No. 6, 1998


Jerzy Barglik, Ivo Dolezel, Bohus Ulrych: Induction heating of moving materials, 515–527

The paper deals with induction heating of a nonferromagnetic electrically conductive metallic strip of a given thickness moving at a constant speed between the coils of an inductor. Its principal aim is to find ratio between the heat caused by time variations of the magnetic field and the heat caused by movement of the strip towards the inductor. The analysis starts from mathematical models of electromagnetic fields in moving and nonmoving media, that are solved by professional FEM based program OPERA 2D. Several illustrative examples are also presented and their results are discussed.

Viktor Valouch: Induction motor stator flux calculation, 529–542

An alternative method of the induction motor stator flux calculation, instead of the voltage integration, is developed and analysed in the paper. The target is to avoid calculated dc flux components, which can deteriorate the performance criteria of an induction motor drive substantially. A few modifications of the method have been proposed to improve the accuracy of the flux calculation even at the lowest stator frequencies in steady states as well as transient processes. The method is based on transfer functions of the first-order filter with some correction coefficients, which make it possible to find a compromise between residual dc components and errors in the estimated stator flux for fundamental frequency, both being desirable as small as possible.

Daniel Mayer, Svatopluk Zacharias: Maxwell stress tensor and forces in magnetic fields, 543–554

The stress tensor was introduced into the theory of electromagnetic field by J. C. Maxwell when he investigated the effects of forces in the electromagnetic field. He started from the idea that a spatially distributed electromagnetic field possesses certain properties of elasticity, and when constructing its model he used the analogy with the theory of elastic continuum which was then already developed in all details. Although these theories are very old now, their presentation in textbooks is incorrect and incomplete. The Maxwell stress tensor in the magnetic field has been used in literature mostly only for Cartesian coordinates. In this paper it is derived with respect to general coordinate systems.

Vit Novak: Hall measurement in a nonstandard sample geometry, 555–560

Rectangular sample geometry with great width-to-length ratio and four potential sensing contacts far from the sample edges is used in the numerical simulation of the Hall-measurement. Hall-angle, charge carrier mobility and density are obtained as functions of measurable parameters. Expressed as polynomials they allow to interpret correctly the real Hall-measurement on the same sample geometry.

Viktor Pekarek: Participation of electron spin in electromagnetic radiation, extra resonant series in the spectrum of hydrogen, 561–569

Photon-electron collisions are investigated beyond the framework of the standard theory, to enlighten upon engagement of electron spin in emission and absorption of light. Besides generating the classical series, the spin mechanism allows the electron to be set in a resonant regime at much higher frequencies, without overwhelming the ionization potential of hydrogen atom. Extra resonant series in the X-rays region are anticipated.

Josef Mrazek: The positive and negative lightning channel at the start of the thermal ionization, 571–585

The process of heating of the propagating lightning channel is studied in order to determine the time and space parameters of the not yet fully heated section of the channel and to find the minimum discharge path, which is necessary for start of the thermal ionization. There are discussed the consequences of the different forming of the positive and the negative discharge channel. Extreme outdoor experiments were provided in order to get information about the behaviour of lighting-like sparks and to confirm by this way the theoretical considerations.

Vaclav Nenicka: Effects of self-organization in dusty plasma and results of the kinetic theory of two- and three-particle velocity distribution functions, 587–605

We briefly summarize the results of the kinetic theory of two- and three-particle velocity distribution functions for gas mixtures, which are important while constructing low-dimensional behaviour models in dusty nonequilibrium plasmas. Using previously developed principles of Born-Bogoliubov-Green-Kirkwood-Yvon (BBGKY) hierarchy of kinetic equations for multi-particle distribution function, basic equations of self-organization onset in dusty plasmas are formulated. It is concluded that for low dimensional description one needs such approaches which are based on mean values in two- and three-point models using kinetic equations for particle distribution or rather the moment equations of turbulent kinetic theory of gases. Those systems of equations are formulated for two- and three-particle moments respecting the dissipative processes on the right hand sides of the kinetic or the moment equation systems. Nonlinearity of such a system is guaranteed by those members of equations, which respect the mutual influence among the dusty and the plasma particle subemsembles. Weighting of this interaction is given by a sum of products of mutual interaction potential forces of macroscopic dusty particles and the surrounding emsembles of microscopic particles (electrons, ions, etc.), then the single-particle distribution function of velocity and the two-particle ones. For real models the last two variables in this product may rather be the moments of those distribution functions for density, momentum, and energy macroscopic fluctuations. Functional properties of such partial differential equation system are very close to behaviour of particle ensembles in Brussellator and also to more complicated structures with self-organization effects.

Alexander S. Kravchuk: Study of contact problem for disk and plate with wearing hole, 607–613

The problem of elastic interior contact of a rigid disk and isotropy plate with wearing cylindric hole was developed. It is described an explicit approximate solution of the contact problem, received an analytical expression for principal tangential stress along axis of force action.

Stanislav Bartos: Will IGCTs become superior over IGBTs in traction AC drives? 615–622

Due to recent surprising advances in semiconductor technology, several types of power switching devices are nowadays available for the fabrication of voltage source inverters. Next to the well-known IGBTs, the Integrated Gate-Commutated Thyristors (IGCTs) seem to have now become a viable alternative. The present contribution intends to examine the principal differences and discuss main practical properties of the two devices, which is essential for a technically sound design of modern voltage source inverters.