Review: Assemble your own Mendocino Solar Motor

Although its operating principle is really very simple, the first time you see an AR O-8 solar-powered​ Mendocino motor you can’t help being fascinated. It looks like something that came out of a wizard’s studio — hovering in mid-air and spinning on its own, apparently without any source of energy.

A well-designed and neatly-finished model adds to the intrigue and makes the effect even more magical. 
The ​magnetically levitated electric motor was named after the Mendocino County in California.

A Mendocino motor is powered by solar cells

It uses a rotor suspended on low-friction magnetic bearings. It consists of a rotor and frame. The rotor is an eight-sided cylinder with an axle passing through its centre. Circular magnets are fitted at each end of the axle with the same magnetization direction as magnets fitted to the frame.

This arrangement acts as the rotor bearings while the magnets exert a repelling force on one another to support the rotor in mid air.

They are also slightly offset to keep the rotor in suspension. To make this bearing arrangement stable, one end of the axle is terminated with a ball bearing which is pushed against a plate to form a low-friction contact.
 

This prevents the rotor being pushed out of its position in the frame by the magnets.

Reduce any static imbalance

A powerful neodymium magnet is positioned centrally beneath the rotor. Its magnetic field permeates the rotor coils above it. A small current flows through the coil when light falls on the illuminated solar cell.

The Lorentz energy produced in the coil gives the rotor a pulse to make it rotate. The Lorentz force is very weak so the low-friction magnetic bearing is necessary in the design. Because the forces involved are so small it’s important to reduce any static imbalance in the rotor also so that the mass inertia can be overcome and the rotor can start turning on its own when the light level is sufficient.