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

• US 6,504,338 B1
• Filed: 07/12/2001
• Issued: 01/07/2003
• Est. Priority Date: 07/12/2001
• Status: Expired due to Fees

• Current Assignee: HVAC Modulation Technologies LLC (Acacia Research Corporation)
• Original Assignee: Varidigm
• Inventors: Eichorn, Ronald L.
• Primary Examiner(s): Nappi, Robert E.
• Assistant Examiner(s): Duda, Rina I.
• Application Number:
  US09/904,428
• Publication Number:
  US 20030011342A1
• Time in Patent Office: 544 Days
• Field of Search: 318/727, 318/798, 318/799, 318/802, 318/805, 318/599, 318/811, 388/929, 388/930, 417/18, 417/20, 417/22, 417/44.1, 417/44.11, 417/45, 236/DIG.4
• US Class Current: 318/727
• CPC Class Codes: G05D 7/0676 by action on flow sources G05D7/0688, G05D7/0694 take precedence
• CPC Class Codes: Y10S 236/09 Fan control
• CPC Class Codes: Y10S 388/929 Fluid/granular material flow rate, pressure, or level
• CPC Class Codes: Y10S 388/93 Load or torque

Abstract

A method and apparatus for achieving constant air flow rate economically utilizes an induction blower motor with control settings determining the motor excitation voltage. A memory stores, for a variety of motor speeds, a graph of the speeds plotted against a proportionality constant of flow rate to fan speed for the selected motor system on one axis and motor control settings on the other axis. The only monitoring necessary to achieve the constant flow rate is thus the sensing of fan rotational speed. The measured fan speed is compared against the proportionality constant needed for the selected constant air flow rate and a motor excitation voltage is derived to achieve that proportionality constant. When the system load changes significantly, thereby causing significant fan speed change, a cascaded control loop is used whereby the speed changed induced by each control voltage adjustment is monitored until the desired constant flow rate is again attained at the new load level.

Claims

1. A method of achieving a selected 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;

b) determining proportionality constants of flow rate to motor speed for various motor speeds over the operating range of the motor;

c) providing a graph of curves for the various motor speeds plotted against the proportionality constants and the excitation voltages in a memory accessible by the motor controller;

d) setting the controller at a first excitation voltage;

e) measuring a rotational speed of the motor;

f) deriving the proportionality constant for the measured motor speed to determine a fan speed corresponding to the selected air flow rate.

2. The method of achieving a constant mass air flow rate for a blower motor of the induction type according to claim 1 further comprising:

adjusting the excitation voltage according to the determined fan speed to achieve the desired air flow.

3. The method of achieving a constant mass air flow rate for a blower motor of the induction type according to claim 2 further comprising:

deriving a second proportionality constant according to a speed of the motor at the adjusted excitation voltage.

4. 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).

5. The method of achieving a constant mass air flow rate for a blower motor of the induction type of claim 4 further, comprising:

checking the proportionality constant for the measured motor speed by interpolating between the curves to determine a control setting corresponding to the selected air flow.

6. 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.

7. Apparatus for achieving a constant mass air flow rate for a blower motor of the induction type according to claim 6 further comprising:

means for inputting variety of constant CFM rates.

8. Apparatus for achieving a constant mass air flow rate for a blower motor of the induction type according to claim 6 further comprising:

means for sensing changes in a line voltage and adjusting excitation to the motor based on the changes in line voltage.

9. Apparatus for achieving a constant mass air flow rate for a blower motor of the induction type according to claim 6 further comprising:

means for sensing temperature changes in the motor and adjusting excitation to the motor based on the changes in temperature.

10. Apparatus for achieving a constant mass air flow rate for a blower motor of the induction type according to claim 6 wherein the memory is a look up table.

11. 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.

12. The apparatus for achieving a constant mass air flow rate for a blower motor according to claim 11 further comprising:

a motor of the induction type exhibiting nonlinearity between motor rotational speed, motor control voltage, and air flow rate, the motor being operably connected to the tachometer and the motor controller.

13. A method for achieving a constant mass air flow rate for an induction blower motor according to claim 11 further comprising:

performing an algorithm having the steps in order of;

a) select air flow rate value;

b) set motor speed value;

c) change control motor voltage until motor speed value is reached;

d) derive system constant for current control voltage and current motor speed value;

e) divide air flow rate value by derived system constant to derive new motor speed value; and

f) iterate steps c)-e) until motor speed value and system constant balance at selected air flow rate value.

14. A method for achieving a constant mass air flow rate for an induction blower motor according to claim 11 further comprising:

performing an algorithm having the steps in order of;

a) enter air flow rate value;

b) set first motor speed value;

c) step motor control voltage until first motor speed value is measured;

d) derive first system constant for current motor control voltage and first motor speed value;

e) divide air flow rate value by first system constant to derive second motor speed value;

f) subtract second motor speed value-first motor speed value to derive third motor speed value;

g) divide third motor speed value by x to get fourth motor speed value;

h) step motor control voltage up until fourth motor speed value is measured;

i) derive second system constant for current motor control voltage and fourth motor speed value;

j) divide air flow rate value by second system constant to derive fifth motor speed value; and

k) iteratively step up motor control voltage, measure motor speed and step through motor speed values until motor speed value and system constant balance at air flow rate value.