At the beginning, this article details the manufacturing procedures for varistor materials. Starting from the initial composition of two large series of varistors (those with two additive oxides and those with five additive oxides), there is a major overview of the main stages of the technological process and the equipment used for the production, emphasizing the technological changes that were made. The article continues with the study of the influence of the sintering pressure and the sintering temperature on the electrical properties of the varistor materials made before. There were two experimental series of 7 varistors, one series based on 2 additive oxides and one based on 5 additive oxides. Each varistor of these series was sintered at another temperature, the fundamental purpose being to determine an optimal sintering temperature for each chemical composition. A second activity consisted of manufacturing two more series of varistors with the same chemical composition (2 oxides and 5 additive oxides), which were sintered at two different pressures, for having a set of conclusions on the influence of sintering pressure on the electric performances. All conclusions are underlying a new process for manufacturing metal oxide based varistors.

Metal oxide varistors are applied today inside modern surge arresters for overvoltage protection for all voltage levels. Their main issue is the thermal activation of their crossing current, which could lead to complete destruction by thermal runaway. This article presents a new technological solution developed in order to increase the thermal stability of metal oxide varistors. It consists in connecting in parallel two or more similar varistors (for dividing their current), having a thermal coupling between them (for equalizing their temperatures and forcing them to act together and simultaneously as much as possible). Starting from a finite element computer model performed for each situation (varistor standalone or parallel), up to real measurements, the thermal stability of the equipment was analyzed in permanent and impulse regime. Experiments were carried out in the same conditions. Experimental data obtain from two disk varistors corresponds very well to simulations, proving that parallel connection of varistors, combined with a thermal exchange between them is an efficient technical solution for thermal stability improvement, even if not apparently economically justified.

Due to high mechanical inertia and rapid variations in wind speed over time, at variable wind speeds, the problem of operation in the optimal energetic area becomes complex and in due time it is not always solvable. No work has been found that analyzes the energy-optimal operation of a wind system operating at variable wind speeds over time and that considers the variation of the wind speed over time. In this paper, we take into account the evolution of wind speed over time and its measurement with a low-power turbine, which operates with no load at the mechanical angular velocity ωMAX. The optimal velocity is calculated. The energy that is captured by the wind turbine significantly depends on the mechanical angular velocity. In order to perform a function in the maximum power point (MPP) power point area, the load on the electric generator is changed, and the optimum mechanical velocity is estimated, ωOPTIM, knowing that the ratio ωOPTIM/ωMAX does not depend on the time variation of the wind speed.

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In the paper are being analyzed the magnetic losses that occur in the induction machines of low and medium power while being supplied by inverters realized with IGBT transistors. The analysis method is based on the over-position effect principle. As a working hypothesis is being neglected the saturation effect. During analyzes is being deduced the computing relations which allow the quality and quantity determination of the magnetic losses in the situation of the deforming supplying system. The theoretical results obtained using the computing program CALCMOT are back-up-ed by the experimental checks realized on two tri-phased induction motors with the powers: 0.37 [kW] / 1500 [rpm], respectively 1.1 kW]/1500[rpm].