How does a simple Turbo Trainer Work?

My job in the real world is to support the development of Engine Test Facilities for Jaguar LandRover (JLR) in the UK. Specifically I have a team that support and develop software systems for data delivery, although I know a lot about the non-computer systems. I have developed and deliver a training pack for new employees joining the business as Engine Test Bed Operators that covers a wide range of material.

One of the topics is the dynamometers we use, these being a mix of Eddy Current machines and AC motors. During the discussion I mention my turbo trainer as a type of Eddy current dyno, but with permanent magnets rather than electro-magnets.

Wikipedia has a page about what Eddy Currents dynamometers are here: link.

This image shows the principle. Basically the disk passing through the magnetic field causes currents to be induced in the disk. These currents (eddy) generate an opposing force to the direction of motion and hence this resistance is how power is absorbed. The force is function of the velocity (and of course the magnetic strength and resistance of the disk). Basically for a given design doubling the speed means doubling the power dissipated. the resistance dissipated as heat by the currents flowing through the resistance of the metal.

 

I had not really given the exact mode of operation of my turbo trainer a lot of thought but a couple of things coincided recently and I partially disassembled it to fix an issue.

Firstly I was in my garage to re-build my Giant Defy as a turbo (only) bike after it was possibly damaged when I got knocked off in Nov 2015. Secondly, as I was in the garage I partially disassembled my turbo trainer to try and sort out why it was stuck on a quite high load setting - in some respects training at a higher load makes it a little easier when actually riding on the road but at the same time the load can be just too high and make the turbo sessions unpleasant.

I took the cover off and this was the view of the end of the roller:

It is possible to see the disk and a small cooling fan. I think the disk has holes to aid cooling.

The end cover has 6 magnets in a fixed position:

I had assumed that the cable adjuster moved some magnets radially - low load was with the magnets further out and high load with the magnets fully covering the disk. This was clearly not the way this works. I removed the 3 screws and pulled the fan and the disk off the shaft to find  second set of magnets:

These magnets are mounted on an adjustable plate that rotates about 30deg. This rotation moves these magnets into full alignment or out of alignment, this increasing or decreasing the effective magnetic field and hence the eddy currents and hence resistance. This is a really simple and effective design. Here is a schematic:

 

What's also interesting is that this simple set of magnets and metal disk is capable of absorbing around 400W+ (I don't know what my trainer is rated at but I know that I can't get any where near the limit).

Some minor fettling of the assembly and lubrication of the cable adjuster seemed to sort eveything out and I can now easily adjust the load (via magnet positions).

So one better working turbo trainer later I can "train" better.