Multi-criteria based design approach of multiphase permanent magnet low speed synchronous machines

This paper presents a design methodology dedicated to multiphase Permanent Magnet Synchronous Machines (PMSM) supplied by Pulse Width Modulation Voltage Source Inverters (PWM VSI). Firstly opportunities for increasing torque density using the harmonics are considered. The specific constraints due to the PWM supply of multiphase machines are also taken into account during the design phase. All the defined constraints are expressed in a simple manner by using a multimachine modelling of the multiphase machines. This multimachine design is then applied in order to meet the specifications of a marine propeller: verifying simultaneously four design constraints, an initial 60-pole 3-phase machine is converted into a 58-pole 5-phase machine without changing the geometry and the active volume (iron, copper and magnet). Firstly, a specific fractional-slot winding, which yields to good characteristics for PWM supply and winding factors, is chosen. Then, using this winding, the magnet layer is designed to improve the flux focussing. According to analytical and numerical calculations, the five-phase machine provides a higher torque (about 15%) and less pulsating torque (71% lower) than the initial three-phase machine with the same copper losses.