Impeller Trim

[GaTechTheron]

This seems like a rather basic topic, but I could not find where it has been addressed on these forms.

Question:

When pumps are operated on electric motors, the motors never operate at synchronous speeds due to the principle inwhich they work. (Back EMF demands current draw to compensate for load). Because this effect varies from different types of motors, don't manufacturers need to take into consideration the specific motor that will be used when manufacturers are trying to determine impeller trim? If they do, are impellers trimmed based on full load motor speed, or on some interpolated speed based on motor load at either pump runout or operating point?

I hope this was clear. Thanks for your time.

 

[BigInch]

Due to the nature of electrical field interactions, assuming common number of poles, cycles, voltages ... , the net effect is that motors are more similar than different, resulting in the fact that there is probably no more than a 3% difference in motor speeds between full load and no load.

You should use the speed shown on the reference pump curve.

Are you going to eliminate 2 poles or something?

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[Artisi]

Yes, a pump manufacturer or reseller would or should consider the full load motor speed in calculating impeller trim and so should you as the end user if you are selecting the motor based on the pump curve. On smaller units it is not so important but as pump sizes increase careful consideration becomes necessary and on very large diameter impellers it can be critical as 5 - 6 RPM could plus a motor into overload - remember power increases as the cube of the speed change. Of course all of this depends on the margin between maximum pump demand and the full load power rating of the motor.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

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.

 

[Clvet]

Most pumps I have seen have some fudge factor in their sizing(how often do you see a pump sized within a foot of head of where it's supposed to be?) and taking account for the small variance in RPM is probably part of that.

 

[BigInch]

.. carefull there. ice getting thin.

Let your acquaintances be many, but your advisors one in a thousand’ ... Book of Ecclesiasticus

 

[rmw]

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.

rmw