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Presently, condition monitoring and fault diagnostics in the three-phase voltage-source inverter (VSI) system are already considered essential to improve maintenance efficiency and increase reliability levels. This paper presents a new open-switch fault real-time diagnosis method in the VSI based on the phase currents. First, on the basis of real-time measured two phase currents, the sum of the absolute value of the normalized phase currents is calculated, whose average value is less relevant to the transient condition such as load variations and current frequency variations. Second, the current zero-cross detection method is employed to avoid the influence of current distortion caused by the transient condition. Then, the fault detection signal is built to precisely localize the faulty switches, and the real-time fast diagnosis for three-phase VSI with the open-switch fault is realized. Finally, the experiment results verify the reliability and effectiveness of the proposed method.