It should come as no surprise that biking on the island of Hawaii is difficult. The entire island is one big steep hill. One day while biking up a long steep hill, I thought how nice it would be to have an electric motor assist to get up the hill. These were the days that I used my bike to go shopping.
I briefly considered getting a kit, or retrofitting some existing motor, but no, that would be too easy. I wanted to design one myself….
I don’t always do things the easy way.
Fast forward a year and a half, and the design is finished! (Again, I never do it the easy way)
The three phase brushless outrunner design is both simple and powerful, and the perfect sort of thing for a low-speed, high torque situation.
I used FreeCad to do all of the modeling. The design has six coil posts (two for each phase), and eight neodymium magnets in the outer rotor.
The design is intended to be 3D printed in a type of plastic, with metal bearings and shafts plus magnet wire. Do a search for 3d printing service. You will have to upload the .stl files included to get a quote on the price from whatever service you choose. The type of material chosen for this design is flexible. The parts were overbuilt to facilitate weaker and cheaper plastics.
By being plastic, none of the magnetic fields are concentrated which will result in loss of peak torque. However the motor won’t be subject to hysteresis losses through magnetization of a steel core. (i.e. it is a coreless design)
I’ve considered how this might perform while working as a generator. A three-phase bridge rectifier would be required, as well as some kind of voltage regulation.
The free model includes all of the parts, however none of the drawings or wiring diagrams are included. The paid model includes technical drawings, a wiring diagram, and a parts list.
Both versions have the FreeCad Source files
Warning: This is a VERY DIY project. Neither version comes with any guarantee of suitability for any purpose. This design could end up being a big expensive paperweight. It is intended as a hobby project, and it is up to the end user to make it work for the purpose that they see fit.