Tracking down the cause of plasma cutter interference

So far so good, the plasma cutter is working alright. However, the PowerPlasma 60 is causing interference with the motor controller and forcing a restart of the motor controller when the plasma cutter is triggered.

The pc seems to be fine, its 5 feet away and optoisolated, and the parallel port doesn't seem to be the issue.

Here is the setup. A generator powers the plasma cutter and air compressor. The motor controller and PC are powered through an inverter. The plasma cutter is grounded at the generator through the third green wire and by a wire connected to the back of the case staked to ground. The plasma cutter cable runs along with some of the motor wires for several feet, as they are going to the same place. They should be separated by a few inches but the motor controller wires are not shielded.

The manual mentions that the plasma cutter is a pilot arc device, but its very difficult to gauge if it is working. HF starting with a cnc maching could be the problem but I don't know how to test. The internals of the machine has a pilot arc circuit and a HF start circuit. I might just manually disable the HF start and see if that fixes the problems. In theory it shouldn't be on anyways, but the front panel layout is not very helpful. Also, according to the manual, the Auto/Standard switch is suppose to be a toggle vs. momentary switch, but I couldn't tell the difference.

So, is there an obvious fix? Is the PowerPlasma 60 actually a pilot arc machine? How about inductance on the wires?

Loki,

Running on a generator, could be a cause of some strange frequencies, depending on how "clean" the power is.

The HF generates the pilot arc. It sounds as if you are grounding the bolt on the back of the machine correctly.

But you hit upon the key: Shielding. I am not the expert on electronics or on shielding the electronics, but I can tell you that unshielded cables can be affected by the surge from the High Frequency. I think that Mike will weigh in on this. But I do think that the HF could be the real cause. It should be fixeable though.

The HF will generate some EMI/RF. As Mark said, the cables should be shielded and grounded.

Also, the quality of the generator could be an issue. What model is it? You have the plasma running on the generator and the PC/controller on a battery and inverter?

Is this a manufactured controller or homemade? Either way, some more decoupling might be required. Also a grounded cage for it (look up Faraday cage on google).

You need to shield all motor controller cables and be sure the ground or drain is earth grounded on one side only. What type of motors are you using? Steppers, servo … Some of these motors require some high end wires with separated shielded twisted pairs and they run the motor power wires separately from the position feedback wires to reduce the possibility of problems. The generator may be causing the issue when it reacts to the surge demand thus causing a rapid fluctuation in power. Can you hook a recording meter or scope to it to monitor the output current and frequency? To further isolate the controls side of your system you may need to run a low cost one to one transformer on the incoming power side. I have used this method for servo driven systems when there seemed to be no other way to mitigate dirty power or outside interferences. Do you have any wiring diagrams or pictures of your system? Do you only get the feedback problem when the arc fires with the motion stationary and/or moving? Can you start the arc manually then power up the controls side to see if you have positive control after the start? Sounds like a great project you have going on here. Hope we can help.
Wayne

We are using stepper motors.

The motor controller is a Xylotex 4 channel device.

The generator is an older model. and can be viewed in this video. Its in the first few seconds. I would be partly inclined to blame the generator because it already busted a capacitor on an air compressor.
http://vimeo.com/7151604

The inverter is an RV model which is being used to power the motor controller only. It shouldn't be causing the problem.

From the picture it should be clear that I need to move the motor controller away from the plasma cutter and shield the motor controller cables. I could also try adding a ferrite core to the cables so any inductance spike picked up could be smoothed out in theory.

Right now the cheapest and easiest method of shielding the cables seems to be wrapping the wires in overlapping aluminum or copper tape, then attaching to ground. Keeping the tape away from the work table and making sure there is a continuous shield will be the biggest issues. After that, I can do similar shielding between the motor controller and the plasma cutter. Maybe a metal mesh or something.

http://openfarmtech.org/images/thumb...controller.JPG

Can we see the compressor a bit more? Camera went past it pretty fast in the start of the video.

The compressor is running really hard. And if no air to the cutter it will not fire the pilot arc.

Video helps us out a lot if you are working on a CNC plasma cutter table.