I have a similar problem. My lathe uses a 3HP motor that is not rewireable to delta from star to run on 240V 3-phase, and it is so built into the machine that it would be major surgery to remove/replace. I found that inverters will only convert single-phase in to three-phase out at the same nominal voltage - no voltage conversion. Basically, they rectify incoming A.C. to D.C., then chop that to generate the output. So, you need to use a transformer to bring single-phase 240 to 400 or so to feed the inverter. In my case, someone kindly offered me an inverter that had been hacked to change the input circuit slightly. At heart it is quite a good ABB inverter, but the input is now a single-phase input voltage-doubler rectifying arrangement to give a higher voltage on the D.C. bus, which then gives me an appropriate output voltage. Works ok, but as I crank the lathe speed up via its variable-speed pulley arrangement, motor load goes up, and the D.C.bus voltage starts to sag - the capacitor is being charged at only 50Hz because of the voltage doubler but is being discharged at 150 Hz, in effect, so the voltage drops during input cycles and eventually the inverter protection kicks in and complains about input voltage faults. The original input circuit uses a full-wave rectifier so that even with single-phase you get a 100 Hz input. Like you, I need to look out for a 240-400V transformer!