Brushed motor rpm to brushless kv comparison

[Druxus]

Let me start by saying this is for a taigen tiger 1 tank. I would post on some of the tank forums but the general consensus is that brushed motors are plenty good. Lipo batteries also seem to not popular items either.

I'm not looking to make my tank into a rocket, just have it a trailable speed (say 4-5mph). It runs about around 4mph right now.

So after a full run my motors are anywhere from 150°-170° which tells me it's not geared right. And it gets kinda buggy up some inclines. There are 2 different gearboxes 1 is 39:1 and the other is 90:1. I am running the 39:1 with the faster 390 motor. The 390 are supposed to have a rpm of 25,000 on a 6 cell nimh (I am guessing because that's what ships with the tank).

So if I take 25,000rpm/7.2 volts=3472kv. So am I looking for 380 brushless motor around 3500kv?
Is that right or doesn't it work like that for brushed?
I would like to go brushless but I don't really want to go on smaller brushless buying spree.

[Voodoobrew]

That's for the same speed. You need more then double that with the 90:1 gearbox

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[Druxus]

Ok looking at Holmes hobbies site I'm really thinking that the claimed 25,000 rpm is a bunch of crap. A Holmes 11t crawl master pro gets up to 2675kv.

[JohnRobHolmes]

Thats a five slot you are looking at. A three slot 11t would be around 5000kv.


I would recommend you keep the KV under 3500 for phase amp reasons. Above that speed, and the ESC needs to pass a lot of current for the needed torque at the wheel. Even with geardown, we can't get around the heat problems that arise with higher kv motors in stalling situations. The upside is that the same speed on a brushless motor will hold RPm better than brushed, so you could probably go lower on KV and still net the same wheelspeed.

[Voodoobrew]

Originally Posted by JohnRobHolmes

Thats a five slot you are looking at. A three slot 11t would be around 5000kv.


I would recommend you keep the KV under 3500 for phase amp reasons. Above that speed, and the ESC needs to pass a lot of current for the needed torque at the wheel. Even with geardown, we can't get around the heat problems that arise with higher kv motors in stalling situations. The upside is that the same speed on a brushless motor will hold RPm better than brushed, so you could probably go lower on KV and still net the same wheelspeed.

The smaller 380 motors might handle the higher rpms better?



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[Voodoobrew]

Originally Posted by Druxus

Let me start by saying this is for a taigen tiger 1 tank. I would post on some of the tank forums but the general consensus is that brushed motors are plenty good. Lipo batteries also seem to not popular items either.



I'm not looking to make my tank into a rocket, just have it a trailable speed (say 4-5mph). It runs about around 4mph right now.



So after a full run my motors are anywhere from 150°-170° which tells me it's not geared right. And it gets kinda buggy up some inclines. There are 2 different gearboxes 1 is 39:1 and the other is 90:1. I am running the 39:1 with the faster 390 motor. The 390 are supposed to have a rpm of 25,000 on a 6 cell nimh (I am guessing because that's what ships with the tank).



So if I take 25,000rpm/7.2 volts=3472kv. So am I looking for 380 brushless motor around 3500kv?

Is that right or doesn't it work like that for brushed?

I would like to go brushless but I don't really want to go on smaller brushless buying spree.

We're you considering changing to the 90:1 gearbox or was that just extra information?

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[Voodoobrew]

If you get the Holms 380 revolver on 9.6v sidewinder 4 or mamba x mini you ger 2500rpm and more torque.

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[JohnRobHolmes]

Originally Posted by Voodoobrew

The smaller 380 motors might handle the higher rpms better?



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The limitation is mosfet current capability, not a limitation in the motors. Higher kv requires higher phase current to produce an equal torque at the motor.

Using a much larger esc could offset the issue.

[Voodoobrew]

Originally Posted by JohnRobHolmes

The limitation is mosfet current capability, not a limitation in the motors. Higher kv requires higher phase current to produce an equal torque at the motor.

Using a much larger esc could offset the issue.

Are you talking about brushless or brushed? I think he was talking about brushed

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[Druxus]

I gave the other 90:1 as info for another option. The pinion on the 39:1 is really small already and doubt I can find a smaller one. I don't want to get some brushless motors for this gearbox and end up with hot motors yet. With the 90:1 I could put a bigger pinion on and gain some speed, but the reduction seem so high that it still seems like I would need some crazy high kv motor.

I was thinking about some outrunners but I haven't found any with higher kv numbers.

[Druxus]

Originally Posted by Voodoobrew

If you get the Holms 380 revolver on 9.6v sidewinder 4 or mamba x mini you ger 2500rpm and more torque.

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Wow, I didn't know he had 380 sized revolvers. Now revolvers have a lot more torque right? Possibly enough to go up in pinion size to make up for a lower kv?

[Voodoobrew]

I would go up in voltage first.

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[Voodoobrew]

And yes they probably have triple the torque of a brushed 380 motor. Considering a tiny snub nose has about twice the torque of a sealed 550 motor.

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[Druxus]

Alright I think that's my path then, the 380 revolvers it is then.

Thanks for the help!

[Druxus]

A update.

I ordered 2 Holmes 380 revolver motors and when they came in I was a bit worried. They are so much smaller then the 390 motors that were in already I was thinking I may have made a mistake. Boy was I wrong, these little guys are very powerful, and the tank is a lot faster then it was before. Which still makes me question the brushed motor kv numbers. After 2 5000mah packs back to back motor temps are 105f and esc's are 115f.

In setting up the esc I was wondering about what to set the timing at for outrunner motors. Looked online but didn't find anything specific for outrunners and crawling type stuff.

[JohnRobHolmes]

Use lowest timing for outrunners.

Glad to hear its working out well!