# Axial-flux alternator kit: magnet rotor plates, 3-phase stator, coil bobbins, hall RPM pickup

> How do I build a small axial flux generator with 3D printed parts and neodymium magnets?

- Category: Stirling engines › Generators & instruments
- Kind: Hardware
- Basis: Classic coreless axial-flux layout: 12 magnets / 9 coils (3-phase) on Ø90, or 8 / 6 on Ø70; magnets Ø10 × 3 N42 from the library; sized to the Ø8 flywheel shaft and 608 bearings
- Page: https://xemstudio.com/objects/axial-flux-generator-kit/ · JSON: https://xemstudio.com/api/geometry?object=axial-flux-generator-kit · Plain text: https://xemstudio.com/objects/axial-flux-generator-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Rotor: Ø90 plate with 12 Ø10 × 3 magnets on a 72 mm circle, poles alternating N-S around the ring; a second rotor opposite (N facing S across the gap) doubles the flux. Gap rotor-to-rotor 8 mm with a 4 mm stator between. Steel backing plates (1 mm) behind the magnets add ~40% flux.
- Stator: 9 coils on the same 72 mm circle, wired 3-phase (coils 1-4-7, 2-5-8, 3-6-9 in series), rectified with a 3-phase bridge. ~150 turns of 0.3 mm enamelled wire per bobbin ≈ 1.5 Ω; at 600 rpm on the 12-pole rotor expect roughly 4–6 V open circuit per phase and a watt or two into a load: enough for the fan and LEDs, the demonstrator's purpose.
- Frequency = rpm × poles / 120 = 600 × 12 / 120 = 60 Hz. Volts scale with rpm and turns; current with wire gauge; power with magnet count and area. Hall sensor (A3144 or SS49E in the printed mount) facing a rotor magnet gives one pulse per magnet: rpm = pulses × 60 / 12.
- LES: the flywheel alternator on the generator builds; the chain/belt to a NEMA 17 used as a generator is the lazy alternative (the NEMA 17 entry covers it).

## Sizes

| part | size | magnets_or_coils | shaft |
|---|---|---|---|
| rotor 90 | Ø90 × 5 + hub | 12 × Ø10×3 | Ø8 |
| rotor 70 | Ø70 × 5 + hub | 8 × Ø10×3 | Ø5 |
| stator 90 | Ø90 × 4 | 9 coils | clears Ø14 |
| stator 70 | Ø70 × 4 | 6 coils | clears Ø11 |
| bobbin | Ø10 × 8, Ø13 flanges | 150 t × 0.3 mm | — |
| hall mount | 14 × 10 × 20 | TO-92 | M3 |

## Where the files are

- Wikipedia: [Axial flux motor](https://en.wikipedia.org/wiki/Axial_flux_motor) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Alternator](https://en.wikipedia.org/wiki/Alternator) — web — CC BY-SA 4.0 (text)
- Hugh Piggott: [A Wind Turbine Recipe Book (axial-flux alternator design)](http://scoraigwind.co.uk/) — web, PDF — Author's site (verify at link)
- Long Earth Society: [Engine and rig builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (6 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/axial-flux-generator-kit/index.json

- rotor-90-12x-10x3-b8: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/rotor-90-12x-10x3-b8.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/rotor-90-12x-10x3-b8.stl) — rotor plate Ø90 × 5: 12 pockets for Ø10 × 3 magnets (alternating poles), Ø8 bore hub with set screw (print two for a dual-rotor)
- rotor-70-8x-10x3-b5: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/rotor-70-8x-10x3-b5.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/rotor-70-8x-10x3-b5.stl) — rotor plate Ø70: 8 magnets Ø10 × 3, Ø5 bore
- stator-90-9coil: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/stator-90-9coil.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/stator-90-9coil.stl) — stator plate Ø90 × 4: 9 coil pockets (3-phase, 12 pole / 9 slot), 3 mounting ears
- stator-70-6coil: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/stator-70-6coil.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/stator-70-6coil.stl) — stator plate Ø70: 6 coils (8 pole / 6 slot)
- coil-bobbin-10x8: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/coil-bobbin-10x8.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/coil-bobbin-10x8.stl) — coil bobbin Ø10 core × 8 long, Ø13 flanges, wire slot; ~150 turns of 0.3 mm wire
- hall-sensor-mount-to92: [STEP](https://xemstudio.com/files/hardware/axial-flux-generator-kit/hall-sensor-mount-to92.step) · [STL](https://xemstudio.com/files/hardware/axial-flux-generator-kit/hall-sensor-mount-to92.stl) — TO-92 hall sensor (A3144 / SS49E) mount: 4.2 × 1.6 slot, M3 foot; faces a rotor magnet for RPM

## Questions

**How many magnets and coils for a small axial flux generator?**

12 magnets and 9 coils (3-phase) is the standard small layout; 8 and 6 for a 70 mm rotor. Poles must alternate N-S around the rotor; coils = 3/4 of poles for 3-phase.

**How much power from a 90 mm axial flux generator?**

With 12 Ø10×3 N42 magnets, dual rotors and 150-turn coils at 600 rpm, roughly 1–3 W; it scales with rpm², magnet area and turns. Enough to light LEDs and run a 40 mm fan from a Stirling engine.

**What gap between rotor and stator?**

As small as the stator thickness and wobble allow: 4 mm stator with 1.5–2 mm air gaps each side (8 mm rotor to rotor). Every mm of gap costs flux.

## Fits with

- [Neodymium disc magnets, common sizes](https://xemstudio.com/objects/neodymium-disc-magnets/)
- [608 ball bearing](https://xemstudio.com/objects/608-bearing/)
- [Crank discs, connecting rods and flywheels for model engines](https://xemstudio.com/objects/crank-conrod-flywheel-kit/)
- [Roller chain sprockets: #25 (6.35 mm) and #35 (9.525 mm), 10–40 T](https://xemstudio.com/objects/roller-chain-sprockets/)
- [NEMA 17 stepper motor](https://xemstudio.com/objects/nema-17-stepper/)
- [Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471)](https://xemstudio.com/objects/shaft-collars-clips-and-shafts/)
- [Instrumentation: DS18B20 and K-type probes, hall sensor, encoder disc, dial-indicator stems](https://xemstudio.com/objects/instrument-mounts/)