No. 2, 2003


Miroslav Chomat, Ludek Schreier : Compensation of unbalanced three-phase voltage supply in voltage source inverter, pp.113-128

A control method for the compensation of unbalanced voltage supply that eliminates pulsating current in the DC link of a voltage source inverter is investigated. Such a pulsating current can have detrimental effects on operating properties of the drive as well as on the quality of power supply in the electrical vicinity of the inverter. A feasible range of control parameters is determined based on the required level of immunity against unbalanced voltage supply and on the parameters of the system. The impact of the control method on the power factors and current amplitudes in individual input phases for the available range of control parameters is analyzed and discussed. Examples of numerical simulations illustrate operation of the system under investigation both with and without the compensation method used.

Ondrej Kovarik, Jiri Nohava, Jan Siegl : Mechanical properties of plasma sprayed alumina coatings, pp.129-145

Mechanical properties of plasma sprayed grey alumina on low-carbon steel substrates were investigated. The investigation of the properties of composite material ``coating-substrate'' included the measurements of Young's modulus of the coating, microhardness profile, residual stress on the top of the coating, residual stress profile in substrate and fatigue behaviour investigation. Fatigue samples were periodically loaded as a cantilever beam on a special testing machine. Failed samples were observed in SEM to determine failure processes in the coating. The Young's modulus of the coating was measured by the four-point bending method. Samples were tested both in tension and compression at low (300 N) and high (800 N) loads. Our experiments revealed that the average fatigue lives of coated specimens were nearly 2 times longer than those of the uncoated specimens. The Young's modulus of the coating varied between 27 and 53 GPa with an average value of 43 GPa. Loading in tension caused decrease in Young's modulus of the coating while loading in compression lead to increase in Young's modulus. Increase in the lifetime of coated samples was likely due to compressive residual stresses in the substrate, originating from the spray process. Failure of the coating consisted of several processes, among which the most important are splat cracking, splat debonding and coalescence of cracks through the voids in the coating.

Viktor Valouch : Linearized model of electrical ARC furnace suitable for analysis of flicker mitigation, pp.147-156

Hitherto, the flicker has been compensated for mainly by using Static Var Compensators (SVC). Recently, some studies and tests have been done to prove advantages of active filters (AF) over SVC in this field. The Unified Power Quality Conditioner (UPFC) consists of the parallel and series AF, so it offers more possibilities to solve instantaneous active and reactive power fluctuation than either the parallel or series AF alone.

For the analysis of the control systems of AFs and UPQCs, and synthesis of its parameters the frequency domain approach or transfer functions are usually used. For some simulation studies the Cassie-Mayr formula describing the function of EAF was successfully used. Unfortunately this representation is not linear.

It was found that the simulation of the EAF is possible even in the frequency domain by using a linearized model based on this formula. Based on equations presented a linearized model of the power system with the EAF and UPQC has been developed and analysed in the frequency domain.

Viktor Pekarek : New aspects of thermodynamic affinity between solids and gases, pp.157-174

An innovative approach to solid-phase thermodynamics allows drawing a narrow comparison with traditional apparatus of gases, towards establishing conjoint thermodynamic relations. The correlation between the adiabatic exponent in gases and the coefficient of transverse contraction in solids bridges the gap between the two phases, to explain the mechanism of heat dissipations. The theory interrelates elastic and heat parameters and confirms the validity of Mayer's and Poisson's equation in solid phase.

Vaclav Nenicka, Jan Hlina, Jiri Sonsky, Ludek Krejci : Application of the correlation analysis to identification of coherent structures in free plasma flow, pp.175-201

The work presents the results of identification of structures with vortex and dissipative nature in free plasma flows for flow rates 20-100 l/min and electric currents 120-200 A. For the identification of such dynamic behaviour we used a sequence of 8 CCD images scanned within the time interval of 100 msec. The analysis is based on calculating correlation coefficients, that respect radiation dynamics of the system in the neighbourhood of the point under consideration. Comparing the time response of matrixes of correlation coefficients for the sequence of seven images, we can study evolution of dynamics via a two-dimensional image of a three-dimensional free flow of the jet. The construction of the vectors underlying the calculation of correlation coefficients allows us to include the dynamic system of free flow radiation for a certain approximation of a multi-point system.

Jiri Nohava : Wet slurry abrasion tests of ceramic coatings deposited by water-stabilized plasma spraying, pp.203-214

Wear resistance of plasma sprayed ceramic and metal coatings was studied. The gray alumina, chromia and metal coatings were produced by water-stabilized plasma torch WSP and the influence of the deposition method on the wear properties of the coatings was evaluated. Two methods for determination of wear resistance of WSP sprayed coatings were combined: wet slurry abrasion test and scratch test. Scanning electron microscope observations of the damaged samples were used to determine the main wear mechanisms and to justify superior wear resistance of chromia coating.