Moving-magnet linear alternator kit (free-piston Stirling power take-off) — dimension sheet
Source: Xem Studio, https://xemstudio.com/objects/linear-alternator-kit/  (CC BY 4.0)
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
Catalog version: 2026-10-20

Sizes: see table below
- 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.

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

Files elsewhere (not hosted by Xem Studio; license as stated by the publisher):
- NASA (NTRS, public domain): Linear alternator design for free-piston Stirling convertors (2005) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20050182934
- NASA (NTRS, public domain): Sunpower/NASA 80 We free-piston Stirling convertor (2002) — PDF — US Government work, public domain — https://ntrs.nasa.gov/search?q=20020047256
- Wikipedia: Linear alternator — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Linear_alternator
- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine
- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/

Free files (CC BY 4.0, Xem Studio; STEP + STL, 7 parts): https://xemstudio.com/files/hardware/linear-alternator-kit/index.json
API: https://xemstudio.com/api/geometry?object=linear-alternator-kit