For enthusiasts involved in crafting drones, RC vehicles, or devices powered by brushless DC motor (BLDC motor), comprehending motor Kv and motor poles is fundamental. These factors serve as the foundation for defining a motor's characteristics and estimating its performance. This article aims to elucidate these concepts, detailing their calculation methods, measurements, and the interplay between them.

Section 1: Calculating Brushless Motor Kv

Understanding Motor Kv in Brushless Motors

Motor Kv signifies the relationship between a brushless motor's rotational speed in a no-load state and the peak voltage. Measured in RPM/V, Kv is derived by dividing the unloaded motor's rotation speed by the applied voltage. In practical terms, it's the back electromotive force (EMF) that should be considered rather than the applied voltage. Within a brushless motor, the back EMF counters the current from the power source, induced by coil movement within the motor's magnetic field. This phenomenon, explained by Faraday's law, generates an opposing voltage or EMF due to coil movement within a magnetic field. The motor's back EMF is directly proportional to the rotor's speed: as RPM increases, so does the back EMF.

Understanding the Kv Rating

The Kv rating estimates the motor's rotations per volt applied. This rating facilitates comparisons between motors of the same physical size but with varied performance due to internal differences. Generally, as the winding count in the coils increases, the Kv decreases. Mechanically, low Kv motors incorporate more windings of thinner wire, carrying more volts at lower current. High Kv motors, however, feature fewer windings but thicker wire, capable of handling higher currents with fewer volts. The forthcoming equations in the subsequent section illustrate this concept.

In drone applications, low Kv motors operate at lower RPMs, generating more torque suitable for larger propellers and larger drones. Conversely, high Kv motors function at higher RPMs, accommodating small, fast-spinning propellers.

Section 2: Determining the Number of Poles in a Brushless Motor

Understanding Motor Poles

Motor poles consist of magnets aligned around the rotor's circumference within a brushless motor (figure 1). These magnets or poles play a pivotal role when an electric current powers the coils in the stator, prompting rotor rotation as the magnets repel the like electromagnets.

Poles vs. Pole

Pairs Each rotor magnet possesses a north and south end, with only one end interacting with the stator. Each set of N and S magnets constitutes a 'pole pair.' For every pole pair, there exist two poles. Hence, an 8-pole motor comprises 4 pole pairs. Consequently, pole numbers are typically even (2, 4, 6, 8, etc.), ensuring each magnet has an opposing pole.

Calculating Motor Poles

Determining the number of poles involves visually counting the magnets, usually visible inside the motor or accessible by removing the casing.

Section 3: Relationship Between Motor Poles and Kv

Motor Kv and the number of poles both significantly influence motor speed or RPM. Higher RPM increases Kv, while a greater pole count corresponds to lower RPM. The correlation between the number of motor poles and Kv is inversely proportional. This relationship is practical, as larger motors with more poles require high torque and operate at low speeds, resulting in a high pole count and low Kv. Smaller motors, featuring fewer poles, operate at high RPMs, thus possessing low pole counts and high Kv.

Conclusion

Grasping brushless motor poles and motor Kv is pivotal for understanding BLDC motor characteristics and performance. This knowledge aids in selecting the most suitable motor for drones or electric aircraft, thereby enhancing performance and efficiency. It directly impacts flight time, payload capacity, and range.