Wyniki 1-2 spośród 2 dla zapytania: authorDesc:"Milan ŠEBOK"

Analysis of transformer condition by frequency and time methods DOI:10.15199/48.2019.09.08

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Maintenance diagnostics of transformers considering to the influence of short-circuit currents during the operation should be carried out to increase the reliability in real trouble-free process. The short-circuits in operation are commonly created by different line faults, etc. in mechanical damage of insulation, an electric insulation, an electric insulation breakdown on over voltage, wrong operation and in the next case row [1]. The short-circuit currents cause a progressive serious disrepair of the transformer, because there are high currents in it which are awfully rising winding temperature, what it can cause damage their insulation. Much more danger is high electro-magnetic forces, which can be the reason for the devastation of transformer. Considering a significance of power three-phase transformers in the electric system, their price and possible damages arising in accidents, it is necessary to pay attention to higher prevention of these devices. Windings of the transformers should be designed to avoid various mechanical or thermal deteriorations caused by short-circuit currents occurring in operation. Besides the permanent deformation effects of shortcircuit currents, there are also gradual aging process of the electrical device, which can worsen its mechanical properties. Heat shocks can cause decrease of mechanical strength of transformer and consequent unexpected damage of transformer during the operation. To prevent a damage state of transformers, it is performed different types of the measurements that should illustrate an actual condition of the measured equipment. It is therefore important to choose a suitable diagnostics for the right prediction of such conditions. The theory of mechanical forces effect on transformer winding during short-circuit For a better comprehension this relation between transformer damage and short-circuits currents effects, it is needed focused on mechanical forces effect on tr[...]

The delay line with a surface acoustic wave for an oscillator of electric signals in some sensors DOI:10.15199/48.2018.03.38

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As selective elements of oscillators of harmonic vibration besides the band-pass filters are also used delay lines (DL) and resonators as a perspective a coustic-electric components based on surface acoustic waves (SAW). From the stability point of view we can classify these oscillators among the ones with volume acoustic waves and LC oscillators. In the article, the DL with SAW for single - mode oscillators. The oscillator with delay line The basic principle of function of oscillator with delay line is represented on the next figure (Fig. 1). The delay line (4) with SAW plugged in the feedback of amplifier (2) is a basic element. Circuits (1, 3) serve to match impedance of the abovementioned to the impedance of the electronic circuitry. Fig.1. Bloc diagram of oscillator. Because the previous signal from the input to the output of DL is delayed, examined oscillators are ones with delayed feedback. The theory of these oscillators is well known in the literature [1]. Different situation is in the case of DL with SAW, where DL themselves have narrow transmission band and their parameters differ fundamentally in this range. Physical distinctiveness of functioning of listed components which are related with phenomena of excitation, extension and reflection of SAW cause that in addition to delay, the DL´s have specific frequency dependencies of input and output admittances. Their replacement with the lumped elements RLC circuit or broad - band DL with outer selective LC is mentioned only very general. The oscillator with mismatched delay line The used delay line with SAW as a selective element of oscillator can be symmetrical or non-symmetrical. Symmetrical DL is characterized by the same input and output IDT while non-symmetrical DL has the input IDT with small number of electrodes (broadband) and output IDT with big number of electrodes (narrowband). In the contribution we will only deal with the symmetrical DL. For simpl[...]

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