Hello
Miles
Butted up: I believe the optimum would be – “
not touching”, but as we see with 9C hubs – they do in fact touch. Not critical, but a noticeable gap decreases the flux field proportionally, and when you have back iron behind it – the shape of flux is reduced rather diabolically:
Not what we want.
Stator Saturation: That’s part of it – absolutely. Good point!
The two key factors I was looking for are:
Iron Core losses and
Magnetic Strength which are directly related to performance. However – Stator saturation plays a large role!
The shape physical of the magnet determines the field shape. So back iron and Stator are designed with magnetics together as a system; they are optimized as one. I actually think that the weaker magnets work well with the wheel because the iron losses are reduced – so when we start adding more pull-strength, we’re creating a problem both for the stock stator and the back iron because it will want to resist rotation, like a
stepper motor. The FEMM model shows much higher density at the choke-points when the fields pinch, specifically at stator corners and at magnet joins. This reduces the effectiveness of the stator to push against the magnets. I’ve tried increasing the back iron to ridiculous thicknesses, but the Stator appears to gate the gain.
Relative to ironless AF studies, even those with back iron rings, increasing magnetic field strength directly increases power and efficiency. I have to conclude, like you – that it’s partly Stator Saturation and Eddies. We can keep making magnets larger and more powerful, but we won’t ever see the full benefit.
The second part is the relationship between the physicality of the individual magnet and the magnetic strength. All things being the same – two magnets of the same material and grade, make one twice the volume (say by thickness) and you have essentially twice the lifting/holding power. This is a good thing; we want that! The other way to affect output is to change the material out for something stronger; N35 vs. N52. Let’s play with these factors for a moment…
I suggest that the 9C motors are using N35; a wild guess – I could be wrong, but allow me to make a point: It’s the weakest commercially available RE magnet we can lay our hands on and the least expensive. From the
Neodymium Magnet Physical Properties previously linked, N35 is between 11.7-12.1 KGs; we’ll pick
11.9 KGs as the average. Now we want to replace it with the best that money can buy cos I won the Washington State Lotto Uber Mega Millions so we’re gonna order up N52 at 14.5-14.8 KGs, or
14.65 KGs as an average. The cost between these two increases exponentially – unlike the field strength; you don’t need me to tell you it’s not worth double the price. The fact is N52 is only going to improve the field strength about
23% over N35. N’uff said. :wink:
So now we’re back to changing the physicality of the magnet to cheaply increase the power, the torque/Watt ratio. I have modeled over 200 Ironless AF configurations and when I double the magnets’ thickness – you can just about bank that the output is going to double; it’s almost linear (there are associated losses which grow, even in the ideal world). The trade-off is we now have double the magnet mass, and a larger current draw to overcome
momentum.
I think this is why hub motor designers want to keep the magnets thin; find the right balance. With that in mind – if we could afford to create the custom sized magnet as directly replace it with stronger magnets, the problem with Stator Saturation would rear it’s pesky lil’ head. I can’t see spending money making a custom-sized magnet for a 23% gain,
even if I could spend your monies.
My science may not be perfect – but it is how I understand it given I’m not a physicist or chemist or mathematician. I’ve goofed with FEMM long enough to trust some of what it reports, however it falls down at times with back iron material changes. You’d think there would be a noticeable difference between 1018 alloy and 1008 but the deltas were lost in the noise. I have to conclude that the 9C motor is optimized for one magnetic shape and charge, and the only reasonable change we can easily make is in the amounts of winds/turns. We can’t really affect these very much either cos the copper used is pretty good quality. If we look to change laminations – we’ll now were remaking the core, and if we go that far,
I’d begin fresh and make it the way I’d want: Now we’re off topic.
However I do wish to say to the OP that I had exactly the same idea some months ago, and one day decided to go through the rigor and model it thoroughly and completely as a cheaper alternative to Ironless AF. On that note, when I win the lotto for real, I’ll be making AF motors... right next door to my brewery.
Now if I could just find the winning ticket stub…
Full of flux,
KF