Keep the air gap as small as the stator allows, because the magnetic field across it falls as the gap grows: for two opposed magnets of thickness t the field is about B = Br ÷ (1 + g ÷ 2t), so with 3 mm magnets widening the gap from 4 mm to 8 mm cuts the field, and the voltage with it, by 29%. Thicker magnets suffer less, and copper loss in the stator then decides what share of the shaft power reaches the battery.
Part of Advanced Wind Turbine
The air gap is the space between the facing magnets that holds the stator coils; it sets how strong a field the coils see and so how much voltage they make.
The Gap Dilutes The Field
The carrier says the gap between magnets and coils must be as small as possible, 3 to 5 mm total, that wider gaps dramatically reduce output, and that precision spacers should hold it. A simple model shows how much, and why thin magnets are punished most.
Take two opposed magnets, each of thickness t and remanent field Br, facing across a gap g, with steel behind them. If the steel carries the flux without loss and flux leakage is ignored, the field in the gap is:
B = Br ÷ (1 + g ÷ (2t))
This is a simplified magnetic-circuit estimate (recalled), not a corpus formula. It gives the fraction of Br that survives:
| Gap g | t = 3 mm | t = 6 mm | t = 10 mm |
|---|---|---|---|
| 3 mm | 0.67 | 0.80 | 0.87 |
| 4 mm | 0.60 | 0.75 | 0.83 |
| 5 mm | 0.55 | 0.71 | 0.80 |
| 8 mm | 0.43 | 0.60 | 0.71 |
| 12 mm | 0.33 | 0.50 | 0.63 |
Voltage is proportional to field, so the same fractions apply to it. Widening the gap from 4 mm to 8 mm takes 0.60 to 0.43 for 3 mm magnets, a fall of 29%; for 6 mm magnets 0.75 to 0.60, a fall of 20%; for 10 mm magnets 0.83 to 0.71, a fall of 14%. A real machine loses more than the model, because flux also leaks sideways between neighbouring poles when the gap is large, which the model ignores. The conclusion matches the carrier’s warning: the gap matters most for the thin magnets of the sort cut from hard drives (Magnet Selection (NdFeB vs Ferrite)).
The Stator Sets The Minimum Gap
The coil has thickness, and the gap between facing magnets must hold it plus clearance on each side. Seventy turns of 18 AWG wire (1.02 mm) fill at least 70 × 1.02² = 73 mm² of cross-section; in a coil window 10 mm wide that is 7.3 mm thick. With 1.5 mm of clearance on each face, the total gap is 7.3 + 3 = 10.3 mm. From the table’s model with t = 10 mm and g about 10 mm, the field is 1 ÷ (1 + 10 ÷ 20) = 0.67 of Br; with 3 mm magnets 1 ÷ (1 + 10 ÷ 6) = 0.38.
Read the carrier’s “3 to 5 mm” as the clearance between each magnet and the stator surface if your coils are as thick as this, or as the whole gap only if the coils are very flat. The corpus does not say which. Whichever you build, there is a design trade between more turns (more voltage, thicker coil, wider gap) and a narrower gap (more field, thinner coil). Stator Winding (Coil Design) shows the voltage side.
Efficiency: Where The Shaft Power Goes
A coreless stator has no iron in it, so core loss is negligible (recalled). The main losses are copper heating and the rectifier diodes. With a three-phase bridge delivering a DC current I_dc, each phase carries about 0.816 × I_dc rms, a recalled result for the bridge’s conduction pattern.
Worked example at a phase current of 10 A
Phase current 10 A rms means I_dc ≈ 10 ÷ 0.816 = 12.2 A. At a battery voltage of 14 V the electrical output is 14 × 12.2 = 171 W. The diodes drop about 1.4 V: 1.4 × 12.2 = 17 W.
- 18 AWG phase resistance 0.57 Ω (from the stator page’s assumptions): copper loss 3 × 10² × 0.57 = 172 W. Efficiency = 171 ÷ (171 + 172 + 17) = 48%.
- 14 AWG, 0.23 Ω: loss 3 × 100 × 0.23 = 68 W. Efficiency = 171 ÷ (171 + 68 + 17) = 67%.
These use assumed coil geometry and a single operating point, so they show the shape of the trade, not the efficiency of a particular alternator. Thinner wire wins at low wind and thicker wire wins in strong wind, as the Coil Winding page says. Copper loss also warms the stator; epoxy and the N-grade magnets’ 80 °C limit set the ceiling (Permanent Magnets).
Method: Set And Hold The Gap
Step 1: decide the gap from the stator’s thickness plus clearance. Step 2: make spacers of that exact length from a rod or tube, three or more, and check them with a micrometer or gauge strip. Step 3: assemble with spacers between the stator and each rotor, draw the discs in on the threaded rods (Rotor Disc Construction), and check the clearance all round by sliding a strip of the clearance thickness. Step 4: spin by hand; any rub means the gap is uneven. Step 5: brake or lock the shaft with the bearings in place; the magnets pull the discs toward the stator, so the bearings must hold the end load.
Common Mistakes
- Letting the gap grow for easy assembly — each millimeter costs voltage.
- A stator that is cast thick — it fixes a large minimum gap.
- A warped disc — the gap changes as it turns.
Safety
The discs pull toward each other and the coils
If the spacers fail or the rods loosen, the discs close on the stator and crush it, throwing fragments. Lock the nuts, check them after the first week and each month, and keep fingers out of the gap while it is open.
What’s Next
- Learn next: Tower & Mounting for carrying the finished machine
- Combine with: Permanent Magnet Alternator for the whole machine