Structure Of A DC Motor

The control structure of a brushless DC motor: A brushless DC motor is a type of synchronous motor, meaning its rotor speed is affected by the speed of the stator's rotating magnetic field and the number of rotor poles (P), N = 120f/P. With a fixed number of rotor poles, changing the frequency of the stator's rotating magnetic field changes the rotor speed. A brushless DC motor is essentially a synchronous motor with added electronic control (driver). It controls the frequency of the stator's rotating magnetic field and feeds the rotor speed back to the control center for repeated correction, aiming to achieve characteristics close to a DC motor. In other words, a brushless DC motor can maintain a certain rotor speed within its rated load range even when the load changes.

A brushless DC driver includes a power supply section and a control section: the power supply section provides three-phase power to the motor, while the control section converts the input power frequency as needed. The power supply section can accept direct DC input (typically 24V) or AC input (110V/220V). If the input is AC, it must first be converted to DC by a converter. Whether the input is DC or AC, the DC voltage must first be converted into a three-phase voltage by an inverter to drive the motor coils. The inverter typically consists of six power transistors (Q1-Q6) divided into upper arms (Q1, Q3, Q5) and lower arms (Q2, Q4, Q6), connected to the motor as switches to control the flow of electricity through the motor coils. The control unit provides PWM (Pulse Width Modulation) to determine the switching frequency of the power transistors and the timing of inverter commutation. Brushless DC motors generally require speed control that ensures the speed remains stable at a set value without significant fluctuations when the load changes. Therefore, the motor is equipped with a Hall sensor that senses the magnetic field, serving as a closed-loop speed control and also as a basis for phase sequence control. However, this is only used for speed control and cannot be used for positioning control.