Depends on shape of the power curve and the spec requirements.

Motor must be sized for end of curve. Say that is 399hp so you buy a 400hp motor. Your operating point requires 300hp. Shutoff requires 200hp.

Full load speed of your motor is 1180 rpm so the pump curve is set up at 1180. But 3/4 load speed is 1185 and that's the speed the motor is going to run at. 1/2 load speed is 1190 RPM, that can be a pretty large increase in head at shutoff. yes big inch, the sound of cracking......


Mr GaTech, when the pumps are sized, they are sized to deliver the performance shown on the curve, so the manufacturer has to take into account the reduced RPM due to motor slip.  When he sells you the pump and hands you a curve, that pump will have been tested with a real motor, and by some specs with the motor that will operate the pump and the impeller has to be trimmed (if required to be trimmed) to a size that will deliver what the curve says it will.

Most curves I see have a pump rating at a speed slightly slower than synchronous speed, say 1760 for 1800 rpm or 1470 for 1500 rpm.