Motors and ratings
Brushless Motor Ratings
Brushless motor ratings seem to have a lot of people confused out there. I will simplify things so you will understand how to see if the brushless motor that you are thinking about is the correct one for your RC vehicle. Brushless motor ratings and Specs come at us with a few different names and each means something different.
Below I will explain the different Brushless RC Motor Specs such as Kv, Motor turns, Watts, Amps, etc. I will start off by saying that it doesn’t matter what RC vehicle you drive the brushless motor ratings apply for all of Brushless RC Motors. I will go into even further detail about choosing the correct motor for your RC depending on what and how you want to drive.
Brushless Motor Ratings and Specs – What is Kv ?
Understanding Brushless Motor Ratings
I will start off with Kv ratings, almost all brushless motors have the Kv ratings stamped somewhere on them. Some motors will have Kv ratings on the motor can, others on the motor leads, some you will only see on the motor’s spec sheet. Kv is NOT “kilivolt” like a lot of people believe.
Kv actually refers to the RPM of the motor per volt “without a load”. This means that if you have a brushless motor with a Kv rating of 2500 Kv then you would multiply that number by the voltage of your battery to get your total RPM. This number will obviously be less once you have the motor inside your RC vehicle but this is the RPM that the motor is capable of reaching.
A 2500 Kv motor and a 2S Li-Po battery at 7.4 volts
(2500 x 7.4) = 18,500 RPMs
the same Kv motor on a 3S Li-Po battery at 11.1 volts
(2500 x 11.1) = 27,750 RPMs
A 4800 Kv motor and a 2SLi-Po battery at 7.4 volts
(4800 x 7.4) = 35,520 RPMs
the same Kv motor on a 3S Li-Po battery at 11.1 volts
(4800 x 11.1) = 53,280 RPMs
Understanding CastleBrushless Motor Ratings, Kv, Amps, Watts, etc.
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.
Here is a good video from Nitro-planes that explains Kv as well.
RC Brushless Motor Ratings – Motor Turns
Just like in Brushed motors 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.
Example: 5T is a faster motor (more RPMs that is) than a 12T but it has less torque.
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.
Example: 21.5T is a slower motor (less RPMs that is) than a 10T but it has more torque.
Another good thing to understand about Brushless Motor Ratings is that sometimes it will actually be better to have a motor with a lower turn rating because your RC might need more torque to perform correctly.
Good examples of this are heavier RC like RC Trucks, 4 x 4 RC Trucks, SC Trucks, Heavy RC Planes, Heavy RC Boats, and RC Cars that are geared for off the line speed.
Here are a few RCs that benefit from more torque:
Traxxas E-Revo, Traxxas Slash, Traxxas Stampede, Traxxas E-Maxx, HPI Savage Flux, HPI Vorza Flux, and just about any RC that you need off the line speed.
Remember Torque is a huge help with jumps!
Be aware that a lot of ESCs will have a motor turn limit on them, even some brushless ESCs will have a limit. Pay close attention to this when you choose your ESC and Brushless Motor combo. If you try to use a motor that is out of the Brushless motor specs of your ESC than you will end up frying your ESC.
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.
You can normally find the estimated 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 RC vehicle, and gear ratio or 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%. It will be a good safety cushion to make sure that you don’t burn up your brushless power plant.
Brushless Motor Ratings – Watts = Horse Power
Watts are the power rating or the horsepower equivalent of your Brushless RC 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.