Cooling non-hub motors

This post was a good thought for a new thread (so I'll take the liberty):

oofnik said:

Hey guys, random idea that just occurred to me... since we don't all have fancy 5-axis CNC machines, but we would still like to keep our little RC motors cool, we can use impellers designed to keep vacuum cleaner motors at operating temp. A wide variety is available on eBay. Just take a look at these search results. I asked a few sellers for diameters and am awaiting replies. Shouldn't be too hard to adapt most of these to RC motors.

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I saw this online yesterday (maybe overkill):

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=97619

Small and compact handy inflator/deflator includes three adapters for most inflatables.
Plugs into a vehicle cigarette lighter port
Convenient on/off switch
6 foot, 8" long power cord
Operating pressure: 0.5 PSI; Pump dimensions: 5" L x 5" W x 3-7/8" H
Net weight: 0.65 lbs.

ITEM 97619-1VGA $7.99

I'm keen to experiment with water cooling, sometime next year...

A non hub motor should have enough airflow that it won't overheat unless the efficiency is really bad or the motor is undersized. My 1/2 a shilling.

Dang that HF one looks like just the thing at the right price. With a thermostat it shouldn't be over kill even if it draws a lot of power. The on-off noise might get annoying though. How many amps does it eat I wonder.

I'm still trying to figure out how to get ducting mounted from my square outlet fan to my 1018.

johnrobholmes said:

A non hub motor should have enough airflow that it won't overheat unless the efficiency is really bad or the motor is undersized. My 1/2 a shilling.

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Aren't RC motors supposed to be "undersized"? :lol:

A main concern with RC motors seems to be operating at lower speeds such as sustained climbs up hills(like for those that go mountain-biking or just have huge hills). The efficiency decreases and the heat generated increases, which would seem to kill a typically small RC motor. But, I haven't tried it, so I don't really know.

swbluto said:

A main concern with RC motors seems to be operating at lower speeds such as sustained climbs up hills(like for those that go mountain-biking or just have huge hills). The efficiency decreases and the heat generated increases, which would seem to kill a typically small RC motor. But, I haven't tried it, so I don't really know.

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Yes, that's why you need lots of power... (to maintain speed within the efficient operating range on hills), or multiple gearing.

johnrobholmes said:

A non hub motor should have enough airflow that it won't overheat unless the efficiency is really bad or the motor is undersized. My 1/2 a shilling.

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Well, the thing to keep in mind I think is that R/C motors (outrunners specifically) are mounted at the front of the plane where air is constantly rushing over and through at 50mph+ to keep them cool. These motors would have relatively zero airflow in an ebike setup, justifying some sort of forced air cooling.

http://www.rcgroups.com/forums/showthread.php?t=894630

Here's someone with a similar issue and trying out water-cooling. It's an RC boat, so there's not the same access to air as an RC airplane.

I was thinking that using a "tube" to contain the motor at least partially inside and to pump air through would be a great way to cool off. Furthermore, the tubing itself can be wrapped around a cooler medium to cool the air to a cooler temperature(such as ice water) which would make the cooling even more effective.

How about using peltiers (thermo-electric coolers) themally coupled to the motors casing....
http://cgi.ebay.com.au/45W-Thermoel...3286.c0.m14&_trkparms=66:2|65:3|39:1|240:1318
You would thermally glue them to the motors casing using something like this
http://cgi.ebay.com.au/Thermo-Therm...ihZ015QQcategoryZ4660QQtcZphotoQQcmdZViewItem



Using these would create a temperature differential between the casing and the air inside it. The cooler air inside would then draw the heat out of the motor windings based on the temperature differential between the copper windings and the air. The DC resistance of the coils lowers, and efficiency goes up. No different to the task performed by a regular heatsink on the outside of a motor, except it can pull the heat away faster than a passive or a fan blown heatsink.

The other advantage over air-blown cooling is that your motor windings are not limited to the ambient temperature of the day (They are used in car fridges afterall). They can cool your motor well below ambient so on a hot day you could have a nicely chilled motor. This is based on the fact that they create a temperature differential rather than an absolute temperature. Fan cooling cannot achieve this.

They are cheap, light, tiny for their wattage and we already have massive batteries to run them. They are usually 12V so you just string them in series to match your battery pack voltage choosing the combined wattage you want based on your motor's power/efficiency. This saves the hassle of needing DC>DC converters of which only the more expensive ones would have the current capacity.

This will not work on hub motors without some serious engineering (brushes), but would be a simple addition to a static mounted motor shell. A peltier can pull alot more heat away than a big fan. They are silent and solid state making them quite resistant to vibration. They are used in some seriously overclocked PC's.

Its a shame they all seem to be flat which is bad for surface area contact on a round casing but the themal glue should do a decent job of transfering the heat through the gaps it fills.

The user would have to decide if the Amp Hours used to drive them would be worth the saving in Ah's from a cooler more efficient motor. You would of course put a switch inline so you could turn them off unless the weather is hot enough to justify their use, or your climbing a big hill.

PS: It seems you can already buy a finished product combined with a fan and heatsink. Popular in RC car racing http://www.muchmore.nl/downloads/ctxt_manual[3].pdf



http://www.towerhobbies.com/products/much-more/mmrc3010m.html

Have you seen an outrunner spin? I haven't but I think the outside of it spins which would seem to eliminate all possible contact with the case during operation.

Wouldn't a TEC reduce the wasted power of the motor by the same amount required to run itself, therefore having no effect on efficiency?

That's before taking into account the terrible efficiency of the devices. The only thing I see it doing is cooling down the motor so it lasts longer.

I think you'd have to use a peltier in conjunction with water-cooling, anyway....

Heat-sink finned or water-cooled inrunners and fan-cooled outrunners seem the most reasonable options, to me.

swbluto said:

Have you seen an outrunner spin? I haven't but I think the outside of it spins which would seem to eliminate all possible contact with the case during operation.

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Edited: Yes true for outrunners. Inrunners would be a must.

The outer case, apart from one end, spins with the motor shaft - so, you can only mount the motor, or access the armature, from one end of the case.

Miles said:

The outer case, apart from one end, spins with the motor shaft - so, you can only mount the motor, or access the armature, from one end of the case.

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A rapidly spinning outer might be quite good at losing heat to the air, even without aircraft type airflow past it. AFAIUI though, the heat is generated in the inner, stationary, part - that's where the windings are. The bigger problem may be getting the heat from that to the rotating part rather than from the rotating part to the air.

It may be that an inrunner is better for bike applications.

Nick

Tiberius said:

A rapidly spinning outer might be quite good at losing heat to the air, even without aircraft type airflow past it. AFAIUI though, the heat is generated in the inner, stationary, part - that's where the windings are. The bigger problem may be getting the heat from that to the rotating part rather than from the rotating part to the air.

It may be that an inrunner is better for bike applications.

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Yes, unless you ventilate the inside, most of the heat from the armature would have to pass through the magnets to be dissipated.

One possible advantage that inrunners have, for bike use, is that they can be sealed and still be effectively cooled... I don't know how much of a problem an open motor case would be, in practice, though.

swbluto said:

Aren't RC motors supposed to be "undersized"? :lol:

A main concern with RC motors seems to be operating at lower speeds such as sustained climbs up hills(like for those that go mountain-biking or just have huge hills). The efficiency decreases and the heat generated increases, which would seem to kill a typically small RC motor. But, I haven't tried it, so I don't really know.

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Yes, they are indeed small. My point is that if the motor overheats, it is probably a good idea to either gear it down or select a larger motor. Additional cooling can help, a higher efficiency motor can help, but using a larger motor can help too. I would rather use a larger, more efficient motor instead of using additional cooling. Much more reliable IMO.



I think that inrunners are better suited. More robust construction, easier to cool, more efficient by design, easy to seal, and they can be made into a pancake style.

johnrobholmes said:

Additional cooling can help, a higher efficiency motor can help, but using a larger motor can help too. I would rather use a larger, more efficient motor instead of using additional cooling. Much more reliable IMO.

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It's a fair point.

One advantage of the smaller motor would likely to be greater efficiency at low power output.

Concentric cooling fins on an inrunner, should be worth doing - minimal extra weight and no additional energy requirement.

I think that inrunners are better suited. More robust construction, easier to cool, more efficient by design, easy to seal, and they can be made into a pancake style.

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Agreed

I have already busted a few cheap outrunners too, flexy bell tends to hit magnets on stator when rig hits bumps. Not a problem with a supporting 4th ring bearing, but then the RPMs are limited and you have a lot of extra drag and bearing maintenance to worry about. Also, ring bearings make taking apart the motor a bitch, a bearing press or pull must be used.

One thing to remember here; We may be trying to solve a problem that doesn't exist.

What I mean by that is, I have not had any cooling issues with two different outrunners and my recumbent motor is hidden with very poor airflow. One thing that really helps is mounting the motor to an aluminum plate or housing. That pulls heat from the motor. But, even without that, they still run relatively cool.

A comment was made regarding motors being used in airplanes in a stream of air. True. However, most outrunners and inrunners for that matter, are also designed with helicopters in mind. Heli motors are not in the air stream. Heck, they are normally under the canopy surounded by a bunch of equipment. Yet, they run great with a simple can fan or even without a fan. Plus, helis pull alot of current continuously and they have no problem with heat build-up normally.

Also, the liquid cooling of RC boat motors is required because of the HUGE drag of water and the high RPM tiny motors that boat guys pull crazy wattage out of. High performance boats often use tiny motors that are 25mm diameter or smaller, spinning up to 60,000 RPM pulling over 3kw continuous. Their motors are also inside a water-tight boat hull that has no airflow.

The only heat issues I have seen using RC motors in their rated output has been with cheapo motors.

Cooling does increase efficiency. That is why I run a fan on my motors. However, heat is just not a big issue with decent quality RC motors. The only problem I could see is climbing up steep hills with a motor that is far too small. But, that would be the exception, not the rule.

I pull my kids with my Burley trailer up steep hills pulling 4KW to 5KW without any problem. With both my kids in the trailer, the total load is over 400 pounds. Yet, my motor runs fine with just mild warming present and that is geared for 40mph top speed. Yet, I pull them at 15 to 20mph up VERY steep hills without any problems. I did the same with my AXI as with my Plettenberg.

Cooling will only be an issue with poor quality motors or a motor that is way too small.

My .02

Matt

Ben said:

Wouldn't a TEC reduce the wasted power of the motor by the same amount required to run itself, therefore having no effect on efficiency?

That's before taking into account the terrible efficiency of the devices. The only thing I see it doing is cooling down the motor so it lasts longer.

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You may well be right. Im not up to speed with the maths for the gains in efficiency given by ?degrees cooler windings.

It is true that you would be countering X watts of heat dissipation caused by resistive loss in the windings, with Y+Z watts of extra battery power being consumed by the peltiers - with Z being the peltiers inefficiency.

The theory would only work if the motor's efficiency saving was greater than Y+Zwatts. In other words you would need a greater return in power saving than your investment of battery consumption.

Probably unlikely if the motor is sized correctly. Could be feasible if switched on only for steep hill climbing on small ungeared motors.

That's probably true. My concern spawned from one of those cheapo motors I ordered(I'm learning, so I don't want to bust something expensive. ) is that the "danger" thermal output was about 700-800 watts according to peakeff.com and it appears it's an issue if ran there for a minute or two. According to my vehicle simulator using some estimated parameters, it seems like it could easily reach that if I start going up a 4% hill or more and my new future environment has an untold amount of hills so I'm naturally concerned about cooling these things. But I imagine the highest hill I'll end up climbing will be a 50-100 feet tall with coasting on the other side, so maybe it won't be a problem? I suppose it might be running at 900 watts generated-heat for a minute and it'll have a minute to cool off while I coast down the other side.

Ben said:

The only thing I see it doing is cooling down the motor so it lasts longer.

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Exactly! Burning up a motor means no more motor.