Hi,
As indicated by you the primary current is 48 A , but actually the current will be below 10A because your welder operates like in a short circuit condition at that time the output voltage will be down near say 5V @ 300A so the power will be 1500W max
No. That is not at all true in any way, shape, or form. Arc welding machines just simply don't work like that. I've welded with many machines, both transformer and inverter, and no arc welders work like this.
TIG and MMA/Stick welders operate on a Constant Current control system where the output current remains rock solid while welding but the arc voltage will fluctuate depending on the length of the arc and what type of shielding gas is being used or type of flux powder on the rods(stick).
For example, when TIG welding on DC, with pure argon shielding gas, and using a standard arc length: 100A output will result in ~14V across the arc plasma. At 325A the output will be about 23V. Changing the shielding gas will change this, for example, using an argon/helium mix, or pure helium, will result in a higher arc voltage, and a hotter arc for the same output amperage. The hotter helium arc comes in handy for TIG welding thick aluminum on AC.
Stick welding on the other hand, will generally result in an arc voltage approximately 10V higher than TIG welding with argon at the same amperage. Again, this is dependent on the type of rods used(the different fluxes vaporize into different shielding gasses) and the length of the arc(different rods also require the use of different arc lengths). This is why if you go and look at the specs of any production switch-mode welding machine(Miller, Lincoln, and Everlast are some examples) the rated output current for welding in stick mode will always be less than the rated output current for TIG, while the voltage for stick will be higher than the voltage for TIG. With a little bit of math done on these ratings, the output wattage will be pretty close when maxed out in either mode. For example, the Everlast 325EXT is rated for 325A@22.9V in TIG mode while only rated for 250A@30V in stick mode. Both of these are right around 7.5kW.
As a general rule, the voltage when TIG welding with argon will be approximately: Vweld = Iweld * 0.04 + 10
For stick welding it is generally Vweld = Iweld * 0.04 + 20
Obviously these will only be approximations and since these welding modes are constant current, the voltage will fluctuate in order for the system to maintain a steady current output. These rules also don't work at very low currents because there is a minimum voltage require to maintain the arc across the gap through the shielding gas. These equations are pretty accurate in the amperage range where most welding is done.
On a separate note, I don't exactly follow your logic either. Short circuiting the secondary side of a transformer will cause the primary current to hit it's maximum amount. This maximum will be limited by stray resistance in the windings, how well the primary and secondary windings are coupled together, and whether or not the primary switching devices and input source(usually the mains) are able to supply that amount of current. It will essentially cause extremely fast saturation of the transformer core if the energy from the secondary cannot be dissipated somehow and things will go wrong very quickly. The current-controlled switching methods in a SMPS welding machine prevent meltdowns under short circuit conditions by monitoring the primary winding's current ramp when the switches are active. In the case of a short on the secondary, the maximum allowed primary current is hit very quickly and that switching cycle is terminated. The resistance of a TIG welding arc in argon is approximately 65-75mOhms, and while that might be considered a dead short in lower power systems, it's just normal operating conditions for a welder.
Is it possible to construct a spot welder with 1000 amps or more with a SMPS? Because the voltage is just 1 or 2 volts but huge amps.
The best will then use a low switch frequency for less stress, like 20 Khz and so it is more easy to get a lower voltage per winding.
You know kees, I'm not really sure if that's possible or not. I've never personally seen a SMPS spot welder, only transformer based ones. I can't think of any reason why it wouldn't be possible though.
All of the ones that I've ever seen run the AC output current directly from the transformer's secondary into the two electrodes. It is just resistance welding so there is no need to rectify the output. I've seen them built with amperage selection being done using either a tapped transformer or some sort of SCR based Phase-Angle Control on the primary to regulate the output current. You are correct about the voltage in that it is very low. Often the transformers only have a couple to a handful of turns for the secondary, even being large iron 50/60Hz units. Quite a few of the ones I've seen though have had magnetic shunts placed between the primary and secondary windings to limit the magnetic coupling slightly. I would assume these are there to help allow the secondary voltage to drop very low and draw a large amount of current without overloading the primary. The shunts would also limit the output current as well since they are limiting the magnetic coupling between the windings. I'm not sure if this would be needed in a SMPS style machine or how that would work though. Like I said, I can't think of any reason it wouldn't work in theory. Since the output can be AC, that would help simplify things quite a lot, although I'm not sure if the high frequency would have any effect on how the weld formed. The biggest problem I've seen with trying to build very low voltage power supplies is the rectification of such low voltages. Since no secondary rectification would be required for a spot welder, I would imagine getting the transformer correct would be the most difficult part.
On an note unrelated to these posts. I have finally gotten some free time between work and family and life in general that I have gotten back into researching and planning for this welder project. I know that it has been multiple years since I started this thread and I have essentially zero to show for it so far, but I'm hoping that this time will be different. The big thing now is that being able to afford the components to build such a machine is now within my reach so I'm hoping that will help me get this off the ground.