The Principle Of DC Motors

The control principle of a brushless DC motor is as follows: To make the motor rotate, the control unit must first determine the position of the motor rotor based on the Hall-sensor. Then, according to the stator windings, it determines the sequence in which the power transistors in the inverter are turned on (or off). The AH, BH, and CH transistors (called upper arm power transistors) and the AL, BL, and CL transistors (called lower arm power transistors) in the inverter sequentially flow current through the motor coils, generating a clockwise (or counter-clockwise) rotating magnetic field. This magnetic field interacts with the rotor's magnets, thus causing the motor to rotate clockwise/counter-clockwise. When the motor rotor rotates to a position where the Hall-sensor senses another set of signals, the control unit turns on the next set of power transistors. This cycle continues, allowing the motor to rotate in the same direction until the control unit decides to stop the motor rotor, at which point the power transistors are turned off (or only the lower arm power transistors are turned on). To reverse the rotor direction, the power transistors are turned on in the reverse sequence.

The basic switching pattern for power transistors can be illustrated as follows: AH, BL → AH, CL → BH, CL → BH, AL → CH, AL → CH, BL. However, it is absolutely forbidden to switch them as AH, AL, BH, BL, or CH, CL. Furthermore, because electronic components always have a switching response time, the switching time of the power transistors must take this response time into account. Otherwise, if the upper arm (or lower arm) is not fully closed before the lower arm (or upper arm) is opened, a short circuit will occur, causing the power transistor to burn out.

When the motor starts rotating, the control unit compares (or calculates via software) the command (composed of the speed set by the driver and the acceleration/deceleration rate) with the speed of change of the hall-sensor signal to determine which group of switches (AH, BL, AH, CL, BH, CL, or ...) should be turned on, and for how long. If the speed is insufficient, the on-time is longer; if the speed is excessive, the on-time is shorter. This part of the operation is handled by PWM. PWM (Pulse Width Modulation) determines the speed of a motor, and generating such PWM is key to achieving precise speed control.

High-speed control must consider whether the system's clock resolution is sufficient to handle the processing time of software instructions. Furthermore, the way Hall-sensor signal changes are accessed also affects processor performance, accuracy, and real-time performance. For low-speed control, especially low-speed starts, the Hall-sensor signal changes more slowly. Therefore, the signal acquisition method, processing timing, and appropriate configuration of control parameters based on motor characteristics become crucial. Alternatively, speed feedback can be modified to use encoder changes as a reference, increasing signal resolution for better control. Smooth motor operation and good response also depend on the appropriateness of P.I.D. control. As mentioned earlier, brushless DC motors use closed-loop control; therefore, the feedback signal tells the control unit how far the motor speed is from the target speed-this is the error. Knowing the error requires compensation, which can be achieved through traditional engineering control methods such as P.I.D. control. However, the state and environment under control are actually complex and changeable. If robust and durable control is required, the factors that need to be considered are probably beyond the complete control of traditional engineering control. Therefore, fuzzy control, expert systems and neural networks will also be incorporated into the important theories of intelligent P.I.D. control.