Definitive Tests on the Heating and Cooling of Hub Motors

[speedmd]

They solved the spoke issue a half century ago. Key hole the spoke holes worst case. If your cutting standard extrusions for cooling fins, you could notch the fins also at the spoke locations to allow spoke access.

 

[Kodin]

Pretty much exactly what I had in mind. ID probably attempt it with a chunk of ALU one of these days but not this year. Too many projects to build a rotor from scratch.

 

[macribs]

Placing heat sinks in between the flanges of the motor should be an easy mod that most people can do on their own, as it does not require any special tooling. A good adhesive and you should be golden. The heat sinks would be pretty well sheltered between the flanges and with ferro fluid inside the hub as well it should make for a significant cooler motor even hard pressed in hot weather.

Get a round object and place a flat object against it, like a bottle or a jar on a counter top. This is just like placing flat heatsinks upon the rounded surface that is between the flanges, the surface contact would be poor. As much contact as possible is important for thermal pathway, but perhaps this is an issue of least importance on your suggestion.

"Good adhesive" Is also a very broad and important topic, which adhesive is used and it's thermal properties are both important topics, not any old glue or epoxy will do. The heatsinks will be under lots of centrifugal force as the wheel spins, might be a bad time if a few heatsinks happen to come off. Good adhesion for adhesives usually requires very well cleaned, paintless surfaces, not super simple and will surely be at least mildly involved. In addition, what do you do with the wheel if you break a spoke? The heatsinks would block the spoke head paths. The problem with this whole adhesion approach is that in order to do it effectively you'd probably need to start with an unspoked hub, but once the heatsinks were in place how would you lace the hub? Really huge flange with high spoke hole drillings and low profile heatsinks? Straight pull spokes?

These have been some of the engineering problems I have tried to find solutions to for the past few years. The conclusion I came to was that rounde bottom heatsinks could be put in place followed by a large hose clamp to keep it all in place. The design that I've been most interested start off with two hose clamps (one placed at the edge of each hub flange) and a sort of plastic wind scoop that goes over the heatsink/hose clamp object. The hose clamps would slide into the wind scoop object. Part of the goal in it's design that I had in mind was to paint the hub black, the heatinks, and the wind scoop itself, the reason being that it would hopefully mask the absurd aesthetic appearance that would otherwise be very easy to spot. I've imagined several different ways of installing the heatsinks with the round hose clamp. So far my favorite approach is to use thermal tape on the bottom of the heatsinks so you can enhance thermal pathway and keep the heatsinks in place while you put the hose clamp thing on. Naturally, something to consider, is that all this would be fairly costly and increase the weight of the motor.

I am sure there is a lot of expansive exploration to do on this ferrofluid subject as well as a vented hub motor, but I think that if were going to compare apples to apples, a vented motor with some version of a external fan blade would be worth comparing to finned oil/ferrofluid motor.

All valid points BOS. And surely a concern. But lets start in the other end. What is the goal of the heat sink? To add thermal mass and larger surface area. Will that require us to lay donw heat sinks wall to wall between the flanges? Probably not when ferro fluid is added inside. If a heat sink with a large base is chosen I am sure one could even freehand the curve into the base with a dremel. Or simply get custom made heat sink with the correct curves. There are various over clocking shops that do custom computer heat sinks.

How to secure the heat sinks? My initial thought was to keep it as simple as possible. Wrap to loops of wire and tighten firmly.

Adhesive? Use what people have had success with when replacing magnets. A little dremel work where the heat sinks goes to rough things up, wash with acetone as it vapor. Glue it together inside a vacuum bag, two sheets of building plastic or whatever and remove air.

If spoke(s) need replacement pry the heat sink loose and replace spoke(s), pick up your dremel and repeat the process.
I am thinking even 4 heat sinks between the flanges will work.

But my very first step will be to try out ferro fluid and some flat black paint and a little "red" to prevent corrosion inside the hub. I also like the one tip of venting the hub through a vent line that keeps pressure from building up.
For me ferro fluid might be all that is really needed, considering the Norwegian weather.

Vented side covers is not my first choice due to the weather, and the amount of salt that is used on our roads 4-5 months of the year.

 

[Kodin]

The rotors are already decent heat-sinks; it's the thermal resistance of the air inside the motor that makes the motor overheat. If you use ferrofluid or oil bath, that resistance drops significantly. By heatsink fins, I was thinking one more flange in the middle to act as a 3rd "fin", with slots cut through every spoke hole. This would be a new rotor with magnets transferred over or new ones glued in. No thermal epoxy would be as effective as one contiguous piece of aluminum. Further adding complex finned heatsinks en-masse to the rotor will also add weight and momentum, not to even get into balancing issues... At some point you need to draw the line at what is effective for the effort required as well as the drawbacks involved. Let's see what we get from further ferrofluid and/or oil bath testing then see whether outer case modifications can make it even better. The big one for me would be if a small amount of ferrofluid improves cooling without increasing drag. If that's the case, that's my perfect thermal medium.

 

[macribs]

@Kodin Absolutely agree - ferro fluid seems to be the most promising way to go right now. I wonder if Justin will test other ferro fluids or maybe the viscosity is not an issue?

Hopefully Justin and his team gets a hold of a Hi-speed camera. A peak inside a rotating hub motor with ferro fluid inside would an ES blockbuster.

Hm still no takers for the ES-TV? The ultimate e-bike channel......

 

[Kodin]

Wait... Wouldn't that be counter-productive to the cause? I thought the idea was that after all your blood sweat and tears you actually get off the couch and ride the bike you built rather than watch TV... :wink:

Has anyone tried running a vent through their axle before rather than putting a hole asymmetrically on a side cover somewhere? (Hopefully some of you have already experimented with oil or water cooling?)

 

[fechter]

One nice thing about ferrofluid is the magnets will tend to keep it in place. You just need enough to fill the gap between the magnets and the stator.
It shouldn't run out like oil if the bike is laid on its side. I suspect you could get ferrofluid in different viscosities. We'd want the runniest kind.

A ferrofluid (from the Latin ferrum, meaning iron) is a liquid which becomes strongly polarised in the presence of a magnetic field. It is a colloidal mixture comprising extremely small magnetic particles suspended in a synthetic oil. The particles are coated with a a soap or detergent to prevent them from clumping together.

Here's a place that has something that's probably close:
https://www.ferrotec.com/products/ferrofluid/audio/

More possible sources:
http://www.wondermagnet.com/ferro.html
http://www.magnet4less.com/index.php?cPath=24_36

One thing is that if it doesn't work out, it will be quite difficult to remove all of it. Stuff is really messy and hard to get off magnets.

 

[Kodin]

Easiest way I found to remove the sludge, (essentially equivalent to coarse ferrofluid) off the pickup magnet in my transmission was to use a more powerful magnet to pick it up. (Wrap it in something like a plastic bag so it doesn't stick permanently to the more powerful magnet). Just watch your fingers; some of the small magnets I have are only 1/4" diameter and specced to exert 7 pounds of pull on contact with a piece of steel. That's a pretty significant PSI to have part of your finger or hand trapped between, especially if they snap together suddenly. Also, wear eye protection as I've seen magnets shatter when snapped together.

 

[diff_lock]

You could try to use a strobe to observe what is going on with the fluid before going for a high speed camera.

The Sony RX-100 IV does 1000fps with a couple seconds of capture time and pretty good res (1136 x 384).

 

[Kodin]

There's also some really nice global-shutter machine vision cameras out there that can do high speed, and you don't get those terrible tearing and warping artifacts from high speed movement across the frame.

 

[diff_lock]

There's also some really nice global-shutter machine vision cameras out there that can do high speed, and you don't get those terrible tearing and warping artifacts from high speed movement across the frame.

How high how much?

Examples?

 

[Kodin]

Example from kickstarter:
https://www.kickstarter.com/projects/1521768569/edgertronictm-the-first-affordable-high-speed-vide

That one is USD $5495, can do 17,000 FPS or so. There's other models out there that can A: do 1080P, and B: do high-speed at higher resolutions as well. I'm still trying to track down the article I read a while back where someone used one to film multirotors in-flight.

Either way, he was using a Point Grey unit for his stuff.

http://www.ptgrey.com/

 

[justin_le]

So, time to give all you waiting folks an update. I got a lot of wind tunnel tests done the last few weeks, and things are verrry interesting. First off, after opening up the hub motor to clean it out from this first run with ferrofluid I noticed something that could be telling. Of the ferrofluid that didn't fly out from the unsealed gap in the side covers, well most of it stayed in these perfect ridges right at the junction between each pair of magnets, where naturally the field gradient is the strongest:



This immediately suggests that the reason why so little ferrofluid seems required to conduct the heat away. If it concentrates just as small ridges between each magnet, then these ridges could act like "thermal wipers" as they circle around the stator core, scraping heat from the surface while presenting very little viscous drag to the system since the actual fluid contact area is quite small. Until we get a camera in the air gap it'd be hard to say that's exactly what's going on, but it seems pretty plausible and certainly paints a pretty picture, kinda like a wave soldering action.

On the rest of the motor, there was obviously ferrofluid stains on the stator iron and on the side cover flanges where it initially spilled out of, but nothing at all present further up the side plates near the axle, bearings, wires etc. confirming that all the leak was from the poorly sealed side plates.

Next up I'll want to do the reverse, rather than overfilling the air gap and letting it leak we'll keep adding say 1-2mL at a time and then measuring the thermal transfer coefficient, and find out at what point you get diminishing or no returns.

So this test took a little while. After emptying out and cleaning the motor, I was careful to properly seal the side plates and viewing window this time with silicone and then drilled a small ~3mm hole in the side plate for the fluid injection. This was applied with a syringe, first 1mL at a time up to 6mL, and then in 2mL increments until I ran out.



I did all the heat transfer tests with the motor spinning at 200rpm and the wind tunnel at ~20kph, with 150-160 watts of heat being generated in the stator core from a 30A field weakening current from the controller.

Here's the raw numeric data along with the calculated conductivity values. The shell to ambient varies a bit because in some cases I had the back end of the wind tunnel blocked by an obstruction so the internal air flow was slower, but that doesn't matter too much because the numbers of real interest here are the core to shell values, not the shell to ambient. With no fluid the conductivity is 3 °C/watt, adding 1mL doesn't change this at all, 2mL shows the slightest increase (3.3 °C/watt), and then from 3 to 4mL it jumps right up to 6 °C/watt and by 6mL is plateaued at about 7.


The results plotted in a graph are pure gold and tell the story perfectly.


There's really no need for more than 5mL of fluid in here to get the full thermal transfer effect. After 6, it's not just diminishing returns, it's no extra returns. But what you do get above 6mL of fluid is extra drag on the motor. In conjunction with all the thermal measurements I also ran the no-load drag test with each fluid fill, typically after the wind tunnel tests so the motor cores were still fairly warm, around 50oC. Here is the resulting motor drag at both 200rpm and 400rpm.



I think most of the variation can be attributed to the lack of temperature consistency when the measurements were made, but it's apparent that from 4mL and below there is no change in the drag, from 5mL onwards we start to see it increase, and it keeps on increasing right up to the 16mL measurement point. From this data we can easily conclude that there is a definite "sweet spot" for the fluid fill, in this case with MXUS motor it's around 5-6mL, which as allows for full thermal transfer with almost no extra motor drag. If you put more in, say 10mL, you won't see any increase it heat dissipation but the rolling resistance of the hub is increased by like 25%.

In order to find this ideal fill on other hubs the easiest approach would be to look at your no load current draw, then add the fluid in small increments at a time until you notice the slightest increase in your no-load amperage. At that point, stop.

Also, it's interesting to look at the thermal camera IR images of the side cover plates. In the first 3 tests from 0-2mL, the side plate temperatures were quite uniform, with the spot closest to the axle (Spot1) being slightly warmer.


However, with the ferrofluid above 4mL, then there was always a clear temperature gradient with the outside of the rotor and side plate (spot 3) being 2-3 degrees hotter than the middle section, just as we'd expect if the heat flux was coming to the motor shell from the perimeter magnet ring and then working it's way out from there.

also note the viscosity lowers as the fluid warms up. there is another "loosening" effect that is not temp related (less particles clumping) after being "worked"
make sure to do the drag test on a "cold" motor as-found after a fresh fill and then repeat the test after the fluid has been worked/warmed

Indeed, I could see this even with 5-10 degree temperature variations, so in order to better characterize this effect I ran the motor full throttle at 60V, first from a cold stator until it self heated, and then after heating the stator to 75oC and recorded the no load current draw as it cooled down. Here's the X-Y scatter plot of that. Going from room temp to 70 °C results in a 30% decrease in the motor drag. Since a lot of that drag is from core losses rather than the fluid, then the % decrease in viscosity from the fluid itself would be a lot higher.

Maybe you can do another run with sealant on the side covers to see if the ferro fluid is forced outside the hub via axle/wires/bearings?
I think it has been mentioned once or twice that oil filled hub will create internal pressure inside motor that will keep pushing fluid outside the motor - I wonder if the same goes for ferro fluid or will the magnets keep oil in place so much that leaking is not a problem with proper sealed side covers?

So here's the best part, after a week's worth of testing and running that motor again and again in the wind tunnel test chamber with the side plate flanges sealed (but the injection hole left wide open), there wasn't a single drop of leaked ferrofluid anywhere to be seen.

We'll also do a test with conventional ATF oil fill which splashes on the copper windings.

Coming up!

And finally we'll try to pot the stator in a thermally conductive resin and see how well that improves the ferrofluid approach by better coupling the windings to the iron stator.

Coming up!

Expect all this to take a couple months.

On track!

 

[Nathan]

Would you be able to repeat these tests on an inrunner motor and see if their is any gains also ?

 

[justin_le]

Would you be able to repeat these tests on an inrunner motor and see if their is any gains also ?

Nope

 

[BSATOM]

Does the potting make hall replacement impossible?

 

[macribs]

Amazing work Justin. Tons of kudos and mucho respecto.
From what I can see in your tests over heating is now a thing in the past. No need to drill holes or to add heat sinks.
FF seems like a Simple, well working and clean solution for everyone, should there be any reasons not to use Ferro Fluid?

So who will jump into this and sell pre loaded syringes with high quality Ferro Fluid? If all that is needed is 5+ ml there should be possible to make a buck or two selling small quantities FF in pre loaded syringes.

 

[mrbill]

Hi Justin:

Thanks for performing these tests and reporting the results. The 5-6ml ferro-fluid injection appears to be helpful and efficient, at least with a DD out-runner. I look forward to learning your results to continued testing with ATF fluid and potted stators and which method or combination provides the best long-term results.

Would you be able to repeat these tests on an inrunner motor and see if their is any gains also ?

Nope

Hi Nathan:

Although Justin's reply is curt, if you think about what he's trying to accomplish by using the ferro-fluid, you can see that benefit is moot for an in-runner, and may be counter-productive. With an in-runner the stator has good thermal contact with the outer shell of the motor. Creating a broader heat path to the magnets of an in-runner only risks exposing the magnets to higher average temperature without providing a good heat sink. The motor axle is not a good heat sink.

 

[Mammalian04]

I am trying not to clutter this thread with useless posts but wow! Nice work! Ok, I will go back to reading only and non-cluttering.

 

[flathill]

Amazing work Justin. Tons of kudos and mucho respecto.
From what I can see in your tests over heating is now a thing in the past. No need to drill holes or to add heat sinks.
FF seems like a Simple, well working and clean solution for everyone, should there be any reasons not to use Ferro Fluid?

So who will jump into this and sell pre loaded syringes with high quality Ferro Fluid? If all that is needed is 5+ ml there should be possible to make a buck or two selling small quantities FF in pre loaded syringes.

You can already buy small qtys of FF for driver rebuilds. The good one is branded "Ferrotec"

http://www.parts-express.com/catalog/pdf/2015/Parts-Express-2015-Catalog-Pg-164.pdf


Justin, later....can you fill the gaps and see how much fluid your motor takes and compare (so the magnets and stator all are smooth)? The FF should also last longer if it is not be sheared on "sharp" edges. I think I only put a little over 2mL in my ultra motor. MXUS has similar tight packed magnets but the ultra is a little tighter. Quality FF is not cheap as you know. I like the wiper idea thou. There will always be "wipers" even if everything is perfectly smooth/flush. I'm thinking mostly about your custom motor with large magnet spacing. FF may be too costly to add unless you fill the gaps by potting the magnets also, but then you need some gaps for the fasteners....so you would have to pot with dummy fasteners in place so u form thru holes. Potting the magnets for reinforcement is common in the hobby world as the RC outrunners have spaced mags

 

[Kodin]

Justin, if you have the facilities to, please confirm whether the potting affects KV and inductance of the motor. I'd expect that most thermal epoxies affect inductance, capacitance, and/or magnetic flux behavior... It'd be nice to know real-world info.

 

[macribs]

@flathill

I remember I asked about "potting" once before, but I can't remember for sure. "Potting" was covering magnets and gaps with epoxy or adhesive? I think Doctorbase did that with some 3m adhesive, I will try to find back to that post. Seemed that he had tried various blends to find the one that worked best for him.

I am really excited about FF because of the hash climate and conditions where I live, cutting vents into side covers is not really feasible. Due to heavy rain, large amounts of salt being spread on the roads in the winter and early spring.

 

[justin_le]

Amazing work Justin. Tons of kudos and mucho respecto.
From what I can see in your tests over heating is now a thing in the past. No need to drill holes or to add heat sinks.
FF seems like a Simple, well working and clean solution for everyone, should there be any reasons not to use Ferro Fluid?

Well, we still don't really know too much about the long term compatibility and stability in the motor environment, in terms of possible reactions with the magnet adhesives, the side cover sealants, the and any varnishes/coatings on the motor stator. Because there are quite a range of motor manufacturers and production processes out there the only way we'll really find this out is by having a larger number of people testing this stuff in the field on their hub motors and reporting back.

And that's pretty much the next step for us. I ordered 200 pcs of 10mL syringes last week and we're just waiting on delivery of a few liters of the compound in order to prepare a batch of easy to packaged samples for those wanting to be onboard at this phase.

Justin, if you have the facilities to, please confirm whether the potting affects KV and inductance of the motor. I'd expect that most thermal epoxies affect inductance, capacitance, and/or magnetic flux behavior... It'd be nice to know real-world info.

Sure the kV comes out inevitably every time you run the motor, but I really really doubt the presence of a filled epoxy potting will have any effect at all on the key motor parameters like inductance, kV etc. It's quite rare for materials to have any notable effect on magnet field behavior, and the few that do (iron, nickle, magnetite etc.) aren't usually present as a thermal filler in potting resins. The highest performing thermal epoxy that I've got samples of (>4 W/m.K) uses powered metalic aluminum as the thermal enhancer, and there is some slight chance that the electrical conductivity of the filler could be an issue for hi-pot isolation testing etc, but I wouldn't think for motor characteristics.

. I'm thinking mostly about your custom motor with large magnet spacing. FF may be too costly to add unless you fill the gaps by potting the magnets also, but then you need some gaps for the fasteners....so you would have to pot with dummy fasteners in place so u form thru holes.

Ha it's funny that you bring this up since that was one of the main things we were working on last week once it became more clear how the FF behaved on the rotor. Recently we were using small wooden shim spacers as a positive gap fill between the magnets


But then if the fluid sits mostly at the junction between the magnets, then that puts a thermally insulating material right under the ferrofluid and could impeded the effective heat flow to the steel. So we went for a hunt to find an aluminum metal strip as a gap spacer instead and it turns out that 3/32" tig welding rod was exactly the perfect size:


When we put FF on the gapped magnet rotor, it tends to collect around the full perimeter of the magnet, still highest between the two pairs, but not quite in the neat little ridges we saw in the ungapped scenario.


I think we will need a larger relative amount of FF in this case with gapped magnets to get the same thermal benefits, and that'll be an experiment we run in due course.

 

[BSATOM]

I am on board for the in the field FF testing. I have a new MXUS 4t that I have not used ATF in yet, but have beefed the wires. I am waiting on a 100a breaker so I can hit 7000w peak. I will watch for the post on how you will distribute the FF syringes. I suppose via Paypal.
I am still curious about FF "vs" ATF, so I am watching for that post.

OT, but I recently "watched" Justin's presentation of his ride across Canada while modding an ebike in my living room. That was very interesting. I also watched the EBR video tour of Grintech. Hell yeah I love the nuts and bolts tours like that. Those pressure control skateboards are pretty cool. I have a funny vision of people on electric unicycles peppering the Playa...

 

[Kodin]

Quick thought here, have you done any testing with thicker ferrofluid solutions to see if there's any difference? Honestly if we are simply filling until light contact is made, a thicker solution may not have any drawbacks since we're riding the threshold of thermal contact vs. drag... Any consideration toward testing different viscosity options?