What would the effects of an anti-grav device be?

It’s not necessarily a perpetual motion machine. If the anti-gravity mechanism requires a larger energy input than you could get out of the wheel, you’d at least be keeping up with the laws of conservation of energy. It would only be perpetual motion if your anti-grav device required less energy than the wheel generated.

Well, it certainly can’t exist until we figure out how/why gravity functions as a fundamental force…