Brushless Motor: Selecting the Magnets

(see also, Brushless Motor)

The force of the magnets is categorised in grades: N35, N45, N50, N52 are very common, whereas N52 is the strongest. I have the impression, that suppliers are not very accurate with their specifications. Absolute values are difficult to measure (it also depends on magnet size), so you can’t check, if sellers exaggerate. They probably don't even know, what they are selling, but since the price is so cheap, you can't complain. Also the variation between the "same" magnets can be huge - I've measured +/-20%.

Select the strongest magnets:

I'm using this simple setup to compare magnetic forces of magnets relative to each other. It allows to select the strongest. The magnet is placed onto a wooden bar on top a small scale. The scale is tared together with the magnet, and then a probe with a ferromagnetic block is pushed beneath the magnet. I’m using some ferrite as a probe, but an iron bolt probably works a s well. Make sure that the position of the magnet and the position of the probe is always the same. The scale shows the force for the magnet. Since the scale contains magnetic components, it's important to measure the magnets at some distance from the scale, and keep away other metals.

It would be possible to calculate absolute values by applying the law of the lever, but for only comparing and selecting magnets, this isn't important. However it is important to repeat the measurements exactly the same way - I’ve used tape and some indication marks. In addition it matters to measure all the magnets with the same orientation of polarity (e.g. south up, north down). The measurements change, if the same magnet is flipped. I'm not sure about the reasons, but it could be the earth magnetic field, or some magnetic field in my room, or the ferromagnetic probe remains a bit magnetised, or the field lines on North and South of the magnets are not perfectly symmetric, …

For getting the brushless motor running, this selection isn't relevant. It's an optimisation, but since the setup is so little effort and I've ordered 20 magnets (8 spare / waste), I've done it.

Balance the magnets:

A similar setup can be used to measure and compare their weights. That’s more important, since an unbalanced rotor can generate big vibrating forces, and it makes a horrible noise. The ferromagnetic probe isn’t needed here. But the bar can be reused in order to keep the magnet away from magnetic components of the scale. The scale is tared without the magnet, and then the magnet is placed onto the bar.

I’ve first measured the weight of the 12 strongest magnets, and then created a ranking from 1 (lightest) to 12 (heaviest). The scheme below shows, how to assemble the magnets sorted by their weight. It’s probably not THE optimum, but it’s a good and simple guess.