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The use of permanent magnet synchronous machines (PMSMs) is becoming more popular [ 1 ] thanks to their compactness, high torque density and increased flux weakening performance [ 2 ]. Other advantages can be gained by equipping PMSM with concentrated winding in the stator. In fact, using concentrated winding can be beneficial regarding easy manufacturing and consequently lower production costs, modularity [ 3 ] and fault tolerance. More Fractional-Slot Concentrated Winding (FSCW) is an increasingly attractive option for the stator winding of synchronous permanent-magnet machines. Despite of the extensive studies conducted so far, there seem to be further interesting margins for expanding and improving such technology. A well-known drawback of their adoption is the occurrence of large magneto-motive force (MMF) harmonics, which produce eddycurrent losses in rotor permanent magnets. Moreover, it is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on condition that the number of slots Z is an integer multiple of 3 times the maximum common divisor between Z and the number of pole pairs p. This thesis is intended to optimize the multi-layer winding design through a new general systematic methodology having the form of a multi-objective quadratic programming problem. The use of a multilayer design, with coils of different phases wound around the same tooth, is a possible countermeasure to mitigate problems as large magneto-motive force (MMF) harmonics, which produce eddy-current losses in rotor permanent magnets. In addition, this thesis explores the possibility to synthetize a symmetrical FSCWs having unconventional slot-pole combinations, i.e., with a number of slots and a number of poles that are commonly believed incompatible. It is shown that a multi-layer FSCW with (theoretically) arbitrary slot-pole combinations can be implemented and a methodology to synthetize it optimally is given. The performance of FSCW designs with conventional and unconventional pole-slot combinations is comparatively assessed. A study case is provided to illustrate the possible practical advantage of choosing an unconventional pole-slot combination. Fractional-Slot Concentrated Winding (FSCW) is an increasingly attractive option for the stator winding of synchronous permanent-magnet machines. Despite of the extensive studies conducted so far, there seem to be further interesting margins for expanding and improving such technology. A well-known drawback of their adoption is the occurrence of large magneto-motive force (MMF) harmonics, which produce eddycurrent losses in rotor permanent magnets. Moreover, it is commonly assumed in the existing literature that a symmetrical three-phase FSCW is feasible only on condition that the number of slots Z is an integer multiple of 3 times the maximum common divisor between Z and the number of pole pairs p. This thesis is intended to optimize the multi-layer winding design through a new general systematic methodology having the form of a multi-objective quadratic programming problem. The use of a multilayer design, with coils of different phases wound around the same tooth, is a possible countermeasure to mitigate problems as large magneto-motive force (MMF) harmonics, which produce eddy-current losses in rotor permanent magnets. In addition, this thesis explores the possibility to synthetize a symmetrical FSCWs having unconventional slot-pole combinations, i.e., with a number of slots and a number of poles that are commonly believed incompatible. It is shown that a multi-layer FSCW with (theoretically) arbitrary slot-pole combinations can be implemented and a methodology to synthetize it optimally is given. The performance of FSCW designs with conventional and unconventional pole-slot combinations is comparatively assessed. A study case is provided to illustrate the possible practical advantage of choosing an unconventional pole-slot combination.

The design of the stator winding has direct influence on the electric machine performance. As technology has advanced, researchers and motor manufacturers have worked on possible ways to improve the stator winding layout to achieve several targets depending on the application which namely consist of minimization of harmonic losses for improved efficiency, torque pulsation reduction, acoustic noise and vibration reduction,... For this purpose, an extensive range of literature have presented several design strategies for both synchronous and induction motors acting on winding connection types, configurations, conductors type and shape..., to achieve the required electrical performance. It has been shown that - depending on the application - some winding configurations can be more problematic than others as they produce additional undesirable parasitic effects. However, in some cases, the choice of the winding layout is limited by the required application which leads to look for solutions to cope with the winding configuration drawbacks. The works presented in this thesis will deal with two different aspects of electrical machine windings that need to be further explored which consist of: fractional slot concentrated winding optimization in three-permanent magnet machines and study and improvement of the winding in terms of skin effect is large-medium voltage synchronous machines. Regarding the first topic, potentials and limits of the FSCW optimization through multi-layer arrangement will be presented to mitigate three problems: unbalanced magnetic pull (UMP), torque ripple and permanent-magnet eddy-current losses. As for the second topic, an extensive study of the eddy-currents losses in the stator equipped with form wound coils made of flat conductors embedded in open slots will be covered. Such study is important as it helps to identify the main causes of problems to avoid it through a suitable design. The design of the stator winding has direct influence on the electric machine performance. As technology has advanced, researchers and motor manufacturers have worked on possible ways to improve the stator winding layout to achieve several targets depending on the application which namely consist of minimization of harmonic losses for improved efficiency, torque pulsation reduction, acoustic noise and vibration reduction,... For this purpose, an extensive range of literature have presented several design strategies for both synchronous and induction motors acting on winding connection types, configurations, conductors type and shape..., to achieve the required electrical performance. It has been shown that - depending on the application - some winding configurations can be more problematic than others as they produce additional undesirable parasitic effects. However, in some cases, the choice of the winding layout is limited by the required application which leads to look for solutions to cope with the winding configuration drawbacks. The works presented in this thesis will deal with two different aspects of electrical machine windings that need to be further explored which consist of: fractional slot concentrated winding optimization in three-permanent magnet machines and study and improvement of the winding in terms of skin effect is large-medium voltage synchronous machines. Regarding the first topic, potentials and limits of the FSCW optimization through multi-layer arrangement will be presented to mitigate three problems: unbalanced magnetic pull (UMP), torque ripple and permanent-magnet eddy-current losses. As for the second topic, an extensive study of the eddy-currents losses in the stator equipped with form wound coils made of flat conductors embedded in open slots will be covered. Such study is important as it helps to identify the main causes of problems to avoid it through a suitable design.

Lo scopo di questa tesi è presentare alcuni metodi efficienti (dal punto di vista computazionale) per il calcolo degli effetti dovuti alle correnti parassite (eddy currents) in macchine elettriche rotanti in media tensione. Due applicazioni in particolare sono state considerate nel dettaglio. Inizialmente viene analizzato il fenomeno delle correnti parassite indotte nell'albero di motori asincroni a due poli e il conseguente effetto sulle prestazioni della macchina, focalizzandosi in particolare sul fattore di potenza. La seconda parte della tesi concentra la sua attenzione sullo studio dell'avviamento da rete di motori sincroni con rotore massiccio. Per ciascuna applicazione vengono introdotte alcune procedure di calcolo, facenti uso di opportuni modelli numerici basati sul metodo degli elementi finiti, per mezzo delle quali vengono adeguatamente calcolati i parametri dei circuiti equivalenti di macchina, tenendo conto degli effetti legati alle correnti parassite. I modelli numerici sono opportunamente definiti, in modo tale da ridurre al massimo la complessità delle geometrie e il conseguente onere computazionale. I risultati delle procedure innovative qui proposte sono confrontati con i dati provenienti da prove sperimentali sulle macchine oggetto di studio e con analoghi risultati di calcolo dedotti tramite le procedure comunemente utilizzate. Il confronto fra questi dati ha dimostrato che gli approcci di calcolo introdotti in questa tesi permettono di ottenere risultati con un elevato livello di accuratezza e una netta riduzione dell'onere computazionale. The efficient computation of eddy-current effects in medium voltage electric machines is discussed in this dissertation. Two particular cases are considered. Firstly, the effects of shaft eddy-currents on two-pole induction motor performance is addressed, with special focus on the power factor. In the second part of the thesis the start-up calculation of a large synchronous motor with solid rotor is analyzed. For each application a special calculation procedure is introduced. These procedures adopt a set of suitable finite-element models to properly compute the machine equivalent circuit parameters that are mainly influenced by eddy-current-related phenomena. By suitably choosing finite-element models boundary conditions and excitations their geometry is simplified to the maximum possible extent, in order to reduce the computational burden. The results of the new calculation methods are compared with experimental data and with analogous results obtained from commonly-adopted calculation procedures. The comparison proves that the proposed approaches can lead to high accuracy levels with very remarkable computational savings.

Negli ultimi anni, le macchine elettriche ad avvolgimenti concentrati hanno ricevuto sempre più attenzioni principalmente per le loro caratteristiche di un elevato rapporto coppia - peso e basso costo di produzione. Per contro, rispetto alle più diffuse macchine ad avvolgimenti distribuiti, sono note per avere un forte contenuto armonico spaziale che può dare origine a vari inconvenienti tra cui aumento delle perdite e forze radiali indesiderate tra rotore e statore (UMP). Nella presente tesi viene presentato un rigoroso metodo di valutazione del contenuto armonico di un avvolgimento concentrato. Sono analizzate in particolare l'efficacia nel produrre l'armonica fondamentale, le perdite prodotte nei magneti e le forze agenti tra rotore e statore. Infine viene proposto un metodo per l'ottimizzazione del contenuto armonico per ciascuno di questi aspetti e la sua applicazione ad una macchina prototipale. In the last years, fractional slot concentrated winding (FSCW) electric machines have received increasing attention due mainly to their characteristics of high torque-weight ratio and low manufacturing cost. Conversely, if compared with traditional distributed winding machines, FSCW machines are known to produce a high spatial harmonic content, which may lead to several inconveniences as higher losses and undesired unbalanced magnetic pull (UMP) between rotor and stator. In this thesis, a rigorous method for the evaluation of the harmonic content of an FSCW is presented. In particular, a sound analysis is focused on the effectiveness in producing the fundamental harmonic, the arising of magnet losses and UMP. Finally, a method to optimize the harmonic content for each of these aspects is presented along with the application to a prototype machine.

Nowadays, the pollution caused by the massive use of fossil fuel is a well-known critical issue makes the design of the electrical machines a crucial task because the vast majority of the industrial or household applications are integrated with this kind of technology; hence improving the efficiency of electric motors is expected to result in an extraordinary benefit in terms of energy saving. On the other side, the increasing use of renewable energies (like wind energy, hydropower, tidal energy), combined with the distributed generation concept, call for new electric machine technology and for modern design approaches. This doctoral thesis has been mainly focused on finding analytical procedures to model and analyze some types of electrical machines which are of interest for renewable and distributed generation. The use of analytical approaches is, in some cases, fundamental because numerical methods, mostly based on Finite Element Analysis (FEA), are very inefficient in terms of required time and computation resources. The types of electrical machines considered in this work are various. The attention is first placed on the slotless surface permanent magnet (SPM) topology (with different types of rotor magnetization). Wound-field synchronous generators with both three phase or multiphase stator are then considered. For the various kinds of the machines taken into account, some modeling, design and analysis studies have been conducted in the attempt to fill some gaps in the existing technical literature. In the first part of the thesis the purely analytical modeling of slotless SPM machines is addressed. In the work, it has been considered how the stator slotless design can be combined with different surface permanent magnet rotor topologies. The main efforts of the study have been addressed to the purpose of finding an explicit analytical expression to compute slotless machine torque and no-load back-EMF, covering all the SPM rotor topologies of interest. The subsequent part of the thesis, is about the analytical computation of end-coil leakage inductance in concentric windings. For a good dynamic modeling of machines equipping this kind of winding, it is useful that their mathematical model is implemented and that model parameters are identified with good accuracy. The proposed technique shows a very good accordance compared with a 3D FEA simulation and is also validated though tests conducted on a dedicated experimental set-up. Multiphase machines are attractive for many energy-saving fault-tolerant applications thanks to their higher efficiency and intrinsic resilience to faults. A challenging task in the modeling of multiphase machines for design and simulation is identifying the self and mutual inductances due to leakage fluxes. In this thesis is also presented a novel approach for the leakage inductance determination in multiphase machines based on routine tests combined with very simple 2D magnetostatic FEA simulations. The subsequent part of the thesis is about the design of wound-field synchronous generators specifically required to operate in a distributed generation system where significant fluctuations are known to frequently occur in both voltage and frequency. Design provisions are investigated to improve the generator resilience to these grid disturbances. In the last part of this thesis the attention is focused on multiphase alternators interfaced to DC distribution systems through multiple rectifiers. As a new finding presented as a part of this investigation, it is shown that a short circuit fault occurring on AC/DC rectifier terminals generates a strongly oscillatory behavior with much larger current peaks than could be predicted with conventional models regarding short circuit transients in DC networks.

In questo lavoro di tesi vengono rispettivamente trattati: Un metodo di dimensionamento preliminare basato sul criterio di omotetia dedicato a macchine elettriche asincrone caratterizzate da diverse polarità e dal medesimo sistema di raffreddamento. Un algoritmo ibrido basato sul calcolo agli elementi finiti e analitico per la verifica prestazionale di macchine elettriche a induzione alettate ed esposizione del metodo di ottimizzazione e delle analisi specifiche agli elementi finiti transitori per la definizione delle perdite addizionali negli avvolgimenti statorici e rotorici. Descrizione delle scelte progettuali, del processo produttivo e delle analisi agli elementi finiti di un di un generatore eolico a magneti permanenti interno. In this thesis are respectively treated: A preliminary dimensioning method based on homothetic criteria devoted to induction machines that are characterized by the same polarity and cooling system. A hybrid algorithm based on the finite elements analysis and the analytical formulas that is able to estimate the performance of electrical induction machines furthermore there is described the optimization process and the transient finite elements analysis used to define the additional losses in the stator and rotor windings. Description of the design choices, the production process and the finite element analysis of a wind generator with internal permanent magnets.

L’attività di ricerca si è focalizzata sulla determinazione di metodi analitici e analitico-numerici per la soluzione di alcuni problemi “aperti” di interesse nella progettazione e l'analisi di macchine elettriche. La tesi include una gamma piuttosto ampia di argomenti, ma è stata sviluppata seguendo una singola linea di indagine unitaria, che consiste nel tentativo di cercare e, dove possibile, definire e implementare, metodi matematici sufficientemente veloci ma accurati per eseguire calcoli su macchine elettriche in modo da evitare l'uso massiccio di tecniche basate su analisi agli elementi finiti. Queste infatti sono note per essere precise ed affidabili, ma allo stesso tempo richiedono ingenti risorse computazionali e sono quindi poco adatte quando è necessario esplorare un gran numero di varianti di progetto, come nel caso di programmi di ottimizzazione progettuale basati su algoritmi genetici. I metodi agli elementi finiti non sono stati esclusi, naturalmente, ma piuttosto utilizzati come “benchmark” per valutare la validità delle tecniche di calcolo alternative proposte, ove non fosse possibile effettuare un confronto diretto con dati sperimentali. I tipi di macchine elettriche a cui si sono applicati i metodi di calcolo sviluppati in questa tesi sono diversi e vanno dai motori a magneti permanenti superficiali con cave statoriche a motori a magneti permanenti superficiali con statore slotless ,dalle macchine sincrone a riluttanza alle macchine sincrone a rotore avvolto, per finire con motori a induzione a gabbia di scoiattolo. Tutti questi tipi di macchine sono sempre più importanti nelle applicazioni odierne, sia nel settore industriale che per i sistemi di trazione. Per affrontare lo studio di queste macchine si sono di volta in volta sviluppati approcci di calcolo analitico diversi, anche in funzione degli obiettivi da raggiungere. Esempi di questi approcci analitici impiegati sono i seguenti: la teoria della “winding function” applicata al calcolo della coppia di macchine a magneti superficiali; la teoria della “permeance function” applicata al calcolo delle macchine con eccentricità di rotore ; l'approccio mediante circuito equivalente magnetico o reti di riluttanze in combinazione a tecniche basate sulle mappe conformi per lo studio dei motori sincroni a riluttanza; la soluzione delle equazioni di Laplace e di Poisson per determinare il campo magnetico nelle macchine a magneti permanenti superficiali slotless e a gabbia di scoiattolo. Inoltre, l'applicazione degli algoritmi di calcolo proposti è illustrata mediante opportuni casi di studio ed i risultati ottenuti sono sempre convalidati mediante confronto con tecniche di calcolo alternative (in particolare l’analisi agli elementi finiti)