Pulse Motor Rotor Design:
Magnets, Materials and Balance

The rotor is the soul of your pulse motor — it's what spins, what you tune, and what you stare at with quiet satisfaction when everything clicks into place. Getting the rotor design right is crucial: a poorly balanced or poorly magnetized rotor will fight you at every step. Let me walk you through everything I've learned about building a great pulse motor rotor.

Rotor Types

There's no single "correct" rotor design — builders use all sorts of approaches depending on materials available, size requirements, and build goals:

• Bicycle wheel — the classic choice. Large diameter, easy to add magnets to the rim, runs smoothly on existing bearings. Cheap if you have a spare wheel.
• MDF or plywood disc — easy to cut on a jigsaw, holds magnets well with epoxy, inexpensive. Heavier than ideal but great for a first build.
• PVC wheel — lightweight, non-magnetic (important!), can be turned on a lathe or purchased as a blank. Papa Bale's preferred material for larger builds.
• 3D-printed wheel — precise magnet slot positioning, but limited by material strength and print size. Good for small experimental rotors.
• Acrylic or polycarbonate disc — transparent (impressive for videos!), lightweight, non-magnetic. More expensive but beautiful for demonstrations.

Whatever material you choose, it must be non-magnetic. Metal rotors will interact with your coil field in unpredictable ways. Stick with wood, plastic, or non-ferrous materials.

Magnet Selection

Neodymium magnets (rare earth) are the standard choice for pulse motor rotors, and for good reason: they pack more magnetic field into a smaller, lighter package than any other affordable magnet type.

• Grade N35 — Good starting point, widely available, affordable
• Grade N42 — About 20% stronger than N35, my preference for most builds
• Grade N52 — Maximum strength, but more expensive and more brittle; not necessary for most builds
• Ceramic/ferrite magnets — Much weaker but cheap and available; usable for very small builds but not ideal

For shape: disc magnets are easiest to mount in circular holes or surface-mounted with epoxy. Bar magnets can work but require more careful orientation. Ring magnets are elegant for shaft-mounted designs.

Magnet Placement and Polarity

This is where many beginners make their first critical decision. For a standard pulse motor, mount all magnets with the same pole facing outward (all north or all south). This ensures consistent triggering and coil interaction.

How many magnets? Even numbers work best: 4, 6, 8, 12. More magnets = more pulses per revolution = smoother rotation, but less momentum between pulses. 8 magnets is a sweet spot for most builds. For a Bedini SSG starter build, 8 magnets evenly spaced is the community standard.

Spacing must be perfectly even. Mark your wheel positions with a compass-drawn circle and a protractor (or print a template). Off-center or uneven magnets cause timing irregularities that no amount of coil positioning can fix.

Rotor Balance

A magnet motor rotor that isn't balanced will vibrate, consume extra energy fighting its own wobble, and potentially damage bearings. Here's how to balance it:

1. Mount the completed rotor on its shaft or spindle
2. Spin it free and watch where it comes to rest — that side is heavy
3. Add a small piece of tape to the light side and spin again — adjust until it stops randomly (no preferred stopping position)
4. Replace the tape with a small permanent adjustment: drill a small hole on the heavy side, or add a tiny weight on the light side
5. Repeat until the rotor balances freely in any position

Professional balance requires a balancing machine, but tape-and-spin is adequate for most DIY pulse motor builds.

Bearing Choice

Low-friction bearings are essential for pulse motor efficiency. Skateboard bearings (608-size, cheap and widely available) are excellent. Avoid anything with excessive preload or stiff seals. The rotor should spin freely for at least 20–30 seconds from a single hand-flick if bearings are correct.

Some builders use brass bushings in oil — smooth and quiet, but requires maintenance. Ball bearings are more consistent for long-term builds.

Air Gap

The gap between the magnet face and the coil face (air gap) is a critical tuning parameter. Smaller gap = stronger electromagnetic interaction = more torque. But too small and the magnet or coil mount can snag. Start with 3–5mm and experiment closer once the basic motor runs.