Transformer welder: my puzzle

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DeBangis

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Morning to you all.

It always wonders me why the transformer welding machine does not electrocute when someone touch it (or terminal of one of the mains wires connected to it)??

Any explanations and insights are really appreciated.
 
Because the voltage is kept low such that a hazardous current doesn't flow through the human body.
The energy to create the weld comes from a very high current flowing when you effectively create a short circuit by striking an arc.
That's what the transformer in your welder is doing; converting the high voltage, low current mains supply to the low voltage high current for welding.
I hope that helps.
 
Ok, a bit of a question from me!!

How is it that the secondary windings of a welder are separate from the primary ones, AND, apart from the welders designed by "experts" many years ago, the secondary is not connected to the case of the machine in any way. [It is not earthed]

So, the secondary is entirely isolated from both the primary and earth too..

BUT, you kneel on damp earth, and say scratch your face with the end of the rod, and you will get a shock alright!! [Guess how i know...

john..
 
Because the voltage is kept low such that a hazardous current doesn't flow through the human body.
The energy to create the weld comes from a very high current flowing when you effectively create a short circuit by striking an arc.
That's what the transformer in your welder is doing; converting the high voltage, low current mains supply to the low voltage high current for welding.
I hope that helps.
In theory....

In practice you will find a few [old] AC machines with an OCV of about 90v [so peaks of 127v] or even 100V OCV

Very many DC machines have an OCV of 90V too.

They will shock you ok!!

This is why what everyone calls the "welding earth cable" is not, it is the "welding return lead"

The welding earth cable is the cable that you are supposed to have between the thing you are welding and a good earth, say the frame of the building.. {this can cause more problems, but not ones for the welder to care about]

Anyone know or guess what these problems are/were..

john..
 
You should have the welding earth/return as close the the point of work as possible , to prevent the current returning by the building earthing conductors, I cant remember the number of times I had to change cables because there was a bad connection in the return path, and all the return current going back through 6mm2 earth cable and melting the PVC off the cable , worse when its a 3 or 4 core flex

I aways thought that most electric welder's worked on around 36V DC, and you can get a shock from them if you are working in a damp or wet area,
 
You should have the welding earth/return as close the the point of work as possible , to prevent the current returning by the building earthing conductors, I cant remember the number of times I had to change cables because there was a bad connection in the return path, and all the return current going back through 6mm2 earth cable and melting the PVC off the cable , worse when its a 3 or 4 core flex

I aways thought that most electric welder's worked on around 36V DC, and you can get a shock from them if you are working in a damp or wet area,
Hi Poni,

The actual arc voltage when you are welding will be around the 18 to 40 volt mark, but the open circuit voltage will be a minimum 50V, but in the real world anything less than 80V, makes the set horrible to use, so that is the norm.. Pipe welding you want a good 90V due to the type of rods used..

Your burnt out cables....

Many years ago, the geniuses that designed welding sets decreed that the SECONDARY windings be bonded to the case of the machine and hence earth.

Now, off at a tangent a minute... The thing you are working on [we are talking large fabrications here] must be earthed to the frame of the building or other good earth with the "welding earth lead"

The idea is, to prevent a PD between the fabrication and anything else. Not just to protect against the welding voltage, but other electrical equipment might be in use too, and the lead to it, could get cut or squashed, whatever and liven up the fabrication.

Not only that, but say Bert Scroggins was drilling a hole with his class one Black and Decker. If the entire fabrication was not solidly earthed, he would not be too happy when the welder went to strike the arc either [for reasons i will explain in a second]

Back to welding....

What would happen then, if someone would forget to connect the "return lead" and start welding.

Now, remembering that the fabrication is solidly earthed, AND, that the welding set secondary windings were bonded to the case of the set [which was in turn earthed] what would happen, is that the entire welding current, [for manual processes could be up to about 600A] would wizz off to the "good earth" make its way to the MET, and then back through the wiring of the building back to the case of the welding set..

This, as you have seen! causes huge damage... Would not just be the supply to the set either.

Say Bert Scroggins had escaped being shocked [as he was hiding in the bog at the time] but had left his class one black and decker drill laying on the fabrication. The wiring to that could well act as a "parallel path" for the welding current too, and the wiring for it all the way back to the MET could be damaged too..

You will be glad to hear that this sort of welder is now obsolete, but i have seen a few still in use..

john..
 
You will be glad to hear that this sort of welder is now obsolete, but i have seen a few still in use
maybe not as obsolete as you think, I have seen this happen up to about three years ago and as I have left now that industry now it could still be happening, this kit is expensive and lasts a long time, (genny sets) the new inverter type should be better

I thought that you needed to keep the welder return as close to the point of work as possible to avoid damaging the other services and the kit fitted (like your drill) I have seen load cells blown in the past when the hopper was being repaired
 
Several years ago car repairers had to be really careful not to damage electronic components. There was only normally the ECU and audio then, so goodness knows what precautions are needed now.
 
Several years ago car repairers had to be really careful not to damage electronic components. There was only normally the ECU and audio then, so goodness knows what precautions are needed now.
Plasma cutting is what you want to worry about. My own Lincoln PC65 will cut 18mm plate with a current of 65A at a voltage of 106V. The open circuit voltage is a lethal 540V made even worse as you have HF start at about 10,000V..

john..
 
Several years ago car repairers had to be really careful not to damage electronic components. There was only normally the ECU and audio then, so goodness knows what precautions are needed now.
What will happen when the brain box damaged? We had an electrical burn out in our auto some times back; but fortunately the auto electrician checked the brain was okay.
 
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