Electricity generating exercise bike build log
Channel: HeloSpark
Date: 24/02/2018

This is a video of my axial flux generator and human powered exercise bike building process.

Originally this exercise bike used a electromagnetic breaking - basically eddy currents have been generated in the aluminium surface of the flywheel by magnets, which as per Lenz's law slowed down the motion hence you needed to burn off some energy to keep it moving - hence exercise.

But eddy currents are pretty useless, they only create heat, so I decided to hack it and make it actually generate electricity.

While using some off-the-shelf low RPM permanent magnet generator could have worked, I decided to build my own axial flux generator - I have long wanted to make one anyway.
It's fairly popular especially in the DIY wind generator community.

Using Faraday's law of induction, Ohm's law and some elementary school math I have estimated that the output power will approximately match my (human) power, which should be about 100-150 Watts continuously (unfortunately as later turned out, it didn't quite live up to the calculation, due to some overestimates I have used).

As per Faraday's law of induction when magnetic flux is cutting through a conductor (like a copper coil) voltage is generated and if there is a closed circuit electric current will flow and therefore it can deliver power.
Meanwhile as per Lenz's law the coil will generate magnetic field to oppose motion, therefore I will also need to put some exercise into it.

In this design one of the rotors is the original flywheel of the exercise bike (it's made of iron, which is a good conductor of magnetic flux), the other is a cutting disk (made of steel).
The stator is made from resin with 6 coils casted into it 60 degrees apart in alternating directions.

The single phase design probably is a bit weird, most axial flux generators are 3 phased to reduce the vibration and noise and make the power requirements mostly flat, I have gone with a single phase to reduce the amount of magnets and complexity of the design, and also to induce higher voltages.
I could have get away with it, because I have a relatively high RPM (as opposed to wind generators): more than 300RPM and a very heavy flywheel.
Single phase also forced a coreless design for the coils.

Output power:

Depends on the pedaling speed and load (also on the strength of the exerciser :P)
It's usually about 40W output (more is needed from human power, because the resistance of the coils is quite high, therefore the efficiency is not the best). I have been able to go up to a 100W output, but it requires very fast pedaling.
Voltage is around 10 - 50 volts.
Current is around 2 - 6 Amperes

What I use it for:

It is primarily an exercise bike, so exercise. Besides that I can:
 - Make a hot cup of tea
 - Charge all my USB devices
 - Charge 12V batteries
 - Operate inverters with small loads


What to improve:

This project was a great learning experience, couple of things I would reconsider if I were to redone the project:
 - More coils (as was clearly visible in the video there was large gaps between the coils reducing the overall power, creating funky waveforms, etc. I was forced to limit the radius of the coil, otherwise the central hub (that connects the two rotors) would not have fit)
 - Thicker wires (to decrease lost energy)
 - Three phase (reconsidering again)