Analysis, Design and Optimization of Innovative Electrical Machines Using Analytical and Finite Element Analysis Methods

The interest on rotating electrical machines adopting permanent magnets (PMs) has increased during the past few decades, representing now a fashionable design option in a number of fields as industrial processing, transportation, actuators, household appliances and power plants. The issues related to an increasing electrical energy demand and consumption, have generated a tendency to research electrical drives with high efficiency, pushing electrical machines technology to further improvements. The introduction of permanent magnets based on rare earth, experienced since the 1960's, gave a great input in the development of innovative machine topologies. On the other hand, the increase and the instability of rare earth PMs price, especially between 2010 and 2013, have directed the research of rare earth free alternatives, or machines using a smaller amount of PMs. Actually, the current trend in the industrial and academic research focused on developing high performance electric motors and generators, among different electrical machines, highlights the Synchronous Reluctance (SynRel) and the Permanent Magnet Synchronous Reluctance (PMASynRel) motors as best candidates to satisfy the future energy and efficiency requirements. This thesis is comprehensively dedicated to theoretical and experimental analysis and design of the Synchronous Reluctance (SynRel) and the Permanent Magnet Assisted Synchronous Reluctance (PMASynRel) machines. In particular, it will be focused on electrical machines which power ratings are ranging from fraction of Watts to some hundred kWatts, for vehicular traction and house-hold appliances. SynRel and PMASynRel motors exhibit many technical advantages, like simple and robust structure, high torque density, high efficiency, small space required for PMs, high degrees of freedom in the design, high operating speed range, high overload capability, low back EMF (null in case of SynRel motors), leading to a safe behavior in case of inverter failure. Furthermore, thanks to the appropriate vector control algorithm, the performance in terms of efficiency and torque become highly competitive. The subject matter covered in the thesis is organized into three Parts, each including a certain number of chapters. % At the beginning of each Part, a brief summary is proposed aiming to describe the main content of that Part, the goals and the anticipation of the main results. Part I includes seven chapters summarizing the research activities carried out during the Ph.D. period. The first six chapters are dedicated to electrical machines for vehicular traction, while chapter 7 investigate on motors for house-hold appliances. Chapter 1 is mainly introductory and meant to provide the basics information to understand which are the pros and cons and the features of the machines under study. Chapter 2 summarize the design criteria and the electrical requirements of ferrite PMASynRel machine for traction application. The electromechanical performance have been evaluated and compared, in terms of torque and power. A brief description of the electric supply system in order to accurately and efficiently manage the motor for achieving the requested performance is presented. Chapter 3 highlights the influence and benefits of using ferrite magnets on the machine performance, emphasizing the importance of a careful evaluation of the magnet volume in order to increase the performance while reducing the used quantity. Chapter 4 is devoted to the sensitivity analysis of the machine performance in terms of torque ripple with respect to the geometrical design. An optimization algorithm has been performed in order to investigate and determine a rotor geometry which maximize the torque and reduces the torque ripple. The impact of the geometrical parameters is taken into account and the sensitivity of the optimal solution to the geometry variation is pointed out. This chapter highlights the difficulty to get a robust geometry as far as the torque ripple reduction is concerned. Finally, a few experimental results on a Synchronous Reluctance motor prototype will be presented, compared with Finite Element Analysis simulations for validation. Chapter 5 deals with the design and optimization of a high speed PMASynRel motor considering the driving cycles of an electric vehicle. A procedure is employed to evaluate the most effective design area, which has to be considered for the global optimization. Both results and advantages of the adopted methodology are highlighted. Further analysis on traction machines are going to be presented in Chapter 6. A comparison between ferrite and sintered NdFeB PMASynRel, SynRel and a Surface mounted PM (SPM) machines performance is deeply investigated. Chapter 7, the last of this first part, will highlight the advantages in using SynRel and PMASynRel motors for house-hold appliances. The main purpose of this chapter is to discuss the features of these motors as a valid substitute to commercial motors actually used for washing machines application. Part II is dedicated to the analytical modeling of SynRel machines with the challenge of predicting accurately the air-gap field of the machine taking into account the effect of the rotor flux barriers. This Part is divided into two chapters. Chapter 8 explains the hypothesis on which the analytical model is based, the calculation of stator Magneto Motive Force through winding function and describes the reluctance network equivalent circuit for a SynRel motor with one and two flux barriers per pole. The computation of the parameters of the model, the air-gap flux density and finally some comparison with Finite Element Analysis are presented. In Chapter 9 SynRel motors with split-phase stator winding sets supplied by multiple inverters have been investigated as an increasingly attractive solution for fault-tolerant, rugged, magnet-free vehicle traction drives. As an extension to the previous chapter, an analytical procedure to model and simulate a SynRel motor, with a split-phase stator winding, through a magnetic equivalent circuit (MEC) technique, has been introduced. As an output, the air-gap flux density of the SynRel motor can be computed at any operating point. Part III, finally, presents some experimental measurements carried out for two prototypes of SynRel and PMASynRel machines, with the purpose of comparing the results achieved in the motor optimization presented in Chapter 7.

In questi ultimi anni l’interesse per le macchine elettriche rotanti facenti uso di magneti permanenti ha riscontrato uno sviluppo sempre pi `u crescente. Tali macchine rappresentano un mondo alternativo alle tradizionali macchine sincrone e ad induzione, e vengono considerate ad oggi soluzioni promettenti in svariati settori, come quello industriale, per il trasporto, come attuatori, elettro domestici e per l’impiego in impianti di potenza. I problemi legati all aumento della domanda di energia elettrica ed al suo consumo, hanno generato una tendenza alla ricerca di azionamenti ad alta efficienza, spingendo la tecnologia delle macchine elettriche classiche ad ulteriori miglioramenti. L’introduzione dei magneti permanenti che utilizzano terre rare, gia dagli anni 60, hanno incentivato e permesso lo sviluppo di diversi tipi di macchina innovativi. Tuttavia, l’aumento e l’instabilita del prezzo delle terre rare, tra il 2010 ed il 2013, ha diretto la ricerca verso soluzioni di macchine alternative senza magneti permanenti, o con una quantita ridotta di tali materiali, pur soddisfando le specifiche di progetto. Al momento, la ricerca industriale e quella accademica sono entrambe focalizzate allo sviluppo di motori e generatori elettrici con elevate prestazioni, tra i diversi tipi di macchine elettriche esistenti, i motori sincroni a riluttanza (SynRel) ed a riluttanza assistita da magneti permanenti (PMASynRel) risultano essere degli ottimi candidati per il soddisfacimento delle specifiche energetiche e di efficienza, sempre piu stringenti, che verranno richieste ai motori nel prossimo futuro. Questo lavoro di tesi e interamente dedicato all’analisi teorica e sperimentale ed alla progettazione di motori sincroni a riluttanza (SynRel) e motori sincroni a riluttanza assistita da magneti permanenti (PMASynRel). In particolare, l’attenzione sar`a p osta su macchine elettriche in un campo di potenza che varia dalle centinaia di Watt alle decinedi kiloWatt, principalmente per applicazioni come veicoli elettrici ed elettro domestici. Tali macchine presentano una serie di vantaggi tecnologici che le portano ad avere prestazioni, soprattutto nel campo degli azionamenti a velo cit`a variabile (VSD), competitive rispetto ad esempio alle macchine ad induzione tradizionali o quelle a magneti permanenti. La struttura semplice e robusta, l’utilizzo ridotto di magneti permanenti, i gradi di liberta nella progettazione combinate ad un’elevata densita di coppia, alta efficienza elevate caratteristiche di sovraccarico ed un ampio campo di velocita, sono tutte caratteristiche che hanno permesso di collo care le macchine SynRel e PMASynRel in una posizione di rilievo. Inoltre, grazie all’aumento dei convertitori moderni a frequenza variabile e sistemi di controllo digitale, le prestazioni di questo tip o di motori, in termini di coppia ed efficienza, sono diventate altamente competitive rispetto ai tradizionali azionamenti con motori ad induzione.