A triple closed-loop control system for a sensorless brushless DC motor based on disturbance observer

On the basis of a traditional double closed-loop of speed and current control, an additional power closed-loop module with a varying frequency strategy in the commutation states is presented. The triple closed-loop control system can reduce the phase current ripple and torque ripple of the sensorless BLDC motor effectively. The output values which are observed by the disturbance observer (DOB) and the reaction torque observer (RTOB) control the power module. The DOB can obtain the information about the compared whole torque. The RTOB can distinguish the variation range of the torque ripple in the commutation states from that in the steady states. Different control strategies are adopted in different states of operation. The higher frequency signal of the power closed-loop is triggered when the torque ripple becomes larger. Finally, the validity of the presented method and good performance of the improved system are verified by the simulation results.