John in CR wrote: I seem to recall that he tested it on DD's too, and the efficiency hit made it not worth it.
Do you recall why? Could the increased drag be that much of a hit? Remember the cooler you keep the windings the less losses....Your 93% motor wont measure 93% hot. Are you the same John from diyaudio?
Speaker building was a hobby that seems a lifetime ago, though just 5 years or so ago, so it depends which John there.
Regarding the 93% efficient motors, there's exactly 0% chance oil is going in there as it is likely to mess up the series/parallel switching.
If people are so interested in cooling their motors, why don't they run smaller wheels and adjust voltage back to the same speed? On top of the greater efficiency resulting from the lower torque requirement, they'll also get much better cooling from the higher rpm. 90%+ who ventilate don't make an effort to optimize it, and a spinning pizza pan with some big round holes in it certainly doesn't make much of a fan blade.
The issue isn't whether a liquid cools better than air, since that's obvious. For me the question is whether it's more effective overall compared to a substantial fresh air flow, and in determining effectiveness you have to consider increased drag, risk of the more direct heat transfer through the magnets, the overall rate of heat rejection, etc. In comparing to ventilated you've got to consider that if you do it right and achieve flow that the air carries heat out with it, which totally bypasses the step of outer shell to the environment.
Here are my concerns about the oil in a DD approach:
1. Drag- The decrease in top speed should give some idea.
2. Halls- I haven't seen a response when I asked before.
3. Magnets- Is there more risk of heat damage to magnets with such a direct route for heat?
4. Optimum fill- This might be the hardest to determine, since at speed the oil is forced to the perimeter
by centrifugal force. I'd think just enough to just touch the lamination edges at speed would minimize drag
while maintaining a good thermal pathway. The best fill might even be even less so at low speed where efficiency
is low during acceleration that the oil falls onto the stator. Then at high speed and high efficiency where drag
if touching the stator would be great, centrifugal force pins the oil to the perimeter offering no drag at all and
almost no cooling effect, but it's not really needed anyway. OR since it's the windings that are the heat source
does the fill need to be much greater to fill to the copper even at speed despite the greatly increased drag.
5. Erosion- Obviously that moving oil is going to wear on parts not designed for a liquid. Other than the hall wires,
I'd be concerned most about the potential for the oil to get under the edges of the epoxy holding the magnets
and whether it could do harm getting between the laminations especially starting at the edges facing the magnets.
On top of these concerns I don't have a good visualization of the oil's behavior at speed with it inside the rotor, a short spinning cylinder, and having the stator, a very coarse stationary object, in such close proximate all around. If the liquid gets turned into a fine spray that would be ideal and greatly reduce drag.
Heat problems have causes that liquid cooling won't solve, so better cooling is just a bandaid. Addressing the cause, which is typically over-gearing or over-saturating the stator, is the real solution. How else could one of the biggest loads on the forum zip around at 50mph+, and climb mountains for over 3 years with a sealed motor that is very little different than those most DD hubs? I'm not even talking about recent addition to the stable of the 93% peak efficiency hubbie I'm still testing. Forget the western medicine approach and cure the problem instead of living with it and using bandaids that come with risks and compromises. Shoot, I don't even run a temperature sensor. Strap a 100lb backpack on most of these guys backs and let's see how their ebikes perform both on the highway and in the mountains. Liquid cooling won't be enough to keep up.
PS- Regarding drag, my son swears he feels a bit of drag with the motor I ventilated that he runs on his ebike. I can feel it too when I ride it, and if I had to guess I'd say it's about 50W of fan blade resistance. That's air, so just imagine the drag of a liquid at the speeds we run.