The utility model relates to an open -phase protector for motor comprises condenser sampling reduction voltage circuit, diode rectifier circuit, relay and ac contactor. The utility model discloses reliable operation, longe -lived, low power dissipation, the various abominable operational environment of adaptation, open -phase protection ware circuit is simple, easily the preparation, low price, installation wiring convenience, can not bring negative effects for the motor, thoroughly solve the motor caused the damage because of the loss of phase problem.