A Novel Duty Cycle Control Strategy to Reduce Both Torque and Flux Ripples for DTC of Permanent Magnet Synchronous Motor Drives With Switching Frequency Reduction

Conventional switching-table-based direct torque control (DTC) presents large torque and flux ripples as well as variable switching frequency. Many methods have been proposed to tackle the aforementioned problems, among which duty cycle control is a kind of very effective approach. It is known that by adjusting the duty ratio of the active vector selected from the switching table, the torque ripple can be reduced. However, most of the prior duty cycle control methods only focus on the torque performance improvement and fail to take the flux ripple reduction into account. Furthermore, the methods obtaining the duty ratio are usually complicated and relies heavily on the accuracy of machine parameters, which negates the merits of simplicity and robustness of conventional DTC. This paper proposes a unified switching table to select three vectors rather than two vectors in prior arts; hence, lower flux ripple and more sinusoidal stator current can be obtained while maintaining the torque performance. A very simple but effective method is proposed to obtain the durations of the three vectors. The influence of one-step delay caused by digital implementation is investigated. By arranging the switching sequence of the three vectors appropriately, the switching frequency can be significantly reduced. The superiority of the novel method is confirmed by a comparative study with its counterpart using two vectors only. Both simulation and experimental results obtained from a 1-kW permanent magnet synchronous motor (PMSM) DTC drive are presented to validate the effectiveness of the novel duty cycle control strategy.