1,720,956 research outputs found
Comparison of two three-phase pll systems for more electric aircraft converters
No full textThe more electric aircraft power system is characterized by variable supply frequency, in general between 360 and 900 Hz. All equipment on board the aircraft have to operate delivering high performance under this variable frequency condition. In particular, power electronic converters need accurate control algorithms able to track the fundamental phase and frequency in real time, both in normal and unusual conditions. Phase-locked loop (PLL)-based algorithms are commonly used in traditional single-and three-phase power systems to provide phase and frequency estimations of the supply. Despite the simplicity of those algorithms, large estimation errors can arise when power supply voltage has variable frequency or amplitude, presents unbalances or is polluted with harmonics. To improve the quality of the phase and frequency real-time estimations, a robust PLL algorithm, based on a prediction-correction filter, is presented in this paper and compared with a discrete Fourier transform based procedure. The performances of the two algorithms, implemented in a floating-point DSP, have been compared through an experimental validation obtained on a laboratory power converter prototype. © 2014 IEEE
Comparison of two three-phase pll systems for more electric aircraft
Full text linkThe More Electric Aircraft power system is characterized by variable supply frequency, in general between 360Hz and 900Hz. All equipment on board the aircraft have to operate delivering high performance under this variable frequency condition. In particular, power electronic converters need accurate control algorithms able to track the fundamental phase and frequency in real time, both in normal and unusual conditions. Phase Locked Loop (PLL) based algorithms are commonly used in traditional single and three phase power systems to provide phase and frequency estimations of the supply. Despite the simplicity of those algorithms, large estimation errors can arise when power supply voltage has variable frequency or amplitude, presents unbalances or is polluted with harmonics. To improve the quality of the phase and frequency real-time estimations, a robust PLL algorithm, based on a prediction-correction filter, is presented in this paper and compared with a Discrete Fourier Transform (DFT) based procedure. The performances of the two algorithms, implemented in a floating-point DSP, have been compared through an experimental validation obtained on a laboratory power converter prototype
Real-Time Estimation of Fundamental Frequency and Harmonics for Active Shunt Power Filters in Aircraft Electrical Systems
No full textA novel algorithm for fundamental frequency and harmonic components detection is presented in this paper. The technique is based on a real-time implementation of discrete Fourier transform, and it allows fast and accurate estimation of fundamental frequency and harmonics of a distorted signal with variable fundamental frequency. It is suitable for active shunt filter applications, when fast and accurate tracking of the reference signal is required to achieve a good control performance. The main application for the algorithm is aircraft ac power systems, where the fundamental frequency can be either fixed on 400 Hz and its actual value fluctuates around the nominal value, or variable in the range 360-900 Hz. Hence, a real-time estimation of fundamental frequency is essential for active filter control. The proposed algorithm has been at first implemented in Matlab/Simulink for computer simulation, and it has been compared with a Phase Locked Loop (PLL) algorithm for frequency detection and the synchronous dq reference method for harmonic detection. Experimental tests have been carried out in order to validate the simulation results. The distorted current absorbed by a nonlinear load is analyzed and processed by means of a digital implementation of the algorithm running on the active shunt power filter control DSP, in order to calculate the active filter compensating current
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
A DFT-based phase locked loop for phase and amplitude tracking in aircraft electrical systems
No full textPhase locked loop (PLL) algorithms are commonly used to track sinusoidal components in currents and voltage signals in three phase power systems. Despite the simplicity of those algorithms some drawbacks arise when signals have variable amplitude or are polluted with relevant harmonic content and measurement noise. These are typical conditions in aircraft AC power systems, where the fundamental frequency can be variable in the range 360-900 Hz. To improve the quality of phase and frequency estimates in such power systems a novel PLL scheme based on a real-time implementation of discrete fourier transform (DFT) is presented in this paper. The DFT algorithm calculates the amplitudes of three consecutive components in the frequency domain that are used to determine an error signal to be minimized by a PI controller that estimates the fundamental frequency. The integral of the estimated frequency is the estimated phase of the fundamental component that is fed back to the DFT algorithm. The proposed algorithm can be then reviewed as a PLL in which the phase detector is performed via a DFT-based algorithm. A comparison between the performances of standard PLL and the proposed DFT-PLL has been realized using computer simulations and experiments
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
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