1,720,980 research outputs found
Hierarchical EMC Design for Inverters in Motor Drive Systems
No full textPower electronics applications are usually accompanied by high voltage and current amplitudes, and steep voltage and current transients. The EMI (Electromagnetic Interference) issue is regarded as the main side effect of power electronics applications. Noise level prediction is a vital task for filter design, but many difficulties are encountered. For instance, a large amount of experience is needed to build equivalent circuits, the use of simplified models often requires sacrificing details in the high frequency range, individual approaches must be developed for various topologies, etc.. A new method for EMI prediction of power electronics applications is thus desirable. The universal method should overcome the following common characteristics of power electronics circuits: (a) the large difference between the time constants, and (b) the long time required to reach steady state. The time domain approach is very time-consuming. The assumptions used in the frequency domain approach, namely, periodic noise sources and a fixed propagation path, are also not valid. For example, the slopes of voltage and current transients depend on the operating points, and the assumption of periodic noise sources does not apply. Because the junction capacitors of power rectifiers and switches change with the reverse voltage, the values of the components in the noise propagation path also change. In this thesis, a hierarchical approach is proposed for the EMC design of inverters in motor drive systems. It combines the advantage of the time domain and frequency domain approach to achieve a fast, universal and accurate result. The approach is validated by observations in the time domain and the frequency domain. The proposed approach has three steps. In the first step (functional level), a simple model of switches in the system is developed. The operating points of each switching transient and all time intervals are derived and the narrowband signals of the EMI noise can be derived. The second step (transient level) results in detailed transient waveforms which take the variation of the nonlinear switching transient into account. In the third step (propagation level), the noise propagation through the system is described by the transfer ratio, and the EMC performance is evaluated. The approach is described in detail, and then a PWM voltage source inverter feeding an induction motor is analyzed using this approach. This approach is also applied to a resonant inverter that operates under ZVS conditions. The experimental results are compared to calculated results. Two approaches are proposed for EMI suppression in this thesis. The first approach is by adding a passive filter on the dc-bus. It can be concluded that the same noise suppression performance can be achieved using a dc-bus filter as the conventional ac side filter. The advantage of this approach is that the connections can be made very short which can significantly improve the suppression. A new active filter called the "fourth leg compensator" is proposed in this thesis. The fourth leg inverter generates a signal to compensate the transients of other three legs. It can suppress the fast transients of common mode voltages while reducing the amplitude of the common mode voltage by 50%. A method to determine the values of the additional components is described. It is shown that the CM voltage can be compensated, even with nonideal coupling in the CM transformer and when leakage inductance is present.Electrica Sustainable EnergyElectrical Engineering, Mathematics and Computer Scienc
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Variations on the Author
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Apparatus for measuring the strength of an electromagnetic field
No full textThere is disclosed an apparatus for measuring the strength of an electromagnetic field. The apparatus comprises a plurality of antennas arranged such that the field is received from all directions, each antenna supplying an RF signal as output. The apparatus also comprises a plurality of transforming modules, each of said modules being fed with the RF signal supplied by one of the antennas, each of said modules transforming the impedance of the RF signal it receives as input, so as to supply another RF signal as output. The apparatus also comprises a plurality of converting modules, each of said modules being fed with the RF signal supplied by one of the transforming modules, each of said modules converting the RF signal it receives as input into a DC current it supplies as output, said DC current varying substantially as a logarithmic function of the said RF signal
Using transfer ratio to evaluate EMC design of adjustable speed drive systems
Full text linkThis paper proposes a way to evaluate the conducted electromagnetic compatibility performance of variable speed drive systems. It is considered that the measured noise level is determined by two factors, the level of the noise source and the conversion efficiency of the propagation path from the source to the measurement equipments. They are corresponding to the two roles played by the converter. On the one hand, a converter provides the noise source and generates the noise current and voltage on the motor side with the cable and the motor. On the other hand, it acts as the propagation path with the DC bus and the rectifier to spread the noise generated on the motor side to the line side. The transfer ratio is defined as the ratio between the CM current on the motor side and the CM current on the line side. It can be used to evaluate the EMC design of a converter because it is independent of the cable and the motor. A simplified model is used to explain this characteristic. It can be measured when the converter is powered off. Verification is carried out by experimental results obtained from a 12-kVA laboratory system
Investigation of EMI noise transfer characteristic of variable speed drive system
Full text linkElectromagnetic compatibility is becoming an essential consideration for variable speed drive systems. Many methods are proposed to predict the conducted electromagnetic interference level. They ask to model all components of the drive system including cable and motor. In this paper, it is supposed that the measured noise level is determined in two phases. The first is the main noise source level and the second is the conversion efficiency from main noise source level to measured noise level. The noise transfer characteristic is investigated to understand the conversion mechanism. A transfer ratio is defined afterwards which is independent of the cable and motor. It can be used to evaluate the EMC design of a converter. A simplified model is used to explain this characteristic. Laboratory tests on a 12 kW PWM drive system are carried out to verify this hypothesis. Thorough test results are included in this paper
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