Thanks. I have also found
AquaPhase and
Add-a-Phase as possible alternatives but need to do more reading.
The Ronk requires that you make absolutely sure the output is balanced, otherwise you must de-rate the motor. If you think that might be problematic, I wouldn't go that way.
The AquaPhase is a larger version of an already used technology called a Voltage Doubler. Many VFD manufacturers use it on small VFDs so that you can plug them into a 115V outlet and get 230V 3 phase for a motor. There are several people out there like AquaPhase doing this for larger motors as well, but there are problems with it, that's one reason why you don't see the major mfrs doing it. The number one problem is, as you will notice on this one, they are very difficult to get UL listed. In fact I don't know of one that is UL listed for 240-480V conversion. One problem with it is that the voltage doubler is essentially "external" to the VFD portion (even though it's in the same box, it's a separate circuit). In that unit, the output is around 678VDC, and UL considers anything over 600V as "Medium Voltage", which falls under a completely different class of rules. VFDs do this internally, but by keeping it all in the same circuit, they get away with it. It's a technicality, but an expensive one for the mfrs.
If UL is not a requirement for you, you may want to pursue it. But a
LOT of small VFD manufacturers have come and gone in the last 25 years. People who have products install from the "gones" are stuck holding the bag on support and repairs. There is another version of this concept from a company in the UK called
Drives Direct that uses a standard Teco VFD as the basis and adds that Voltage Doubler. But they too cannot get UL on it, so they don't sell many in the US.