OK, let's see, first, the best way to do this is to obtain a torque-speed curve from the pump mfgr.
The next job is to figure the motor rated torque at base speed. Use the formula HP = Torque x RPM/5252 . This will be the torque available from the motor from base speed (50hz) DOWN to whatever slow speed begins to cause overheating in the motor due to the fan slowing down. This is typically 1/3 to 1/5 of base speed on constant torque applications.
Above the base speed of the motor up to at least 125%, the motor will operate at constant horsepower. Examining the formula above, you can see that, in order for hp to remain constant, the torque must reduce in inverse proportion to the increase in speed. So, for operation at 60hz the torque would drop to 50/60 times the base speed torque, at 70hz, the torque would drop to 50/70 times the base speed torque, and so on. Motors, depending on their design, cannot hold this constant horsepower characteristic up to all speeds. At some point, even the horsepower starts to drop and torque falls even faster. Of course, when overspeeding anything, the mechanical integrity of the rotor must be considered. An exploded rotor is not a pretty sight as any old DC motor guy can attest!
Now that you have the torque characteristic of the load and the motor, you should compare the two. The point at which the load torque equals the motor torque is the maximum possible speed point. If you crank the numbers thru the above hp formula, you will find that, at that point, load hp exactly matches motor hp. Isn't that a curious coincidence!
This might get a little long but, in the field, you may not have these torque curves available. So, using what we have, we know that the 182amp motor is drawing only 165 amps. Unfortunately, motor amps are the vector sum of magnetizing amps and torque amps. You can safely assume that modern efficient AC motors draw about 25% of nameplate full load amps with a free shaft. This is necessarily all magnetizing amps since there is essentially no shaft torque. In this case, that would be 45 amps. 45 x 45 = 2025. 165 x 165 = 27225. 182 x 182 = 33124. Using these numbers to find the torque amps at the present pump load 27225 - 2025 = 25200. Taking the square root of 25200 gives us 159 amps. Now solving to find full load torque amps 33124 - 2025 = 31099. The square root of 31009 gives us 176 amps. Dividing 159 by 176 gives us the % of full load torque which is 90%. You can see that, near full load, simply dividing nameplate amps by total motor amps at load has very little error, but, the further from full load you get, the more error creeps in using the short cut. Using the longer method I just demonstrated will give you accurate numbers regardless of load.
Considering that this motor is only at 90% torque and constant displacement pumps generally have constant torque characteristics regardless of speed, I would expect that you could get about 11% overspeed before running out of motor. I get this figure by dividing 100 by 90 to get 1.11.
Hope this is somewhat clear and helps someone out there to understand this tricky business.
There is one thing that troubles me a bit about this thread. In my experience, a turbine vane pump is not a constant torque positive displacement load but rather a variable torque load with flow (displacement) dependent upon rotor speed. I would definitely check the speed-torque curve for the pump before proceeding. If it ends up that the torque is not constant over the speed range but increasing with speed as in a centrifugal pump, you will not get 11% overspeed but somewhat less due to the rapidly rising load torque in the overspeed range.
Either way, you will know when to stop increasing speed when the total motor amps equal the nameplate amps. Even at overfrequency, the motor amps will be the indicator that the motor is max'ed out.