Monthly Archives: June 2013

First steps with electromagnets

Having made a platform to mount them on, it’s time to start thinking about the electromagnets themselves. It doesn’t take me long to realise I know next to nothing about this area. My starting point is just what I remember from high school physics lessons – which were quite some time ago! (What did we do before Google?)

If nothing else, I have plenty of questions; how strong? how many? bought or home made? what voltage? how much current? wire gauge? number of turns? core material? power dissipation? positioning? mounting? The list goes on and on…

Fortunately, I’ve got somewhere to start – and that’s the original resistance unit, which uses a single neodymium magnet. I know where the magnet is positioned relative to the flywheel, and I if I can measure the strength, I should know what I’m aiming for with the electromagnet(s).

To measure the magnetic field, I can use a hall effect sensor. I happen to have an Allegro A1302 in my parts drawer which fits the bill. This is a very simple to use 3 terminal device, requiring just 5V power and ground, and producing an output voltage that is proportional to an applied magnetic field.

Recording the output voltage for various magnets will provide a means to compare the magnetic field strengths. The output of the A1302 is specified at 1.3mV per gauss, but the units aren’t a consideration for me at this stage, just the relative differences.


I’ve created a very simple breadboard circuit to provide the regulated 5V for the sensor using a LP2950 LDO regulator, and 12V power (from my bench supply) to an electromagnet. To help with consistency, I’ve routed a small groove in a block of MDF, and stuck the sensor to the bottom with double sided tape. This provides a gap of 2mm between the face of the sensor and the top of the groove, meaning I can place any magnet against the block, and will always be comparing the same air gap between sensor and magnet.


The following graph illustrates the relative field strengths of various magnets. Just for starters I have the original neodymium Tacx magnet, another neodymium magnet salvaged from an old hard disk drive, a fridge magnet, and a small £5 electromagnet from eBay.

It soon became clear that the strong neodymium magnets were saturating the sensor at a distance of 2mm, so for the purposes of this initial test I increased the distance to ~8mm with a small block of foam.


Hmm, that eBay magnet isn’t off to a very promising start! Next I’ll try winding some of my own electromagnets to see how they compare.


Magnet testing platform

With the original permanent magnet assembly removed, the next job was to make a platform for mounting the electronics and electromagnets.


For a first prototype, I used a piece of MDF, which would be easy to machine and shape, and has fairly good dimensional stability against environmental changes.

This was turned to a round plate (of around 140mm diameter) on the lathe. I then put a chuck in the tailstock, and used a selection of Forstner bits to drill out the internal steps required to mate with the resistance unit and support the bearing. I also needed to remove some material to fit around the body of the resistance unit, and drill the holes for the locating lug & mounting screws .

Once the overall shaping was complete I sealed all the edges with some diluted PVA glue. While probably not as good as a dedicated “MDF sealer” this will hopefully prevent any swelling from water ingress on the bare faces.

The plate was then attached to the resistance unit with the original self tapping torx screws.


I’m quite happy with the resulting fit. Assuming this approach works, I’d like to 3D print the final assembly, so have been taking detailed measurements along the way, and working on a model in OpenSCAD.


Next post should hopefully get back to some electronics ;) Thanks for reading!

Resistance unit teardown

In order to start experimenting with electromagnetic resistance control, I first needed to remove the original cable operated magnet assembly from the resistance unit.

The first step was to remove the resistance unit from the frame, by undoing a pair of hex bolts. (Not strictly necessary – but will make the following steps much easier).

Next the black cap can be prised off the flywheel with a small screwdriver, exposing the 13mm retaining nut underneath.


Note: there is not enough exposed thread to make it obvious, but this is a left-handed thread. Fortunately, I had found this out via Google already – else I may have been stuck at this stage for some time ;) The flywheel body needs to be securely held with a strap wrench, or in a large vice, while the nut is turned in a clockwise direction to loosen and remove it (the opposite direction to a standard thread).

pullerI then supported the flywheel between two blocks of wood, and gave the end of the shaft a tap with a rubber mallet to knock it out of the flywheel… and then a heftier tap, and another… Ok, so this wasn’t moving easily! I then put it on my drill press table, and applied as much pressure as I dared trying to shift it, but still no go.

In the end I had to resort to a sharp smack with a lump hammer, which fortunately didn’t damage the threads. As I’ll doubtless be taking the flywheel on & off repeatedly during development, I’ll invest a in gear puller (right) to make future attempts less brutal.

With the flywheel off, you can easily see the non-ferrous disk on the inside. This disk passes through the magnetic field, creating the eddy currents and the resulting opposing magnetic field that results in the braking force.


The picture below shows the sliding magnet assembly that sits behind the flywheel. The small plastic collar sits fairly loosely on the shaft and doesn’t appear to serve much purpose. Am assuming it’s there just to create some cooling airflow?


Once the 4 torx screws are removed, the plastic magnet assembly just pulls off the main body, revealing the left hand side bearing.


Is a bit hard to see from the photo, but there’s a lip around the inside of the assembly that holds this bearing in place.


Next up, I’ll replace this assembly with a platform for mounting and experimenting with various electromagnets..