How to increase centrifugal impeller speed

[ghensky]

1. How do the pump manufacturers determine the pump speed or RPM?
2. A motor with a constant speed rotates the impeller at the same speed. Hence higher the motor speed, the larger the impeller RPM.
3. Higher the motor hp, larger the motor speed. So one way to increase the pump RPM is to upgrade the motor hp and speed.
4. A motor can be sped up by more than 100% with a carefully chosen VFD.

Are the above statements correct? if not, why? Thanks for your comments/answers in advance.

 

[MikeHalloran]

2. true.
3. false.
4. Mostly false. VFDs can overspeed AC motors, but the current, and hence the power, must be derated. It's just not good practice to size a pump so that it needs an overspeeded motor to get the performance you want, and it's not as easy as you think it is.



Mike Halloran
Pembroke Pines, FL, USA

 

[ghensky]

In relation to #2 above, to gain more pump heads (assuming the impeller size has already maxed out) instead of adding a VFD, can a higher constant speed motor be used? thanks for your answers.

 

[MikeHalloran]

Please detail exactly what problem you are trying to solve. Thanks.



Mike Halloran
Pembroke Pines, FL, USA

 

[MFJewell]

Pump affinity laws should answer your questions. Finding a motor that fits your calculations is up to you.

 

[Compositepro]

ghensky, from your questions it is apparent that you have a fairly common misconception about electric induction motors. A 5 Hp motor will not always output 5 Hp. It is limited to putting out 5 Hp or it will overheat. The load on the motor determines the power output of the motor. The motor will slow down slightly as it is loaded but attempts to maintain the speed which is determined by the powerline frequency.

Mike, your statement #4 is not correct. A VFD can overspeed a motor, within mechanical stress limits, and its power output will increase. The maximum torque of the motor is limited by its current rating for thermal reasons. So double the speed at constant torque will give double the horsepower. But again, the load determines the power required from the motor, and doubling the speed of a pump requires something like 8 times the power, depending on a number of other factors.