| ACTA TECHNICA CSAV |
The paper deals with problems connected with the arrangement of conductors in the rotor slots of large asynchronous generators for the pumped storage hydro-plants with the aim to find the value of the Joule losses in these conductors and evaluate the possibilities of their minimization. After a short description of technical details concerning the design of such rotor windings, attention is paid to the analysis that is performed by both analytical and numerical (realized by professional program QuickField, Tera Analysis, USA) methods. The used methodology is supplemented by an example illustrating the dependence of the losses on the number of conductors in the rotor slot. Although the analytical calculation is based on certain simplifications, discussion of the results shows a very good accordance between both presented kinds of solutions.
The electric field on the surface of a semiconductive coating of high-voltage bushing is analysed, to determine the tangential component of electric field intensity. This component is crucial for the existence of electric discharges. Since discretization of thin layers by numerical calculation is much complicated, the problem is solved via a circuit model, using the two-port theory. The solution can be applied to a bar of a stator winding of turboalternators, in the area of coil ends. The method can also be used by examining the electric field on polluted outer insulators.
It is shown in the present paper that the diffusion coefficients of carbon in steels at high temperatures can be measured by the use of the chromatograph coupled to a thermobalance. This method is suitable especially in the case of steels with higher content of carbon. The values of D's measured in low carbon Fe-C-Cr steels, or those measured at insufficiently long time period of oxidation anneal, have rather informative character only. The measured D values are compared to carbon diffusivities D(bci) calculated on the base of Wagner's type model. The measured and calculated diffusion coefficients agree mutually in the 47% difference frame and reveal the same dependence trend on the chemical composition of steels.
An example of creep behaviour of a simple discontinuous aluminium matrix composite, namely Al-30SiCp composite (p means particulate) processed by powder metallurgy is given. The creep behaviour is described phenomenologically. Anomalously high values of the apparent activation energy of creep and the apparent stress exponent of minimum creep strain rate are shown to follow from the true threshold stress decreasing with increasing temperature more strongly than the shear modulus. The threshold stress effect and the load transfer effect are discussed. The likely origin of the true threshold stress and, especially, of its strong temperature dependence is identified. Disappearance of the true threshold stress at high testing temperatures is interpreted in terms of transition from athermal to thermally activated detachment of dislocations from fine alumina particles in the composite matrix. Finally, the creep behaviour of discontinuous composites with aluminium matrix strengthened by additional fine particles is illustrated by some examples and briefly discussed. It is shown that specifically the oxide dispersion strengthening of aluminium matrix generally results in rather dramatic increase in creep strength of the composite as characterized by the minimum creep strain rate.