I am not a transformer person professionally.... so don't really know what I'm on about .
But ripping apart the originals, it was not hard to get some simple rules together, so that designing something that will work from the get go, seems not that difficult.
I found that around the 2800sqmm of torroid core does 1turn per volt at 50hz.... thats all you really need to know. If you go conservative from there you will get even lower idle currents.
The next thing to notice is that contrary to popular belief, loading up a tranny does not saturate it if it was not saturated at idle... so the amp turn under load is not an issue, only at idle. Once you have got clear of saturation at idle, you cannot saturate the core unless you lower the freq, or raise the voltage.
Here we can raise the voltage .. or so it seems. The battey voltage can change, so that can cause saturation if you run at full tilt perhaps, other than that, you need to be mindful that at full charge and equalization, your at 60v, so your peak inputs on the scope will appear higher, but in practice this does not seem to matter much if your using the rule of thumb windings.
So I am not an expert on transformers, but I can make them work ok. My normal run of the ,mill twin cores easily do 16kw-20kw on motor start ups, and pretty happy at 6kw cont with some cooling if we are running for 20 mins or more.
The 28:240v ratio seems to work perfectly at 230v ( where I am happy ). The lights don't dim or flicker with 2-3kw loads switched in or out at night time. which is just crazy for a 2 dollar chip. I still am amazed by the performance of this pretty innocuous chip.
I would like to think I deserved to get some recognition for the beautiful performance of these things, but really it is the little 8010 that does all the wizardry, and I'm really just an onlooker hanging on for the ride.
Some folks have taken a slightly different route than I did, and seem to have met with some ugly problems. I don't know why. They seem to well know what they were doing, but it all turned up side down for them, their reliability was shot to hell, and just getting it to work seemed out of their reach. It is only that they knew I had over a dozen working unit/s out in the field that they persevered with it I think.
So I don't know what I did right, but it is very right somehow...... just lucky I guess.
.........oztules