PURPOSE: To permit to stop a motor most quickly within a range in which an abnormal load is not applied to the motor by a method wherein the motor is stopped when the temperature of the motor is reduced to a given value and the temperature rise of the motor upon restarting is limited so as not to exceed an allowable value.

CONSTITUTION: When the amount of air is controlled mechanicallly by the compressor and a low load operation is continued, the temperature of the motor 20 is reduced by the self-cooling thereof and an electromagnetic contactor 22 is demagnetized by the operation of a motor temperature switch 36 provided in the circuit of air discharging solenoid valve 23, therefore, a contact 22A is cut the motor 20 as well as the compressor are stopped. According to this method, the motor 20 stops a temperature lower than a given temperature, therefore the temperature of the motor 20 may be restricted to a temperature below the allowable value even if the motor temperature is risen by the restarting. Accordingly, the motor 20 may be stopped most quickly within a range in which an abnormal temperature load is not applied to the motor 20.