ACTA TECHNICA CSAV |

The corrosion behaviour of Ni, Ni-Cr and Ni-Fe coatings deposited by a water-stabilized plasma gun was investigated.

The experimental methods included

- measurement of potentiodynamic polarization curves of as-sprayed coatings (i.e. in contact with the substrate)
- accelerated corrosion test of self-standing deposits, i.e. coatings separated from the substrate.

Significant characteristics derived from the polarization curves, such as the critical passivation current density and the passivation potential, are quite different from those obtained previously in studies of high-alloy steel coatings, whereas the corrosion rates are of the same order of magnitude. The results indicate that the plasma deposited coatings of nickel and its selected alloys are suitable for corrosion protection if their thickness is sufficient to avoid through-porosity and if provisions are made for good adherence of the coatings to the substrate.

We address a sample axisymmetric Maxwell problem involving radiation of a thin loop antenna, wrapped around a metallic cylinder, into a conductive medium. This class of problems finds application, for example, in the construction of automatic drilling devices in oil industry. After replacing the antenna with a point source, the problem is transformed into a Bessel equation and solved analytically in terms of Hankel functions. The derivation is presented quite in detail since details of the procedure are often neglected in the literature. In the end we describe the way public-domain Fast Fourier Transform (FFT) routines can be used for the evaluation of the results (typically rescaling of transformed quantities is necessary). A few numerical results are presented.

Systems with harmonically varying components of linear damping coefficients are investigated. The possibility of initiating a combination resonances is considered for the case of a linear positive damping. Contrary to the systems excited parametrically by varying the stiffness of springs, the combination resonances can occur at the parametric frequencies which are close to the sum and difference of two natural frequencies of the abbreviated system. As an example, a system with two degrees of freedom is analysed to show the necessary conditions for suppressing self-excited vibration through parametric anti-resonance.

The paper presents closed-form expressions for the harmonic components of the space-vector modulated waveforms both in the B6 and B4 inverters. The analysis is based on space-vector decomposition technique for finding the symmetrical vector sequences. From the Laplace transform of the symmetrical voltage vectors we can derive relations for the harmonic spectrum. The harmonic spectra in the B6 and B4 inverters are compared and verified with the measured results. As shown, there is an excellent agreement between the analytically calculated waveforms and the experimentally determined ones.

A tuned damper (dynamic absorber) consisting of a linear part and
one-sided or both-sided stops is attached to the main oscillating system
with one degree of freedom, acted upon by i) external periodic force
*F*_{0} cosw*t*, or ii) self-excited element characterized by
the Van der Pol expression (-*b*_{se}+*d*_{se}*x*_{1}^{2})*x*_{1}.
The nonlinear forces aroused in the stops are expressed by the Hertz formula
*F* = *k*_{h} *y*^{3/2} completed by the term describing the loss of
energy during impacts. The efficiency of one-sided and both-sided stops,
the distance of stops, the tuning of a linear part of the absorber,
the stiffness of the Hertz stops and the damping in these stops for
both forced and self-excited systems are investigated via
the numerical simulation of differential equations of motion, to
obtain the response curves of forced vibrations and bifurcation diagrams
for self excitation. Time history, phase plane trajectories and the Poincare
mappings complete the response curves and bifurcation diagrams.

A mathematical model based on partial differential equations allows studying the behaviour of a switched reluctance motor in various operational modes. Look-up tables containing measured values were used to express non-linear functions of phase currents and torques. Though a position sensor on the shaft is usual for correct switching the closed loop control, a sensorless control may be applied in some cases. Here the position identification was realised by an additional 2kHz signal, while the initial rotor angle was determined by a short voltage pulse. Time responses of main quantities were gained by simulation via the MATLAB programme.

Magnetohydrodynamic devices for transport, feeding or stirring of liquid metals are often used in special industrial technologies. One of such devices is an electrodynamic feeder for accurate dosing of molten non-ferrous metals. Design of these relatively expensive devices has to be based on reliable numerical models. Simulation of their behaviour in various operational regimes (heating, pumping) represents a complicated coupled task characterised by interaction of nonstationary electromagnetic, temperature and flow fields. The paper deals with the static characteristics of such a device and possibilities of their influencing. The theoretical analysis is supplemented with an illustrative example and discussion of the results.