Abstract: In order to improve the reliability of dual Y shift 30 degrees permanent magnet synchronous motor, a logic dynamic model of the motor is constructed based on the twelve-switch inverter. On this basis, an inverter open-switch fault diagnosis method is proposed. As the measurement of the traditional motor phase voltage increases potential fault source of the system, a second-order sliding mode observer with no voltage sensor is proposed to estimate the motor voltage in real time. The voltage prior information provided by the logic dynamic model is compared with the actual output value of the voltage containing the fault information, and the fault information is extracted and the fault switch is accurately positioned. The validity of the method is proved by simulation data.