Strange welder fault..

[flyingsparks]

Got a strange welder issue, here, I havn't been to look, just had my brother on the phone with this problem!

Welder works fine when the item being welded is on the floor with the earth on to that.

When trying to weld onto the steel work of the building, which is obviously bonded to the electrical supply, it isn't welding properly. I believe it is not as hot as it should be for the settings on the welder.

He has been welding to the steel work before with no problems, but now it has suddenly started playing up! Any ideas why this is happening would be appreciated! Cheers Chris

 

[bernardgreen]

When trying to weld onto the steel work of the building, which is obviously bonded to the electrical supply

The "ground" side of the welder's transformer has to be connected to the the work piece by a very low impedance cable to reduce to the minimum the voltage drop. The impedance of the "ground" path from work piece via steel work of the building and the electrical earth to the welder will be introducing large and variable voltage drops so the arc will be running at a lower and probably very variable current.

The heavy welding current may also be damaging the electrical earthing if that is the only return to the welder for the welding current.

 

[RF Lighting]

Sounds like the ground cable from the welder is shorted to the case of the welder, and therefore the electrical earth.

The welder should be taken out of service immediately as it could cause massive damage to the electrical earthing system.

 

[ricicle]

The earth clamp of the welder needs to be clamped as close as practical onto the workpiece. If, as it sounds, the electrical earth is being used then untold damage will be likely on the installation earthing and possibly bonding wires. A fire could also start due to extreme overloading of these wires !

 

[flyingsparks]

Thanks for all the replys..

I did ask about the earth clamp, and it was near to the work area and did have a good connection to the work piece. (Clamped to bare metal, not painted metal etc)

The welder appeared to be working correctly when used in the workshop, which is downstream (electrically) of the barn where the fault is happening.

I asked him to check the bonding conductors etc, and there were no obvious signs of heat damage etc, as I had thought the same.

Possible welder casing shorting to the earth clamp is something which could be checked at this stage I guess, as to why it worked ok in the workshop and not in the barn, perhaps moving the welder coincidently removed the fault?


Any more ideas much appreciated!

 

[Spark123]

Is it a remote supply, i.e. a long distance of fairly small cable? Could be as simple as the impedance of the supply causing a too high volt drop.

 

[Peet]

Is your test piece, that welds okay, on the floor of the workshop or the floor of the barn? Reason for asking is I'm wondering is the welder on a long extension lead? And that's the difference. Different welders cope with mains voltage drops differently.

Some other thoughts -

You don't say what kind of welder it is. Single phase or three? Is it older transformer based, or newer inverter based? If its older transformer style then depending on the setup its output maybe isolated from its input and so (I'd guess, although I've never tried it) you could happily hook it onto the earthed steel work as the current will be confined to the job. Although what the rest on the installation will sense about it I'm not sure, with possible leakages etc.
The TIG machine (larger older inverter based one) here doesn't have either of its terminals connected to mains earth with it fired up but what happens after the torch is struck might be a different matter. If you do have a defined connection from an output terminal, through the machine to mains earth then that begs the question 'which one of the terminals'? Usually in DC welding the torch is negative and the 'earth' clamp is positive. In AC it can be anything from that to 50/50(?).

To me it doesn't sound like you've got welding like currents flowing in the mains earth system. Currents of 100A are not uncommon in welding and if you're seeing a drop off in heat input to the material, make a guess as to what kind of current is present. Usually you can guess the current by seeing its effect if you've been welding the material for a while. So say if you've got maybe 80A (as it sounds like you've got something like but its a little short of whats expected) then I'd say you'd see the damaging effect of that on the earth core of the welder with anything over a short blip of current.

Surely with the fault you've seen, if it was including the mains earth system then any RCD present would have tripped out as you now have an imbalance of phase and neutral currents?

Just had a natter with a chap who knows more about welders than I ever will. I mentioned the specific fault to him and he said he'd seen some strange effects of the work piece being at mains earth, usually the HF behaving strangely. ie not going where it should do. Other than that he said check the integrity of the earth setup on the welder. If the work piece is big enough it is possible to still weld it without an apparent earth connection although the current must return by some path somewhere. Sometimes he's seen the internals of MIG machines burned out if the torch has been shorted out. And then the obvious one - is the work piece you're welding significantly bigger than your previous pieces and you've underestimated the current?

Hope that helps

 

[Adam_151]

Sounds like the ground cable from the welder is shorted to the case of the welder, and therefore the electrical earth.

The welder should be taken out of service immediately as it could cause massive damage to the electrical earthing system.

I'm lead to beleive that welders were commonly manufacturered with an earthed secondary at one time, and the the standards commitees realised the problem and the british standard now insists that they are kept floating.

Seen issues on a site where a TP ceefrom outlet, and SP ceeform outlet along with 2 rotary isolators had been made onto pieces of mild steel in the maintance workshop, steel had been earthed. The assembly was then taken out to factory floor, bolted to side of bench, and the two SWAs glanded in. Found quite a few with all the insulation on the earth path between teh ceeform sockets and the mild steel back plain burnt and crispy! I tested a few examples of welders would couldn't find any particular type with an earthed secondary

 

[flyingsparks]

Thanks again for all your replys.

The barn supply is rather under rated really, and in the future I am due to be re newing it. Currently it is only a 6mm and it runs probabily 70m ish. It is on a 20A MCB at the workshop.

There are no RCD's on the supply.s

The welder is a single phase MIG welder, not that old probably about 3 years, and its a transformer type not an inverter. It has a 32A plug/supply. I believe it is a 300A welder, and he had the heat setting on pretty hot, so could be welding at perhaps 250A.

Welding on to steel staunctions of a shed which go into the grouind so yes there is a large amount of metal work in the ground.

The test piece laying on the concrete floor which welded ok was on the floor of the barn, not workshop, so one would think the volt drop etc would have been an issue for this piece too if it was an issue when welding to the steel of the barn.??

So yes I know there are some issues with his set up, 32A welder on a 20A supply, Long cable run, no RCD but all in all this is how the set up is at the moment and will be renewed in the future.


Thanks all, more ideas and things to try will be great!!

 

[Peet]

If the 20A MCB is staying in then its not pulling more than that. So, if my flat cap is on straight, 6mm will drop about 5v per conductor over 70M. So a total of about 10v at the other end. About 5% on a 240v supply. I don't reckon it's an issue especially when you've confirmed the test piece was also in the barn.

It's an interesting question and one I've never actually thought about. What happens if/when the workpiece is attached to mains earth.

Have you asked the girlfriend/wife? It's amazing how many times I've been stumped by something only to have had a cup of tea thrust at me, asked how it was going, for her to say have you tried the x/y/z/thrunging sprocket/crossbeam ont' treadle. It's amazing how many times they've been right!

 

[flyingsparks]

Well here's a sort of update. Brother temporarily disconnected the bonding conductor to the steel work of the barn (rightly or wrOngly) and hey presto it's welding fine...

 

[securespark]

Yikes.

 

[Peet]

Would you ask him to reconnect it and verify that the weld deteriorates again, if he already hasn't. And also what current is he welding at? What make and model of welder is it?