Calculating Motor Kv & Motor Poles: A Comprehensive Guide
Posted by sheryl
from the Agriculture category at
15 Nov 2023 07:36:58 am.
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Section 1: Calculating Brushless Motor Kv
Understanding Brushless Motor Kv
Motor Kv represents the relationship between a brushless motor’s rotational speed, in a no-load situation, and the applied voltage. Measured in RPM/V, Kv is determined by dividing the unloaded motor’s rotational speed by the applied voltage. In reality, it’s the back electromotive force (EMF) that's significant rather than the applied voltage. The back EMF, induced by coil movement in the motor’s magnetic field, opposes the current from the power source, following Faraday’s law. This back EMF is directly proportional to the rotor’s speed: as RPM increases, so does the back EMF.
Interpreting the Kv Rating
The Kv rating indicates a motor’s rotations per applied volt. This rating assists in comparing motors of similar size but different performance due to internal variations. Generally, as the number of windings in the coils increases, the Kv decreases. Lower Kv motors consist of more windings of thinner wire, carrying more volts at lower current. Higher Kv motors have fewer windings but thicker wire, capable of carrying higher currents with fewer volts. The forthcoming equations illustrate this concept.
Low Kv motors operate at lower RPMs, generating more torque, ideal for larger drones with larger propellers. Conversely, high Kv motors function at higher RPMs, fitting smaller, faster-spinning propellers.
Section 2: Determining the Number of Poles in a Brushless Motor
Understanding Motor Poles
Motor poles are magnets aligned around the rotor’s circumference within a brushless motor. Each set of north and south magnets constitutes a ‘pole pair’, contributing to two poles. Hence, an 8-pole motor comprises 4 pole pairs. Usually, pole numbers are even (2, 4, 6, 8, etc.) to ensure each magnet has an opposing pole.
Calculating Motor Poles
Identifying the number of poles involves manually counting the visible magnets inside the motor or accessing them by removing the casing.
Section 3: Relationship Between Motor Poles and Kv
Motor Kv and the number of poles affect motor speed or RPM. Higher RPM increases Kv, while a greater pole count corresponds to lower RPM. This inverse correlation is practical: larger motors with more poles necessitate high torque and operate at low speeds, leading to a high pole count and low Kv. Smaller motors, with fewer poles, operate at high RPMs, resulting in low pole counts and high Kv.
Conclusion
Grasping brushless motor poles and motor Kv is vital for understanding BLDC motor characteristics and performance. This knowledge assists in selecting suitable motors for drones or electric aircraft, directly impacting efficiency, payload, flight time, and range.
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