Definitive Tests on the Heating and Cooling of Hub Motors

[pendragon8000]

OK, final test:

love your work mate, any notable increase in no-load power use?
and its considerably more effective than 'twice the vented cover' - as the lower delta t between vented with fan and ambient vs just vented makes heat extraction even harder. it'd be great to see some higher power tests, particularly how much more cont. power you can push with the fan vs un-vented. Shit mate, I wish I had more time - would love to come over and lend a hand with such tests!

testing is time consuming. help wold be welcome but yeah I know you're busy. I hope we can work on some projects together in the future.

I'm pretty flat out untill the weekend when I will test the smaller diameter UDI out in the real world. I am send off 3 large UDI units to different ES members tomorow on my lunch break. then picking up some black ABS filament and non contact laser thermometer. then I'll dial in the new filament incase its melting temp is a bit different and start printing those to fill the orders I have already.


I'm sorry if I'm off topic with any of my posts here. I dont want to spam.

Justin, thanks for your research and testing on this thread.

edit:
Snowchyld, no noticable increase in no load power.

 

[ebikedelight]

I notice many people claim that painting the inside of the hub side covers, black, will result in noticeably more heat dissipation. This is another subject that im not convinced is true.

I have read some opinions about this subject , from thermal dynamic specialists, who claimed there is no real noticeable benefit to doing this. ..which makes sense to me.

the inside of the hub motor assembly, is completely dark anyways and it doesnt make sense to me that painting the inside hub covers, in a setting where its already dark, will have added benefits. Also, it can be very difficult to get spray paint to stick to aluminum surfaces, especially when those surfaces get very hot and are in constant movement. IMHO..spray painting the inside of a hub motor black, is not useful, and could even result in the paint flaking off/ chipping away , under stress , and having negative repercussions in regards to the coils, magnets, etc.

 

[Arlo1]

I notice many people claim that painting the inside of the hub side covers, black, will result in noticeably more heat dissipation. This is another subject that im not convinced is true.

I have read some opinions about this subject , from thermal dynamic specialists, who claimed there is no real noticeable benefit to doing this. ..which makes sense to me.

the inside of the hub motor assembly, is completely dark anyways and it doesnt make sense to me that painting the inside hub covers, in a setting where its already dark, will have added benefits. Also, it can be very difficult to get spray paint to stick to aluminum surfaces, especially when those surfaces get very hot and are in constant movement. IMHO..spray painting the inside of a hub motor black, is not useful, and could even result in the paint flaking off/ chipping away , under stress , and having negative repercussions in regards to the coils, magnets, etc.

. Its emsitisvity and I think its not just the fact it's black but any paint or some other coatings help absorb and give off (radiate) heat. Some colors and coatings will be better then others.

 

[ebikedelight]

I notice many people claim that painting the inside of the hub side covers, black, will result in noticeably more heat dissipation. This is another subject that im not convinced is true.

I have read some opinions about this subject , from thermal dynamic specialists, who claimed there is no real noticeable benefit to doing this. ..which makes sense to me.

the inside of the hub motor assembly, is completely dark anyways and it doesnt make sense to me that painting the inside hub covers, in a setting where its already dark, will have added benefits. Also, it can be very difficult to get spray paint to stick to aluminum surfaces, especially when those surfaces get very hot and are in constant movement. IMHO..spray painting the inside of a hub motor black, is not useful, and could even result in the paint flaking off/ chipping away , under stress , and having negative repercussions in regards to the coils, magnets, etc.[/quote .

quote :

Its emsitisvity and I think its not just the fact it's black but any paint or some other coatings help absorb and give off (radiate) heat. Some colors and coatings will be better then others.

end quote.

it just doesnt make sense to me....we are talking about the inside of the hub motor, where its dark anyways...so wy would a certain color of spray paint, result in pulling substantial heat in those conditions ? We arent talking about radiation heat dissipation ....like having the sun beat down on a white surface or black surface

In fact, I think painting the inside of the hub motor, would not help at all, since its putting a thicker layer of substrate, that the inside heat now has to try and penetrate thru, to get to the outside of the hub side covers, via thermal conduction.

Maybe im wrong, but I dont think its a big help...I think its a waste of time, that acutally has more negatives then positives.

If you havent checked out my thread , under general disccusion, on heat sink mods, take a few moments and read thru it.

I think we have found a simple, effective way of cooling , for most people, without using messy fluids, electric fans and open holes in the hubs

 

[macribs]

For a cooling system that can remove vast amount of heat from the hub without adding complexity in form of pumps, hoses, oil filled leaky hubs or perforated side covers that attracts dust and debris I think there is only one durable solution. Heat pipes. Heat pipes combines with cooling fins/heat sink inside and outside of the motor as well as a impeller to "fan cool" the outside heat sink and have cooling gas convert to cool fluid in seconds and thereby sucking the heat out from a hubmotor. All that with minimum structural weakening of the covers.

One would use cooling fins on the inside of the covers, those fins would be connected to outside heat sink via heat pipes that goes through holes in side covers. These holes are then blocked by the heat sinks larger diameter so only the heat pipes goes through covers. No dust, sand water etc.

The fins have embedded heat pipes filled with cooling liquid that will circulate when heated. Liquid inside heat pipes turns to vapor when heated and will be pressed to the other end of heat pipe (outside hubmotor) where a larger heat sink rapidly cools down the vapor into liquid. As liquid cools it will return inside by capillary action. And so it goes. A closed loop cooling system with few setbacks and lots advantages. No leaky hub that puts oil stains on your floor. No worries about grinding paste inside a perforated hub.

Cool air being funneled over outside heat sinks/fins/heat pipes should cool instantly and that would create circulation inside the pipes and cool down both fins, cover, inside temperature and stator/magnets/copper.

Such cooling solution would also includes insides fins, heat pipes, outside fins and an impeller. It would use principles from thermal convection, conduction, radiation and a "wind blown" outside heat exchanger that forces liquid to rapidly travel through the closed loop bringing inside heat outside.

This system would be relatively light weight using light weight high thermal capacity material. No extra complexity regarding electric pump, large radiators.

 

[Arlo1]

it just doesnt make sense to me....we are talking about the inside of the hub motor, where its dark anyways...so wy would a certain color of spray paint, result in pulling substantial heat in those conditions ? We arent talking about radiation heat dissipation ....like having the sun beat down on a white surface or black surface

In fact, I think painting the inside of the hub motor, would not help at all, since its putting a thicker layer of substrate, that the inside heat now has to try and penetrate thru, to get to the outside of the hub side covers, via thermal conduction.

Maybe im wrong, but I dont think its a big help...I think its a waste of time, that acutally has more negatives then positives.

If you havent checked out my thread , under general disccusion, on heat sink mods, take a few moments and read thru it.

I think we have found a simple, effective way of cooling , for most people, without using messy fluids, electric fans and open holes in the hubs

Again its not sun light. Its IR heat the metal has to absorb and emit the word is Emissivity and it makes sense when you understand it. There is a video of guys testing a turbo car intercooler with this and they prove painting it black helps it remove heat better then raw aluminum. I would say if I didn't put oil inside my motors I would find the best coating to use. https://www.google.ca/search?q=emissivity&rlz=1Y3TXLS_enCA628CA628&oq=emisti&aqs=chrome.1.69i57j0l3.4426j0j4&sourceid=chrome-mobile&espv=1&ie=UTF-8

 

[ebikedelight]

it just doesnt make sense to me....we are talking about the inside of the hub motor, where its dark anyways...so wy would a certain color of spray paint, result in pulling substantial heat in those conditions ? We arent talking about radiation heat dissipation ....like having the sun beat down on a white surface or black surface

In fact, I think painting the inside of the hub motor, would not help at all, since its putting a thicker layer of substrate, that the inside heat now has to try and penetrate thru, to get to the outside of the hub side covers, via thermal conduction.

Maybe im wrong, but I dont think its a big help...I think its a waste of time, that acutally has more negatives then positives.

If you havent checked out my thread , under general disccusion, on heat sink mods, take a few moments and read thru it.

I think we have found a simple, effective way of cooling , for most people, without using messy fluids, electric fans and open holes in the hubs

Again its not sun light. Its IR heat the metal has to absorb and emit the word is Emissivity and it makes sense when you understand it. There is a video of guys testing a turbo car intercooler with this and they prove painting it black helps it remove heat better then raw aluminum. I would say if I didn't put oil inside my motors I would find the best coating to use. https://www.google.ca/search?q=emissivity&rlz=1Y3TXLS_enCA628CA628&oq=emisti&aqs=chrome.1.69i57j0l3.4426j0j4&sourceid=chrome-mobile&espv=1&ie=UTF-8

I would imagine Paint is a insulate no matter the color....and nobody seems to have brought that up.

there are countless opinions on this subject though...

heres someone elses opinion, dealing with vehicles...which makes sense to me

When it comes to heat transfer through a metal substrate, which is what you have here; heat source, pads and rotor, conducted through the caliper to the surface, the paint will function as an insulator, inhibiting heat rejection. If your looking for the maximum heat rejection, don't paint them. If you want to increase heat rejection increase the surface area of the caliper or increase the convection mechanism

 

[made_in_the_alps_legacy]

@ justin le :
I see that your experiments orient themselves on the "heat capacity" - based on that, I was wondering if there would be an option to lodge some Pockets of melting wax Inside the hub : the heat transferred by the oïl would dissipate in those pockets by melting the wax, and as long as the wax is not entirely melted in those pockets, the temperature would remain constant (latent heat)

 

[madin88]

I would imagine Paint is a insulate no matter the color....and nobody seems to have brought that up.

there are countless opinions on this subject though...

heres someone elses opinion, dealing with vehicles...which makes sense to me

When it comes to heat transfer through a metal substrate, which is what you have here; heat source, pads and rotor, conducted through the caliper to the surface, the paint will function as an insulator, inhibiting heat rejection. If your looking for the maximum heat rejection, don't paint them. If you want to increase heat rejection increase the surface area of the caliper or increase the convection mechanism

the influence about insulation has been brought up. thats why i mentioned brunishing or anodizing instead of paint but i do not think the problem of insulation is so big - IMO the advantages are higer - even with thick paint coat.
as Arlo1 already explained, we are talking about RADIATION HEAT in IR area, and colors have big influence to this. as example shiny metal does about 5 times poorer job than matt paint!

 

[ebikedelight]

I would imagine Paint is a insulate no matter the color....and nobody seems to have brought that up.

there are countless opinions on this subject though...

heres someone elses opinion, dealing with vehicles...which makes sense to me

When it comes to heat transfer through a metal substrate, which is what you have here; heat source, pads and rotor, conducted through the caliper to the surface, the paint will function as an insulator, inhibiting heat rejection. If your looking for the maximum heat rejection, don't paint them. If you want to increase heat rejection increase the surface area of the caliper or increase the convection mechanism

the influence about insulation has been brought up. thats why i mentioned brunishing or anodizing instead of paint but i do not think the problem of insulation is so big - IMO the advantages are higer - even with thick paint coat.
as Arlo1 already explained, we are talking about RADIATION HEAT in IR area, and colors have big influence to this. as example shiny metal does about 5 times poorer job than matt paint!

are there any major ehub manufacturers coating the inside of their hubs with matt paint and providing tech details on their data sheets that it cools 5x better within a sealed/ closed , dark scenario , then just non finished inside aluminum surfaces ?

If so, then I would have to consider my opinions on this subject have been wrong.

 

[justin_le]

there are countless opinions on this subject though...

Perhaps, but there is a reason why I titled this particular thread "Definitive Tests on the Heating and Cooling...", because a whole bunch of opinion and subjective "my motor felt cooler doing X" reports don't really help set any records straight.

it just doesnt make sense to me....we are talking about the inside of the hub motor, where its dark anyways...so wy would a certain color of spray paint, result in pulling substantial heat in those conditions ? We arent talking about radiation heat dissipation ....like having the sun beat down on a white surface or black surface

Look at any container designed for the opposite purpose, minimizing heat transfer from the inside to the outside, like a thermos or other vacuum flask. The inside surfaces of those are almost always silvered with a mirror finish so that there is a minimal possible radiative heat transfer between the layers. We want to maximize the heat transfer, so it would make sense to do the opposite. Paint the stator black so that it emits infrared heat energy as much as possible, and paint the side covers black so that they can absorb this IR heat as easily as possible to conduct it over to the air outside.

In a still environment, the heat dissipation by radiation like this can easily exceed the amount of heat moved by conduction through air, but if there is a reasonable amount of airflow then usually convective heat loss is quite a bit more significant. That's why I don't think the painted side covers will have that much of an effect since the air inside is already pretty turbulent, but rather than just speculating, here is this:

A pair of MXUS side plates with a flat black paint job ready for reassembly and a repeat of the last experiments. If all goes well we should have the set of 100, 200, 300, 400, and 500 rpm conductivity tests for this finished by end of Friday, and then we'll see if the difference is that material.

In fact, I think painting the inside of the hub motor, would not help at all, since its putting a thicker layer of substrate, that the inside heat now has to try and penetrate thru, to get to the outside of the hub side covers, via thermal conduction.

It's true for sure that a layer of paint will have some insulating effect, but it is also very easy to estimate the magnitude of this. Typical hub motor has a surface area of ~1000 cm^2 (0.1m^2), a decently thick layer of paint would be about 0.1mm, and the thermal conductivity of plastics is on the order of 0.1 w/mK, and I'd expect paint to be similar:

Conductivity = Area / Length * coefficient = 0.1 m^2 / 0.0001 m * 0.1 W/mK = 100 Watts / degree

So even if the motor is shedding 200 watts, the presence of the paint over the casing would increase the temperature by just 2 degrees. Presumably the benefits of added radiation transfer will outweigh this small insulating effect, but you are correct that it does add some small impediment to the heat flow.

Maybe im wrong, but I dont think its a big help...I think its a waste of time, that acutally has more negatives then positives.
If you havent checked out my thread , under general disccusion, on heat sink mods, take a few moments and read thru it.

Yes, I saw that earlier. I suspect you could achieve almost exactly the same effect by using a proper thermally conductive adhesive to bond the finned heat sink to the side plate and avoid drilling the hole in the first place. Same analysis, if you have say a 1" diameter (.0005m^2) of side plate (assume 3mm thick aluminum), then the thermal conductivity of that chunk of metal you are drilling out is:

Conductivity = 0.0005m^2 / 0.003m * 210 W/mK = 35 watts / degree. Now multiply this by the number of "holes/heatsinks" that you have and you can see that the metal side plate pieces you were removing under the heatsinks were barely any impediment at all to the flow of heat from the inside to the outside of the shell. The joint between the heat sink and the side plate may have been, but not the side plate metal itself.

I'm sorry if I'm off topic with any of my posts here. I dont want to spam.

No it's great so long as you are collecting and posting actual measured data as you've been doing, then this can help to either corroborate or challenge some of the numeric results that we've been seeing in our tests here.

 

[justin_le]

@ justin le :
I see that your experiments orient themselves on the "heat capacity" -

I wouldn't quite say that. Heat capacity is just one of the critical thermal parameters that previously I had been deducing form the temperature rise data rather than measuring directly, so I thought it would be a good idea to empirically measure these capacities in order to make sure that the model is inline. ie. if the thermal model has a best fit when the stator is assumed to be 2000 J/K, but I only measure it to be 1000 J/K, then something is not right in the model representation.

based on that, I was wondering if there would be an option to lodge some Pockets of melting wax Inside the hub : the heat transferred by the oïl would dissipate in those pockets by melting the wax, and as long as the wax is not entirely melted in those pockets, the temperature would remain constant (latent heat)

I can see the rationale on why this is done with lithium batteries, where there is a very clear consequence to getting the cells beyond a certain temperature and it's worth an extra weight/cost penalty to have a phase change material to clamp the upper temperature.

For a motor? I dunno. Going from 25 degrees to say 85 degrees a typical 9C style motor shell (1500 J/K) absorbs 90,000 Joules. Waxes by contrast have a heat of fusion around 200 Joules / gram. To absorb that same amount of energy from a wax liquifying you would need 90,000 J / 200 J/g = 450 grams, exactly a pound's worth of wax in pockets in the motor. For 1 lb of extra weight, you could do a lot of things that would help actually shed the heat rather than simply delaying how long it takes before motor heats up.

Such as....

For a cooling system that can remove vast amount of heat from the hub without adding complexity in form of pumps, hoses, oil filled leaky hubs or perforated side covers that attracts dust and debris I think there is only one durable solution. Heat pipes.

This is something that I would really really like to try, and last time I was at the metal scrap yard they had an entire dumpster bin full of laptop heatsinks and heatpipes ready for the pickings, they seemed to be egging me on to buy at scrap copper prices.

But I don't quite understand this

One would use cooling fins on the inside of the covers, those fins would be connected to outside heat sink via heat pipes that goes through holes in side covers. These holes are then blocked by the heat sinks larger diameter so only the heat pipes goes through covers. No dust, sand water etc.

You don't need the heat pipes to move heat from the inside of the side plate to the outside of the side plate, since the side plate is already in close to a perfect isotherm, and that's not where the heat is being generated which is on the copper windings and core. If you had a heat pipe, I would think it would be to move heat directly from the non-rotating stator core, out through the axle, and then to a finned sheet to convect the heat to outside ambient air. It would be nice to use a heat pipe to go directly from the stator to the side covers to spread the heat to ambient, but I can't think of how you'd thermally made from a static to a rotating frame.

 

[speedmd]

Hollow Axle would allow you to pump into/out of the stator legs as on a auto crankshaft. Can do the same with heat pipes if the axle is large enough diameter. Ideally twin holes to create a loop in a liquid cooled setup.

 

[macribs]

You don't need the heat pipes to move heat from the inside of the side plate to the outside of the side plate, since the side plate is already in close to a perfect isotherm, and that's not where the heat is being generated which is on the copper windings and core. If you had a heat pipe, I would think it would be to move heat directly from the non-rotating stator core, out through the axle, and then to a finned sheet to convect the heat to outside ambient air. It would be nice to use a heat pipe to go directly from the stator to the side covers to spread the heat to ambient, but I can't think of how you'd thermally made from a static to a rotating frame.

We might be talking past each other due to translation issues. So I will try one more time to explain.
What I mean is that the cover itself is not rotating alone, is it rotates with the rest of the outer parts of the motor. If you punch holes in the cover and stick heat sinks on the inside and on the outside of the cover [heat sinks a mounted to the cover itself both outside and inside motor] you could connect the inner sink to to outer sink with a heat pipe. The inner heat sink will be hotter then the outside because of wind, ambient outdoor temp and the stream of air from the impeller over the outer heat sink. That will let the cooled down liquid trickle down to the inner part of heat sink and cool down heat sink and the ambient temperatures, as the liquid heats it will vapor and so the loop will go.

There might be other places that are even more suited for placements.

 

[ebikedelight]

there are countless opinions on this subject though...

Perhaps, but there is a reason why I titled this particular thread "Definitive Tests on the Heating and Cooling...", because a whole bunch of opinion and subjective "my motor felt cooler doing X" reports don't really help set any records straight.

it just doesnt make sense to me....we are talking about the inside of the hub motor, where its dark anyways...so wy would a certain color of spray paint, result in pulling substantial heat in those conditions ? We arent talking about radiation heat dissipation ....like having the sun beat down on a white surface or black surface

Look at any container designed for the opposite purpose, minimizing heat transfer from the inside to the outside, like a thermos or other vacuum flask. The inside surfaces of those are almost always silvered with a mirror finish so that there is a minimal possible radiative heat transfer between the layers. We want to maximize the heat transfer, so it would make sense to do the opposite. Paint the stator black so that it emits infrared heat energy as much as possible, and paint the side covers black so that they can absorb this IR heat as easily as possible to conduct it over to the air outside.

In a still environment, the heat dissipation by radiation like this can easily exceed the amount of heat moved by conduction through air, but if there is a reasonable amount of airflow then usually convective heat loss is quite a bit more significant. That's why I don't think the painted side covers will have that much of an effect since the air inside is already pretty turbulent, but rather than just speculating, here is this:

A pair of MXUS side plates with a flat black paint job ready for reassembly and a repeat of the last experiments. If all goes well we should have the set of 100, 200, 300, 400, and 500 rpm conductivity tests for this finished by end of Friday, and then we'll see if the difference is that material.

In fact, I think painting the inside of the hub motor, would not help at all, since its putting a thicker layer of substrate, that the inside heat now has to try and penetrate thru, to get to the outside of the hub side covers, via thermal conduction.

It's true for sure that a layer of paint will have some insulating effect, but it is also very easy to estimate the magnitude of this. Typical hub motor has a surface area of ~1000 cm^2 (0.1m^2), a decently thick layer of paint would be about 0.1mm, and the thermal conductivity of plastics is on the order of 0.1 w/mK, and I'd expect paint to be similar:

Conductivity = Area / Length * coefficient = 0.1 m^2 / 0.0001 m * 0.1 W/mK = 100 Watts / degree

So even if the motor is shedding 200 watts, the presence of the paint over the casing would increase the temperature by just 2 degrees. Presumably the benefits of added radiation transfer will outweigh this small insulating effect, but you are correct that it does add some small impediment to the heat flow.

Maybe im wrong, but I dont think its a big help...I think its a waste of time, that acutally has more negatives then positives.
If you havent checked out my thread , under general disccusion, on heat sink mods, take a few moments and read thru it.

Yes, I saw that earlier. I suspect you could achieve almost exactly the same effect by using a proper thermally conductive adhesive to bond the finned heat sink to the side plate and avoid drilling the hole in the first place. Same analysis, if you have say a 1" diameter (.0005m^2) of side plate (assume 3mm thick aluminum), then the thermal conductivity of that chunk of metal you are drilling out is:

Conductivity = 0.0005m^2 / 0.003m * 210 W/mK = 35 watts / degree. Now multiply this by the number of "holes/heatsinks" that you have and you can see that the metal side plate pieces you were removing under the heatsinks were barely any impediment at all to the flow of heat from the inside to the outside of the shell. The joint between the heat sink and the side plate may have been, but not the side plate metal itself.

I'm sorry if I'm off topic with any of my posts here. I dont want to spam.

No it's great so long as you are collecting and posting actual measured data as you've been doing, then this can help to either corroborate or challenge some of the numeric results that we've been seeing in our tests here.

Very interesting....

My opinion was that by removing the 1" thick sections of aluminum side cover { by drilling holes} directly lateral of the coils , and then using thermal conductive adhesive to attach heat sinks directly above those coils, over the holes, would result in better/ faster heat dissipation, because the base plate of the aluminum heat sink is only 1/4" or less....which I assumed would mean alot less resistance for the heat to have to transfer thru , to get to the outside cooling fins of the heat sink...so I went from having a thickness of 1-1/4" of aluminum { the side cover + the thickness of the heat sink base plate} to a thickness of just 1/4" { thickness of just the heat sink base plate} ....which resulted in 1" less material the trapped heat had to fight to make it to the outside cooling fins

But according to your tests , I would have gotten better cooling effects to have not drilled out the hub and just attached the heat sinks to the same area ? I would have never assumed this would be the case.

Justin, could you tell us the exact brand paint you are using to paint the inside of the hub cover ?

Also, do you not think there may eventually be a problem of the paint flaking off the aluminum , under the stress of the heat and rotation ? Sometimes paint does not like to stick to aluminum very well.

 

[ebikedelight]

Justin, the more I think about this, the less sense it makes to me.

You are claiming that the inside of the hub motor, would have better cooling results, if the side cover material was made of 1-1/4" thick material, compared to 1/4" thick material ?

This seems illogical to me, since we are talking about conduction , and trying to get the heat inside the motor , to the outside cooler air, as quickly and efficiently as possible, thru conduction of the side cover material.

By me drilling out holes, directly lateral to the coils, where the heat is generated/ stored , I was able to reduce the thickness by 1" , in which the heat had to penetrate to get to the outside cooling air . So I basically added significant surface area { cooling area} thru my mod , while reducing the conductive resistance
{ thickness of material } that the heat had to penetrate from the inside to the outside, by more then 75% , in those areas where I drilled out the `1" thick side cover hub and replaced it with 1/4" thick heat sink base plate.

Doing it your way, would have meant that I increased the side cover thickness in those specific areas , to 1-1/4" thick instead of my current 1/4" thick ...

 

[Punx0r]

Its emsitisvity and I think its not just the fact it's black but any paint or some other coatings help absorb and give off (radiate) heat. Some colors and coatings will be better then others.

The forum seems to have ate my earlier post, so here it is again:

This paper offers something interesting on object colour and radiative cooling: http://physlab.lums.edu.pk/images/a/a5/Cooling_paint.pdf

Basically, a black painted surface performs better (more emissivity, cools faster) than bare metal surface, but no better than one painted white: The reflectivity in the visible spectrum is not necessarily related to that in the infra-red spectrum.

So, paint = good and nothing wrong with black paint if the IR emissivity is good, but it seems the classic school physic experiment leading us to believe black is inherently better is miss-leading

 

[Arlo1]

Wow this page exploded

I would imagine Paint is a insulate no matter the color....and nobody seems to have brought that up.

wrong This part is science you need to read the research and understand. Sure you can build up 10 or 20 layers of paint and uselessly waste paint then maybe it will start to insulate. But we are talking about IR absorption and radiation.

[youtube]f1QL9veQaNg[/youtube]

 

[ebikedelight]

well i hope Justins experiments prove once and for all, that painting the interior of the hub side covers, gives significant cooling results. I would be glad to admit my opinions on this subject have been wrong .


This would be great news, that may allow most of us to greatly increase our hub motors cooling effects, and efficiency, just thru 2 simple mods..

painting the inside hub covers

adding heat sinks to the outside in key areas, directly above/adjacent to where the heat is generated/ stored.

 

[justin_le]

well i hope Justins experiments prove once and for all, that painting the interior of the hub side covers, gives significant cooling results.

Well I have serious doubts that will be the case. I think what we will see is at low speeds, like 100-200 rpm, that there will be a small but measurable difference, and that at higher RPM's it will be a wash since the convective effects will dominate.

This would be much like what was shown in the black intercoolers mythbusted video. When you have still air over a surface, then a significant if not a majority of the heat conduction away can happen via radiation if the material has high emissivity. This why we had the Satiator painted black, it runs double digit degrees cooler with a black coating than left as a plain aluminum finish. But when there is even a modest amount of air blowing over a surface, then the heat taken away from that convection is way more than what the black coating will radiate, and at some level of forced air flow you would get worse conductivity with the painted surface because of the small additional insulation effect is higher than the radiative gains

What this would mean in practice is that painting the insides black will see benefits not so much in reducing the heat buildup when you are running the motor at power, but it in increasing the speed at which the motor cools down whenever you stop for a break. And if you've ever stopped to wait for a hub motor to cool down after gunning it up a long hill, then you'd appreciate this happening at a much faster pace.

But lets wait for the actual data before concluding if it's worth it.

Justin, the more I think about this, the less sense it makes to me.
You are claiming that the inside of the hub motor, would have better cooling results, if the side cover material was made of 1-1/4" thick material, compared to 1/4" thick material ?

No, that's not what I said at all. If the side plate was 1-1/4" thick it would take a LONG time to overheat the motor because of all the heat capacity, but the final steady state temperature would be more or less the same. As far as heat conduction goes, for all intents and purposes aluminum is perfect, the temperature on the inside surface of the side cover is going to be within a fraction of a degree of the temperature on the outside surface.

What I was saying is that you observed you were getting better subjective results when you had the heatsinks going over holes in the side plate compared to being attached to the side plate directly with no holes, and I don't doubt this is the case. But you concluded this was because the extra resistance to heat flow through the side cover metal, and what I was saying is that it's not the aluminum side cover that was blocking the flow, but the very small air gap and junction _between_ the side cover and the heatsink. When you drilled the holes in the plate, then you didn't have this thin still air gap anymore, so you'd expect better results. Making a good thermal junction between two nominally flat pieces of metal is a tricky, you really need to have two very flat mating surfaces with a super thin film of gap filling thermal grease or adhesive, and a look at how CPU heatsinks are secured in computers will show what this entails.

Doing it your way, would have meant that I increased the side cover thickness in those specific areas , to 1-1/4" thick instead of my current 1/4" thick ...

Again I have no idea where the 1 1/4" reference came from.

 

[ebikedelight]

on the 1-1/4" reference , thats my misunderstanding...you mentioned 1" in a prior post , but I see that is round diameter...not thickness. My error.

I should have put a micrometer on the ehub side cover, and measured it thickness.

Anyways, we appreciate all you do for the ebike community.

I agree with you, that painting the inside of these hub motors black, isnt gonna give significant benefits...but hopefully we will be proven wrong.

 

[Cowardlyduck]

Great stuff Justin. I, like many, look forward to the results of these tests.
Even if the benefits of black side covers are only realised during low speeds, or stopped conditions, this would still benefit nearly all E-Bikers anyway.
I'm not sure if you saw it previously, but this kind of removal of heat is similar to what I found with my first forced air cooling mod with small fans mounted inside the hub.

What I found was that the fans helped very little with initial heat build-up under load, but were able to remove it far faster once stopped, coasting, or using lower power levels.
http://endless-sphere.com/forums/viewtopic.php?f=30&t=56965&start=25#p946731

For my (and many others) kind of riding in hilly off-road conditions this is a great way to remove the heat gained from steep hills.

My next attempt is coming along nicely, and I expect to see improved cooling with the higher CFM fans I am using.

I will try and do the same test run I did to plot the graph above with this motor as a point of comparison once I get it up and running. I won't be able to vary the voltage on this setup though as I'm running the fans in series directly off the battery this time.

Cheers

 

[madin88]

Basically, a black painted surface performs better (more emissivity, cools faster) than bare metal surface, but no better than one painted white: The reflectivity in the visible spectrum is not necessarily related to that in the infra-red spectrum.

So, paint = good and nothing wrong with black paint if the IR emissivity is good, but it seems the classic school physic experiment leading us to believe black is inherently better is miss-leading

the optimal motor paint would be all black inside and white on the outside, because white only absorbs about 20% of the heat coming from the sun (and i believe also from the environment like hot asphalt) while black absorbs about 90% of this heat thats over 4 times more.
if the motor sits in the sun we pretty sure will measure temperature differences about 10-20°C. if the motor always is in move and has airflow, the benefits are almost negligible (just like it is with black vs white painted cars).


Justin, i very appreciate your work and sharing of results. looking forward for the test.

on one of my mxus motors i have installed a blower wheel from a 140mm PC fan and little heatsinks (which also work as a radial fan). the air will be pushed over windings, through the stator teeth and airgap against the opposite sidecover. i know the gaps are very small, but i believe it will be best direction of airflow. further i have thought of removing the chinese wood sticks in the stator teeth and replace them with small aluminum pipes, but a guy has told me this will increase eddy losses in the motor and result in more heat if the magnets spin. is this right?

 

[made_in_the_alps_legacy]

@justin, thxs for your quantitative assessment : I have been surprised by the amount of wax you came up too : it’s quite realistic rather than pure science fiction.
This being said, I am afraid it will still take weeks if not months for u to test and validate a kind of “very best of” cooling designs involving :
• increased convective internal heat transfer (by fluid density enhancement – oil – what u do, and/or increased area),
• increased conduction heat transfer (by reduced wall thickness and material change- like ebikedelight),
• increased convective external heat transfer (by area enhancement, fins, pins,flow enhancement impeller…)
If you need support based on several years of heat transfer engineering experience, feel free to PM me. For now, I will stop trolling this topic to leave the intended space for experiments.

 

[Punx0r]

the optimal motor paint would be all black inside and white on the outside, because white only absorbs about 20% of the heat coming from the sun (and i believe also from the environment like hot asphalt) while black absorbs about 90% of this heat thats over 4 times more.

As detailed in the paper I linked to, it does not appear that the inside of the motor must be black. It seems reasonable that the spectrum emitted by a motor winding at ~100°C will be very different to that coming from the sun...

White on the outside may give a double-advantage: Reflecting the majority of sunlight, but not impeding emission of heat from inside the motor. This is where the paper comes in: Normally we would assume the motor must be black to get good emission, but then we would also have high absorption from the sun. The paper says white will emit just as well at <100°C (it's example based on the cooling of hot water).

As Justin suggests though, we could be debating how to optimise what is in reality a very small effect!