Axial-flux generator (wind turbine): voltage and cut-in speed
For wind turbine builders: magnets sit on two steel discs, the coils cast in resin between them. The tool estimates the voltage per coil and phase, the DC voltage at the rectifier and the speed above which a battery is charged. The estimate can be calibrated with your own measured value.
Free, no account needed
Abbreviations and symbols on this page (6)
- Axial-flux generator
- Generator in which the magnetic flux runs parallel to the shaft through flat coils: magnets on two steel discs, the coils cast in resin between them (design after Hugh Piggott).
- rpm
- Revolutions per minute.
- Br
- Remanence: flux density a permanent magnet retains in the material. Outside, at the pole face, the field is lower.
- Cut-in speed
- Speed above which the rectified voltage exceeds the battery voltage and charging current flows.
- U, V, W; U1, U2
- Terminal marking according to IEC 60034-8: U, V, W are the three phases, 1 = start, 2 = end.
- Bridge rectifier
- Six diodes turn three AC voltages into one DC voltage; about 2 × 0.7 V is lost per current path.
| Flux density in the gap (estimate) | 0.343 T |
| Frequency | 40 Hz |
| Voltage per coil (peak) | 9.26 V |
| Line voltage at 400 rpm | 34 V |
| DC voltage (open circuit) | 44.6 V |
| Charging starts at 12 V | 117 rpm |
| Resistance per phase | 1.626 Ω |
Formula
Flux density at the gap centre: two opposing block magnets (formula supermagnete.de) B ≈ 2 · B_block(gap ÷ 2). Speed v = 2π r · rpm ÷ 60. Voltage per coil (peak) ≈ 2 N B l v (two active sides). Coils of one phase in series; star: U_line = √3 · U_phase. DC voltage ≈ 3/π · U_line,peak − 2 × 0.7 V. Frequency f = rpm ÷ 60 · magnets ÷ 2.
Example
12 magnets 46 × 23 × 10 mm (Br 1.2 T) per disc, 9 coils with 70 turns, gap 16 mm, radius 100 mm, star: at 400 rpm about 34 V between two terminals, charging into 12 V starts at about 117 rpm. Rough estimate, calibrate with a measurement.
Notes
- The estimate neglects leakage flux and the steel discs; deviations of ±50 % are possible. Enter a measurement (open-circuit voltage at a known speed), and the tool then calculates with that factor.
- A common design has the cut-in voltage roughly equal to the battery voltage; above that the battery holds the voltage and limits the speed (forum solar-electric.com).
- Number of coils divisible by 3 (three phases). In one example Hugh Piggott recommends connecting the coils of each phase in series and the three phases in star.
- Strong magnets on steel discs attract each other with great force: assemble only with threaded rods and spacers; spinning rotors and charging current are dangerous.