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Old 03-27-2009, 06:11 AM   #1
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Default Let's make KV clear

For some time now I have been looking at discussions where people are speaking about motors KV as if they show the power of that motor.

NO, NO, NO. Kv does not show the motor power.

KV means the revolutions per minute that motor will spin for each volt of the batterie connected to it.

As you understand you can have a 1/18 motor with 8000kv to connect to a 3S lipo and you can have a 1/8 motors with 1700kv to connect to the same 3S lipo.

Nobody doubts that the 1/8 motor will move a 1/8 buggy fast, and nobody believe the 1/18 motor can do it even with a lot more KV.

Why is that? because what we need to make the cars move is power and this power are WATT or HorsePower as you prefer (is possible to make an equivalence between them) not KV.

WATTS should be what manufactures should tell us.(for example GM tells us that) We just need to know the KV to get a correct final ratio to allow the motor to show all its power.
GM for example for each 1/10 brushless motor gives three levels of power depending of the number of cells we connect.
They give an amount of power for 4 NiMh cells, for 5 cells and for 6 cells. For the same motor (with the same KV) the power can vary from 350 to 600 watts.

This everybody understand, if to the same motor we connect a 5S lipo the buggy will have plenty more power compared to the same motor connected to a 3S lipo.

Diferent manufactures can achive the same power output with diferent kv numbers. So it is only good make comparisons between motors of the same manufacturer and with motor of the same size. I mean you can compare a CC motor for 1/8 buggy based on there KV, the higher the number the more fast (powerful) the motor will be. (with the same cells). But you can not compare a CC motor for 1/18 with a CC motor for 1/8 based on their KV.

I hope I have been clear. I am just trying to help. So that no one will be disapointed if bought a motor with more kv from one manufaturer and his buggy is slower than another one with a motor with less KV from another manufacturer.

Manufacturer should tell us the power of their motors but they do not like to do it.

As there is not a standard to measure this output power even when manufacturers tell us their motors power that does not mean it is OK for a direct comparison. Some manuf. show power before eficiency is taken in consideration, other calculate the power connected to a fresh battery other connected from a constant powersuply.

Well this is difficult but I just wanted to make some light on this problem.

Hope have helped.
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Old 03-27-2009, 06:47 AM   #2
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With inconsistencies between similiar S-rating battery packs let alone differing C-ratings rating "motor power" seems an impossibility.
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Old 03-27-2009, 07:58 AM   #3
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I just think it would make more sense to use a universal system for rating these things
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Old 03-27-2009, 08:15 AM   #4
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I think it would be easy, but who has the power to rule that.
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Old 03-27-2009, 08:31 AM   #5
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I think people are more interested in different Kv numbers in their class size, than a 1/18th motor vs a 1/8th motor. a 4800Kv vs 5600Kv for example.

Since a typical 1/10th vehicle uses 7.4v, and a typical 1/8th used 14.8v would that double the apparent Kv of the motor, as it is getting twice the voltage as Kv is just RPM per volt?

So a 14.8v battery running a 2200kv motor would be similar to a 7.4v battery pushing a 4400Kv motor?

Of course that is just RPM, nothing on torque.

Big motor make more torque than small motor
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Old 03-27-2009, 08:41 AM   #6
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the hacker car motors list their motors in watts, still doesn't help much unless your comparing other hackers though. kv rating are more like what voltage your gonna run
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Old 03-27-2009, 08:47 AM   #7
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If manufacturers would have a standard dyno to show power, that is very much needed. For example, Hacker claims their 13.5 motor can output 370W. I tested that motor on a dyno I made, it didn't even come close, rather about 215W. On the other hand, Novak claims their 13.5 only puts out 165W, but on the same dyno, put out about 210W. So I feel Hacker over-rates their motors while Novak under-rates their motors.

Also, motors of the same wind and class from the same manufacturer can put out different results. I tested a friends Novak 13.5 motor and it spun at about 24k RPMs, and I tried my own 13.5 motor and it only spun up to 22k RPMs, however it had more torque.





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Old 03-27-2009, 08:49 AM   #8
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I would think the manufacturers have to stick to certain guidelines to call a motor 13.5T and 17.5T as those are considered SPEC for alot of classes.
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Old 03-27-2009, 08:53 AM   #9
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Quote:
Originally Posted by Lightin' View Post
With inconsistencies between similiar S-rating battery packs let alone differing C-ratings rating "motor power" seems an impossibility.
THIS is why Kv rating is better. At least you know the RPM range you'll be in, we already know there's PLENTY of power on tap.
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Old 03-27-2009, 09:06 AM   #10
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THIS is why Kv rating is better. At least you know the RPM range you'll be in, we already know there's PLENTY of power on tap.
I don't think so. I think a power rating @ RPM, torque rating, efficiency rating @ RPM, and max RPM ratings are all necessary data.

I can make a Novak 13.5SS show a higher than stock kv rating and get it up to 30k RPMs @ 30deg boost, but it will have poor torque, poor efficiency, and even the power rating will be lower.
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Old 03-27-2009, 09:13 AM   #11
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The problem is we really can't trust the manufacturers power ratings because they are using different dynos and different methods to calculate their ratings. The Hacker claims a good 205W over the Novak, but in reality, they are pretty darn equal.
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Old 03-27-2009, 09:20 AM   #12
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But do people really look at Kv ratings much?

1/10th it's pretty much 8.5T, 10.5T, 13.5T etc.....as most racers run sensored stuff. You typically don't hear people saying they are running a 5500Kv motor, unless it's sensorless stuff.

1/8th scale it's typically 2200Kv or 2650Kv (going off of Castle 1/8th stuff)
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Old 03-27-2009, 10:38 AM   #13
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Originally Posted by mattnin View Post
I don't think so. I think a power rating @ RPM, torque rating, efficiency rating @ RPM, and max RPM ratings are all necessary data.

I can make a Novak 13.5SS show a higher than stock kv rating and get it up to 30k RPMs @ 30deg boost, but it will have poor torque, poor efficiency, and even the power rating will be lower.
We don't care about efficiency, we want a motor that smokes a nitro, and ALL of them do. The Kv rating tells us what battery we need per-motor to get close the same RPMs. We already KNOW they have plenty of power. Maybe later, when it becomes a class all on its own, we "Might" want to know the differences. But at this point, it's not gonna help sell them, so it's not likely to be a big enough reason for the manufacturers to change.
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Old 03-27-2009, 10:39 AM   #14
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Quote:
Originally Posted by y2kgtp View Post
But do people really look at Kv ratings much?

1/10th it's pretty much 8.5T, 10.5T, 13.5T etc.....as most racers run sensored stuff. You typically don't hear people saying they are running a 5500Kv motor, unless it's sensorless stuff.

1/8th scale it's typically 2200Kv or 2650Kv (going off of Castle 1/8th stuff)
In 8th scale, YES, because we want motors that perform in the same RPM range as Nitro engines. That way, we know where to gear it. We already KNOW they're plenty fast.
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Old 03-27-2009, 11:20 AM   #15
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R40VICTIM, you would care about the efficiency rating if your motor got smoking hot after 3 minutes. That is what efficiency tells you. And claimed power rating does help sell motors.

Like I said earlier, I can make a Novak 13.5SS have an extremely high KV rating by adjusting timing, but the efficiency is so low that the motor could be smoking hot after one heat and the gearing would have to be lowered.

In my own testing, at 0 degrees timing, the Novak 13.5SS had 3243KV. At 30 degrees timing, the KV jumped up to 4054KV. So just by looking at the KV numbers, are you going to say the motor was performing better at 4054KV? No, because we had to sacrifice efficiency and starting torque to get it there. KV rating is a poor metric comparing motors because it does little to show the overall performance of the motor.



In these dyno tests, the Novak 13.5 at 0 deg boost reached 105 deg Fahrenheit after the dyno run, at 30 deg boost it reached 144 deg
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