What is KV?

[justatree]

What is it and what is better, the numbers that go down or the higher numbers?

 

[WoodiE]

@justatree there isn't a "better" or "worse" Kv, it really depends on what your goal is for that particular RC. In general Kv is the number of RPM's per 1v applied. A brushless motor with a high Kv would offer higher top speed but less torque. While a lower Kv motor would offer less top speed but more torque.

 

[justatree]

But then how come arrma's bls is a 4000kv (makes more top speed like you mentioned) but then has a lower top speed than the Blx (3600kv) then that would be a different story? Would that then go into the amps on the esc?

 

[WoodiE]

The 4000 Kv motor is faster - at the same voltage. The 4000 Kv in the BLS models are rated for 2s (7.4v) while the 3600 Kv motor in the BLX is rated for 3s (11.1v).

4000Kv x 7.4v = 29600 RPM
3600Kv x 11.1v = 39960 RPM

The higher the RPM the faster top speed.

 

[BFH]

Another thing to consider is gearing and load. If the blx is geared taller than the bls and the power to pull it. It will have a higher top speed. Add 5 lbs to the vehicle and it won't reach that many rpm unless it's geared very low.

One motor at 3000kv might have more power than another 3000kv motor. The weaker one might just have weaker magnets allowing it to spin higher under no load.

The amount of watts a motor puts out is a better way to judge power. The kv just tells you the rpm range that the power is delivered.

 

[mrbelloso]

Great explanation WoodiE

 

[springboardz]

@BFH and @WoodiE - good explanations

 

[upangued]

Dumb question, but how do you know what lipo the motor is rated for? I'm trying to find a cheap BL set up for another project on eBay. I know the one I was looking at says 2-3s but I thought that was for the esc.

 

[justatree]

owch
jk

 

[Mike_D]

What if a fella wanted to take the motor out of his e Revo 2.0 and put it in the Kraton? Would that be stupid? And can the Kraton eSc deal with the Revo motor or would that have to be changed ?

 

[RxMonkey]

What if a fella wanted to take the motor out of his e Revo 2.0 and put it in the Kraton? Would that be stupid? And can the Kraton eSc deal with the Revo motor or would that have to be changed ?

I’m gonna guess the blx185 can handle whatever traxxas puts in a revo factory. I’m almost certain it’s not more than 6s. My general impression is that Arrma is waaaaay more overbuilt and durable than traxxas. Only possible issue I could see is if the traxxas motor is a high kv (3000-4000kv) and makes the truck work too hard, lacking torque with a heavier truck overheating/stressing electronics.

 

[RxMonkey]

The 4000 Kv motor is faster - at the same voltage. The 4000 Kv in the BLS models are rated for 2s (7.4v) while the 3600 Kv motor in the BLX is rated for 3s (11.1v).

4000Kv x 7.4v = 29600 RPM
3600Kv x 11.1v = 39960 RPM

The higher the RPM the faster top speed.

Can you help me understand. The wisdom I have heard is volt up, gear down. Meaning to gain performance gear your vehicle lower and use a higher voltage setup to achieve more performance. Logically this is using gearing and input to reduce the load on the system.

For these Arrma trucks I am confused a little. As I understand to make the truck run cooler you use a lower Kv motor and gear up. Only thing I can figure is that volt up refers to voltage applied to the system not the motor itself. By using a motor with lower kv and gearing up you’re actually reducing load by utilizing a motor with more torque that can handle higher gearing with less stress or load and still achieve speed/performance similar to a high kv motor geared moderate to high.

 

[2fast4u]

This may be interesting to some.

http://learningrc.com/motor-kv/

 

[Jerry-rigged]

Can you help me understand. The wisdom I have heard is volt up, gear down. Meaning to gain performance gear your vehicle lower and use a higher voltage setup to achieve more performance. Logically this is using gearing and input to reduce the load on the system.

For these Arrma trucks I am confused a little. As I understand to make the truck run cooler you use a lower Kv motor and gear up. Only thing I can figure is that volt up refers to voltage applied to the system not the motor itself. By using a motor with lower kv and gearing up you’re actually reducing load by utilizing a motor with more torque that can handle higher gearing with less stress or load and still achieve speed/performance similar to a high kv motor geared moderate to high.

You have a pretty good handle on it. One other thing to notice, is that when people buy lower KV motors, they will often get a bigger can size - so the motor can make more total power. And one last (sometimes overlooked) observation - the truck takes the same Power (watts or HP, which every you prefer) to go 50mph regardless of motor size, voltage, gearing, or whatever. So if you have two motors - one is running at 95% max power to get the car to 50mph, and another that is running at 50% max rated power, guess which will be running cooler and have a longer life?

 

[2fast4u]

You have a pretty good handle on it. One other thing to notice, is that when people buy lower KV motors, they will often get a bigger can size - so the motor can make more total power. And one last (sometimes overlooked) observation - the truck takes the same Power (watts or HP, which every you prefer) to go 50mph regardless of motor size, voltage, gearing, or whatever. So if you have two motors - one is running at 95% max power to get the car to 50mph, and another that is running at 50% max rated power, guess which will be running cooler and have a longer life?

What he said !
Well spoken.

 

[RxMonkey]

You have a pretty good handle on it. One other thing to notice, is that when people buy lower KV motors, they will often get a bigger can size - so the motor can make more total power. And one last (sometimes overlooked) observation - the truck takes the same Power (watts or HP, which every you prefer) to go 50mph regardless of motor size, voltage, gearing, or whatever. So if you have two motors - one is running at 95% max power to get the car to 50mph, and another that is running at 50% max rated power, guess which will be running cooler and have a longer life?

Excellent thank you! So in your example a 1600kv motor geared higher to run the same top speed as the stock 2050kv would be cooler (less stressed) assuming same can size?

Seems to me many folks change to a lower kv motor. Arrma has to use a higher kv motor stock so the marketing dept can put a sexy top speed on the box but it isn’t necessarily ideal for keeping temps low for most users.

 

[RxMonkey]

This may be interesting to some.

http://learningrc.com/motor-kv/

I appreciate the article my man. It did help me understand a little better. Lemme tell you I am not an engineer and trying to read that really drives the point home for me. Lol. I was having flashbacks to physics classes. Aaaahhh. It get to a point where I do want to understand more but my lord are electric motors a complex topic. This isn’t even touching the windings like Y3 or whatever those designations are for the brushless.

 

[Jerry-rigged]

Excellent thank you! So in your example a 1600kv motor geared higher to run the same top speed as the stock 2050kv would be cooler (less stressed) assuming same can size?

Seems to me many folks change to a lower kv motor. Arrma has to use a higher kv motor stock so the marketing dept can put a sexy top speed on the box but it isn’t necessarily ideal for keeping temps low for most users.

In your example, I think the answer is NO. For otherwise identical motors, the lower KV motor (generically) has a lower max power vs a higher KV motor. So to get the same "Actual" power, you must run it at a higher % of max output. But there are so many variable issues that it is hard to put a fine point on in.

And yeah, some people change to a lower KV motor. Some people change to a higher KV motor. Everyone has their preference. Gear it right, and it will work.

As for Arrma Marketing, yeah, that 60mph is not for general use, but just for short speed runs on pavement. the 2050kv motor is pretty much in line with (+-150kv) what 90% of 1/8 RTR cars run, so I don't feel it is a marketing 'trick' either. The motor is solid, you just can't expect to run it (or any motor) near max output without overheating it. If you really want to run 60mph, offroad, for a full battey without heat issues, you need a much LARGER motor, not just a lower KV one.

 

[RxMonkey]

In your example, I think the answer is NO. For otherwise identical motors, the lower KV motor (generically) has a lower max power vs a higher KV motor. So to get the same "Actual" power, you must run it at a higher % of max output. But there are so many variable issues that it is hard to put a fine point on in.

And yeah, some people change to a lower KV motor. Some people change to a higher KV motor. Everyone has their preference. Gear it right, and it will work.

As for Arrma Marketing, yeah, that 60mph is not for general use, but just for short speed runs on pavement. the 2050kv motor is pretty much in line with (+-150kv) what 90% of 1/8 RTR cars run, so I don't feel it is a marketing 'trick' either. The motor is solid, you just can't expect to run it (or any motor) near max output without overheating it. If you really want to run 60mph, offroad, for a full battey without heat issues, you need a much LARGER motor, not just a lower KV one.

That helps me understand it better. I found the hobbystar 4092 runs a good bit cooler than the stock motor even geared up to 15t pinion. I’m not trying to knock the stock motor by any means, it’s a beast I just hate checking the temps and finding it 170-180+. Since I got the dual fan setup it’s much much cooler. Like 160 tops unless I get really carried away.

Also I didn’t mean to sound like I was dissing Arrma. I adore their trucks 100%. I mainly meant that they almost have to run a high kv motor to keep up with competition. It’d be hard to sell a kraton advertised at 40+ mph next to a revo 60+ mph when in truth either can be geared to hit whatever speed just a matter of motor and gearing.

 

[Jerry-rigged]

So you new motor falls into what I was talking about back in post #14 - Most people that buy lower KV motors also buy BIGGER motors. That new motor make something like an extra 1200w of power over the stock motor (per THIS thread). This is why the motor runs cooler, more so than the lower KV. And the extra power is because of the longer motor can, not the lower KV.

And I didn't take it as bashing Arrma, as I think we all know pretty much all MFGR's stretch the truth with their MPH on the box claims. At least Arrma CAN hit that speed, even though you can't run like that for a full battery.