Current control of unbalanced electrical systems

The vector modeling approach is employed to describe the behavior of unbalanced three-phase (three-wire), unbalanced two-phase and single-phase systems. In the case of single-phase systems, a fictitious circuit is introduced in order to create the vector model. Continuous-time and discrete-time current controllers based on the vector approach are presented. The current control scheme is based on two controllers, one for the positive sequence and another for the negative sequence. The controllers are implemented in the synchronous reference frame and in the stationary reference frame. In the stationary reference frame, the use of the same integral gains for both controllers simplifies the equations of the control law especially for single-phase systems. Simulation and experimental results obtained with a pulsewidth modulation voltage-source inverter supplying unbalanced systems are presented     

PVC and LDPE for packing minimally processed garlic

Minimum processing associated with an adequate packing system has shown good results in preserving a large group of perishable horticultural produce. The present work was done to determine the effect of the number of layered of low‐density polyethylene (LDPE) or polyvinyl chloride (PVC) film on the quality of minimally processed packed garlic (Allium sativum), known for its short shelf‐life. Polystyrene trays containing 200 g garlic bulbils were wrapped in one to four layers of PVC or one or two layers of LDPE and stored at room temperature. Changes in weight, bulbil colouration, in‐package gas composition and apparent quality were monitored for 14 days. Best results were obtained by wrapping the packages in four layers of PVC film, with insignificant weight loss (5%), changes in colour tone and saturation, and the in‐pack atmosphere (O₂ = 0.01% and CO₂ = 13.5%) inhibited fungal growth, extending shelf‐life to 13 days at 25 ± 2°C. On the other hand, the high in‐package moisture in LDPE wrapping favoured fungal growth, rapidly depreciating bulbil quality. The shelf‐life of minimally processed garlic can be increased by maintaining an adequate in‐pack atmosphere, which can be achieved by using four layers of PVC or a another film of similar permeability. Copyright © 2002 John Wiley & Sons, Ltd.     

Electrochemical and morphological characterization of poly(12-pyrrol-1′-yl dodecanoic acid)

12-Pyrrol-1′-yl dodecanoic acid was prepared and electrochemically polymerized on ITO electrodes by three different methods. The electrochemical and morphological properties of poly(12-pyrrol-1′-yl dodecanoic acid) films were investigated by cyclic voltammetry, galvanostatic charge–discharge curves and AFM, which characteristics varied as function of deposition method. Films deposited by potentiodynamic and galvanostatic methods showed capacitive properties and are promising for application in polymeric capacitors.     

Palladium and Molybdenum Mono and Bimetallic Catalysts on Modernite for Direct NO Decomposition Reaction

Palladium and/or molybdenum catalysts supported on mordenite were prepared and characterized by XRD, UV–vis, DRS, textural properties analysis, TPR, TPD and chemical analysis. The Mo-catalyst rapidly deactivated after the first minutes reaction. The Pd-catalysts were active in the NO decomposition reactions. The incorporation of Mo in the Pd-catalysts improved their catalytic properties.     

Semibatch and continuous fructooligosaccharides production by Aspergillus sp. N74 in a mechanically agitated airlift reactor

BACKGROUND: Fructooligosaccharides are important sweeteners produced by sucrose biotransformation. Although fructooligosccharides production has been reported widely, most studies have been carried out at laboratory level. This study evaluates semibatch and continuous fructooligosaccharides production by Aspergillus sp. N74 at bench scale in a mechanically agitated airlift. RESULTS: Sucrose biotransformation to fructooligosaccharides was carried out with biomass harvested after 24 or 48 h of culture. For 6.21 ± 0.33 or 9.66 ± 0.62 g biomass dry weight L⁻¹, the highest FOS yields were obtained at batch operating 62.1 and 66.4% after 26 or 6 h of reaction, respectively. Reduction in fructooligosaccharides yield was observed for both biomass concentrations at semibatch operating, while a comparable yield was obtained during continuous operating (62.1% for 6.21 ± 0.33 g L⁻¹ and a dilution rate 0.016 s⁻¹, and 62.8% for 9.66 ± 0.62 g L⁻¹ and a dilution rate 0.032 s⁻¹). Nevertheless, 1‐kestose formation was favored with biomass harvested after 24 h under any operating mode. CONCLUSION: Biomass concentration, reaction time and operating mode have a notable effect on fructooligosaccharides yield and composition. 1‐kestose, the most valuable fructooligosaccharide, was obtained in greatest proportion at a biomass concentration 6.21 ± 0.33 g L⁻¹. Under the different operating modes, Aspergillus sp. N74 mycelia and the reactor described are presented as a feasible alternative for scaling up fructooligosaccharides production. Copyright © 2008 Society of Chemical Industry     

Development of an in-house method for the incorporation of model migrants in polyethylene films and determination of diffusion constants in food

Two methods were tested and compared for the additivation of low density polyethylene (LDPE). The aim was to obtain highly contaminated plastic films to enable the study of migration (diffusion and partition phenomena). One of the methods involved immersion of low density polyethylene (LDPE) films in a concentrated solution of the selected substances. The other method involved achieving close contact of plastic films with a polyethylene wax contaminated with the selected compounds. The PE-wax method provided better results as regards the final concentration and homogeneity of the contaminants in the plastic films (deviations between replicates for each plastic film was lower than 10% for both migrants tested). This method was therefore considered suitable for preparing a homogeneous additive release system. Finally, to test the applicability of the method, concentration profiles were studied in chocolate spread placed in contact with the contaminated films, and diffusion coefficients were estimated in this foodstuff. Estimated diffusion coefficients were 4.6 × 10⁻⁰⁷ cm²/s for DPBD and 3.2 × 10⁻⁰⁷ cm²/s for triclosan in the same conditions.     

Poly(3-hexadecylthiophene)/montmorillonite clay nanocomposites: Influence of preparation and type of clay on their structures

A nanocomposite of poly(3-hexadecylthiophene) (PHDT) with montmorillonite (MMT) was prepared by in situ polymerization (PHDT/MMT) and nanocomposites of PHDT with organomontmorillonite clay (OrgMMT) were prepared by both in situ polymerization (PHDT/OrgMMT) and by the solvent casting method (PHDT/OrgMMT-sc). The nanocomposites were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). PHDT/OrgMMT was also characterized by transmission electron microscopy (TEM). FTIR spectra indicate the presence of the clay in the polymer matrices and show variations on the frequency of the Si–O–Si stretching vibration. The XRD diffractograms indicate that PHDT/MMT and PHDT/OrgMMT-sc are intercalated nanocomposites, while PHDT/OrgMMT is mostly exfoliated, but present also some intercalation, as confirmed by TEM.     

Sensor arrays to detect humic substances and Cu(II) in waters

The interaction between poly(o-ethoxyaniline) (POEA) adsorbed onto solid substrates and humic substances (HS) and Cu²⁺ ions has been investigated using UV–vis spectroscopy and atomic force microscopy (AFM). Both HS and Cu²⁺ are able to dope POEA and change film morphology. This interaction was exploited in a sensor array made with nanostructured films of POEA, sulfonated lignin and HS, which could detect small concentrations of HS and Cu²⁺ in water.     

Predictive Current Control Strategy With Imposed Load Current Spectrum

This paper presents a modified predictive current control strategy which allows one to have control over the spectrum of the load current. The proposed method uses a model of the system to predict the behavior of the current for each possible voltage vector generated by the inverter. For that purpose, at each sampling interval, signal predictions are evaluated using a cost function that quantifies the desired system behavior. The cost function used in this work evaluates the filtered error of the load currents. The inclusion of a filter for the load error allows one to manipulate current spectra. Thus, by designing this filter appropriately, the load spectrum can be shaped. The performance of the proposed control strategy is verified by simulation and experimental results.