ACTA TECHNICA CSAV |

A numerical model of an adjustable-speed power unit utilizing a doubly fed machine with a 12-pulse cycloconverter in the rotor circuit was developed. The synchronous frame vector control scheme with impressed rotor voltages was employed to control the produced active and reactive powers of the unit. Both steady-state and transient operation of the system were numerically simulated and analysed. First, the behaviour of the system at a constant speed was investigated to evaluate the quality of the control. Second, the operation of the system connected to the power network through a long power line was simulated. The machine was driven by a Francis turbine regulated so that its speed was optimal with respect to the demanded active power.

The paper presents harmonic analysis of the voltage waveforms of the four-switch voltage source inverter feeding an induction motor drive.The four-switch inverter uses space-vector modulation strategy.

This proposed method is based on the Laplace transform approach of the periodic waveforms, and all the calculations can be carried out into effect straightly by algebraic method with high accuracy. Using the relation between the Laplace transform of the periodic waveform and Fourier coefficients we can derive the coefficients of the Fourier series. From the analytical relations we can evaluate harmonic spectra for the different space vector modulation techniques.

A considerable increase of current Young's moduli of plasma-sprayed ceramics with increasing uniaxial pressure, due to elastic closing of microcracks, has recently been proved experimentally. This effect is modeled within the framework of the anisotropic nonlinear theory of elasticity and a qualitative agreement with experiments is shown.

Within the general framework of Method of lines, numerical solution of the nonstationary compressible Euler equations in 1D, 2D and 3D is splitted into three steps: First, space discretization is performed by the first order finite volume method using several approximate Riemann solvers. Second, smoothness and Lipschitz continuity of RHS of the arising system of ODE's is analyzed and its solvability is discussed. Finally, the system of ODE's is integrated in time by means of implicit and explicit higher-order adaptive schemes offered by ODE packages ODEPACK and DDASPK, by a backward Euler scheme based on the linearization of the RHS and by higher-order explicit Runge-Kutta methods. Time integrators are compared from several points of view, their applicability to various types of problems is discussed, and 1D, 2D and 3D numerical examples are presented.

Two examples of parametrically excited chain systems are considered. On the first chain system it is shown that only parametric resonances of the first kind and no combination resonances can occur. From this it follows that for such a system when being self-excited the self-excited vibration cannot be favourably influenced by parametric excitation. On another chain system the conditions for a favourable effect of parametric excitation are presented.

Three problems are dealt with: stress propagation, subsidence trough
formation and diffusion process. According to the experimental evidence, those
problems may be treated as being of a stochastic nature. This approach
represents an alternative when solving the above soil mechanics problems.
Diffusion, well known when solving the primary or hydrodynamic consolidation,
seems to draw a special attention. It intervenes with loose soils and at
higher stress levels (*i\. e.*, with media of highly dissipative nature)
and is much more widespread then controlling the pore water pressure
dissipation. Further research in this interesting field of behaviour is needed.

Stress wave propagation in thin wall structures is of interest in many areas of applied mechanics. The advanced ultrasonic non-destructive testing of shell structures is conditioned by understanding of waves propagating in thin-wall solids.

Ultrasonic waves were generated by focusing the pulse laser beam on the surface of the thin cylindrical shell. The full-field visualization of the displacements caused by generated waves was carried out by double-pulse holointerferometry (DPHI) with a ruby laser as a source of light.

The second part of the paper is focused on the study of influence of the shell curvature on the wave propagation. This research was carried out by recording the time history of the ultrasonic waves in different points on the shell surface by the pair of miniature piezoelectric transducers.

The aim of the paper is to contribute to ultrasonic non-destructive testing of thin shell structures.

Power systems under ferroresonance are studied by the nonlinear dynamics approach. The Lyapunov exponents are used to identify the regions in parameter space with various types of dynamical behaviour. Transition to chaos is investigated in detail for a dissipative system, and it is shown that the ferroresonant circuit is led to chaos by the Feigenbaum route to chaos. In the case when a transformer is considered virtually lossless, a ferroresonant circuit cannot be considered as dissipative, but conservative, and the system exhibits a quasi-periodic trajectory for a certain initial condition, and chaotic trajectory for an other initial condition. The influence of harmonics on chaos in a ferroresonant circuit is also investigated.