Control structure of brushless DC motor.

Control structure of brushless DC motor.

DC brushless motor is a type of synchronous motor, which means that the speed of the motor rotor is affected by the speed of the rotating magnetic field of the motor stator and the number of rotor poles. When the number of rotor poles is fixed, changing the frequency of the stator rotating magnetic field can change the speed of the rotor. DC brushless motor is a synchronous motor with electronic control (driver), which controls the frequency of the stator rotating magnetic field and feeds back the speed of the motor rotor to the control center for repeated correction, in order to achieve a way that is close to the characteristics of a DC motor. That is to say, brushless DC motors can still control the rotor of the motor to maintain a certain speed within the rated load range when the load changes.

The DC brushless driver includes a power supply unit and a control unit: the power supply unit provides three-phase power to the motor, and the control unit converts the input power frequency according to demand.

The power supply unit can directly input DC power (usually 24V) or AC power (110V/220V). If the input is AC power, it must first be converted to DC through a converter. Whether it is direct current input or alternating current input, before entering the motor coil, the direct current voltage must be converted from the inverter to a three-phase voltage to drive the motor. Inverter is generally composed of six power transistors (Q1-Q6) divided into upper arm (Q1, Q3, Q5)/lower arm (Q2, Q4, Q6) connected to the motor as a switch to control the flow through the motor coil. The control department provides PWM (Pulse Width Modulation) to determine the frequency of power transistor switching and the timing of inverter commutation. DC brushless motors are generally intended for speed control where the speed can remain stable at the set value without significant changes when the load changes. Therefore, the motor is equipped with a Hall sensor that can sense the magnetic field, which serves as a closed-loop control for speed and also as the basis for phase sequence control. But this is only used as speed control and cannot be used as positioning control.