Abstract:
A method of controlling a blower motor in a furnace includes applying control signals to the blower motor; determining that a reference airflow through a portion of the furnace is present; determining a reference control signal at which the reference air flow is present; receiving a requested airflow; computing a requested control signal in response to the reference air flow, the reference control signal and the requested airflow; and using the requested control signal to control the blower motor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein generally relates to air blowers, and in particular to a method and system for providing variable speed blower control. 
         [0002]    Heating, ventilation and air conditioning (HVAC) systems typically use a blower driven by a blower motor to supply air through ducts. HVAC systems are typically designed to provide an amount of airflow expressed as cubic feet per minute (CFM) (cubic meters per second in SI units) in certain modes. For example, low heat, high heat, cooling and continuous fan may all utilize different airflows. There is a need to simply and efficiently control the blower motor through different modes of operation. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0003]    An embodiment is a method of controlling a blower motor in a furnace, the method comprising: applying control signals to the blower motor; determining that a reference airflow through a portion of the furnace is present; determining a reference control signal at which the reference air flow is present; receiving a requested airflow; computing a requested control signal in response to the reference air flow, the reference control signal and the requested airflow; and using the requested control signal to control the blower motor. 
         [0004]    Another embodiment is a system for handling air including a blower; a blower motor; and a controller for controlling the blower motor, the controller applying control signals to the blower motor; determining that a reference airflow through a portion of the furnace is present; determining a reference control signal at which the reference air flow is present; receiving a requested airflow; computing a requested control signal in response to the reference air flow, the reference control signal and the requested airflow; and using the requested control signal to control the blower motor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
           [0006]      FIG. 1  depicts an exemplary furnace having an evaporator coil; and 
           [0007]      FIG. 2  is a flowchart of an exemplary control process. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0008]    Referring to  FIG. 1 , the numeral  10  generally designates a gas-fired condensing furnace employing the blower motor control of the present invention. Condensing furnace  10  includes a steel cabinet  12  housing therein burner assembly  14 , combination gas control  16 , heat exchanger assembly  18 , inducer housing  20  supporting, inducer motor  22  and inducer wheel  24 , and circulating air blower  26 . Combination gas control  16  includes a hot surface igniter (not shown) to ignite the fuel gas. 
         [0009]    Burner assembly  14  includes at least one inshot burner  28  for at least one primary heat exchanger  30 . Burner  28  receives a flow of combustible gas from gas regulator  16  and injects the fuel gas into primary heat exchanger  30 . A part of the injection process includes drawing air into heat exchanger assembly  18  so that the fuel gas and air mixture may be combusted therein. A flow of combustion air is delivered through combustion air inlet  32  to be mixed with the gas delivered to burner assembly  14 . 
         [0010]    Primary heat exchanger  30  includes an outlet  34  opening into chamber  36 . Connected to chamber  36  and in fluid communication therewith are at least four condensing heat exchangers  38  having an inlet  40  and an outlet  42 . Outlet  42  opens into chamber  44  for venting exhaust flue gases and condensate. 
         [0011]    Inducer housing  20  is connected to chamber  44  and has mounted thereon an inducer motor  22  together with inducer wheel  24  for drawing the combusted fuel air mixture from burner assembly  14  through heat exchanger assembly  18 . Air blower  26  is driven by blower motor  25  and delivers air to be heated in a counterflow arrangement upwardly through air passage  52  and over heat exchanger assembly  18 . The cool air passing over condensing heat exchanger  38  lowers the heat exchanger wall temperature below the dew point of the combusted fuel air mixture causing a portion of the water vapor in the combusted fuel air mixture to condense, thereby recovering a portion of the sensible and latent heat energy. The condensate formed within heat exchanger  38  flows through chamber  44  into drain tube  46  to condensate trap assembly  48 . As air blower  26  continues to urge a flow of air, upwardly through heat exchanger assembly  18 , heat energy is transferred from the combusted fuel air mixture flowing through heat exchangers  30  and  38  to heat the air circulated by blower  26 . Finally, the combusted fuel air mixture that flows through heat exchangers  30  and  38  exits through outlet  42  and is then delivered by inducer motor  22  through exhaust gas outlet  50  and thence to a vent pipe (not illustrated). 
         [0012]    Cabinet  12  also houses a controller  54  and a display  56 . Controller  54  may be implemented using a microprocessor-based controller executing computer program code stored on a computer readable storage medium. A thermostat  55  communicates with controller  54  to designate operational modes and temperature. Thermostat  55  may be an intelligent device that communicates requested air flow rates as described in further detail herein. A pressure tap  58  is located at primary heat exchanger inlet  60 , a pressure tap  62  is located at condensing heat exchanger outlet  42  and a limit switch  64  is disposed in air passage  52 . In a non-condensing furnace, pressure tap  62  would be disposed at primary heat exchanger outlet  34 , since there would be no condensing heat exchanger  38 . 
         [0013]    A first airflow pressure tap  90  is positioned near the outlet of blower  26 . A second airflow pressure tap  92  is positioned downstream of the first pressure tap  90 , near the outlet of primary heat exchanger  30 . First pressure tap  90  and second pressure tap  92  are fluidly coupled (e.g., via tubing) to a pressure switch  94  in the cabinet  12 . Pressure switch  94  is designed to change state (e.g., close) upon a predetermined pressure differential between pressure taps  90  and  92 . The predetermined pressure differential between pressure taps  90  and  92  is indicative of a predetermined reference airflow, CFM REF , through the furnace and provided to ducting coupled to the furnace. Pressure switch  94  provides a signal (e.g., a 24 VAC signal) to controller  54  indicating that the predetermined pressure differential has been reached. 
         [0014]    A cooling coil  82  is located in housing  80  on top of furnace cabinet  10  and is the evaporator of air conditioning system. The cooling coil  82  has an inlet  84 , where subcooled refrigerant enters, and an outlet  86 , where superheated refrigerant leaves, as is conventional. In response to an input from heating/cooling thermostat  55 , air blower  26  urges air flow upwardly through cooling coil  82  where heat exchange takes place. As a result of this heat exchange, cool air is delivered to the conditioned space and superheated refrigerant is returned to the outdoor condensing section (not illustrated) via outlet  86 . In the outdoor condensing section the refrigerant is subcooled and returned to inlet  84 . This cycle continues until the thermostat is satisfied. 
         [0015]    In operation, the controller  54  controls blower motor  25  by providing a control signal to the motor  25 . The control signal may be a pulse width modulated (PWM) signal indicating a duty cycle for blower motor  25 . In exemplary embodiments, the control signal is a 12-bit PWM control signal. It is understood that analog control signals may be used, or different types of digital codes may be used to provide the control signal. Controller  54  determines the appropriate control signal in response to a transfer function stored in controller  54 , and described in further detail herein. 
         [0016]      FIG. 2  is a flowchart of a process for providing variable speed control for blower motor  25 . The process begins at  200  where the blower motor  25  is turned on. This may be in response to a request from thermostat  55 . At  202 , the controller  54  gradually ramps up the blower motor torque by adjusting the control signal (e.g., increasing PWM) to the motor  25 . At  204 , controller  54  determines if the pressure switch  94  has changed states to indicate that the predetermined differential pressure between pressure taps  90  and  92  has been reached. If not, the process returns to  202  to step increase torque at blower motor  25 . 
         [0017]    When the pressure switch  94  changes states, flow proceeds to  206 . Controller  54 , in response to a signal from pressure switch  94 , stores the current control signal value as a reference control signal. In this manner, controller  54  knows that a predetermined reference airflow, CFM REF , is obtained when the reference control signal is applied to the blower motor  25 . The reference control signal is stored in the controller at  206 , along with the reference airflow, which may be stored in the controller  54  prior to executing the method of  FIG. 2 . 
         [0018]    At  208 , the controller receives a request for a requested airflow. The requested airflow may be communicated from thermostat  55  or determined by the controller  54  based on the selected mode of operation. For example, a standard mode may use  350  CFM/ton (0.165 m 3 /s/ton), a dehumidifying mode  275  CFM/ton (0.130 m 3 /s/ton), a super dehumidifying mode  200  CFM/ton (0.094 m 3 /s/ton) and a maximum mode  400  CFM/ton (0.189 m 3 /s/ton). At  210 , controller  54  uses a transfer function to compute the appropriate control signal to apply to blower motor  26 . Controller  54  calculates a requested control signal (e.g., PWM) needed to supply the requested airflow based on fan laws, as PWM is proportional to motor torque, and motor torque is proportional to the square of the CFM. As controller  54  knows the reference airflow, the reference control signal and the requested airflow, the transfer function is employed to solve for the requested control signal. Accordingly, the requested control signal is computed as a function of reference airflow, the reference control signal and the requested airflow. At  212 , the requested control signal is applied to the blower motor  25 , to achieve the requested airflow. 
         [0019]    While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.