Force on welding cables

[Anonymous]

Posted by Phil Whitley on 13/07/2013 18:56:24:

The current in the welder cables will only be equal and opposite if you coil the electrode holder and earth cables together, and even then it would not be as perfect as a three core cable on an extension., The heating effect quoted on your extension lead is due to the fact that a coiled lead is unable to radiate heat due to load, whereas an extende cable is. There is no heating effect due to inductance or the fact that it is a coil.

So, if I understand correctly the current in the positive and negative leads of a welding set will be different to each, other depending upon how the two cables are placed?

Andrew

[Phil WhitleyParticipant @philwhitley94135]

Sorry Andrew, I didnt explain myself very well. Because the live and neutral in a three core cable are in close proximity to one another, and the current being carried by them is equal and opposite, the induction effect is cancelled out. with a welder there are two single core cables, not in close proximity to one another, so the inductance effect operates. If you have ever used a clip on ammeter you will know that it has to be placed round the individual cores of a cable to make a reading. It works on the inductance effect, and if you place it round a three core extension, or similar, you will get no current reading because of the above.

The current in a welder electrode holder and earth/work leads will be the same value in amps, but will be traveling in opposite directions. You can actually look for inductance effects using an ordinary compass held near a conductor. In a perfect world, you would get no reading near an extension lead, but a massive swing of the needle near a welder lead or other single core current carrying conductor.

[Sub MandrelParticipant @submandrel]

I was given the tip of using an extension lead (uncoiled) with my (puny) welder and I now get about 1 trip per 30 switch ons instead of 1 in 2 or 3.

<edit> my simple arc welder has an A/C arc, is this why I've never seen cable twitch or am I just being boring and watching what I'm doing instead of trying to weld while watching the cables?

Neil

Edited By Stub Mandrel on 14/07/2013 20:31:04

[Anonymous]

Phil: Quite so, we agree, although I suspect that in practice the inductance values are quite low, and unlikely to be an issue at 50Hz.

I have used 50Hz clip-on current probes, but most of my current measurements have been taken using current transformers or closed loop Hall effect devices as I generally need a high bandwidth, into the hundreds of kHz or low MHz. Of course the same principle applies, only one conductor!

Andrew

[jason udallParticipant @jasonudall57142]

The phrase common mode signal springs to mind..
The three core mains lead is “more”like a twisted pair transmission line..plenty of L&C but as said at 50Hz negligible effect

Again with the welding cables forming a loop of significant area then you will have a noticeable field..the same conductors run as a close pair will show little external field..simularly Non inductively wire wound resistors…showing much lower or nil inductance…

[Sub MandrelParticipant @submandrel]

> plenty of L&C

Inductance and capacitance? No such thing mate

**LINK**

Neil

[jason udallParticipant @jasonudall57142]

Thanks Neil.
I was not aware of his writings.
The lumped LC model works for me..produces useful answers even if the theory is “wrong”…caps never stored charge they store energy in the measurable force’s bewteen the plates..inductance is still a bit grey at least to me…

[Sub MandrelParticipant @submandrel]

> I was not aware of his writings.

Best to remain aware, rather than actually trying to understand them…**LINK**

Neil

[Anonymous]

Blimey, that's put the catt among the pigeons; haven't heard his name for years. When I was a subscription reader of 'Wireless World' he was a regular in the letters column, plus some articles. Largely along the lines of he was right and the rest of the world was wrong. Which is fine, but at some point one needs to come up with some hard evidence………..

The company set up to exploit wafer scale integration, Anamartic, started well, but failed to deal with a crash in memory prices, meaning that there was little point in using partially working ICs. While I was working for Pi Research, way back when, we eventually moved into the old Anamartic offices in Milton Hall, near Cambridge.

Whether you subscribe to the Catt or Maxwell view of the world, dealing with issues of interference in electrical circuits is fairly simple, provided you follow a few basic rules and think about how the currents and voltages change. In my experience some people, and companies, always have trouble with these problems, and others rarely do.

Regards,

Andrew

[Peter TuckerParticipant @petertucker86088]

When an electron moves it generates a circular magnetic field. This field rotates in an anticlockwise direction when the electron travels towards an observer and clockwise when moving away. When a current flows through two parallel cables in close proximity the circular magnetic fields interact, with the current flowing in the same direction the circular fields merge drawing the cables together, conversely with the currents flowing in opposite directions the fields thrust at each other pushing the cables apart. I would suggest this is the cause of the welding cable twitch.

Peter.

[Donhe7Participant @donhe7]

When working in the agricultural industry in the 1960s, we attended a meeting of the local Agriculture Bureau at which we were shown a coiled extension cable which had fused together due to being used to power a reasonably heavy load, but still within its' rated value, but whilst coiled on a sheetmetal spool, so I've very severe doubts about the wisdom of using a coiled extension lead.

Back to welding cables, if anyone has ever looked at a welding lead lying through a sprinkling of iron (or steel) filings, after the arc has been struck, and the cable not moved from its position during the welding run, you would see a series of filings arranged in a number of rings around the cables.

I first saw this effect whilst using an AC welder in this situaion, and was surprised at the time, it is quite intriguing, when seen for the first time

donhe7

[John OlsenParticipant @johnolsen79199]

I haven't particularly noticed my welding cables twitching, and would probably attribute it to my own nervousness if I did…however it is an effect that can occur. A cousin of mine once worked for a company that sold special kickless cables for the large spot welders used for putting car bodies together. These just used a construction that kept both conductors close together, along with carrying a cooling water supply for the electrodes. Of course the currents were quite a lot larger than most of us would be using for welding.

It is not unknown for overhead power transmission lines that are a little loose to swing when a heavy current is drawn. They swing apart when the current is drawn, but as a result can swing towards each other when the current flow stops. In extreme cases this can result in them shorting together. The results are left to your imagination.

John

[Ian S CParticipant @iansc]

John, We in Canterbury used to have a problem than resembled the latter stage of your example, when we got a good nor-wester, the HT lines that were slung on poles about 50 m from my place would start swinging, and when the sparks started we knew it wasn't long before the transformer would blow, and lights out fot the rest of the day. They cured that one this year, it all went underground. Ian S C

[Sub MandrelParticipant @submandrel]

In the UK we use a cunning technique called 'spacing the wires and not having the poles too far apart' which seems to avoid these problems

Neil

[Ian S CParticipant @iansc]

The tempory cure was to put wooden battens across the wires as spacers, I think it was 2 between each pole. Wind speeds of over 100 Kph are not that uncommon here. Ian S C

Edited By Ian S C on 20/07/2013 10:56:43

[Author]

Posts