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The lifetime of power semiconductor devices strongly depends on their mission profile. The main influence limiting the lifetime are thermal cycles causing thermo-mechanical stress. Using the example of a three-phase voltage source inverter (VSI) feeding an induction (IM) or a permanent magnet synchronous machine (PMSM), this paper will present a thermal control strategy to increase the lifetime of the power semiconductors within the VSI. Especially within traction applications, the occurring operation points are very unsteady and rarely predictable. Therefore a thermal control is designed to dynamically react on changing operation points. The performance of the proposed thermal control using both machine types is proved at a real setup for load cycles similar to traction applications. By measuring the junction temperatures of the power semiconductors, a lifetime estimation is done to compare the different control strategies.