System and methods for driving a blower with a motor

• US 5,592,059 A
• Filed: 12/01/1994
• Issued: 01/07/1997
• Est. Priority Date: 05/27/1992
• Status: Expired due to Fees

• Current Assignee: Regal Beloit Electronic Motors Incorporated
• Original Assignee: General Electric Company
• Inventors: Archer, William R.
• Primary Examiner(s): Wysocki, Jonathan
• Application Number:
  US08/348,514
• Time in Patent Office: 768 Days
• Field of Search: 318/254, 318/471, 318/439, 318/138, 318/55, 318/66, 318/68, 318/98, 318/772, 318/774, 318/806, 62/115, 62/160, 62/180, 62/208, 62/211, 62/215, 388/909, 388/929, 388/930, 388/934, 388/432
• US Class Current: 318/400.08
• CPC Class Codes: F24F 11/30 for purposes related to the operation of the system, e.g. for safety or monitoring
• CPC Class Codes: F24F 11/41 Defrosting; Preventing freezing
• CPC Class Codes: F24F 11/52 Indication arrangements, e.g. displays
• CPC Class Codes: F24F 11/64 using pre-stored data
• CPC Class Codes: F24F 11/76 by means responsive to temperature, e.g. bimetal springs
• CPC Class Codes: F24F 11/84 using valves
• CPC Class Codes: F24F 11/86 by controlling compressors within refrigeration or heat pump circuits
• CPC Class Codes: F24F 11/871 by controlling outdoor fans
• CPC Class Codes: F24F 11/88 Electrical aspects, e.g. circuits
• CPC Class Codes: F24F 2110/10 Temperature
• CPC Class Codes: F24F 2140/50 Load
• CPC Class Codes: H02H 7/0856 characterised by the protection measure taken
• CPC Class Codes: H02P 6/085 in a bridge configuration
• CPC Class Codes: Y10S 388/904 Stored velocity profile
• CPC Class Codes: Y10S 388/934 Thermal condition

Abstract

A system for driving a blower of a heating, ventilating, and/or air conditioning (HVAC) system. The blower discharges heated or cooled air to a space for conditioning the air in the space by changing its temperature. A motor drives the blower at a speed or torque defined by a motor control signal thereby to control air flow rate of the HVAC system. The system includes a temperature sensor generating a temperature signal representative of the temperature of the air discharged to the space by the blower. In response to the temperature signal, a control circuit generates the motor control signal to cause the motor to operate at a minimum speed or torque until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature. After the temperature of the discharged air reaches the reference temperature, the control circuit generates the motor control signal to control the motor speed or torque as a function of the difference between the temperature of the discharged air and the reference temperature whereby the air flow rate of the HVAC system is increased as the temperature difference increases.

Claims

1. A system for driving a blower of a heating, ventilating, and/or air conditioning (HVAC) system, said blower discharging heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature, said system comprising:

a temperature sensor positioned in the discharge air path, said temperature sensor generating a temperature signal representative of the temperature of the air discharged to the space by the blower;

a motor having first and second operating modes driving the blower at a speed or torque defined by a motor control signal thereby to control air flow rate of the HVAC system said motor operating at a minimum speed or torque in the first operating mode and operating at a variable speed or torque greater than or equal to the minimum speed or torque in the second operating mode; and

a control circuit generating the motor control signal in response to the temperature signal to cause the motor to operate in the first operating mode until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature, said control circuit thereafter generating the motor control signal to cause the motor to operate in the second operating mode, said variable motor speed or torque of the second operating mode being a function of the difference between the temperature of the discharged air and the reference temperature, said motor maintaining the air flow rate of the HVAC system substantially constant in the first operating mode and increasing the air flow rate of the HVAC system as the temperature difference increases in the second operating mode.

2. The system of claim 1 wherein the HVAC system includes a heat exchanger for changing the temperature of the air discharged to the space by the blower, and wherein the temperature sensor is positioned remotely from the heat exchanger.

3. The system of claim 1 further comprising a temperature regulation circuit for generating an error signal representative of the difference between the temperature of the discharged air and the reference temperature, and wherein said control circuit has an input for receiving the error signal, said control circuit generating the motor control signal as a function of the error signal.

4. The system of claim 3 wherein the temperature regulation circuit comprises a differential amplifier circuit for comparing the temperature signal and a signal representing the reference temperature thereby to generate the error signal, said differential amplifier circuit including a limiting circuit to limit the error signal such that when the HVAC system is heating the discharged air the motor control signal is limited to increase the speed or torque of the motor as a function of the error signal when the temperature of the discharged air is increasing above the reference temperature and is limited to decrease the speed or torque of the motor as a function of the error signal when the temperature of the discharge air is decreasing above the reference temperature.

5. The system of claim 3 wherein the temperature regulation circuit comprises a differential amplifier circuit for comparing the temperature signal and a signal representing the reference temperature thereby to generate the error signal, said differential amplifier circuit including a limiting circuit to limit the error signal such that when the HVAC system is cooling the discharged air the motor control signal is limited to increase the speed or torque of the motor as a function of the error signal when the temperature of the discharged air is decreasing below the reference temperature and is limited to decrease the speed or torque of the motor as a function of the error signal when the temperature of the discharge air is increasing below the reference temperature.

6. The system of claim 3 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, and further comprising an override circuit generating an override signal when the heat pump system operates in the defrost mode, said control circuit receiving and responsive to the override signal for generating the motor control signal to cause the motor to operate at a speed or torque greater than the minimum speed or torque whereby the motor operates above the minimum speed or torque and independently of the difference between the temperature of the discharged air and the reference temperature when the heat pump system is operating in the defrost mode.

7. The system of claim 6 wherein the heat pump system includes a heat exchanger for heating the air discharged to the space by the blower, said temperature sensor being positioned remotely from the heat exchanger, and wherein the override circuit includes an additional temperature sensor positioned adjacent the heat exchanger generating an additional temperature signal representative of the temperature of the air adjacent the heat exchanger and includes a differential amplifier circuit for comparing the additional temperature signal and a signal representing the reference temperature thereby to generate the override signal, said control circuit receiving and responsive to the override signal for generating the motor control signal as a function of the difference between the temperature of the air adjacent the heat exchanger and the reference temperature when the heat pump system is operating in the defrost mode.

8. The system of claim 3 wherein the motor has a stationary assembly and a rotatable assembly in magnetic coupling relation thereto, the stationary assembly including windings adapted to be energized in at least one preselected sequence, the rotatable assembly being in driving relation to the blower, and further comprising power switching devices responsive to the motor control signal for selectively connecting a power supply to the windings in the preselected sequence to produce an electromagnetic field for rotating the rotatable assembly at a desired motor speed or torque, said motor control signal being a pulse width modulated signal representative of the desired motor speed or torque and having a duty cycle corresponding to a desired level of energization of the windings thereby to control the speed or torque of the motor, the duty cycle of the motor control signal being a function of the magnitude of the error signal.

9. The system of claim 8 wherein the HVAC system comprises a system for heating the discharged air and wherein the temperature regulation circuit has a gain, said control circuit defining a maximum temperature of the discharged air greater than the reference temperature as a function of the gain, the difference between the maximum temperature and the reference temperature corresponding to a maximum duty cycle of the motor control signal.

10. The system of claim 8 wherein the HVAC system comprises a system for cooling the discharged air and wherein the temperature regulation circuit has a gain, said control circuit defining a minimum temperature of the discharged air less than the reference temperature as a function of the gain, the difference between the minimum temperature and the reference temperature corresponding to a maximum duty cycle of the motor control signal.

11. The system of claim 1 wherein the HVAC system comprises a system for heating the discharged air having a first heating mode and a second heating mode, said first heating mode being defined by the temperature of the discharged air being less than or equal to the reference temperature, said second heating mode being defined by the temperature of the discharged air being greater than the reference temperature, said first and second heating modes corresponding to the first and second operating modes of the motor, respectively, whereby the air flow rate of the HVAC system is increased or decreased as the temperature of the discharged air increases or decreases, respectively.

12. The system of claim 1 wherein the HVAC system comprises a system for cooling the discharged air having a first cooling mode and a second cooling mode, said first cooling mode being defined by the temperature of the discharged air being greater than or equal to the reference temperature, said second cooling mode being defined by the temperature of the discharged air being less than the reference temperature, said first and second cooling modes corresponding to the first and second operating modes of the motor, respectively, whereby the air flow rate of the HVAC system is increased or decreased as the temperature of the discharged air decreases or increases, respectively.

13. The system of claim 1 wherein the HVAC system includes a system control generating a two state system control signal in response to the temperature of the air in the space, said system control signal having a DEMAND state and a NO DEMAND state, each said state corresponding to a difference between the temperature of the air in the space and a set point temperature, and wherein the control circuit is responsive to the DEMAND state of the system control signal for initially generating the motor control signal to cause the motor to operate in the first operating mode and, after the temperature of the discharged air reaches the reference temperature, generating the motor control signal to cause the motor to operate in the second operating mode.

14. The system of claim 13 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, said system control circuit generating an override signal when the heat pump system operates in the defrost mode, said control circuit receiving and responsive to the override signal for generating the motor control signal to cause the motor to operate at a maximum speed or torque.

35. The system of claim 1 wherein the HVAC system comprises a system for heating the discharged air and further comprising a temperature regulation circuit for comparing the temperature of the heated discharged air to the reference temperature, and wherein the control circuit is responsive to the temperature regulation circuit for generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is less than or equal to the reference temperature and for generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is greater than the reference temperature whereby the air flow rate of the heating system is increased as the temperature of the discharged air increases above the reference temperature.

36. The system of claim 1 wherein the HVAC system comprises a system for cooling the discharged air and further comprising a temperature regulation circuit for comparing the temperature of the cooled discharged air to the reference temperature, and wherein the control circuit is responsive to the temperature regulation circuit for generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is greater than or equal to the reference temperature and for generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is less than the reference temperature whereby the air flow rate of the cooling system is increased as the temperature of the discharged air decreases below the reference temperature.

37. The system of claim 1 further comprising a temperature regulation circuit for comparing the temperature of the heated or cooled air discharged by the blower to a heating reference temperature and to a cooling reference temperature, and wherein the control circuit is responsive to the temperature regulation circuit for generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is less than or equal to the heating reference temperature and greater than or equal to the cooling reference temperature and for generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is greater than the heating reference temperature or less than the cooling reference temperature whereby the air flow rate of the HVAC system is increased as the temperature of the discharged air increases above the heating reference temperature or decreases below the cooling reference temperature.

15. A heating, ventilating, and/or air conditioning (HVAC) system comprising:

a heat exchanger for heating or cooling air;

a blower for discharging heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature;

a temperature sensor positioned in the discharge air path remotely from the heat exchanger, said temperature sensor generating a temperature signal representative of the temperature of the air discharged to the space by the blower;

a motor having first and second operating modes driving the blower at a speed or torque defined by a motor control signal thereby to control air flow rate of the HVAC system, said motor operating at a minimum speed or torque in the first operating mode and operating at a variable speed or torque greater than or equal to the minimum speed or torque in the second operating mode; and

a control circuit generating the motor control signal in response to the temperature signal to cause the motor to operate in the first operating mode until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature, said control circuit thereafter generating the motor control signal to cause the motor to operate in the second operating mode, said variable motor speed or torque of the second operating mode being a function of the difference between the temperature of the discharged air and the reference temperature, said motor maintaining the air flow rate of the HVAC system substantially constant in the first operating mode and increasing the air flow rate of the HVAC system as the temperature difference increases in the second operating mode.

16. A motor having first and second operating modes for driving a blower of a heating, ventilating, and/or air conditioning (HVAC) system, said blower discharging heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature, said HVAC system having a temperature sensor positioned in the discharge air path, said temperature sensor generating a temperature signal representative of the temperature of the air discharged to the space by the blower, said motor comprising:

a stationary assembly and a rotatable assembly in magnetic coupling relation thereto, the stationary assembly including windings adapted to be energized in at least one preselected sequence, the rotatable assembly being in driving relation to the blower;

power switching devices responsive to a motor control signal for selectively connecting a power supply to the windings in the preselected sequence to produce an electromagnetic field for rotating the rotatable assembly at a desired motor speed or torque thereby to control air flow rate of the HVAC system, said rotatable assembly rotating at a minimum speed or torque in the first operating mode and rotating at a variable motor speed or torque greater than or equal to the minimum speed or torque in the second operating mode; and

a control circuit generating the motor control signal in response to the temperature signal to cause the motor to operate in the first operating mode until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature, said control circuit thereafter generating the motor control signal to cause the motor to operate in the second operating mode, said variable motor speed or torque of the second operating mode being a function of the difference between the temperature of the discharged air and the reference temperature, said motor maintaining the air flow rate of the HVAC system substantially constant in the first operating mode and increasing the air flow rate of the HVAC system as the temperature difference increases in the second operating mode.

17. The motor of claim 16 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, and wherein the control circuit includes a circuit receiving and responsive to an override signal for generating the motor control signal to cause the motor to operate at a speed or torque greater than the minimum speed or torque when the heat pump system operates in the defrost mode.

18. A method of operating a system for driving a blower of a heating, ventilating, and/or air conditioning (HVAC) system, said blower discharging heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature, said method comprising:

positioning a temperature sensor in the discharge air path for sensing the temperature of the air discharged to the space by the blower;

generating a temperature signal representative of the sensed temperature of the air discharged by the blower;

driving the blower with a motor having first and second operating modes and operating at a speed or torque defined by a motor control signal thereby to control air flow rate of the HVAC system, said motor operating at a minimum speed or torque in the first operating mode and operating at a variable speed or torque greater than or equal to the minimum speed or torque in the second operating mode;

maintaining the air flow rate of the HVAC system substantially constant until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature by generating the motor control signal to cause the motor to operate in the first operating mode; and

thereafter increasing the air flow rate of the HVAC system as the difference between the temperature of the discharged air and the reference temperature increases by generating the motor control signal to cause the motor to operate in the second operating mode, said variable motor speed or torque of the second operating mode being a function of the temperature difference.

19. The method of claim 18 wherein the HVAC system includes a heat exchanger for changing the temperature of the air discharged by the blower and wherein the step of positioning the temperature sensor in the discharge air path comprises the steps of positioning the temperature sensor remotely from the heat exchanger and sensing the temperature of the discharged air thereby to generate the temperature signal.

20. The method of claim 18 further comprising the steps of generating an error signal representative of the difference between the temperature of the discharged air and the reference temperature with a temperature regulation circuit and generating a motor control signal for defining the speed or torque of the motor as a function of the error signal.

21. The method of claim 20 further comprising the steps of comparing the temperature signal and a signal representing the reference temperature thereby to generate the error signal and limiting the error signal such that when the HVAC system is heating the discharged air the motor control signal is limited to increase the speed or torque of the motor as a function of the error signal when the temperature of the discharged air is increasing above the reference temperature and the motor control signal is limited to decrease the speed or torque of the motor as a function of the error signal when the temperature of the discharge air is decreasing above the reference temperature.

22. The method of claim 20 further comprising the steps of comparing the temperature signal and a signal representing the reference temperature thereby to generate the error signal and limiting the error signal such that when the HVAC system is cooling the discharged air the motor control signal is limited to increase the speed or torque of the motor as a function of the error signal when the temperature of the discharged air is decreasing below the reference temperature and the motor control signal is limited to decrease the speed or torque of the motor as a function of the error signal when the temperature of the discharge air is increasing below the reference temperature.

23. The method of claim 20 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, and further comprising the steps of generating an override signal when the heat pump system operates in the defrost mode and generating the motor control signal in response to the override signal to cause the motor to operate at a speed or torque greater than the minimum speed or torque whereby the motor operates above the minimum speed or torque and independently of the difference between the temperature of the discharged air and the reference temperature when the heat pump system is operating in the defrost mode.

24. The method of claim 23 wherein the heat pump system includes a heat exchanger for heating the air discharged to the space by the blower and further comprising the steps of generating an additional temperature signal representative of the temperature of the air adjacent the heat exchanger, comparing the additional temperature signal and a signal representing the reference temperature thereby to generate the override signal, and generating the motor control signal in response to the override signal as a function of the difference between the temperature of the air adjacent the heat exchanger and the reference temperature when the heat pump system is operating in the defrost mode.

25. The method of claim 20 wherein the motor has a stationary assembly and a rotatable assembly in magnetic coupling relation thereto, the stationary assembly including windings adapted to be energized in at least one preselected sequence, the rotatable assembly being in driving relation to the blower, and further comprising the steps of selectively connecting a power supply to the windings in the preselected sequence by switching power switching devices in response to the motor control signal to produce an electromagnetic field for rotating the rotatable assembly at a desired motor speed or torque and pulse width modulating the motor control signal at a duty cycle corresponding to a desired level of energization of the windings thereby to control the speed or torque of the motor, the duty cycle of the motor control signal being a function of the magnitude of the error signal.

26. The method of claim 25 wherein the HVAC system comprises a system for heating the discharged air and wherein the temperature regulation circuit has a gain, and further comprising the step of defining a maximum temperature of the discharged air greater than the reference temperature as a function of the gain, the difference between the maximum temperature and the reference temperature corresponding to a maximum duty cycle of the motor control signal.

27. The method of claim 25 wherein the HVAC system comprises a system for cooling the discharged air and wherein the temperature regulation circuit has a gain, and further comprising the step of defining a minimum temperature of the discharged air less than the reference temperature as a function of the gain, the difference between the minimum temperature and the reference temperature corresponding to a maximum duty cycle of the motor control signal.

28. The method of claim 18 wherein the HVAC system comprises a system for heating the discharged air having a first heating mode and a second heating mode, said first heating mode being defined by the temperature of the discharged air being less than or equal to the reference temperature, said second heating mode being defined by the temperature of the discharged air being greater than the reference temperature, said first and second heating modes corresponding to the first and second operating modes of the motor, respectively, and wherein the step of increasing the air flow rate of the HVAC system comprises increasing or decreasing the air flow rate as the temperature of the discharged air increases or decreases, respectively in the second heating mode.

29. The method of claim 18 wherein the HVAC system comprises a system for cooling the discharged air having a first cooling mode and a second cooling mode, said first cooling mode being defined by the temperature of the discharged air being greater than or equal to the reference temperature, said second cooling mode being defined by the temperature of the discharged air being less than the reference temperature, said first and second cooling modes corresponding to the first and second operating modes of the motor, respectively, and wherein the step of increasing the air flow rate of the HVAC system comprises increasing or decreasing the air flow rate as the temperature of the discharged air decreases or increases, respectively in the second cooling mode.

30. The method of claim 18 wherein the HVAC system includes a system control generating a two state system control signal in response to the temperature of the air in the space, said system control signal having a DEMAND state and a NO DEMAND state, each said state corresponding to a difference between the temperature of the air in the space and a set point temperature, and further comprising initially operating the motor in the first operating mode in response to the DEMAND state of the system control signal and, after the temperature of the discharged air reaches the reference temperature, operating the motor in the second operating mode.

31. The method of claim 30 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, and further comprising the steps of generating an override signal when the heat pump system operates in the defrost mode and generating the motor control signal in response to the override signal to cause the motor to operate at a maximum speed or torque.

38. The method of claim 18 wherein the HVAC system comprises a system for heating the discharged air and further comprising the step of comparing the temperature of the heated discharged air to the reference temperature, and wherein the step of maintaining the air flow rate comprises generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is less than or equal to the reference temperature and the step of increasing the air flow rate comprises generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is greater than the reference temperature whereby the air flow rate of the heating system is increased as the temperature of the discharged air increases above the reference temperature.

39. The method of claim 18 wherein the HVAC system comprises a system for cooling the discharged air and further comprising the step of comparing the temperature of the cooled discharged air to the reference temperature, and wherein the step of maintaining the air flow rate comprises generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is greater than or equal to the reference temperature and the step of increasing the air flow rate comprises generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is less than the reference temperature whereby the air flow rate of the cooling system is increased as the temperature of the discharged air decreases below the reference temperature.

40. The method of claim 18 further comprising the step of comparing the temperature of the heated or cooled air discharged by the blower to a heating reference temperature and to a cooling reference temperature, and wherein the step of maintaining the air flow rate comprises generating the motor control signal to cause the motor to operate in the first operating mode when the temperature of the discharged air is less than or equal to the heating reference temperature and greater than or equal to the cooling reference temperature and the step of increasing the air flow rate comprises generating the motor control signal to cause the motor to operate in the second operating mode when the temperature of the discharged air is greater than the heating reference temperature or less than the cooling reference temperature whereby the air flow rate of the HVAC system is increased as the temperature of the discharged air increases above the heating reference temperature or decreases below the cooling reference temperature.

32. A method of operating a heating, ventilating, and/or air conditioning (HVAC) system, said HVAC system having a heat exchanger for heating or cooling air, a blower discharging the heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature, said method comprising:

positioning a temperature sensor in the discharge air path remotely from the heat exchanger for sensing the temperature of the air discharged to the space by the blower;

generating a temperature signal representative of the sensed temperature of the air discharged to the space by the blower;

driving the blower with a motor having first and second operating modes and operating at a speed or torque defined by a motor control signal thereby to control air flow rate of the HVAC system, said motor operating at a minimum speed or torque in the first operating mode and operating at a variable speed or torque greater than or equal to the minimum speed or torque in the second operating mode;

maintaining the air flow rate of the HVAC system substantially constant until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature by generating the motor control signal to cause the motor to operate in the first operating mode; and

thereafter increasing the air flow rate of the HVAC system as the difference between the temperature of the discharged air and the reference temperature increases by generating the motor control signal to cause the motor to operate in the second operating mode said variable motor speed or torque of the second operating mode being a function of the temperature difference.

33. A method of operating a motor having first and second operating modes for driving a blower of a heating, ventilating, and/or air conditioning (HVAC) system, said blower discharging heated or cooled air through a discharge air path to a space thereby to condition air in the space by changing its temperature, said motor having a stationary assembly and a rotatable assembly in magnetic coupling relation thereto, the stationary assembly including windings adapted to be energized in at least one preselected sequence, the rotatable assembly being in driving relation to the blower, said method comprising:

selectively connecting a power supply to the windings in the preselected sequence by switching power switching devices in response to a motor control signal to produce an electromagnetic field for rotating the rotatable assembly at a desired motor speed or torque thereby to control air flow rate of the HVAC system, said rotatable assembly rotating at a minimum speed or torque in the first operating mode and rotating at a variable motor speed or torque greater than or equal to the minimum speed or torque in the second operating mode;

positioning a temperature sensor in the discharge air path for sensing the temperature of the air discharged to the space by the blower;

generating a temperature signal representative of the sensed temperature of the air discharged by the blower;

generating the motor control signal in response to the temperature signal to cause the motor to operate in the first operating mode until the temperature of the discharged air as represented by the temperature signal reaches a reference temperature, said motor operating in the first operating mode maintaining the air flow rate of the HVAC system substantially constant; and

thereafter generating the motor control signal to cause the motor to operate in the second operating mode, said motor operating in the second operating mode increasing the air flow rate of the HVAC system as the difference between the temperature of the discharged air and the reference temperature increases, said variable motor speed or torque of the second operating mode being a function of the temperature difference.

34. The method of claim 33 wherein the HVAC system comprises a heat pump system having a heating mode, a cooling mode and a defrost mode, and further comprising the steps of receiving an override signal and generating the motor control signal in response to the override signal to cause the motor to operate at a speed or torque greater than the minimum speed or torque when the heat pump system operates in the defrost mode.