Many friends are confused about the number of poles of brushless motors, do not understand exactly how to detect, why the rotor magnetic field will change, in the end what role, Vacuz to give you an analysis of it! Hope it can help everyone!

stator and rotor

In fact, you can detect the change of rotor pole magnetic field to distinguish the number of brushless motor pole, rotor magnets are generally arranged alternately in accordance with the N pole and S pole, when the brushless motor Hall sensor, detect the magnets from N pole jump to S pole, or S pole jump to N pole, Hall’s output will have the corresponding up and down level change, which means that the rotor position has changed. Whether the brushless motor winding is energized or not has no effect on Hall detection.

The speed of brushless motor magnetic field, also called synchronous speed, is related to the frequency of the three-phase current and the number of poles p. If the stator winding has only one pair of poles (pole pair p=1), that is, only two poles, for a rotating magnetic field with only one pair of poles, the three-phase current changes once and the synthetic magnetic field rotates once. If it is 50hz AC, the synchronous speed of rotating magnetic field is 50 rpm or 3000 rpm. If the stator winding of brushless motor has two pairs of magnetic poles (pole pair p=2), that is, there are four poles, it can be proved that the current changes one cycle and the synthetic magnetic field rotates 180 degrees in space. From this, we can calculate the synchronous speed of the rotating magnetic field of the P pair of poles as n=60f/p per minute.

When the number of poles of a brushless motor is certain, if the frequency of the alternating current is changed, the synchronous speed of the rotating magnetic field can be changed, which is the fundamental principle of frequency regulation, because the magnetic poles of the motor appear in pairs, so it is usually expressed by pole pairs.

The brushless motor senses the change of the magnet by Hall element to determine the position of the magnetic pole pair of the brushless motor, and drives the rotor of the brushless motor to reach the magnetic field through the controller to detect the speed and pole pair.

BLDC Motor

These are the common methods of detecting the number of pole pairs in brushless motors, and the changes in the magnetic field. Mastering these basics will help to better understand the rotor structure of brushless motors.

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