Analytical Prediction of Optimal Split Ratio for Maximum Torque Per Copper Loss in AFPM Machines

The axial flux permanent magnet (AFPM) synchronous machine has advantages of high torque density and high efficiency. In the design process, the ratio of inner to outer diameter, that is, the split ratio, directly affects the electromagnetic performance of the AFPM machine, for example, torque, copper loss and efficiency. Due to the three-dimensional magnetic circuit, a fast prediction of the optimal split ratio can significantly reduce the computation load for machine optimization. This paper proposes an analytical method for quickly predicting the optimal split ratio λ_opt for the maximum torque per copper loss for the AFPM machine. Different from the existing analytically predicted optimal split ratio λ_opt for the maximum torque, that is, λ_opt ≈ 0.58, it is found that the optimal split ratio λ_opt for the maximum torque per copper loss depends on the stator slot number Q, that is, λ_opt increases with a larger Q, whilst the rotor pole number 2p has little influence on λ_opt. The accuracy of the proposed prediction method is verified by finite element analysis, and the absolute error is ≤ 1.1%.