1,720,963 research outputs found
Evaluation of DC-link voltage switching ripple in multiphase PWM voltage source inverters
Get PDFThis paper presents a generalized approach towards the dc-link voltage switching ripple analysis in two-level multiphase PWM voltage source inverters with balanced load. Since voltage ripple is one of the crucial sizing criteria for dc-link capacitor, simple and practical equation for designing the dc-link capacitor, based on the maximum (peak-to-peak) value of the dc-link voltage ripple, has been proposed for multiphase inverters. The amplitude of the dc-link voltage switching ripple is analytically derived as a function of operating conditions. The effect of the number of phases on the dc-link capacitor size is investigated as well. It is found that considering the same total output current, the dc-link capacitor size is reduced increasing the number of phases up to seven. However, from the point of view of dc-link capacitor size, there are no benefits of further increasing the number of phases. Reference is made to two commonly used modulation strategies – sinusoidal PWM and continuous symmetric centered PWM (i.e., space vector). The mathematical models derived aim to provide precise dc-link capacitor sizing and hence improve the power density of the whole system. Comparison of different phase numbers has been made. Proposed theoretical developments are verified by simulation and experimental tests
Evaluation of DC-link voltage ripple in five-phase PWM voltage source inverters
Get PDFThis paper presents the analysis of the DC-link voltage switching ripple in five-phase PWM voltage source inverters with balanced load. The analysis is particularly important for the design and selection of a DC-link capacitor making it possible to meet desired electrical performance of the inverter system. Simple and practical equation for designing the DC-link capacitor is proposed relating the value of capacitance to the maximum value (peak-to-peak) of the DC-link voltage ripple. The amplitude of the DC-link voltage switching ripple is analytically derived as a function of modulation index, and the amplitude of the output current and phase angle. In order to show peak-to-peak voltage ripple distribution, different diagrams are introduced. Reference is made to continuous symmetric centred PWM modulation (i.e., space vector modulation SVM). In order to verify proposed developments, simulations have been carried out by Matlab/Simulink considering full range of modulation index and output phase angle
Dc-link current and voltage ripple harmonics in three-phase three-level flying capacitor inverters with sinusoidal carrier-based PWM
No full textThis paper concerns with the analysis and calculation of the low-frequency DC-link input current and voltage ripple harmonic components in three-phase three-level flying capacitor inverter topologies. As a first simplified approach, reference is made to sinusoidal carrier-based PWM, and sinusoidal load currents with unity power factor, well representing most of the grid-connected applications. The proposed method can be straight-forwardly extended to centered PWM with any output power factor angle. Based on the ripple requirements of DC-link voltage, an original simple and effective equation for designing the DC-link capacitor has been proposed, since design guidelines only for the flying capacitors are reported in literature. Numerical simulations have been carried out by Matlab/Simulnik in order to verify the proposed developments
Evaluation of DC voltage ripple in three-phase PWM voltage source inverters
No full textDetermination of dc-link voltage switching ripple in three-phase PWM voltage source inverters (VSI) is important for the selection and design of the dc-link capacitor. In this paper the complete analysis of the dc-link voltage ripple is given for three-phase PWM VSIs, in case of dc source impedance and balanced load. The dc voltage ripple amplitude is analytically determined as a function of amplitude and phase angle of output current, and modulation index. In particular, peak-to-peak distribution and maximum amplitude over the fundamental period of the dc voltage switching ripple are obtained. Guidelines for designing the dc-link capacitor are given on the basis of the dc-voltage ripple requirements. Simulations results carried out by Matlab/Simulink are provided to verify the analytical developments
BULLYING AND STRATEGIES FOR CONTRASTING THE PHENOMENON IN ITALIAN SCHOOL
No full textAfter about twenty years, Italy can be considered one of the European Nations that has more invested in terms of support for inclusion and prevention for confronting the problem of bullying in the school.
This article aims to set the problem of bullying in the framework of Italian school restructuring process. Starting from a literature review about bullying since 1970s to date, on the bases of the outcomes of some studies previously conducted, the author aims to explain because the phenomenon of bullying it is accompanied by raising the mandatory school. Through the research of eminent scholars, she argues that the crisis of values and the loss of the perspective for the future of teenagers increases the possibility of violent relationships among peers in school, where they spend much of their time.
An interpretative model on bullying is therefore highlighted using the “dramaturgic metaphor” of Goffman and focusing the role of viewer/witness (often the same classmates) for breaking of the violent triangle where perpetrator and victim are similarly victims of the same cruel play.
Finally it describes the strategies devised by the Ministry of Education which are currently applied in the schools of the Italian peninsula in a perspective of preventive and rehabilitative education both on potential protagonists ‒ victim and bully ‒ both on the viewers ‒ all those adolescents just look at the “violent drama” for fun or for weakness, without interrupting it and prevent the recurrence.
In the light of empirical evidences, it is suggested that such programs accompained from informal practices should be encouraged
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
An Output Ripple-Free Fast Charger for Electric Vehicles Based on Grid-Tied Modular Three-Phase Interleaved Converters
Get PDFAn off-board dc fast battery charger for electric vehicles (EVs) with an original control strategy aimed to provide ripple-free output current in the typical EV batteries voltage range is presented in this article. The proposed configuration is based on modular three-phase interleaved converters and supplied by the low-voltage ac grid. The ac/dc interleaved three-phase active rectifier is composed of three standard two-level three-phase converter modules with a possibility to slightly adjust the dc-link voltage level in order to null the output current ripple. A modular interleaved dc/dc converter, formed by the same three-phase converter modules connected in parallel, is used as an interface between the dc link and the battery. The use of low-cost, standard and industry-recognized three-phase power modules for high-power fast EV charging stations enables the reduction of capital and maintenance costs of the charging facilities. The effect of coupling on the individual input/output inductors and total input/output current ripples has been investigated as well, considering both possible coupling implementations, i.e., inverse and direct coupling. Numerical simulations are reported to confirm the feasibility and the effectiveness of the whole EV fast charging configuration, including the proposed control strategy aimed to null the ripple of the output current. Experimental results are provided by a reduced scale prototype of the output stage to verify the ripple-free output current operation capability
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
