Definitive Tests on the Heating and Cooling of Hub Motors

[justin_le]

Tight tolerances in mass-produced assemblies becomes expensive very quickly.

Indeed, already the air gap at ~0.5-0.6mm is about as tight as we'd want things for mechanical and assembly tolerances, and is the source of occasional rubbing and interference. I suppose if you had a super thin precision gap and then used compressed air to make this into a viable air bearing then you could... STOP! The point of this thread is "Definitive Tests on the heating and cooling of hub motors" and not "Endless Banter on the heating and cooling..." (unless of course such idle banter is to suggest trying ferrofluids, in which case )

So back to being focused on things that can be tested and the results they show. We've taken a lot of time in the last week getting the wind tunnel and associated measurement equipment revamped so that we can have the testing and analysis be largely automated, and rather than it taking 3-4 days for me to do all the experiments to make a conductivity vs rpm/speed plot, we should be able to get it down to just a few hours.

One of the main steps in this effort is to consolidate all of the recorded info into a single data file, rather than a multitude of IR camera images, CA logs, and logs from the BACdoor controller. For this I've repurposed a CA3 with special firmware as our main acquisition device. The PAS input and Speedometer inputs are configured to show the wind speed and motor RPM, while the torque sensor input was converted into a 2nd temperature sensor for measuring ambient. So now this one device is able to record motor watts, motor RPM, motor core temperature, ambient temperature, and wind speed:



Previously to get the shell temperatures I was using an infrared imaging camera, but that makes the data analysis and especially the transient analysis quite tedious since the data is captured as image files rather than numeric. So instead we purchased three MLX90614 discrete IR temperature detectors with a tight 10 degree field of view.

View attachment 2
We have them mounted on magnetic bases so that we can position the sensors to different parts of the shell, including directly on the edge view of the rotor rather than just the side plates as we were doing with the thermal camera. The plan in general would be to have one on the edge, one on the end of the side plate, and another nearer to the center of the side plate.

A small arduino circuit then intercepts the CA data stream and appends the 3 different IR shell temperature readings to the end of each line, so that what finally comes to the computer is a data stream with the motor RPM, motor heat power, the tunnel wind speed, the motor core temperature, the ambient wind temperature, and 3 IR shell temperatures, all primed for easy transient (rather than steady state) analysis.


Previously we weren't actually using an anemometer during the test as we had a correlation of fan RPM vs wind speed, but that doesn't account for the effect of the sample blockage in the test chamber. So I happened on this inexpensive smartphone anemometer at local shop which is designed to plug into a phone's stereo/microphone jack. It has a tiny circuitboard in it to get power from AC signals on the audio output jack for running an optical encoder, and with a bit of hacking of the board it was easy to make it run off DC instead with a DC pulse signal sent straight to the CA.


We compared with displayed speed readings that it showed on the phone app with the pulse period of the encoder as measured on an oscilloscope to get the calibration curve that they were using, which is a simple straight line + offset:

 

[spinningmagnets]

Endless banter? your "throw away" banter is more useful than some peoples research. Post on, bro...anything that pops into your head.

 

[Kodin]

Actually... compressed air for an air bearing might be viable for cooling a stator as well... Sadly it's not viable as I doubt there's an effective way of generating enough pressure and volume in a bike-sized vehicle running on batteries. Still intriguing though!

 

[liveforphysics]

Nice test rig, and great idea with the ferro fluid tests.

If it works out, tomorrow I will do some before and after ferro fluid dyno runs. I was thinking perhaps just holding 1.5kW motor output with the dynos fans blowing. Maintain that state for 15min and take thermal images, then add ferro fluid until no-load just starts to rise, and re-dyno for 15min at 1.5kW and take thermal images again to compare with the first set.

If more of the outside of the case it hotter after ferro fluid, it would indicate the internal motor temps are lower and the motor is capable of greater continous power.

 

[justin_le]

If it works out, tomorrow I will do some before and after ferro fluid dyno runs.

Sweet, I would love to see some independent testing results too. Just make note of the approximate quantity needs. For the 45mm wide MXUS stator, ~200 mm diameter, we needed just 5mL of FF. So that works out to about 0.02mL / cm^2 of motor air gap area.

I was thinking perhaps just holding 1.5kW motor output with the dynos fans blowing. Maintain that state for 15min and take thermal images, then add ferro fluid until no-load just starts to rise, and re-dyno for 15min at 1.5kW and take thermal images again to compare with the first set.
If more of the outside of the case it hotter after ferro fluid, it would indicate the internal motor temps are lower and the motor is capable of greater continous power.

You gotta keep in mind that at steady state the shell temperatures will be the same whether there is FF in there or not, from a black box perspective if a given amount of watts needs to escape the shell it will be at a given temperature. But transiently while the motor is heating you are right that with better thermal linkage between stator and shell then the shell will get warmer sooner and you should be able to see this, although it may only be a few degrees, not nearly as significant as the difference in the actual stator temperature inside the hub. This is a perfect usage case to play around with the themal aspects of the trip simulator here:
http://www.ebikes.ca/trip-simulator-test
Create a long slope incline using say the MXUS 4504 hub, set the cursor somewhere along the slope to look at the core and shell temps, then switch to the 4505_Ferrofluid option and compare the differences. In this example you see that the shell temp goes from 46 °C to 51 °C with the addition of FF, while the core temp has dropped from 146 °C down to 113 °C


You wouldn't necessarily presume that a 5 °C increase in your shell temperature was associated with a 33 °C decrease in the core!

 

[liveforphysics]

I have an internal winding embedded temp sensor as well as the FLIR images. I suspect it will show something.

The core takes so long to heat from your data, perhaps extending the test to 1hr of 1500W (Im fed from a 250A capable power supply) would reach the full temp steady state saturated point, at which just comparing the outside surface temps from FLIR would be most useful.

Im am open and willing to do a better test plan if someone has one.

Her air cooled base line isn't quite stock. I added some heatsinks and gave it a light coating of the high emissivity paint doc recommended. The heatsinks function is mainly just to stir the air inside the hub, ideally causing more turbulence and hence heat exchange, lowering the core to shell deltaT a bit. I already gave away my 2nd one of the 1500w leafmotors, or i would swap on a full stock one to do the testing.

 

[justin_le]

I have an internal winding embedded temp sensor as well as the FLIR images. I suspect it will show something.

Embedded temp sensor on the windings is by FAR the most useful thing. The FLIR images are great if you want to the compute or derive conductivity values, but for seeing the effectiveness of a cooling strategy embedded winding sensor is the way to measure.

The core takes so long to heat from your data,

This is a fact that I feel isn't appreciate enough. Most people, and _especially_ the higher power hot rodding people, don't ever come close to running their systems long enough to ever see steady state temperatures at their supposed comfortable power levels. The short term temperature rise is almost all dictated by the heat capacity of the steel and copper in the motor core.

perhaps extending the test to 1hr of 1500W (Im fed from a 250A capable power supply) would reach the full temp steady state saturated point, at which just comparing the outside surface temps from FLIR would be most useful.

You mean just comparing the embedded temp sensors? The outside surface temps at steady state after an hour or so will be very similar, identical in fact if you really are at steady state. The FLIR only plays a useful distinguishing roll earlier on in the test before you reach steady state.

 

[John in CR]

If more of the outside of the case it hotter after ferro fluid, it would indicate the internal motor temps are lower and the motor is capable of greater continous power.

If the same heat is generated inside the motor, then the outside shell will be the same temperature. From ambient it will have gotten there more quickly at the beginning of the run.

Assuming the stator is cooler like in Justin's testing, then with the same 1.5kw input at the same rpm, then I'd actually expect the outside shell to stabilize at a slightly lower temperature. That's because the cooler stator would reduce the copper losses.

Great to hear you're getting into it too. I look forward to seeing how the FF benefit holds up with higher rpm and heat. I have a feeling that the ferro fluid manufacturers need to come up with a specific hubmotor formulation.

 

[madin88]

Create a long slope incline using say the MXUS 4504 hub, set the cursor somewhere along the slope to look at the core and shell temps, then switch to the 4505_Ferrofluid option and compare the differences. In this example you see that the shell temp goes from 46 °C to 51 °C with the addition of FF, while the core temp has dropped from 146 °C down to 113 °C

this is not bad!
because of you justin we now know that we can expect an enhancement of thermal conductivitiy between stator and shell of about 20% with total black painted inside. Fan blades should add approx another 20%.
With the small heatsinks i have installed additionally (they should work like a radial fan pushing air through the air gap in a circle), i believe TOTAL thermal conductivitiy between stator to shell will be about factor 1,5 better as a stock motor has.
I know FF would improve by factor 2 or more, but this i always can do later. or should i worry about the paint on stator and magnets when adding FF? And what about the thing with the glued magnets when they get soaked with oil? I get the creeps if i think of that!

this is coming together soon:

i was not able to find the krylon paint here anywhere in EU, so i decided to use a different matt black

 

[Offroader]

madin88, I would add heat sinks inside the stator as well, you'll have to cut them to fit inside the circle but this would remove a good amount of heat from the stator and hopefully pass it to the motor cover.

Those fan blades will really help push the heat over the heat sinks to get the heat out of the stator.

 

[madin88]

madin88, I would add heat sinks inside the stator as well, you'll have to cut them to fit inside the circle but this would remove a good amount of heat from the stator and hopefully pass it to the motor cover.

Those fan blades will really help push the heat over the heat sinks to get the heat out of the stator.

i have thought of adding small heat sinks to the inside of the stator and also some with pinned fins to the other sidecover as well, but this would take another big amount of time and the other downside is it will add weight. so i decided to stop "MXUS modding project" at that point
with the custom axle, heatsinks and paint the motor has now about 250-300g more

@ Luke
do you put FF in that painted Leaf? I have a bit worry if it will work well togehter with the paint. Could it be that FF dissolves it or rub it off over time?

 

[justin_le]

because of you justin we now know that we can expect an enhancement of thermal conductivitiy between stator and shell of about 20% with total black painted inside. Fan blades should add approx another 20%.

No no, you must respect the data and not invent numbers like this which I never quoted, since you're then propagating myth again and the whole point of this thread is to stamp that out with objectively measured results. Please re-read these results posts:
https://endless-sphere.com/forums/viewtopic.php?p=1095507#p1095507
https://endless-sphere.com/forums/viewtopic.php?p=1069665#p1069665

The effect of black paint on a vented motor was less than a 5% decrease in thermal resistance (0.154 to 0.147 oC/watt), not 20%, and the effect of internal inside blades was more on the order of 10%. This could probably be enhanced with more optimized fin/blade design, but until you confirm it you shouldn't quote a number.

The good news is that it's easy for you even with no more instrument than a CA3 and a temperature probe inside the hub to confirm how much your internal conductivity has changed. It just takes a bit of patience and I really encourage you to do this to quantify the effects of your mods, otherwise there is little point in posting here. All you do is first run the unmodded motor propped off the ground and power it unloaded for several hours until it reaches steady state temperature as a result of core heating. Note the temperature of the windings and the no load power from the CA at the end of his run, and if you have the means also measure the side cover temperature (IR temp gun or just a fast response thermometer that you touch to the surface right after th run). Then repeat the exact same thing once you have completed your modifications.

The net thermal resistance from core to shell or core to ambient is derived from the temperature difference divided by the no load watts into the motor. That will tell you if the combination of painting and internal fins is a factor of 1.15, 1.25, or 1.5 as you're hoping to believe. Definitive tests... I'm not the only one here who can do this.

 

[flathill]

increasing the internal surface area will also help with a sealed motor

Adding fins to the inside of a sealed motor is helping because you are adding surface area, so don't orient for increased stirring, orient them for minimized windage losses.

a stirring fan inside a sealed motor won't do much at all besides reduce hot spots (increase temp uniformity), but can help stop magnets from demag due to heat soak. Never go on a hard run and just let you bike sit. Make sure to run around at low power for a minute unless you have stirring fans. You might not even notice if some of the mags are partially demagged but it will hurt performance and how smooth the motor runs

 

[macribs]

@Justin I am sorry to say I believe it was me that first posted the 20% number of black matt paint on the forum.
I pulled that number from memory and clearly I was totally wrong, so madin is repeating my wrong I think.

So sorry for spreading wrongful information. Next time I will be sure to do a proper check up of facts before posting and not spread wrongful info because of laziness.



Edited:
I found back to that post, I was spreading wrongful info. Just a little lower numbers. But still wrong.

When you first open the motor to upgrade wires and spray varnish do you also paint the inside of covers flat matt black to to better heat dismission? If I remember correct it was a gain around 7-10% better heat radiation with flat matt black color over the stock unpainted covers because they stock covers reflex the back to the motor more.

 

[fechter]

One thing I have observed in the past is infrared thermometers can be very inaccurate if the target is too reflective (at IR wavelengths). A bare aluminum surface will read significantly different than a painted surface. It's almost like bare metal is a mirror and you're reading the temperature of what's in the reflection. Best to verify against a thermometer stuck to the surface. Even a thin coat of any paint seems to prevent this.

I'd love to see Luke's high power test. The IR thermographs may not be as scientifically relevant, but they sure look cool. You may find surprises if you look carefully. That's what science is all about.

 

[Offroader]

because of you justin we now know that we can expect an enhancement of thermal conductivitiy between stator and shell of about 20% with total black painted inside. Fan blades should add approx another 20%.

No no, you must respect the data and not invent numbers like this which I never quoted, since you're then propagating myth again and the whole point of this thread is to stamp that out with objectively measured results. Please re-read these results posts:
https://endless-sphere.com/forums/viewtopic.php?p=1095507#p1095507
https://endless-sphere.com/forums/viewtopic.php?p=1069665#p1069665

The effect of black paint on a vented motor was less than a 5% decrease in thermal resistance (0.154 to 0.147 oC/watt), not 20%, and the effect of internal inside blades was more on the order of 10%. This could probably be enhanced with more optimized fin/blade design, but until you confirm it you shouldn't quote a number.

The good news is that it's easy for you even with no more instrument than a CA3 and a temperature probe inside the hub to confirm how much your internal conductivity has changed. It just takes a bit of patience and I really encourage you to do this to quantify the effects of your mods, otherwise there is little point in posting here. All you do is first run the unmodded motor propped off the ground and power it unloaded for several hours until it reaches steady state temperature as a result of core heating. Note the temperature of the windings and the no load power from the CA at the end of his run, and if you have the means also measure the side cover temperature (IR temp gun or just a fast response thermometer that you touch to the surface right after th run). Then repeat the exact same thing once you have completed your modifications.

The net thermal resistance from core to shell or core to ambient is derived from the temperature difference divided by the no load watts into the motor. That will tell you if the combination of painting and internal fins is a factor of 1.15, 1.25, or 1.5 as you're hoping to believe. Definitive tests... I'm not the only one here who can do this.

I like this idea on testing as I realized I had no way to quantify just how much my custom air cooling mod has done. I just wish there was a little more information about doing it as I can't really understand exactly how to do it from your post here. Do you have to run the hub motor for several hours? How exactly do you determine steady state temperature?
How do you determine the speed or watts to run the motor at?

 

[gensem]

Justin, for high performance looks like venting is still the way to go.
Edit: Just saw that you already tested most of if not all hole placements for intake/out take.

 

[markz]

Venting, cooling lines etc will be the last thing I do to my MXUS 3kw v2 motor.
In the meantime, its phase wire upgrade, maybe axle/bearing upgrade if its not too much money.
I'd want to get a baseline on what the motor can do before the cooling mod.

 

[justin_le]

I like this idea on testing as I realized I had no way to quantify just how much my custom air cooling mod has done. I just wish there was a little more information about doing it as I can't really understand exactly how to do it from your post here. Do you have to run the hub motor for several hours?

In general yes. With a more advanced analysis you could look at the shape of the temperature transient to determine the heat conductivity without waiting for steady state, but this way (in steady state) the math is super easy and clearcut if you just wait long enough.

How exactly do you determine steady state temperature?

You determine the temperature with a temperature sensor. A lot of vendors are now including a temperature sensor inside their motors which is great in which case use that, but otherwise you can easily open up a motor and glue your own thermistor sensor right on the copper windings.
Steady state just means that the temperature is no longer increasing, and for me I set roughly a 15 minute comparison window. So if you look on the CA and see the motor core temp is 42.3 oC, then you come back in 15 minutes and it's 43.1 oC, then you're not at steady state since the core is still slowly warming up. But if instead after 15 minutes and it was still at 42.3oC (+- like 0.2 degrees say) then you can conclude this is the steady state temperature and note the value.

How do you determine the speed or watts to run the motor at?

You don't determine that. Here I was suggesting you just run the motor unloaded and rely on the internal core losses to dictate the watts of heat generated and just live with / use that. Typically this will be in the range of 30-80 watts or so, it's not as much heat as you generate in copper losses when you have a loaded motor and the core temperatures will be a fair bit lower than when you are running at full power, but the beauty of thermal conductivity measurements is that to first order this doesn't really matter. The results that you get comparing with 50 watts of heat in the core are going to directly correlate as though you were generating 200 watts of heat (ie running the motor at 1000 watts, with about 800 watts going to output power and 200 watts in copper/core losses)

I just wish there was a little more information about doing it as I can't really understand exactly how to do it from your post here.

Perhaps it's easier with a hypothetical example, done in the garage where it's 20 degrees room temperature. Suppose I've got a Crystalyte H3525 motor on an ebike with a 48V battery pack and a V3 CA showing volts, watts, speed, and motor temperature.
1) I Prop the wheel off the ground so it can spin freely
2) I Use tape or zip tie or CA autocruise settings so that I can run the bike at WOT unattended. The motor in this case spins at ~45 kph (365rpm) and draws about 55 watts of power from the pack
3) I let the bike motor run spinning on the stand or whatever for several hours, occasionally checking the CA3 to see the motor core temperature until it's clear that the temperature is no longer increasing.
4) When the temperature is steady state, I now look at the CA3 again, and see that on average the motor is pulling 57 watts and the core temperature has stabilized at 47 degrees
5) Now the math. Thermal conductivity from motor core to ambient air in this scenario is:
57 watts / (47 degrees - 20 degrees ambient) = 2.1 Watts / degree.

Now we get excited, drill holes in the plate, add vanes and heat sinks, paint the insides black etc. and repeat steps 1-3. After reaching steady state, the CA is reporting 56 watts and the core is sitting at 39 degrees, but as it's late at night the garage has cooled a bit as well to 17oC. The thermal conductivity after these mods is:
56 watts / (39 degrees -17 degrees ambient) = 2.54 watts / degree.

So the modifications lead to an improvement of 21%. THAT's how you quantify the effects that you're trying to achieve. The really important detail is that you don't use external brakes or have anything else that is putting drag on the motor, since then the watts showing up on the CA screen will include not only the heat being generated inside the hub, but also the work being done producing heat outside the motor too. The key to getting these results is that you know to first order just how many watts of heat are being produced inside the motor.

 

[liveforphysics]

I dyno'd it! The FF works in a non-subtle way, enabled a big boost in continuous power.

I have everything on videos that need to be edited together, but I got a late start on dynoing it because I was hoverboarding with Tony Hawk this evening. Not a bad reason to be late in getting to something.

The friend behind me holding the gopro is Thomas, he is the man who you see flying in this video:
[youtube]L75ESD9PBOw[/youtube]

Tony Hawk rocked the board like a champ.
Image1
Image2
Image3

ATB,
-Luke

 

[madin88]

Sorry for that posting with incorrect values. After reading all these tests and graphs i had the wrong numbers in my mind. And it was not your fault macribs!
Now after reading again i catched that painting the covers and sides of the stator with windings will improve things about 6%. with the entire inside painted black (magnets and the part of the stator facing them as well) the motor will have about 9-10% better thermal conductivity. with the fan blades you noticed an additional improvement of about 10%.

that means the improvement in thermal conductivity from core to shell on my motor likely will be about factor 1,2 better as i would have left it stock. Well, thats not much but better than nothing and the most important thing for me the motor has remained SEALED.

Thank you Justin for the detailed description of the procedure. i think this can be done also with the Adaptto unit, but i believe it will take hours to get the motor to a steady temp with only no load losses used for producing heat..
i more probabaly will i do a test in practice like riding up the long steep grade near my home with full throttle and see how things compare between this modded 3t and stock 3t motor. there are not many cars so most of the time the street will be mine. with a speed limit in combination with full throttle from standstill till the end point, the generated heat should be quite similar and i expect to get accurate numbers. i will put them into a chart like core temp, ambient temp, time it takes to cool down from value x to value y..
---
I have seen that hoverboard in TV yesterday. there was "back to the future day" with celebrations all around the world nice!

 

[markz]

that painting the covers and sides of the stator with windings will improve things about 6%. with the entire inside painted black (magnets and the part of the stator facing them as well) the motor will have about 9-10% better thermal conductivity. with the fan blades you noticed an additional improvement of about 10%.

Gloss or Flat black?

 

[madin88]

that painting the covers and sides of the stator with windings will improve things about 6%. with the entire inside painted black (magnets and the part of the stator facing them as well) the motor will have about 9-10% better thermal conductivity. with the fan blades you noticed an additional improvement of about 10%.

Gloss or Flat black?

https://endless-sphere.com/forums/viewtopic.php?f=2&t=48753&start=525#p1031975

 

[liveforphysics]

Before and after FF video.

[youtube]QerQ5B-8DfQ[/youtube]

 

[spinningmagnets]

Trans-scription from the above video as an index of events

00:05 just had the motors thermistor reading 1.4 (no FF added yet), which is the lowest I've seen it
00:14 see what the motor looks like now (44C-45C)...a bit hotter...here we go...seems to be hotter plates on my motor
00:52 Just had the thermistor reading 1.2 kilo-ohms...you can see the power going into it to heat it this fast...
01:16 that's full throttle...and this is the outside of the motor (50C-53C) no FF yet. (throttle off, motor RPMs start winding down)
02:34 (sideplate reaches 54C) assymetrically hot covers...or, asymmetric emissivity on the covers...(55C) not very hot either
02:50 and the thermistor is now at 1.94K
03:00 OK, we got the wheel up now...I'm measuring "no load" before adding FF (eco mode 30-MPH)...now I'm in normal mode...
03:40 (58-ish MPH) that is not very stable...try boost...(boost mode, 63-MPH)...
04:04 looks like eco's the most stable, so I'll use that for trying to tune the amount of FF to put in my motor
04:22 OK, here's 3 ml of FF. (adds FF to motor) Wow, its boiling...[it wasn't boiling, its just the air from the fans]
05:06 OK, I'm gonna wipe that up (FF drip) and then measure the no load
05:16 OK, gonna do a max power pull. I've let the motor cool off until it's (the thermistor) at 26...almost 27 kilo-ohms

05:22 I've added FF to the motor...[I spilled a bit, but ended up seeing a no load increase at ~5ml and stopped adding more]

05:28 I'm gonna do another full power XXX...I think thats all visible
06:02 thats full throttle...(boost mode 47-MPH)
07:10 (39C-42C) (indistinct words)
07:25 give it more throttle...(motor continues to run)
08:08 (boost mode 47 MPH)...(dyno readout shows 49-MPH)...(48-MPH)...(indistinct words)
09:04 ...not a very stable WOT reading...(also, readout shows 41A-42A continuous)...(motor continues to run)...indistinct words
09:45 (51C...as motor slows down, heat rapidly rises to 60C)...indistinct words
11:00 (48 MPH) I left it pinned in boost...outputting about...2.25 HP...drawing about...26-27 hundred watts (42A X ~65V according to screen)...
11:30 this is the temperature that it seems to roughly stabilize at...
11:54 [FF works for cooling, I have no idea if it will cause long term corrosion or dissolve the magnet glue or wire varnish]
...indistict words...