Pseudo six-step modulation with optimal flux tracking for control of high-speed permanent magnet synchronous machines (PMSMs)

As fundamental frequency increases with speed, the ratio between the permissible switching frequency and the fundamental frequency, also known as pulse number is reduced dramatically in high-speed electric machine drives. In electric drives with low pulse numbers, the state-of-art pulse width modulation (PWM) methods, such as space vector modulation (SVM) and synchronous optimal modulation (SOM) cannot yield both good transient performance and low harmonic distortions in steady state. Therefore, in this paper, a novel and simple pseudo six-step modulation (PSSM) technique is proposed. The concept of this method is to synthesize the reference voltage vector based on optimized switching vector patterns (OSVPs) to track the optimal stator flux (OSF) in six steps per cycle for minimum current THD. As a result, the proposed method can respond to the demand change within 1/6 cycle during transients since the stator flux is directly controlled. Meanwhile, in steady states, the applied switching vectors would approach to the corresponding OSVP, hence the low current distortions close to the offline optimal results can be obtained. The effectiveness of the proposed method has been validated by extensive simulations on a high-speed PMSM drive with rated speed of 30,000rpm.