Optimal Split Ratio for High Torque Density PM Machines Based on Analytical Determination

The split ratio, defined as the ratio of the rotor diameter to the stator diameter, is a key parameter due to its significant impact on the performance of electric machinery. This article investigates the optimal split ratio for achieving high torque density (i.e., minimization of active weight) in permanent magnet (PM) machines. Preferentially, an analytical determination is carried out to formulate an optimal split ratio in relation to the active weight. As a result, it is revealed that the design parameters most closely related to the split ratio are the number of poles and residual flux density, while the number of slots has very little effect on it. To corroborate these findings, a large-scale design optimization is performed by using the differential evolution (DE) algorithm. Taking advantage of the optimization datasets, the formulated optimal split ratio is validated and subsequently refined for enhanced accuracy. Finally, the theoretical analyses derived in this work are verified by finite element analysis (FEA) and experimental results.