No. 2, 2005


Viktor Valouch : How to control active power filters for harmonic voltage mitigation in multibus industrial power systems, pp.107-130

The effectiveness of the bus voltage and branch current detection control strategies of the APFs used in multibus power systems is analysed and compared. The multiple harmonic optimization problem for both the control methods is formulated in such a way that for the whole selected range of harmonics a single control gain is searched. The obtained results are not far from those achieved by the application of the repeated solution of the single harmonic problem, which represents the most effective way to suppress the node voltage harmonics, but it is rather difficult to realize, except the cases where only a few voltage harmonics should be compensated for.

Jiri Klima : Fourier approximation of voltage space-vectors in three-level VSI with SVPWM, pp.131-143

This paper proposes an analytical model for calculating the harmonic spectra of a three-level voltage source inverter (VSI) with space-vector PWM (SVPWM). A three-level inverter has a large number of switching states compared to a two-level inverter and, therefore, the SVPWM algorithm is more complex. The proposed model is based on the Fourier series approximation of the voltage space vectors in complex ab plane.

From the analytical relations we can visualize the derived Fourier series both in the complex plane and in the real phases by means of the programme MATCAD.

Jiri Sonsky : Self organized structures in plasma jets-methods of detection, pp.145-166

The goal of this work is to provide information about novel diagnostics methods and their parameters which we have developed for detection of structures studied in plasma jets. These methods differ considerably in spatial and temporal resolution of the plasma jet dynamics records. Records analysis, and namely the combination of analysis records from two or more methods, can be used for observation of the structures velocity, frequency spectra analysis and their dependence on autonomous or excitation mechanisms.

Developing diagnostic methods based on the radiation flux tracking is a suitable way to check operation of plasma jets without affecting the technological process. The information about the plasma jet dynamics is scanned at chosen points and saved in the form of radiation records. The identification of structures requires choosing the layout and the number of scan points, or scanning the whole image of the luminous structure. The reported results relate to the experimental study focused on investigating the fine scale structure of turbulent plasma jets, detection of self-organized structures and their evolution using several methods of imaging. The results are obtained in a visible region of optical radiation of plasma jet. The usage of linear arrays of photodiodes and linear CCD chips to capture the plasma jet luminosity results in higher frame rates and longer time sequences which are more sufficient for reconstructing frequency spectra and improving the accuracy of wavelet transformation. Planar CCD chips are used to capture single view or both front and side views of the jet. Bi-directional imaging can be used for 3-D reconstruction, needed for identification of several types of structures, as discussed later. The data obtained by evaluation of source pictures are mainly used to investigate macroscopic velocities, various types of instabilities, and characteristic frequency spectra.

Ondrej Fischer, Miros Pirner : Modal analysis and analysis of forced vibrations of multispan bridges, pp.167-176

Dynamic tests of bridges usually compare the measured bridge response under resonance with the calculated natural mode. In general, it is not easy to obtain good agreement between the measured stationary excited amplitude and the calculated mode-shape, especially with continuous multispan bridges and in the spans which are distant from the point of excitation. On a simple calculated example the authors show that this discrepancy can be caused by structural damping which makes the natural mode and the shape of the resonance amplitude different.

Viktor Pekarek : Thermo-mechanical anomaly developed in metals under elastic stress, pp.177-194

The aim of the paper is to prove that the Young moduli determined by tear tests in metals are adiabatic. Recent findings suggest that the forces arising in metals under elastic strain are non-conservative. Only a smaller part of incoming mechanical energy can be stored as the energy of strain capable of elastic response, while a larger part is transformed to heat. Hidden and unobserved, the anomaly distorts interpretation of standard tests in metals and calls the authenticity of computer simulations into question.

Pavel Zaskalicky, Maria Zaskalicka : Behaviour of the two-phase permanent magnet synchronous motor supplied by rectangular voltage, pp.195-206

Permanent magnet synchronous motors are attractive for domestic and industrial application due to several reasons. They are chemically inert, which makes them suitable for applications in aggressive environments. Strontium-ferrite magnets have high specific electric resistance, so they do not experience thermal problems due to eddy-current losses. Their low residual flux density permits the construction with high air-gap, what is needed for small pump applications, where the rotor of electric motor is exposed to water. But permanent magnet motors have some drawbacks. The most important of them is the inability to start directly on the mains. Permanent motor must be equipped with an electronic converter. The paper shows the analytical method to determine electric and mechanic quantities when motor is supplied by no harmonics voltage.