What is Kv in a Drone Motor? (Explained)

You might have heard people talk about Kv ratings of drones and be wondering what in the world they are talking about. Not to worry, in this article, you’ll learn all you need to know about Kv ratings of drones and you may even be able to contribute a thing or two when you hear people talking about Kv ratings. Read on to learn more about Kv ratings. 

“Kv” (not to be confused with “kV” the abbreviation for kilovolt) talks about the constant velocity of a motor. Therefore, the Kv rating for a motor is an indication of the number of revolutions per minute (RPM) of the motor when a given voltage is supplied.

How does this apply to drones? You probably know that motors are one of the key components of a drone, which in turn makes the Kv ratings of a drone’s motor very important. We’ll be talking more about Kv ratings below and how this rating has an impact on the performance of your drone.

What is Kv in the Motor of a Drone? #

Remember we said Kv rating is an indication of the number of RPM that a particular motor can make when a given voltage is applied. Usually, the voltage applied is 1 Volt current. This voltage is usually applied without any load attached to the motor. 

Let’s use the Hyperion 3025/6, which is specified to have a Kv rating of 1255, as an example. The Kv rating of 1255 means the motor of the Hyperion 3025/6 can rotate at 1255 RPM when you apply 1 Volt current without any load. There is a reduction of the RPM of the motor when you attach a propeller to it because of air resistance.

How Kv Rating Varies #

The Kv rating of a motor will depend on the exerted power (or in other words, the torque level) of the motor. Also, the number of copper wire windings on the rotating coil of the motor (the armature) will also determine the Kv rating. In general terms, the higher the number of copper windings on the rotating coil of the motor, the higher the Kv value and vice versa.

Also, the Kv rating of a motor is dependent on magnetic strength. An increase in the magnetic strength of the motor will also lead to an increase in the Kv value.

Why the Kv Rating of a Drone Motor Matters #

There are several reasons the Kv rating is important for you to know. We’ll go into a few of them here:

For Avoiding Overloading Incidents #

One of the applications of the Kv rating of a motor is in the determination of the speed of the motor at a particular voltage. Let’s use a motor with a Kv rating of 900 as an example. If said motor is powered by a lithium polymer battery that has a Kv value of 11, then the motor would rotate at 9900 rpm (900 x 11 = 9900). 

Any change in the applied voltage will have to be related to the change in the rpm. And this will ultimately require an adjustment in the propeller of the motor in order to avoid the risk of overloading the motor.  

For Distinguishing Two Similar Motors #

Two motors may be very similar in their configurations but have different windings (copper windings on the coil of the motor). So an easy way to make a comparison and distinguish the two similar motors is by the Kv rating. Since Kv ratings depend on the number of windings of the motors, the two otherwise similarly configured motors will have different Kv ratings.

Reducing the Risk of Overheating #

As a rule of thumb, motors that have high Kv ratings should be used for lighter drones while motors with low Kv motors should be used for heavier drones. It’s important to follow this basic rule as using motors with high Kv ratings for heavy drones or motors with low Kv ratings for lighter drones will increase the risk of overheating and consequently burning out the motor. 

We know you are curious as to why this is so and are eager for more explanation. Everything will be made clear below as we are still going to talk about how to choose motors for drones based on Kv ratings. But the short explanation is pairing a high Kv motor with a heavy drone (which will have a large propeller) will lead to the motor trying to rotate the propeller at a high rpm. This requires more torque and consequently, more current. Ultimately, a lot of heat will be generated which will most likely lead to premature failure of the motor. Yeah, not cool. 

Brushless Motor Kv Rating Explained #

Brushless and brushed motors are the two most popular types of motors. While both of them are designed on the same physical principles, there are significant differences in their structure, control, and performance. 

A brushed motor utilizes wound wire coils (brushes), also referred to as the arms tire as a two-pole electromagnet while a brushless motor uses a permanent magnet as its external rotor. The advantage of removing the brushes in brushless motors is improved operating efficiency, less overall maintenance, and lower electric noise generation, among others. As a result, brushless motors have gained a lot of popularity and have become the motor of choice in many drones.

The Kv motor rating of a brushless motor is very important as it determines other factors and parameters of the motors such as the efficiency, purpose of the drone, agility of the drone, flight time, dimension of the propeller, flight time, and so on. 

You’ll also be able to understand the torque of a brushless motor based on the Kv rating as the torque will be determined by the strength of the magnet. Also, your knowledge of the Kv rating of a brushless motor will come in handy when you are trying to determine which motor should be used in a particular aircraft. For example, an FPV racing quadcopter will require a motor (better if it is brushless) that has a high rpm (for high speed). So this means you need a motor with a high Kv rating but a propeller with a small diameter. 

On the other hand, when you encounter a motor with a low Kv value, you’ll most likely want to use it in a heavy aircraft in order to get more torque to spin the heavier prop.

How to Choose the Right Kv Motor for a Drone #

By now, you should have a good understanding of the Kv rating of motors. This heading will focus on how to choose a Kv motor for a drone, something you may already have an idea of if you’ve been paying attention to everything so far. 

I’d like to quickly remind you that the Kv rating of a motor is determined by the number of copper wire windings on the motor (for brushed motors) or the strength of the magnet (for brushless motors). The higher the number of copper wirings or the strength of the magnet, the higher the Kv value, and vice versa.

The size of your drone is the major determining factor of the Kv rating of the motor you are going to want to use. If your drone is light, then it makes the most sense to use a high Kv motor. Lighter drones will also have small propellers and by using a high Kv motor, the motor will want to spin the propeller faster, an act that will be successful due to the small size of the prop. As a result, high Kv motors are used in lighter drones or racing drones where speed is prioritized. 

On the other hand, heavier drones, which will have larger props, will require a low Kv motor. A low Kv motor will be able to rotate the motor without any problem and will also be able to generate the high torque needed to lift the motor. But should you use a high Kv motor for a heavy drone, the motor will attempt to rotate the larger propeller at a high speed. This will require more power from the motor and force the motor to draw more current. As this happens, overheating will happen and the motor will end up being damaged.