Study On Torque Ripple Suppression Technique Of Brushless DC Motor

As the backbone of the future motor field, permanent magnet brushless DC motors(BLDCM) have attracted widespread attention due to the simple structure, high starting torque, high efficiency, saving electric energy, etc. But the torque ripple problem of BLDCM prevents the application in high precision servo system. The main reasons of the torque ripple are cogging torque, non-ideal trapezoidal Back-EMF, commutation and diode freewheeling of non-commutation zone. As the key problem, commutation torque ripple usually accounts for 10%~15% of the average torque, sometimes even can reach 50% of the average torque. Speed and current dual closed-loop control and direct torque control(DTC) are the most common control methods of BLDCM. Based on the two kinds of control methods, the corresponding control strategies to decrease and inhibit the torque ripple are studied in this dissertation.Firstly, the detailed analysis of torque ripple caused by commutation and the diode freewheeling of the inactive phase is studied. The influence on torque ripple with different PWM schemes is compared. Based on the conclusion that commutation torque ripple can be eliminated when the DC link voltage is equivalent to four times of back-EMF, the main circuit of the motor is improved. Instead of a fixed-voltage DC power supply, a novel Quasi-Z-Source converter and switches selection circuit are employed in the DC link voltage control strategy. This method also combines with the PWM_ON_PWM modulation strategy to eliminate the diode freewheeling of the inactive phase.Simulation models of this control strategy is constructed in the MATLAB/SIMULINK circumstance. The results of simulation experiments show that the control strategy based on Quasi-Z-Source converter can reduce torque ripple to 3.1% of the average torque. It can also improve the average output torque of the motor, especially suitable for the torque ripple suppression in high speed zone.Secondly, the direct power control(DPC) is applied to BLDCM. Traditional DTC is a kind of mature technology applied in BLDCM and has achieved good torque ripple suppression effect. According to the difficulties of flux linkage and torque estimation in DTC, the DPC method is proposed. The power observation model is established without the flux calculation. And the space voltage vector selection table is also improved. The results of simulation experiments show that the DPC method can effectively minimize torque ripple as well as the DTC method. The torque ripple can be reduced to 5.8% along with more simple control algorithm and faster dynamic response.