Welding monitoring and earthing systen

Gents,

Bit of a headscratcher here that I could do with a bit of help with:

There is a rotating turntable used for the purpose of being connected to an automatic welding machine.

 Situated on the turntable is typically a welding workpiece & three single phase sockets that are then used to deliver power to a three phase preheating unit preheating unit. 

 The power to the sockets is delivered through slip-rings, as is the welding return current for the welding machine.

 This supply to the slip rings also carries on providing power to another turntable.

 This turntable is earthed as required for fault protection on the pre-heating circuit; as if a fault arose then protection device would be required to operate.

 The trouble comes around when the welding machines AVM (Arc voltage monitor which decides the height of the arc welding needle from the workpiece) are used. As they are both connected to the workpiece and the workpieces are both connected to each other through common earthing it is believed that the systems may cause malfunctions with each other when both workpieces are being used.

I have made a rough sketch below, the small writing that you probably can't see either says: "Workpiece", "Welding return boss" or Pre heating strip"

https://imgur.com/oEPIrKs

  • I presume that these are automated TIG welders. Without much more detail of how these are meant to work it is difficult to comment usefully, but is that a resistor in the return line (the rectangle)? More information, please. It is usual to use a constant current supply for TIG, so perhaps this is some kind of current monitoring, which could be anywhere in the circuit.

    Regards

  • Sorry David, that is just a plug. I'm not too sure on the welder type, I can find out.

  • It rather depends if the welders themselves are earth free - I suspect they are actually,  in the sense of no direct connection between the welding return and the mains earth. However, there will be unavoidable capacitive return paths.

    What are the symptoms of the problem you are trying to fix ?

    If the welders are high frequency striking types you may need to add RF choking in the mains earths (A very low value inductance, pretty much a dead short for 50Hz fault currents so fuses or RCDs or whatever blow OK if required, but allows the two earthed platforms to 'flap about a bit' and discourage current loops in the earthing wiring at high frequencies.) Another approach is to have common mode choking of the welder leads - more or less forcing the current in one lead to a given welder to be in balance with the current in the other lead to the same welder (think like the RCD winding when in balance but again for high frequencies.)

    All this is tosh of course if the welding waveform is DC or 50Hz AC but I'm only aware of arc voltage monitors for height control on plasma cutters and very fancy rigs for welding aluminium so maybe.

    If you could get a bit more info on the set-up it may or may not become clearer ;-)

    Mike.

  • Thanks Mike, I'll have a proper read into this tomorrow, I know it's a fairly fancy setup as it's a robotic welder and the welding unit is Polysoude