Home About us Contact
|
Home About us Contact | ||
|
|
||
Medium Frequencies (medium + frequency)
Selected AbstractsSome remarks concerning the measurement of the ferromagnetic losses under non-sinusoidal conditionsEUROPEAN TRANSACTIONS ON ELECTRICAL POWER, Issue 2 2002Andrei, ugulea The present standards concerning the measurement of the losses in ferromagnetic media, namely the IEC publications 404-2 (1978), 404-;3 (1982) and 404,10 (1988), refer only to the magnetic flux densities between 1 T and 1.85 T, if the magnetic sheets are grain oriented, and 1 T and 1.5 T if the magnetic sheets are nonoriented. The time variation of the magnetic fluxes must be almost sinusoidal at industrial frequencies of 50 Hz or 60 Hz and medium frequencies between 400 Hz and 10,000 Hz. In the above-mentioned standards there are no references concerning the measurement of losses under non-sinusoidal conditions. The paper deals with some theoretical aspects concerning the power-flows under non-sinusoidal time variation of the electromagnetic fields. The theoretical conclusions are applied to the study of the losses in ferromagnetic media. It is shown that new norms or standards are necessary. Some practical suggestions are given at the end. [source] Capital Asset Pricing Model and the Risk Appetite Index: Theoretical Differences, Empirical Similarities and Implementation Problems,INTERNATIONAL FINANCE, Issue 2 2009Marcello Pericoli We perform a thorough analysis of the Risk Appetite Index (RAI), a measure of changes in risk aversion proposed by Kumar and Persaud (2002). Building on Misina's study (2003), we first argue that the theoretical assumptions granting that the RAI correctly distinguishes between changes in risk and changes in risk aversion are very restrictive. Then, by comparing the RAI with a measure of risk aversion obtained from the Capital Asset Pricing Model (CAPM), we find that the estimates are surprisingly similar. We prove that if the variance of returns is sufficiently smaller than the variance of asset riskiness, then RAI and CAPM provide essentially the same information about risk aversion. We also show, however, that RAI and CAPM suffer from exactly the same implementation problems , the main one being the difficulty in measuring ex-ante returns. At high and medium frequencies, the standard method of measuring ex-ante with ex-post returns may generate negative risk aversion and other inconsistencies. Hence, future research is needed to address this problem. [source] An iterative defect-correction type meshless method for acousticsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 15 2003V. Lacroix Abstract Accurate numerical simulation of acoustic wave propagation is still an open problem, particularly for medium frequencies. We have thus formulated a new numerical method better suited to the acoustical problem: the element-free Galerkin method (EFGM) improved by appropriate basis functions computed by a defect correction approach. One of the EFGM advantages is that the shape functions are customizable. Indeed, we can construct the basis of the approximation with terms that are suited to the problem which has to be solved. Acoustical problems, in cavities , with boundary T, are governed by the Helmholtz equation completed with appropriate boundary conditions. As the pressure p(x,y) is a complex variable, it can always be expressed as a function of cos,(x,y) and sin,(x,y) where ,(x,y) is the phase of the wave in each point (x,y). If the exact distribution ,(x,y) of the phase is known and if a meshless basis {1, cos,(x,y), sin, (x,y) } is used, then the exact solution of the acoustic problem can be obtained. Obviously, in real-life cases, the distribution of the phase is unknown. The aim of our work is to resolve, as a first step, the acoustic problem by using a polynomial basis to obtain a first approximation of the pressure field p(x,y). As a second step, from p(x,y) we compute the distribution of the phase ,(x,y) and we introduce it in the meshless basis in order to compute a second approximated pressure field p(x,y). From p(x,y), a new distribution of the phase is computed in order to obtain a third approximated pressure field and so on until a convergence criterion, concerning the pressure or the phase, is obtained. So, an iterative defect-correction type meshless method has been developed to compute the pressure field in ,. This work will show the efficiency of this meshless method in terms of accuracy and in terms of computational time. We will also compare the performance of this method with the classical finite element method. Copyright © 2003 John Wiley & Sons, Ltd. [source] Effects of Hf content and immersion time on electrochemical behavior of biomedical Ti-22Nb- xHf alloys in 0.9% NaCl solutionMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 5 2009B. L. Wang Abstract The aim of this study was to investigate the effects of Hf content and immersion time on the electrochemical corrosion behavior of the Ti-22Nb- xHf (x,=,0, 2, 4, and 6 at%) alloy samples in 0.9% NaCl solution at 37,°C and neutral pH range, utilizing the potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. From the polarization curves, all these alloys exhibited typical passive behavior, which was indicated by a wide passive region without the breakdown of the passive films and low corrosion current densities. In addition, the values of the corrosion current densities and passive current densities decreased with increase in the Hf content. The EIS results, fitted by RS(QPRP) model, exhibited capacitive behavior (high corrosion resistance) with phase angles closed to ,80° and high impedance values at low and medium frequencies, indicating the formation of a highly stable film on these alloys in the test solution. The resistance of the passive films improved with increase in the Hf content and immersion time. All these observations suggested a more noble electrochemical behavior of the Ti-22Nb- xHf alloys compared to the Ti-Nb binary alloy. [source] Alternating-current conductance and capacitance of a diamond diode in the presence of deep-level impuritiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 2 2010Ana Rodrigues Abstract The alternating-current conductance and capacitance of a diamond film/aluminium diode was measured over the temperature range 230,330 K. Loss tangent curves for the frequency range 100 Hz,1 MHz exhibited three maxima. Doping of the CVD diamond was not uniform, indicating the presence of deep-level impurities. Results were analysed for the temperature range 230,270 K, at which it is possible to observe processes that are supposed to occur in grains and grain boundary regions of polycrystalline diamond. A loss tangent curve was simulated using an equivalent circuit model composed of resistances and capacitances corresponding to depletion regions and the bulk and of the electrical characteristics of the two phases of diamond. This model explicitly includes deep impurities. The curve-fitting parameters for the loss tangent plot demonstrate that at medium frequency (4 kHz, activation energy 0.2 eV) the bulk resistance is high (100 k,) and the maximum is related to the presence of microcrystals, which exhibit low conductivity. At high frequency (800 kHz, activation energy 0.5 eV) the bulk resistance is low (1 k,), suggesting that the maximum is associated with grain boundary regions. The low values of the relaxation times obtained (10,28 and 4×10,27 s) are justified. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of chronic hypoxemia on peripheral muscle function and oxidative stress in humansCLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 2 2004Marion Faucher Summary Transient re-oxygenation of humans suffering from chronic obstructive pulmonary disease (COPD) allows the assessment of the consequences of chronic hypoxemia on peripheral muscle and metabolism apart from the effects of de-conditioning. The subjects performed maximal voluntary contractions (MVC) of flexor digitorum and vastus lateralis muscles and sustained infra-maximal contractions. COPD patients repeated the whole challenge during a 50-min oxygen breathing period and after recovery to baseline hypoxemia. We measured the compound evoked muscle mass action potential (M-wave) and the medium frequency (MF) of surface electromyography (EMG) power spectrum. Blood lactate (LA) and potassium (K+), erythrocyte-reduced glutathione (GSH), and plasma thiobarbituric acid reactive substances (TBARS) were also measured. Compared with a control group, COPD patients had lower MVCs, an attenuated decrease in MF during exercise, lower resting level of GSH, no posthandgrip TBARS increase and no GSH consumption. Reoxygenation (1) increased MVCs, (2) accentuated the MF decline and (3) elicited a posthandgrip TBARS increase and GSH consumption. Thus, we conclude that chronic hypoxemia exerts specific muscular effects: a reduced force production, an attenuated ,muscle wisdom', and the suppression of the exercise oxidative stress. [source] | ||||||||||