the plasma starts, my stepper motor drivers burn out. I've already destroyed 6 SLA7078MRP :-(.
I would like to mention :
1/ During the sutting, the plasma nozzle, the metal part and the earth clamp are completly insulated from hobbycncpro board and the steel frame of my CNC.
2/ when I'm using another selfmade driver board (based on IRFZ20 and NJM3517 stepper drivers), everything works perfectly (except that it is not fast enougth for a plasma )
Does anyone know how to solve this issue ?
Thanks a lot.
Regards
JC

[jimcolt]

I can help solve it.....what brand and model plasma cutter do you have? Are your plasma torch leads separated from your drive cables? Did you use shielded cables to the drive motors? Can you drive a ground rod next to your machine?

If you can answer these questions...I can help steer you in the right direction. Without knowing about your plasma...I would guess you have an older unit, or a low cost import with high frequency start.

Jim Colt

[JC22]

thanks for your reply Jim,
My plasma is a TIGMIG CUT50-M like this one :
http://cgi.ebay.fr/TIG-MIG-PLASMA-CU...item3efb859473

Are your plasma torch leads separated from your drive cables? Yes they are.

Did you use shielded cables to the drive motors? Not yet

Can you drive a ground rod next to your machine? I'm not sure to catch your question... sorry, could you please rephrase it

a low cost import with high frequency start => It must be this latter :-)

Thanks
Regards
JC

[jimcolt]

A ground rod.....an earth driven ground. Also....what plasma? Is it high frequency start?

Jim

[Alan Bradford]

The High Frequency that starts the plasma torch is basically a lightning bolt, and it will/can couple through the grounds, motor leads, motor frames, gear rack, PC and LCD Screen cases, joysticks, limit switches, and any thing else connected to the system.
After you ground and shield everything, you may still toast Steppers and an occasional PC or two.
The hobby grade of CNC's were not designed for a High Noise (Electrical) environment.
It reminds me of building a boat out of screen mesh...You can plug all the holes to make it float but is it worth it?
Noise immunity is designed into the pc boards and is a fundamental part of the initial design in Industrial grade equipment. It costs money and so is left out of the hobby grade stuff.

The safest and maybe cheapest fix is to get rid of the cheap plasma and get a non High Frequency Start unit. Of course I will recommend a Hypertherm PowerMax unit (I used to work for them and still bleed black).

Good Luck,

[Alan Bradford]

PS I forgot to mention that I have spent many, many hours trying to get hobby grade units to work with HF based Plasma Units.
After burning out piles PC's, Flashcut Boxes and Stepper motor/drivers (Fried Geckos anyone!!!), my customer dropped the idea and warned all his customers to not use HF plasma units.

Good Luck,

[JC22]

Thanks for your reply Alan,
It is now very clear for me that HF is my enemy !
However my plasma supports both HF and contact start. I will open it and try to find out if there is a way to turn off the HF start.

When I choose this plasma I thought HF was better for a CNC because it's avoid the plasma nozzle to touch the metal part being cutted (and so it's avoid the nozzle erosion). finally I was wrong !

In case I could find out how to switch off the HF start on my plasma, I will need a system to make the nozzle touched the cutting part to start the plasma and once the plasma is ON, it will bring the nozzle up for 1 or 2 millimeter. That's it ?

Thanks again for sharing your experience.
Regards
JC

[JC22]

Hello again,
There is still something I can not understand : How do you explain that when I'm using my other self made driver board, I do not have problems (and I already made a few cut with this board [but it is really not fast enougth for a plasma]). Any idea ?

Regards
JC

[jimcolt]

A Hypertherm "non high frequency" start torch does not have to touch the plate to fire the arc....it uses a moving electrode inside the torch to create an ionizing spark.....completely eliminating the high frequency circuit. It will fire an intense pilot arc, strong enough to attach the arc through rust, paint, or other non conductive surfaces. This technology is o all of the Hypertherm Powermax plasma systems. On Hypertherms industrial class plasma systems....High Frequency start is used....as these systems arealways installed on industrial quality machines that have adequate filtering and protection from RF noise interference.

Your torch likely can be used with high frequency start.....or with scratch...or contact start.....both difficult to use on entry level cnc machines.

Jim Colt

Thanks for your reply Alan,
It is now very clear for me that HF is my enemy !
However my plasma supports both HF and contact start. I will open it and try to find out if there is a way to turn off the HF start.

When I choose this plasma I thought HF was better for a CNC because it's avoid the plasma nozzle to touch the metal part being cutted (and so it's avoid the nozzle erosion). finally I was wrong !

In case I could find out how to switch off the HF start on my plasma, I will need a system to make the nozzle touched the cutting part to start the plasma and once the plasma is ON, it will bring the nozzle up for 1 or 2 millimeter. That's it ?

Thanks again for sharing your experience.
Regards
JC

[JC22]

Thanks for your reply Jim and your explanation for the ionizing spark,
Finally do you suggest me to buy another plasma torch to connect to my plasma cutter ?

"both difficult to use on entry level cnc machines" => In fact, the start of my plasma is driven by a relay connected to the // port of the PC (via a transistor). it offers a galvanic insulation between the PC and the Plasma. When M3 or M4 G code commands are interpreted, pin14 of my // port is set to '1' which closes the relay and starts the plasma. (if you look carefully to the picture of the plasma cutter front panel, you will see a port called 'CONTROL'. When you short cut the 2 pins of this control port, the plasma starts)

Regards,
JC

[Alan Bradford]

Hi JC,
If you do not have problems with your other self made driver then the noise problem may be in the board design of the faster one.
Somehow the HF is getting into the board and the prime suspect is the motor leads and/or motor.
HF can couple throught the leads and motor just by being close to the torch leads.
Try running the torch leads overhead as an experiment. Put a metal shield around the motor and make sure it and the machine frame are grounded.
HF can couple into the motor through the gear rack and pinion and motor shaft. I would run a #10 (min) wire from the torch station and another one from the bridge to a leg on the machine.
From that point run a #8 (min) to your ground rod.
Do this before trying anything else.

If that does not help then you will need to look at the board design. I would guess that the driver transistors/IC's are what burned out.
That is the first thing the HF hits when entering the drive from the motor leads.
They pack a lot of smoke.
Here is where the design problem gets tricky. Keeping the HF out without bogging down the drive motors. That is filter design beyond my pay grade.

Most of the hobby grade stuff was designed for CNC milling and other types of mechanical type cutting/metalworking. Noise is not an issue.
Only plasma cutting and Mig welding have this type of HF generator to start/sustain torch ignition. A very small segment of their market.

Dont get me wrong, I am not bashing the hobby stuff, just trying to explain the diffrence.

Jim Colt has a hobby grade cutter in his garage and has cut stuff for me when my machine has been down, or torn apart for experiments.
The cut quality is excellent. Yes half of it it the plasma torch (Guess what he cuts with???), but the machine motion also has to be smooth and accurate for a good cut quality.

Hobby and Industrial both have their place. Hobby is less expensive, but you could not run it 24/7 and expect it to last more than 6 months or a year.
On the other hand you would not cut a 30 foot by 300 foot 1 inch steel plate with stepper motor drives. (A Korean Ship Builder has 2 of these cutting side plates for container ships)

I make my money in the Industrial segment, but I have worked with a couple of Hobby manufacturers as well.
You still only get what you pay for.

Good Luck

[JC22]

Thanks a lot Alan,
I will try out your advice and let you know if it works or not (but I would say that your previous post almost frozen me ;-) I don't want to burn out my PC!)
Thanks again !
Regards
JC