Safe Turn-off Strategy for Electric Drives in Automotive Applications

This paper proposes a strategy to safely turn off an electric drive system in case of a fault without destroying the power module and the electric machine. Classic turn-off strategies like Active Short Circuit (ASC) and Freewheeling (FW) are adopted as state of the art for electric vehicle applications. However these methods cause either high currents in the machine or over-voltage in the DC-link capacitor, the system should be designed to withstand these unwanted effects making them more expensive. The novel method proposed in this study mitigates both over-voltage and over-currents, thereby achieving a smooth transition from torque control to a safe state. The proposed method can be implemented almost cost neutral with respect to the state of the art methods. In this method, voltage-vectors are identified with respect to the position of the current-vector, which can either charge or discharge the DC-link capacitor hence keeping both the DC-link voltage and stator-currents to a safe value. The proposed strategy is analysed through simulations with a combined inverter and machine model. The simulation model includes skin-effect loss models which are essential for the accurate calculation of the DC-link voltage. Measurements were done on an electric drive system to validate the strategy.