A Novel Modular Stator Fractional Pole-Pair Permanent-Magnet Vernier Machine With Low Torque Ripple for Servo Applications

In recent years, high-precision servo systems have attracted much attention due to the development of aerospace and electronic industries. Given the requirement of the servo systems, the machine torque ripple, including the no-load cogging torque and the load torque ripple, must be limited under a low level to achieve the high precision. The ironless permanent magnet-machine (ironless PMM) is usually adopted in high-precision servo systems due to its characteristics of zero cogging torque and low torque ripple. However, ironless PMM has a lot of drawbacks such as low torque density and large PM consumption, making the system size large and expensive. The permanent-magnet vernier machine (PMVM) is also studied for servo applications for its low torque ripple and large torque density due to the adoption of the so-called magnetic gearing effect. However, its torque ripple is still apparently larger than that of the ironless PMM. In order to achieve low torque ripple and large torque density at the same time, a novel modular stator fractional pole-pair PMVM (MFP-PMVM) is proposed and analyzed in this article. The cogging torque and torque ripple of the proposed MFP-PMVM is 93% and 70% lower than that of the conventional PMVM without an obvious decrease on the rated torque. The proposed MFP-PMVM exhibits a low torque ripple close to the ironless PMM while the torque density and PM utilization of the proposed MFP-PMVM is 163% and 14 times higher than that of the ironless PMM. Based on the finite-element method (FEM) results, the superiorities of the proposed MFP-PMVM are verified.