Optimization of multiphase permanent magnet synchronous machines on system level with reduced losses

The requirements of an electrical machine (EM) in Electric- (EV) and Hybrid Electric Vehicles (HEV) are high power density, a stator and rotor field with low harmonic content and best acoustic behavior. Six-phase permanent magnet synchronous machines (PMSM) have significant benefits over conventional three-phase PMSMs, such as reduced current per inverter phase leg, less fundamental losses because of a better winding factor and increased fault tolerance. Despite their advantages, considering the same installation space, the torque per ampere increase of a six-phase PMSM is very marginal. Therefore, the main focus of this dissertation lies in increasing the power density of the multiphase electric drive, where optimizations and investigations are done on a system level. System level simulation models, which consist of the power electronics, different control strategies and the electrical machines are developed and studied. Considering the electrical and mechanical phase shift between the two three-phase winding sets, different winding concepts of the sixphase EM are developed, in an effort to gain a better understanding of their advantages and disadvantages. Detailed analyses are performed, in order to understand the sources and influences of the space harmonics and high frequency inverter-induced time harmonics on the losses and performance of the EM. In particular, this dissertation will examine the EM torque, iron losses, magnet losses and copper losses for both inverter switched currents and sinusoidal currents supply. The results show that Pulse Width Modulation (PWM) Interleaving control strategy for a multiphase phase inverter eliminates the dominant harmonic frequencies in DC link capacitor voltage and current ripple thereby reducing their root mean square (RMS) as well as peak amplitudes. Therefore, the size of the DC link capacitor can be reduced while maintaining the same current and voltage ripple. The focus lies in understanding the physical and control difference between each three-phase inverter module due to PWM Interleaving strategy, which leads to high frequency harmonics producing only harmonic airgap fields, increasing and decreasing losses in different parts of the electrical machine. Compared to the conventional six-phase design, an alternative dual three-phase winding concept, where the two three-phase windings are 180° spatially wound from each other, is shown to reduce the influence of these harmonic frequencies, thereby reducing the losses in the electrical machine. A time effective combination of simulation and interpolation methodology is suggested for the simulation of the electrical machines, coupled with a voltage source inverter (VSI). The interpolation results, when compared with the time consuming Finite Element Method (FEM) simulation results, show very good accuracy. In an attempt to find the electric drive with the best redundancy, examinations of the fault tolerance of the different six-phase windings concepts are performed. Open circuit fault (OCF) simulations for a complete three-phase winding are executed and their performances in post fault operation are studied. One of the investigated multiphase electrical machines is built as a prototype, in order to validate the simulation results with measurements. The final part of the dissertation contains the measurement results and their comparison with the simulation results.