No. 2, 2000


Pavel Solin, Ivo Dolezel, Martin Skopek, Bohus Ulrych: Stationary temperature field in a non-magnetic thin plate heated by transversal harmonic electromagnetic field, 109–128

The paper deals with the computer simulation of the induction heating of a thin non-ferromagnetic well electrically conductive plate whose thickness is very small with respect to its remaining sizes. Heat is produced by the transversal electromagnetic field generated by two symmetrically arranged inductors carrying alternating current of prescribed frequency. As the problem can only hardly be solved by classical numerical methods such as FEM or FDM because of problems with real possibilities of discretisation (co-existence of 2D and 3D subdomains in the investigated region), an alternative method is suggested based on solution of a system of integral equations of the electromagnetic field and balance equations of the temperature field. The task is formulated as a weakly coupled problem (no back influence of temperature on the material parameters is considered) and all calculations are carried out by means of a special user program written by the authors. Theoretical analysis is supplemented by an illustrative example and evaluation of its results. Discussed are also further possible improvements of the presented methodology.

Ludek Schreier: Contribution to DC current braking of induction motors, 129–147

The work deals with some problems of the braking of induction motors by DC current. The attention is paid to the possible influence of the saturation of the iron on the process of the braking. The saturation considerably decreases the values of the static torque in comparison with that determined under the assumption of the constant main inductance. It is shown that this fact only slightly influences the length of the braking period, which is due to the prevalent influence of the non-linearly involved in the machine equations. The decrease of the values of the torque in the dynamic characteristic can prevail substantially over the decrease of the values in the static characteristic that is given by the saturation. Therefore, the time of braking is determined especially by the waveform of dynamic characteristic for the given parameters of the braked motor and the given value of the moment of inertia. In same cases the saturation may decelerate the drop of the speed within the final part of the braking period.

Dusan Balara, Jaroslav Timko: The state-differential method for adaptation of parametric models, 149–161

In some technical applications, parameters of systems are estimated either on-line or off-line using various estimation techniques. Based on similarity with neural network, parametric estimator can be designed, which structure is identical to mathematical model of estimated system. Computational units of estimator may be called neurons and their adaptation rules should be local operating. However, particular neurons may have different nonlinear or inertia transfer functions which complicates derivation of adaptation rules. Application of widely used gradient descent method yields in different adaptation rule for each type of neuron or even for each weight of neuron. In this paper, new adaptation method the State-Differential method for neural parametric estimators is proposed. This method is based on comparison of output error of neuron in two time steps and is independent on transfer functions of neurons. Propagation of error signal to inner layers of neurons is not discussed in this work.

Marian Pasko, Krzysztof Debowski: A method optimizing the working point of three-phase three-wire system with nonlinear load, 163–174

The working point of a three-phase three-wire system with a sinusoidal voltage source and a nonlinear load is optimized imposing an additional condition: we require that the active power consumed by nonlinear loads be the same before and after the optimization. This approach allows for reducing the RMS value of the source currents and for minimizing their deformations caused by nonlinear load. The solution is obtained by Lagrange's factors and by a suitable measurement experiment. The last point is to determine the parameters of the compensator in order that it may generate an active current of the voltage sources. This working point can be obtained by means of the LC, RLC or (RLC, -R) compensator.

Vaclav Bartos: Switching phenomena in asynchronous machines by different stator and rotor resistance, 175–186

The phenomena associated with connecting the stator or rotor of asynchronous machine to a feeding voltage are analysed. In contrast to the standard approaches, the slips for a maximum torque by feeding from the stator- and rotor side are different, since different stator- and rotor resistances are taken into consideration. The solution via the Laplace transformation shows how can the above conditions modify the roots of the characteristic equation.

Ryszard Nawrowski: The thermal fields in nonequilibrium plasma generator, 187–197

The paper presents the mathematical model of the coupling between electric field and thermal field in plasma generator. The equations of momentum, composition and energy are taken in consideration. An algorithm of numerical solutions of nonequilibrium plasma equations is presented. The algorithm is used for the simulation of stationary plasma jet in the divergent nozzles. In the paper the results of calculation of thermal field in argon plasma generator are presented.

Ales Tondl: Self-excited vibration quenching in a rotor system by means of parametric excitation, 199–211

This contribution deals with the possibility of self-excited vibration quenching by means of parametric excitation in a rotor system. The conditions for full vibration quenching are formulated. In Appendix I the stability conditions for a general system governed by differential equations with vector coordinates and complex coefficients are discussed.

Miros Pirner: Long-term monitoring of the response of structures, 213–219

The purpose is long-term monitoring of civil engineering structures and research of thin-walled reinforced structures. A motorway three-span prestressed concrete bridge is outlined. Recording apparatus and measurement methodology are described, including the results of measurement, number of cycles and their sorting.