Force on welding cables

In the recent thread about demagnetisers someone mentioned welding cables "twitching" when the arc was struck, supposedly because of the interaction between the current in the cable and the Earth’s magnetic field. This seemed unlikely to me, I thought it was more likely to be due to interaction between the cables, so I decided to do some figuring, with the help of formulas in Wikipedia.

The Earth’s magnetic field (according to Wikipedia) varies between 25 and 65 micro Teslas depending where it is measured. The force on L metres of wire due to a current I Amps in a field B Teslas is given by the formula B x I x L Newtons. So the maximum force on 1 metre of cable carrying 100 Amps in the maximum Earth’s field is (if the cable runs E-W perpendicular to the field) 0.0065 Newtons. If the cable was parallel to the field (running N-S) there would be no force.

The “Ampere” force per metre between two parallel wires 1 metre apart each carrying A Amp is 0.0000002 Newton-metres per Amp-squared. The force is given in Newton-metres since it is inversely proportional to wire spacing. So two parallel cables 1 metre apart and carrying 100 Amps have a force between them of 0.000002 x 10,000 = 0.002 Newtons, about a third of that due to the maximum Earth’s field. However if the current was increased to 200 Amps, the force between the wires would be 4x greater; and if the cables are closer together it would be greater still. Another factor is that the Ampere force is in the same direction whether the welding current is DC or AC, whereas the net force in the Earth’s field would be zero for an AC welder.

So it looks like the force in the Earth’s magnetic field is of the same order as the “Ampere force”, depending on the actual current and configuration of the cables. But what’s puzzling is that neither force is very big! A one-kilogram weight exerts a force of 9.81 Newtons, so a Newton is about a tenth of the force exerted when you hold up a bag of sugar (in fact about the weight of an apple). The electromagnetic forces on a metre of welding cable seem to be measurable in apple pips – so why do the cables twitch? Are the peak welding currents much bigger? Or is there some other mechanism at work?

Well said that man.... you lost me after....'The Earth’s magnetic field (according to Wikipedia) varies between 25 and 65 micro Teslas depending where it is measured'...

Never was into electrickery, when I served my apprenticeship I was only allowed to pull the main breaker to 'off' & then padlock it off. If any other work was required then I had to send for the 'sparky' even to change the direction of motor rotation or pull the fuse (although being able to change leads myself & pull the fuse) otherwise there would have been a 'everyone out' scenario.. 'the good old days'... don't get me wrong I understand & can work with 'Ohms law etc & can change sockets/switches/lights etc but anything further has always been 'get the sparky, even more so these days,what with legislation & Part P requirements, more so with house insurance requirements per se.

George

It's probably from the pucker effect when you strike the arc relating to the size of the electric bill ?

And always at the South Pole!

Martin.

John

You are not far off with your calculation, however I suspect that the thing that is missing is you are assuming steady state. On switch on there is a dynamic interaction between the field due to the cable current and the earth field. The field is changing. There is a reluctance to this change and it takes energy to achieve it. For mechanics, think inertia and the difference between the force required to accelerate a mass. Once a steady state (constant velocity) is arrived at the only force is that to overcome frictional losses. On switch on the current stores energy in the magnetic field and so does work. Once a steady field has been set up the only energy require is that to overcome the losses due to the resistance of the cables. The cables should also twitch an switch off too, when the magnetic field dumps energy by collapsing. Is this any clearer?

regards Martin

I noticed some time ago that when using my 225 A Lincoln AC stick welder that the front sheetmetal panel jumps/oilcans/moves a small amount (maybe a mm or two) when the arc is struck. I think this physical movement is due to the transformer core moving the panel ( it is screwed to it) when the field current is first applied, acting like the armature of an electric motor when current starts flowing. I guess if the cables were observed near this panel or stretched out taut away from it they could be seen to give a small twitch. Maybe this happens on larger welders and the mechanical movement is what people are seeing.

Some large welders have contactor relays that have a large armature movement, again this motion could cause panels etc to move, and any attached cables.

JD

Edited By Jeff Dayman on 11/07/2013 12:11:09

I just switch my welding plant on and use it without any thought other than a good strike and weld.

I feel the need to add to this as the "instigator" of this.

the welding cable "twitch".. is no more than that a twitch,,

the cables when observed are normally loosly laid on the floor and as cables do not flat and straight,

some "coil" is left in the cable and it will reach equilibrium between gravity and spring in the cable...when the current flows this small force due to electro magnetism disturbs the equilibrium ( like a moving coil meter or in a galvanoscope) and the cables twitch seeking a new equilibrium .

My "obsevations" saw more effect due to earths field than to ampere forces...but that was what the (crude) experiment was looking for....

..Intresting though isn't it......

Edited By jason udall on 11/07/2013 13:25:03

how to demagnetise somthing a bit bigger

http://www.redlandsfortnightly.org/papers/subm.htm

I have a big industrial 440v 32 amp per phase TiG set and I reckon I can get as much heat off the gimbal on the 'lecky meter as the torch.

I've seen this effect at work, and always put it down to the fact they're MIG sets and the filler wire is passing through the cable too. But now you've mentioned it, it also happens on the earth cable.

Seen it but never thought about it.

My trouble at home with my Boxford 240/440 V industrial stick welder is the power supply MCB. I've had to take it out on the workshop circuit and replace it with a fuse. Every time I plugged the welder/transformer in the domestic Type B MCB popped due to the sudden ramp up in current as the field built up. I would have preferred to use an industrial Type C MCB, but it doesn't fit in the old consumer unit I have.

Regards,

Richard.

Just wondering, what effect of the magnitic field in the cable has on the reinforcing wire in the floor. The only time I'v noticed movement in the cable, is if some is left hanging on the hook on the side of the welder, due to the elecro magnetic field in the loops of cable. Ian S C

Have you never wondered why the screws in Mains sockets get loose?. 50 Hz vibration from field interactions.

Martin

"The spooky effect..."

Nah, that's quantum mechanics. :0)

Hi

On the effects of magnetism and welding cables I had the misfortune to destroy the main and ballance wheel springs in an expensive watch.

Using an "Air Arc" gouging torch at high amperage with the lead draped over my shoulder and several times coiled round my arm the springs were heated hot enough to to soften the springs and render the watch useless.

Eric

I had the same problem with my old Interlas / Miller machine. I ended up fitting a Type C 63A MCB into the consumer unit (MK Sentry). This worked most of the time but occasionally (frequently enough to be annoying), it would still trip out when I closed the contactor. This would normally happen when I was all garbed up and poised for the weld.

This secondary problem was due to the remanence of the steel transformer ie the residual field left at the instant of turn-off when the contactor last opened. If the next time the contactor closed, the applied voltage was in the same sense as the residual field, the transformer could (did) saturate for the first cycle or two, at which point the current was limited by a combination of the supply impedence and the rest of the welder primary circuit.

The only cure for this problem is to make or procure a soft starter for the transformer. There are several ways to do this but in my case I used a low-ish value resistor (something like 10 Ohms) and a timer relay (a few hundred milliseconds) to get the transformer fluxed up and balanced before the main contactor closes. Works a treat and is a lot cheaper than an industrial phase control soft start unit.

If you make the resistor too small you don't achieve any useful limiting. If you make it too large it doesn't drive the transformer enough.

Your solution (wired fuse) is commendably simple but in my case the house was already on an RCD so I wanted to protect against damaging it by controlling the surge current to something sensible.

Muzzer

"Have you never wondered why the screws in Mains sockets get loose?. 50 Hz vibration from field interactions."

This is a complete myth, although I have seen it reported in a letter to a trade magazine, who should have known better, and not published it. The twitch effect in welding cables is caused by the cables coiled on the floor assuming (or trying to) the shape of the magnetic field they are creating. try putting a piece of string round one cable in a coil of welding cable, lift it to test the weight, then get an assistant to strike and maintain an arc, and try lifting it again, you should be able to feel the magnetic attraction to the other coils. It is not a good idea to leave welder cables coiled while you are working, as they can overheat. This is not the case with extension leads however, as the current in the live conductor equals that in the neutral, but they are traveling in opposing directions, and so cancel each other out. If your extension lead overheats, you're overloading it!

Phil

Surely the current flowing in the welder cables is equal and opposite too? If it isn't, what additional path does some of the current flow down?

The current in an extension cable may well be equal and opposite, so for normal domestic style extension cables the magnetic fields tend to cancel, greatly reducing the inductance, but it doesn't change the resistance. If you leave your extension cable rolled up and run it below rated current it will still overheat and melt the wire insulation. As an example my 30m extension cable quotes a maximum load of 720W fully wound, but 3120W fully unwound.

Regards,

Andrew

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.