Hi everyone. Great forum you've got here with some very knowledgeable people. I've got some questions about building a miniature PMA for use with a small hydro turbine.
I've got a turbine prototype built, and have tested it under normal operating conditions. I attached weighted discs to simulate the load that a PMA rotor would put on it. First question: how much extra load/drag will be induced with a real PMA vs. just spinning weights on a disc? Is there additional drag from the magnets' field cutting through the coils? With a ~50g disc the shaft speed is about 200rpm, and a ~80g disc slows it down to about 160rpm. (I know the rotational inertia is what's important, but I'm simplifying and using mass. The radius of both discs is similar, and will be close to the PMA dimensions).
My main questions are about the PMA. There is a lot of info out there for wind turbines and other large applications, but not much on building very small axial flux PMAs (I've seen a couple small radial flux).
This hydro system uses very little water, and very little head. Therefore very little power will be produced, but that's ok because its job is to power a small car (probably will use a pager motor…). The system needs to be as efficient as possible because the water is limited. Since the car will be using a DC motor, the 3 phase AC needs to be rectified. Assume I'm using very efficient Shottkey diodes, I'm still going to have about .9V drop, right? Therefore I want as much AC voltage out of the PMA as possible so the % power loss is less.
In order to make such a small PMA produce a high voltage, this means a LOT of turns in each coil. My plan is to use 1/2x1/8†N52 neo magnets, and either an 8 magnet/6 coil config, or a 12 magnet/9coil. I created a spreadsheet with different configurations. I think the optimal setup will be 12 magnet rotors (~80g total) and 9 coils. My calculations show ~450 turns/coil to produce 9V, and ~600 turns/coil to produce 12V. Since the power draw of the pager motor will be very minimal (<500mA at 9V), I can use pretty fine wire in the stator. I found the ideal wire gauge (for compact coils with low resistance) to be 30AWG which gives a coil resistance of ~7ohm for 450 turns, and ~10ohm for 600 turns. Am I on the right track here?
Basically, am I right in trying to maximize the voltage to minimize losses across the rectifier? Power is not the main concern here, efficiency is. Will that many turns be too much resistance in the stator? Am I going to see a major decrease in shaft speed with a real PMA vs a dummy weight? Thanks in advance!
