2 Speed Motor

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fharczuk

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May 26, 2009
43
Eatontown, NJ
I just installed a 2 speed motor for my pump.

Since low runs at 1/2 speed of full could I expect the GPM would be pretty much exactly 1/2?

Full speed the pressure at the filter is 18 psi compared to 2 psi at low.

Since the pressure is not proportional could I say that at high speed the pump is pumping much more that the filter can most efficiently filter. Would this be the bell curve that motor manufacturers have with their pumps. Reading into this more my question above could I be getting more that half the flow at low speed.

Everything is working great but I'm a techy and am curios, and you guys on this forum seem to know everything? :)
 
Low speed does indeed move half as much water (GPM) as high speed. The dramatic reduction in filter pressure is normal. Filter pressure comes from resistance to flow, essentially friction, which increases far more rapidly than the flow rate increases. I've forgotten the exact formula, I think resistance to flow increases by the square or the cube of the flow rate.
 
Wow 1/6 the power.

Is there any reason why I would need to use the high speed? My Polaris with separate pump seems happy at the low speed. I measured the rotation according to the Polaris manual and its even on the high end. I know the Skimmers will not be the greatest but I am experimenting with shutting off the bottom drain to increase the flow at the skimmers.
 
I run mine on high while the polaris booster is running. I am not sure if this is strictly necessary (I also noticed the booster seems OK with main at low), but I think it may help get the suspended particles that the cleaner stirs up into the filter.
 
I run mine on low speed 24/7 and only toggle over to high speed for 1-2 min. backwash. My skimmer functions well on low speed if the skimmer valve is fully open and the main drain valve is 3/4 to 1/2 open. I manual vacuum to the filter via skimmer plate using low speed with the main drain valve only 1/4 open (or less). This works great because at low speed there is almost none of the "blow back" of fine sediment into the pool that normally occurs at high speed while vacuuming. I have been amazed at how much better my sand filter works on a day to day basis using the low speed...the water moves so gently that filtration is greatly increased over my previous set-up of a single speed one (1) hp motor where some of the fines were passing through the filter due to the force of the water.

And yes, low speed moves 1/2 as much water (it takes twice as long to move the same amount of water), but with the new high efficiency 2 speed motors with capacitor start/capacitor run low speed, it only takes 1/8 the power to move 1/2 the water! These new motors offer 20% energy savings over previous 2-speed motors. On low speed, my 3/4 hp full rated motor is only 0.10 hp and pulls 2.2 amps @ 115V...barely noticeable on the monthly electric bill though running 24/7 in summer.
 
The electrical usage on low speed for a standard two speed motor is in theory 1/4th of the usage on high speed, but no motor is perfect enough to actually achieve that. The real world usage is typically more like 1/3rd of the usage on high speed. To make a fair comparison, you then need to multiply that by two, so you are moving the same total amount of water. The overall electrical usage ends up being around 2/3rds of what it would be on high speed to move the same total amount of water.
 
The CEC measurement data actually shows that most two speed pumps use about 1/4 the power on low speed than high. Some are better some are worse but on average about 25%.

However, I think there is some confusion from the previous posts on the application of pump affinity laws. These are stated as follows:

GPM Low Speed = GPM High Speed * (RPM Low Speed / RPM Hi Speed)
Head Low Speed = Head High Speed * (RPM Low Speed / RPM Hi Speed)^2
HHP Low Speed = HHP High Speed * (RPM Low Speed / RPM Hi Speed)^3

HHP, the hydraulic HP, is what goes down by a factor of 8 but the pump affinity equations do not account for efficiency in the pump and motor so adjustments are required to calculate the change in input power. 1/2 speed on a typical pump tends to be about 50% less efficient than high speed because twice the windings and fields are involved. So the actual power draw of low speed is about a 1/4 of high speed for most pumps. Some pumps are better some are worse but again, you can see this in the CEC measurement data.

Also, you can explain the difference in pressure by using the head loss formula so the pressure should decrease by a factor of four. However, you need to take into account the height of the filter gauge so the adjustment needs to be:

(18 PSI + 3'/2.31) / 4 - 3'/2.31) = 3.5 PSI

Gauge accuracy is no better than +- 1 PSI and some even worse so it is pretty close to what is observed.
 

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Keep in mind though that the power factor decreases with low speed so your actual power consumption is less than what the 3A would indicate.
 
With AC waveforms, the voltage and current can become out of phase in the inductive load of a motor. This creates a situation where there is both real and imaginary power but the power company only charges for the real power since the imaginary power is sent back on the line and is not consumed. To account for this effect, the true formula for real input power is:

Real Power = V * A * PF

where PF is the power factor and is between 0 and 100%. At high speed, the power factor is usually over 90% but on low speed it will go down some because effectively the motor in underloaded. So my guess is that the input power is closer to 440 watts instead of 660 watts. The other 220 watts is imaginary power.

The best way to measure pump power is via the electric company's power meter. This will measure only real power. If you take a reading with and without the pump running, you can find out just the pump power.
 
I believe that unit is only rated for 125v maximum and should only really be used for plug in pumps. Are you using it to measure 240v power? While it may work, for safety reasons, I don't think I would recommend that anyone should do this.
 
I run this pump: neiko-tools-usa-1-hp-water-pump-with-strainer-review-109-99-t20674.html

The draw is weel below the 15 amp rating of the Kill A Watt. I have tested my Pentair 240v pump with it but thats trickier. I had to cut a 14ga. extention cord in half and down to 3 feet. then run just one of the phases thru the Kill A Watt. I then double our rate of 11.2 cents/kwatt hr. to get a accurate Dollor readout as the Kill A Watt can only see half the draw.
 
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