Ques about spot welder OCV

Hi
Why do small spot welders have an OCV in 4-6 Volt - at first glance
one would think that using a higher voltage would make it easier as
you could use thinner tongs,leads etc - 600 amps at 6V is the same
as 300 amps at 12V (P=VI)
I cant remember much transformer theory - does it have to do with
efficiency ?
Tim
Reply to
TMN
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A spot welder depends on current between the tongs to get the job done. Even a small hand held unit needs several thousand amps to do a couple sheets of .049" steel using .062" diameter tips. To do weld nuts, thicker stock, or larger diameter spots just ups the amperage.
To get to those currents with reasonable KVA ratings, the closed circuit voltage is around 1/2 volt. There is a balance between current, voltage, transformer rating, and losses in the arms and windings.
Open circuit voltage is not really relevant for the actual weld but a higher OCV makes it easier to punch through any residual oil film or other contaminants.
TMN wrote:
Reply to
RoyJ
NO it has to do with I squared R . ...lew...
Reply to
Lew Hartswick
What matters is power produced at the point that needs to be melted. It is a product of current and resistance of the joint (which changes as the metal melts). Therefore, all that matters, given a particular joint, is the current produced.
i
Reply to
Ignoramus4770
Right, and in order to do that, most of the resistance needs to be in the joint. That is why the secondary winding & the tongs need to be such heavy guage - to keep their resistance down and avoid dissipating power in them.
Bob
Reply to
Bob Engelhardt
OCV is irrelevant to a spot welder -- all that counts is how much current you can pump through the spot and the spot's resistance. While it's mathematically equivalent, you're much better off thinking in terms of I^2R than VI. By the time you could see that 12V, the current would be much, much higher than 300A because the resistance very low.
Reply to
Joe Pfeiffer
1)Clearly there needs to be a large resistance differential between the tongs and the joint otherwise everything gets hot.
2)Athough p=vi = v^2/r =I^2R I^2R is the key as the heat is proportional to the square of the current. (for the same reason power lines are high voltage)
Thanks you guys have jogged my memory !
But why do most plans on the net say to aim for about 4-5V OC ?? Miller at
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have nice spotwelding primer and show OCV for their units from 1.6 - 3.5V
Thanks Tim
Reply to
TMN
...
I'd say it's because their secondary winding and tongs are "designed" such that their resistance is too high and too much voltage is dropped there. For instance,
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#4 wire for the winding and brass for the tongs!! #4 is much too small and brass is much too resistive. I'm impressed that it works at all.
Bob
Reply to
Bob Engelhardt

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