Definitive Tests on the Heating and Cooling of Hub Motors

[justin_le]

Justin,
What's the most time or mileage you've run FF in a motor? My only concern about running FF is the life of it.

So far I've been clocking about 150km / week with it on my own commuter bike and that goes back almost a couple months, so we're upwards of 1000 km at this stage. I'll be swapping this motor out soon for a my hub with drilled out side holes to see how well it behaves in the vented/exposed motor where it will inevitably mix with rain water in this wet time of the year. But I haven't yet had a chance to setup the continuous running test jig operating 24/7, been focused on some other tests lately but I will get into this.

I have a side question...
if you get FF on permanent magnets, how DO you get it off? will it dissolve in brake cleaner or acetone? what washes it off or breaks it down?

Oh that's a great question that I totally forgot to address. The stuff we've been using comes off super easily with an absorbent paper towel. I've now filled and cleansed out close to a dozen different motors with the Statorade during the course of the wind tunnel testing and cleaning has never posed any challenges. The capillary forces pulling the fluid into cloth fibers are much stronger than the magnetic forces holding it back. There's no need for any solvent dilution or anything like that unless you wanted to get it squeeky clean.

What's also nice is that unlike oil fill, there's no mess in disassembling the hub with the small amount of FF. You take it apart and 95% of the fluid is right on the magnet junctions. Wipe it off with a cloth/towel, then wipe the bit that's left on stator, and the hub is pretty clean again. Removing actual iron/magnetic particles from a motor is a real bitch, this is nothing like that.

 

[John in CR]

...The stuff we've been using comes off super easily with an absorbent paper towel. I've now filled and cleansed out close to a dozen different motors with the Statorade during the course of the wind tunnel testing and cleaning has never posed any challenges. The capillary forces pulling the fluid into cloth fibers are much stronger than the magnetic forces holding it back. There's no need for any solvent dilution or anything like that unless you wanted to get it squeeky clean...

That's awesome and surprising news. I figured it would be tougher to get off the magnets than iron dust, which has been a royal pain every time I've had to do it. I need to put a list of parts I need to buy from your shop and get my order in.

Since it cleans off easily, I'll go straight to one of my high efficiency hubbies. It's magnet backing iron is cast in a thick aluminum shell, and I figured out a simple way to give it some heat transfer fins in the center when I cut off the rim and give it spoke flanges converting it from a scooter hubbie to an ebike hubbie. Miles' spreadsheet predicts peak efficiency of over 92% at just 400rpm, and it makes only 1kw of heat at 100Nm of torque if it's kept cool enough, so Statoraid and more than doubling the external surface area (all between and including the spoke flanges) should make it a hell of a 10kg ebike hubbie. The only minus I see is the low pole count gives it only 20 "magnetic wipers", but hopefully the low tooth count puts the 54mm long "wipers" in better thermal contact for longer.

 

[spinningmagnets]

I think that drilling out one of the screw holes for the disk brake so that it goes all the way through is probably the most ideal. Then the port naturally stays covered when the disk is screwed on

Here are pics of doing that to a Bafang BPM to create a port to fill with ATF:

https://endless-sphere.com/forums/viewtopic.php?f=16&t=51237#p758155

 

[justin_le]

@Justin_le a while back there was a thread about adding an impeller to help cool the hub motor. Any thoughts on that?

Yes, I touched on it here:
https://endless-sphere.com/forums/viewtopic.php?p=1112026#p1112026
and here
https://endless-sphere.com/forums/viewtopic.php?p=1112341#p1112341

Well while cleaning up around my bench the other week I stumbled on a 3D printed 'under disk impeller' device that Pendagron8000 had shipped here earlier this year but I thought I had misplaced, it's one of these devices that is form fit for the disk mounting boss of a Crysalyte 'H' series side cover, as detailed here:
https://endless-sphere.com/forums/viewtopic.php?f=2&t=48753&p=1031080#p1030705

So I decided to use this opportunity and do a similar set of comprehensive comparison tests as what I'd done with the MXUS motor, only using the Crysatlyte H3525 hub.

I drilled out six 3/4" holes on the disk side towards the center, and nine 3/4" holes on the freewheel side towards the perimeter. This is a much smaller total drilled out area compared to what I did with the MXUS motor and is more representative of the kinds of mods people have been typically doing.



As a matter of curiosity, I also tried making radial facing scoops to more explicitly capture forwards moving air into the disk cover holes in order to see if that had any better or worse characteristics to the 'impeller' system. This was done by cutting sections of a 90 degree bent elbow of PVC conduit pipe and gluing them to the side plate like so:


And the whole thing was put in the wind tunnel, run with ~ 100 watts of heat generation until steady state reached at 4 different speeds and RPMs, and run first with the vent holes taped up, then with the vent holes open but no external air flow enhancers, then with the under disk impeller added, and finally the radial scoops on there instead.
View attachment 6

And what do we see? Stock motor has a resistance of 0.61 deg/watt at 10kph dropping down to 0.35 deg/watt at 40kph. With the 3/4" holes opened up on both side covers, this drops by about 0.13 deg/watt across the speed range. Both the radial scoops and the U.D.I have almost identical effects, showing very little improvement at all over just the plain holes at the lower 10-20 kph speeds, but becoming noticeably better at higher speeds. At 40 kph, the bare holes have a resistance of 0.22 deg/watt, adding the U.D.I. improves this by 20% to 0.183 deg/watt. At 40 kph the scoops meanwhile improve on bare holes by 24%, to 0.177 deg/watt
View attachment 2

Now for fun how does the addition of Ferrofluid affect the results? I then repeated the tests with 6mL of Statorade added to the hub, in one case with the holes taped up again and in another with the holes open and the U.D.I. attached. In this experiment, the performance of Statorade with no vent holes at all was substantially better than the vented motor at lower speeds, even with the flow enhancers, and it's only the final test at 40 kph where the vented motor with scoops or impellers catch up, and look like this beyond 40kph speeds they will cross over to be more effective:


And as with the case with the MXUS experiments, when we combined the Statorade PLUS the vented motor with flow enhancement, the overall effects combine and we get excellent low speed and high speed thermal conductivity. At 40 kph, we're down to 0.117 deg/watt, versus 0.349 deg/watt on the stock motor, so a factor of 3 total improvement in heat flow.

The results in tabulated form are here

 

[madin88]

thanks for the further tests.
when using statorade and drilling additionally holes to the covers, it will improve things by another 50-100% (hopefully i'm right this time with the numbers :lol: ), but how would it look after some miles in real world with dust and small debris inside the motor gum up with the FF?
IMO this is something you do not want to have so either statorade or drilling, but not both.

look what i have done with a MXUS 45H rotor:

i turned off almost 1kg of material and increased the surface area. the back iron now is about 2,5-3mm thick at the bottom of the fins. the fins are about 3mm deep. since a paper clip does not show magnetism on the outer surface, the thickness should be sufficient.
with custom made sidecovers with integrated spoke flange (similar to the prototype of Justins Grin Tech motor, or Bionx), i believe the MXUS 3000 could be turned into a 7kg motor making hub drive MUCH more attractive 8)

 

[danielrlee]

Justin, excuse me if this has already been discussed, but have you observed any efficiency gains when running a hubmotor with FF? With the ferric element of the fluid directly coupling the magnetic field to the windings, rather than it being weakened in bridging an airspace, does this make for more efficient operation?

 

[macribs]

@Justin again thx for all your hard work. I bow to you in respect. Once again we see that statorade is the one stop solution for cooling hub motors.

@Madin Nice work, I really wonder how those cooling fins would affect the colling when filled with statorade. Any way to calculate the increase in outer surface after milling the fins? Have you considered removing some of the flanges as well, like those CL motors? I am guessing it could be close to another 0.5 kilo or so less weight. With weight loss like that you need to consider tubeless ghetto style, and the lightest 2 ply tire you can find. We are talking significant weight savings here madin. Would love to see you take it to the max and shave off every little bit of material you can. Using light small rim, light 2 ply tire and the total weight savings over stock motor in m/c rim/tire would probably be a major contributor to better handling.

 

[Samd]

I drill the shoulders on my mxus motors right next to where you would expect to find the stator/rotor gap.
I've seen no risk of the burr entering if done slowly with a piece of tape on the drill bit for a depth gauge at 5mm.
There's about a 4mm gap once through so if done carefully there is no risk to the windings beneath from pushing the drill bit down inside.
It's so much faster than opening the motor and can be sealed with a gob of black silicone goo in a few seconds after injection.
I know you can crack the motor open, but you have to accept the risks and inconvenience associated with that.

 

[spinningmagnets]

Thank you so much for doing this and posting the results. A single test is truly worth a thousand opinions.

I feel vindicated that the impeller (which has a lot of room for design improvement) is a worthwhile addition when a hubmotor is ventilated. However...I am incredibly impressed by the Statoraide in a sealed motor.

madin 88 posted pics of the insides of the new BionX DD motor, and...surprise, surprise...they painted the insides of the sideplates with black paint. Maybe only a few degree of temp improvement, but...costs only pennies and no moving parts to wear out, why not?

For me, I am very impressed by black paint on the insides of the sideplates, plus FF in the magnetic gap. This is a game-changer.

 

[John in CR]

Samd, No need to put the hole there if only for adding the Statorade. It will go straight to the magnets where it belongs all by itself. That hole location with a plug is a great idea for those who ride in heavy rain to be able to drain water that gets in.

 

[KalSteve]

Hey Samd, I see you are using the Ferrotec EFH1. I was looking at giving that a go as it is relatively easy to get hold of and fairly cheap. Had a look at the specs and decided not to when I saw the fluid carrier is a light hydrocarbon with a flash point of 92 C. In a sealed hub if everything goes wrong it could make a bomb. Any reports of FF cooled hubs exploding yet?

 

[Cowardlyduck]

In a sealed hub if everything goes wrong it could make a bomb.

:lol:
Everyone using that particular FF please point a camera at your hub when testing.
An exploding hub is something I'm yet to see. :lol:

 

[John in CR]

thanks for the further tests.
when using statorade and drilling additionally holes to the covers, it will improve things by another 50-100% (hopefully i'm right this time with the numbers :lol: ), but how would it look after some miles in real world with dust and small debris inside the motor gum up with the FF?
IMO this is something you do not want to have so either statorade or drilling, but not both.

look what i have done with a MXUS 45H rotor:

Nice work on your rotor, both nice additional surface area as well as weight reduction...A kilo, WOW! Are you going to remove some unnecessary spoke flange between the spoke holes too? While it gives up some surface area, especially if you direct more air flow toward the motor, it would help that work between the flanges pay the most dividends.

I'm with you on the either/or thing for holes and FF. It may be easier than I thought to clean out, but I only want to open motors once or never, at least until years later when the bearings wear out.

FWIW, the ultimate proof that you haven't changed any necessary portion of the back iron is verifying that the Kv hasn't changed.

I hope 50% holds up. I'd love to see Justin run one test with double the heat input and see temperature stay at or below the control test. Even better, he said heat transfer increased by a factor of 3, then apply 3 times the heat to the system. I realize the thermal resistance proves better heat transfer, but I think my problem with it being used to quantify heat transfer is more than just semantics. This is a case where I'd really like to be proven wrong.

John

 

[justin_le]

thanks for the further tests.
when using statorade and drilling additionally holes to the covers, it will improve things by another 50-100% (hopefully i'm right this time with the numbers :lol: ),

That's correct in that it will increase the ability of the motor core to shed its heat to the outside air by 50-100%. Just don't phrase it in a way that says the motor performance will increase by 50-100%, that would be quite misleading! Motor performance for the most part will feel just the same, but the temperature of the stator will be quite a bit cooler in use, and as a result you can put more continuous heat into the core without causing it to overheat.

but how would it look after some miles in real world with dust and small debris inside the motor gum up with the FF?

That's a great question that only by testing it out will we know. Here in Vancouver the air is clean and there is practically no dust and debris to get in, just rain, but if someone goes out trail riding in the dirt then that would be a better way to know. People have been using the phrase "gummed up" quite often on this thread but I think that's a bit of a loaded statement, we're talking just a few mL of liquid, and if it were to get contaminated with a bunch of foreign particulates I doubt there would be any effect on the motor. It would just get pushed aside and would loose its effectiveness at cooling. I'll have an update on rain + statorade at the end of the holidays but I can't do any dust testing really till summer.

IMO this is something you do not want to have so either statorade or drilling, but not both.

I wouldn't be one to say that based just on speculation. I would bet that if you already have drilled holes in the motor, then there is little to loose by adding Statorade as well. You'll see another large improvement in the cooling, and in a worst case scenario where the few mL of Statorade ceases to flow freely then you'd be back to a the same general motor you had before the addition, with a small amount of extra gunk in the motor that doesn't do anything good or bad.

look what i have done with a MXUS 45H rotor:
i turned off almost 1kg of material and increased the surface area. the back iron now is about 2,5-3mm thick at the bottom of the fins. the fins are about 3mm deep. since a paper clip does not show magnetism on the outer surface, the thickness should be sufficient.

That looks absolutely awesome, and yeah the back-iron thickness in the DD motors is so often quite a bit thicker than needed for magnetic reasons because they use it structurally to take the side cover screws. Just be sure to paint the steel with a good anti-rust paint since that freshly exposed steel will rust pretty quick.

Hey Samd, I see you are using the Ferrotec EFH1.

Just to be clear the ferrofluids made for educational/demo purposes are definitely aiming for different properties than the FF's for commercial use. The flashpoint of the two flavours of Statorade for instance are listed at >200 oC and >240 oC, and have quite a bit lower saturation magnetizations than the EFH1. So if you are going to be purchasing independently, I'd recommend getting from a supply intended for voicecoil speaker cooling or similar rather than for "gee wow" demos. Flathill had a lot of comments earlier in the thread about the range of FF types out there.

I feel vindicated that the impeller (which has a lot of room for design improvement) is a worthwhile addition when a hubmotor is ventilated.

I guess the point I was trying to make is the fact that it's an "impeller" is pretty moot, since non-impelling cups with a similar exposure area had the same or better performance improvement on the vented holes. The impeller profile is actually shaped to pull air "out" of the inside diameter holes, while I'd wager a bet with anyone that if we put a smoke stream in the tunnel that this side became the intake holes instead. (Still don't have that fog machine darn it, otherwise I'd have done that as a matter of course).

With the ferric element of the fluid directly coupling the magnetic field to the windings, rather than it being weakened in bridging an airspace, does this make for more efficient operation?

No, it makes zero nadda no difference whatsoever in the electromechanical properties of the motor. We're talking having a fraction of a gram of nanopartical magnetite (which is pretty weakly magnetic in the first place) distributed over like 200-300cm^2 of air gap space. I think I've mentioned this earlier in the thread but might as well repeat it.

 

[themadhatter106]

After a month of daily riding with minimal air holes (4 x 1" holes on each side) I opened up the motor to take a look at the fluid and it doesn't look contaminated at all.



I think FF and air holes is a winning combination.

Sure, it will probably eventually get contaminated, the more and larger air holes you have the quicker this will happen. If it gets contaminated just clean it out and add more. I regularly pull 6kW on my 9C with it staying below 100C no problem.

 

[macribs]

Great to hear from first hand experience how ferrofluid works with vented side covers. Maybe now the "gum up" part will loose traction here?

 

[madin88]

@Madin Nice work, I really wonder how those cooling fins would affect the colling when filled with statorade. Any way to calculate the increase in outer surface after milling the fins? Have you considered removing some of the flanges as well, like those CL motors?

yes, the surface area should be about 80-100% larger because the fins have the shape of an equilateral triangle
for stability i think its not a good idea to remove more material from the spoke flanges. i don't know what kind of steel the rotor is made of and the holes for the sidecover bolts and the spoke holes already makes it weaker at some areas..

I think FF and air holes is a winning combination.

depends on the situation. imagine you ride in dry areas with lots of dust awhirl from the wheels. the FF would here probably gum up in no time and convert the rotor into a grinding machine.
for myself i prefer to have a waterproof and long-living motor and this is only possible when keeping the motor closed.

@madin Wow! What tool did you use for that?

IIRC we worked with a turning tool used for cutting threads

 

[LI-ghtcycle]

Great info on this thread! Spinning Magnets mentioned the FF to me, and it sounds like something I would love to try!

Is the Statorade Kerosene based?

Thanks again for doing this Justin! You must have so much fun testing all these things! Wish I had your job, keep up the good work!

 

[LI-ghtcycle]

Thanks!

 

[Papat92]

Just did the Statorade mod to my MXUS 3kW mounted on a Fatbike.
Drilled a small 5mm hole in the flange and injected 8ml of ferrofluid type A.

The difference is simply huge!
I can keep full throttle for many kms before reaching the temp limit while before I had to lower down the input only after one km or so.
Also the cooling down is much faster after lowering the power.

The Fatbike is mainly used to commute to work via 16km of sand on the North Sea beach.

MXUS fed by an Adaptto max-e with quite high settings: 140A battery and 350A motor... battery is 18s-20Ah lipo. 10kW peak power!

Thank you very much Justin for this excellent work!

 

[LI-ghtcycle]

thanks for the further tests.
when using statorade and drilling additionally holes to the covers, it will improve things by another 50-100% (hopefully i'm right this time with the numbers :lol: ), but how would it look after some miles in real world with dust and small debris inside the motor gum up with the FF?
IMO this is something you do not want to have so either statorade or drilling, but not both.

look what i have done with a MXUS 45H rotor:

i turned off almost 1kg of material and increased the surface area. the back iron now is about 2,5-3mm thick at the bottom of the fins. the fins are about 3mm deep. since a paper clip does not show magnetism on the outer surface, the thickness should be sufficient.
with custom made sidecovers with integrated spoke flange (similar to the prototype of Justins Grin Tech motor, or Bionx), i believe the MXUS 3000 could be turned into a 7kg motor making hub drive MUCH more attractive 8)

Wow, now that's what I'm talking about! 8)

Now imagine that you are using that hub as a mid-drive, and now you can take off that "unnecessary" spoke flange altogether, might also make for some interesting options for active cooling the outside, just brainstorming, but what if there was a contained ring around that outer hub, with a coolant, just enough that drag is minimal, or maybe that is just overly complex (I tend to start there and get more simple in my designs later lol).

 

[amberwolf]

Those "unnecessary" flanges are also heatsinks, so you probably wouldn't want to remove htem. But if youdidn't need them to be structural, you could groove them for more surface area, perhaps .

 

[LI-ghtcycle]

Those "unnecessary" flanges are also heatsinks, so you probably wouldn't want to remove htem. But if youdidn't need them to be structural, you could groove them for more surface area, perhaps .

Good point!

I drew up a design with a "water bath" cooling system that would make use of a shallow sump and radiator with the idea of using the outside of the hub as a crude "water wheel" to splash water on the outer surface, but then I realized any cooling that would accomplish would probably be at the cost of too much complexity and resistance (if it was able to "pump" water, then it's too much drag).

Air cooling a sealed hub motor is very interesting and practical in my thinking, I am really liking the performance with just the addition of a tiny amount of liquid like the Statorade added to a sealed system, and wonder if would be realistic to expect that liquid to last for the life of the bearings, i.e. all things being equal, under normal use, with-in the motor's designed continuous power, if so, I believe this will possibly give equal footing to our beloved hub (or "frock" motors as some would say :wink: ) with more easily cooled in-runners!

Maybe combining greater surface area to the outside of the hub (perhaps using some easily mounted flexible aluminum fin material that would require no machining, just clamp on would be most cost effective as Justin has mentioned, but even more so like this material

I have always hoped that a more efficient hub motor that cooled better would allow us to get similar performance to what is possible with the very small high RPM Turnigy or Astro motors, but sort of like how a "N/A big block V-8" performs compared to a supercharged 4 Cyl that is lighter, but also working at much higher levels of stress?

Sorry if this is getting too far off topic.

 

[Cowardlyduck]

Ordered some Statorade the other day. I got one of each type, so will try and see if there is any difference when it arrives.

I will be adding the FF to my already vented and fan cooled motor's. I realise that's a bit risky in terms of contamination, but so far everyone's (and mine) aversions to doing it have been theoretical only.
I don't mind being a bit of a guinea pig and seeing how it goes...hopefully the worst that happens is a muddy/oily mess. :lol:
The fans should then not need to be used as often, meaning less dirt/dust being sucked into the motor, and when I do run them cooling performance should be further improved meaning they don't need to run as long.

The 2 motor's I will be adding it are my HS4080 and Leaf Motor '1500W' and are vented like this;
HS4080:

Leaf Motor:

It will be interesting to see how much of the FF is still contained in each motor, but it won't really be possible to measure. I think the Leaf motor will be more inclined to release the FF given the holes are closer to the perimeter. There might be an optimal amount less than 5ml that still improves thermal performance, but also does not fling out the holes.

I also just bought some thermal grease to seal up the side covers:

I was just going to get the cheapest stuff I could find, but this stuff has a thermal conductivity of >4.63 W/m-k so should be pretty good and hopefully a bit more effective than the cheap stuff.

Cheers

 

[macribs]

CD and chance you could run the motors on stand kind of like Justin did? That way oil drippings will tell if statorade is leaking or not.
Aside from running motors on stand it will be hard for you to collect data. If you log motor temp as well you can find temp before statorade and after as well. Cos I don't expect a full test lab with wind tunnel like Justin, your could be as simple as putting the bike on a beer box and make sure rear wheel is lifted from the ground. If you got a working thermistor it would be nice, if not use the semi-scientific "touch to the skin" :lol:


Very excited to see how statorade works out for you and your vented motor, with your fans we might get to see optimal cooling