Investigation on Slot–Pole Combinations of a PM Vernier Motor with Fractional-Slot Concentrated Winding Configurations

This paper presents a new method to find available slot–pole combination of a permanent magnet (PM) vernier motor with fractional-slot concentrated winding (FSCW) configurations instead of the conventional design rule. To this aim, for a common structure of PM vernier machines with FSCW, the air gap flux densities including modulation flux due to vernier effects are investigated from the magnetic view points and then a general condition to effectively use the modulation flux is derived. Under the obtained condition, the specific design condition for slot–pole combinations are established for the most popular FSCW configurations with the coil spans of ±2π/3. Using the established condition, all available vernier structures including those which could not be found by the previous rule are obtained, and the back-electromotive force (EMF) of each structure is analytically estimated to check the vernier effects. During these procedures, it is also revealed that some of general FSCW-PM motors possess vernier effects and thus can be classified into vernier motors. To verify the proposed ideas, the characteristics of back EMF are analyzed through finite element (FE)-simulations for various models, and compared with their analytical calculation results. Finally, the characteristics of torque regarding to slot–pole combinations of the vernier motors are discussed