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Thread: Pump & impeller size question. SOLVED!

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    msgtdan's Avatar
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    Pump & impeller size question. SOLVED!

    This question is just for my general knowledge and not related to a problem. I've read here a lot about pumps and matching impellers to motors. A good match is most efficient, an under-sized impeller is okay but not ideal, an over-sized impeller is bad for the motor. So on a two speed pump why doesn't the motor have a problem when on slow and the impeller is over-sized for the motor hp? I assume it has to do with the different RPM's and maybe different windings but I'm not sure. Please educate me.
    Dan D
    Used 2003 Aqua Leader 27'x52", 17,800 w/10" hopper, SwimPro SW256T 250# sand filter, Hayward PowerFlo LX pump 1hp impellor, Emerson 1 1/2 hp motor, setup Aug 2012 Summer 2011 used Summer Escapes Ring pool 14' x 42", Intex 1600gph sand filter, HTH 6 way test kitTF-100 w/stirrer, Well water @ FC=0, PH=7.2, TA=290, CH=320

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    mas985's Avatar
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    Re: Pump & impeller size question

    It has to do with the pump affinity laws. When a pump motor runs at half speed (low speed), the flow rate is half of what it is on high speed. But also, the head pressure drops by a factor of 4. Hydraulic power (power delivered to the water) is related to the product of both flow rate and head so at half speed, the hydraulic power drops by a factor of 8 and so does the shaft power. However, because of the way a two speed motor is designed, the efficiency drops by 50% on low speed so the motor input power drops only by a factor of 4. A VS pump does not suffer as much from this drop in efficiency because the line frequency is changed instead of the winding configuration.
    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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    msgtdan's Avatar
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    Re: Pump & impeller size question

    So to restate in very simplistic terms, because resistance to flow as well as flow itself is lower you won't damage the motor. The pump is most efficient at full speed, loses some efficiency at low speed but overall power consumption is lower therefore saving money. If that's an accurate restatement I think I understand. Thank you. I'd read about the lower flow and flow resistance but hadn't put it together with the over-sized impeller comments. The two are usually discussed separately and I hadn't seen any discussion of the two speed motors with impeller sizing. I knew there had to be a reason and it was bugging me that I hadn't figured it out.
    Dan D
    Used 2003 Aqua Leader 27'x52", 17,800 w/10" hopper, SwimPro SW256T 250# sand filter, Hayward PowerFlo LX pump 1hp impellor, Emerson 1 1/2 hp motor, setup Aug 2012 Summer 2011 used Summer Escapes Ring pool 14' x 42", Intex 1600gph sand filter, HTH 6 way test kitTF-100 w/stirrer, Well water @ FC=0, PH=7.2, TA=290, CH=320

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