A three phase UPS would have nothing particularly special. There would just be 3 DC to AC inverter circuits coordinated from the same timing or waveform generator. It wouldn't really matter much of it was a square wave output or sinewave. Nor would it matter if it is line interactive or online dual conversion.
I suppose some UPS designer might go cheap and use just 2 phases at 90 degrees and try to make a "tee" source instead of a normal "wye" source. I'd stay away from those.
Re: Three Phase UPS#129450 03/24/0502:33 PM03/24/0502:33 PM
Thank you for comments and internet link. I think I better understand. Our Municipal emergency 911 building has a 3Ph-3Ph,480 volt UPS. The 30 battery, battery bank (408 VDC) cabinet has two cables leading to the adjacent UPS inverter. I wasn't initally aware that the inverter could create 480V,3Ph,4W,AC from 2 wire 408V DC. I knew it was able to convert DC to AC.
Re: Three Phase UPS#129452 03/27/0507:58 PM03/27/0507:58 PM
Although you already have received plenty of answers - along with a linked site for references (thanks to all who responded!!!), I have a few Inverter Circuits posted at the Tech. reference area.
For Polyphase output, the Clock Circuitry would likely be cascaded "side by side", with one Clock set at "0 Point", another one set at "+120°", and the other one set at "-120°". These would drive independent "Push-Pull" Amplifiers, and they would drive a (likely) Delta 3 Wire Primary.
I have Schematics in the "Cook Books" which show Clocks for 2 and 3 Phase outputs.
I'll see what can be referenced for Polyphase UPS types.
P.S. sorry for the delayed response!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!