A static converter would be just as good as a VFD, in this application. Even simpler would be to wire in a capacitor to the third phase on the motor. It would not be as stable as a VFD, or even as a phase converter. But with the flywheel action of a couple of grinding wheels, plenty good enough. VFDs do well in a number of applications, especially where spped should be varied. But for a grinder????? I can't say what value cap to use, but a couple or three motor starting caps would fit the bill. I have the conversion factors, somewhere, on paper. But not handy. . .
The big issue is to ensure the motor is wired for 240. Connect Ph1 to Ph1, Ph2 to Ph2, and a cap to Ph3 from either line. If it runs backward, wire to the other line. A little research will tell what value cap. Use a Non-Polarized electrolytic. A Radio Shack electrolytic will blow up in your face. Use something motor rated.
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