An apparatus for controlling an air distribution system by controlling the speed of a motor associated with a blower to maintain a rate of air flow in the system at substantially a target air flow rate. The apparatus comprises means for controlling a supplied frequency to the motor; means for providing a speed signal representative of the speed of motor; means for providing a frequency signal representative of the supplied frequency from a controller to the motor; means for providing a load signal representative of the load of the motor; means for calculating an air flow rate signal based on the speed signal and the load signal; means for controlling a supplied frequency in response to the target air flow rate signal and the calculated air flow rate signal. The apparatus can apply three different types of algorithm for controlling and compensating the motor.