There is no issue with using a VFD after an RPC, but could be an issue with a static converter. There was a discussion on this at one of the RPC manufactures site, but I lost the link. The plus side, is that the VFD does not need to be oversized if one was to use a 3 phase input VFD on single phase power. I have built some systems running the VFD running off a RPC and there have not been any issues or VFD glitches. Sizing the RPC correctly may come into play.