{
 "version": "2026-10-20",
 "license": "Dimension sheets are written by Xem Studio from the cited sources and are free to use (CC BY 4.0, attribute 'Xem Studio'). Linked files belong to their publishers under the license shown; Xem Studio does not host them. 'license_verified: false' means the license shown is from memory or the publisher's page as last read and should be confirmed at the link before commercial use.",
 "slug": "linear-alternator-kit",
 "name": "Moving-magnet linear alternator kit (free-piston Stirling power take-off)",
 "kind": "hardware",
 "category": "Stirling engines",
 "group": "Free-piston",
 "year": null,
 "question": "How does the linear alternator in a free-piston Stirling engine work and what parts does it need?",
 "tags": [
  "linear alternator",
  "free piston",
  "moving magnet",
  "generator",
  "nasa",
  "sunpower",
  "stirling",
  "free piston",
  "long earth society"
 ],
 "aliases": [
  "free piston stirling linear alternator",
  "moving magnet linear alternator design",
  "linear generator coil magnet",
  "stirling convertor alternator",
  "linear alternator 3d print"
 ],
 "overall_mm": {
  "length": null,
  "width": null,
  "height": null
 },
 "official_cad": false,
 "xem_status": "free-files",
 "les": true,
 "free_files": 7,
 "url": "https://xemstudio.com/objects/linear-alternator-kit/",
 "sheet_txt": "https://xemstudio.com/objects/linear-alternator-kit/sheet.txt",
 "sheet": {
  "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",
  "overall_mm": {
   "length": null,
   "width": null,
   "height": null
  },
  "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."
  ],
  "table": {
   "columns": [
    "part",
    "size_mm",
    "note"
   ],
   "rows": [
    [
     "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"
    ]
   ]
  }
 },
 "sheet_text": "Moving-magnet linear alternator kit (free-piston Stirling power take-off) — dimension sheet\nSource: Xem Studio, https://xemstudio.com/objects/linear-alternator-kit/  (CC BY 4.0)\nBasis: 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\nCatalog version: 2026-10-20\n\nSizes: see table below\n- 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).\n- 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.\n- 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.\n- 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.\n\npart  size_mm  note\nmagnet carrier Ø40  ID 38, OD 47, 20 long  12 × Ø10×3 radial\nmagnet carrier Ø30  ID 28, OD 37  8 magnets\ncoil annulus  ID 50 / OD 70 / 12  300 t × 0.4 mm\ncoil annulus small  ID 40 / OD 58 / 12  for Ø30\nback iron  ID 72 / OD 84 / 30  steel ring\npiston rod  Ø5 × 120  E-clip grooves\n\nFiles elsewhere (not hosted by Xem Studio; license as stated by the publisher):\n- 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\n- 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\n- Wikipedia: Linear alternator — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Linear_alternator\n- Wikipedia: Free-piston engine — web — CC BY-SA 4.0 (text) — https://en.wikipedia.org/wiki/Free-piston_engine\n- Long Earth Society: Engine builds — web, video — LES site — https://longearthsociety.com/engineering/\n\nFree files (CC BY 4.0, Xem Studio; STEP + STL, 7 parts): https://xemstudio.com/files/hardware/linear-alternator-kit/index.json\nAPI: https://xemstudio.com/api/geometry?object=linear-alternator-kit\n",
 "sources": [
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Linear alternator design for free-piston Stirling convertors (2005)",
   "url": "https://ntrs.nasa.gov/search?q=20050182934",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "NASA (NTRS, public domain)",
   "title": "Sunpower/NASA 80 We free-piston Stirling convertor (2002)",
   "url": "https://ntrs.nasa.gov/search?q=20020047256",
   "formats": [
    "PDF"
   ],
   "license": "US Government work, public domain",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Linear alternator",
   "url": "https://en.wikipedia.org/wiki/Linear_alternator",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Wikipedia",
   "title": "Free-piston engine",
   "url": "https://en.wikipedia.org/wiki/Free-piston_engine",
   "formats": [
    "web"
   ],
   "license": "CC BY-SA 4.0 (text)",
   "license_verified": true,
   "hosted": false
  },
  {
   "publisher": "Long Earth Society",
   "title": "Engine builds",
   "url": "https://longearthsociety.com/engineering/",
   "formats": [
    "web",
    "video"
   ],
   "license": "LES site",
   "license_verified": true,
   "hosted": false
  }
 ],
 "xem": {
  "status": "free-files",
  "sheet_free": true,
  "mesh_price_usd": null,
  "step_price_usd": null,
  "tolerance_mm": null
 },
 "related": [
  "free-piston-core",
  "spiral-flexure-bearings",
  "neodymium-disc-magnets",
  "axial-flux-generator-kit",
  "shaft-collars-clips-and-shafts",
  "nasa-fpse-references"
 ],
 "faq": [
  {
   "q": "How does a free-piston Stirling generate electricity?",
   "a": "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."
  },
  {
   "q": "What happens if a free-piston engine has no load?",
   "a": "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."
  },
  {
   "q": "How much power from a small linear alternator?",
   "a": "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."
  }
 ],
 "files_license": null,
 "offers": [
  {
   "file": "magnet-carrier-40-12x10x3",
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   "license": "CC BY 4.0 (Xem Studio)",
   "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",
   "note": "moving-magnet carrier: Ø40 bore sleeve, 12 radially-magnetized Ø10 × 3 discs in pockets around the OD (alternate poles), 20 long"
  },
  {
   "file": "magnet-carrier-30-8x10x3",
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   "license": "CC BY 4.0 (Xem Studio)",
   "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",
   "note": "moving-magnet carrier Ø30 with 8 magnets"
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  {
   "file": "coil-annulus-50-70-12",
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   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-50-70-12.stl",
   "note": "stationary coil bobbin: ID 50, OD 70, 12 wide; ~300 turns 0.4 mm wire"
  },
  {
   "file": "coil-annulus-40-58-12",
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   "license": "CC BY 4.0 (Xem Studio)",
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   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/coil-annulus-40-58-12.stl",
   "note": "coil bobbin ID 40, OD 58, for the Ø30 carrier"
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  {
   "file": "back-iron-ring-72-84",
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   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-72-84.stl",
   "note": "back-iron envelope (laminated or mild-steel ring) ID 72, OD 84, 30 long"
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  {
   "file": "back-iron-ring-60-72",
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   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/back-iron-ring-60-72.stl",
   "note": "back-iron envelope ID 60, OD 72"
  },
  {
   "file": "piston-rod-5x120-grooved",
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   ],
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   "stl": "https://xemstudio.com/files/hardware/linear-alternator-kit/piston-rod-5x120-grooved.stl",
   "note": "Ø5 × 120 piston rod with two E-clip grooves for the flexure hubs"
  }
 ],
 "markdown": "https://xemstudio.com/objects/linear-alternator-kit/index.md"
}