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Author Topic: Motor cooling solution?  (Read 1831 times)

nigezero

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Motor cooling solution?
« on: July 31, 2014, 08:46:27 AM »

Hey all; it's clear that simple cost effective cooling solutions are key for sustaining ongoing hi power applications from what I'm reading. I'm sure heaps of people are innovating but struck me that I haven't seen a thread yet of such solutions. 

So; here's a pretty logical looking possibility courtesy Gizmag; wrap it in copper mesh-claims are 500x cooling area compared to fins!

http://www.gizmag.com/silent-power-pc-passive-cooling/33145/
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protomech

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Re: Motor cooling solution?
« Reply #1 on: July 31, 2014, 09:12:59 AM »

I'm not sure this would work well in motor applications.

You need moving air through the fins to evict the heat.

More surface area is fine, but there's not going to be a ton of moving air through the mesh. Or that's what it looks like to me.
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Doug S

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Re: Motor cooling solution?
« Reply #2 on: July 31, 2014, 09:38:06 AM »

More surface area is fine, but there's not going to be a ton of moving air through the mesh. Or that's what it looks like to me.

I'd agree with protomech. I know some manufacturers of air-cooled engines have REDUCED the number of fins and achieved better cooling; it's a balancing act between surface area and airflow. Better ways of increasing surface area exist such as bead blasting or anodic treatment to roughen the surface up.

Also, that copper mesh doesn't CONDUCT heat very well because the individual copper wires are very thin. A nice thick fin conducts heat away from the motor very fast, all the way to the end of the fin, so all the surface area is effective at transferring the heat into the airstream. That copper mesh would probably heat up very close to the source of heat but not transfer the heat to the rest of the mesh very well, so much of the increased surface area wouldn't be hot enough to transfer heat to the airflow.

Fins are good. Make them relatively thick to conduct heat quickly, rough them up for good surface area, space them and orient them so you get good airflow between them, anodize them a nice dark black so they radiate infrared energy away well, and you're about as good as you're going to get without going to a water jacket.
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trikester

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Re: Motor cooling solution?
« Reply #3 on: July 31, 2014, 10:18:50 AM »

Quote
anodize them a nice dark black so they radiate infrared energy away well,

I agree with everything said except this statement. At the frequency of the IR put out by a hot motor surface the black doesn't radiate any better than plain aluminum. At the frequency of the sun's IR, however, black is very effective, that's why solar water heater pipes are black. It has just been customary to anodize heat sinks black to the point that it would look strange to see them any other color. The most important factor in radiating the low IR is a non-shiny surface. Bead blasting works well for making the surface rough, among other methods. Just the process of anodizing usually breaks the shiny surface and helps.

My own fast cooling method wouldn't help at highway speeds but when I'm going to do a long hot dirt climb I carry a spray bottle of distilled water. The evaporation of the water sprayed on the motor, when it's approaching 250*F, brings the temp down super fast. I've thought about installing a spray bar system using an electric pump so that I don't have to stop to spray the motor. However, I wouldn't have to use it often enough to make that complication seem worthwhile. I use distilled H2O so that the evaporation doesn't leave a mineral deposit on the motor. :-[

Trikester
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Doug S

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Re: Motor cooling solution?
« Reply #4 on: July 31, 2014, 10:42:30 AM »

Trikester, your analysis is astute and correct. Let me strengthen your point: "Black" is a color, which refers to visible light, which isn't relevant with respect to heat radiation until an object is literally starting to glow (dull red, at first, well beyond the temps we're talking about). But, black anodized aluminum DOES have a higher emissivity in the IR range than un-anodized aluminum, so it DOES radiate a bit better in the IR range. And I'm sure you're aware that the blackbody radiation curve is quite broad, so even if an object isn't visibly glowing, there can still be a fair amount of energy in the visible band. I work in the electronics industry, and they do charge more for a black-anodized heat sink, because it does perform measurably better than an un-anodized heat sink. It's not a huge improvement, a few percent, but as EV owners we understand the benefit of a few percent improvement.
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Le Z Turbo

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Re: Motor cooling solution?
« Reply #5 on: July 31, 2014, 12:40:59 PM »

It seems to me that the first restriction I'm hitting on my bike is the ESC temperature (electronic speed control, below the saddle with the big radiator) and not the motor temp. Same for you ?
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dkw12002

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Re: Motor cooling solution?
« Reply #6 on: July 31, 2014, 11:52:30 PM »

In industry, to increase surface area dramatically for bonding you would acid or laser-etch the metal. I'm not sure it would stay etched very long in the case of an electric motor housing being exposed to dirt, water, and air though. Maybe an electroplating process or zinc dip would protect it.
« Last Edit: July 31, 2014, 11:54:27 PM by dkw12002 »
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trikester

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Re: Motor cooling solution?
« Reply #7 on: August 01, 2014, 12:00:50 AM »

I was a co-founder and head of engineering of a successful high power switchmode power supply company. We found that black and clear aluminum heatsinks worked equally well, as long as the aluminum was not shiny, and that it didn't pay to have them dyed black. All anodize is clear to begin with so black costs more because it is another step to dye the anodized surface which is aluminum oxide. The radiation of hot electrical surfaces is quite far below the beginning of light visible to the human eye. The sun's IR radiation is much closer to visible, and black works very well there.

It's interesting to realize that a black object exposed to the sun becomes a frequency converter. It readily absorbs the higher frequency IR radiation from the sun but because the object isn't heated to anywhere near the sun's temperature it does not re-radiate the lower frequency IR out through the black surface as well as it absorbed it. This IR frequency conversion makes black solar heaters work very well.

BTW - It is a big mistake to actually paint a heatsink black because the layer of paint acts as an insulating barrier to heat radiation. Better to rough it up and leave it natural in that case. The black dye used on an anodized surface does not degrade the heat transfer like paint does, so there is no down side to black dyed anodize, except extra cost.

Trikester
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Doug S

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Re: Motor cooling solution?
« Reply #8 on: August 01, 2014, 12:21:25 AM »

Trikester, take a look at this data sheet of a heat sink we use in one of our products:  http://media.digikey.com/pdf/Data%20Sheets/Ohmite%20PDFs/W_Series.pdf

I'm not trying to pick a fight, I really just want to understand better. They offer two finishes for each of these heat sinks, degreased or black anodized. In all cases, their claim is that the anodized one DOES dissipate heat a little bit better than the degreased one. Do you think the anodizing process adds surface area (perhaps they acid etch to prepare the surface or something), raises the emissivity, or do you think they're just plain wrong? The degreased one that we use retails for $2.90, the anodized one goes for $2.99, so the price difference isn't much.
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trikester

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Re: Motor cooling solution?
« Reply #9 on: August 01, 2014, 10:46:44 AM »

My guess is that the natural color one is smoother than the black one. I didn't see if it was clear anodized or not. Just the process of clear anodizing will break the surface shine which may be just from the prep etch and/or the anodizing process itself. One definitely wants the surface to appear dull for good radiation. A shiny surface (especially a polished one) reflects the IR back into the body of the heatsink just as it shines in the visible light we see coming back to our eyes.

Given the same surface roughness we could not measure a difference between black and clear when testing our heatsinks. As you say, the radiation of a black body is broad so there might have been a difference that was less than our measuring capability. However, a body at say 200*F is a very low frequency radiation in the IR spectrum.

BTW - It was an engineer who worked for me that first brought all of this to my attention, many years ago. I had not given it much thought before that because I was so accustomed to seeing and using black aluminum heatsinks.

Trikester

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krash7172

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Re: Motor cooling solution?
« Reply #10 on: August 01, 2014, 12:14:28 PM »

Isn't the primary mode of heat transfer forced convection in this case?
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firepower

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Re: Motor cooling solution?
« Reply #11 on: August 01, 2014, 12:30:36 PM »

Still think black anodized would look better than gold. Both heatsinks should be matching.
 
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trikester

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Re: Motor cooling solution?
« Reply #12 on: August 01, 2014, 09:58:26 PM »

I agree that I like the looks of black heatsinks. They have that look of doing the job they are supposed to do.

Nobody commented on my practice of spraying distilled water on my motor when it goes into heat limit on a slow, long, steep climb. Has anybody else done this with their motor? It could work with the controller also since it is also water proof. Evaporating water is a great coolant!

Trikester
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Doug S

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Re: Motor cooling solution?
« Reply #13 on: August 01, 2014, 10:07:26 PM »

Trikester, I've thought about doing the same thing, and was glad to hear that you've performed the experiment. I (obviously) don't take my SR in the dirt, but I do ride in so cal, and it gets warm here. So far I've only triggered the high-temp alarm once, and only to the point where the light began to flash, not until it started cutting power. But I agree, evaporative cooling is VERY effective, and it does seem logical and straightforward to mount a small reservoir, a windshield-washer pump and a switch somewhere near the handlebars to provide a spritz now and then to keep the motor comfy. It seems like there'd be hardly any risk in doing so, since the motor has to be sealed well enough to ride in the rain.

I guess my only question would be, if the motor's getting warm, don't you imagine that the controller and the batteries are also working pretty hard? If you add a way to keep the motor cool, you might just move the heat stress to another part of the system. Granted, those parts of the system should be able to protect themselves too, but it's just a variable that the engineers didn't design into the system, and it might not be obvious what all of the effects might be.
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RickSteeb

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Re: Motor cooling solution?
« Reply #14 on: August 02, 2014, 06:03:55 AM »

I agree that I like the looks of black heatsinks. They have that look of doing the job they are supposed to do.

Nobody commented on my practice of spraying distilled water on my motor when it goes into heat limit on a slow, long, steep climb. Has anybody else done this with their motor? It could work with the controller also since it is also water proof. Evaporating water is a great coolant!

Trikester

If the heatsinks will ever be exposed to direct sunlight while in operation , then the bare Aluminum or clear anodize sounds like a wise choice!  Low-E is not an issue with sufficient convection/fan airflow...

[Worked at Raytek for 5 years in the '70s ;]   8)
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