A 3 phase motor (or single phase for that matter) will become a generator when the motor is spun faster than the synchronous speed it is designed around. So if you have a 4 pole 60Hz motor, the synchronous speed is 1800RPM and as soon as you start spinning it faster, it is becoming a generator. 15% faster would probably be extreme because all you really need is the same amount as the slip speed of the motor (as a motor). So for example if the motor has a slip speed of 1725RPM, the slip is 1725/1800 or 5% and to become a generator of the same amount of power, you need to run it at about 1895RPM.
But that is ONLY going to happen if the stator windings are excited. So essentially, the motor has to be connected to the grid in order for it to become a generator. There is a way to make SOME motors self-excite using capacitors. To make it work however, the motor has to have a lot of "residual magnetism" which, unfortunately, is not a design specification that you can investigate or ask for from a manufacturer. So what that means is, it might work, it might not; you won't know until you try. If you want to look in to that, do a search on "self exciting AC induction generator" (without the quote marks).
