The Basics of drone motor KV Rating:

Drone/UAV motors KV rating has a notable impact on their desirable performance. Understanding the KV rating of a motor, its variation and application are a critical skills for drone technicians and users.

What does KV rating stand for?
In a word, it is the rpm constant of a motor. It indicates the number of revolutions a motor makes upon the application of 1 Volt current without any load attached to the motor. So, it is the number of revolutions per volt in the free load condition.
For example, Hyperion 3025/6 has a KV rating of 1255. It means that the motor spins 1255 rpm at the application of 1V without load.
Attachment of a propeller on the motor reduces the rpm due to the air resistance.

How KV Rating Varies
KV rating is dependent on the exerted power from a motor. More specifically, the torque level of a motor. The number of copper wire windings on the armature determines the KV value. KV value increases with the decrease in the number of winds and vice versa.
KV rating is also affected by magnetic strength. The rise of magnetic strength raises KV value. Most motors are manufactured with 12N14P (1) magnetic arrangement.

Application of KV Rating
For Avoiding Overloading Incidence
KV rating of a motor helps to determine the speed of the motor at a given voltage. For example, a motor of 980 KV which is powered by a Li-Po battery of 11.1 KV would spin 10878 rpm (980×11.1=10878) without any load. The change in voltage is related to the change in rpm and thus requires an adjustment in the propeller to avoid any risk of overloading.

For Distinguishing Two Similar Motors
KV rating is very useful in making a comparison between two similar motors which have close configurations but different windings.

Reducing the Risk of Overheating
Proper choice of the motor based on KV rating is crucial to reducing the risk of overheating and thus burning out of the motor. Commonly, motors with high KV ratings are suitable for lighter drone/quad. Also, low KV motors are for heavy quad/drone. Alteration of this is not good at all for drone and UAV motors.

When a high KV motor is paired with a large propeller, it will try to spin the propeller with high rpm which requires more torque. The motor will consume more current, and thus excess heat will be generated. It causes excess strain on the motor and leads to premature failure of the motor.

So, KV rating helps in choosing a suitable motor for a particular use like drone or UAV construction where precision, as well as when high performance is needed.

Practical Application with Example
Usually, brushless motors are used widely in the construction of drones because of more advantages over brushed motors. With brushless motors, high rpm may be achieved with minimal noise and heat generation. For this reason, brushless motors are very popular in the area of drone manufacturing.

KV ratings can vary widely. It may range from 40 KV to 1900 KV. Quadcopters and tricopters use motors of high KV rating. On the other hand, Octocopters use low KV motors. Practically, higher KV motors are best suited for handling smaller propellers. Usually, drones use Li-Po battery of 3 to 6 cells. The voltage of this battery is dependent on the number of cells. So, voltage lies between 11.1 to 22.2 V.

KV rating is an important factor in the way of determination of the motor type for a particular Drone, Racing drone, Quadcopter, FPV engines, etc. It says which motor suits which type of aircraft. For example, an FPV racing quad requires an engine of high speed. So, it is provided with a motor of high KV rating and a propeller of small- diameter.

KV rating is also related to the frame size of the drone. For a particular frame size, there is a specific range of motor size, KV rating, and propeller size. For example, if the frame size is 150mm or smaller, then KV rating of 1306 or smaller size motor is prescribed with a propeller length of 3” or smaller (using 4S Li-Po Battery). For a 350mm frame size, 1600 KV motor and 7” propeller is advised.

Price Comparison
Lower KV motors are costlier than higher KV motors. Lower KV motors contain extra magnetic arrangement to expand torque and increase the productivity of the motor. It increases the overall cost.

1. Stator pole/magnet pole configurations
12N, 14P (DLRK[1]) – Common for higher torque applications. Noted commonly for its smooth and quiet operation. Winding Pattern is AabBCcaABbcC (lowercase implies reverse in winding direction).