Advanced spectrum estimation methods for signal analysis in power electronics

Modern frequency power converters generate a wide spectrum of harmonic components. Large converters systems can also generate noncharacteristic harmonics and interharmonics. Standard tools of harmonic analysis based on the Fourier transform assume that only harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics are variable and very long. A novel approach to harmonic and interharmonic analysis, based on the "subspace" methods, is proposed. Min-norm harmonic retrieval method is an example of high-resolution eigenstructure-based methods. The Prony method as applied for signal analysis was also tested for this purpose. Both high-resolution methods do not show the disadvantages of the traditional tools and allow exact estimation of the interharmonics frequencies. To investigate the methods several experiments were performed using simulated signals, current waveforms at the output of a simulated frequency converter, and current waveforms at the output of an industrial frequency converter. For comparison, similar experiments were repeated using the fast Fourier transform (FFT). The comparison proved the superiority of the new methods. However, their computation is much more complex than FFT.     

Thermophysical and Magnetic Properties of Carbon Beads Containing Cobalt Nanocrystallites

Magnetic Co-beads were fabricated in the course of a three-step procedure comprising preparation of a metal–acrylamide complex, followed by frontal polymerization and finally pyrolysis of the polymer. The composites obtained were composed of cobalt nanocrystallites stabilized in a carbon matrix built of disordered graphite. The crystallite size, material morphology, fraction of the magnetic component, and thus the magnetic properties can be tailored by a proper choice of the processing variables. The samples were subjected to an alternating magnetic field of different strengths (H = 0 to 5 kA · m⁻¹) at a frequency of f = 500 kHz. From the calorimetric measurements, we concluded that the relaxation processes dominate in the heat generation mechanism for the beads pyrolyzed at 773 K. For the beads pyrolyzed at 1073 K, significant values of magnetic properties, such as the coercive force and remanence give substantial contribution to the energy losses for hysteresis. The specific absorption coefficient (SAR) related to the cobalt mass unit for the 1073 K pyrolyzed beads (SAR = 1340   W ·g^-1 _cobalt){({\it SAR} = 1340 \, \, {\rm W} \cdot {\rm g}^{-1 }_{\rm cobalt})} is in very good conformity with the results obtained by other authors. The effective density power loss, caused by eddy currents, can be neglected for heating processes applied in magnetic hyperthermia. The Co-beads can potentially be applied for hyperthermia treatment.     

Functionalization of textile materials by alkoxysilane-grafted titanium dioxide

Modern materials, including textiles for specific applications, have to satisfy growing requirements. Regulations concerning man and natural environment protection against harmful substances emission, UV radiation, and electromagnetic field radiation become more and more stringent. Intensive development of nanotechnology offers great possibilities to create novel—conforming to the requirements—multifunctional materials based on textile substrates. Nanoparticles of metal oxides, e.g., titania (TiO₂), belong to a group of compounds having photocatalytic properties, which are able to absorb UV radiation and provide antibacterial barrier. The aim of research works was the modification of selected textiles using nanoparticles of metal oxides. Nano-TiO₂ and modified nano-TiO₂ with aminosilane were applied. In the first stage, the works concerned the methodology development of such nanoparticles incorporation onto selected textile substrates. Commonly used techniques, such as padding and spraying were used as well as sol–gel coating. The evaluation of microstructure of textile fabrics covered with nanostructural titanium dioxide was performed using high-resolution SEM and TEM electron microscopes. Assessment of modified textiles pertained to the determination of protective properties against UV radiation and photocatalytic and antibacterial properties. Absorption spectra of textiles were determined using double beam type of UV–Vis Jasco V-550 with integrating sphere attachment. The same apparatus was used to determine ultraviolet protection factor (UPF) of textiles according to the standard EN 13758-1:2002. Textiles modified with nano-TiO₂ demonstrated high absorption of UV radiation in a full wavelength and their good photocatalytic properties were also confirmed.     

Special features of the magnetic abrasive machining

It is shown, using electron microscopy, magnetic and friction methods, that a mixture of magnetic γ-Fe₂O₃ and abrasive powders, properly processed, can be used as a material for magnetoabrasive machining and polishing of variously shaped components. __________ Translated from Novye Ogneupory, No. 1, pp. 53–55, January, 2006.     

Measurement of IEC Groups and Subgroups Using Advanced Spectrum Estimation Methods

The International Electrotechnical Commission (IEC) standards characterize the waveform distortions in power systems with the amplitudes of harmonic and interharmonic groups and subgroups. These groups/subgroups utilize the waveform spectral components obtained from a fixed frequency-resolution discrete Fourier transform (DFT). Using the IEC standards allows for a compromise among the different goals, such as the needs for accuracy, simplification, and unification. In some cases, however, the power-system waveforms are characterized by spectral components that the DFT cannot capture with enough accuracy due to the fixed frequency resolution and/or the spectral leakage phenomenon. This paper investigates the possibility of a group/subgroup evaluation using the following advanced spectrum estimation methods: adaptive Prony, estimation of signal parameters via rotational invariance techniques, and root multiple-signal classification (MUSIC). These adaptive methods use variable lengths of time windows of analysis to ensure the best fit of the waveforms; they are not characterized by the fixed frequency resolution and do not suffer from the spectral leakage phenomenon. This paper also presents the results of the applications of these methods to three test waveforms, to current and voltage waveforms obtained from simulations of a real DC arc-furnace plant, and to waveforms measured at the point of common coupling of the low-voltage network supplying a high-performance laser printer.     

Thermophysical and Magnetic Properties of Carbon Beads Containing Nickel Nanocrystallites

Ferromagnetic and superparamagnetic nickel nanocrystallites, stabilized in a carbon matrix, were prepared by a three-step procedure including formation of a Ni acrylamide complex, followed by frontal polymerization and pyrolysis of the polymer at various temperatures. It was found that the procedure applied enables fabrication of magnetic beads containing metallic nanocrystallites embedded in a carbon matrix. The size of the crystallites, their morphology, volume fraction, and magnetic properties can be tailored by the pyrolysis temperature. The size of the crystallites affects their behavior in an external magnetic field, i.e., a heating process is the most effective for a sample pyrolyzed at 873 K. The revealed H ⁿ-type dependence of the temperature increase rate, (dT/dt) _t=0, on the amplitude of the magnetic field indicates the presence of both superparamagnetic and ferromagnetic particles in all the samples studied since n > 2. For the superparamagnetic particles, the heating mechanism is associated with Néel relaxation. For the lower values of the magnetic field amplitude, H <  H ₀, the relaxation losses dominate whereas for the opposite case, H >  H ₀, the magnetic hysteresis is the main source of thermal energy losses. The composites containing magnetic Ni nanocrystallites entrapped in a carbon matrix can be potentially applied for hyperthermia treatment.     

Crystallization of Te-Si-Pb glasses exhibiting double T g effect

Results of calorimetric, X-ray and transmission electron microscopy investigations of the glasscrystal transition in Te₈₀Si_20–xPb_x glasses are presented. The investigated glasses exhibit a double-stage crystallization and a double glass-transition temperature, T _g, within a composition range of 2.5 to 10 at % Pb. The second glass transition appears after partial crystallization of the glass. The double glass-transition phenomenon is discussed in relation to the crystallization process.