Fixed or Variable Speed Pool Pump?

[EEVblog]

Finally got my pool circuit monitoring enabled.
The pool pump takes twice the energy per day than my heat pump hot water tank.

But I'm not convinced that a variable speed pump will make much energy difference overall. I'd just have to leave it on for a lot longer for a given volume/filter/chroine production. Convince me otherwise...

Madimac claim you save up to 80% in running costs with a variable spped controller. Someone else in one of my video comments mentioned big savings, but I'm not convinced.
It's not like the pump is magically more efficient, you are just reducing the speed/flow rate, so need longer to filter the same volume of water.
Sure, maybe some increased efficiency in pipe fluid dynamic perhaps, but I'm not sold.
Someone convince me....

https://madimack.com/au/blog/madimack-inverter-plus-energy-saving-single-speed-pool-pump-controller

[IanB]

Pumps generally have an optimal efficiency at an optimal design point of flowrate and pressure (head). So the most efficient system will have a pump that is selected to be the right size for the duty required of it, and which turns on and off as needed.

Most of the energy required by a pump is to move the required volume of liquid in the system against the elevation change and resistances in the pipework and filters etc. It is possible that a smaller pump running for longer periods at a lower flowrate will use less energy, but it is not possible to know how much less without a detailed hydraulic analysis.

(All things being equal, a pump running twice as long at half the flowrate will consume the same amount of energy overall. All things are not equal in reality, but the differences may not be that much. It all depends...)

If you have an existing pump and are considering replacing it, then you need to figure in the cost of replacement against any potential cost savings obtained.

[EEVblog]

Pumps generally have an optimal efficiency at an optimal design point of flowrate and pressure (head). So the most efficient system will have a pump that is selected to be the right size for the duty required of it, and which turns on and off as needed.

Most of the energy required by a pump is to move the required volume of liquid in the system against the elevation change and resistances in the pipework and filters etc. It is possible that a smaller pump running for longer periods at a lower flowrate will use less energy, but it is not possible to know how much less without a detailed hydraulic analysis.

Exactly, which is why I'm skeptical of these blanket 80% reduction claims, and of people telling me to get a variable speed pump.

If you have an existing pump and are considering replacing it, then you need to figure in the cost of replacement against any potential cost savings obtained.

It's literally a brand new pump. I would not replace the pump of course, I'd get a variable speed controller for the existing one if I went that route.
I do believe my current pump is fairly optimum for my head, pipework and volume.

[IanB]

It's literally a brand new pump. I would not replace the pump of course, I'd get a variable speed controller for the existing one if I went that route.

A relatively low power fixed speed motor would typically be a single phase AC induction motor. Whereas for variable speed, you would more likely have a VFD and a three phase motor. Is it actually possible to retrofit a variable speed controller on a single phase motor?

[Alex Wolf]

Any machine has a sweet spot where its efficiency is at maximum. This electromechanical and hydrodynamic machine isn't an exception. Therefore if that pump controller finds the sweet spot automatically, it may work. But that 80% efficiency increase claim sounds like a BS for me. Yep, 'up to': maybe in case of their product was compared versus the very bad and dirty cheap water pump. But in common case, doubt it.

Whatever it was, if the manufacturer is confident enough, I think he will not refuse to sponsor a video verifying his claims of effectiveness, providing a sample for independent testing by some of the most authoritative and recognizable people in the modern electronics world. If he will refuse, then we know the correct answer to the question of the topic. 😏

[EEVblog]

A relatively low power fixed speed motor would typically be a single phase AC induction motor. Whereas for variable speed, you would more likely have a VFD and a three phase motor. Is it actually possible to retrofit a variable speed controller on a single phase motor?

Madimack sell them, and like I said, claim up to 80% power saving.
https://madimack.com/au/blog/madimack-inverter-plus-energy-saving-single-speed-pool-pump-controller

The link to their test report with UNSW has gone missing.
I only found this:
https://www.scimex.org/newsfeed/energy-guzzling-pool-pumps-can-be-tamed-new-research

[IanB]

Madimack sell them, and like I said, claim up to 80% power saving.
https://madimack.com/au/blog/madimack-inverter-plus-energy-saving-single-speed-pool-pump-controller

OK, so that link claims "up to 80% energy and cost savings". That means a pump that was previously consuming 5 kWh per day (for example), would then be consuming 1 kWh per day (80% less). I call bullshit.

If it were possible to do that, then it would mean that 80% of the power being consumed by the pump was being wasted to no good purpose. No competent designer would design a system with such a low efficiency, unless the energy costs were negligible and irrelevant, or the designer didn't care.

[johansen]

I Assume the market for this is 2hp pumps that circulate water through the filter and an electric heater.

Most pumps in hot tubs in the usa are two speed pumps, a small 4 pole winding with a separate run capacitor ans a large 2 pole winding with another cap.


The 80% savings from cutting the rpm in half is legit, in theory you can run a pump at half speed on 1/7th the power..  The claims about real energy savings are dubious because the waste heat just heats the water

Half speed at 1/8th the power means you cam move the same volume of water for 1/4the the cost. This is legit, its how friction works in fluids like fans and pumps, where the kinetic energy in the moving fluid is lost

[IanB]

The 80% savings from cutting the rpm in half is legit, in theory you can run a pump at half speed on 1/7th the power..  The claims about real energy savings are dubious because the waste heat just heats the water

I'm sorry, but as a chemical engineer who knows something about pumps and hydraulics, I know of no such theory. If you could make such huge savings by running a pump slower, then there are lots of accountants who would like a word with you.

The only scenario that makes sense is if the motor is originally vastly oversized and is simply wasting most of its power against a dead head. Is it possible that everyone installs a 2 hp pump even when only 0.5 hp is needed, because the 2 hp pump is a standard off-the-shelf item?

[IanB]

To put this in perspective, let's suppose you need to move 10 m³ of water in 24 h. If you run at half the flowrate, then you will only move 5 m³ of water in the same 24 h period, and you won't have met the system requirements.

On the other hand, suppose the pump is only running for 6 h every 24 h period, and off for the other 18 h. Then you could use a smaller pump and have it running for 12 h, or an even smaller pump and have it running continuously for 24 h. But in that case you could just specify and install a smaller pump in the first place. It makes no sense to install a bigger, oversized pump, and then install a variable speed controller to slow it down. The cost of the bigger pump and the speed controller is much more than just installing a right-sized pump in the first place.

[johansen]

The 80% savings from cutting the rpm in half is legit, in theory you can run a pump at half speed on 1/7th the power..  The claims about real energy savings are dubious because the waste heat just heats the water

I'm sorry, but as a chemical engineer who knows something about pumps and hydraulics, I know of no such theory. If you could make such huge savings by running a pump slower, then there are lots of accountants who would like a word with you.

The only scenario that makes sense is if the motor is originally vastly oversized and is simply wasting most of its power against a dead head. Is it possible that everyone installs a 2 hp pump even when only 0.5 hp is needed, because the 2 hp pump is a standard off-the-shelf item?

The 2hp 2 speed pool pumps need to be that big to run the jacuzzi jets.

And yes, 1/2 speed is about 1/7th the mechanical power on a zero static head, centrifugal pump, which is what these systems are. The 2 feet the water has to be lifted is negligible.

There are pump motors with separate windings for 2,4,6,8 poles. Geometrically the windings decrease in size and the 6 and 8 pole windings take up very little space in the 100hp sized motors and it takes a tiny fraction of the power to run the pumps at 1/4th full rpm. These centrifugal pumps circulate coolant through a certain 60 megawatt thing with control rods.

You dont seem to be reasonable, so go argue with hazen-williams instead.
https://www.engineeringtoolbox.com/amp/hazen-williams-water-d_797.html

Or boat designers. You need around 8 times the hp to push a boat twice as fast

[IanB]

The 2hp 2 speed pool pumps need to be that big to run the jacuzzi jets.

And yes, 1/2 speed is about 1/7th the mechanical power on a zero static head, centrifugal pump, which is what these systems are. The 2 feet the water has to be lifted is negligible.

There are pump motors with separate windings for 2,4,6,8 poles. Geometrically the windings decrease in size and the 6 and 8 pole windings take up very little space in the 100hp sized motors and it takes a tiny fraction of the power to run the pumps at 1/4th full rpm. These centrifugal pumps circulate coolant through a certain 60 megawatt thing with control rods.

So we have here a pump that does dual duty? One is to run jacuzzi jets, where a high flowrate is needed (or the jacuzzi won't jacuze). And another is to circulate water through some system that needs a lower flowrate?

This seems far from the situation described by Dave, which is a pool pump. No mention of a jacuzzi.

[Alex Wolf]

Half speed at 1/8th the power means you cam move the same volume of water for 1/4the the cost. This is legit, its how friction works in fluids like fans and pumps, where the kinetic energy in the moving fluid is lost

This statement sounds like an attempt to violate the law of conservation of energy. We know that the efficiency of a single-phase electric motor is approximately 60-70%, regardless of its RPM (as long as they are within the nominal range). Putting aside the effects of cavitation at higher flow/rotation speeds, according to your statements, the efficiency of the electric motor must double to provide enough torque at half speed to pump the same volume of water. And by the way, revolutions per minute don't have a linear relationship with the torque (useful work) of the electric motor, but the power consumption does. The only case where this would be possible is if the level of cavitation was absurdly high before at a higher flow, and the system throughput is prohibitively low for a given electric motor power. I don't think this has anything to do with the Dave's case, knowing how thoroughly he researches a matter before doing anything.

One is to run jacuzzi jets, where a high flowrate is needed (or the jacuzzi won't jacuze).

BTW, Jacuzzi jets are an excellent example of an absurdly high level of cavitation by design. 🙂

[IanB]

This statement sounds like an attempt to violate the law of conservation of energy. We know that the efficiency of a single-phase electric motor is approximately 60-70%, regardless of its RPM (as long as they are within the nominal range). Putting aside the effects of cavitation at higher flow/rotation speeds, according to your statements, the efficiency of the electric motor must double to provide enough torque at half speed to pump the same volume of water. And by the way, revolutions per minute don't have a linear relationship with the torque (useful work) of the electric motor, but the power consumption does. The only case where this would be possible is if the level of cavitation was absurdly high before at a higher flow, and the system throughput is prohibitively low for a given electric motor power.

Well you can't get something for nothing of course, but it's not the motor but the pump. A centrifugal pump approximately follows power laws, where the flow is proportional to the speed, the head is proportional to the square of the speed, and the power consumed is proportional to the cube of the speed.

That means that if you run a pump at half the speed, it will consume 1/8th of the power. It will also produce a quarter of the head and half the flow, which all fine and dandy, except that now you only have a quarter of the head and half the flow available. If your system design calls for all of the head and all of the flow, then your pump has just stopped doing its job.

[Alex Wolf]

Well you can't get something for nothing of course, but it's not the motor but the pump. A centrifugal pump approximately follows power laws, where the flow is proportional to the speed, the head is proportional to the square of the speed, and the power consumed is proportional to the cube of the speed.

At idle or under load? Still sounds like an oversight in calculations that try to violate the law of conservation of energy. If you produce the same amount of useful work with half the energy consumption, this can only mean doubling the efficiency and nothing else. Are centrifugal electric pumps so poorly designed that when their speed is halved, their efficiency doubles (leaving all other elements of the hydraulic system behind the scenes)?

Edit: I mean that usually the nominal speed is designed to be as close to peak efficiency as possible. Doubling the efficiency by simply reducing the speed sounds very doubtful in the general case.

[Siwastaja]

Don't know about pool pumps exactly, but blanket claims about magically better efficiency might well be true. The problem isn't fixedness, it's usage of poor efficiency motor types. We saw this in hydronic heating loop circulation pumps: the older types consumed 70W at maximum speed, and the replacement variable frequency types (don't know if they are internally BLDC or what) consume like 20-25W at the same flow rate and pressure. The difference grows even bigger at reduced flow rates. The old type is simply a cost-optimized capacitor-run single phase AC induction motor with a lot of slip. It's same as with fan motors: a 10-15W BLDC "computer fan" produces same pressure / air flow as a 50W mains fan.

To answer your question, you should not start from the specifications of the new variable speed pump, but instead try to find out what is the efficiency of your current fixed speed pump.

Remember that in circulation systems the physical work to lift water is zero, and you can consider the whole power consumption as losses - friction and electric. Therefore, it is entirely possible, and not even difficult, to get to very low consumption. To me your numbers sound high. If the current pump is of inefficient type, replacing it would be the first step. The next step would be to reduce mechanical losses, probably by first increasing the filter area. If that is not sufficient, pipes can be replaced by larger diameter size.

There is also synergy in doing these steps. Say, you decrease pressure drop of the filter by making it larger, halving mechanical losses. But the old fixed-speed pump has a narrow efficiency sweet spot, so power consumption does not drop to half. Add a VFD pump (and the point here is not the speed, it's the commutation strategy), and it will have much wider sweet operating region. It's like comparing switch mode supplies where one does have pulse skipping and other does not, in light load conditions.

[Alex Wolf]

The same thing as discussed above about the sweet spot, but clearly in the form of curves:



Source 1: https://chemicalengineeringworld.com/pump-performance-curve/

Source 2: https://hardhatengineer.com/centrifugal-pump-curves/

So if the pump controller is smart enough and equipped with pressure and flow sensors, it can automatically peak the efficiency of the entire system. It is impossible to predict what specific gain in efficiency will be without knowing the parameters of the current system.

[Kleinstein]

In the circulating system the energy needed is for friction. Depending on th flow regime the needed pressure is linear (laminar flow) to quadratic  (turbulent flow approximation)  going up with the flow rate. So half the speed and 1/4 the energy would the upper limit.

The other point is the efficiency of the pump. With a centrifugal pump the suitable pressure should go down quadratic with the speed and should thus reasonable match the turbulent flow case. The question is however if the motor allows for good control (e.g. switching the poles for 1/2 the speed). 1 phase induction motors are tricky to control.

For dave it looks like the pump is already running quite long, much of the time that solar power is available. So there is not really an option to run longer and thus slower. It would be a thing only if not that much filtering is needed.

For the circulating pumps in heating systems there is  an additional effect from the motor: the old style fixed power pumps where rather low efficiency (e.g. 30% range) induction motors. Induction motors don't work well that at the small size (some 50 W) and with large slip they don't blow up even if blocked. The modern speed controlled pumps are usually BLDC motors with much better efficiency.
With the pool pump the motor to start with should be quite a bit better.

[EEVblog]

For dave it looks like the pump is already running quite long, much of the time that solar power is available. So there is not really an option to run longer and thus slower. It would be a thing only if not that much filtering is needed.

4hrs per day. I just lowered that to 3hrs.
Correct, running longer outside of solar periods defeats the purpose unless we had battery storage.

[EEVblog]

The same thing as discussed above about the sweet spot, but clearly in the form of curves:

My pump maker only gives the Head graph.
https://s3-ap-southeast-2.amazonaws.com/astralpools-au-2/MEPS/AstralPool_Pumps_CTX_MKII_190922_(1).pdf

[EEVblog]

Well you can't get something for nothing of course, but it's not the motor but the pump. A centrifugal pump approximately follows power laws, where the flow is proportional to the speed, the head is proportional to the square of the speed, and the power consumed is proportional to the cube of the speed.

At idle or under load? Still sounds like an oversight in calculations that try to violate the law of conservation of energy. If you produce the same amount of useful work with half the energy consumption, this can only mean doubling the efficiency and nothing else. Are centrifugal electric pumps so poorly designed that when their speed is halved, their efficiency doubles (leaving all other elements of the hydraulic system behind the scenes)?

That's my question. Ultimately people are saying that a variable frequency pump is (say) 4 times more efficienct than a single speed pump for a given amount of useful work. I can't buy that at face value.

[EEVblog]

Astral makes this claim for their variable speed pumps vs their fixed speed pump:
https://s3-ap-southeast-2.amazonaws.com/astralpools-au-2/MEPS/Viron_XT_MKII_Datasheet_190922_(1).pdf

Increasing or decreasing the speed of a
pump simply changes the flow rate by the
same rate. However, the actual energy used
to change the speed and flow varies
dramatically. Halve the speed of your pump
and the flow rate will also halve but, the
energy consumed drops by more than 85%.
Compared to conventional single speed
pumps, the Viron XT pump will save you
money every year in operating costs.
These two unique features in the new Viron
XT range offers the most flexible and
adaptable pump on the market and will
often allow you to save money by choose a
smaller model that will out perform other
larger model pumps.

[nctnico]

Madimack sell them, and like I said, claim up to 80% power saving.
https://madimack.com/au/blog/madimack-inverter-plus-energy-saving-single-speed-pool-pump-controller

OK, so that link claims "up to 80% energy and cost savings". That means a pump that was previously consuming 5 kWh per day (for example), would then be consuming 1 kWh per day (80% less). I call bullshit.

Not so quick. I'm looking into upgrading the ventilation system in my home and I keep getting across the claim that using a 'DC' brushless motor is 80% more efficient compared to using an AC motor (for the same amount of airflow). Even on official government websites to help people save energy. I assume pool pumps have a power, pressure and flow rating. Comparing these, should give all the answers you need.

[EEVblog]

My fixed speed CTX360 vs the variable speed Astral pump.
Both similar head at say 300L/m if you use the orange "overdrive" mode.

And their own claimed power consumption figures shows
Variable: 514kWh/yr for 303L/min for 8m head
Fixed: 1353kWh/yr for 360L/min for 8m head.

So correcting the variable one for 360L/min gives 610kWh/yr
So better than half the energy consumption at the same flow rate, why?
Just because of the way the motor is driven?
In which case why not simply make the fixed speed pump with the variable drive control but fix it at one speed? 
It makes no sense for a company to make a fixed speed pump at half the efficiency if it can make it by just changing the motor drive type
They do have a whole range fo fixed speed pump models though, I'm only comparing the CTX model I have.

But in any case, how does one get an Energy Star rating of 1 and the other get a rating of 6? 

[EEVblog]

And then in the same veriable speed datasheet they show even less of an advantage vs fixed speed 

367kwh vs 414kwh is not exactly selling a huge advantage?