Build each rotor from a flat mild-steel disc of 6.4 to 9.5 mm, with 12 magnets glued on at 30° spacing in alternating north and south faces, then hold each magnet with a mechanical retainer as well as epoxy. Set the two discs facing each other with opposite poles aligned, spaced apart on threaded rods, and bring them together on the stator only with spacers, because the pull between them is strong enough to injure.
Part of Advanced Wind Turbine
A rotor disc is the steel plate that holds the alternator’s magnets in place and closes the magnetic path between them.
The Disc Is A Magnet Holder And A Magnetic Return
The Generator Building page specifies two rotor discs of mild steel, 1/4 to 3/8 inch (6.4 to 9.5 mm) thick, with the stator between them. Steel does two jobs: it carries the magnets, and it guides the flux from one magnet to the next, so use mild steel and not aluminum or wood. Permanent Magnets gives the layout: magnets alternate N and S around the disc, evenly spaced, so 12 magnets sit 30° apart. The carrier requires the magnets on opposite discs to face each other north to south across the gap, an attracting configuration.
Method: Make And Mark The Disc
Step 1: Cut and flatten. Cut a 300 mm disc from the plate. A torch-cut disc warps, so grind or machine the magnet face flat; the air gap is only 3 to 5 mm (Air Gap & Efficiency).
Step 2: Centre-drill for the shaft hub and drill the holes for the spacer rods, matching on both discs.
Step 3: Mark the magnet positions. Choose a pitch circle radius. At r = 0.11 m, the spacing between magnet centers is 2π × 0.11 ÷ 12 = 57.6 mm; a 30 mm wide magnet then leaves a 27.6 mm gap to its neighbour. Make a template with slots at each position, and use one template for both discs so they match.
Step 4: Test polarity with a compass (Magnet Selection (NdFeB vs Ferrite)) and mark each magnet’s outward face before gluing. Alternate N and S around the disc.
Step 5: Glue. Clean the steel, roughen it, and set each magnet in epoxy, placing them one at a time through the template. A magnet released too near its neighbour will flip. Let the epoxy cure before handling.
Step 6: Retain the magnets mechanically. The permanent-magnets page requires retention beyond epoxy. Options: a shallow pocket or a ring of steel around each, or a layer of fiberglass cloth and resin over the face (the stator gap then includes its thickness).
Step 7: Mirror the second disc, so the magnet at each position faces the opposite pole of the one on the first disc.
Step 8: Check flatness and wobble by spinning the disc on the shaft against a fixed pointer. The runout must be a small fraction of the gap.
Worked Example: Weight And Retention
A 300 mm disc 6.4 mm thick has a volume of π × 0.15² × 0.0064 = 0.000452 m³, and at about 7,850 kg/m³ for steel (a recalled handbook density) a mass of 3.6 kg. The pair weighs about 7.1 kg before magnets and hub, which the shaft and bearings carry (Tower & Mounting).
The centrifugal force on a 20 g magnet (an assumed mass) at a radius of 0.12 m and 250 RPM is 0.02 × (250 × 2π ÷ 60)² × 0.12 = 0.02 × 685 × 0.12 = 1.6 N; at three times that speed, 9 times more, or 14.8 N. That is small beside the pull of a neodymium magnet on steel (recalled, tens of newtons for a magnet of this size). The reason for mechanical retention is not the spinning load: it is that a glue joint weakened by heat, vibration or an impact releases the magnet unexpectedly.
Assembly: Pull The Discs Together Safely
The two rotors attract each other with great force through the stator. Assemble on a bench with long threaded rods and nuts at the spacer holes. Slide the first disc onto the rods, place the stator on its supports, and draw the second disc down by turning the nuts evenly, with spacers set to the gap. Never let the two discs approach by hand. To open the assembly, back the nuts off evenly.
Common Mistakes
- Uneven magnet spacing — it creates uneven voltage and vibration.
- A warped disc — the gap varies as it turns, and the magnets can strike the stator.
- Mismatched poles across the gap — the field cancels and the output falls to almost nothing.
Safety
Closing two magnet discs can break fingers
Strong magnets on two steel discs attract across the gap with a force that grows sharply as they close. Keep fingers away from the gap, use threaded rods to draw the discs together, wear eye protection, and keep tools and watches away from the assembly. A magnet that comes loose at speed is a projectile: fit the retainer.
What’s Next
- Learn next: Air Gap & Efficiency for setting the gap between discs
- Combine with: Stator Winding (Coil Design) for the stator they sandwich