A Novel Negative-Sequence Current Injection Method for Reducing LVPMM Electromagnetic Thrust Ripple Considering Static Longitudinal End Effect

The thrust force ripple of a linear motor includes positioning force and electromagnetic thrust ripple. Compared to the conventional rotary motor, linear vernier permanent magnet machines (LVPMM) break at both ends, leading to three-phase asymmetry. This article focuses on the elimination of the electromagnetic thrust ripple caused by the special structure of the LVPMM, and an asymmetrical current control strategy using in power grids is introduced to the drive system. However, unlike power grids working at a fixed frequency, the drive system of the LVPMM needs variable speed operations, and some improvements are made to the original method. High bandwidth of current loop is guaranteed, and the stability of the control system is achieved with the proposed method. Experimental results obtained are presented. It verifies that the asymmetrical current control strategy reduces electromagnetic thrust ripple by more than 67%.