Abstract: A method includes: step 1: configuring a detection state to be a pure induction mode with no voltage, keeping this mode for t1ms; step 2: configuring the detection state to be a mode of positive and negative saturation excitation square waves, keeping this mode for t2 ms; step 3: when a duration of each mode ends, outputting a characteristic quantity based on sampling data through an algorithm module, and processing the characteristic quantity by a software to complete a state determination; and step 4: in case of a sudden large current, adjusting the detection state to a new detection mode, to respond to the sudden large current, and the software performing process after data collection is completed and a process performed by the algorithm module is completed.