Optimization of a Halbach array for torque ripple reduction in a Yokeless And Segmented Armature axial flux machine

This paper presents the optimization of a Halbach magnet array applied to an axial flux machine (AFM) in a 12-pole, 18-slots yokeless and segmented armature (YASA) topology, evaluated in the torque–speed characteristics diagram. AFMs offer significant advantages in terms of compact design and high torque density compared to other permanent magnet machine topologies. However, noise, vibration, and harshness (NVH) performance is strongly influenced by cogging torque, electromagnetic torque ripple, and tooth forces. While Halbach magnet arrays are well established in high-performance radial flux machines, only limited research has investigated their influence in AFMs. A Halbach array concentrates magnetic flux on one side of the magnet arrangement, leading to increased air-gap flux density and a more sinusoidal magnetic field distribution. By using a Halbach array, the magnetic field distribution in the air gap becomes more sinusoidal, thereby reducing harmonic components. Previous studies have primarily focused on further torque enhancement or mass reduction through the elimination of back iron. Given that AFMs already exhibit high torque and power density, the objective of this paper is the reduction of torque ripple while minimizing the required magnet mass and maintaining the specified performance criteria. In the optimization process, in addition to the segmentation of the pole and transition magnets, the magnet height as well as the required thickness of the rotor yoke are optimized. For the design and optimization of the Halbach array, two-dimensional (2D) and three-dimensional (3D) finite-element (FE) models are combined with surrogate modeling techniques. In addition to the impact on torque ripple, further potential benefits of the Halbach configuration, including improvements in efficiency and reductions in overall motor weight, are analyzed and discussed.