Abstract:
A method is provided for controlling operation of electrical heating elements in a heating, ventilating, and air conditioning (HVAC) system in accordance with fan operating conditions, the method comprising the steps of: monitoring speeds at which a fan motor is operating; communicating status signals indicative of fan motor operating speeds; and upon receiving a status signal indicating the fan motor is operating at a speed exceeding a predetermined maximum speed, communicating a control signal instructing a heater interlock to interrupt power supplied to at least one electrical heating element.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a divisional of and claims priority to U.S. Non-Provisional Patent Application No. 11/728,632 filed on Mar. 27, 2007 by Robert W. Helt and entitled “Heater Interlock Control for Air Conditioning System,” the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    Not applicable. 
       REFERENCE TO A MICROFICHE APPENDIX 
       [0003]    Not applicable. 
       BACKGROUND 
       [0004]    In certain types of heating, ventilating and air conditioning (HVAC) equipment, electric heating elements are incorporated in the equipment in combination with a motor driven blower and, possibly, a cooling type heat exchanger, such as an evaporator coil for a vapor compression cooling circuit or heat pump circuit. One problem associated with utilizing electric heating elements in an air conditioning system of the general type mentioned herein is the requirement to provide for positive shut-off of the electric heating elements if the system blower or air circulation fan motor is operating in a range of operating conditions which will result in hazardous heat buildup. For example, if the blower or circuiting fan motor is operating at a relatively low speed, or has shut-off for any reason, unwanted and rapid heat buildup or overheating of the system may occur. 
         [0005]    Moreover, regulatory requirements for air conditioning systems which utilize electric heating elements can result in extensive testing for various blower or air circulating fan motor operating conditions. However, if a system control can be provided which would block or interrupt power to the electric heating elements when the blower or circulating fan motor was operating outside of a predetermined range of operating conditions, regulatory testing requirements could be reduced, system reliability increased and the chance of a hazardous operating condition could be avoided. It is to these ends that the present system has been developed. 
       SUMMARY 
       [0006]    The present disclosure in one embodiment provides a method for operating an air conditioning apparatus, said apparatus including a cabinet, an air blower including an electric blower motor for propelling air from an air inlet to an air outlet of said cabinet, and at least one electric heating element disposed in an air flowstream propelled by said blower through said cabinet, said apparatus further comprising a first temperature sensor for sensing the temperature of an enclosed space being supplied with conditioned air by said apparatus and a control system including a system controller operably connected to said first sensor, and to a second temperature sensor for sensing the temperature of air being discharged from said apparatus, said control system further including a heater interlock operable to prevent energization of said heating element, said method including the steps of: monitoring at least one of an operating condition of said blower motor and a limit temperature of air being circulated by said blower; and causing said heater interlock to prevent energization of said heating element dependent on one of said operating condition and said limit temperature. 
         [0007]    The present disclosure as provide a method for controlling operation of electrical heating elements in a heating, ventilating, and air conditioning (HVAC) system in accordance with fan operating conditions, the method comprising the steps of: monitoring speeds at which a fan motor is operating; communicating status signals indicative of fan motor operating speeds; and upon receiving a status signal indicating the fan motor is operating at a speed exceeding a predetermined maximum speed, communicating a control signal instructing a heater interlock to interrupt power supplied to at least one electrical heating element. 
         [0008]    Those skilled in the art will further appreciate the above-mentioned advantages and features of the present system and method, together with other important aspects thereof upon reading the detailed description which follows in conjunction with a drawing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is perspective view of an air conditioning apparatus utilizing a control system and method in accordance with the present disclosure; and 
           [0010]      FIG. 2  is schematic diagram illustrating major components of a control system in accordance with the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    In the description which follows, like parts are marked throughout the specification and drawing with the same reference numerals, respectively. The drawing figures and elements thereof may be in somewhat generalized or schematic form in the interest of clarity and conciseness. 
         [0012]    Referring to  FIG. 1 , there is illustrated an air conditioning apparatus  10 , commonly known in the art as an air handler. The apparatus  10  comprises a substantially rectangular hollow cabinet  12  and is arranged as an upflow type apparatus wherein a bottom wall  14  of the cabinet  12  has a suitable large opening  15 , see  FIG. 2  also, to allow air flow in a generally upward direction, as indicated by arrow  16 . Air flows from bottom wall  14  upwardly and out through an opening  18  in a transverse top wall  19 . 
         [0013]    Within cabinet  12  there is disposed a suitable heat exchanger, such as an air conditioning or cooling coil  20 , disposed between the aforementioned air inlet opening and an air circulating fan or blower  22 . Blower  22  is driven by a suitable electric motor  24  which may be controlled in accordance with description that follows herein and in accordance with the disclosure. Air conditioning apparatus  10  also includes additional heat exchangers or heating means comprising electric resistance grid type heaters or heating elements  26 ,  28  and  30  which are illustrated three in number by way of example. One or more electric heaters may be utilized in an air conditioning control system and method in accordance with the disclosure. Electric heaters  26 ,  28  and  30  are disposed between an outlet opening  23  of blower  22  and the cabinet air outlet opening  18 ,  FIG. 1   
         [0014]    In the manufacture of air handlers or air conditioning apparatus of the general type illustrated and described, various types of blower drive motors may be utilized, including variable speed motors with serial communication, that is, communication between the blower motor and a controller may be by way of a four-wire interface and the air handler controller may include a microprocessor which will signal the desired blower speed required to satisfy the demand for conditioned air flowing through the cabinet  12  to an enclosed space. The motor  24  may also be a so-called constant torque type motor whereby the aforementioned controller may be set to select a constant torque setting from a plurality of available settings. The aforementioned air handler controller typically provides a suitable signal to the desired motor input connection. Still further, the motor  24  may be a so-called PSC (permanent split capacitor) motor whereby the controller may select one of three motor speeds and provide a signal for controlling the operation of one or more relays. 
         [0015]    Referring now to  FIG. 2 , the apparatus  10  is illustrated somewhat schematically and associated with a control system  10   a  for providing conditioned air to an enclosed space  32  from which conditioned air may be returned to the apparatus  10  via suitable duct means, as indicated by the dashed line  33 . A temperature sensor  34  and a humidity sensor  36  may be disposed in the air conditioned space  32  and suitably connected to a thermostat control device  38  which is provided with low voltage power in a conventional manner. Thermostat  38  may also provide output signals via respective conductors, including a first stage heating output signal via conductor  40 , a second stage heating output signal via a conductor  42 , a third stage heating output signal via a conductor  44 , at least a first cooling stage output signal via a conductor  46 , a continuous fan operating mode signal via a conductor  48  and, possibly, a heat pump operating signal via a conductor  50 . Alternatively, serial or parallel digital communication signals may be sent between thermostat  38  and a controller described hereinbelow. The control system  10   a  and apparatus  10  illustrated in  FIG. 2  would, typically, include a vapor compression compressor and condenser unit, not shown, operably connected to the evaporator or cooling coil  20  and configured for either cooling only or, possibly, heat pump operation. The output signals conducted from thermostat  38  are input to a microcontroller for the system  10   a,  generally designated by the numeral  52 . Microcontroller  52  may be of a type commercially available, such as a Model AT Mega 128 commercially available from Atmel Corporation, San Jose, Calif. A suitable human operable interface  53  including a visual display  53   a  and control and/or configuration command input means  53   b  may also be operably connected to microcontroller  52 . Microcontroller  52  is provided with suitable electric power from a source which may also supply power to the thermostat  38 , but not shown in the drawing, such a source being well known to those skilled in the art. 
         [0016]    Referring further to  FIG. 2 , microcontroller  52  provides output signals by way of respective conductors as follows. Conductor  54  provides a heater interlock relay signal to a heater interlock relay  56 . Output signal conductor  58  provides a control signal to a heater relay  60  connected to heater  30  by way of the relay  56 . A status signal indicating the condition of relay  60  may be input to microcontroller  52  via conductor means  62 . 
         [0017]    In like manner, second and third stage heaters  28  and  26  are operably connected to respective relays  64  and  66  which receive control signals from the microcontroller  52  by way of conductors  65  and  67 , respectively. Relay status signals are returned to the microcontroller by way of conductors  63  and  68 , as indicated. Assuming that a PSC type motor is the embodiment of the motor  24  shown in  FIG. 2 , suitable motor control relays  70 ,  72  and  74  may be provided with control signals by way of conductors  71 ,  73  and  75  whereby the microcontroller  52  may command operation of the blower motor  24  at selected speeds. An input signal to the microcontroller  52  may be provided by way of a conductor  76  which is connected to a motor controller  78  which also receives operating signals from the relays  70 ,  72  and  74  for operating the motor  24  at the selected speed. 
         [0018]    Although the specific configuration of the motor control circuit  78  and the associated relays  70 ,  72 , and  74  illustrated in  FIG. 2  may be that for a PSC motor, control signals on conductors  71 ,  73  and  75  may be sent directly to motor controller  78  to set a motor speed control signal or a motor torque control signal commensurate with the air flow demand for the conditions which exist as determined by the sensors  34  and/or  36 , for example. The microcontroller  52  may, for example, issue a message to the blower motor controller  78  to set the control mode and receive a status signal of motor speed in return, set motor speed and receive a torque signal, set motor torque and request a demand signal, set air flow demand and request direction of rotation of the blower motor, set demand ramp time and request status of the demand ramp rate, set motor torque percent and request status regarding the air flow limit, and set blower coefficients. Air flow limits may also be set by the microcontroller  52  via the motor controller  78 , for example. 
         [0019]    The system  10   a  may be preset to operate in the selected mode depending somewhat on the type of motor  24  being used and including the types of motors described hereinbefore. However, for variable speed motors and variable torque motors certain limits are required to be set within and controlled by the microcontroller  52 . For motor speeds above and below the preset limit speeds, for example, the heaters or heating elements  26 ,  28  and  30  are not allowed to operate. For example, if the motor  24  is not energized the controller  52  will send a signal to the heater interlock  56  to prevent conducting electrical power to the heating elements  26 ,  28  and  30 , even if any one of relays  60 ,  64  or  66  is closed. Still further, if the motor  24  is not operating the blower  22  at a predetermined minimum speed sufficient to provide a certain volume rate of airflow through the cabinet  12 , one or all of the heating elements  26 ,  28  and  30  will be prevented from operation by actuation of the interlock  56 . Also, blower motor speed is continuously monitored and, if an overspeed condition exists, possibly indicating blockage of air flow into or out of cabinet  12 , the interlock relay  56  may also be operated to shutoff power to the heating elements  26 ,  28  and  30 . Still further, the status of the heaters  26 ,  28  and  30  may be confirmed by the status of the respective relays  66 ,  64  and  60 . Additionally, a temperature sensor  80  may be disposed in cabinet  12  to measure system discharge air temperature from apparatus  10  and communicate a signal regarding same to microcontroller  52  by way of conductor means  82 . Microcontroller  52  may be programmed such that system discharge air temperature in excess of a predetermined value, or the rate of change of discharge air temperature in excess of a predetermined value, may be effective to cause microcontroller  52  to shut off operation of the heating elements  26 ,  28  and  30 . Such shutoff of heating elements  26 ,  28  and  30  may be carried out by actuation of the respective relays  66 ,  64  and  60  or by the interlock  56  if any one of the relays should fail. 
         [0020]    Accordingly, a signal from the motor  24  and/or its controller  78  to microcontroller  52  determines the status of the motor, that is, energized at a selected speed or selected torque setting or deenergized. The heater power relays  60 ,  64  and  66  also transmit signals or otherwise communicate to the microcontroller  52  indicating their status, that is, for example, stuck or failed open, actuated to allow power to flow to the respective heating elements  26 ,  28  and  30  and the contact elements welded or stuck together to prevent opening a power circuit between a power source, not shown, and the respective heating elements. Still further, the interlock  56  may communicate its status via a conductor  85  to microcontroller  52  to indicate whether it is in a condition to prevent power being applied to the heating elements  26 ,  28  or  30  or vice versa. 
         [0021]    If the thermostat  38  issues a call for heating, signals are sent via conductors  40 ,  42  or  44 , or possibly all three, which will cause microprocessor  52  to transmit a signal to motor controller  78 , possibly via relays  70 ,  72  and  74 , to energize motor  24  at a selected speed. Typically, there are no on or off delays in energizing the motor  24  with respect to the signals sent to the relays  60 ,  64  and  66  to energize one or more of the heaters  26 ,  28  and  30 . However, if more than one stage of electric heat demand is called by thermostat  38 , relays  60 ,  64  and  66  may be energized at intervals of about 0.5 seconds, respectively. If a signal is presented to the controller  52  only at conductor  40 , motor  24  may be energized for about 0.5 seconds before interlock relay  56  is closed to allow energization of the selected heating element, for example. A similar delay in signal transmission may be carried out when the call for heat has been satisfied to enable capture or transmission of residual heat from the heating elements to the circulating air. 
         [0022]    Other modes of operation may include operation when a signal is provided on conductor  48  for continuous operation of the motor  24  and a combination of the electric heating and heat pump operation in the heating mode is initiated wherein the microcontroller  52  will effect energization of the respective heating elements and provide for operation of the heat exchanger  20  to reject heat. The controller  52  will recognize that this mode of operation requires operation of the blower  22  at the higher of the electric heat or so-called mechanical heat air flow requirements, immediately. The controller  52  is also capable of detecting a fault condition in heater interlock  56 . If the heater interlock relay feedback signal via conductor  85  indicates the interlock relay contacts are closed when they should be open or if any of the relays  60 ,  64  and  66  signal the controller  52  that the relay contacts are closed when they should be open, such signals will cause the controller  52  to run the blower motor  24  at maximum heat speed and report a fault condition via the interface  53 . Moreover, if the interlock relay  56  is stuck closed, the microcontroller  52  may ignore requests for heating, for example. Still further, anytime the microcontroller  52  should malfunction and denergize the blower motor  24 , the heater interlock relay  56  is also required to interrupt power to the heating elements or heaters  26 ,  28  and  30 . 
         [0023]    The construction and operation of control system  10   a  for an air conditioning system in accordance with the disclosure is believed to be within the purview of one skilled in the art based on the foregoing description. Commercially available components may be utilized to provide the functions described herein. Although preferred embodiments of the disclosure have been described in detail, those skilled in the art will recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.