The paper reports on measurements of thermally stimulated depolarization current (TSDC) that is applied to characterize changes in medium voltage rotating machine enamelled wires that have been exposed to ageing under a pulse width modulated (PWM) waveform. An elevated temperature polarization process is used which provides various information on the relaxation processes in multiple coated magnetic wires such as the disorientation of dipoles and the release of charges from trapping sites. The thermoelectrets were prepared at various polarizing fields and measurements of TSDC are carried out both as a function of time and temperature over the range of 23 to 2000C. The various trapping levels, which have discreet energy differences having distinct peaks in the thermograms, are discussed on the basis of space charge polarization.