Variable Speed Pumps - Cheaper? Better? Alternatives

[AlanH]

I've been looking at pumps for our DIY build. An energy efficient pump is especially important since we have a battery backup system (power cuts up to 3-4 days) and will be installing a larger, totally off-grid system in the near future. (Unreliable power at $0.25/KwH the main reason). Without getting into the specifics of the pool and required pump, I have some general thoughts and questions to run by forum members.

I've had very little success finding 12-24 volt DC pumps suitable for a pool installation (suggestions welcome). Variable speed pool pumps may be an option as a last resort. They are more attractive to me than 2 speed pumps since they seem to operate at much lower power levels(eg. 160 watts - 230Vx0.7A for the Intelliflo). My hesitation at the moment are reports of the controller failing (especially the Intelliflo), no warranty service in Nicaragua, the addition (and inefficiencies) of a step-up transformer (from 115 to 230V) to our system, and a cost of over $2000 to get one here.

Given the energy efficiencies and cost savings reported for these pumps I began searching for "variable speed pump" alternatives. I came up with an item called "AC Drives" (variable frequency drives, variable speed drives). These are the controllers that allow one to control the speed (rpm) of an AC motor. They have been around for decades and are not quite the newest, greatest thing that the pool industry would have you believe (well, maybe for pool pumps, but not by a long shot for other pump applications). For .5 to 1 HP pump motors they cost $100-$200, have very sophisticated control features, and wire easily between an AC source and the pump.

Questions for forum members:

1. Could the appropriate AC Drive be paired with a (less expensive) single speed pool pump to make a "workable" variable speed pump (probably at half the cost or less than current VS pumps; and the AC drives being a considerably more mature product with a better warranty and more easily fixed/replaced)?

2.The AC Drive literature references the pros and cons of such controllers--such as moving pumps off their best efficiency operating point by changing their RPMs and unexpected electrical issues (harmonics?) and vibrations due to currents and RPMs not in original engineering specs--but generally concludes that the energy savings far outweigh the downside of such issues particularly for low voltage and low HP pumps (i.e. the 115-230 volt, .5 to 3HP pumps found in most pools). Do Pentair, Hayward, Jandy and StaRite VS pumps also face these issues? Have they engineered their pumps in any special way to mitigate/minimize these issues? (thus, perhaps, justifying a higher cost?)

3.Related to the point above about a pumps best efficiency, I believe that a pump's motor is closely matched with the wet-end impeller (correct me if I wrong). As above, the literature mentions that there will be inefficiencies due to an impeller operating at an RPM it wasn't designed for. I couldn't find anything in layman's language about such inefficiencies or if they are show stoppers. Is this also a problem though, for the current crop of pool VS pumps and, for that matter, 2 speed pumps? Are the impellers designed differently? If so, could (should?) a VS or 2 speed impeller be swapped into the wet end of a single speed pump with an AC Drive? Or, are the inefficiencies simply not "big enough" to worry about in relation to energy cost savings?

Thanks in advance for your thoughts and comments.

 

[susa]

look into inverters, I've tested successfully hookup via inverter (Honda equipment) the IntelliFlow VF, tons of different options exist based on your final off-grid solution.

 

[mas985]

#1 Yes but unless the pump motor is an inverter duty motor, the life of the motor will be significantly reduced. Many pump motors are not inverter duty so you may end up replacing the motor in a very short period of time.

#2 Not sure what literature you are referring too but with residential pool pumps that use diffusers, running outside of the BEP is not a huge problem. Plus lower HP pumps (< 2 HP) do not create as much stress on the motor shaft so this is not usually an issue. But again, the motor needs to be inverter duty which Hayward, Pentair and Jandy motors are inverter duty.

#3 Yes a motor and wet end are matched for the HP rating of the motor. But I don't agree with the second statement about RPM. A two speed wet end is identical to a single speed wet end so the impeller design is also identical. Where you lose efficiency is not in the wet end but in the motor. A motor operating at low speed will have about half the efficiency of high speed. This due to the way the motor is wound for two speeds and the losses in windings. However, a VFD changes speeds by changing the line frequency as well as apparent voltage so the efficiency of the motor can be maintained through a wider range of speeds.

I would encourage you to take a good look at two speed pumps. Although they do not save as much energy on low speed as VS, they save enough to make the overall energy savings substantial.

 

[AlanH]

Susa - good to know you can run an Intelliflo from an inverter. Do you know the type/model of inverter and whether the output was pure sine wave (like true AC grid power) or not? Some inverters output power in a "square" or quasi-sine wave that may be ok for light bulbs, TVs, power tools, etc but less so for some electrical/electronic components/circuits such as may be found in the Intelliflo's control circuitry.

Mark - I'd like to follow through on your answers (very helpful, and appreciate the correction in #3, thank you).

#1 AC Drives (VFDs) and inverter duty motors can be bought in matched combos (or mix and match).
#3 Since, "Where you lose efficiency is not in the wet end but in the motor" and "a VFD changes speeds by changing the line frequency as well as apparent voltage so the efficiency of the motor can be maintained through a wider range of speed", could a matched VFD and inverter duty motor attached to an appropriate wet end (used or new) still provide for a more energy efficient pump than current 2 speed pumps on the market? (let's leave cost out of the equation for the moment).

Thanks.

 

[mas985]

#1 True but you will also need an inverter rated cable and possibly an outdoor enclosure. Last time I looked into the total cost of everything, you could get an VS for less cost.

#3 Yes all VFD drives will be more efficient at lower speeds than a two speed pump on low speed but since both use such low power, even if the VS is twice the efficiency, it will still take many years of operation for the VS to become more cost effective than the two speed because of the up front cost difference.

Also, all VFDs use pulse width modulation. This creates a lot of harmonics which have the potential to cause arcing in the windings and/or bearings which is why a inverter duty motor is required. In fact, you could never get the efficiency out of a drive that tried to simulate a true sine wave because of the smoothly varying voltage. With a VFD, the PWM waveform is always at +/- the peak DC voltage (rectified AC input) and the "effective" voltage of the waveform is controlled by the duty cycle. The frequency is controlled by the alternating voltage waveform.