Constant CFM control algorithm for an air moving system utilizing a centrifugal blower driven by an induction motor

A method of achieving a constant mass air flow rate for a blower motor of the induction type exhibiting nonlinearity between motor rotational speed, motor control voltage, and air flow rate, comprising:

• a) controlling motor speed through a motor controller by varying an excitation voltage to the motor at selected control settings representative of varied excitation voltages;
• b) determining proportionality constants of flow rate to motor speed for various motor speed curves over an operating range of the motor;
• c) providing a graph of motor speed curves plotted against the proportionality constants of step b) and control settings of step a) in a memory accessible by the motor controller;
• d) setting the motor controller at a first setting for a selected constant mass air flow rate;
• e) measuring a rotational speed of the motor;
• f) deriving a proportionality constant for the measured motor speed to determine a control setting corresponding to the selected air flow;
• g) adjusting the RPM setting according to the proportionality constant of step f);
• h) adjusting the control setting to increase or decrease motor speed to achieve the desired RPM setting;
• i) deriving a second proportionality constant when the measured speed of the motor has achieved the desired RPM setting;
• and
• j) iteratively adjusting the rotational speed of the motor by repeating steps e)-g).

Apparatus for achieving a constant mass air flow rate for a blower motor of the induction type exhibiting nonlinearity between motor rotational speed, motor control voltage, and air flow rate, comprising:

• a) a motor controller having a microprocessor for receiving a selected constant CFM rate value;
• b) a memory having stored therein motor speed curves plotted against proportionality constants of fan speed to air flow on a first axis and motor excitation voltages on a second axis, the memory operably connected to the microprocessor;
• c) means for controlling an excitation voltage to the motor, the means for controlling being operably connected to the microprocessor;
• and
• d) a tachometer for sensing the speed of the motor, the tachometer being operably connected to the microprocessor.

Apparatus for achieving a constant mass air flow rate for a blower motor comprising:

• a) means for inputting a selected constant CFM rate to a motor controller;
• b) a motor controller having a microprocessor for receiving the input constant CFM rate;
• c) memory means having stored therein motor speed curves plotted against proportionality constants on a first axis and motor excitation voltages on a second axis, the memory means operably connected to the microprocessor;
• d) means for controlling the excitation voltage to the motor, the means for controlling operably connected to the microprocessor;
• and
• e) means for sensing a speed of the motor, the means for sensing a speed operably connected to the microprocessor.