Impact of Rotor Losses in a 12-Slot 10-Pole Axial Flux PM Machine

In spite of their several advantages, the fractional-slot PM machines suffer high contents of space harmonics in the air-gap MMF distribution. The MMF space harmonic amplitude and frequency depend on the particular combination of number of slots and poles. The main consequence of such harmonic contents is the induced losses in the rotor. Moreover the slot opening can produce considerable rotor losses due the variation of permeance corresponding to the stator slots. This paper presents the impact of the rotor losses on the performance of a 12-slot 10-pole axial flux PM machine. The influence of both MMF harmonics and slot harmonics is considered. Comparisons between prediction and experimental results are included in the paper

Multi Polar Direct Drive Permanent Magnet Synchronous Machines for Renewable Energy

This doctoral study mainly considers the Wind Energy resource and focuses on the electromagnetic aspects of generators for direct drive solutions in this field. Despite the strong employing by many constructor of geared technology in Wind Turbines Generators, a transmission/gear box reduces both, eciency and reliability: the losses due to the transmission potentially compromise the sustainability of the electromechanical system. A better solution is the multipolar low speed direct drive train. Permanent magnet machines offer compactness and the absence of brushes with respect to traditional machines. Material in the armature can be saved using fractional slot windings. This type of machine exhibits many advantages such as short end windings, high slot fill factor, high efficiency and power density together with electrical redundancy, which allows the modularization of the active part and fault-tolerance capabilities. However, space harmonics of the magneto-motive force (MMF) in fractional slot windings lead to considerable rotor losses. The computation of these losses according to standard procedures (e.g. Steinmetz equation or models for massive body within homogeneous field) is not satisfactory. The MMF harmonics move asynchronously with respect to the rotor, inducing currents in any conductive rotor parts, e.g. the metallic iron yoke which supports the magnetic poles and the rare earth permanent magnets (PMs), acting on their working temperature which is strictly related to the performance of PMs. The reduction of the residual ux density causes a decrease both in the back electromotive force (EMF) and in the electromagnetic torque. The reduction of the magnetic eld increases the risk of an irreversible demagnetization of the PMs. The MMF space harmonic amplitude and frequency depend on the particular combination of number of slots and poles. The amount of such losses increases dramatically with the size of the machine becoming a crucial aspect in the design of a large diameter (v 2 - 3m) multipolar direct drive generators with PMs. A proper selection of the winding of the machine,fixing the number of slot and poles represents an improvement in the sustainability of the electromechanical conversion process: losses are limited and a higher eciency is achieved. If efficiency is better even less active material is wasted. This part of the doctoral study has been accomplished at the Electric Drives Laboratory at the Department of Industrial Engineering of the University of Padova during a partnership with Leitwind A.G.(in Sterzing, Italy), which operates in the Wind Energy Market. The aim is to design a large diameter generator (v 4m), which will be employed in the prototype of a 3 MW wind turbine. The will of Leitwind to implement a method to predict rotor losses in large machines with permanent magnet and fractional slot windings is the key basic point to be solved. The existing LW15C generator for LTW77 (1.5 MW rated power) wind turbine is analyzed by means of analytical relations and finite elements: it represents the starting point of this study. A method to calculate rotor losses due to the high harmonic contents of the fractional slot winding is applied. Such estimated losses are then compared with the results of experimental test benches with "full-scale" prototypes. The same study is then repeated on SFA motor (500 kW rated), employed in ropeways transport in Leitner A.G. plants. Both Leitner and Leitwind belong to the Leitner Technologies Group. Test bench results are finally compared with the values from the analysis. Starting from both the experience on LW15C generator and SFA motor the design of LW30A prototype generator for Leitwind 3MW wind turbine is chosen. Dierent topologies of PM machines with fractional slot winding are then investigated and compared with the switching ux configuration (SFPM), both with rare earths and ferrite PMs. The possibility to integrate a huge magnet quantity in the armature of the SFPM machine and the ux concentration principle can lead to a cost eective solution which must be carefully evaluated without considering the performance of the machine only. The structure of the rotor is robust and simple, like in classical reluctance machines. The robustness against PMs demagnetization is a crucial point to investigate. To extend the scenario of renewable energy, the wave energy resource is brie y overviewed and some topologies for linear direct drive generator in this field are investigated. In order to increase the thrust density the possibility of employing a double-sided structure is analyzed. The employing of ferrite PMs is also considered: despite their low energy with respect to rare earth magnets they have a lower impact on environment and human health. The extraction process of ferrite is similar to the iron one, while rare earths must be separated from radio elements. Main contributions of the thesis To the knowledge of the author, the contributions of this thesis for the upcoming researchers in electrical machines for renewable energy eld are: • The application of straight lined model and of the current sheet method to calculate rotor losses in large direct drive permanent magnet machines. The results of the test bench activity on real machines are then described and compared with the prediction. • Selection of number of slots and poles for fractional winding direct drive large generators. • Comparison of the switching ux machine with other well known machine topologies, including demagnetization behavior and ferrite PMs, more sustainable than rare earth ones. • Investigation on dierent linear modules topologies for wave energy, including double side switching ux topology. Outline of the thesis Chapter 1 provides an overview on renewable energy conversion field and presents the company Leitwind A.G. Chapter 2 describes the analysis and modeling of the LW15C generator for 1.5 MW wind turbine. Then a similar study has been performed for the SFA motor for ropeways applications. Both analytical and finite elements model have been developed/implemented and are compared to the results of experimental test bench activities. The same models are applied to the analysis of the new LW30A generator. Chapter 3 faces the topic of rotor losses in order to model them with the straight lined model and the current sheet method. The rotor losses computation is considered and described. Predicted values are compared with the test bench ones. Chapter 4 establishes a scaling law for rotor losses in fractional slot PM machines. Chapter 5 provides the selection of the number of slots and poles to design the new LW30A generator for 3.0 MW wind turbine. The validity of the Index of Rotor Losses obtained from the straight lined model is investigated and applied. Chapter 6 describes the comparison of fractional winding machine with a small switching ux permanent magnet machine, both with rare earth and ferrite PMs. The demagnetization behavior of the different machine topologies is analyzed. Chapter 7 considers linear drives for Wave energy conversion with different topologies, including the double side switching flux machin

Contribution au développement d'outils de conception de machines synchrones à aimants permanents en vue de l'intégration convertisseur-machine : étude des machines électriques double étoile à coupleur magnétique intégré pour une application aéronautique

A l’heure où la distribution de l’énergie électrique reste encore en discussion pour les nouvelles générations d’avions de transports civils ou militaires, une remise en cause du niveau des tensions continues disponibles pourrait porter les bus de tension de 270Vdc à 540Vdc. De fait, les équipementiers devront proposer des produits facilement adaptables à ces deux niveaux de tension. Par ailleurs, la course au rendement nécessite de revoir les systèmes actuels en proposant des versions plus compactes, fonctionnant à des meilleurs niveaux de rendement. L’atteinte de ces objectifs peut passer par une rupture technologique qui devrait s’opérer dans le domaine de la conversion d’énergie avec l’avènement, d’une part, des composants « grand gap », d’autre part, l’exploitation de structures innovantes de convertisseurs de puissance autore configurables. Les systèmes associant plusieurs convertisseurs et plusieurs machines, appelés communément Systèmes Multi-Convertisseurs Multi-Machines (SMCMM), jouent également un rôle prépondérant grâce aux avantages qu’ils présentent tels que, la modularité, la sûreté et l’accroissement de puissance. Néanmoins, si a priori, l’innovation technologique porte sur le convertisseur statique, le rendement d’un système électromécanique alliant convertisseur et machine ne correspond pas en général au produit des rendements de chacun des composants qui le constituent. En effet, le fonctionnement global fait apparaître de nouvelles problématiques. Dans ce contexte, la ligne directrice de ces travaux de recherche porte sur une méthodologie générale et le développement d’outils qui permettent d’étudier ces systèmes dans leur globalité. L’enjeu scientifique de cette étude consiste à adapter au mieux la machine à son convertisseur, optimiser la qualité du couple (amplitude et ondulations), donc à dimensionner et optimiser une machine qui réponde non seulement à la fonction principale visée par l’application, produire un couple mécanique à une vitesse donnée, mais qui satisfasse aussi une, voire plusieurs fonctionnalités requises pour l’utilisation d’onduleurs reconfigurables en tension. Afin de constituer une palette d’outils qui permettra de développer une méthodologie générale d’analyse des SMCMM, un outil de génération et de caractérisation systématique des bobinages et des aimants permanents est développé. Ce premier outil couplé à un modèle de type champ, basé sur la résolution analytique des équations du champ magnétique, est capable de fournir les performances électromagnétiques de la machine en fonction des critères du concepteur. Ensuite, un second outil qui permet d’appliquer la théorie d’une vision de conception par l’adéquation des sources du champ dans une machine, est développé. Nous pouvons alors rechercher la possibilité de maximiser le couple en adaptant, soit les bobinages, soit les aimants permanents. Pour étendre les précédents résultats à un cas général, un problème d’optimisation est formulé. Pour cela, un problème inverse à variables mixtes, relations complexes et non linéaires, est résolu avec un algorithme de type « boîte noire ». Les travaux se focalisent ensuite sur l’intégration de la fonction coupleur magnétique, puis sur la mise en évidence des conditions de fonctionnement optimal d’une machine synchrone à aimants permanents montés en surface et à double étoile (MSAPDE), alimentée par deux convertisseurs en parallèle reconfigurables en tension à commande entrelacée. Cette démarche est une première approche concrète de l’intégration machine convertisseur. Les courants induits dans les parties conductrices de la machine en mouvement sont modélisés afin de vérifier en fonction de la fréquence, leurs effets sur la fonction de coupleur magnétique. Finalement, après des simulations numériques qui permettent d’analyser et de classifier les avantages et les inconvénients de plusieurs solutions de machines, la réalisation de deux démonstrateurs de MSAPDE à coupleur intégré est initié