9c Hub Motor Temperature Limits

dozentrio

10 kW
Joined
May 26, 2009
Messages
516
Location
Canada
I'm running a "1606" 9c motor, from Grin Cyclery ( ebikes.ca ) and am starting to become interested in the true limits of it.

The magnet wire coating isn't a large concern because I'm rewinding a motor with high temperature coating. The hall sensors I'm not too worried about, as they're easy and cheap to replace. My main concern is the magnets. Does anyone know what temperature you can take these too before they become damaged? I know there's different grades of these rare earth magnets, but so far I've only heard people say they're good from between 80 to 100 C. That's alarmingly low, and I wouldn't be surprised if I've already surpassed those temperatures once or twice. Does anyone have any information on the temperatures these magnets can take? Also, any guesses about the relationship between temp at windings, and temp at magnets? Suppose my thermister measures 120 C at the windings, can I safely assume the magnet temp is still only 90C or something?
 
Vent the motor as much as possible, and you reduce the thermal coupling between stator and magnets.

I have a hunch the grade of magnet is going to fluctuate between batches in hubmotors if you're looking for a fixed do-not-exceed number.
 
I know many people have drilled holes in all kinds of hub motors for air cooling, but I'm still somewhat concerned. Won't water vapour, rain, moisture, etc, damage the motor over time?
 
dozentrio said:
I know many people have drilled holes in all kinds of hub motors for air cooling, but I'm still somewhat concerned. Won't water vapour, rain, moisture, etc, damage the motor over time?

i'm worried too, but also for the risk of having some debris or small rocks entering in there and getting trapped between stator and rotor and damaging them. Looks a bit extreme to me. Drill holes ok, but maybe having a little mesh of some kind to prevent debris from entering but allowing vapor to escape would reassure me.
 
Hugues said:
dozentrio said:
I know many people have drilled holes in all kinds of hub motors for air cooling, but I'm still somewhat concerned. Won't water vapour, rain, moisture, etc, damage the motor over time?

i'm worried too, but also for the risk of having some debris or small rocks entering in there and getting trapped between stator and rotor and damaging them. Looks a bit extreme to me. Drill holes ok, but maybe having a little mesh of some kind to prevent debris from entering but allowing vapor to escape would reassure me.

I wonder why the air-drilling always seems to include such big holes....surely a lot of small holes will let in the same amount of air???
 
I'd like to know the max temperature also. I think I maybe have damaged one of my motors by running 5Ah through it in 10 minutes on 35C outside temperature multiple times. It gets too hot to touch for longer than 1 second.
 
The Mighty Volt said:
Hugues said:
dozentrio said:
I know many people have drilled holes in all kinds of hub motors for air cooling, but I'm still somewhat concerned. Won't water vapour, rain, moisture, etc, damage the motor over time?

i'm worried too, but also for the risk of having some debris or small rocks entering in there and getting trapped between stator and rotor and damaging them. Looks a bit extreme to me. Drill holes ok, but maybe having a little mesh of some kind to prevent debris from entering but allowing vapor to escape would reassure me.

I wonder why the air-drilling always seems to include such big holes....surely a lot of small holes will let in the same amount of air???

Nope. That's not how air works.
 
Motor temp limit, for any motor is 1 degree less than when it melted. It's weather dependent since we are air cooled, so even stating numbers measured inside the motor can have different results in different weather. But even cold weather is no garantee, I've melted down several motors on days under 50F.

I find I have little to zero problems if I keep a motor's temperature under 200F, measured inside, 150f measured outside. Those are the limits I shoot for when reliability is needed.

At the DR, when I blew smoke out of my motor, it was measured very soon after at 450F. The smoke was mostly the twine inside the motor that ties the windings in place igniting. Presumably that happened at 451F. I have heard of people getting away with 350 F for a while. Clearly I had a running motor at 400f+, but not for long of course. No telling at what exact temp a particular hall sensor will fail, but motor winding varnish seems to take at least 250-350, at least for a while. Goes without saying that the wiring insulation needs to be able to take high temps to go there, so you need good stuff.

If you plan on playing with more than 1500w, then it's definitely time to put a thermometer on or in the motor.
 
I'm actually just starting to monitor/record temps on my ventilated (3/4" outer perimeter holes) 9c rear 8x8 motor. I got my first data point last night. At 15A (100V) the motor stabilized at 58.1C while in motion, and climbed to 62.1C when I stopped for a bit. This is with a 20.0C ambient temp at 900' above sea level. It may take me a year to get enough data points to come to any conclusion.
 
My biggest worry is that high temperatures will -damage- but not ruin the magnets. Then I'll be getting less power without knowing it, because the motor still seems to work. And THAT would be an atrocity! not even -knowing- you're not getting the most out of your motor.
 
liveforphysics said:
Nope. That's not how air works.

I once read a patent case about a rubber mat which was filled with tiny tiny holes. At high-centrifugal forces, eg, in a washing machine, the holes opened and allowed water to flow through them, thus enabling easy washing of the mat. Once the machine stopped spinning, the holes closed in again, and the mat resembled, for all intents and purposes, a contiguous sheet of rubber.

Is this not possible with air? Lots of small holes, a motor hub being spun at high revolutions, with the holes located on the opposite covers so that air moves from an area of high pressure to an area of lower pressure, on the other side, such that it is pulled across the windings and thus creates a cooling effect??
 
dozentrio said:
My biggest worry is that high temperatures will -damage- but not ruin the magnets. Then I'll be getting less power without knowing it, because the motor still seems to work. And THAT would be an atrocity! not even -knowing- you're not getting the most out of your motor.

High temperature does damage magnets. So does striking them with a hammer.
 
The Mighty Volt said:
liveforphysics said:
Nope. That's not how air works.

I once read a patent case about a rubber mat which was filled with tiny tiny holes. At high-centrifugal forces, eg, in a washing machine, the holes opened and allowed water to flow through them, thus enabling easy washing of the mat. Once the machine stopped spinning, the holes closed in again, and the mat resembled, for all intents and purposes, a contiguous sheet of rubber.

Is this not possible with air? Lots of small holes, a motor hub being spun at high revolutions, with the holes located on the opposite covers so that air moves from an area of high pressure to an area of lower pressure, on the other side, such that it is pulled across the windings and thus creates a cooling effect??

For a given area (one large hole), you will pass "X" amount of air. If you create smaller holes with the same total area, you will pass less than "X" amount of air since there are more losses (more total hole perimeter per given area). In engineer speak, you have more weir flow than orifice flow, therefore more friction losses. Think about your sliding screen door. Many holes, little air flow. If you just had one large hole of equivalent area, it would pass much more air.
 
I think I've actually damaged the magnets some in my front motor as it doesn't seem to have the same power as my rear motor with the exact same windings/controller/battery. I guess I'd have to do a test to make sure.
 
Hi Itchyknackers...thanks for that. I wasn't disputing what LFP had said, never would....I know very little physics :D ......but, forgive me for being argumentative.....a screen door doesn't revolve at 500+ rpm!! :D Anyways just an idea, I thought I might be able to overcome the small hole size by making more of them and using un-equal air pressures on either side to create a flow.
 
el_walto said:
I think I've actually damaged the magnets some in my front motor as it doesn't seem to have the same power as my rear motor with the exact same windings/controller/battery. I guess I'd have to do a test to make sure.

Check no-load speeds.
 
el_walto said:
I think I've actually damaged the magnets some in my front motor as it doesn't seem to have the same power as my rear motor with the exact same windings/controller/battery. I guess I'd have to do a test to make sure.

Some front motors may not have the same power as their rear equivalents- a good source of information on this would be the HT/HS series.
 
The Mighty Volt said:
Hi Itchyknackers...thanks for that. I wasn't disputing what LFP had said, never would....I know very little physics :D ......but, forgive me for being argumentative.....a screen door doesn't revolve at 500+ rpm!! :D Anyways just an idea, I thought I might be able to overcome the small hole size by making more of them and using un-equal air pressures on either side to create a flow.


It's unfortunate with respect to keeping debris out of a motor (which in practice really isn't an issue), but you're never going to be able to use a bunch of small holes to get the same airflow that is possible with large holes.
 
liveforphysics said:
The Mighty Volt said:
Hi Itchyknackers...thanks for that. I wasn't disputing what LFP had said, never would....I know very little physics :D ......but, forgive me for being argumentative.....a screen door doesn't revolve at 500+ rpm!! :D Anyways just an idea, I thought I might be able to overcome the small hole size by making more of them and using un-equal air pressures on either side to create a flow.


It's unfortunate with respect to keeping debris out of a motor (which in practice really isn't an issue), but you're never going to be able to use a bunch of small holes to get the same airflow that is possible with large holes.

Too bad. That was the idea, small holes preventing little rocks getting in and bouncing around in there. Oh well. PS I am not sure that stones getting into the wheels is all that likely, except maybe on very very heavy ground.
 
Re magnets. I melted down a heinzmann motor real good. The magnets all came unstuck from the disk inside. Since they are flat disk magnets, I thought at least I had some really strong fridge magnets. But they wouldn't stick to steel anymore. Cooked good.
 
The best solution here would be to force air out of the holes that you drill (whether they be big or small, though don't make them too big! Those side covers are actually what ultimately hold your spokes in place and your wheel together)

Have a powerful turbine pump push air into a relatively large cup shaped device on the end of your axle. The cup is stationary, but the wheel moves, so there needs to be a small gap (as small as possible) between this cup and your side covers. Then drill your input holes under this cup. The air being forced in through here is filtered for dust and particles. The other holes will constantly have outwards airflow, thereby stopping any nasty particles (such as uranium) from going IN. +1up nuclear industry for this idea.

Actually it's ChrisLuxembourg's idea, and his cup thingy covers nearly the entire side panel. Quite neat, although I'm uncertain about the end effectiveness since i think he's not using the idea anymore. http://www.youtube.com/watch?v=Kvm7v6TMykU#t=02m10s
 
Interesting idea. For those of us with small bearings ( :cry: ) I wonder if we could drill the non-wire side axle and force air into the axle. Wouldn't even need to run a tube down the axle, since the axle would act as the tube. Hmm.. I guess I'd have to do some before/after testing to see if it actually cooled more or less than passive cooling.
 
liveforphysics said:
el_walto said:
I think I've actually damaged the magnets some in my front motor as it doesn't seem to have the same power as my rear motor with the exact same windings/controller/battery. I guess I'd have to do a test to make sure.

Check no-load speeds.

I agree with Luke. Weaker the field the faster the motor and also the more currect it will induce because of the low BEMF from the weak magnetic field from the damaged magnets.
 
itchynackers said:
Interesting idea. For those of us with small bearings ( :cry: ) I wonder if we could drill the non-wire side axle and force air into the axle. Wouldn't even need to run a tube down the axle, since the axle would act as the tube. Hmm.. I guess I'd have to do some before/after testing to see if it actually cooled more or less than passive cooling.

I kind of like this idea. It'd be even better if the axle allowed enough space to have both an in and an out tube, so you dont have to mod the side covers at all. Though I doubt that you could get enough airflow to make a notable difference. A tube running forced air in via the axle and then a series of holes around the outer edge of the cover near the inside of the stator could work though? maby have the blower wired up to only switch on when internal temps reach say 80degC?

On second thought it'd probably work better to have the tube as the 'out', ie it sucks air in via the holes in the cover, so you get the coolest possible air flowing over the area you're trying to cool. Though now I cant get the image of rocks, gravel and even small birds being sucked into the holes on the side of my motor out of my head :shock: .
 
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