Patent Application
20080237217
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.
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 invention has been developed.
The present invention provides improved controls for an air conditioning system of a type which includes electric heating elements and one of several selected types of blower or air circulating fan electric drive motors whereby the electric heating elements are prevented from operation under certain motor operating conditions. For example, if the blower motor is operating at a speed less than a predetermined speed an interlock or relay is actuated which will be prevent operation of the electric heating element or elements. Still further, if the blower drive motor should fail, power to such heating elements would also be locked out or interrupted by the control system of the invention.
The present invention further provides a method for operating an air conditioning system utilizing electric heating elements and one of a selected type of blower or circulating fan drive motor wherein, depending on the control signals imposed on the motor control system, or a signal from the motor control system, a so-called interlock relay is actuated to prevent energization of the electric heaters. Still further, for applications of air conditioning equipment with variable speed motors driving an air circulation fan or blower, electric heater operation may be inhibited at a predetermined maximum blower speed in the event of a restricted air inlet to or air outlet from the system cabinet.
Those skilled in the art will further appreciate the above-mentioned advantages and features of the invention, together with other important aspects thereof upon reading the detailed description which follows in conjunction with a drawing.
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.
Referring to
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 invention. 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 invention. Electric heaters 26, 28 and 30 are disposed between an outlet opening 23 of blower 22 and the cabinet air outlet opening 18,
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.
Referring now to
Referring further to
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
Although the specific configuration of the motor control circuit 78 and the associated relays 70, 72, and 74 illustrated in
The system 10a 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.
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.
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.
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.
The construction and operation of control system 10a for an air conditioning system in accordance with the invention 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 invention 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.
