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Drives and electric motors are sometimes paired in order to improve efficiency and process control or to eliminate gears and reduce maintenance costs. Sometimes the addition of the drive to an existing installation has not been the success that was originally envisioned and substantial derating has occurred or the original motor or drive replaced. One industry standard for product certification requires that there be a 30°C margin in the temperature rise if a motor intended for use on a drive is acceptance tested on sinusoidal power. The intent of this temperature margin is to allow for the additional heating losses due to inverter harmonics. Machine design, especially the ventilation, has a significant impact on the difference in temperature rise due to the increased harmonic losses. This paper will discuss a number of different ventilation methods present in motors. The results of some tests done on machines using the same load on both sinusoidal power and inverters are also presented.