Lifetime prediction involves various aspects of uncertainties due to limited mission profile data, lifetime models, electro-thermal modeling errors, etc. The predicted lifetime results may have considerable discrepancies with different assumptions of the uncertainties. This study aims to investigate the impact of different lifetime model assumptions for the IGBT switches in a motor drive application with a specific representative operation profile. Moreover, the sensitivity of the IGBT switching frequency and operating grid voltage to the predicted lifetime is also studied. The outcomes serve as a first effort to quantifying the uncertainties in the reliability analysis of power electronic systems for motor drive applications.