HallA, HallB, and HallC are three Hall sensors installed in different parts of the motor, used to check the relative polarity in the rotor area of the three windings (A/B/C). During the entire operation of the motor, when the forward magnetic induction line crosses the Hall sensor, the sensor outputs "1", and vice versa, outputs "0". Because three Hall sensors are uniformly installed at a 360 degree electrical angle, when the motor runs for one electrical equipment cycle time (360 degrees), each Hall sensor‘s pulse signal will generate 2 rotations, a total of 6 pulse signal rotations. Because the sensors are evenly distributed, each rotation is exactly 60 degrees apart from each other, which is the electrical angle of the motor‘s second commutation phase distance. According to the continuous loading of different pulse signals from the Hall sensor, the rotation time is determined, and then the relative winding is powered on, so that the motor can operate stably.

    As shown in the figure, the self induced electromotive force working voltage on the unconnected power phase winding is checked in each section (60 degree electrical equipment angle). When the self induced electromotive force working voltage passes through the "center" or "zero crossing", it is detected that a zero crossing occurs. The control system learned that the rotor is currently located at the center of the 60 degree electrical equipment angle range and has a spacing of 30 degrees for the next phase change of the electrical equipment angle. The timer records the time for each section (60 degree electrical equipment angle) as T60. When zero crossing is detected, the timer will be loaded with a preset value of half of T60. The timer generates a request timeout, which will cause a termination and complete the next phase change. This type of control measure is called sensorless operation of BLDC motors.

    There are various ways to inspect the position of the rotor for sensorless operation of BLDC motors. For example, BEMF measurement method, magnetic flux measurement method, various system software data signal observer method, data signal introduction method, etc. At present, the most commonly used method is the BEMF method, which completes commutation by checking the zero crossing point of the self induced electromotive force of the disconnected phase. Other phase sensorless control measures will be discussed in the subsequent chapter directory.

    The following table shows the three-phase bridge driver of a DC brushless motor (as shown in Figure 1), the power switch situation matched with the six step commutation in each electrical equipment cycle time (360 degrees), and the correlation between the direction of current and the rotor position.

    Above is the basic principle of brushless DC motor waveform driver.