Tig HF starts keep burning up my wire speed controller- Help

Hey Guys, I've built a cold wire feeder for my tig torch. Basically its a 24 volt dc spoolgun with a tig torch mounted to it that feeds the wire directly into the weld puddle. I adjust the wire feed speed and weld amperage with two potentiometers mounted on the gun. It works well and has significantly increased my travel speed on long straight runs. The problem I am having is that the high frequency arc start keeps burning up my 24volt motor controller. I believe the arc is jumping to the weld wire through the drive rollers, motor housing, down the wires to my speed controller and lights out. I've done some research on diodes that might help be with this problem but Im over my head in trying to design some protection circuit for the speed controller. Any ideas on how I might be able to isolate the controller from the welding current/ arc start. Any help/advise would be greatly appreciated Thanks-Nate

The short answer is isolation.

Energy is traveling back to ground and your controller is in the path.

The power supply, motor and controller need to be isolated from all grounds.

Check motor windings with ohmeter to make sure no leakage to frame.

Use an isolated power supply and make sure every part of the power and control do not conduct energy to ground.

The wire being grounded may cause energy to run up the wire so others need to comment on how the energy between torch and target interacts.

We thinks the wire also needs to be isolated so no ground path.

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Ahh, ok, Thanks, hadn't thought of the complete ground isolation solution. I went and looked at it. The problem with my design.... I'm using a 120vAc to 24Dc transformer to bring power to the motor controller and thereby the 24v dc gearmotor( wirefeed). If I remove the ground wire from the transformer(power supply), I thing the white neutral wire 120ac would act as the ground being its tied in at the house panel. I don't think I can completely isolate the wire feed system from ground with my set up. Would there be a way to block or buffer the errant incoming volt/amps from the tig torch to protect the motor controller??

NClay said:

I don't think I can completely isolate the wire feed system from ground with my set up. Would there be a way to block or buffer the errant incoming volt/amps from the tig torch to protect the motor controller??

Click to expand...

First off, I'm gonna take an educated guess here... the component that's probably failing first, is your bridge rectifier... and it's probably burning up because it's operating at 24v, but getting hammered with HF energy in the KV range.

The HF arc starter is a massive noise generator. Tony's spot on, you gotta isolate it... but galvanically isolating only does PART of it... you've got lots of EMI rattling around that out will be very difficult to 'keep out' of your low-voltage drive electronics.

first off, replace your bridge rectifier with another version that has a much HIGHER PIV rating. Next, put snubbing on it- you'll need capacitors going between each of the legs... some ceramic disc capacitors... try starting around 0.01uf 1kv ceramic discs.

If your drive control system is very, very simple, it will have very few things sensitive to HF noise... in industrial control design, simplest is always best... but for all things, you'll need to make sure that the environment (which includes EMI both external and internal to your process) doesn't kill stuff...

...because wierd stuff happens. Nature doesn't care about what you want- she does what SHE wants.

You may find that the presence of HF causes the bearings in your drive mechanism to erode. It's basically an EDM process that occurs. Same thing happens to gearbox bearings in 3rd-rail and catenary-power railway equipment powered by variable frequency AC traction drives. Everyone understands that when an axle's contact brush goes bad, that the gearbox and bearings become a return current path, but the circulating AC of the traction inverter wreaks havoc on anything metallic in reach of it's field. Anything that does, becomes a haven of 'eddy currents'.

In a class once, I had some students that didn't understand what all the grounding was about, and couldn't figure out why so many of their systems had bearing and seal failures, blown circuit boards and sensors, so I got a paint pen and started drawing Kilroy, but writing "Eddy Lives Here" on gearboxes, truck frames, bolsters and nearby cabinets... They started maintaining the grounding better, and those weird issues went away...

One of the first things that comes to mind is that if the HF is coming back into the mechanicals it may be getting into the controller through the motor leads. I suspect a good start would be to use a non conductive coupler between the reel and the gear motor.

The basic ways HF interference travels are conducted, radiated, and coupled. In many cases adding ferrites around leads will attenuate coupled interference as well as radiated interference that uses the wires as an antenna. Coupled interference can be reduced by separating noise emitters from "victims" and making sure that wires that belong together are either twisted together or tied to keep them as close together as possible to keep interference common mode (easier to filter) vs differential mode which is a bitch to deal with.

DaveKamp's suggestion about uprating critical components to ones with a higher voltage rating is an excellent one. You may also with to uprate any capacitors in that part of the circuit as well.

EMI can never be totally eliminated in cases like this but it can be reduced and controlled to the point that it doesn't interfere with proper operation. It is impossible to accurately review potential flaws in your setup without knowing the intimate details but I believe there is enough expertise on PM to help with a gradual back and forth of questions and feedback from you.

Just a thought, have you tried using lift arc start instead of high frequency?( If it's a setting in your machine) It's not that much of a pain and might avoid a lot of your issues.

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Awesome guys, thanks for all the good ideas. You've given me a lot to think about. I'm going to upgrade the bridge rectifier and add the capacitors to start. This was exactly the info I was looking for. Thanks again