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Old Oct 25, 2006, 10:48 PM
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What does Kv really mean ?

If a motor is rated at, say, 1000 Kv and you give it 12V, then presumably the number 12,000 RPM is somehow significant. Is this the speed that the motor runs at without a load ? The speed of highest power ? The most efficient speed ?

Dumber question: How does your run-o-the-mill brushless ESC know what speed to run it at ? It doesn't know the motor's Kv, and it doesn't know how fast the motor is actually turning (right ?).

Thanks!
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Old Oct 26, 2006, 12:05 AM
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The ESC "know" the speed of the motor, simply because the motor actualy produce electricity when it turn, just like a generator. Let say your 1000kV motor turn 12,000rpm, then it'll produce a 12V sin wave going at 12,000Hz (for a 2 pole motor), your ESC "see" that and act accordingly. Since your power source is "let say" 13V, there's still a 1V difference between the source and the motor, some current is going trought the motor. If your motor work hard and goes down to 6,000rpm (6,000 /1000kV = 6V), there's now a 7V difference between the source and motor, lot of current (power) is going in the motor. That phenomena is called: "counter-electromotive force".

The KV value is the speed the motor need to turn so it produce 1V of counter-electromotice force. Since the motor is not 100% efficient you can say that the 1000Kv motor will turn slightly slower than 12,000rpm when running on 12V with no load.

Sometime you see (sensorless brushless ESC) on castle creation products for example, some ESC (novak) use a sensor on the motor shaft to know what speed it turn. Sensorless ESC "sense" the couter-electromotive force to know the speed of the motor. However that force is weak when the motor turn at low speed, making low speed drivability difficult for such ESC.

-Hugo
TrueRC Canada
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Old Oct 30, 2006, 12:19 AM
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kv or RPM/Volt is way of determining the use of a motor. It varies with motor type: inrunner or outrunner, number of poles, number of turns, winding resistance etc. The kv can be used to find the maxium voltage that can be applied to a certain motor. As an example a 5000kv motor may have a RPM limit of 60,000 (set by the manafacturer) so the means 60,000/5000 = 12volts would be the maxium voltage. The same motor as above with more windings may have a kv of 2500, this would then be 60,000/2500 = 24volts maxium. If this motor was rated at 240watts then the 5000kv version would use 240/12 = 20amps and the 2,500kv one 240/24 = 10amps max under load.
This is a very basic example but gives you the idea.
Eddie
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Old Nov 24, 2006, 06:04 PM
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If a motor is rated at 1000 Kv and you give it 12V, then 12,000 RPM is the peak epm it will reach.

The significant in an immediate sense is
1. the kv directly relates to the torque constant Kt which is expressed as in/oz/amp ie torque per amp my recall is 1352/kv = kt in in/oz
2. max power is at 50% of peak rpm and max efficincy will depend on the type of motor - for good brushless in runners use 88-90% for car motors use 78% of peak rpm

Your esc just runs the motor as hard as you push the throttle. If its brushless you need to set the timing and operating frequency to suit the motor. If its brushed your controller doesnt care it just puts out the power you throttle it to modified per any adjustments you make in the esc.

at the Kv is rpm/v - the esc has only aminor effect on the motors speed - the rpm/v figure and the actual volts in your pack determine the motor speed
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Old Dec 02, 2006, 10:45 AM
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Buster,
Let me try to help a little. The kv of a motor will also tell you which prop you can effectively use based oh what the watts rating is. The larger prop you put on the motor the higher the amps, and the watts that it will pull trying to drive the prop up to the max rpm or kv the motor is rated at. If the motor is rated at 1000kv and say 150 watts, then you put a 10x6 prop on it, and it pulls 150 watts at 12volts and wide open throttle then that is a good fit. If on the other hand the 10x6 prop causes the motor to over heat, and pull more watts than 150, then eventually it will probably fail.

By the way, watts are amps x volts equals watts. 12amps x 12volt equals 144watts.

good luck,
Darrell
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Old Apr 08, 2008, 02:28 AM
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Quote:
Originally Posted by Hugeone
The KV value is the speed the motor need to turn so it produce 1V of counter-electromotice force. Since the motor is not 100% efficient you can say that the 1000Kv motor will turn slightly slower than 12,000rpm when running on 12V with no load.
-Hugo
TrueRC Canada
What is the precise definition of the KV value for brushless motors? In the above explanation, what is meant 1V of counter-electromotive voltage, is it the peak amplitude value or the effective value? (Like 120V in the mains -- it is the effective, not the amplitude value). Actually, I guess the shape of the voltage is not precisely sine, it may somewhat depend on the shape of the magnets and poles (though possibly this is a small distortion, and a tiny difference in the effective vs. amplitude relation)

I tried Wikipedia, but don't know the precise term to look for.

And, in practice -- if one runs a brushless motor in idle, how significantly lower will be RPM than KV*Voltage -- 90%, 80% ?


And how KV is defined for brushed motors? Through its idle speed, or through average (or effective) terminal voltage?

Nick
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Old Dec 15, 2009, 01:36 AM
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Okay, so for the really really dumb people (me in other words...), if I were to get a Losi 1/18th mini desert buggy, and I had my choice of a 7.4V 9400Kv, 7400Kv, or 6000Kv brushless system, which one would I use to get the most speed?
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Old Dec 15, 2009, 06:29 AM
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If you stick with the same battery on all of you're choices then the 9400kv motor would generaly give you more speed as it as more RPM per volt.

BUT

the 7000kv motor mite be able to run with a 3S battery as the 9400kv motor may not and then the 7000kv motor setup with 3S would be faster.

But unlikely that the 7000kv would run on 3S.
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Old Aug 23, 2013, 06:23 PM
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This article certainly complements the subject being discussed: http://en.wikipedia.org/wiki/Motor_constant
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Old Aug 24, 2013, 07:23 PM
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This thread is 7 years old and hasn't been updated in 4 years...
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Old Aug 26, 2013, 03:41 AM
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Quote:
Originally Posted by wparsons View Post
This thread is 7 years old and hasn't been updated in 4 years...
nice input though
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Old Sep 20, 2013, 02:56 PM
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Quote:
Originally Posted by wparsons View Post
This thread is 7 years old and hasn't been updated in 4 years...
What's your point? I just found this thread through Google while looking for an explanation of kv (I'm new to R/C) and the Wikipedia article was very helpful.
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Old Apr 15, 2014, 07:54 AM
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Hi Guys,
If I have a Tarot 4114/320KV motor, then which ESC would be suitable for it, and which props do I need?
Further more: is there any difference between ESC and ESC? (I've seen expensive against the "same" (different maker) but less cheaper.)

Thanks!
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Old Apr 15, 2014, 10:18 AM
Did you check the FAQ already?
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Quote:
Originally Posted by richiepic View Post
Hi Guys,
If I have a Tarot 4114/320KV motor, then which ESC would be suitable for it, and which props do I need?
Further more: is there any difference between ESC and ESC? (I've seen expensive against the "same" (different maker) but less cheaper.)

Thanks!
This is about a plane, right? Maybe you will have better response in the planes section.
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Old Apr 15, 2014, 11:28 AM
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Originally Posted by SoloProFan View Post
This is about a plane, right? Maybe you will have better response in the planes section.
Not really. I'm looking for something like this:
http://www.jamcopters.cz/en/tarot-t8...kv-ou-c86-p335

But I'd like to understand the motor-ESC-reliability threesome.
What makes you think that my question is plane related?
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