Wind the stator with three-quarters as many coils as there are magnets on one disc, such as 9 coils for 12 magnets, and join them in three series groups, because that ratio puts neighbouring coils 240 electrical degrees apart and so makes three-phase output. Output voltage scales directly with turns per coil and with speed: 70 turns of 18 AWG wire in the worked example below reach battery-charging voltage at about 281 RPM, and raising it to about 109 turns would bring that down to 180 RPM.
Part of Advanced Wind Turbine
The stator is the fixed disc of copper coils in which the passing magnets induce voltage; its coil count, turns and wiring decide the alternator’s cut-in speed and its output.
Three-Quarters As Many Coils As Magnets Makes Three Phases
The carrier’s rule is coils = 3/4 × the number of magnet poles for a three-phase alternator, so 16 magnets give 12 coils. The Permanent Magnets page pairs 8 magnets with 6 coils, 12 with 9, 16 with 12 and 20 with 15. The rule works because of an angle. In the 12-magnet, 9-coil layout one coil spacing is 360° ÷ 9 = 40° of rotation; with 6 pole pairs, each degree of rotation is 6 electrical degrees, so neighbouring coils are 40 × 6 = 240° apart electrically, which is the same as −120°. Three phases, 120° apart, come out. Coils 1, 4 and 7 are 3 × 240° = 720° apart, which is a whole number of cycles, so they are in phase and are joined in series as Phase A, as in Generator Building. The same arithmetic holds for 8/6 (60° × 4 pairs), 16/12 (30° × 8) and 20/15 (24° × 10).
How Voltage Depends On Turns And Speed
For one coil, the peak voltage is V = N × B × A × ω_e, as the Coil Winding page writes it, with N turns, B the flux density at the coil face (0.3 to 0.5 T on that page), A the effective area the magnet sweeps, and ω_e the electrical angular speed. The electrical frequency is f = pole pairs × RPM ÷ 60.
Worked example: 12 magnets, 9 coils, 70 turns, 200 RPM
The effective area A is not in the corpus; here A = 6 × 10⁻⁴ m² (a 30 × 20 mm magnet face) is assumed to show the method.
- Frequency: 6 × 200 ÷ 60 = 20 Hz; ω_e = 2π × 20 = 125.7 rad/s.
- One coil: 70 × 0.4 T × 6×10⁻⁴ m² × 125.7 = 2.11 V peak.
- One phase of 3 coils in series: 3 × 2.11 = 6.33 V peak.
- Star connection, line to line: √3 × 6.33 = 10.97 V peak.
- After a bridge rectifier, two silicon diodes conduct in series, about 0.7 V each (a recalled value), so DC ≈ 10.97 − 1.4 = 9.6 V.
That is under a 12 V battery’s charging voltage of 14 V or so (Charge Controller gives 14.4-14.8 V for absorption). Voltage rises in proportion to speed, so the cut-in speed is the RPM at which the peak line voltage reaches 14.0 + 1.4 = 15.4 V:
200 × 15.4 ÷ 10.97 = 281 RPM.
The Coil Winding page’s table gives 180-280 RPM for 60-80 turns of 18 AWG, so the example falls at the top of its range. To reach cut-in at 180 RPM, scale the turns by the speed ratio: 70 × 281 ÷ 180 = 109 turns. That needs wire thin enough to fit the same coil space, which raises resistance.
Resistance And Heat Set The Wire Gauge
From the Coil Winding page’s table, 18 AWG has 2.10 Ω per 100 m and 14 AWG has 0.83 Ω per 100 m. If a 70-turn coil takes 0.13 m of wire per turn (an assumed mean turn length), a coil holds 9.1 m, so:
- 18 AWG: 0.19 Ω per coil, 3 × 0.19 = 0.57 Ω per phase.
- 14 AWG: 0.076 Ω per coil, 0.23 Ω per phase.
At a phase current of 10 A the copper loss in all three phases is 3 × 10² × R: 3 × 100 × 0.57 = 172 W with 18 AWG against 3 × 100 × 0.23 = 68 W with 14 AWG. The thin wire gives voltage at low speed and wastes power in strong wind, while thick wire does the opposite, as that page also says. Do not choose by voltage alone.
Method: Build And Check The Stator
Step 1: wind one coil on a jig with the chosen wire and turns (Coil Winding). Tension must be even; crossed wires thicken the coil and widen the gap. Step 2: test it. Spin a magnet rotor past it at a known RPM and read the voltage. Scale N to your target. Step 3: wind all the coils the same way, and number them 1 to 9. Step 4: connect 1-4-7, 2-5-8 and 3-6-9 in series as three phases, joining the three far ends in a star point. Step 5: measure the resistance of each phase. They should match closely; a low one means shorted turns, an infinite one a break. Step 6: spin the rotor and measure each phase voltage. All three should be about equal. A phase noticeably lower than the others usually has one coil connected the wrong way round: swap its leads. Step 7: cast the stator in fiberglass resin or epoxy for rigidity, as the carrier says, to a thickness that keeps the air gap small (Air Gap & Efficiency).
Common Mistakes
- Counting turns by eye — a coil with 10% fewer turns unbalances the phases.
- Winding some coils the opposite way — the phase voltage drops and the machine hums.
- Forgetting the diode drop — the cut-in speed rises.
Safety
Voltage rises with speed and the rotor stays live
An unloaded alternator at 250 RPM gives about 14 V peak from this example, and at three times that speed in a gale, about 41 V peak on an open circuit. Connect the output to the battery and dump load before the rotor turns, and never touch bare coil leads while it turns.
What’s Next
- Learn next: Rotor Disc Construction for the matching magnet discs
- Combine with: Magnet Selection (NdFeB vs Ferrite)