The interest in high speed electrical machines for automotive applications has increased recently. This includes electrically-assisted turbochargers (EAT), air compressors for fuel-cells and waste heat recovery (WHR) generators. For these kinds of application working in elevated temperature environments, the losses in the machine and in the power electronics should be carefully analyzed. In this context, this paper presents a coupled electronic and magnetic simulation and its application on a high-power high-speed motor. The coupling between the two models is mainly linked to the phase currents and their impact on the electronic driver. In addition, motor performances must be considered to optimize the overall system efficiency. However, optimizations of electronic driver and electric motor are very time-consuming since they are based on finite element methods for magnetic simulations and on Kirchhoff equations resolution for electronic simulations. Therefore, this paper presents a fast coupled model to increase the computation speed and its application to optimize a system using a high-power high-speed motor with special focuses on motor rotor losses and overall efficiency.