How to build a PMA permanent magnet alternator generator for wind turbines
DIY guide to building a Dual Rotor Axial Flux Permanent Magnet Alternator
We have created a series of videos to demonstrate the process:
Step 1: Winding the coils and connecting them together.
Step 2: Casting the coils in resin to make the stator.
Step 3: Building the Rotors
This layout provides a great starting point for designing your own PMA. Click the image to open the PDF file in a new window:
This layout shows the internal coil connections for 3 phase. Click to view:
How many Turns per coil?
The number of turns per coil is a flexible design variable. Higher turn counts produce higher open voltages. Higher turns will also lower the cut-in speed at which the alternator will start charging a battery bank. Higher turns per coil also increases the resistance in the coils which lowers the efficiency, so this becomes a balance of trade-offs. For Drag style VAWT windturbines which have lower rotational speeds such as a Lenz II, a good place to start is with about 70-80 turns per coil. Lift style wings such as our WindRazors will produce much higher rotational speeds, so a good starting point might be about 50-60 turns per coil. HAWT have even higher rotational speeds and might use 40-60 turns. The open output voltage is directly proportional to the number of turns which makes it easy to design an alternator once you have a base line to go by. We've already done this for you. Here is some data on this 9" dual rotor PMA:
With 60 turns per coil, at 150 rpm, the rectified DC voltage is 16V. This is 0.103 volts/rpm. So at 100 rpm, the alternator will produce 100 X .103 = 10.3 Volts. This linear nature has been confirmed with tests.
Likewise, a stator with 70 turns per coil will produce 18VDC at 150rpm. Which is .120 volts/rpm.
Notice the linear relationship between the volts-per-rpm and the number of turns per coil.
60 turns: .103/60= .00171
70 turns: .120/70= .00171
So, if you want to know how many volts/rpm and alternator will make with 95 turns per coil for example. The answer is:
95 x .00171 = .162 volts/rpm. Or 16.2 volts at 100 rpm.
This linear nature of the voltage, speed and number of turns per coil has been observed with tests.
What size wire?
Simple, BIG! You want to use the biggest wire possible!! Bigger wire has less resistance and will produce a more efficient alternator. As current flows through a resistor, heat is generated. More resistance will simply produce more heat. Heat is energy that will never make it to your battery bank and will be lost.
Here's the challenge. We have limited space in the PMA for coils and the number of turns needed to produce the voltage profile desired. So, we have to balance the trade-offs and compromise between stator size, number of turns per coil, and wire size. For this 9" PMA, we find that 15awg wire is a good general size when using 60-80 turns per coil. Here is a performance trick. 15awg can be a single strand of wire, or it might be made from multiple strands which collectively add up to the same cross sectional area. For example, in the tutorial video, you will see us use 2 strands of wire. We are essentially making a 15awg conductor from 2 strands of 18awg. The reason we do this is because we can get more copper packed into the same area if we use multiple strands rather than one large wire. The ultimate goal is to use the largest wire we can to reduce the resistance. Search the web for a chart of awg wire sizes, then do some math to come up with multi strand combinations that result in the same cross section area. Perhaps use 6 strands of 22awg wire....etc.
We sell wire in 10lb spools. This 9" PMA will general need about 3-4 lbs of wire so you can easily make 2 stators will a spool.
How much power will it make?
The power output is determined by the amount of power input less losses.
Power out = Power in - losses
For a wind turbine, the input power is based on the wind speed and the swept area of the blades. We addressed the wind power in more detail on this page:
The complete charging system. Fixed voltage on battery bank state of charge. Current flow, cut-in speed
Power output = power input, less losses. Resistance losses, heat.
Rotor configuration: N-S-N-S poles, steel disk material, dual rotor, Stacking alternators