Ever been confused by all the terms, labels and ratings attached to a brushless RC motor? You wouldn’t be alone. Not every RC fan is a talented physicist and without explanation, a lot of us are happy just to let these words and numbers wash over us without really taking them in. Understanding this lot can be of real benefit, however, whether it’s for assessing the worthiness of a motor in a RTR vehicle, picking an upgrade part or deciding what motor you want for a totally custom model. So let us clear up those grey areas and answer that key question: what terms should you know, and what do they mean?
We’ll start with the most basic one: RPM. RPM stands for ‘Revolutions per Minute’ and in effect measures how fast a mechanical part can spin. This is relevant to motors because the way they produce their power involves the use of spinning parts, therefore higher RPM means faster power production and essentially more speed for your hobby vehicle or craft. It’s no use for rating a motor, however, as motors will run different RPM depending on the electricity supply and the amount of resistance (load) faced.
This is where the KV rating comes in (not to be confused with kV, which stands for kilovolts). It measures the RPM produced per Volt of electricity supplied, assuming zero resistance or load. (To work out the full RPM you’ll get, minus load, just multiply the KV rating by the number of Volts delivered by your battery pack.) Unlike RPM, this fact about a motor will not change so long as it is working properly. That’s why it’s most commonly used as a measure of a brushless RC motor.
But hold on a minute – this isn’t a simple case of ‘bigger is better’. While a motor with a higher KV rating is literally faster, it can only buy this speed increase with a loss of torque or ‘acceleration power’. We’ll spare you the details, but because of how electric motors are built, they have to strike a balance between more strength at lower speeds or less strength at higher speeds. Which one you prioritise will depend on the type of model you have/are making, and how you want to use it.
Another term you might come across is ‘Motor Turns’, which is directly related to the KV rating and relative strengths (top speed vs. torque) of a motor. The ‘turns’ are wire windings around the motor’s rotor poles, but the real thing you need to know is that the more of them there are, the more torque the motor will have – at the expense of its top speed. Conversely, the fewer there are, the higher the model’s top speed will be, but the lower its torque.
Right, time to take a look at electrical current and, more specifically, Amps. Amps are a measure of the ‘flow’ of electricity (much like ‘gallons per hour’ measures the flow of water) and are vital for the safety of an RC electrical setup. Why? Because every motor will have a maximum current rating (or ‘amperage’) and if that is exceeded then the motor is at risk of damage and overheating. Of course, motors only ever pull as much electricity as they need to use, but if what they need is significantly more than what they were designed for (i.e. they are too small for their application) then they can burn out pretty fast. So if your last motor died of overloading, be sure to compare its maximum amperage with any you’re looking to buy!
There’s also Watts to consider, which are the electric motor’s version of horsepower. Like Amps, if you run more than the recommended Watt rating on your motor’s specification sheet (i.e. work it too hard) you make it vulnerable to damage. For context, you calculate Watts by multiplying amperage with voltage.
Still with us? It’s a lot to take in if you haven’t looked into these terms before, but we’re nearly there! The last one to go through is the efficiency rating of a motor. Now, this is pretty important, because it basically tells you the motor’s quality. A high quality motor will convert almost all of your electricity into kinetic power with very little lost as heat, which gives the motor a high efficiency rating. The closer to 100% the better.
If you play on the budget end of the spectrum, however, motor efficiency can be a matter of safety as well as power. Basically, if you’re running a motor with 70% efficiency, then 70% of the electricity supplied by your battery is going to power your vehicle – but the other 30% is being used purely for the production of heat. It’s an inevitable by-product, but too much of it and things start to get risky. A 550 Watt motor that only runs at 70% efficiency, for example, will have a whole 165 Watts (30%) going to producing heat. Those kinds of temperatures can melt cheaper vehicle parts, so watch out!
You’ve made it! That’s all the terms you’re likely to come across in relation to a brushless RC motor (that you need to worry about anyway). So let’s have a quick recap on the practical applications of these labels. In terms of top speed versus acceleration/torque, a high KV rating and low number of motor turns mean a good top speed but less acceleration power or strength, whereas a low KV rating and high number of motor turns means lots of torque and strength, but a lower top speed. With Amps and Watts, the higher the maximums for these are, the less likely you are to exceed them. And lastly, the more efficient your motor is, the better; it doesn’t only mean more bang for your electrical buck, but less heat and more safety too.
Want to know more about the practical side of brushless motors? We have articles explaining how to find the perfect ESC and battery pack to match your motor, how to choose the right size and spec of motor for your particular application, what could be causing those overheating issues and much, much more. So stay tuned, stay savvy and keep enjoying RC hobbies!