Design optimization of a high performance five-phase slotless PMSM

Multi-phase slotless permanent magnet synchronous motors (PMSMs) are cogging torque-free motors with fault tolerant capability developed for high speed applications such as aerospace and marine. Slotless PMSMs can offer a higher torque density when they have a trapezoidal back EMF and supplied with nonsinusoidal current. In this paper, a five-phase slotless PMSM with an external rotor is designed for high torque density. For this purpose, the optimal portions of the time and space harmonic components for producing the highest torque in five-phase PMSMs are calculated mathematically. A closed form analytical model for this motor is then introduced. Based on the analytical model, a multi-objective optimization using genetic algorithm (GA) technique is performed to optimize the motor back EMF waveform and achieve the highest torque density. The detailed finite element analyses are executed to validate the analytical results.