Magnets

I'm waiting on some parts right now to build the next stage, so i thought id go over the magnets that i'm using while i wait. I'm using Neodymium disk magnets (Neodymium Iron Boron, generally Ne2Fe14B, to be precise). They are slightly larger than a dime, and quite strong. Neodymium was chosen both because is stronger than most (all) other magnets and because this type of magnet doesn't loose its strength when it gets knocked as easily as some other types.  This is the same type of magnet in most hard-drives.  They are also fairly light, and although relatively speaking they loose their magnetism at a low temperature with respect to other magnets, that temperature is still fairly high. (the Curie Temp is 590deg-F for mine. So probably high enough. Max operating temp of 176deg-F)

The 'Grade N42' number on the package tells us what material the magnets are made of, with higher grades generally corresponding to stronger magnets. The place i bought these from sells N35-N52 (in the 'normal' temperature range type), so Grade N42 (the one i got) is sort of middle of the road. This grade has a "Residual Flux Density" (another name for Remanence i believe), commonly Br,  of 1.3-1.32 Tesla.  Using this we can approximate the field density (or magnetic flux density) as we move away from the magnet, seen below. The surface field here (at distance = 0mm) matches nicely with the manufacturers data, which is comforting.

As we need large changes in magnetic field to generate power, from this plot we obviously want to keep the magnet as close to the coils as possible. Currently, i'm using a spacing of 1-2mm, which seems reasonable on a bike wheel. As the coils i'm using are not perfectly flat, (some windings are further away than others) ill use 7mm as my 'average' distance.

If we take one of the plots of the voltage generated by the coil passing a magnet,

we can integrate to get an idea of what the field strength (generally denoted by a capital 'Phi', sub 'b', measured in Webers (Wb), or magnetic flux) looks like to the coil as it passes.

This is sort interesting, and shows a max field strength of about  3.15uWb in each turn. The coils i'm using have a diameter between 1cm and1.5cm, so the magnetic flux density seen can be calculated to be about  30mT. This is a bit lower than what we would have expected from the ideal equations (about 50mT or so), but in the right area so i think its OK.

In any case, none of this is that important. The magnetic field plot is sort of neat but not that meaningful. You should just buy the strongest magnets you can, and then place your coils as close to them as possible.