PURPOSE: To protect a fan motor system used primarily for air blowing of an air conditioner against overcurrent, overvoltage, and loss of synchronism when a motor is rotated at a speed out of a specified range by an external force.

CONSTITUTION: A fan motor system is provided with a control circuit 9 which drivingly controls a motor 2 by a PWM inverter circuit 11 and is provided with a speed detector means 10 for the motor 2. Also a current is interrupted when the speed of the motor 2 is smaller than that a first speed S1, and passed when it is larger than or equal to a second speed S2. In addition, when it is larger than a third speed, current is interrupted and, when it is smaller than or equal to a fourth speed, current is passed.