Brushless Motor Rating
Voltage directly influences the RPM of the electric motor (brushless motors are rated by kV, which means 'RPM per Volt'). So if you have a brushless motor with a rating of 3,500kV, that motor will spin 3,500 RPM for every volt you apply to it. On a 2S LiPo battery, that motor will spin around 25,900 RPM. On a 3S, it will spin a whopping 38,850 RPM. So the more voltage you have, the faster you're going to go. 

Brushless Motor Ratings and Specs –
What is Kv?         Brushless motors have Kv ratings   Kv actually refers to the RPM of the motor per volt “without a load”. 
 A brushless motor with a Kv rating of 2500 Kv  multiply that number by the voltage of your battery and you get your total RPM. 

Our x class build is setup for a 6s battery would be 

motors are U8 Pro KV135.      recommended prop. G29x9.

What this all means is that if you have a motor with a higher Kv then it will have more top end speed but less efficiency than a lower Kv motor at the same voltage .  So a higher Kv motor produces more heat and it will handle less voltage than a lower Kv motor.

If you have a motor with a lower Kv then it will have a lower top end speed but it will have better efficiency than a higher Kv Motor at the same voltage. So a Lower Kv motor is more efficient than a higher Kv motor so it produces less heat.  This means that it will be able to handle more voltage and that means more possible speed.

I usually like to suggest using a motor that is balanced in the middle of the two extremes, it will give you the best of both worlds.  That is of course unless you have a very specific goal in mind for your RC.  If you are a speed freak like I am then you can opt for the higher Kv rating.  If motor heat is an issue then it is sometimes advisable to use a lower Kv rating with a higher voltage battery to get the same effect.  

The big thing to remember when using Kv for your Brushless Motor Ratings is that your Brushless Motor and ESC will each have a maximum input voltage (battery cell count) that is allowed.  So if either your motor or ESC has a lower maximum voltage then you must use this to calculate your top RPMs.  If you go over the recommended voltage then you have a high chance that something will fry in your setup.

RC Brushless Motor Ratings – Motor Turns

 The “turns” of a motor refers to the amount of wire windings around each of the motor’s rotor poles.  The number of wire turns will effect the torque and speed of the motor.   The lower the number of turns equals higher top end speed and lower torque/acceleration, alternately the higher the  number of turns equals lower top end speed but higher torque/acceleration.

So a motor with a lower turn rating will have less acceleration and torque but higher top speed than a motor with a higher turn rating.  

This also means that a motor with a higher turn rating will have more acceleration  and torque but less top speed than a motor with a lower turn rating.

If you try to use a motor that is out of the specs of your ESC than you will end up frying your ESC. Pay close attention to this when you choose your ESC and Brushless Motor combo. Be aware that a lot of ESCs will have a motor turn limit on them, even some brushless ESCs will have a limit. 

RC Brushless Motor Ratings – Current Rating / Amps

The max current rating refers to the maximum amount of current that a motor is able to handle safely.  This current is measured in Amps.  The continuous current rating of a motor is the Amps that a motor can handle safely over a long period of time.
Find  current rating of a motor on the factory specs sheet.  However the actual current that a Brushless motor will draw depends on a few factors, Kv rating, voltage of batteries, weight of Drone, and  prop size. 

These all come into play because the harder that a motor needs to work in order to reach its Max RPM/top speed the higher the Amp draw will be.  This is why it is a great idea to find an ESC that has a current rating that is higher than your motor’s by at least 20%. 

Brushless Motor Ratings – Watts = Horse Power

Watts are the power rating or the horsepower equivalent of your Brushless  Motor.  

(It takes 746 Watts to equal one Horse Power by the way, just a quick bit of info). 

Watts equals Amps multiplied by your battery voltage or (Amps x Volts).

Your brushless motor should have a watt rating on its spec sheet, something like “180W”.  This means that that is the amount of “horse power” that it should produce safely.  Running anything over this rating could damage your motor, especially over a long period of time.

The Motor will also have an efficiency rating, for brushless this will be around 80 – 90%.  So if your battery is supplying the ESC with 180 watts then your motor will only be producing around 85% of that or (180 x .85) = 153 watts. 

The rest of the wattage is turned into heat, so in that example there would be (180 – 153) = 27 watts of heat coming off the motor.  But hey that’s only 27 watts right? Just be aware that some soldering irons only use 25 watts and that is enough to melt solder so even that much can hurt. 

Heat is the enemy of your setup, and you want to try and eliminate what ever heat you can. 

A cooler running motor will give you much less trouble.  To reduce heat you can change your gearing or prop size, use a more efficient motor, reduce your voltage or amps, or try a motor heat sink and motor fan. 

 More efficient motors usually cost more and most anything else that you change will result in less power.  So most people try the heat sink and fan option because let’s face it nobody wants to give up any power in their RC setup right? 

If you will be running your motor “hot” the heat sinks and fans will help to pull the heat from your motor much quicker and keep them working longer.  This is not a perfect solution but it will definitely help and not hurt.