***Why do brushless motors have a kv rating?***

**"kv Rating" has nothing to do with Torque, current, power, thrust , lift or drag**

It is based on number of magnets, number of stator windings per rotation, number of phases per pole and has no indication of power.

>**It is purely the rotational velocity whichg enerates the back EMF voltage to match the applied voltage. This match only occurs at no load and drag reduces this ratio up to 10% with increase towards rated voltage depending on inherent losses.**   (e.g. eddy current, friction , generally small compared to power avail. 

**Power is a function of current and load only** is rated with EITHER a linear load or the nonlinear load of the aerodynamic prop. or an incremental linear load **in terms of gm/W or gm/A** where gm is the prop thrust.

Background thumbnail on theory (over simplified)

- It is based on the laws of Physics defined by Maxwell and in more depth by Heaviside, and Lorenz who proved that this Force on charge q is a product of the sum of the E field and the velocity of the B field.

So the vector equations says.  **F=q(E+vxB)**

The **Lorenz force**, F acting on a particle of electric charge q with instantaneous velocity v, due to an external electric field E and magnetic field B.  This force is what we call the Electromagnetic Force and is matched by the Back EMF at no load.

The Angular Velocity per Volt is a more complex one with the number of stator poles and rotor poles giving a ratiometric conversion and the commutation of the motor current is automatically reversed just an adequate number of arc seconds after the null magnetic field to ensure no dead stop. ( design/process failure)
[![enter image description here][1]][1]

**Thus the magnetic charge velocity is proportional to the Field strength which is due to Voltage and is also referred to as Back EMF field strength**


  [1]: https://i.sstatic.net/2VAkF.jpg