Variable Speed Pumps

[PoolDoc]

Do you know if whether V*A gives a close approximation to watts, or whether the power factor is large enough (or small enough) to make that inaccurate?

[Extreme21]

Mark,

Thank you for the response. The chart is very helpful.

Matt

[mas985]

Do you know if whether V*A gives a close approximation to watts, or whether the power factor is large enough (or small enough) to make that inaccurate?

V*A is certainly an upper limit because it assumes a PF of 1 but over a pump's head curve, the motor load can vary from around 40% to 100% and the power factor is dependent on load. At peak load the PF is usually not better than 0.9 but I haven't measured it directly on a pump. The following paper shows some of the effects but in general, I don't think you can count on the PF being close to 1 for an induction motor. There is always a reactive part to the power.

http://www1.eere.energy.gov/manufact...s/10097517.pdf

The best way to measure pump power is by using the power company meter on the side of the house. You can get a measurement with the pump vs off and get a much better estimate as long as nothing else changes in the house between measurements. I have a digital meter and it is pretty easy to get the power difference but you can do the same with the old style analog meters too.