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Thread: Motor to impeller matching

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    Motor to impeller matching

    Recently I replaced my 3/4 HP single speed motor with an AO Smith two speed 3/4HP motor on my Hayward Superpump. I noticed that the new motor was running hotter than I expected, after looking at the wet end closer, it is a 1HP wet end, with presumably, a 1HP impeller. The service factor on the motor is 1.5, so it should be fine turning a 1HP impeller, but what about energy usage? Would the pump draw significantly less electricity spinning the smaller impeller? Currently the pump draws 5.5amps at low speed, 11amps at high. The old 3/4hp pump drew 12.5 amps. The new pump model is a AO Smith/Century B793.
    17,000 Gallon plaster pool. Hayward Perflex EC65 DE Filter. 3/4 HP Hayward Superpump converted to dual speed. CircuPool RJ45. Skimmer, floor drain, two returns.

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    mas985's Avatar
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    Re: Motor to impeller matching

    The service factor on the motor is 1.5, so it should be fine turning a 1HP impeller
    Not necessarily. Most impellers are full rated meaning they are meant for a full rate motor. When the same impeller is put on an up rated motor, the label HP must be higher than the full rated motor so their THP is the same. If you have a 3/4 HP full rated motor, it will be undersized for a 1 HP impeller.


    but what about energy usage? Would the pump draw significantly less electricity spinning the smaller impeller?
    Yes, it is the impeller which determines how much a motor will draw. If there were no differences in motor efficiency, it wouldn't matter what size motor was put on the impeller, they would all use the same energy. However, smaller motors tend to be less efficient so they will draw slightly more current than the larger motor. But this is somewhat offset by the fact that larger motors will have a smaller power factor so more reactive power some of which is lost in the windings.
    Mark
    Hydraulics 101; Pump Ed 101; Pump/Pool Spreadsheets; Pump Run Time Study; DIY Acid Dosing; DIY Cover Roller
    18'x36' 20k plaster, MaxFlo SP2303VSP, Aqualogic PS8 SWCG, 420 sq-ft Cartridge, Solar, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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