ACTA TECHNICA CSAV |

Operating regions of an adjustable-speed power unit with a doubly fed machine connected to the power grid are determined. The control range of the rotor voltage phasor is determined by means of analytical relations, considering maximal permissible values of currents and voltages of both the machine and supply cycloconverter. The results of simulation are shown to illustrate operation of the system.

High-speed superexpresses (HSST) developed by Japanese airlines (JAL) are based on the electrodynamic principle of magnetic suspension. A superconductive winding placed in the vehicle induces strong magnetic field. A drive realized by linear synchronous motor with ``long stator'' (LSM) provides its movement. The track contains short-circuited coils and interaction between them and superconductive coils in the vehicle produces its suspension. The paper includes a mathematical model for traction electrodynamic suspension device HSST represented by a system of linear differential equations with coefficients varying in time. Numerical analysis of this model yields the velocity-dependent lift and drag forces acting on the system. The time distribution of the lift force exhibits certain oscillations that may be suppressed by suitable placement of several superconductive levitation windings in the vehicle. The results obtained are in a good agreement with the knowledge found by various authors on prototype vehicles. The presented analysis of dynamics of the traction system based on electrodynamic suspension is important for investigating service properties of the HSST systems, securing its continual run, suppressing parasitic phenomena (i.e. elimination of vibrations) and optimization of its overall design.

Thermally sprayed ceramic coatings contain a high density of microcracks which are responsible for small Young's moduli, nonlinear behaviour at higher compressive or tensile stresses, and for low fracture toughness. The extension of an initial surface crack in the direction to the interface, where the crack is repelled by the metal substrate with higher Young's modulus, is studied using the methods of fracture mechanics and finite elements. It is shown that high tensile stresses induced by the crack in the interface can lead to a local decohesion along the interface so that the crack can continue and deviate into the interface.

Computer modelling of assembly and disassembly of structural parts that are coupled with interference fit by means of induction heating is a rather complicated task. Besides three physical fields, we have to solve also the thermocontact problem. The paper solves the task in quasi-coupled formulation as an electromagnetic-thermoelastic problem in 2D arrangement. Its solution is based on time stepping with built-in internal iterative processes for improvement of accuracy of parameters of linearized equations describing all three involved fields that are in each time step solved by the finite element techniques. The suggested methodology is illustrated on an example of fixing and disassembly of a drill in a chuck. Considered are real characteristics such as radial interference between the drill shank and chuck, power of the inductor, physical parameters of used materials, conditions of cooling and other quantities responsible for reliable operation of the system.

The paper analyzes the behavior of a basic one-mass self-excited system with a dynamic absorber. The amplitudes of absorber's mass are constrained by two soft stops situated symmetrically to the equilibrium position. The self-excitation of the basic system is due to the vortex shedding.

The soft stops are characterized by additional stop stiffness and by the damping that is supposed to depend on the stop deformation and on the signum of relative velocity of absorber's mass to the results of numerical simulation of the differential equations of motion. The effects of varying system's parameters are investigated and the efficiency of the dynamic absorber is evaluated.

The paper deals with the problem of solving differential equation set for nonlinear electromechanical converter with nonlinear magnetic circuit. It is important to present the model which enables taking into account the saturation effect. As an example an induction motor has been considered.

Some questions associated with development of procedure to determine inaccuracy that appears in analog-digital conversion of random signal are discussed. Equations for calculation of metrological characteristics of measuring channel (MC) of expert-diagnostic system were obtained. These equations are based on application of stochastic linearization method.

Practical realization of even order elliptic filters,
including low-pass, high-pass, band-pass and band-stop units, is proposed using the
biquadratic OTA-C and OTA-RC structures of type *a* and *b*. The calculation of
particular elements, arrangement of the block diagrams and resultant frequency
characteristics are presented.