[JRaef]

I have a 5500 watt 230v vacuum pump. I do not have 3 phase in my shop so thought I would run it off of a 7.5KW VFD. Does anyone think that there would be a problem with this? I plan to run my overload relay on the motor to the VFD dcm/for connection. That way if the motor overloads there will not be a sudden load drop from the VFD which, I believe, would possibly let the smoke out of the VFD. Any Advice?

As a general rule, VFD mfrs will tell you to at least DOUBLE the VFD current compared to the motor current. There are two reasons: when drawing current for a 3 phase motor from a single phase source, the single phase current seen by 4 of the diodes in the bridge instead of all 6, is 1.732x higher. Then in addition to that, there is more DC bus ripple from rectifying single phase, so you need more capacitance to smooth that ripple out. So by doubling the size of the drive, you effectively cover those added requirements. it's not always that simple, because even with extra caps, if there is no DC bus choke, the caps will run hotter, so many mfrs suggest even more de-rating, to 65%, or de-rating the ambient temperature to 25C (about 77F), which can be onerous.

In THEORY, you could set up the VFD so that it current limits at 50% of it's capacity, and some people do that. But the way most VFDs handle current limitation is to artificially override the frequency command as a way to shed load. That can be problematic in machine tool applications. On a vacuum pump? You might get away with it depending on the use. If it is pulling a vacuum on a chamber, running slower just means taking longer. But if it's for a vacuum holding application, less holding force might be an issue for you.