A permanent magnet alternator makes electricity with magnets on two steel discs sweeping past copper coils on a stator between them, with no brushes and no field current, and its voltage rises in direct proportion to speed. For a direct-drive wind turbine use the axial-flux layout with 12 magnets per disc and 9 coils, which gives usable output at 150 to 250 RPM, and rectify its three-phase output to charge a battery.
Part of Advanced Wind Turbine
A permanent magnet alternator (PMA) converts shaft rotation into alternating current using fixed magnets as the field, which suits a slow, direct-driven wind rotor.
The Magnets Are The Field, So Voltage Follows Speed
An axial-flux PMA, as the carrier describes it, has two steel disc rotors with magnets on their inner faces, and one stator disc between them wound with copper coils. As the magnets pass the coils, the changing flux induces an alternating voltage by Faraday’s law (Electromagnetic Induction). The Generator Building page notes that sandwiching the stator between two rotors roughly doubles the flux through each coil compared with a single-sided design.
Because there is no field winding to adjust, the voltage is proportional to the rotation speed. Halve the RPM and the voltage halves. The frequency follows too: f = (number of poles ÷ 2) × RPM ÷ 60. A 12-magnet rotor at 250 RPM gives 6 × 250 ÷ 60 = 25 Hz; at 150 RPM it gives 15 Hz. That variable frequency is why the output is rectified to DC and fed to a battery, not to ordinary AC loads.
| Part | Role | Detail |
|---|---|---|
| Rotor discs (2) | carry the magnets and complete the magnetic path | steel plate Rotor Disc Construction |
| Magnets | the field | Magnet Selection (NdFeB vs Ferrite) |
| Stator | holds the coils in epoxy or fiberglass | Stator Winding (Coil Design) |
| Air gap | the clearance between magnets and coils | Air Gap & Efficiency |
| Rectifier | turns three-phase AC to DC | six diodes for three-phase |
Matching The Alternator To The Rotor
Direct drive means the alternator turns at the rotor’s speed. The Generator Building page quotes cut-in speeds of 150-250 RPM for the 12-magnet, 9-coil layout. For the 2.4 m rotor in Blade Length, Chord, & Twist at a tip speed ratio of 6, the wind needed for a given RPM is wind = RPM × π × D ÷ (60 × 6):
- 150 RPM: 150 × π × 2.41 ÷ 360 = 3.2 m/s.
- 250 RPM: 250 × π × 2.41 ÷ 360 = 5.3 m/s.
So a generator that only reaches battery voltage at 250 RPM leaves a turbine idle below about 5 m/s. Cut-in decides how often the turbine charges the batteries (Average Wind Speed & Power Curve). The remedy is more turns per coil or more poles, not larger blades.
Wind PMAs follow the rotor; hydro generators follow the water
A hydro alternator can be geared to the speed it likes and held steady by a dump load. A wind alternator is tied to a rotor whose speed changes with every gust. That is why the wind alternator is wound for a low cut-in and protected from overspeed instead of being regulated to a constant speed (Generator & Electrical).
Method: Build Order
- Decide the rotor speed from the blades.
- Choose magnets, then the pole count (12 magnets per disc, 9 coils per the Generator Building page).
- Wind and test one coil, then all of them (Coil Winding).
- Build the rotors and the stator, then assemble with spacers to set the gap.
- Spin it by hand or with a drill and measure the open-circuit voltage at a known RPM.
Common Mistakes
- Winding for 12 V at a speed the rotor never reaches.
- A rectifier without an overspeed plan — an unloaded PMA overspeeds and its voltage rises with speed.
- Ignoring the pull between rotors — see the rotor article.
Safety
Strong magnets, and voltage that rises with speed
Neodymium magnets on opposing discs pull together with enough force to crush fingers; assemble with spacers and threaded rods, never by hand. An unloaded alternator in a gale produces a voltage far above its battery voltage, so wire it to the battery and dump load before the rotor turns (Dump Load & Overspeed Protection).
What’s Next
- Learn next: Magnet Selection (NdFeB vs Ferrite) to choose the field
- Combine with: Permanent Magnets for sourcing and handling