Abstract:
A control system and method for controlling operation of a compressor in a device is disclosed. The system includes a line voltage measurement circuit which measures the difference in voltage between a first line voltage and a second line voltage being applied to the device, and a controller having means for storing a first and second threshold voltage, a comparator having an output for comparing the voltage difference to the first and second threshold voltages, and compressor control means for controlling operation of the compressor based on the output of the comparator.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a control system for controlling the operation of a compressor in an appliance. 
     In HVAC systems, the compressor control mechanisms are typically operated off of the 24 Volt secondary winding of a transformer. Compressors need a high line voltage to operate properly. However, line voltages are susceptible to fluctuations, and in particular, going low as a result of poor connections, power failures and peak cooling demand times. A low line voltage causes the compressor to draw more current than necessary, causing it to overheat and burn out. Current compressor configurations do not allow the compressor control mechanisms to monitor the line voltage directly. As a result, there is no means to de-energize the compressor when line voltages get too low in order to prevent the compressor from overheating and burning out. Given the possibility of low line voltage occurrences, the cost for replacing compressors can become significant. Further, a malfunctioning compressor can render the HVAC system inoperable, further, increasing the cost of repair and/or replacement. 
     Accordingly, there is a need for a compressor control that prevents malfunctioning of the compressor due to fluctuations in line voltage. 
     SUMMARY OF THE INVENTION 
     A control system for controlling operation of a compressor in a device powered by a first and second line voltage is disclosed. The system comprises a line voltage measurement circuit for measuring a difference in voltage between the first line voltage and the second line voltage before and during the energizing of the compressor, and a controller in communication with the line voltage measurement circuit and the compressor, the controller comprising storage for storing at least a first threshold voltage, a comparator having an output for comparing the voltage difference to the first threshold voltage, and a compressor control mechanism for controlling operation of the compressor based on the output of the comparator. The compressor control comprises a switch for switching the compressor off when the voltage difference is less than the first threshold voltage for a predetermined period of time. The controller storage mechanism also stores a second threshold voltage, and the compressor control mechanism comprises another switch for switching the compressor off when the voltage difference goes below the second threshold voltage. The controller comprises a counter having a count representing a period of time, the count being initialized when the voltage difference is less than the first threshold voltage and incremented for as long as the output of the comparator shows the voltage difference less than the first threshold voltage. The compressor is switched off by the switch of the controller when the period of time equals the predetermined period of time. 
     The controller also may comprise an indicator for indicating when the compressor is turned off. In another embodiment, a control system comprises a line voltage measurement circuit for measuring a difference in voltage between the first line voltage and the second line voltage before and during the energizing of the compressor, and a controller in communication with the line voltage measurement circuit and the compressor. The controller comprises storage for storing a second threshold voltage, a comparator having an output for comparing the voltage difference to the second threshold voltage, and a compressor control mechanism for controlling operation of the compressor based on the output of the comparator. The compressor control mechanism comprises a switch for switching the compressor off when the voltage difference goes below the second threshold voltage. The controller may further comprise an indicator for indicating when the compressor is turned off. 
     A method for controlling operation of a compressor in a device powered by a first and second line voltage and having a controller is also disclosed. The method comprises measuring a difference in voltage between the first line voltage and the second line voltage before and during the energizing of the compressor, storing at least a first threshold voltage in the controller, comparing the voltage difference to the first threshold voltage, and controlling operation of the compressor based on the comparison between the voltage difference and the first threshold voltage. The method further comprises counting the period of time during which the voltage difference is less than the first threshold voltage and switching the compressor off when the period of time exceeds a predetermined period of time. The method may also further comprise storing a second threshold voltage in the controller and switching the compressor off when the voltage difference goes below the second threshold voltage. The method further comprises indicating to an operator of the device when the compressor is turned off. 
     In another embodiment, a method comprises measuring a difference in voltage between the first line voltage and the second line voltage before and during the energizing of the compressor, storing a second threshold voltage in the controller, and controlling operation of the compressor based on the comparison between the voltage difference and the second threshold voltage. The method further comprises switching the compressor off when the voltage difference goes below the second threshold voltage. The method may further comprise indicating to an operator of the device when the compressor is turned off. 
     While the principal advantages and features of the present invention have been explained above, a more complete understanding of the invention may be attained by referring to the description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 displays a schematic of a compressor control system in accordance with the present invention. 
     FIG. 2 displays a flow chart outlining a method of controlling the compressor of FIG. 1 in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While the invention will be described with respect to HVAC systems, it can be appreciated that it is applicable with respect to other systems using compressors, such as those using combustion motors or circulator motors. 
     Referring to FIG. 1, various components of an HVAC system are shown generally as  10 . Such components include a line voltage measurement circuit  12 , a compressor relay circuit  14 , and a controller  16  in communication with the line voltage measurement circuit  12  and the compressor relay circuit  14 . Line voltage, typically in the amount of 120 Volts, is applied to the system  10  via a first line voltage terminal  18  and a second line voltage terminal  20 . The line voltage measurement circuit  12  includes an operational amplifier  22  and a pair of divider resistors. In particular, the operational amplifier  22  includes an inverting pin  24  connected to a first divider resistor  26 , which is in turn connected to ground  21 , and a non-inverting pin  28  connected to a second divider resistor  30 , which is in turn connected to the output of the operational amplifier  22 . The inverting pin  24  also connects to the first line voltage terminal  18  via a first line voltage resistor  32 , while the non-inverting pin  28  connects to the second line voltage terminal  20  via a second line voltage resistor  34 . The output signal of the line voltage measurement circuit  12  represents an attenuated version of the difference in voltage between the first line voltage terminal  18  and the second line voltage terminal  20 , which is measured thereby which is then input to the controller  16 . 
     The controller  16  includes an analog-to-digital (A/D) converter (not shown) which measures the positive peak of each cycle of the sine wave of the output signal received from the line voltage measurement circuit  12 . The controller  16  further includes memory (not shown) for storing at least a first threshold voltage, preferably 80 percent of rated voltage against which the voltage difference measured by the line voltage measurement circuit  12  is compared. The controller  16  also preferably includes a counter (also not shown) which is initialized when the voltage difference is less than the first threshold voltage and is incremented until it reaches a predetermined number, which equates to the amount of time, preferably eight seconds, the voltage difference remains less than the first threshold voltage. Once the counter reaches a predetermined number, the compressor (not shown) is at risk of overheating. The memory of the controller  16  may also store a second threshold voltage representing a base voltage, preferably a value sufficiently below 80 percent rated voltage, over which the voltage difference must be maintained in order to prevent immediate risk of overheating of the compressor. It can be understood that the first and second threshold voltages may change depending on the line voltage applied to the system  10 , and also the manufacturer&#39;s specifications for various compressors. The controller further includes a compressor control mechanism, preferably in the form of software, for controlling operation of the compressor based on the output of the compared voltages and the counter. In particular, once the voltage difference is less than the first threshold voltage for longer than the predetermined time or the voltage difference goes below the second threshold voltage, the compressor control mechanism switches the compressor off to prevent overheating. The controller preferably includes an indicator, such as a light emitting diode, for indicating when the compressor has been locked out. The controller  16  further includes a diagnostic output signal for transmission to ground via a third resistor  56  and third diode  53 , to indicate why the compressor shut down. 
     The compressor relay circuit  14  includes a first compressor contractor relay coil  36  and a second compressor contractor relay coil  38 . The first compressor contact relay coil  36  is connected in parallel with a first diode  40 , and the combination is in turn connected to the controller  16  via a first transistor  42  and a first resistor  44 . The second compressor contractor relay coil  38  is connected in parallel with a second diode  46 , and the combination is in turn connected to the controller  16  via a second transistor  48  and a second resistor  50 . The compressor relay circuit  14  further includes a first compressor contractor terminal  52  and a second compressor contractor terminal  54  connected to the controller  16  and output signals Y 1 . and Y 2  From a thermostat (not shown) through a first compressor contact  56  and a second compressor contact  58 , respectively. Once the first and second compressor contractor relay coils  36  and  38  are sufficiently energized, the first and second compressor contacts  56  and  58  close to permit the transmission of the Y 1  and Y 2  output signal of the thermostat to the first and second compressor contractor terminals  52  and  54 . The terminals  52  and  54  lead to the compressor contractor coil (not shown), which when sufficiently energized switches line voltage to the compressor. 
     In a preferred embodiment, controller  16  is a MSC68HC705P6, first and second divider resistors  26  and  30  are 24 kOhm resistors, first and second line voltage resistors  32  and  34  are 1 Mohm resistors, first and second contractor relay coils  36  and  38  are SPAT relay coils, first and second diodes  40  and  46  are IN4004 diodes, first and second resistors  44  and  50  are 10 kOhm resistors, and first and second transistors  42  and  48  are 2N3904 transistors. 
     The system  10  of FIG. 1 will now be described in operation with respect to FIG.  2 . At  100 , the line voltage measurement circuit  12  measures the difference in line voltage between the first line voltage terminal  18  and the second line voltage terminal  20 . At  102 , the controller  16  compares the line voltage difference measured by the line voltage measurement circuit  12  with the second threshold voltage stored within the memory of the controller  16 . At  104 , the controller  16  checks whether the line voltage difference is below the second threshold voltage. If so, at  114 , the controller  16  sends a signal to the compressor contractor relay coils  36  and  38  to de-energize. If not, at  106 , the controller  16  checks whether the line voltage difference is less than the first threshold voltage. If not, the line voltage measurement circuit  12  takes another measurement of the difference in the line voltage between the first line voltage terminal  18  and the second line voltage terminal  20  at  100 , and at  102 , the controller  16  makes another comparison. If so, at  108  the controller  16  initiates the counter and at  109 , another check is made whether the line voltage difference is less than the threshold voltage. If not, steps  100  through  108  are repeated. If so, at  110 , the controller  16  checks whether the counter has reached a predetermined number. If not, at  112 , the counter is incremented. If so at  114 , the controller  16  sends a signal to the compressor contractor relay coils  36  and  38  to de-energize, and at  116 , the compressor is locked out. 
     The foregoing constitutes a description of various features of a preferred embodiment. Numerous changes to the preferred embodiment are possible without departing from the spirit and scope of the invention. Hence, the scope of the invention should be determined with reference not to the preferred embodiment, but to the following claims: