A new monitoring system able to give information on the insulation aging of an AC motor is presented. The principle is based on experiments on earlier aging of many specimens of a standard magnet wire under thermal and electrical stresses. These measurements show a correlation between the breakdown voltage, the probability of failure at rated voltage and the change in capacitance. In motor windings, the turn to turn capacitance increase yields changes in the high frequency resonances. The new system consists in detecting small variations of these resonances on a working machine fed by the grid or by an adjustable speed drive