An alternator-powered e-bike provides insight into the rotor’s magnetic field

For anyone involved in building small electric vehicles, it has become very interesting to be able to make a cheap high-power electric motor from a humble car alternator. It’s a conversion made possible by the advent of affordable three-phase motor controllers, and it’s well presented by [austiwawa]electric bike build video (embedded below).

The bike itself is a simple conversion in which the motor powers the rear wheel via an additional sprocket. He tried a centrifugal clutch with limited success, but retired it for the final build. Where the appeal of this build lies is in its examination of the placement of the Hall effect sensor.

Most alternator conversions work without sensors, but for better control, it’s worth adding these magnetic sensors to allow the controller to sense rotation more directly. He first placed them on top of the stator coils and found them inefficient, the big discovery coming when he looked at the rotor. The electromagnet in the rotor of a car alternator has triangular poles with the field concentrated in the center of the stator, so moving the sensors halfway up the stator solved the problem. Something to note, for anyone converting an alternator.

If you want to give it a try, a year ago we published an introduction to turning car parts into engines.

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