I linked a couple from eBay:
Re: I recently lost the ability to back feed when the utility installed a smart meter. I figured the my house for the most part was using more then the 800+ watts my PV was providing, but they called me on it, and I was feeding the grid when my load was low .... Yo Scott, I am wonder which...
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That was for 120/240V conversion, either isolated or not works. You need 120/120 isolated, or 120 to 120/240 isolated
I think the second listing can provide 2000VA isolation, either 2, 120V output in parallel or in series for 120/240V
You would wire 100V and 20V in series to get 120V.
I power up one winding with AC, check voltage of others. When I find two leads from separate (isolated) windings and determine the voltage each produces to it's other lead, I then join the two windings together at one end, measure between the other leads to see if I got them in parallel or in series. That way I can label them +/- or red/black and figure out what series/parallel connection each needs. You just have to be careful not to short a winding, or parallel two different voltage windings. A low value fuse or breaker on the AC source while figuring it out would be good. A 12VAC or 24VAC transformer as source would limit risk/damage.
With transformers, there is a current limiting per winding (ampacity and heating), also VA limit through the magnetic core. To some extent you can pull more current from one winding if the other isn't loaded, less heating of the core, but the wire has its resistive heating limit too.
I'm setting mine up with a fan controlled thermostat and manual reset overtemp thermostat opening a relay (due to high current, 9000VA). You can get a manual-reset thermostat which will handle 10A, maybe 20A. One or two of those could protect both primary windings in parallel, or each primary winding separately, depending on current handling.
250V, 20A, $7 delivered:
Specification:Product Name : Temperature Control SwitchModel: KSD302TAction Temperature: 90°CAction Type: N.C (Normal Close)Rated Voltage/Current : 250V/20APin Width : 6.3mm/0.25inchReset Type : Manual Package Content :1 x Temperature Control Switch, 4 x 6.3mm Copper terminal, 4 x Protective...
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Airflow is good too. Does your inverter have a fan which comes on when it works harder? That warm air ducted over the transformer would probably keep it cooler than still air.