How Hall Sensor Works in Brushless DC Motor?

The work principle of the BLDC motor is similar to that of the brushed DC motor. The brushed DC motor adopts the electric brush and the commutator to realize commutation of the current in the winding. On the contrary, the BLDC motor realizes commutation of the winding current in the electronic way.

Commutation of the BLDC motor is controlled electronically. To ensure the BLDC motor to move, the stator winding should be powered on according to certain sequence. In order to identify which winding will gain electricity first according to the electrification sequence, it is important to know the rotor's position, which is detected by the Hall sensor embedded in the stator. When the rotor magnetic pole passes nearby the Hall sensor, it will give out a high level signal or a low level signal, indicating that the north magnetic pole or the south magnetic pole is passing by the sensor. According to the combination of the three Hall sensor signals, the precise sequence of commutation can be pinned down.

The torque of the DC electric motor react with the current in the winding via the permanent magnetic field. In the brushed DC motor, the commutator realizes commutation of the armature current and seeking of a suitable magnetic field by switching to the armature current. In the BLDC motor, the Hall sensor detects the position of the rotor's rotating magnetic field and provides the corresponding winding excitation through the logic and driving circuit. On the whole, the winding responses according to the magnetic field of the electric motor's permanent magnet, thus generating the required torque. Fig. 1 presents the basic components of a three-phase, eight-magnetic-pole (four pairs of magnetic poles):

The rotating permanent magnet, when rotating by the double-magnetic-pole digital Hall sensor, will cause changes to the double-pole digital Hall sensor's state. As shown in Fig. 1, the two south poles of the eight-pole magnet BLDC motor are apart from each other by 90°, while the Hall sensors are apart from each other by 120°. At the moment, the electric phase angle between Hall sensors is 30° apart. When the south pole approaches, the double-pole digital Hall sensor changes its working state. When the first digital Hall sensor swifts to the working status at the 0° electric angle, the second digital Hall sensor works at the electric angle of 30°. The third digital Hall sensor works at the electrolysis angle of 60°.

When the north pole passes by the double-pole digital Hall sensor, the digital Hall sensor will swifts to the release state, every north pole of the rotating eight-pole magnet is apart from the adjacent south pole by 45°. Therefore, when the digital Hall sensor will change from the working state to the release state when the magnet rotates by 45°. Fig. 2 shows the output level state under the control of the digital Hall sensor in 3-phase BLDC motor.
The output of the above three double-pole digital Hall sensor is adopted as the coder of the rotor position. The magnet position and pole information can be sent to the logic circuit as the information to disconnect the H-shaped bridge power tube. Fig. 3 shows the typical driving circuit of the three-phase BLDC motor.
In Fig. 3, R1, S1, T1 can be driven by the above signal, when R2, S2 and T2 are driven after phase shift of the above signals. Hence, according to the position of the rotating magnet, every pair of power tubes will correspondingly connect or judge to provide the current for the electric motor's winding in the right sequence and at the right time.