Pump replacement help - Larger motor replacement safe?

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chenss2000
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Joined: Sat 05 Mar, 2011 02:22
My Pool: Small pool and spa. 3/4 hp whisperflo, DE.
Location: Los Angeles

Pump replacement help - Larger motor replacement safe?

Postby chenss2000 » Sat 05 Mar, 2011 02:39

I currently have a 3/4 hp whisperflo. It started to make noise due to the bearings, so I called home warranty to repair it since I just bought the house couple months ago. They came with a 2 hp motor and convinced me it would work since my impellers are for the smaller motor. Good thing is it would not fit in the space so they are bringing a smaller motor on Monday.

Is going with a larger motor a good idea? Will it use more power even with the smaller impeller? I am trying to confirm the size of the smaller motor, but really I just want the same size since everything works (including the spa). I definitely don't want to use more power than I need to.

Thanks


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mas985
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Location: Pleasanton, CA, USA

Pump replacement help - Larger motor replacement safe?

Postby mas985 » Sat 05 Mar, 2011 12:33

Even though oversized, a larger motor will usually work fine with a smaller impeller. In fact, it could have better efficiency overall because of the large winding gauge. The power draw of any motor in a pump application is determined mainly by the load of the impeller. So even if you oversize the motor, it will still draw approximately the same power as the smaller motor. But because of the larger windings, the I2R losses tend to be less with a larger motor.

The only downside, although not really a negative, is that the power factor starts to decrease when oversized by more than 50% or so. This just means that there is some reactive power on the line which is more of a negative for the power company than the home owner.
Mark
Hydraulics 101 ; Pump and Pool Spreadsheets ; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater
chenss2000
Pool Newbie
Pool Newbie
Posts: 2
Joined: Sat 05 Mar, 2011 02:22
My Pool: Small pool and spa. 3/4 hp whisperflo, DE.
Location: Los Angeles

Pump replacement help - Larger motor replacement safe?

Postby chenss2000 » Sat 05 Mar, 2011 12:45

Is going from 3/4 hp to 2 hp too large of a jump? It sounds like a larger motor will perform based on the impeller. So if the impeller rated lower then the motor will only work that hard. By power factor, is that the service factor? So 2 hp with a power factor of .5 will really yield 1 hp and use approximately half the power?

What is I2R loss?

Thanks for the detailed response. I'm still learning and very new at this.

mas985 wrote:Even though oversized, a larger motor will usually work fine with a smaller impeller. In fact, it could have better efficiency overall because of the large winding gauge. The power draw of any motor in a pump application is determined mainly by the load of the impeller. So even if you oversize the motor, it will still draw approximately the same power as the smaller motor. But because of the larger windings, the I2R losses tend to be less with a larger motor.

The only downside, although not really a negative, is that the power factor starts to decrease when oversized by more than 50% or so. This just means that there is some reactive power on the line which is more of a negative for the power company than the home owner.
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mas985
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Posts: 199
Joined: Tue 08 Sep, 2009 10:40
Location: Pleasanton, CA, USA

Pump replacement help - Larger motor replacement safe?

Postby mas985 » Sat 05 Mar, 2011 18:37

Correct, the impeller size is what primarily dictates power consumption. However, larger motors tend to be more efficient since the windings are larger.

Wiki has a pretty good description of power factor . Basically, it is the cosine of the phase difference between the voltage and current. Normally motors will run at a power factor of greater than 90% but when underloaded, the power factor decreases some. This has a small impact on efficiency.

I2R losses are the loses due to the resistance in the wire. I = current and R = the resistance in the wire. I * I * R (I2R) is the power lost due to the resistance.
Mark
Hydraulics 101 ; Pump and Pool Spreadsheets ; Pump Ed 101
18'x36' 20k gallon plaster/gunite pool, 1/2 HP 2sp pump, Aqualogic PS8 SWCG, 420 sq-ft Cartridge Filter, Solar Panels, 6 jet spa, 1 HP jet pump, 400k BTU NG Heater

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