256 research outputs found
Struggle for Existence: Selection, Retention and Extinction of a Metaphor (Reprint of chapter 3, S. Maasen, P. Weingart, Metaphors and the Dynamics of Knowledge, 2000)
No full textWeingart P. Struggle for Existence: Selection, Retention and Extinction of a Metaphor (Reprint of chapter 3, S. Maasen, P. Weingart, Metaphors and the Dynamics of Knowledge, 2000). In: Fasolo A, ed. The Theory of Evolution and Its Impact. Milan Dordrecht Heidelberg London New York: Springer; 2012: 69-82
Multi Polar Direct Drive Permanent Magnet Synchronous Machines for Renewable Energy
No full textThis 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
Effects of progesterone derivatives, dihydroprogesterone and tetrahydroprogesterone, on the subependymal layer of the adult rat
No full textIndirect evidence suggests that in the subependymal layer (SEL) steroid hormones could be partially involved in the modulation of neurogenesis, but little or nothing is known about a direct effect of these molecules on this cellular system. The possible effect of progesterone (P) and/or its neuroactive metabolites, dihydroprogesterone (DHP) and tetrahydroprogesterone (THP), on the two cellular components of the SEL (i.e., proliferating/migrating neuroblasts and protoplasmic astrocytes) has been analyzed in adult male rat. P, DHP, and THP were administered by intraventricular injections and after 2 days the SEL was analyzed by immunohistochemistry by using anti-glial fibrillary acidic protein (GFAP) and anti-vimentin antibodies, to label the glial compartment, anti-polysialylated form of the neural cell adhesion molecule (PSA-NCAM), anti-Stathmin, and anti- III Tubulin antibodies to label the migrating neuroblasts. Furthermore, the newly formed cells were identified by using intraventricular injections of 5-bromo-2-deoxyuridine (BrdU) detected immunohistochemically. Our results demonstrate that DHP and THP treatments drastically decrease the number of BrdU-labeled cells within the SEL. THP, DHP, and to a lesser extent P, administrations also induce molecular and structural modifications of the SEL glial compartment. On the whole, the present results indicate that neuroactive derivatives of P (i.e., DHP and THP) exert direct effects on adult neurogenesis, strongly affecting both neuroblasts and astrocytes of the SEL
- …
