# Moving-magnet linear alternator kit (free-piston Stirling power take-off)

> How does the linear alternator in a free-piston Stirling engine work and what parts does it need?

- Category: Stirling engines › Free-piston
- Kind: Hardware
- Basis: Moving-magnet linear alternator as in Sunpower / NASA convertors: magnet ring on the power piston oscillates through a stationary coil inside a back-iron; sizes here suit the Ø40 and Ø30 free-piston cores and the library's Ø10 × 3 magnets
- Page: https://xemstudio.com/objects/linear-alternator-kit/ · JSON: https://xemstudio.com/api/geometry?object=linear-alternator-kit · Plain text: https://xemstudio.com/objects/linear-alternator-kit/sheet.txt
- License: dimension sheet CC BY 4.0 (Xem Studio); linked files under their publishers' licenses

## Notes

- Layout: the power piston carries a ring of magnets (poles alternating around the ring, all facing radially out); a stationary annular coil sits in a 1–1.5 mm gap outside it; a soft-iron ring (back iron) outside the coil closes the flux. Piston stroke S moves the magnet ring through the coil, inducing V ≈ N·B·l·v: turns × flux density × wire length in the field × piston velocity (v_peak = π·f·S).
- Numbers for the Ø40 kit: 12 × Ø10 × 3 N42 magnets, B ≈ 0.3 T in a 1.5 mm gap, 300 turns of 0.4 mm wire (≈ 4 Ω), stroke 10 mm at 15 Hz → v_peak 0.47 m/s → roughly 5–8 V peak open circuit and a watt or two into a matched load. Real convertors use laminated iron, Halbach-style magnet arrays and 50–100 Hz; this kit shows the principle and lights LEDs.
- The alternator is also the engine's load control: a free-piston engine without a load over-strokes and hits the stops, so the coil must always see a load (or a tuning capacitor + resistor). Phase the load for resonance: C = 1 / ((2πf)² L_coil) brings the current in phase with velocity.
- Parts: magnet carrier (print, PETG), coil bobbin (print, wind by hand), back-iron ring (mild-steel tube or a stack of washers; printed envelope here is for fit), Ø5 piston rod with E-clip grooves for the flexure hubs.

## Sizes

| part | size_mm | note |
|---|---|---|
| magnet carrier Ø40 | ID 38, OD 47, 20 long | 12 × Ø10×3 radial |
| magnet carrier Ø30 | ID 28, OD 37 | 8 magnets |
| coil annulus | ID 50 / OD 70 / 12 | 300 t × 0.4 mm |
| coil annulus small | ID 40 / OD 58 / 12 | for Ø30 |
| back iron | ID 72 / OD 84 / 30 | steel ring |
| piston rod | Ø5 × 120 | E-clip grooves |

## Where the files are

- NASA (NTRS, public domain): [Linear alternator design for free-piston Stirling convertors (2005)](https://ntrs.nasa.gov/search?q=20050182934) — PDF — US Government work, public domain
- NASA (NTRS, public domain): [Sunpower/NASA 80 We free-piston Stirling convertor (2002)](https://ntrs.nasa.gov/search?q=20020047256) — PDF — US Government work, public domain
- Wikipedia: [Linear alternator](https://en.wikipedia.org/wiki/Linear_alternator) — web — CC BY-SA 4.0 (text)
- Wikipedia: [Free-piston engine](https://en.wikipedia.org/wiki/Free-piston_engine) — web — CC BY-SA 4.0 (text)
- Long Earth Society: [Engine builds](https://longearthsociety.com/engineering/) — web, video — LES site

## Free STEP/STL (7 parts, CC BY 4.0, Xem Studio)

Index: https://xemstudio.com/files/hardware/linear-alternator-kit/index.json

- magnet-carrier-40-12x10x3: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/magnet-carrier-40-12x10x3.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/magnet-carrier-40-12x10x3.stl) — moving-magnet carrier: Ø40 bore sleeve, 12 radially-magnetized Ø10 × 3 discs in pockets around the OD (alternate poles), 20 long
- magnet-carrier-30-8x10x3: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/magnet-carrier-30-8x10x3.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/magnet-carrier-30-8x10x3.stl) — moving-magnet carrier Ø30 with 8 magnets
- coil-annulus-50-70-12: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-50-70-12.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-50-70-12.stl) — stationary coil bobbin: ID 50, OD 70, 12 wide; ~300 turns 0.4 mm wire
- coil-annulus-40-58-12: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-40-58-12.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-40-58-12.stl) — coil bobbin ID 40, OD 58, for the Ø30 carrier
- back-iron-ring-72-84: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-72-84.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-72-84.stl) — back-iron envelope (laminated or mild-steel ring) ID 72, OD 84, 30 long
- back-iron-ring-60-72: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-60-72.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-60-72.stl) — back-iron envelope ID 60, OD 72
- piston-rod-5x120-grooved: [STEP](https://xemstudio.com/files/hardware/linear-alternator-kit/piston-rod-5x120-grooved.step) · [STL](https://xemstudio.com/files/hardware/linear-alternator-kit/piston-rod-5x120-grooved.stl) — Ø5 × 120 piston rod with two E-clip grooves for the flexure hubs

## Questions

**How does a free-piston Stirling generate electricity?**

The power piston carries magnets that oscillate through a stationary coil: V = N·B·l·v. No crank, no rotation; the engine's resonance sets the frequency and the alternator's load sets the stroke.

**What happens if a free-piston engine has no load?**

The stroke grows until the piston hits the stops. Always connect the coil to a load or a tuned capacitor-resistor; the alternator is the throttle.

**How much power from a small linear alternator?**

The Ø40 kit at 10 mm stroke and 15 Hz with 12 N42 magnets gives roughly 1–2 W; power scales with stroke², frequency², flux and turns. NASA's 80 W convertors run at 80–100 Hz with laminated iron.

## Fits with

- [Free-piston Stirling core: displacer, power piston, liner, gas spring, stops, clearance-seal and tuning tables](https://xemstudio.com/objects/free-piston-core/)
- [Planar spiral flexure bearings (Sunpower / NASA free-piston style): Ø40–120, DXF + STEP](https://xemstudio.com/objects/spiral-flexure-bearings/)
- [Neodymium disc magnets, common sizes](https://xemstudio.com/objects/neodymium-disc-magnets/)
- [Axial-flux alternator kit: magnet rotor plates, 3-phase stator, coil bobbins, hall RPM pickup](https://xemstudio.com/objects/axial-flux-generator-kit/)
- [Shafts, D-flats, set-screw collars, E-clips (DIN 6799) and circlips (DIN 471)](https://xemstudio.com/objects/shaft-collars-clips-and-shafts/)
- [NASA and Sunpower free-piston Stirling references (public domain) and design numbers](https://xemstudio.com/objects/nasa-fpse-references/)