BL Motor temp question.

[BrianG]

OK, I've read here often enough that running a BL with gearing that is too high will heat it, the ESC, and the batteries. Easy enough to understand. But what makes a BL motor heat up with too-low gearing? You'd think that with less load that it would remain cooler?

[HotnCold]

I believe over reving the motor.

[Sylvester]

Yes, it makes the motor spin alot more for slower speeds that can be obtained with higher gearing.

[BrianG]

OK, so what if the rpm is kept within its rated rpm?

[coolhandcountry]

Accually what happens with a low load on a brushless is that the rpm goes up and the efficency goes down. So all the extra heat is generated by the lost of efficency. If you keep a brushless lower rpm I find you can have a lower gearing with out the problems of over heating.

[HotnCold]

Originally Posted by HotnCold

I believe over reving the motor.

Woo hoo - i got one right... :005:

[BrianG]

Originally Posted by coolhandcountry

Accually what happens with a low load on a brushless is that the rpm goes up and the efficency goes down. So all the extra heat is generated by the lost of efficency. If you keep a brushless lower rpm I find you can have a lower gearing with out the problems of over heating.

I can see how overall efficiency would technically be reduced, but since there is less current being drawn, there is less heat on the motor (P=V*I), despite the inefficiency. Heating from overrevving makes sense though.

For example, I ordered a Wanderer 8XL (kv=1875) and plan to run that around 16.8v - maybe as high as ~20v in the future. According to the BK site, this motor is rated at 80,000 max rpm. So, even at 20v, that's only 37,500 RPM. I would have to run around 40v (maybe a bit less since the kv rating is the loaded value) to get close to 80k RPM. That being said, I really couldn't overheat from over-revving could I?

I'm not trying to be argumentative; I just to be sure about this stuff. :) The reason I'm going BL is so it is more worry free and plug-n-play than Nitro or brushed setup. I was hoping I wouldn't have to carry my temp sensor around with me to check temps every 5 minutes. I figured as long as the motor was not running free, I'd be fine as long as I made sure the gearing wasn't too tall.

[maxxdude1234]

When bk say the motor's rated to 80,000rpm, they're really only refering to the bearings. A BL motor is in its peak efficiency at around 35,000-40,000 rpm (bigger motors will be less, and smaller more) - but for 540 size motors it is this.
As long as you get your gearing right, you won't need your temp gun at all. If you use the first say 5 runs to gauge the right gearing (checking motor/batt/controller temps) and gear appropriately. You should be fine after that. Obviously if it is a particularly hot day, you might want to keep a constant check on temps anyway.

[HotnCold]

80,000 rpm's - could you imagine that in a r/c car - Holy chit....

[maxxdude1234]

Originally Posted by HotnCold

80,000 rpm's - could you imagine that in a r/c car - Holy chit....

They little mamba motors will do close to 80,000rpm, and reasonably efficiently too. Crazy!!!

[Serum]

I got one 8KV running at 11.1V, but castle recommended not to use it at WOT for more that 4 seconds at a time.. (which is impossible, because with .5 sec. wot, the truck is on it's roof, against a curb or out of sight...) (last option is a rather rare, yet glorious moment)

It SCREAMS for sure.. Too bad, i lost the movies, once the road gets a bit dry, i will go out and shoot a movies of that little thing in action..

I would choose a 9 or a 10XL if you are planning on using 20 cells. an 8XL could do 20cells, but it is quite a stress for both the motor and the controller if you don't use a mechanical brake.

[coolhandcountry]

If you run the brushless closer to the 30,000 rpm mark they have decent power and don't run near as warm. The 40,000 is not bad though. What you planning is going to be good brian.

He said 20 volts serum. Not 20 cells. ;)

[Serum]

LOL, it must be my selective reading...