A new (contributed) hub motor

Posted on December 6, 2008


After a 3 week intensive sprint and with a little help from me, Pierre has finalized his own motor model and it is now available in the repository. He also contributed a couple of small tweaks to my common Lua “libraries”. The GPL wins again. 😉

I helped out as much as I could (with just a few hints in magnetic design) in between my school exams and assignments, and after just a week he had my Lua/FEMM framework running with his new model. After two weeks he had a consistent motor and was running optimization scripts. Not bad at all, considering his haste and the sometimes flaky communication between two non-native English speakers! After 3 weeks he had the basic mechanic and cooling design too. So if anyone asks, you can tell them it is possible to completely design an electric wheel in under one month (I’m not so sure about the output quality, though). Anyway, hats off for a man who knows what he wants and works hard to get it. 🙂

I haven’t checked-in his optimization scripts yet, they are very redundant (extensive “copy-paste-modify” done in a hurry), but I intend to merge that feature into some “M-files” that I want to create for the project (FEMM has a nice integration level with Matlab & Octave). I did however spend some time cleaning up his model code and also refactoring some of mine to better allow multiple models in the framework. It’s all looking pretty usable now.

The new model  is also 3-phase fed, but otherwise it is quite different from what I had been experimenting with. He followed “industrial practice” and designed a single outer rotor version with a more standard slot/pole topology. He chose a base model of 9 slots & 8 poles (all multiplied by a factor of 8, yielding 72 coils & 64 magnets), and all the stator teeth are wound. Additionally, the rotor is very thin and the permanent magnets are embedded into the rotor shell (instead of just glued to its surface).

psabatier_fluxFig.1 – Pierre’s FEMM motor model with flux distribution and two active phases.

psabatier_flux_closeupFig.2 – A close-up of the air gap and rotor, showing the “hammerheads” he developed.

The magnets he chose are custom-designed from supermagnete.de for his dimensions. He picked a high-temperature grade (N40UH) capable of withstanding 180 degrees Celsius, which makes perfect sense in a high-power motor. Unfortunately, higher-temperature Neodymium magnets are a bit weaker in the magnetic field aspect. He chose a path of engineering I am not especially interested in: high-power / high-torque. As a consequence, his motor is to be forcefully cooled (and he did a fine job with that arrangement). Personally, I’d like to build a motor that is so efficient that it needs not be cooled.

Pierre was kind enough to share his initial mechanical design with me, although under a “verbal” non-disclosure agreement because he is seeking patents on this design. So, with his permission, I’m only showing here a large scale overview, which in fact contains nothing more than just common industry knowledge and common sense. His specific insight and choices on thermal management and mechanical assembly are purposely hidden (so don’t ask me for more details).

psabatier_assemblyFig.3 – Pierre’s basic mechanical assembly: the stator structure and the rotor “cap”.

This mechanical assembly does not show any magnetic parts; the stator is empty and ready to receive the ferromagnetic sheet and teeth and copper coils. The same is true about the rotor, it has no magnets or iron sheet in it, it’s just the outer shell.

Unfortunately, I didn’t have much time to follow his work up close, and he was really in a hurry to finish the magnetic and mechanical modeling and deliver the plans for a prototype. That’s right, this baby is going to be manufactured any day now. 🙂

According to Pierre’s simulations, his model shows a higher cogging torque (around 10Nm) than my LRK-based topologies (close to zero). But it gets a full 1000Nm for just 50A per phase, which is actually better than my double rotor model – almost twice the “bang for buck”! I have to investigate the reasons for this.

Speaking of phases, here is the snapshot of his motor (Pmult=8) with one phase active (in pink), so we can see the winding sequence:

psabatier_one_phase_8_spotsFig.4 – One phase active at 8 different spots.

And here is a close-up so we can count the teeth.

psabatier_phase_close_upFig.5 -Three teeth per phase pole.

This is a very curious arrangement to me…In fact, if we reduce Pmult to 1, we can see that the phases are not arranged in pairs unless Pmult is even…

psabatier_asymm_3coils_per_phaseFig.6 – Base, Pmult=1.

I have to find some time to run a few experiments on it… 😉