140 Amp Polarity Reversing switch

Do compact hefty switches exist for switching 140 amp welding cables. I have a MIG welder where you have to swap the cables over between GAS and NO-GAS. This is done by taking the 2 cables off their posts, and swapping them over, thus changing their polarity. They would be swapped over while no load was on them. A simple Knife switch comes to mind, but its a bit bulky.
Any ideas?
BobH

A place I worked at a while back had a consumer product with an unrated contactor which was fine for a real 80 amps RMS continuous.

We never had to use it to switch the load because we used a transformer to generate the 80 amps from a 240 volt supply. The supply was not high power and it was controlled by standard 240V switchgear. If the unrated contactor had been used to switch the power directly I'm sure it would have died quite quickly.

I'm sure the high current contactor was as bulky as any knife switch I've seen. The use of such is no bad call. The thing you are going to be interested in is keeping the contacts clean. You ideally want a wiping action, perhaps silver or gold plate.

For a modern product, I would have to say that a Solid State Relay is the best way forward, but I don't think you'll get what you want off the shelf. I am equally certain that technology is up to the task, it's just that no-one much wants it, so it's hard to come by. If you were developing a product it would be worth developing a supplier for such a thing, because it would be more reliable than any mechanical solution.

I think the unrated contactor was a non-standard Crydom product. It was Bakelite/Phenolic with an open frame about the size of a playing card, perhaps an inch and a half thick.

Edited By Andy Ash on 18/12/2016 14:26:53

As far as I am concerned Andy, this is a "one off" , I don't like the idea of solid state as I would then need another switch to switch it plus a control power pack to energise it. Too complicated I think.
BobH

I completely get that.

If you were thinking about making something yourself, I would say that it is completely within the bounds of a normal ME workshop capability.

The one we used to use had a copper braid onto a rocking top. The solenoid was at one end and the contacts at the other. The pivot was across the middle looking down on top. The switch was double pole, single throw so it had one contact either side.

P.S. Because of the rocking action the contacts would wipe each time the switch was made.

IIRC the contacts were unplated copper, so one of the yearly service items was to dress the contacts with a diamond file.

You could make the body of the switch out of Paxolin on the mill.

P.P.S. Normally in a modern welder you can find a low voltage supply to pick off for control purposes. On my TIG welder I added a gas purge facility and the relay is powered from the welders own low voltage supply.

Edited By Andy Ash on 18/12/2016 15:07:14

I think a changeover switch is going to be quite large and cost a fair bit, would a plug and socket be OK as the plugs and sockets for fork lift truck type batteries are rated at those sort of currents and voltages. Two sockets with opposite polarity could be permanently wired and just plug the torch cable into the required flavour socket. RS seem to have the sort of socket at around £20 each, still going to cost 60 odd quid though.

Mike

Mike, You know ME's - got to be done for low cost. The 2 studs are located in a small recess about the size of 2 matchboxes. The cables are swapped over by taking off a couple of nuts and washers, then bending the cables to the other position and tightening them up again. Invariably, the washer / nut springs off and the gremlins take over !! I could make the terminal eyes into "C" eyes, so that the nuts could stay on the stud, but then I would have to twist the cable thro 180 deg in one plane and 90 deg in the other. With all this bending, the insulation would probably give up in a couple of years.
BobH

Wouldn't it be easier, and a good deal cheaper, to cut the leads a few cm out of the machine, fit ring terminals to each of the cut ends, and bolt them together as necessary? You can buy insulated posts for doing this with car battery leads, that you could attach to the case of the machine.

As noted above, this is not technically a switch ie it will not be required to open or close under load. It would be more correctly referred to as a disconnector or isolator.

You could make something up easily with some Anderson connectors. You could put 2 double sockets side by side, wired straight and reverse polarity and choose which one to plug your ground and torch into. SB175 (175A) would work. These are widely used and well proven. And you hardly complain about the price.

Another solution would be to take 3 of the stackable single way PP180 (180A) connectors eg 2 red and 1 black (connected +ve, -ve, +ve) on the machine and a pair of them for the torch eg 1 red for the torch and 1 black for the ground clamp. Then you could choose which of 2 alternative ways to plug in to the welder.

Note that they also sell handles, boots etc to make a neat and tidy job, as well as a choice of crimp sizes to suit different cable gauges.

Murray

How about arranging six contacts, one pair going to the welding lead connections for the output and two mutually reversed pairs coming from the power source with a pair of short busbars in some sort of insulated holder fixing over the top to make the appropriate connections. Still means faffing about with a screw or at least some sort of quarter turn tension connector but its only one which can be made easy to get at.

Saw something of this ilk a fair few years ago with the connectors in two rows of three, inputs on the middle, with a hollow cover over the whole lot. Busbars on one side to make the connection and a simple rectangular cut out to protect the unused contacts. I want to think that the contacts were half round on the end and the busbars simple rounds pushed into drillings combining self aligning connections and easy manufacture but its too long ago to be sure. I imagine 1/4 - 3/8 or 6 - 10 mm Ø brass for contacts and bus bars in a delrin holder should be up to the job.

Clive.

Edited By Clive Foster on 18/12/2016 17:00:46

These plugs and sockets are available on eBay for under 10 quid for a pair, they are very quick to change as they are just turn to lock. Most industrial machines use these as they are very quick to change a torch and cable which is the bit that suffers most. If you fit the sockets on your machine life could be very easy.

Mike

Edited By Michael Poole on 18/12/2016 17:05:32

Edited By Michael Poole on 18/12/2016 17:09:36

Doesn't Michael Cox's article in MEW 249 describe exactly what you are trying to achieve? Admittedly it doesn't get round the lead swappery but it does the job.

John

MUZZER and Mike, a couple of good ideas there, Mike, have you a fleabay name for those connectors ?

Bob, I just put weld cable connector in the eBay search, plenty of really cheap offers from China and a bit more from UK, some offer plug and socket pairs and even red or black.

Mike

Also try DKJ10-25  and DKZ10-25 for plug and socket

Edited By Michael Poole on 18/12/2016 18:29:25

Edited By Michael Poole on 18/12/2016 18:29:52

That sounds like a better idea than the Andersons. Must admit I've seen these welder cable connectors but never used them myself. They sound ideal.

Mikes connector suggestion is just what you get in better gas/nogas welders. Just make sure you do them up tight else they spark & burn.