Outrunner self-cooling testing

[swbluto]

So, it's been hypothesized that the open-design of an outrunner helps the motor cool itself, and that by spinning the motor, the spinning helps circulate the air and cool it off even faster.

So, this is the motor I'm using (Unlike what's pictured, it also has a skirt bearing and the can has screws in it; it's also colored orangish instead of blue.):

http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=4188

And the voltage I was using was 29 volts.

I spun the motor at no-load which, during testing, consumes somewhere around 4 Amps.

That implies the motor was spinning 29*230 kV = 6670 RPM or thereabouts.

With the courtesy of an extra 200 pounds of load provided by a relative on a bicycle and towing him up a hill, I was easily able to get the motor's winding upto 185 degrees fahrenheit. After detaching the drive system and spinning the motor at no-load for a minute, it decreased to 175 degrees. After doing it for another minute, it decreased to 165 degrees.

Now, for the comparison testing. I measured the motor's winding's temperature, just let it sit, and then measured it again sometime later. Doing this, I recorded 180 degrees when I first began and then recorded 148 degrees 7 minutes later.

This implies the average cooling rate while at rest is (180-148)/7 = 4.57 degrees fahrenheit per minute.
At 6600 RPM, the average cooling rate was 10 degrees fahrenheit per minute.

So it appears that spinning does enhance cooling, though only upto 10 degrees fahrenheit per minute in my testing.

For my application, I kind of wished it was faster because, if the motor is at the "danger zone" of 250 degrees or so, then that implies I might have to wait 8ish minutes to get the motor down to 130-140 degrees using active cooling, or 15ish minutes using natural cooling at rest. While faster, 8 minutes is kind of a long time.

 

[REdiculous]

8)

Interesting findings...I'll have to do something like that for giggles.

 

[boostjuice]

stick a fan on the end of the can.....

 

[John in CR]

swbluto,

I've been looking at the outrunner I bought from you and keep wanting to take it apart and use a dremel with a grinding disk to make those spokes at the can support end blade shaped instead of squared edges, so they will naturally force air through the motor. I want to get some vinyl ester resin to glue those windings in place anyway, and it has to come apart, so I might as well mod away while I'm there. Hasn't anyone done that already?

John

 

[recumpence]

John, you and I think sooo much alike!

I did that (blade shaped can spokes) 6 years ago with my RC helis. It does work, yes! I eventually crashed hard and broke that can. But, admittedly, I really hogged alot of material from those spokes.

Matt

 

[spinningmagnets]

Air compressor pulleys often have bladed spokes. How much it helps, I don't know, but, as long as theres spokes there, it couldn't hurt to have them shaped to move some air.

Is it better for the blades to push air into the motor, or to pull air out of it? Perhaps if it pulls, it would be easier to make a foam dust filter to attach to the other end?

 

[johnrobholmes]

Scorpion has done some interesting centrifugal and bladed fan designs on their outrunners. It has certainly allowed them to bump up the amp ratings on the motors.

 

[swbluto]

Yes, good idea. The next time I take apart the motor (Which hopefully is a loooong time from now), I'll be sure to attempt "blading" the can's "spokes". And then I'll add the temperature testing results. That might take some time though... it might actually be a while before the motor starts making irritating sounds given it's already lasted quite a bit longer compared to my last unskirted outrunner and scooter.

 

[Kepler]

Scorpion has done some interesting centrifugal and bladed fan designs on their outrunners. It has certainly allowed them to bump up the amp ratings on the motors.

One of the reasons I really like Scorpion motors and are supprized not to see more with ebikes. Run a Scorpion motor on the bench at full speed and its like a leaf blower with the amount of air it moves.

 

[AussieJester]

I did that (blade shaped can spokes) 6 years ago with my RC helis. It does work, yes! I eventually crashed hard

/OT: Matt i was curious do you still fly RC helis these days or have you given the game completely away now?
if yes will you ever likely get back into it? Be the RC Heli scenes loss and e-bike scenes gain haha

Cheers

KiM

 

[swbluto]

I was looking at the recent hobbycity blog post at http://www.hobbyking.com/hobbyking/store/blog.asp on the new large motors coming up, and I noticed this interesting discussion...

Humboldt,Ca 7/24/2010
Jason, also please mill the openings in the rotor at at a 45 degree angle so the openings force air though the motors.

...


Jason5001 7/25/2010
Humboldt,Ca: we tried that, it doesnt work. We tried a thin propeller style end-bell but it simply doesnt push any air through past the stator and wire.

 

[spinningmagnets]

...We tried a thin propeller style end-bell but it simply doesnt push any air through past the stator and wire...

I...don't...believe it. If they didn't get at least a tiny increase of airflow,...they're doing it wrong.

 

[swbluto]

...We tried a thin propeller style end-bell but it simply doesnt push any air through past the stator and wire...

I...don't...believe it. If they didn't get at least a tiny increase of airflow,...they're doing it wrong.

Maybe it didn't make a significant difference?

I'm not well-versed on propellers, but the "propeller diameter" in the case of the can seems to be 63 mm(?)(Or 2.8 inches?) and the RPM is 6600. Is that enough to generate any significant airflow with an ordinary prop? It seems that hobbycity suggests a prop with a length between 14 and 18 inches.

However, the propeller might work better at higher RPMs. I don't think the skirt bearing, though, could go that much higher without problems. Maybe it could. *shrugs*

This motor suggest a 4" propeller for it: http://www.hobbycity.com/hobbyking/store/uh_viewItem.asp?idProduct=7338&Product_Name=1404N_2290Kv_9.2g_3.5A_160g_Thrust_Outrunner

At that size, it suggests 3S (or 10V or so) and the prescribed kV is ~2300. That's an RPM of 23,000. Even so, the max rated current is like 3 amps which is the equivalent consumption of 30 watts. Assuming 30% of that is being converted to thrust, that's like 10 watts of airflow. Now shrink that from 4" to 3", and reduce the RPM from 23,000 to 6,000 and I'm going to guess less than 4 effective watts is allocated to cooling. That implies less than 4J could be transferred per second, and the thermal energy of the can is probably a LOT higher than that.

It seems the specific heat capacity of steel is 0.46 J/gK and I think that means the thermal energy of the can is .46 * 800 grams * ~370 K = 136,160 Joules. In a minute, you'd transfer upto 4W*60seconds = 240 J which is a temperature difference of (240)/(.46*800) ~ .5 degree celsius or a degree fahrenheit. Let me know if I've made a wrong assumption in my calcs.

 

[fizzit]

...We tried a thin propeller style end-bell but it simply doesnt push any air through past the stator and wire...

I...don't...believe it. If they didn't get at least a tiny increase of airflow,...they're doing it wrong.

Well I mean if you think about it... these motors need to spin propellers with much larger diameters than the actual motor to generate airflow, because of their low kv. So you would need a lot of blades and pitch to make any airflow, especially because the inside of the motor probably isn't too aerodynamic.

 

[dontsendbubbamail]

I tried forcing air through the motor, but it didn't seem to make a difference. The gray ring fits tightly around he motor so any air that the fan moves has to go through the motor. I slid the fan back on just for the picture.

Bubba

 

[John in CR]

My recent ventilation of controller for cooling showed me how poorly axial fans push air through any resistance. Making the can spokes somewhat blade shaped will have to move some air through, but they can't create much pressure differential to push much air through the restriction inside the motor without throwing crazy power at it like an RC ducted fan.

It sounds like Scorpion went the route that will push or pull air through restrictions, which is a centrifugal fan. Now I just need to source one I can fit to an outrunner like I did on my controller.