Comparative Analysis of Traditional Fractional Slot and Vernier RPMFS Machines

In this paper the authors present the results of a comparative performance analysis for Rotor Permanent Magnet Flux Switching (RPMFS) machines having several different stator slot, stator pole and rotor pole combinations. The investigated combinations of stator slots and poles, as well as rotor poles are chosen taking into account the theory of air-gap magnetic field modulation or magnetic gearing. The presented machines include both traditional fractional slot machines, where the stator winding and the rotor PMs have the same number of poles, and non-traditional Vernier machines, where the stator winding and the rotor PMs have different pole numbers depending on the number of flux modulating stator slots. All analyzed machines are designed for the same rated torque, speed and voltage and have the same outer diameter, slot filling factor, and current density in conductors, in order to have a fair comparison of torque density between machines with different slot-pole combinations. The machines are designed and evaluated by a custom program based on 2D magnetostatic FEA developed by the authors. Although Vernier machines are generally claimed to have a superior torque density in recent publications, results presented in this paper show that the achievable torque density of Vernier RPMFS machines is similar to that of traditional fractional slot RPMFS machines.