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The activities of the preliminary stage of a mediumterm research program aimed at studying the aging processes in the wire insulation of electrical machines supplied by inverters are illustrated. The experimental work allowed to select reference testing procedures and produced the results of tests carried out on twisted pairs of enamelled wires at different temperatures, sinusoidal voltages and frequencies.While the research plan regarded such results just as reference for future comparison to the outcome of steep waveform voltage tests, the available data allowed also to formulate, with reasonable accuracy, a phenomenological model of the aging processes acting in the tested insulation. Such a model could facilitate the implementation of the above research program next stages and appears worth to be extended to real winding insulations.