Ahh! Lancie, I did misunderstand. You are talking about pushing the motor into overspeed on an existing machine simply to get more speed.
The controlling factor here is not motor hp but load hp. Since the motor enters a constant available hp mode above its design frequency and speed, the machine speed can only be increased to the point where load hp equals available motor hp.
Now, if the original system has been designed with lots of extra motor/drive capacity, then you can often get quite abit of extra speed. On the other hand, if the motor is nearly fully loaded at its design speed, not much extra speed is going to be possible before the motor becomes overloaded (that is, load hp exceeds motor hp). Under those conditions the current will rise over nameplate full load amps and, assuming the drive is programmed right, the overload fault on the drive will protect the motor by tripping out.
So, as can be expected, you can't get more out of a motor than it has in it and the drive enforces that rule.
Having said all that, there are a couple of tricks that can be played under desperate production pressures. Remember that motor nameplate limits are based on an assumed ambient temperature of 40 degrees C (104 degrees F). If you find that the motor is not too hot at its fully loaded point, you could trick the overload function in the drive by entering a higher motor full load current than stated on the nameplate. You may end up sacrificing the motor by doing this but, in some cases, you could save a production process and become a hero! Imagine that! Imagine all the praise and wealth that will be heaped upon you! (I've got to stop this!) You get my point!
You could also place additional cooling fans on the motor to keep its temp down or, as I've seen numerous times in paper mills, you could run a little cold water over the motor to keep it cool.
If you examine a NEMA Design B speed torque curve, you will see that, without the thermal limitations, a motor can develop about twice its nameplate hp. Again, this is a high risk game and you may find its simply not worth the risk to play. Another very important consideration is machine safety. You must be absolutely sure that the extra machine speed is safe or forget everything I've told you about overspeed. It is not worth getting someone hurt over a little extra production, I don't care how ugly the production manager gets. A good example is any motor driving a flywheel as on a stamping press. If the press nameplate says max strokes is 50 per minute, do not be tempted to increase it even to 51. A disintegrating flywheel makes bad company, if you get my drift!