[ssutton]

I am setting up a new Longevity 42i on my CNC table and have scoped the arc voltage input to the CANDCNC controller. This is the signal coming out of the plasma cutter after the 20:1 voltage divider. The signal looks like it has excessive spikes on it and I want to hear from anyone that may know if this looks typical for an arc signal.

Attachment 226676

Voltage showing time from transfer of pilot arc to cutting arc on a 500 ms time base

Attachment 226678

Same image with time base expanded for detail


I have heard that some plasma cutters can put out such bad noise that it can damage the control electronics. Any experience with this would be appreciated. I may be able to filter this signal if it is deemed possibly harmful to the control system.

Thanks,
Scott

[jason_alex]

From what I understand that most of the damage done to electronics by plasma cutters is done by the high frequency of industrial plasmas that use high frequency for starting the arc so there are no moving parts in the head. This is why most of them use hardened controls such has Burny, Hypertherm, and pro-motion. I have not seen a set up with candcad before.

Noise on the input to the controls can cause weird motion such and odd random paths being off if I understood my electrician correctly, Isolation transformer can be used to reduce noise but I think they come with a $$$$.

I am fairly sure that is close to accurate, but I could be wrong. Please some one feel free to correct me if I am.

[Torchhead]

The DTHCII and the new High Speed DTHCIV use a Digital Isolated pickup for the divided arc volts. There is no common ground or signal connection to the plasma side from our electronics. The noise he is seeing on the scope is probably ground loop noise (is not really there) and caused because the plasma voltage negative or positive is not "ground" The Scope is ground referenced (ground clip on the probe ) to the AC safety ground wherever it is plugged in. To really measure the DC you need to use either a differential scope probe or a battery operated scope. Any ground line longer than a few inches starts to factor in as an antenna.
This issue was resolved on our Support Forum and the customer is up and running.

Isolation transformers can be helpful if you want to re-establish a ground between two systems. If you pass the safety ground though it has less effect since that can be the source of a lot of noise in the plasma world. The CONDUCTED noise (goes through wires or common connections) is one source and RADIATED noise goes though the air. Air is a rather poor conductor of energy and the source of the noise has to be pretty strong to induct noise of several volts even a few feet away. Signal through the air drops off by the square of the distance. We suggest that it is not a good idea on an HF start unit or plasmas above about 60A to run the plasma cables in the same wire track as the control or motor cables. Wires running close and parallel for long distances have better "coupling" and can transfer noise The smaller plasma cutters ( below 100A ) typically do not have a lot of RFI (Radiated) noise if you earth ground the table.

On our DTHC IV series, all communication to/from the control program (MACH3) for displaying values on the screen and loading settings, is done via our C3BUS RS485. It is a USB to RS485 4 port hub that will handle up to 32 devices, that is placed close to the PC (because USB is highly sensitive to noise) and the RS485 lines running on CAT5 can be hundreds of feet long, RS485 is a differential noise immune serial protocol. We have introduced our Ether-Cut product line that allows you to totally isolate the PC logic signals from the control electronics. You can elect to move the PC up to 50 ft away from the cutting machines with the two communication paths (Ethernet and RS485) both being extremely noise immune.

TOMcaudle
www.CandCNC.com