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Using adjustable speed drives employing fast switching components, to supply low voltage asynchronous motors may, in some cases, be detrimental to their reliability. The premature failures of the insulating materials are often attributed to the existence of partial discharges (PD) in the winding. Nevertheless, a clear evidence of PD existence in such systems is far from being established. Due to the voltage waveforms, their location is not in the turn-to-ground or phase-to-phase insulation but in the turn-to-turn insulation and the classical AC test is no longer applicable. The aim of this paper is to present an off line test procedure and its results able to solve this problem.