On that front, one of the things we noticed in the above test on the H3540 motor with 8mL of statorade is that the core to shell conductivity started off pretty high (above 8 watts/degree), increased a little with RPM at first, but then decreased to a below 6 watt/degree at 400 rpm before increasing again at the 500 rpm point. That seemed peculiar, so we decided to repeat the test with a lot more data points for a finer resolution. Going from 100 to 566 rpm in 14 steps required just a little over 3 hours for the experiment to complete and the results are fascinating. You can see the graph below, blue data is with the 8mL of Statorade. What we see is that up to about 200 rpm the Statorade had a huge effect on the core to shell conductivity, increasing it from ~3.5 to over 8 watts/degree. However, above 200 rpm the conductivity definitely starts to drop steadily with increasing RPM. At 360 RPM it looks like it hits a minimum value and then most abruptly starts to rise again. To me this data looked like a very clear sign that the above 200 rpm the centripetal forces start to exceed the magnetic forces on the ferrofluid which gets increasingly flung out of the air gap or squished flat so that it no longer bridges to the stator core as often for wiping the heat off. We'd hypothesized about that happening in this thread before but never seen it so clearly on display or quantified the rpm's at which that starts to take place.justin_le wrote: Now not only can we characterize motors for their thermal model much faster, we can also do tests with many more data points over the RPM and wind speed range to really understand and model the nuances of heat dissipation.
You guys are like mind readers, that was more or less exactly where I was going to be going with this. Not just repeating at increased the volumes but also experimenting with some different formulations of FF that have a higher magnetic loading. This would be for people who aren't really concerned about motor drag but just want to maximize the cooling effect. In the current Statorade formulation the aim was really to have a solution that cooled the motor with the leased amount of additional drag for those who use a pedal assist ebike, but clearly a lot of the more avid FF users are running more like dirtbikes where this is inconsequential and better cooling at all rpm's would be preferred over reduced drag if that's an option.Cowardlyduck wrote:What I would love to see, is that same test conducted with a lot more FF added. If it's the centrifugal forces flinging it out, would adding 15ml, or even 20ml reduce or even eliminate the disconnect at higher rpms?
Yes I did that at some length characterizing the drag torque with the mL of statorade added, there are numerous posts on the topic back around this time here:macribs wrote:Has anyone quantified the effect of added drag?
Yes and yes. Previously I was reluctant to engage in testing and characterizing a whole bunch of various 3rd party hub motors in this manner because of the huge time and energy commitment that that entailed, but now with this much faster and more automated test sequence we'll greatly reduce the barrier to getting new motors tested, modeled, and put up on the Trip Simulator web app. I'm probably going to first repeat all the tests on the Crystalyte, 9C, and MXUS motors that I had done previously to establish a new baseline and then open it up to many more DD motors in the mix (with and without statorade + hubsinks) once the characterization procedure is totally dialed in.macribs wrote:Does this means we will see new data on motors you already got your hands on? Will you be able to test motors that you still do not have data for?
Yup, this is true, but for sure the gains in reduced heat generation with the smaller wheel needing less torque for the same thrust will more than offset any slight reduction in heat transfer from the stator as a result of the higher RPM causing the FF to loose effectiveness. The smaller wheel diameter would still win the day for staying cool. For instance a 20% reduction in wheel diameter means a 20% reduction in phase current, which means a ~36% reduction in the copper losses and heat generated inside the motor. Even if you loose 10-20% of the thermal conductivity of FF from centripetal forces, you'll still be running cooler overall than with the big wheel.John Bozi wrote:Awesome Justin.
This now adds a bit of a spanner in the works!
Especially for those playing with wheel sizes for their optimal speed. A small wheel needing higher rpm now starts to lose some value if FF is included.
At low speeds the addition of ferrofluid increases the core to shell heat conductivity dramatically, by a factor of ~3. I haven't tested the thermal conductivity from the core to the shell with FF when the motor is completely still though. That would be an easy enough test to do. I think we'd all suspect that with the motor not moving at all the ferrofluid wouldn't have much effect since it's then just a static conductive transfer medium in just a few small spots inside the airgap, rather doing liquid convection transfer wiping over the entire stator surface as happens when the motor is turning. .Now this brings me back to my video test I posted here ages ago, where I recorded just cooling by not moving the motor vs keeping it going at low speeds. With the FF included this would be completely different.
Note that the graph I posted there was showing only the motor core to motor shell conductivity, it wasn't showing the total conductivity to ambient which includes heat flow the shell to the surrounding air, which always increases at higher rpms/speeds. If I redo the graphs showing the TOTAL conductivity from the motor core to ambient, then the increasing shell to air conduction more or less offsets the decreasing effect of Statorade. It's pretty much flat at ~4 watt/degree from 200 to 400 rpm, and then above 400 rpm it starts to tick up again.Imagine now knowing this 360 "ghost" info we have. You are at the top of a mountain road and have a 100 degrees C motor that you want to cool the best possible way on the descent. Do you let fly down at maximum speed or use your brakes and go down slowly with disc brakes to make sure FF is fully engaged!
From this data it seems closer to 200 rpm would be most ideal, but that's splitting hairsKeeping the bike rolling at about 150rpm I guess roughly 20 kph looks ideal to cool.
Indeed and I'm glad you didn't give up on me I always tell people that sooner or later I do the things I say I'll do, but it definitely winds up on the later side than the sooner side.sketchism wrote:This photo gets me excited haha, been a long time coming!
Thank you for those numbers and calulations about the wheel size.justin_le wrote: Yup, this is true, but for sure the gains in reduced heat generation with the smaller wheel needing less torque for the same thrust will more than offset any slight reduction in heat transfer from the stator as a result of the higher RPM causing the FF to loose effectiveness. The smaller wheel diameter would still win the day for staying cool. For instance a 20% reduction in wheel diameter means a 20% reduction in phase current, which means a ~36% reduction in the copper losses and heat generated inside the motor. Even if you loose 10-20% of the thermal conductivity of FF from centripetal forces, you'll still be running cooler overall than with the big wheel.
It's been discussed at some length, I haven't done statorade tests on a cromotor directly yet but you can easily extrapolate from this information on MXUS motor with 45mm wide stators:Offroader wrote:Does anyone know how much statorade should be added to a cromotor? This is a 50mm wide stator? I'm sure this was discussed before but I can't find the info easily. Or should I do the no load current and keep adding?
Correct, it really doesn't matter at all where you put the hole as the Statraode will find its way to the magnets. You just want to avoid drilling into the windings if you are making the hole with the side plate still attached.Does it also matter if you drill the hole to fill it towards the center of the hub motor? I assume it doesn't matter where in the motor it is squirted in from because it finds its way to the magnets?
At one point I was using thermal paste on the side covers thinking that this would help with conducting the heat sideways from the rotor ring to the plates, but even though it initially seemed promising for making a seal we found that the the statorade eventually started to leak out. So no I don't recommend using thermal paste for this. All of our experience with silicone indicates that it seals perfectly with a seal that holds up fine, even if you apply it just on the outside of the seam after the motor has already been assembled.When sealing the side covers, I read where it was recommended to use thermal paste instead of silicone. Do you think it is a good idea to use thermal paste and the thermal paste will be sufficient to stop the leaks?
That test is still coming, but in the meantime I already had a nine continent 2707 motor setup in the tunnel in order to do some repeating of the statorade QTY vs conductivity graphs with this hub, and at the 6mL statorade fill level I decided to run exactly the same 14 point conductivity vs. RPM sweep as I did on the Cyrsalyte H3540 motor to see if it also showed the same decreasing conductivity from 200 to 366 rpm before curbing up again.Cowardlyduck wrote:What I would love to see, is that same test conducted with a lot more FF added. If it's the centrifugal forces flinging it out, would adding 15ml, or even 20ml reduce or even eliminate the disconnect at higher rpms?
Yes that's exactly what I'm thinking assuming that the hypothesis is right, and it will be pretty easy to test and confirm once I get the H3540 motor back in the chamber again. In the meantime though, I completed a full full round of tests with the Nine Continent motor while that was set up, going from 0 to 18mL of statorade in 2mL increments at 100, 200, 300, and 400 rpm, and also doing much higher datapoint tests at 0mL, 6mL, and 18mL of fluid. And then at the end I also added Hubsinks to the mix so that we could see the effect there.macribs wrote:That is very interesting. Could a possible solution be to build up a seem/bead of silicon to help prevent the gap from being large enough to allow for statorade to be flung out from the gap between magnets and rotor?
Here's what we see with the Nine Continent motor at least, which as you saw from my last posts had less of this centripetal flinging effect than the Crystalyte H35XX hub. With 6mL you see this decrease in conductivity at 300 rpm before it increases again at about 450 rpm. But with a full 18mL Statorade added, there is no such effect. At the lower <300 rpm speeds there is basically no improvement in thermal performance to the much higher statorade fill. Here is another perspective showing the core to shell conductivity at at 0mL, 2mL, 4mL... to 18mL of Statorade. It's impressive just how much of an effect even 2mL has at the low 100 rpm point, but then this clearly ceases to do much as the motor spins up faster. Both the 4mL and 6mL fills sortof plateau around 300-400 rpm, while with >8mL the conductivity always increases with motor speed. What is really interesting is that it seems to max out around 10-12mL, and then the thermal conductivity actually decreases a bit with the 14,16 ,and 18mL fill levels. You can see this really clearly in the same data here but plotted with mL Statorade on the 'X' axis, and each of the 100, 200, 300, and 400 rpm data sets. I've got no idea at all why this would be and can't visualize a scenario where higher amounts of ferrofluid would have the effect of reducing the thermal heat transfer. It's not big enough to worry about overfilling, but it's a curiosity that I"m curious to see if will be replicated with other motor series.Cowardlyduck wrote:What I would love to see, is that same test conducted with a lot more FF added. If it's the centrifugal forces flinging it out, would adding 15ml, or even 20ml reduce or even eliminate the disconnect at higher rpms?
I'm glad you picked up on this oddity too because it also struck me as unexpected with no obvious explanation. Have a look at the more recent test data with the 9C motor and just 2mL of fluid. At 100 rpm it really does manage to conductively bridge the air gap pretty well, almost doubling the heat flow from 2.3 to 4.4 watts/degree. Not surprisingly at at the faster speeds of 200 and 300 rpm the fluid is squished flatter against the magnets and has decreasing effect, with just a 0.5 w/degree boots at 300 rpm. It looks like this trend should continue and that at 400 rpm the 2mL of Statorade would barely touch the stator at all and the conductivity should match that of no fluid fill, but this isn't at all what happens. It then picks up again with a 1 watt/degree boost at the 400 rpm point.Punx0r wrote:It's interesting to see the thermal conductivity plot flat-line or dip and then increase again with increasing motor speed.
Is it possible that the ferrofluid is being drawn back into the air gap at higher speeds due to a venturi effect? Or that the increased air turbulence is causing the ferrofluid to foam and re-enter the air gap?
I can't tie up the wind tunnel for months or years doing this, but I am planning at some point to do this kind of experiment just running on the roof of our building so that it's also exposed to varying outdoors conditions too, and then after so many hundreds of hours take it back down to the lab and see if there's been any change. I'll probably run 2 or 3 motors otherwise identical but with different max temperatures, say 80oC, 100oC, and 120oC, and then we'd be able to learn where the thermal rollback should be set from a Statorade longevity perspective.Cowardlyduck wrote: Another totally unrelated test that might be worth doing if you can is a longevity test, to test how long we should expect the FF to last in our motors before needing a top up. Would it be possible to setup a motor that runs through heating/cooling cycles and leave it running in the wind tunnel for a number of weeks or months to simulate 10's of thousands of km's of usage?