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BLDC Motor Control and Speed Regulation II

Update:10-04-2020
Summary:

As is mentioned in the previous article, three-phase BL […]

As is mentioned in the previous article, three-phase BLDC motor was taken as an example. The number of pole pairs of the motor is three, so that each pair of "magnetic poles" are turned on in a certain order, to achieve the effect equivalent to the magnetic field turning. The permanent magnet rotor in the middle always has the tendency to keep the magnetic field in the same direction under the action of the magnetic field, and it will "follow" the rotating magnetic field to rotate. It is equivalent to continuously changing the energizing sequence to achieve the purpose of "rotating" the stator.

To make the three coils turn on and off in sequence requires a certain control strategy. In the figure, H1 \ H2 \ H3 are three Hall sensors placed at the air gap of the excitation coil. As the components for detecting the magnetic field, they can change the voltage according to the direction of the magnetic field, and the output is a digital signal. For example, if the sensor is facing the N pole, it outputs a high level, that is, a logic signal of ‘1’, otherwise it outputs ‘0’. In this way, the output signals of the three Hall sensors are arranged in a certain order, of which two must be ‘1’ and one ‘0’, and we can judge the rotation state of the rotor according to the returned three binary digits.

The use of three-phase control is more difficult than two-phase control, but no dead zone judgment is required. As long as the power is turned on, the Hall sensor will return to the current state of the rotor, and according to this state, the stator coil is energized according to the next timing, the rotor magnetic field and the stator magnetic field must have an angle, and it will turn up. In this way, there is no need to judge whether the BLDC motor is just started, and only need to execute the next command according to the working status returned by the Hall sensor. The command it sends is the on-off of three pairs of coils, which can theoretically be controlled by three switches. In fact, these switches are implemented through transistors. Therefore, it is only necessary to energize or de-energize the three pairs of transistors in a certain order to realize the rotation of the three-phase BLDC motor.