Challenges for including characteristic current as a design parameter in optimization of IPM machines

The magnet flux linkage to d-axis inductance is defined as characteristic current, which is a key factor to determine the Constant Power Speed Range (CPSR) and flux weakening performance of Interior Permanent Magnet (IPM) machines. Therefore, it is critical to predict the accurate value of the characteristic current when considered as an objective or constraint in an optimization process of IPM machines. This paper closely investigates the sensitivity of characteristic current to the variation of magnet flux linkage and inductances due to saturation. A new method is proposed to estimate the characteristic current precisely by considering the variation of direct axis flux linkage under the saturation condition. A systematic procedure is also provided to implement the proposed method in a multi-objective optimization process which can concurrently consider the Finite Element (FE) simulation effort to minimize computational costs. It is also shown that the proposed method can be employed in a design optimization of an IPM machine with the characteristic current smaller, equal or larger than 1pu. Two case studies are considered in this paper to investigate the effectiveness of the proposed method in a design optimization of IPM machines. Results of the experimental tests provide confidence on the accuracy of proposed method and FE results.