Considering a VS motor? Look at the whole system to optimize performance.

setsailsoon

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Folks,

***** EDIT*****

With some great input from our experts I should state up front I've greatly over-simplified this in my initial post by omitting the rest of the system behavior. If you're interested in how this affects pressure and flow through your system read on for some excellent information provided by TFP experts. If like most, you really don't care about this you can just follow @JamesW simplified advice and just put the largest impeller in your pump that the motor can power. But if you care at all you should read all their posts. They're a fantastic explanation with some great insight into your pool's hydraulic behavior.

*****EDIT*****

Since changing to a VS motor I noticed my pool was getting a little dirtier on the bottom than it used to be. I use a suction cleaner and it was now running way too slow unless I speed up to get the wheel speed to specification. I initially thought this would be an easy fix by setting an additional cycle at higher speed but I also remembered @JamesW had indicated I might be able to go up a size on my impeller size. My pump is a Jandy FHPM-1.0 and the next size up is a FHPM 1.5. I looked into this a little further and found next size up is only slightly bigger but would significantly improve flow while staying below the 1.65 HP rating of my new V-Green motor. Here's the pump curve:

1579784222321.png
Looks like I could get surprisingly better flow from this small change in impeller size (20 gpm to 35 gpm). Looking at the pump body and parts diagrams I figured out there is a wide range of impellers that will fit and there are two different diffuser sizes. My impeller was the largest size for the diffuser so I had to go up on that as well. Another huge advantage is at the high speed. Look at what happens at 3450 rpm curve:

1579785056732.png

I go from just under 50 gpm to just under 80 gpm. Of course this isn't exactly right since the increased flow will increase pressure drop in the filter and actually result in a slightly higher pressure. But this is one of the nice things about having a filter that is over-sized. The pressure doesn't really go up much:

1579785892044.png

As you can see my filter is sized such that I'm on the very bottom of the pressure drop curve so going up to about 80 gpm only increases my drop by about 1.5 psi.

So bottom line, when you change just the motor to convert a VS pump it's definitely well worth looking into the impeller size. I've noticed our experts mention many times that when you buy the entire pump a big advantage is that the impeller is optimized for the motor. I think this little exercise proves the point. Even with no impeller change you can still save a lot of operating cost with just the VS motor only approach. But when you want the maximum savings and performance it's a real good practice to look at the whole system, the pump motor, impeller and filter to get the best overall performance. Since I was at the top end of the diffuser my cost was double that of an impeller change but still only $70 so the motor and impeller change totaled $470. Still way lower than a complete pump. To me this also illustrates a huge benefit of having TFP around for ordinary pool owners. It might be difficult for a handy DIY type pool owner to figure this out and I doubt that many pool service companies would do this. Even if they did costs would be prohibitive. TFP has experts that can help guide anybody that's reasonably handy to do this themselves.

I hope this is helpful to others considering an upgrade to a VS motor while using their existing pump.

Chris
 
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mas985

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Head loss in a plumbing system does not remain a constant with flow rate. Everything in the plumbing system contributes to head loss, not just the filter. Head loss in a plumbing system changes by the square of the flow rate and has it's own head curve that is in the form of:

Head (ft) = C * GPM^2

So if the 1 HP was producing 50 GPM, the 1.5 HP would be producing only 54 GPM, not 80 GPM. On half speed, both pumps produce half the flow rate so 25 GPM and 27 GPM.
 

setsailsoon

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Chris, the SS pump motor was 1 HP and you replaced it with a 1.65 HP VS motor?
Well sort of. It was a FHPM 1.0-2 originally but the single speed and 2-speed have the same impeller size. And yes the VS motor is a V-Green 1.65.

Chris
 

setsailsoon

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Head loss in a plumbing system does not remain a constant with flow rate. Everything in the plumbing system contributes to head loss, not just the filter. Head loss in a plumbing system changes by the square of the flow rate and has it's own head curve that is in the form of:

Head (ft) = C * GPM^2

So if the 1 HP was producing 50 GPM, the 1.5 HP would be producing only 54 GPM, not 80 GPM. On half speed, both pumps produce half the flow rate so 25 GPM and 27 GPM.
Mark,

Yes, I was thinking about that after I posted. I'll see if I notice pressures that can confirm this. One thing I do notice is the suction cleaner looks like it's on drugs! I need to adjust it. Wheels are waaaay to fast. Wouldn't that indicate I'm getting more than 10% improvement in flow?

Chris
 

JamesW

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Mar 2, 2011
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A lot depends on the slope of the system curve. With a steep curve, the benefits are not as significant as with a less steep curve.

I would think that it's worthwhile to replace the impeller to the largest size that the motor can take for most system curves.
 

JamesW

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Mar 2, 2011
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To get the best idea, you would need to generate a system curve by changing the gpm and measuring the resulting suction and return head loss (pressure) and then plotting the curve on the same graph as the pump curves to see where the lines cross.
 

setsailsoon

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A lot depends on the slope of the system curve. With a steep curve, the benefits are not as significant as with a less steep curve.

I would think that it's worthwhile to replace the impeller to the largest size that the motor can take for most system curves.
James,

That makes a lot of sense. My suction cleaner performance would indicate I'm not on the steep part of the curve. I would think that "rules of thumb" designs and traditional conservative design practices probably gave me more "fat" than a newer customized and optimized design would have.

Chris
 

setsailsoon

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To get the best idea, you would need to generate a system curve by changing the gpm and measuring the resulting suction and return head loss (pressure) and then plotting the curve on the same graph as the pump curves to see where the lines cross.
James,

Another great point. For people considering a VS motor only approach it would be a real good idea to get the system pressure curve first. It would be easy to do and give a much better prediction of performance. My guess is the first rule of thumb you mentioned "go to the largest impeller size the motor will handle" will almost always be the right answer.

Thanks!

Chris
 

cfherrman

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May 10, 2017
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Hays, Kansas
We're you running your pump with the 1 hp impeller at full speed ever?

With a vsp this is (possibly) more of a waste of time as running it a higher speed with 1 hp impeller or a lower speed with a 1.5 hp impeller is the same cost energy wise.

So if you need more push at full speed, that's great to change, other wise setting a different speed is exactly the same.
 

JamesW

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Mar 2, 2011
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Variable speed pumps get pretty loud at full speed. The TEFC (Totally Enclosed Fan Cooled) motor has a back mounted fan that begins to get very annoying above about 2750 rpm.

I suspect that you can even use an impeller larger than the 1.65 hp model as long as you keep the rpm low enough that the amps used don't exceed the label amps.

For example, you could probably have used the largest impeller available for the pump and then measured the amps to set a top speed.

This would make the pump more like a water feature pump that works well for lower head applications.
 
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setsailsoon

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We're you running your pump with the 1 hp impeller at full speed ever?

With a vsp this is (possibly) more of a waste of time as running it a higher speed with 1 hp impeller or a lower speed with a 1.5 hp impeller is the same cost energy wise.

So if you need more push at full speed, that's great to change, other wise setting a different speed is exactly the same.
CF,

I think I see your point. Yes, I ran the pump at high speed most of the day. Problem for me was I have a solar heater that is mounted on the roof and a suction cleaner. Low speed would work fine for the swg but it didn't flow at all when the solar valve was open and runs the wheel speed way to low for the suction cleaner. So I needed a low speed for "normal" operation and high speed for "solar" operation. Now my max speed will probably be about 2700 rpm and low speed will be around 1600 rpm. I'm still playing around with it to optimize.

Chris
 

mas985

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If your plumbing is closer to Curve-C, typical of 2" plumbing, the flow rates for the two impellers would be around 68/79 GPM. Which is a little over 15% increase.

Also, there may be modifications you can make to allow for a slightly lower RPM when running solar but it depends on the current placement of the VRV.
 

setsailsoon

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https://aquamagazine.com/service/how-to-read-pump-system-curves.html

Here, you can see the big difference between different system curves and how they cross the pump curve at different points.
James,

Great article... But now you've done it! I won't be satisfied 'till I get my system curve established. To do this I'll get enough points for the curve by adjusting the speed. To get flow rates I'll use the bucket timer method at the spillovers (two of them). I hope to do this while Robin's not there. I already take a fair amount of ribbing from her at dinner with friends for the nerd engineer in me. They all had a real blast with my heater repair fiasco! :mad:

Chris
 

JamesW

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Mar 2, 2011
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This gives an idea about how the system curve interacts with the pump curves at different speeds or different impeller sizes.

So, the line is not horizontal. It's more like 45 to 60 degrees depending on where you are in the system curve with the slope getting steeper as you go up.

As we can see, there are multiple factors to consider when choosing a pump, motor, impeller etc.
 

JamesW

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Mar 2, 2011
17,026
When measuring the total dynamic head, you need the suction pressure as well as the return pressure.

Return pressure can be seen on the filter pressure gauge. A suction gauge can be installed to measure the suction.

A flowmeter is good for measuring flow directly. BlueWhite has several good and affordable models.

Mas985 has several good tools for estimating the system that you can use.

Also note that every configuration of the system (solar on/off) etc. will have a different system curve.
 

setsailsoon

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When measuring the total dynamic head, you need the suction pressure as well as the return pressure.

Return pressure can be seen on the filter pressure gauge. A suction gauge can be installed to measure the suction.

A flowmeter is good for measuring flow directly. BlueWhite has several good and affordable models.

Mas985 has several good tools for estimating the system that you can use.

Also note that every configuration of the system (solar on/off) etc. will have a different system curve.
Oh great! More measurements... Robin's gonna have fun with this.

Thanks!

Chris