Torque ripple reduction strategy for magnet suspended reactive flywheel brushless DC motor

Due to its high torque response speed, direct torque control (DTC) without flux feedback makes the torque ripple reduction of magnet suspended reactive flywheel (MSRF) brushless DC motor (BLDCM) possible, which has a small inductance. However, the bang-bang controller of the DTC causes high torque ripples. To solve this problem, the mathematic models of commutation and diode freewheeling of the inactive phase were established and the relationship between torque and phase current was obtained. Then a torque prediction method based on the established models was proposed to reduce the torque ripple effectively, and the stability and robustness were proved. While estimating the back electromotive force of the BLDCM with sliding mode observer (SMO), a smooth and continuous function with one parameter was employed to replace the sign function which leads to a higher back electromotive force estimation accuracy. Simulation and experimental results show that compared to the traditional DTC, the advanced torque control method with torque prediction and advanced SMO can significantly suppress the torque ripples and has almost the same torque response speed.