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!