Design and Optimization of a Low-Torque-Ripple High-Torque-Density Vernier Machine Using Ferrite Magnets for Low-Speed Direct-Drive Applications

In recent years, Vernier permanent magnet machines (VPMs) have gained much interest in academia. However, due to the large leakage flux, the power factor of Vernier machines is too low to be applied in real industry. In this paper, a novel split-tooth, concentrated-winding Vernier machine using ferrite magnets is proposed for high-torque, low-speed direct drive applications in home appliance industry. It uses consequent-pole magnets in the rotor, as well as additional assistant magnets in the stator teeth, in order to reduce the leakage flux and boost the torque density. A systematic way of reducing the torque ripple from 40% to 4% is proposed by carefully choosing the right combination of stator/rotor pole pair numbers. A comprehensive design optimization is conducted to achieve the optimal performance. The magnets in the stator are shaped on both corners to reduce the saturation level. The key findings during the optimization are described in detail. Finally the total number of turns per phase is carefully reduced to meet the torque, saturation, power factor and space requirements. The test results of a prototyped machine show that the proposed design can achieve 34% more torque and 20% less loss with a reasonable power factor of 0.63, compared with an optimized PM synchronous machine under the same volume. It is found that the significant torque boost is attributed to the multiple working harmonics of the air gap flux density.