By Papa Bale · April 6, 2026
The magnets you put on your rotor determine more about your pulse motor's behavior than almost any other component. Too weak and you'll struggle to get triggering and torque. Too strong and you risk eddy current losses, cogging problems, and — with high-grade neodymium — genuine safety hazards at high RPM. Here's everything you need to know to choose the right magnets for your build.
Neodymium (NdFeB) magnets are graded by their maximum energy product — a measure of field strength. The grade is expressed as a number (35, 42, 52) sometimes followed by letters indicating temperature performance (N, M, H, SH, UH). For most hobbyist pulse motor builds, you'll see:
⚠️ Safety Note: N52 neodymium magnets are extremely powerful for their size. At high RPM, centrifugal forces can overcome epoxy bonding. Always use mechanical retention (countersunk screws through the rotor disc, or a retaining ring) for high-speed applications. N52 magnets can shatter when they snap together — always wear eye protection when handling them.
Shape affects both field distribution and mounting complexity:
Round discs are the most common choice for pulse motor rotors. They produce a clean, well-focused field perpendicular to the face. Standard sizes for beginner builds: 20mm × 3mm or 25mm × 5mm. Drill a hole in the rotor disc, apply epoxy, press in, and allow to cure flat.
Blocks offer more surface area for adhesive bonding and can be easier to position precisely on rectangular rotor spokes. The field distribution is slightly different — useful for some SSG variants where the coil geometry doesn't match disc magnets well.
Ring magnets with a center hole are convenient for rotor builds where you want to bolt through the magnet. Less common but useful for certain axial flux designs.
The number of rotor magnets determines the pulse frequency for a given RPM. Common configurations:
For a first build: 4–6 evenly spaced magnets. Measure and mark the positions before drilling to ensure even spacing — imbalance at high RPM causes vibration and bearing wear.
Mount all rotor magnets with the same polarity facing outward — all north or all south. This is crucial. The trigger coil or Hall sensor is calibrated to respond to one polarity; mixing polarities causes erratic triggering and chaotic rotation.
To verify polarity before mounting: hold a known-north compass needle near each magnet in the same orientation. If the north needle is repelled, that face is north. Mark each magnet on the "outward" face with a paint pen before applying epoxy.
Once magnets are mounted, set the stator coil gap to 3–5mm. Closer = stronger magnetic coupling but risk of contact during vibration. Further = weaker interaction. For N42 or N52 magnets, you can often go slightly wider than with N35 and still get strong triggering and drive force.
Don't overlook ceramic magnets for first builds. They're weaker than neodymium — roughly 5–10× less field strength — but they're cheap, widely available, virtually unbreakable, and perfectly adequate for learning. Papa Bale used ceramic magnets in early builds before upgrading. If you're still figuring out the circuit and coil, ceramic magnets remove one variable from the equation.
For quality neodymium magnets, look for suppliers who grade and test their products. Popular options include K&J Magnetics (US), Apex Magnets (US), and various AliExpress suppliers for budget builds. When buying in bulk for rotor assembly, order all magnets from the same batch to ensure consistent grade and dimensions.
Watch Papa Bale mount, balance, and test magnets on real rotor builds — with commentary on what works and what doesn't.