Motor "Sweet Spot" - RC-Monster Forums

BrianG · **Motor "Sweet Spot" -** 06.30.2006, 10:39 AM

I was looking over some rpm charts for various BL motors and noticed they can have quite a high amount of winds, which brings the kv down to as low as ~500. On these motors, is the sweet spot still 35k-38k rpm? If so, then they must be designed primarily for high voltage. For example, motor with a kv of 500 would need 76v to reach 38k rpm. I'd imagine the current would be quite low though.

squeeforever · 06.30.2006, 10:54 AM

Yes, in a car, the "sweet spot" is usually around 35 to 40k even for those motors. Just remember, most of these are intended for planes. From what I understand, the motor on a plane doesn't spin as fast. I think around 25k. Not really sure why.

MetalMan · 06.30.2006, 11:06 AM

Planes need to run limited RPMs because of the propellors that they use. But, they can use gearboxes on the motors, so there's no reason why a plane can't have a motor spinning faster.

Are these motors with a kv of 500 outrunners or (standard) inrunners? Outrunners function differently, and work with a much lower RPM.

BrianG · 11:29 AM

Well, the reason I ask is I was thinking of doing a direct motor-to-diff drive using a lower rpm motor, but if they are still supposed to spin at such high rpms, then it wouldn't be a good idea. I was also thinking of using one of those inline planetary gear drives, but they are quite expensive. I was shooting for a semi-realistic car where the motor is in the front, followed by the "tranny". But, it looks like I'll still be using a spur/pinion arrangement.

BrianG · 06.30.2006, 11:27 AM

Quote:

Originally Posted by MetalMan

Are these motors with a kv of 500 outrunners or (standard) inrunners? Outrunners function differently, and work with a much lower RPM.

The ones I was looking at were from the Feigao charts, so I assume they are inrunner types.

neweuser · 06.30.2006, 01:27 PM

Sweet Spot, i've seen a lot of that in threads, but if my calculations are correct. I'm running around 28,000 right now with 16 cells. Is that a bad thing? If I were to run 20 cells, that would bring me over 30,000+...

squeeforever · 06.30.2006, 01:35 PM

Thats why I'm going with 6S LiPo. That should bring the motor to life....

neweuser · 06.30.2006, 01:49 PM

I'm worried if i go 6s LIPO, my controller will be getting close to limitations...so i think i'm going 5s, what controller are you running squee?

squeeforever · 06.30.2006, 03:02 PM

9920. Chances are I will get my packs in the next couple of weeks so I'll let you know how hot it gets. I wouldn't however worry. With a 10XL, it won't be pull a bunch of amps. Plus, people were running 20+ cells on a 9920 way back when they were only rated for 18 cells.

neweuser · 06.30.2006, 03:11 PM

Sounds good. You just got me a little worried cuz you advised 5s earlier. But now thinking 6s! LOL, never too much power if a system can handle the juice! I'm runnin the same that you are i think. 9920/10xl wanderer?

squeeforever · 06.30.2006, 03:12 PM

Yea, except I have a Feigao. Same thing though.

neweuser · 06.30.2006, 03:18 PM

Ok, PM me when you have a few test runs if you could please. In the end of all though (it'll be a few months), I would like the Quark 125B with a lehner 1950/8 on some lipos. For now, I'm going to look more into lipo, chargers, balancers, and an LVC. When I get some solid info and made decisions, I'll get the lipo first, then a new system...thinking maybe doing a Revo for the next project...

BrianG · 03:21 PM

Quote:

Originally Posted by neweuser

Sweet Spot, i've seen a lot of that in threads, but if my calculations are correct. I'm running around 28,000 right now with 16 cells. Is that a bad thing? If I were to run 20 cells, that would bring me over 30,000+...

The sweet spot is where the motor operates most efficiently. You can probably get a little more than 28k rpm on 16 cells. For one thing, that is assuming a nominal 1.2v per cell, but cells can be as high as 1.3+ on a full charge. Also, the actual kv rating you are using is its "loaded" value, which is dependent on weight, gearing , etc. The 10xl's unloaded value is 1668rpm/v. Those kv values are probably average values too, so there is some variation. Depending on weight and other factors, you can probably get between 30k-32k rpm on 16 cells.

neweuser · 03:23 PM

I see. I did do the calcs at 1.2. But i noticed my cells with no load are at 1.45, 1.43 or so. My gmaxx is i think close to 12lbs. Gearing right now is 20/51 mod 1, I know there is a calculator for all this, but can't find it! Anyone know what rpm's i would be close to? with the above info?

BrianG · 06.30.2006, 03:40 PM

Quote:

Originally Posted by neweuser

I see. I did do the calcs at 1.2. But i noticed my cells with no load are at 1.45, 1.43 or so. My gmaxx is i think close to 12lbs. Gearing right now is 20/51 mod 1, I know there is a calculator for all this, but can't find it! Anyone know what rpm's i would be close to? with the above info?

Funny you should mention that, since I have one: Speed Calculator :)