Use neodymium (NdFeB) magnets for a wind alternator when you can find them, and ferrite only when you cannot: neodymium is about 10 times stronger, comes from hard drives and motors, and lets a lighter rotor start in lighter wind, while ferrite from speakers is weak and needs a larger rotor with more or bigger magnets. Prefer large magnets over a higher grade, and keep standard N-grade neodymium below 80 °C.
Part of Advanced Wind Turbine
Magnet selection is the choice of which salvaged magnet material, size and number will make the alternator field.
Two Magnet Families Cover Nearly Every Build
The carrier names two: neodymium (NdFeB), the strongest available, salvaged from hard drives, speakers and motors, with 12 to 24 magnets arranged in alternating north-south pattern on each rotor disc; and ferrite (ceramic), weaker but cheaper and easier to find, needing larger magnets or more of them. The Permanent Magnets page lists them with Alnico as a third, which is moderate in strength and easily demagnetized, so not suited to a generator.
| Neodymium (NdFeB) | Ferrite (ceramic) | |
|---|---|---|
| Relative strength | strongest, 10 times ferrite (Permanent Magnets); 5-10 times (Generator Building) | weak but stable |
| Typical sources | hard drives, brushless motors, speakers | speakers, cheap motors |
| Temperature limit | 80 °C for N-grade, up to 200 °C for EH grade | 300 °C and above |
| Failure to watch | brittle, chips, rusts if the coating breaks | brittle, cracks on extraction |
The two corpus pages disagree on the ratio: 10 times and 5 to 10 times. Both are quoted; the ratio depends on whether you compare the energy product or the field in a gap, as the section on area below shows.
Grade Matters Less Than Size
The Permanent Magnets page grades NdFeB by the number after N: N35 is entry level, N42 a good balance, N52 the maximum commercial grade. Flux depends on volume as well as grade, so an N35 magnet 50 × 25 × 10 mm will out-perform an N52 magnet 20 × 10 × 5 mm. Hard drives yield two arc-shaped magnets of about 40 × 15 × 3 mm each, usually N48 to N52; a small turbine needs 20-30 drives. The temperature code follows the grade: N is 80 °C, M 100, H 120, SH 150, UH 180, EH 200. A standard N magnet is fine if the alternator never reaches 80 °C.
Why Ferrite Needs So Much More Area
In round numbers, which are recalled and not from the corpus: NdFeB has a remanent field of about 1.2 to 1.4 T and ferrite about 0.4 T, a ratio of about 3, while the energy product of an N42 magnet (42 MGOe, about 334 kJ/m³) is roughly 10 times a ferrite’s. The field in the air gap depends on the first number and on the magnet’s thickness. In the simple model of Air Gap & Efficiency, B ≈ Br ÷ (1 + g ÷ 2t) for a gap g between two opposed magnets of thickness t. For a 4 mm gap:
- NdFeB, 3 mm thick (a hard-drive magnet), Br 1.3 T: B = 1.3 ÷ (1 + 4 ÷ 6) = 0.78 T.
- Ferrite, 10 mm thick, Br 0.4 T: B = 0.4 ÷ (1 + 4 ÷ 20) = 0.33 T.
- Ferrite, 20 mm thick: 0.4 ÷ (1 + 0.1) = 0.36 T. More thickness barely helps, since the ceiling is Br itself.
So the ferrite rotor makes about 0.78 ÷ 0.33 = 2.4 times less flux density, and the only way to make up is area: about 2.4 times the pole area on the disc. That is why the Generator Building page says a ferrite generator would need to be 3 to 4 times larger. These are idealized values for a quick comparison, not predictions.
Method: Choose, Test And Mark
Step 1: Count what you can salvage. The carrier’s 12 to 24 magnets per disc, times two discs. If you have fewer than 24 good magnets, plan a smaller pole count (Rotor Disc Construction).
Step 2: Match the set. Use magnets of equal size and strength, within one batch if you can. Test by seeing how each one holds a steel nail or a steel plate.
Step 3: Find each magnet’s polarity. A compass needle’s north-seeking end is attracted to a magnet’s south face. Mark the faces that attract the needle’s north end with a dot (south) and leave the others unmarked, and then alternate them around the disc.
Step 4: Keep magnets away from heat and moisture. Coat NdFeB in epoxy after it is on the disc.
Common Mistakes
- Mixing grades and sizes on one disc — the weaker magnets spoil the field uniformity.
- Prying a magnet off a hot motor with a torch — N-grade neodymium can lose strength for good above 80 °C. The Permanent Magnets page suggests heating a motor rotor to 150 °C to soften the glue; that is above the limit for N and M grades, and motor magnets are often a higher temperature grade, but if you do not know the grade, do not heat the magnet.
- Ferrite magnets from a microwave — brittle, weak and not worth the effort.
Safety
Magnets that pinch, shatter and pull tools
Neodymium blocks snap together across a gap and can crush a finger or fly off a bench. Handle them one at a time, keep steel tools away, and wear eye protection: they shatter when they collide. Keep magnets away from anyone with a pacemaker.
What’s Next
- Learn next: Stator Winding (Coil Design) for the coils that sweep them
- Combine with: Air Gap & Efficiency to see how thickness and gap interact