Date of conference: 2025.01
Conference: The 16th Joint Conference on Magnetism and Magnetic Materials and Intermag (2025 Joint MMM-Intermag)
Authors: Ki-O Kim, Ye-Na Bai, Jae-Hyun Kim, Myung-Seop Lim
DOI:
Within the same grade, the coercive force of ferrite PM decreases as the residual magnetic flux density increases. The outer rotor surface-mounted permanent magnet motor (SPMSM) can enhance torque density as the split ratio (SR) between the stator and the rotor increases under limited outer diameter conditions. However, as the SR increases, the thickness of the PM decreases. If ferrite PM with high residual magnetic flux density is used within the same grade, irreversible demagnetization may occur due to its low coercivity. Consequently, the available ferrite PM material may vary depending on the SR, affecting the torque density of the outer rotor SPMSM accordingly. In this article, a design method is presented that considers the material characteristics of four types of grade 7 ferrite PMs and the electromagnetic characteristics according to the SR of the outer rotor SPMSM. Using a theoretical approach of the outer rotor SPMSM, the airgap magnetic flux density (AMFD) and back electromagnetic force (BEMF) are analyzed as functions of the SR. Fig. 1 shows the magnetic characteristics of grade 7 ferrite PMs and the variations in AMFD and BEMF according to SR. The initial design, aiming for high efficiency and high power density using ferrite PM materials and the SR, is evaluated through 2D Finite Element Analysis (FEA). Additionally, the issue of low-temperature irreversible demagnetization of ferrite PMs is reviewed. Based on the initial design results, the ferrite PM material is selected, and the SR range for design is determined. Fig. 2 shows the core stack length, PM thickness, PM material, load characteristics, and demagnetization ratio according to the SR. Subsequently, the optimal SR, ensuring high efficiency and power density, is selected through the optimal design. The design results are compared and validated through experiments conducted on the manufactured motor.