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Old 01-26-2012, 09:26 AM   #1
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Default Brushless motor turns and Kv rating

I've always been under the impression that all brushless motors of the same turn (ie, 10.5, 13.5, 17.5, etc) are relatively the same. Maybe brand X is a little stronger than brand Y or one brand is a little more durable than another and so one. However, I've noticed that despite their number of turns being the same, they could have different Kv ratings. How is this possible?

My understanding of KV rating is the speed measurement of the motor in rpm per amount of voltage applied. So the higher the Kv the better.

For example 17.5T: I've seen some with 1900Kv, some with 2200Kv, another with 2250Kv and another with 2300Kv. Are these Kv ratings true? Is this another falacy in motor rating like that of the C rating for lipos?

Someone please explain.
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Old 01-26-2012, 09:33 AM   #2
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I noticed this before also and was wondering. Maybe because different size rotors, and etc.??? I dunno
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Old 01-26-2012, 01:28 PM   #3
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the design of the motors, the quality of the magnets, type of wire used all have an effect of the overall KV of the motors. Unfortunatly there seems to be a 10% tolerance "allowed" by the manufacturers for the KV rates.
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Old 01-26-2012, 02:06 PM   #4
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KV rating is essential the RPM rating of motor. It's supposed to be how many RPM a motor will spin at a given voltage.

RPM is only half of the power equation though. power=RPMxTorque

(keep in mind these ratings are generic to make an example).

Say you have the following motors, all the same turn.

Motor A 2200 KV
Motor B 2250 KV
Motor C 2500 KV
Motor D 1950 KV

However

Motor A 8 oz Torque
Motor B 10 oz of Torque
Motor C 4 oz torque
Motor D 16 Oz of torque

As you can see, Motor C might make a ton of RPM, but it doesn't make very much torque....while Motor D makes a ton of torque, but a lot less RPM. Motor B is pretty stout in both...

If you have motors with identical KV, then the highest torque one is going to make the most overall power, and vice versa.....if they all make the same torque, then the highest KV would be the most powerful.

Everything in a motor is usually a trade off....you give up RPM for torque or vice versa.....power is power, then you just gotta gear it.

KV is a stupid rating that came along with brushless motors. While I understand why we went brushless, why we gave up the standard of showing motor power output in watts is beyond me.

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Old 01-26-2012, 06:15 PM   #5
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Quote:
Originally Posted by EddieO View Post
KV is a stupid rating that came along with brushless motors. While I understand why we went brushless, why we gave up the standard of showing motor power output in watts is beyond me.
I agree.

If motors were rated that way, it would put an end to all those un-necessary threads asking "What's the best motor".
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Old 01-26-2012, 07:17 PM   #6
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yeah, but mfgr's would lie about the power output, or fudge the numbers at different voltages / amp draws to make themselves look good!
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Old 01-26-2012, 08:35 PM   #7
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Yeah, cause you can't do that with KV? KV on most motors is not even done correctly. Many companies simply spin it up at 1 volt take an RPM reading, call it KV....or spin it up to say 5 volts and divide it...

Both of those are incorrect.

It's actually a much different process of spinning the motor at a know RPM and taking a measurement of what the motor produces....

Later EddieO


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yeah, but mfgr's would lie about the power output, or fudge the numbers at different voltages / amp draws to make themselves look good!
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Old 01-05-2013, 07:52 AM   #8
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What kind of test fixtures/stands/set up are their out there to test motor output/torque etc? Has anyone seen plans or information sites about these?
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Old 01-05-2013, 06:05 PM   #9
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Actually, you said it correctly in the first post, KV ratings are the motor's equivalent of C ratings for batteries. They might be useful if an independent lab took random over the counter motors and put them all through the exact same tests, then it might be informative, but that's not the case. Even then, a lower KV motor could beat a higher one depending on timing, gearing, class etc.
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