Back on the topic of test results, I'm finally working on the thing that has me most excited, which is testing out the ferrofluid effects on one of the prototype Grin motors. For those who don't remember, this one has a 2.5mm gap space between all the magnets, and so the behavior of the FF could be quite a bit different than when the magnets are butted up against each other.
In this case, rather than doing it just at 200rpm, I'm testing the stator->core conductivities at both 200 and 400 rpm in case the higher motor speed causes centripetal forces push the FF away from magnets and air gap and flat against the back iron instead.
And here after a full 22 runs in the wind tunnel under all different scenarios I have that data, and it continues to be exciting. Here's the most essential summary.
The 2mm gap space between the magnets rather than having them butted up did not drastically increase the amount of fluid fill required. At 2mL you start seeing some improvement and then it reaches full benefit at 6mL. As well, the concern that at a higher 400rpm rotational speeds the Statorade would be "squished flat" against the back iron rather than bridging the stator/magnet gap also proved unfounded. The 400 rpm curve seems to line up with the 200rpm curve pretty well even with the large gap between magnets which the FF could easily recess into.
You'll also notice here that the final thermal conductivity is even higher than it was on the wider MXUS motor (10 Watts/degree vs ~7 W/K https://endless-sphere.com/forums/viewt ... 8#p1106508
). That's mostly a result of the fact that there I was measuring side cover temperature with the MXUS motor, but here I was using an IR sensor pointed right at the steel back iron ring, with the effect that we'll have higher core->shell conductivity numbers but worse shell->ambient , since the definition of the shell temp isn't quite the same.
The following graphs really help illustrate what was going on. With no practical ferrofluid fill, the motor core temperature with ~95 watts of heat rose to nearly 70 degrees C, while the side cover and steel back iron temperatures were all very close to each other, the side plates ~31oC, and the steel rotor ring ~33oC.
With the exact same test conditions using FF, the motor core only reaches 44 degrees, but the temperatures on the shell are now much more spread out, with the steel ring being at 35 degrees and the side plates 27-29 degrees.
This is what we'd expect with a majority of the heat flow now going through the magnets and back iron, but it's especially exaggerated in this particular hub design. Remember that I actually did most of the design and tooling of this motor in 2010/2011 and at that time heat flow wasn't a high priority on my mind. The goal instead was complete material removal and weight reduction, and so while most motor rotors have a thick (~6mm) ring of back iron surrounded in cast aluminum, in my case I made the steel ring taper as thin as possible before it met the side covers since there was no need conducting magnetic flux over there. And I did away with an aluminum casting altogether, instead putting the spoke flanges on the side covers.
The effect as you can clearly see is that there isn't much of a conductive pathway for heat to move sideways from the steel back iron to the side covers, and so we see this much larger 7-8 degree temperature difference vs ~4 degrees that was measured with the MXUS/9C/Crystalyte motors. This also means that the addition of an inside rotor fin would have more of a beneficial effect on the Grin hub compared to a regular motor that can more effectively transfer to the side covers.
Previously competed in the Suntrip race on a back to back tandem solar powered row/cycle trike
. 550 watt solar roof, dual Grin All Axle hub motors, dual Phaserunner controllers, 12 LiGo batteries, and a whole wack of gear.
Now back in Vancouver learning to be a dad with my Big Dummy Frame (yes This One
, thanks ES!) with GMAC 10T rear hub motor, Phaserunner controller, and 52V 19Ah EM3EV pack
My website: http://www.ebikes.ca
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