Permanent Magnet Axial Flux Generator Design
The Magnet Rotors
The magnet rotors are steel plates roughly ¼-½” thick that hold the magnets for the generator, which are either bolted in place or adhered to the plates and encapsulated in epoxy for extra protection from the elements. An efficient axial flux PMG will have two magnet rotors, one on each side of the stator, though some micro turbines used for low demand purposes might only use one magnet rotor. The magnet rotors don't just hold the magnets, but also help carry and direct the magnetic flux (magnetic field lines) through the coils to generate voltage, similar to how the laminated steel core in a radial flux generator works. For this reason, the rotors need to be made from a ferrous metal like steel.
Notice that the magnets are arranged on the steel plate with their poles alternated. When the magnet rotors are assembled with the stator, the magnets on each plate will have alternating poles facing each other, as well. Because opposites attract, this configuration combined with a small air gap (the distance between the plates) will ensure maximum flux density in the stator.
The Magnets
Generally, 50x25x12mm (2”x1”x1/2”) N42 to N52 grade block neodymium magnets are used for residential axial flux turbines, though some builders have used square shaped, round, wedge, etc. Wedge shaped magnets have the best geometry for fitting within a circular rotor, but are quite expensive - usually too expensive to justify their use in a DIY project. Most builders opt for the rectangular magnets because they’re more affordable and have a suitable geometry for making a compact machine. Ceramic magnets can also be used, but they need extra protection from the elements and are much weaker than neodymiums, so they won't produce the same power without being much larger and heavier. With an air core design, the more compact the assembly is to decrease the air gap between the rotors, the better. There's no steel core in the stator to pull flux into the coils, so this arrangement requires a very thin air gap to work properly.
There’s an optimal ratio of magnets to coil count that should be used. For a 3 phase generator, there should be four magnets on each rotor plate for every four coils. So for example, a 9 coil stator should be coupled with two magnet rotors, each containing 12 magnets. This ensures that the right magnets pass the right coils at the right time for the best performance. In a 3 phase generator, there is always two phases energized at the same time (see image of sine wave below), so as a group of magnets pass the first two phases and energize them, there should be no magnets activating the 3rd phase at that time.
There’s an optimal ratio of magnets to coil count that should be used. For a 3 phase generator, there should be four magnets on each rotor plate for every four coils. So for example, a 9 coil stator should be coupled with two magnet rotors, each containing 12 magnets. This ensures that the right magnets pass the right coils at the right time for the best performance. In a 3 phase generator, there is always two phases energized at the same time (see image of sine wave below), so as a group of magnets pass the first two phases and energize them, there should be no magnets activating the 3rd phase at that time.
A single phase generator can also be built, but would be far less efficient, as can be seen in the image above. The sine wave represents how much voltage is being produced at a given time. Notice in the single phase wave how the voltage always drops back down to zero before reversing, while the voltage may drop slightly with the 3 phase generator, but not back to 0. The more phases there are in a generator, the smoother the output frequency is going to be and the more efficient it will be. Most builders choose a 3 phase configuration.
The Stator Coils
The stator coils are what harness the flux from the magnets and turns it into electricity. They’re made by winding enameled copper wire around a mold attached to a winder, then they're connected together and encapsulated into a thin stator using epoxy. The coils can be any shape, but their inner profile should closely match the shape and dimensions of the magnets being used to minimize waste. If the inner profile is smaller than the magnets, then the extra windings will be a waste because they won’t be carrying any flux. If the inner profile is larger than the magnets, then the extra space is wasted potential that could have been filled with copper. Notice in the images below how the 9 coil stator uses square coils that have an inner profile that perfectly matches a 50x25mm magnet, while the 18 coil stator uses somewhat trapezoidal coils. A few of the windings in each coil of the latter won’t be energized, but tapering the coil allows more to be fitted into a tighter space. Generally, a generator with more coils is more efficient than a generator with less, but they have a larger footprint as a result. So at a certain point, it can be advantageous to make a compromise to build them more compact. The lower potential of the tapered coils can be easily compensated for by using stronger magnets to increase the flux density in the stator.
The coils are usually wired in 3 series strings (start wire from one coil connects to the finish wire of the next in that phase, and so on), which are connected in a star (wye) configuration to form a 3 phase AC generator, as shown in the CAD image above. Wiring the coils in this (star) configuration produces high voltage and low amps at low rpm’s, which makes it a favorable choice for low-medium wind environments. The generator will allow the blades to get up to speed and reach charging voltage much easier. The coils can also be wired in a delta configuration to produce low volts and high amps at higher rpm’s, but this is only beneficial for high wind environments that can produce the torque needed, and the transmission cables can be costly because they need to be thicker to handle the extra current. Switching between star and delta and back during operation is possible, but complicates the installation so it's really not beneficial unless the installation site conditions warrant it.
The number of windings in each coil will determine how many volts it produces at a given rpm. As mentioned in earlier chapters, it’s important to design the generator to match the angular momentum of the blades as much as possible for direct battery charging. This can be a bit tricky, because the winding or turn count is going to affect the size of the coil (specifically the coil legs), and the overall size of the generator. Ideally, the coil leg width should be close to half the width of a magnet, and the coils and magnet rotors have to be arranged so that the rotational plane of the center of the magnets lines up directly with the center of the coils. The best way to figure out how the coils are going to fit within the stator is to wind one coil as tight as possible, then layout the stator on a piece of cardboard or wood and divide it into equal sections. Then place the coil at the desired radius from the center of the stator, and trace around the perimeter of the coil with a pencil. Do that at every section to see if you need to adjust the coil geometry or the size of the stator. It would be a good idea to make a template for the magnet rotors as well. Cut out the locations for the magnets, then place the rotor template over the stator template to see how well the magnets and coils line up.
Resistance
Another factor that has a major influence on the generator’s power curve and how well it matches with the blades is the stator resistance, which depends on the gauge and length of wire used to make up each phase, and the resistance of the wire. Think of electrical current in a wire like water flowing through a pipe; the higher the flow rate for a given size of pipe, the higher the erosion rate and the higher the pressure drop at the other end of the pipe will be. Similarly, higher resistance in an undersized wire will result in more energy lost to heat and a higher risk of overheating the circuit and even causing a fire. Lower resistance will result in more power and efficiency and a generator that will survive longer. But there's always a compromise. Lower resistance requires thicker wire, and when you're turning coils by hand for a micro turbine, an extra fraction of a mm per turn adds up really quick. For turbines ranging from 8ft to 14 ft in diameter, most builders use 14-12 awg wire for winding their coils. Turbines for charging low voltage systems (24V or less) will usually have coils with parallel windings, meaning that the coils were wound with two wires at the same time instead of one, and they were connected in parallel at the start and end of each coil. This is an effective way to lower resistance in the stator to handle the higher current that the low voltage systems will demand, without using a thicker wire that would be harder to wind tightly. Parallel 14 awg is a popular substitute for 11 or 12 awg wire, for example, because it's much easier to wind into a tighter space while providing the same ampacity (ampacity is the current carrying capacity of a wire, dictated by its size and resistance).
Below is a link to one of many online wire resistance calculators. Just enter the length and gauge of wire, and it will produce a resistance value. If you’re using 50x25mm (2”x1”) magnets, then you’re going to use around 0.22m (8.5”) of wire per turn. So if each coil in one phase has 50 turns, and there are 3 coils in the phase, then there’s 150 turns per phase. 0.22 x 150 = 33m. If you’re not using those magnets, then you’ll have to wind a coil according to your desired shape to determine how much wire will be in each phase.
https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
Below is a link to one of many online wire resistance calculators. Just enter the length and gauge of wire, and it will produce a resistance value. If you’re using 50x25mm (2”x1”) magnets, then you’re going to use around 0.22m (8.5”) of wire per turn. So if each coil in one phase has 50 turns, and there are 3 coils in the phase, then there’s 150 turns per phase. 0.22 x 150 = 33m. If you’re not using those magnets, then you’ll have to wind a coil according to your desired shape to determine how much wire will be in each phase.
https://www.cirris.com/learning-center/calculators/133-wire-resistance-calculator-table
Below is the equation for calculating voltage output based on the # of coil windings and magnet specs, but I’ve also created an easy to use worksheet for doing instant calculations, which accounts for other factors like resistance, TSR and rotor area in order to calculate generator output at a given wind speed or rpm and compare it with your chosen blade design to make it easier to find an appropriate match between the two. Again, it's absolutely necessary to match your generator's power potential with the angular momentum of your chosen blades, or vice versa. Failure to do this will ultimately result in failure to design and build a practical working turbine.
https://www.resystech.com/the-diy-wind-turbine-design-guide.html
https://www.resystech.com/the-diy-wind-turbine-design-guide.html
VDC = (A*B*n*rpm*2.72/30)-1.4
Where:
VDC = volts DC
A = magnet pole area
B = magnet flux density
n = turns per phase (turns per coil * coils per phase)
RPM = rotational frequency
Because a 3 phase axial flux PMG produces what’s called ‘wild’ AC, it will need to be rectified to DC for battery charging. This is accomplished with a 3 phase full wave bridge rectifier rated for at least 25% more voltage and current than what the turbine can possibly produce. It can be mounted on the turbine to immediately convert the AC power to DC, but for cost efficiency reasons it’s best to mount the rectifier close to the batteries because once the high voltage 3 phase AC is converted to low voltage DC, the current will be increased substantially, which requires much thicker transmission wires to carry it safely and efficiently.
Where:
VDC = volts DC
A = magnet pole area
B = magnet flux density
n = turns per phase (turns per coil * coils per phase)
RPM = rotational frequency
Because a 3 phase axial flux PMG produces what’s called ‘wild’ AC, it will need to be rectified to DC for battery charging. This is accomplished with a 3 phase full wave bridge rectifier rated for at least 25% more voltage and current than what the turbine can possibly produce. It can be mounted on the turbine to immediately convert the AC power to DC, but for cost efficiency reasons it’s best to mount the rectifier close to the batteries because once the high voltage 3 phase AC is converted to low voltage DC, the current will be increased substantially, which requires much thicker transmission wires to carry it safely and efficiently.
