How Control BLDC

Structure of Brushless Motors
First, let's introduce some concepts. The number of slots N and the number of poles P in a brushless motor. The number of slots N refers to the number of electromagnet poles on the stator, and the number of poles P refers to the number of magnetic poles on the rotor. The simplest 3N2P structure motor is a brushless motor with three coil poles on the stator and two magnetic poles on the rotor. The schematic diagram of the stator structure of a 3N2P brushless motor is as follows:

The stator has three sets of coils: A, B, and C. One end of the three coils is connected to a common point, and the other end leads out three wires a, b, and c. Place a magnet as the rotor in the center, and the simplest brushless motor structure is formed, as shown below:
Of course, this is just the simplest 3-slot 2-pole motor. Commonly used motors increase the number of slots and poles to make the rotation smoother and the torque larger. The connection mode of the coils can be star or delta connection. At the same time, according to the mechanical structure, whether the rotor is inside or outside the motor, it can be divided into outer rotor motors and inner rotor motors.

Driving Principle of Brushless Motors
After understanding the structure of the brushless motor, how does it rotate? We still take the simplest 3N2P brushless motor as an example. Suppose initially, we connect the positive power supply to terminal a, the negative power supply to terminal b, and leave terminal c suspended. Then the magnetic field generated by coil A is directed to the upper left, the magnetic field generated by coil B is directed upward, and the vector sum of the magnetic fields is directed to the upper left. Under the action of the magnetic fields of coils A and B, the rotor magnet will rotate to the direction shown in the figure:
At the next moment, we connect the positive power supply to terminal c, the negative power supply to terminal b, and leave terminal a suspended. Then the vector sum of the magnetic fields is directed to the upper right, and the rotor magnet will rotate from position 1 in the following figure to position 2:
Similarly, in the subsequent sequence of c+a-, a-b+, b+c-, c-a+, a+b-, b-c+ for power supply, the rotor magnet can rotate circularly. After every 6 current switches, the rotor rotates one circle. Since the three coils are 120° apart, it is not difficult to conclude that when two coils are conducting simultaneously, the torque is √3 times that of a single coil.
In the above driving method, two coils are conducted each time, so it is called the "two-two conduction" driving mode. Relatively, there is also a mode in which three coils are conducted simultaneously, called the "three-three conduction" driving mode. For example, when the applied voltage is in the state of a+b-c-, since all three coils will generate magnetic fields, the stator magnet will rotate to the position in the following figure (the N pole is directly opposite to coil A):
Moreover, since the current in coil A is equal to the sum of the currents in coils B and C, the total torque is 1.5 times the torque of coil A. It is not difficult to analyze that the "three-three conduction" driving mode also requires 6 steps to complete one rotation. If we control the coil voltage in sequence according to a+b-c-, a+b-c+, a-b-c+, a-b+c+, a-b+c-, a+b+c-, the stator can also rotate.

Driving Circuit of Brushless Motors
Above, we analyzed how to make a three-phase brushless motor rotate. In essence, it requires applying positive and negative voltages to the lead-out points of the three coils respectively. Generally, a three-phase six-arm full-bridge circuit as follows can be used to achieve this:
For example, in the above figure, if Q1 and Q4 are turned on and the others are not, the current will flow from Q1 through the U-phase winding and then from the V-phase winding to Q4. In this way, one coil is energized. Similarly, by turning on Q5Q4, Q5Q2, Q3Q2, Q3Q6, and Q1Q6 in sequence, the 6-step working mode of the "two-two conduction" is completed. Similarly, the three-phase full bridge can also achieve the "three-three conduction" control mode.
The above full-bridge circuit is just a theoretical introduction. In practical applications, during control, the upper and lower MOS transistors of the same bridge arm should not be turned on simultaneously, otherwise the devices will be burned out. We can first turn off the MOS transistor of the upper bridge arm and then turn on the MOS transistor of the lower bridge arm (or vice versa), which avoids the simultaneous conduction time of the upper and lower MOS transistors. This time difference is generally called the dead time. Many PWM waves output by MCUs can control the size of the dead time, which is convenient for our design.