Bill,
I've never done anything greater than 5x the rating (a 10HP VFD on a 50HP motor) and it took about 3 minutes to accelerate it that way. All it had on it was a clutch assembly that eventually would couple in to a centrifuge. That centrifuge motor was not standard however, they tend to have more rotor than normal because of the expected heat they expect starting X-Line, so it may have been able to accelerate faster or with a smaller VFD if it had been std.
I have heard of someone using a 25HP VFD to start a 200HP pump motor, but I didn't witness it and the guy was kind of a braggart so I didn't put much stock in it. If you think about it, at some point you are going to be at full speed and that means full voltage from the VFD. So the VFD output will need to be able to deliver the full UNLOADED current of the motor, i.e. the magnetization current, with iron losses, windage, friction etc. We know that some unloaded induction motors can draw as little as 20% FLA, so that shows you technically will be able to get it to full speed with a 5:1 ratio in those cases, but 10:1 or 15:1? At 15:1 that means the unloaded current can be no more than 6.67% FLA. I have never seen a motor draw that little even uncoupled.
