Brushless Motor Kv, which stands for “Rotor Velocity Constant”, is a crucial factor in determining a motor’s efficiency, agility, thrust, flight time, and other characteristics. It is defined as the increase in rotor rotations per minute (RPM) when voltage is increased by 1V when the motor is running without any load.
If you are looking for a complete guide on choosing motors for your build, take a look at our post here.
In the RC (radio control) world, brushless DC motors are typically marked with a 4-digit number: XXYY, where “XX” represents the stator width and “YY” represents the stator height. The torque of a brushless motor is directly related to the stator width and height, with wider and taller stators capable of producing higher torque.
5-inch quadcopters commonly use motors that are marked with numbers such as 2205, 2206, etc. These markings only refer to the stator and do not include the rotor, as seen in the image above.
The dimensions and Kv rating of the motor are typically marked together. A typical motor mark would look like “2206/2300”, where the second number refers to the Kv rating of the motor.
The Kv Rating
Kv or the “Velocity Constant” represents the speed at which the motor rotates per volt applied to the motor. For example, if a 3500Kv motor is powered by a 4s (14.8V) battery, the maximum rotation speed of the motor would be approximately 3500×14.8=51800RPM. It’s important to note that this is calculated under ideal conditions and theoretical assumptions (without any load) and in reality, there will always be some load such as props, and due to air resistance, the motor won’t be able to reach that RPM, it will always be a little lower.
Motors with a higher Kv rating will rotate the propellers faster, however, they will have less torque compared to lower Kv motors. That’s why it’s suggested to use larger props with lower Kv motors, as they will provide more torque. On the other hand, higher Kv motors are more efficient with higher RPM, but at the cost of torque.
It’s important to note that when moving away from ideal conditions and theoretical assumptions, the situation can become more complex. Kv is not an indicator of a motor’s power, its ability to handle high current, or its efficiency. Instead, it’s recommended to rely on thrust tests for these calculations.
To understand it more in-depth, when a magnet is moved near a copper wire, electricity is produced in the form of voltage. Similarly, when a motor rotates, it creates a back electromotive force (EMF) that can be measured in volts. It’s best to think of Kv as a motor constant, where the motor generates 1V of back EMF per a certain number of rotations. For example, if a motor generates 1V of back EMF per 2500RPM, it’s a 2500Kv motor. If a motor generates 1V per 1650RPM, its Kv rating is 1650Kv. Therefore, measuring the correct RPM and voltage is the simplest way to calculate the Kv rating of any brushless motor in the RC world.
Let’s talk about Torque
The relationship between Kv and the torque of a motor is important to consider. Kv is used to calculate the current required to achieve a certain torque level. The torque constant (Kt) is inversely related to Kv, meaning that as Kv increases, Kt decreases.
The relationship between current, torque, and Kv are closely related. Motors with lower Kv require less current to rotate larger propellers and therefore have a higher torque (Kt), but are less efficient with increasing RPM. Conversely, motors with higher Kv require more current to rotate larger propellers but are more efficient with increasing RPM.
In summary, motors with higher Kv require more current to achieve the same torque as motors with lower Kv.
Is higher always better?
As previously discussed, the Kv rating of a brushless motor indicates the RPM of the motor per 1V of the voltage applied. However, it is important to note that a higher Kv rating does not always mean that the motor is better. The relationship between Kv and torque, as well as the size of the propeller, should also be taken into consideration when choosing a motor.
- When the voltage is increased, it results in higher RPM but also a higher current. A current that is too high can damage the motor.
- The torque is directly proportional to the current.
- The current drain is highest at lower RPMs (at full throttle) and decreases as RPMs increase. To achieve maximum RPM, multiply the Kv by the battery voltage. However, it’s important to note that rotating the motor too slowly can also damage it.
- A larger prop diameter or pitch results in a heavier load which slows down the motor. The goal is to use a prop that accelerates the motor enough to have a current that is low enough to not damage the motor. This is the formula for maximum power.
- The C-rating of the battery must be high enough to deliver the necessary current to the motor. The result of C * mAh * 0.5 should be greater than the current the motor is drawing.
- The ESC must be able to handle more current in Amps than the motor is drawing. It’s ideal to use a 0.75 factor in the calculation, meaning that a 40A ESC would be sufficient for a 30A motor.