# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/Ddt3vl0Afun
- **Gondola URL:** https://gondola.cc/posts/70844133-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/b81dcf5a49.jpg
- **Posted:** 2026-09-25T16:19:11.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

A rotor spinning silently on sunlight alone, kicked forward a fraction of a second at a time. The radial solar pulse motor is one of the most elegant demonstrations of electromagnetism, and pairing it with a Halbach array makes it measurably better.

The principle is simple. Permanent magnets sit around the rim of a rotor, and one or more stationary drive coils sit just outside that rim. As each magnet approaches a coil, a sensor fires a short current pulse. The coil briefly becomes an electromagnet, pushes the passing magnet onward, then switches off. Momentum carries the rotor to the next magnet, and the cycle repeats.

The core subsystems:

- Photovoltaic cell charging a storage capacitor
- Hall-effect sensor or reed switch detecting rotor position
- MOSFET or transistor switching the coil for milliseconds per pass
- Freewheeling diode clamping the inductive voltage spike at turn-off
- Low-friction bearings and rotor inertia acting as a flywheel

The Halbach array is where the design gets interesting. By rotating the magnetisation direction of each successive magnet by 90 degrees, the fields reinforce on one face and largely cancel on the other. John Mallinson described this one-sided flux effect at Ampex in 1973, and Klaus Halbach at Lawrence Berkeley Laboratory developed it independently around 1980 for focusing particle accelerator beams.

In a radial motor, the strong side faces the coils. More flux crosses the air gap, so each pulse delivers more torque per joule, while stray field leaking into the hub and bearings drops sharply. For a machine running on a few milliwatts of sunlight, that concentration of flux is the difference between stalling at dusk and spinning into the evening.

Every pulse is a small decision about timing, and timing is everything.

#stemantics #electromagnetism #halbacharray #solarpower #engineering

## Stats

- **Views:** 123,406
- **Likes:** 1,229
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- **Comments:** 12

## Tags

engineering, electromagnetism, solarpower, halbacharray, stemantics

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