Electric Motor KV rating doesn't make sense

randyrls

Randy
Hello; I don't understand why the KV rating of motors exists.

I have a background in electronics and have built and used both "Variable Direct Current Drives" and "Variable Frequency Drives" on machine tools.

The R/C motors are brushless 3 phase motors. Correct? I believe that the speed of the motor depends on the frequency of the supplied current and number of windings. Barring overloads, and residual losses the motor being fed by say 50 Hertz should turn at one speed, and if fed by 100 Hertz should go twice as fast. No KV rating needed. There is something lacking in my understanding of these motors.

Can someone enlighten me on where my knowledge is faulty???

I've seen several articles and videos where the KV rating was explained and demonstrated but I still don't grok it.

Thanks for any understanding you can impart!
 

Crazy Goats

Active member
My understanding is, if you multiply the kv rating by your input voltage (12.4 V on a fully charged 3s lipo) you get your RPM. I have generally ignored the kv ratings on motors, except for some light guess work on prop sizes.
 

ElectriSean

Eternal Student
Mentor
This is something I've struggled to understand as well, so thanks for posting and encouraging me to do some research. Your knowledge is correct, AC motor speed is determined by frequency and the number of poles. Kv is not a rating really, but a motor constant. It doesn't refer to the applied voltage, but to the back-emf generated by the motor, which is proportional to speed.

Check out http://learningrc.com/motor-kv/ for a more in depth explanation
 

cswalker

New member
Being new Goats explanation is pretty much how I understand it. No variable needed. The RPM is constant per Volt applied from your battery pack. Then you can subtract your KV Sinks such as propellers and current wind conditions. E.G: a 1KV motor will spin 1 time per minute per 1 volt applied. so if you have a 2200KVM motor, and your battery pack is 10 volts your prop (without kv sinks) will spin 22000 times per minute.
 

Bricks

Master member
Being new Goats explanation is pretty much how I understand it. No variable needed. The RPM is constant per Volt applied from your battery pack. Then you can subtract your KV Sinks such as propellers and current wind conditions. E.G: a 1KV motor will spin 1 time per minute per 1 volt applied. so if you have a 2200KVM motor, and your battery pack is 10 volts your prop (without kv sinks) will spin 22000 times per minute.

This is the way I understand it .... Some models need a low KV to swing a big prop where others need a small fast prop to fly ...AKA Storch will not fly with a 2200KV motor but will fly with a 600 KV motor if you have enough room for prop clearance,
 

Hai-Lee

Old and Bold RC PILOT
The simple analogy is that the Kv rating is just an indication of its maximum available rotational speed.

The main figure to consider is the Wattage or overall power rating. You can get 2 motors with the same power and totally different Kv ratings. For the same power the LOWER Kv motor will have more torque and swing a larger propeller. Conversely the HIGHER Kv motor will have less torque available and will use a smaller propeller.

ESCs are not just 3 phase AC generators but rather 3 phase PULSE generators which get feedback from the load applied as well as the throttle setting to calculate the pulse repetition frequency and hence the rotational speed of the motor. With the revs per volt nature of the rating consider that increasing the applied voltage will increase the applied current as well and hence the power and torque.

Too much voltage and you smoke a motor or ESC. Too much prop and you smoke a motor or ESC. Just follow the manufacturers recommendations and there need not be any confusion.
 
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