ALP300 protector (hereinafter referred to as protector) adopts the latest single-chip microcomputer technology and has features of strong capacity of resisting disturbance, stable and reliable working, digitization and intellectualization and so on. The protector can protect the motor during operation in case of overtime starting, under-voltage, over-voltage, over-load, open phase, unbalance, under-load, ground/electric leak, blocking, short circuit and external faults and so on. The field maintenance personnel may rapidly find the fault cause with the digit display. The protector has RS485 remote communication interface and DC4～20mA analog quantity output to compose the network system with PLC and PC control units and to achieve remote monitoring of motor operation.