If I may, allow me to chime in on a few points here.
I don't know a lot about pool pump motors, because I just had my first pool built earlier this year. However, I did spend about 2 years each working as an applications engineer for a variable frequency drive (VFD, aka inverters) manufacturer and a servo motor/controller manufacturer.
For the Intelliflo, I haven't quite figured out how the work the multiple speeds but I have a theory. I don't think they use frequency adjustments since this would be very inefficient. So the only other thing they could do is use a large winding with multiple taps to create fractional poles. By exciting each set of taps in different combinations, I believe that they are able to get many different RPM settings so even though the display may allow you set an RPM, they probably just choose one that is close enough.
The Intelliflo motor is controlled by the use of a frequency controller. VFD's basically work by modulating the AC frequency being fed to the motor. Low end VFD's are commonly called voltz/hertz drives because the ratio between the voltage and frequency being sent to the motor is relatively constant. VFD's are almost always used with AC induction motors (typically 3-phase) because this is an inexpensive and relative robust solution.
For higher performance applications (think industrial robots and "pick-and-place" positioning), the technology of choice is brushless DC (BLDC) servo control. BLDC can also be accurately called brushless AC, and the terms are often interchangeable. Basically BLDC/BLAC combines a permanent magnet motor (usually associated with DC motors) with variable frequency control. There is no commutator (hence the use of the term "brushless") but the power being sent to the motor is generally AC. The difference between a BLDC controller versus a VFD is that BLDC controllers have the ability to vary the voltage independently of frequency. So, you could conceivably give the motor full voltage and current even at zero frequency. In fact, this is done very commonly when you want the motor to hold a position or provide lots of torque at very low speeds.
BLDC drives are actually very efficient because they only push enough current through the motor to achieve the desired speed or position. It is very common for BLDC motor to run at less than 10% of their rated current with very little loss of efficiency. In fact, lightly loaded BLDC motors may even be slightly more efficient than fully loaded motors because winding resistance increases with temperature. The lower the winding resistance, the less waste heat you create. The efficiency of the controller itself is is typically about 85-90% regardless of the technology, because the power electronics of a BLDC controller is very similar to a VFD. BLDC controllers cost a lot more because there's more intelligence built into them.
Although Pentair calls the Intelliflo a permanent magnet synchronous motor, I'm pretty sure that it is a brushless dc motor. I think it's because PM synchronous sounds sexier, and most people don't have a clue what BLDC means. It's accurate enough, but perhaps a little ambiguous. Another reason may be that most BLDC motor/drives have some type of high-accuracy feedback device. The feedback is required for the controller to know exactly what the shaft position is at any instant, and for calculating instantaneous speed. The feedback loop is what makes BLDC motor a "servo" motor. For obvious reasons, there's no need to know the exact shaft position of a pump motor. So the Intelliflo is basically a BLDC motor that is NOT controlled by a servo loop. Calling it a BLDC motor might be considered misleading because almost all BLDC motors used in industrial applications are servo controlled. If servo control is not required, AC induction motors and VFD's are usually sufficient (and 50 to 90% cheaper), especially for bigger motors. The trickiest part about spec'ing a BLDC system is to properly size the components. There's generally nothing wrong with oversizing the system, but the price difference between a properly sized system and an unnecessarily over-sized system can easily run into the thousands of dollars.
I was wondering why a person couldn't buy and install a motor controller to dial in the rpm and subsequently GPM flow for their pump motor, in effect making a multi speed pump capable of manually matching what the intelli flow style pumps do, for a greatly reduced cost?
In theory, it should be this simple. However, I think most pool pumps are single phase AC and most VFD's are 3-phase output (even though many will work with single phase input). There are some that will work with single phase motors, but I'm not familiar enough with the current crop of products point you to any particular company. Of course, you could retrofit a 3-phase motor to your existing pump's wet end, but you may have trouble finding the correct mounting and shaft configuration. Since the motor mounting specs are not typically published (except maybe as part of the service manual) good luck finding an off-the-shelf 3-phase replacement motor.
Another factor is that pool pump motors probably aren't "inverter-rated". Basically, "inverter-rated" means that the motor windings have better insulation than non-inverter-rated motors. VFD's work by switching a DC voltage on and off very quickly. Using a technique called pulse width modulation, the resultant waveform looks reasonably close to AC. However the rapid switching also causes rapid changes in voltage (often called dv/dt). High dv/dt values coupled with poor insulation resistance in the motor is a recipe for insulation breakdown and premature motor failure. While most modern VFD's work fine with standard motors, lower-cost VFD's may have inferior filtering and waveform control. Most industrial AC motors nowadays are inverter-rated, except for the lowest cost commodity models. I'm guessing that pump manufacturers are probably using the cheapest motors they can get.
And if you make it past those hurdles, be prepared to read some schematics and do some electrical wiring. Even the most basic drives will probably require you to hook up some type of start/stop circuit and possibly an "inhibit" circuit. Plus, you'll have to program the drive. There can be literally dozens of parameters that need to be entered to optimize a drive for a particular motor/application.
I am reasonably sure that pool pump motors are synchronous motors, which means the controller won't work. The fancy continuously variable speed pool motors are also synchronous but they vary the AC frequency, something I don't believe any $60 controller can do.
Generally AC motors are induction motors, which almost by definition are non-synchronous. A $60 frequency controller is not out of the question. Automationdirect.com sells a 1/4 hp VFD for $99, quantity 1. The dirty little secret of the drive industry is that there isn't much of a difference, cost-wise, between a 1/4 hp VFD and a 2 hp VFD. The guts are probably identical. They're priced differently because that's what the market will bear (although I don't know how any company makes money building VFD's these days).
As for horsepower input vs output, the horsepower load on a motor is effectively the pumping horse power which is found from this:
Pumping Horse Power (PHP) = Head * GPM / 3960
This peaks at the best efficiency point and is usually where you get the best GPM / KW as well. The input power has a somewhat linear relationship with GPM but a fairly high intercept so the more flow you have, the more power the pump draws. So for either side the best efficiency point, the efficiency of the pump drops off and thermal losses go up. So a pump with full head or no head will generally run hot. A pump at dead head will still draw about 40% of the peak power but is doing no useful work, PHP = 0, so all of the energy is released as heat. A pump itself has head loss so even a open ended pump will run with a little bit of head loss.
A pump run without load at all (i.e. no water running through it) will be very inefficient so it is not surprising that you would only see 50% power draw. Probably most of the energy is going into heating of the windings and bearings. And the power factor would be very low due to the lack of load.
You know a lot more about pumps than I do, so I'll have to tread lightly here. We have to be careful when we talk about efficiency. It is possible for a process to be very inefficient, but still consume very little power. As work goes to zero, so does efficiency, regardless of how much or how little power is consumed.
A dead-headed pump is obviously extremely inefficient, but I don't know if it draws a significant fraction of it's load rating. If you're getting a 50% reading with an ammeter, then its possible that the ammeter is measuring the magnetizing current in the motor. The magnetizing current tends to flow back and forth. It doesn't do any work and it consequently doesn't consume any energy per se, but it does contribute to motor heating and it has a negative affect on power factor.
I'm not sure, but it seems that typical pump curves are drawn for a motor running at a fixed speed. For that speed, there is a combination of flow rate and head loss that is considered optimal. Moving away from that sweet spot results in lower efficiency. It seems to me, though, that a given motor operating at lower speeds might have an entirely different sweet spot. To best describe efficiency, variable speed pump manufacturers should publish curves that correlate wattage with respect to flow. The problem with Pentair's published curves (see Intelliflo manual, page 47) is that it's almost impossible to know what your total head is at any given speed.
Ikeric provides a set of charts that can show, within reason, the flow at a given RPM and the corresponding energy use of the pump. I am not an electrician so I don't know much about the converter, but it does work. And the pump motor is defintely 3 phase (says so right on the pump in big, bright letters).
Could you provide a link to this? I'd be interested to see it.
I hope I haven't bored or offended anybody. I just wanted to get some facts out there. Motor control is a somewhat tricky subject, and the pump manufacturers are not particularly forthcoming with their details.
