Valentin Ionita

Box-Behnken experimental design for chromium(VI) ions removal by bacterial cellulose-magnetite composites

In this study bacterial cellulose-magnetite composites were synthesised for the removal of chromium(VI) from aqueous solutions. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis and X-ray Photoelectron Spectroscopy (XPS) were used to characterize the bacterial cellulose-magnetite composites and to reveal the uniform dispersion of nanomagnetite in the BC matrix. Magnetic properties were also measured to confirm the magnetite immobilization on bacterial cellulose membrane. The effects of initial Cr(VI) concentration, solution pH and solid/liquid ratio upon chromium removal were examined using the statistical Box-Behnken Design. Because of the possibility of magnetite dissolution during chromium(VI) adsorption, the degree of iron leaching was also analysed in the same conditions as Cr(VI) adsorption. From the factors affecting chromium(VI) adsorption the most important was solution pH. The highest Cr(VI) removal efficiency was observed at pH 4, accompanied by the lowest iron leaching in the solution. The adsorption experiments also indicated that the adsorption process of chromium(VI) is well described by Freundlich adsorption model. Our results proved that the BC-magnetite composites could be used for an efficient removal of chromium(VI) from diluted solutions with a minimum magnetite dissolution during operation.

Correction of Measured Magnetization Curves Using Finite Element Method

Magnetic measurement accuracy for open samples is analyzed using numerical simulations. The finite element method is used for the complete numerical correction of experimental data which are measured with a vibrating sample magnetometer. The magnetization curves are recalculated taking into account the demagnetizing field, the shape of the open magnetic circuit, and the field probe position.

An improved method for the inrush current evaluation in single phase power transformers

The paper presents an automatic procedure for quantitative evaluation of energizing inrush current of a specific range of single phase power transformers: 1.2-25 kVA. Starting from transformer main rated data and core magnetic H-B characterization, the first peak value of inrush current is accurately estimated. Additionally, the harmonic content of its waveform is also investigated offering valuable indicators of this transient phenomenon. The predetermination of the main inrush current characteristics becomes very useful in avoiding its countless undesirable effects on both the transformer and the installation where the device operates. The proposed method offers a fast and flexible solution of the energizing inrush current problem for the main general purpose single phase transformers.

An efficient method for investigating the ferroresonance of single-phase iron core devices

The paper presents a fast and efficient method for determining the solutions of the ferroresonant circuits generated at the switching of different single-phase low-voltage iron-core devices. These are widely encountered in modern electric installations as no-loaded power transformers, magnetic choke or shunt reactors. The here suggested procedure basically relies on the solutions of the nonlinear and non-autonomous differential equations systems that model the commutation (dynamic) process. Starting from the devices manufacturer data, the energy supply installation specification and adopting an adequate description of their magnetic characteristics, the time evolution of the major electrical parameters is numerically predicted and critically analyzed. The power spectrum, phase portraits and Poincaré map of some relevant electrical quantities variation complete the analysis of the ferroresonance phenomenon. Additionally, mitigations strategies for the common undesirable power quality issues associated with the ferroresonace occurrence are proposed and discussed.

Computational Errors in Hysteresis Preisach Modelling

The paper analyzes the influence of the computational errors on the accuracy of magnetic material modelling with scalar Preisach model. The numerical tests on magnetic recording media allow the correct choosing of numerical algorithms for model parameter identification.

Influence of harmonics' initial phases on magnetic losses in non-oriented grains FeSi sheets

The estimation of magnetic losses in a non-oriented grains FeSi sheet by a time-dependent method is used in the paper for underlining the influence of each harmonic initial phase. The initial phase of the first harmonic, usually indicated by a power analyzer to be zero, is very important for the signal shape, the losses being dependent on this parameter. The paper proves that THD and the harmonic amplitudes of the magnetic flux density are not sufficient for the magnetic losses predetermination in distorted regime, the initial phases of the harmonics being of great importance for any computation involving a truncated Fourier series.

Numerical and experimental investigations on the energizing of miniature iron core transformers

The paper analysis both numerically and experimentally the no-load miniature power transformer energizing process. The transient state characterization targets the predetermination of the main inrush current features: amplitude, duration and waveform signature. The whole quantitative investigation is carried out on a particular core-shaped devices (E+I), widely used in small power applications. The here suggested analytical computation method bases on the solution of the nonlinear differential equations that model the commutation phenomenon. Supplementary, a simple and low-cost experimental set-up platform is proposed to validate the theoretical results.

Characterization of soft magnetic materials in a wide range of frequencies

Amorphous and nanocrystalline magnetic cores allow lighter, smaller and more energy efficient designs of many industrial high frequency applications such as: adjustable speed drives (ASD), invertors, uninterruptible and switched-mode power supply (UPS and SMPS). The aim of this paper is to present a complex comparison between two soft magnetic materials, frequently used in electromagnetic devices, that operate in a wide range of frequencies. Aspects regarding their first magnetization curves, magnetic permeability, and energy losses at different frequencies are analyzed. Additionally, a magnetic loss separation study, using Bertotti method, for each investigated material is performed.

Computation of non-sinusoidal hysteresis losses using standardized measured data

A magnetic field having the first and the third harmonic is applied to a FeSi sheet, computing the hysteresis losses in this periodic non sinusoidal regime. The magnetic material is modeled by Preisach classical model, which is identified starting from the family of symmetrical hysteresis cycles measured by an industrial single sheet tester (SST). Various ratios between the amplitudes of the two harmonics, which are in phase or in quadrature, are considered in order to study the effect of the magnetic field distortion on the magnetic losses.

Polyethylene-based magnetic composites

Magnetic polymer composite materials have very good rheological (that are characteristic for polymers used as matrix) and magnetic properties superior to those of polymers (due to their magnetic filler) making them useful in many power applications (electromagnetic shielding, permanent magnets etc.). In this paper the results of an experimental study regarding the manufacture and characterization of some composites which have low density polyethylene (LDPE) as matrix and neodymium (Nd) and neodymium-iron-boron (NdFeB) as filler are presented. The manufacture process (with mass content between 0 and 15 %) and their structure (obtained by electronic and optical microscopy) are presented. It is shown that the samples are inhomogeneous and isotropic and the filler particles form clusters of variable dimensions and distances between them. Then, the first magnetization cycles obtained on neodymium (A) and neodymium-iron-boron (B) samples for three mass concentrations (5, 10 and 15 %) are presented. It is shown for both samples type (A and B) that the hysteresis area cycles increase with filler content being greater for samples B than A. It is shown, also, that the magnetic permeability values increase with filler content, but decrease very quickly with the magnetic field strength.