Life of electric machines is very sensitive to temperature increases especially due to the fragility of their windings as well as to the overvoltage at the motor's terminals. Therefore, it is necessary to find the temperature in the most sensitive areas of the machine. It has been known for a long time that temperature rise is caused by the losses in the machine; thus, various models have been proposed. It has also been shown that the permanent magnet synchronous motor fed by a voltage inverter overheats more than the one supplied by a sinusoidal voltage. Indeed the permanent magnet synchronous motor fed by a voltage inverter deserves special attention. In this paper a new technique based on diffusive representation is used to develop two coupled mathematical models; one for thermal variation in some of the most delicate places of winding and another one for the overvoltage at the motor's terminals. The obtained simulations and experimental results show clearly the validity and the high performance of the proposed models.