No. 1, 1998


Josef Adam, Ivo Dolezel: Magnetic field of a leakage transformer, 1–18

The paper contains an analysis of the 3D magnetic field in an unbounded region generated by the leakage transformer, whose windings located on two neighbour cores are counterconnected in order to eliminate the magnetization current. Results of measurements are compared with calculations carried out by commercial program OPERA 3D in the selected bounded area in the form of graphs and histograms of particular components of the magnetic flux density. The agreement is very good; slight differences caused by chosen discretization of the real arrangement occur only in the vicinity of the field winding. An excellent agreement also shows the calculated short-circuit impedance of the non-coaxial windings, determined from the field energy.

Josef Cadek, Kveta Kucharova: On the simultaneous particulate reinforcement and dispersed particle strengthening in creep of aluminium and aluminium solid solution alloys, 19–35

Simple arrays of silicon carbide particulates, fine alumina particles and both silicon carbide particulates and fine alumina particles in aluminium are assumed to qualitatively analyse the behaviour of dislocations in high temperature creep of Al-SiCp composites, ODS Al alloys and ODS Al-SiCp composites (the subscript p stands for particulate). The analysis suggests that the ODS Al-SiCp composite is, as to its threshold creep behaviour, similar to that of aluminium heavily strengthened by fine alumina particles, i.e. ODS Al, in two respects. First, the rate controlling process of bypassing particles as well as particulates by climb of dislocations is, most probably, due to the athermal detachment of dislocations from the ``interacting'' obstacles, i.e. alumina particles as well as silicon carbide particulates. And, second, the load transfer effect does not play any significant role in the creep behaviour of heavily fine alumina particle strengthened and silicon carbide particulate reinforced aluminium. Instead, another process contributing to the creep strength of SiC particulate reinforced ODS aluminium is assumed to appear. A possible nature of this creep strengthening process is discussed. In particular, an effect of SiC particulates as well as fine Al2O3 particles on the diffusion controlled recovery is considered. However, in heavily dispersion strengthened composite matrix the dominant amount of creep strengthening is due to the threshold effect.

Marcus Alexander: Flicker caused by wind turbines with cage induction generators directly connected to the grid, 37–50

Unlike the generation of electrical energy in thermal power stations, however, the uneven power output of wind turbines involves some problems. Especially in areas with low short-circuit power of the public-supply system wind turbines can cause voltage fluctuations the frequency of which amounts to a few Hertz. Today the evaluation of such voltage fluctuations with respect to their effects on light sources and hence on the human visual perception system is done by means of the internationally acknowledged flicker measurement method. This report describes the method used in Germany in order to find the characteristic flicker values of wind turbines with asynchronous generators directly coupled to the grid. Following this, the results of computer simulations for the examination of this phenomenon are presented.

Vratislav Kafka: General mesomechanical concept of modeling inelastic deformation, continuum damage and localization, 51–75

The author's mesomechanical concept of modeling inelastic processes in statistically homogeneous and isotropic materials is generalized by including description of structural changes and influence of temperature on phase transformation in shape memory materials. A very general form of the model is presented and a number of special models for different materials and loading conditions are shown to follow from the general model as special cases. Particular attention is paid to mesomechanical description of continuum damage and localization.

Ludek Schreier, Petr Medacek: Interruption of stator currents as the cause of unstable operation of three-phase machines, 77–94

The present paper deals with analysing the relations in three-phase machines in the case when stator currents are cyclically interrupted through the agency of a feeding inverter. The process can influence properties of the machines considerably. This is the reason why the equations derived for the stator circuits proved to be substantially different from those when all phase windings are carrying current. Since the rotor disposition remains untouched through the interruption of stator currents, the equations may be applied to various types of three-phase machines. As an example, we solved the case of an induction motor which is fed in such a way that allows for the so-called soft starting. The properties of the motor turned out to be substantially modified in comparison with the case when the motor is fed by harmonic voltage. The motor tends to unstable operation which manifests itself by fluctuations of currents, torque and revolutions.

Zygmunt Piatek: Impedance of overhead transmission line with ground return, 95–107

An analytically-numerical method of self and mutual impedances calculation of circuits with ground return is presented in the paper. An earth is treated as a conductor of a square section area and conductivity g which with Nc-1 overhead line's conductors of conductivity g creates a system of Nc parallel conductors. Between all conductors exist magnetic couplings. Discretizing to triangles as for earth domain as well for overhead conductors domain has been carried out. Therefore the determined impedances take into consideration the skin and proximity effects. In the ending the results of calculations of impedances for the line of one and two conductors have been quoted. The results have been compared with the ones quoted in literature.

Ales Tondl: To the problem of quenching self-excited vibrations, 109–116

The problem of quenching self-excited vibration using parametric excitation is treated. A relatively general system is analyzed theoretically and the conditions, even for full quenching, are presented. Several examples of a system with two degrees of freedom which have been solved on analogue computer confirm the theoretical results.