Comparison of the Power Factor of SMPM and SM Vernier Outer Runner Machines for Traction Applications

Low voltage traction applications demand high torques at low speeds and at a small design space. In order to achieve high torque densities, manufacturers often combine high-speed machines with mechanical gearboxes. In the past decade, flux modulation machines came up, offering low speeds and high torque densities by utilizing the magnetic gearing effect. The idea is to remove the mechanical gear and get a compact direct drive solution. Now, the Vernier machines are a subgroup of the flux modulation machines. However, literature reports very poor power factors for Vernier machines in comparison to PM machines as the main drawback. Nevertheless, the presented studies always assume a fixed speed and only one single current operating point. This is correct for fixed speed applications, e.g. pumps and blowers, but it is not correct for traction applications, because the machine cycles dynamically through the torque-speed curve. Therefore, this paper first investigates the machine design requirements considering the machine control strategies and the needs of traction applications. Furthermore, a method to check if the design fulfills the given requirements by calculating only a single operating point is presented. In addition, the emerging differences between Vernier and PM machines are explained using analytic results. Furthermore, the paper presents a power factor comparison of torque optimized Vernier and PM machines while considering iron losses. Thereby, the comparison covers the whole torque-speed range, including the field weakening area.