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The stator insulation breakdown is a major cause of ac machine failures. Ground insulation defaults are easily detected by classical systems based on leakage current measurements, however the turn-to-turn insulation degradations are more difficult to detect. For large machines, on-line methods, based on partial discharge detection and analysis, give good results but they cannot be used for low-voltage machines fed by adjustable speed drives (ASD). This paper presents a new monitoring system able to detect slight variations of high frequency resonances in the winding of a working machine fed by an industrial inverter. The new system works in association with statistical data on accelerated aging of the magnet wire used to build the machine.