brushless motor control system analysis
1.1 Three-phase brushless DC motor star connection full bridge drive principle
The rotational speed of the rotor of the brushless DC motor 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 rotating magnetic field of the stator can change the rotational speed of the rotor. The brushless DC motor controller includes a power supply section and a control section, as shown in FIG. The power supply section supplies three-phase power to the motor, and the control section converts the power supply frequency as required. The power supply part can be directly input as a direct current or as an alternating current. If it is an alternating current input, it must be converted to direct current by a converter. Whether it is a DC input or an AC input, the DC voltage must be converted from an inverter to a three-phase voltage to drive the motor before being sent to the motor coil. The inverter generally consists of six power transistors, which are divided into an upper arm and a lower arm, and connected to the motor as a switch for controlling the flow through the motor coil. The control section provides a PWM pulse width modulation signal to determine the switching frequency of the power transistor and the timing of the inverter commutation. For brushless DC motors, when the load changes, it is generally expected that the speed can be stabilized at the set value without much change, so the Hall-sensor is installed inside the motor as the closed loop control of the speed. It is also used as the basis for phase sequence control.
The motor rotates the position of the rotor of the motor sensed by the Hall sensor, and determines the order of turning on or off the power transistors in the inverter to control, as shown in Figure 2, AH, BH, CH in the inverter (upper arm power) Transistor) and AL, BL, CL (lower arm power transistor), the current flows through the motor coil in sequence, producing a forward or reverse rotating magnetic field, and interacts with the magnetic field generated by the rotor magnet to make the motor rotate in the forward or reverse direction. . When the rotor of the motor rotates to a position where the Hall sensor induces another set of signals, the control unit turns on the next set of power transistors.
When the motor rotates, the control part determines the conduction time of the power tube according to the speed set by the system. If the system requires acceleration, the time during which the power tube is turned on is increased. If deceleration is required, the time during which the power tube is turned on is shortened. This part of the operation is controlled by the PWM pulse width modulation signal.
