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Artificial defects have been introduced during the manufacturing process inside the composite insulation of stator models of medium-voltage motors dedicated to the application of traction. The dielectric analysis at low frequency (10-3-103 Hz) has been conducted at 30°C for each of the seven different models. The sensitivity of the dielectric properties to humidity is illustrated. This is the reason why the measurements were made with the models in a dried state. The dielectric analysis allowed the detection of most of the seven defects sought. These defects have also been identified. The value of the capacitance at 1 kHz appeared in particular as a reliable marker for identification. The observed differences between models with defect and the reference model are qualitatively justified by dielectric considerations.