Abstract: In order to solve the over-current discriminate problem between the short circuit fault and motor starting in coal mine power supply system protection,designed a phase-sensitive detection protector based on the function of signal input capture in the timer of the STM32 chip.Through the experiment and analysis,the power factor discriminate value of asynchronous motor starting was set 0.4~0.5.The phase relationship of system voltage and current input to the timer pin as square wave converted by hardware circuit.The timer capture the adjacent edge of square wave get level last time,acquire the level duration of the phase difference between voltage and current signals,obtained the system power factor.Tested by the experiment,the protector can accurately measure the power factor of system and use discriminate value realizes the discretion of short circuit fault and the normal starting of motor.

Key words: short circuit fault, motor starting, input capture, STM32 chip, phase-sensitive protector, power factor