Tranformer sizing calculations for resistance/spot welders

Hi all, Nice to see the e-mails on this active group. I am looking for some literature and guidelines for sizing transformers for resistance spot
welders. I used Roman "TX select" software, but wanted to know more about the theory. IS there any book or booklet or PDF file information anybody has? Can you please send it to me ?? Thank you,
Fell free to ask any questions about spot welding I am mush into it. I would like to share.
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Hi!
Nice to see someone else in this field! I'll echo your question.
We haven't paid much attention to the topic of transformer sizing. The equipment we sell (10-100 kA welding current) we size on experience and estimates rather than computer analysis. (We don't make the machines, we only sell them and put together parts from different suppliers to form a working system.)
Obviously, we have to verify that the transformers can supply enough welding current with the selected inverter.
The maximum current can be found in the transformer's datasheet. In our experience, the limit is often the diodes on the transformer in manual applications. Long times between each weld when the user moves the gun or the work piece allows the transformer too cool off.
With automated welding, the limit is more likely to be the kVA of the transformer and the cooling of the electrodes. The robot moves rapidly from weld to weld and never lets the transformer cool off. The necessary kVA can be calculated if you know the current the machine will normally need and how many spots per minutes will be made. In reality, we find that the kVA is choosen either by the buyer or from experience from similar projects.
Let's say I have a inverter that outputs 100 kVA @ 500V and 20% duty cycle. I then see little point in choosing a transformer larger than 100 kVA. A larger transformer gets you a a higher secondary voltage and correspondingly lower current. This is rarely desired.
In most cases, a smaller transformer is quite sufficient and will give a higher secondary current due to the different turns ratio. The turns ratio is determined by the transformer manufacturer and is as much a manufacturing and marketing choice as a techical one. I think the common ratios are something like 40:1, 50:1, 60:1, 70:1 but don't quote me on that.
When I know how large a transformer is needed, I pick a cable for the primary to match this: If I use a 100 kVA transformer per the above data, I get a nominal current of 200A at 20% duty cycle which gives the same thermal load as a constant current of "square root 0.20" x 300 = 89 A. Thus, a 25 sq.mm. cable (or 3 AWG) should be enough.
The conductor on the secondary side is sized by the manufacturer of the machine.
Is this reasoning right or wrong? I don't know, but it seems to work.
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