Bellucci, Massimo (2014) Configurazione innovativa di alternatore destinato alla generazione di energia elettrica da fonte eolica. [Tesi di dottorato]

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Abstract

The aim of the research has been to identify a new configuration of electric generator that can be usefully employed as an alternator in wind turbines. All the variables of the project have been defined and its prototype has been carried out. Powers of the order of few kW were considered and therefore applications related to micro-generation. Reference was made to transverse flux brushless machines ( TFPM ), this because these machines allow to obtain high performance both in terms of torque density that of values of the induced electromotive forces even at low speeds. This also thanks to the presence of an inductor system employing permanent magnets with an high density ( NeFeBo SACO ) and the fact that their geometric configuration allows to overcome the limits of reduction of the pole pitch of the radial flux machines. This resulting in the possibility of achieving generators with a high number of pole pairs and thus to obtain high EMF induced at the ends of the armature windings. Significant are also the advantages of these machines in comparison with those radial flow in terms of simplicity of control, the extended speed range, constant power or torque operation, low inertia and low maintenance . In addition, TFPM machines have a distribution of induction which is not plane and susceptible to strong variations to vary the geometric electric and magnetic characteristics; if this introduces an obvious difficulties in their modeling, on the other hand is well envisage the possibility of obtaining machines that fully match the specific application in which they will be devoted . The same considerations are also valid with regard to the possibility of modulating, in the motoring function, the contribution of each force components (field current interaction, reluctance variation, cogging ) that contribute to the determination of the actual value assumed by the electromagnetic torque and they affect the ripple around the average value. In other words, they affect the contribution of each component to EMF available at the machine terminals in operation as a generator. As concerns the sizing of these machines, as indeed for the design of all electrical machines, it is necessary to know the precise gap-distribution of the induction. However, while for the traditional machines are valid approximations inherent to the distribution of induction that are well verified and which allow significant simplifications of the mathematical models to describe the machine itself, in these , regardless they are AFPM or TFPM , the low predictability of the field distribution and its three-dimensional development, makes it necessary to resort to numerical methods. Only with the aid of these it is possible to determine the actual distribution of induction. In particular, throughout the design and the generalization of the results obtained , which was conducted using numerical code Ansoft Maxwell 3D . This software is able to perform finite element analysis. With particular reference to the last issue highlighted , the software allows to solve the problem posed for each of the configurations obtained at different values of the parameter considered , the range imposed . On the other hand , however, the result of each of them, since functional links existing between the different quantities of interest are not highlighted, make it difficult deductions also only qualitative changes resulting from changes of one or more geometric quantities , electric or magnetic machine. Concerning this machine, it is made even more relevant, once given the high number of variables involved and the high susceptibility of their performance resulting from changes to one or more geometric , electrical or magnetic machine parameter. It was therefore considered that it might be interesting to have an analytical tool that would allow to express, even in a qualitative manner , the functional links between the different variables of interest. For this purpose it has been developed a simplified analytical method ( m.a.s. ) that under suitable simplifying assumptions is used to determine the distribution of induction in the gap, the magnitude of the leakage flux and the achievable torque values. The results obtained with this method are provided in the form of relatioships, allowing to deduce in a complete manner, albeit approximate , some of the interest quantities and to understand how other intervene in determining the characteristics of the machine. The method and the deductions derived from it , have been subjected to validation using the numeric code before described by experimental tests carried out on a stage of prototyped TFPM. By keeping in mind that the actual ability to achieve predicted performance is in fact linked to the ability of the machine to work on thermal regimes resulting from it, has been determined a thermal model for the machines considered. It has been also compared the thermal behavior of this type of machines with the radial flow ones, leading to a thermal equivalent model, which made possible to perform: - Comparison between the thermal power dissipated at constant exchange surface; - Comparison between the surfaces at constant exchange of useful thermal power dissipated; - Checking the numerical congruence. All the obtained results were generalized when possible, and placed in an Excel worksheet . The latter is intended to provide as a first approximation support for the design of the transverse and axial flow machines intended for micro wind generation. In the middle of this work , still remaining the primary goal, is often used for sake of simplicity and clarity , the description of the machine in motor operation, claiming the characteristic of the machine you want to highlight, resorting to its dual behavior in the operation of a motor and a generator and the necessary corrections.

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