Types & Sizes of Brushless Motors: RC Motor KV Ratings Explained

Types & Sizes of Brushless Motors: RC Motor KV Ratings Explained


Motors that have a more KV rating run faster. They do this using less torque, which is the power to accelerate faster. This is a give and takes off more power for less speed and vice versa when it comes to the basic construction of brush-less motors. The torque or a motor depends on its “turns”, which the wires are winding around a rotor pole. The rotor pole is just another component of a motor. The more turns of the rotor pole mean the higher torque that a motor has. The motor torque that a motor has the lower its top speed is. All of these things run really faster or not so depending on the amps and amperage flowing through a conductor. Amp is a measure of electricity flowing from source to destination, and the amperage is the maximum amount of current that a motor can handle. Another way to look at this is to calculate the watts of a motor, which is just Amperage multiplied by voltage. When it comes to keeping a motor running like new, there is the High-Efficiency rating. This tells how much energy is being converted to kinetic motion. The higher the rating the better, because energy not converted into motion becomes heat within a motor. If a motor has lots of plastic parts but is not highly efficient, the plastic parts are liable to melt or become damaged.

Types & Sizes of Brushless Motors: RC Motor KV Ratings Explained

Some people explain the aforementioned understanding of the KV constant as wrong because it does not account for Back-EMF (Electromotive Force). This EMF is directly proportionate to the speed of a rotation speed of a motor. The equation is actually simple, once the expounding of vocabulary gets out of the way. The EMF equals Volts per RPM times Speed (represented as W). When this equation is applied using constant DC (Direct Current), the maximum speed can be factored and figured out. It is called circuit analysis.


☑V = R * I + E = R * I + K
e * w


Once the circuit analysis is figured out, figuring out the top speed of a motor is formulaic. The formula, like many of such formula, is a placement of term relative to one another so they balance.


Once the details of the terms are explained, figuring out how to use them becomes a snap. When it comes to figuring out torque the formula looks like this:

Where T represents the torque, K

t is the torque constant and I stands for the current.

In another formula, K

t equals 1 over K

  1. The formula looks like this:

It can be written like this as well:


The bottom line is that K

v can be written as a formula as long as the terms are clearly understood. The formula for that looks like this:

Even though RC (Radio Controlled) vehicles have an ESC (Electronic Speed Controllers) within their system, it is useful to know what is going on inside. This knowledge actually enhances the enjoyment and performance of RC vehicles.


Just in case the information is needed for comparison, the following facts and formulas apply to a brushed DC motor.

The formula for finding the top speed of a motor looks like this:

Where V stands for voltage, R is the motor resistance, I

0 is a current with no load, K

e is the back-EMF and W represents speed.


Basically, it comes down to the current of a motor having a direct influence on its kinetic motion or speed. The more current, the slower your motor runs. The torque, which has everything to do with the turns of a motor has everything ti do with that. However, these facts and figures only apply in a perfect world where things do not collide. In reality, a motor speed can be null while full voltage is flowing through its components and conduits. Such as when a stick is wedged between the wheels and the chassis of an RTR (Ready Ro Run) vehicle.


When that happens the formula looks like this:

Where I is the back EMF, V is for voltage and R represent resistance.


Understanding the KV of a motor is not just about its speed, volts, and resistance. It really is about knowing how to explain what is going on within the motor. Another way to think about this is to understand that that Kv is equal to the back-EMF constant. The formula for that looks like this:


No matter which type of engine you choose to go for, it is good to know how to fix or at least talk to a professional about fixing your vehicle. It also does not hurt to know what you are up against when it comes to other kinds of motors. In the end, it all comes down to being helpful. The above information should serve that purpose, where RC racing and maintenance is concerned.

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