This is one mistake you will want to avoid. Not only is it very costly but it can take the fun right out of RC and there is no place for this! Primarily this mistake is made when selecting a brushless motor for an RC vehicle. Often times the motor that is selected by a new builder tends to not have the correct kv. Incorrectly selecting the kv for the motor can literally destroy the motor, ESC or battery. Avoid this Brushless Motor Mistake. Let’s dive into this.
Why Is kv Incorrectly Selected?
Well for starters, there are a lot of RC brushless motors available on the market. You can get one in nearly any size, colour, wind type, inrunner, outrunner, sensored, sensorless and with multiple different kv options on the same motor. Simply put, it’s quite complex and there is no one telling you if the motor you selected will work perfectly in your RC to hit the targets and goals that you have set out.
Why Kv Selection is Very Important
When it comes to selecting kv, the importance lies in the total amount of RPM that you require. Not only is the output RPM that you get out of the motor important but also the total voltage you plan to run. As we know, kv represents the RPM per volt applied to the motor. The total RPM we get out of the motor is battery voltage multiplied by the kv. You would need to know what battery you plan to run prior to being able to select the RPM per volt that you need in your RC. In addition you would also need to know the amount of total RPM that you are trying to target within your RC. This value changes depending on the type of RC that you are powering.
Rule for RPM of an RC Car, Plane, Boat
Here we have a general table that represents the total amount of RPM that you could expect for each RC vehicle Classification.
| Vehicle Type | Minimum RPM | Maximum RPM |
| RC Car | 28,000 | 50,000 |
| RC Airplane | 5,000 | 18,000 |
| RC Boat | 22,000 | 32,500 |
It is very possible that your RPM requirement could be less than the minimums that are above. Having too much RPM will increase the load requirement on your power system. If the system can not handle the increased load, this is where we get excessive heat and possible failure. Below we talk about how you can sanity check your selection and overall setup.
Picking a setup that runs at RPM levels above the maximums listed above is only for experienced modelers.
RC Airplane RPM Output Range
For the RC airplane in the above table, you would want to go for a higher RPM when spinning a prop close to a diameter of around 5 inches. You would want to use an RPM on the lower end when you need to spin a prop that is larger than 20 inches in diameter. Higher RPM with a larger prop will take more power from your motor, ESC and battery.
RC Boat RPM Output Range
For RC Boats, hull length comes in to play considerably when looking at the desirable RPM range to hit. Hulls that naturally have more drag (mono hull) would need a larger propeller to push them through the water with enough thrust. This will translate to requiring an RPM output on the low end of the range. However a hull with minimal drag can benefit by having higher RPM output using a smaller prop.
RC Car RPM Output Range
The RPM output range for an RC car highly depends on a few different specifications of the car. These include the overall gear ratio of the vehicle, tire diameter and speed that the vehicle would be setup to hit. See below on how you can check your selection.
How to Sanity Check the kv that you have Selected
In order to boost the confidence that you have in your selection, I would highly recommend going through a sanity check. The idea is to use the kv that you have selected as well as the vehicle specifications to calculate the expected speed that you could achieve. If you discover that your RC 1/10 scale truck is going to go 120mph (190km/h) with your conservative setup, this would be a huge indication that something is wrong.
The same approach would be used for an RC airplane or boat. Instead of using a gear ratio and tire diameter, you would be using propeller pitch and a slippage factor.
Check Out these Calculators that will help identify the speeds for your specific RC:
RC Airplane Calculator
Boat Calculator
RC Car Calculator
Conclusions
Now that you know the importance of selecting the correct kv for your RC, you will be able to pick out a reliable power system! Just make sure you continue to use a heat gun to measure the temperatures of the motor, ESC and battery. Be certain that your operating temperatures are never outside of the maximum specification for each component.