ACTA TECHNICA CSAV |

The paper deals with numerical solution of electromagnetic stirring of molten metal in a cylindrical crucible furnace. The task is formulated as a weakly coupled electromagnetic-thermal-hydrodynamic problem. Its mathematical model consists of the Helmholtz equation for the vector potential describing the electromagnetic field distribution, Poisson equation describing the steady state temperature field and Euler equation describing the linear hydrodynamics of the considered process. The theoretical analysis is supplemented with an illustrative example (electromagnetic stirring of molten aluminium) and discussion of its results.

To assess the applicability of plasma deposited coatings made of typical high-alloy steel for corrosion protection, the properties of steel CSN 17 346 (equivalent of AISI 316) were studied.

Plasma spraying was conducted by a water-stabilized plasma gun. In addition to coatings deposited under standard spraying conditions, deposits with slightly reduced porosity and oxide amount and, on the contrary, others with high porosity and extremely high oxide content were tested.

Two experimental methods were used:

- measurement of potentiodynamic polarization curves
- accelerated corrosion test.

From the polarization curves, the values of the critical passivation current density and the passivation potential were determined. The accelerated corrosion test yielded the weight gain in each test cycle.

The results show that the deposits are less resistant than bulk 17 346 steel. This appears to be mainly a consequence of coating porosity and chromium depletion of steel during its plasma spraying. On the other hand, the electrochemical and corrosion characteristics of the coatings are much more favourable than those of bulk unalloyed low-carbon steel. Acetylene shrouding (or, in general, shrouding by a reactive gas consuming oxygen which penetrates from the atmosphere into the plasma jet) reduces both the porosity and the oxide amount in the deposit and improves slightly its corrosion behaviour.

The velocity distribution function of free electrons in a partially ionized gaseous medium is studied as a solution of an initial-boundary value problem for the linearized Boltzmann equation with a source term. Presence of external electric field is assumed. Global existence, uniqueness, and continuity of the solution are established under specified hypotheses.

A mechanical system consisting of two subsystems which are nonlinearly coupled is considered. The coupling terms in the differential equations of motion are of the second order. One subsystem is parametrically excited, the other being excited only through the nonlinear coupling. The vibration in the parametric resonance is analysed and the effect of nonexcited subsystem on this resonance is investigated.

For a circuit with ground return consisting of a long rectilinear overhead conductor the vector magnetic potential is introduces. Transformation of the differential equations describing the magnetic potential by means of Fourrier's transformation yields the vector potential in the form of an analytical formula. Next, the magnetic field strength represented by improper integrals is determined. Appropriate representation of those calculations reduces them to the calculation of Laplace's transformation, thus yielding analytical formulae describing the magnetic field in the circuit with ground return.

The impedance of a circuit with ground return for a two-layer overhead conductor is represented as the total of external and internal impedances. The external impedance is determined using the vector magnetic potential to compute the induced electric field strength. The internal impedance of the conductor is determined using the solution of the Hemholtz's equation for electric field strength.

Finally a computer simulation in Delphi of the discussed question is presented.

The paper reports on the comparison of numerical (finite element method) and experimental (holointerferometry) results achieved in the study of bending waves propagation in the thin elastic clamped square plate. The impact loading was carried out by the focused ruby laser beam. The agreement between the numerical results and the measurement is very good.

The paper deals with issues of observing the dynamic system when the system status variables necessary for controlling it are unavailable. Described is realization of utilization of the artificial neural network principles as the identifier of status variables necessary for the system control. Simulation results are included to add-on to the theoretical discussion.

The objective of this paper is to provide an overview of ferroresonance phenomena in power systems, their modeling aspects and method of solving the problem. In particular, we will present symptoms of ferroresonance. A special tool for ferroresonant circuit identification and a case study involving laboratory serial ferroresonant circuit with a voltage instrument transformer with solutions to avoid ferroresonance is presented.

The elementary layers' method is one of the methods of taking into account cage parameters change caused by a current displacement. The method consists in creating hypothetical layers along lines of slot leakage field. Physically, a multi-cage construction with a common connecting ring is obtained. It is necessary to consider this phenomenon in an asymmetrical supply, as it is in the case of a single-phase motor. In a paper, a model of a single-phase motor with running capacitor and drop shape slotted cage rotor is presented.

The layers were chosen according the rules for circular and trapezoid cross-section.