Performance Improvement of Open-Winding PMSMs With a Discrete-Time Designed Zero-Sequence Current Controller Based on Virtual Three-Phase Expansion

Open-winding permanent magnet synchronous machines (OW-PMSMs) with a common dc bus have gained increasing research focus in recent years due to its advantages including multilevel modulation and flexible control capability. However, a low impedance zero-sequence electrical path exists in this type of configuration. Significant uncontrolled zero-sequence current will result in unexpected power losses, reduced efficiency, and undesired high-frequency torque ripples. The linear proportional resonant controller has been previously proved to be an effective solution to suppress the zero-sequence current. However, the zero-sequence current regulation performances will be degraded, even become unstable, when the switching to operating frequency ratio is reduced, i.e., when the switching frequency is decreased or the machine operating frequency is increased. This article addresses this issue using a discrete-time controller, which is designed based on virtual three-phase expansion. The exact closed-form hold-equivalent discrete model, including the zero-order hold characteristic and the control delay, of OW-PMSMs is derived. The zero-sequence components are expanded to virtual three phases and, then, converted to direct signals by means of reference frame transformation. A synchronous-frame state-space framework for OW-PMSMs is designed directly in the discrete-time domain, which can obtain improved control performances with decreased switching frequency and increased operating frequency. Relevant experimental validations and performance comparisons based on a 2.3-kW OW-PMSM setup is presented to confirm the effectiveness of the proposed controller.