Patent Publication Number: US-10330099-B2

Title: HVAC compressor prognostics

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to U.S. Provisional Application Ser. No. 62/141,549 entitled “HVAC COMPRESSOR PROGNOSTICS” and filed Apr. 1, 2015, the entirety of which is hereby incorporated by reference herein for all purposes. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure is directed to improving the reliability of HVAC systems, and in particular, to improved systems, apparatus, and methods for monitoring HVAC compressor performance to identify potential or imminent failures before they can occur. 
     2. Background of Related Art 
     Air conditioning and heat pump systems, sometimes referred to as heating, ventilation, and air conditioning (HVAC) systems, employ the vapor-compression refrigeration cycle to cool or warm indoor air. In the case of an air-conditioning system, refrigerant gas is pressurized by a compressor and flows through a condensing coil. A fan blows air through the condensing coil to move heat from the refrigerant into the outside environment, causing the refrigerant to release heat and condense into liquid form. Refrigerant continues to an evaporator where it expands and vaporizes, absorbing heat from, and thereby cooling, indoor air blown through the evaporator by a second fan. In the case of a heat pump, the cycle is reversed whereby heat is moved from the outside environment to indoor air. The compressor and fans are typically driven by electric motors, which may be driven at a fixed speed or at a variable speed. Variable speed motors may be driven by a variable speed drive (VSD) which utilizes an inverter to vary the frequency of alternating current power delivered to the motor. 
     Over time, HVAC components are subject to wear and tear or other faults which can cause reduced efficiency, component failure, damage to other system components, or even system shutdowns. A technician responding to a service call faces a number of challenges when troubleshooting an HVAC system, particularly if the underlying problem is intermittent or temperature-related. For example, an HVAC system might fail on a hot and humid day, but if the technician arrives on a cooler day where the failure does not occur, it may be difficult to determine the cause of the failure and the proper remedy. In another scenario, a compressor nearing the end if its service life may be functional albeit at a lower efficiency, which increases operating costs to the homeowner in a manner which is not immediately apparent. In yet another scenario, a refrigerant leak can cause diminished performance, can damage compressor internals, and can be harmful to the environment. 
     An HVAC system which identifies potential problems before they occur would help maintain the overall effectiveness and reliability of the HVAC system, assist service technicians during troubleshooting, prevent customer dissatisfaction, and be a welcome advance in the art. 
     SUMMARY 
     In one aspect, the present disclosure is directed to a method for determining an operational condition of a compressor operatively coupled to an electric motor. The method includes receiving information indicative of an actual input power of the electric motor; receiving information indicative of a compressor speed, a compressor saturated suction temperature, and a compressor saturated discharge temperature; determining a mapped input power from the received compressor speed, the compressor saturated suction temperature, and the compressor saturated discharge temperature; and determining the operational condition of the compressor from the actual input power and the mapped input power. 
     In some embodiments of the method, receiving information indicative of an actual input power includes receiving the information from an inverter drive coupled to the electric motor. In some embodiments, receiving information indicative of a compressor speed includes receiving the information from an inverter drive coupled to the electric motor. 
     In some embodiments, the method includes receiving a sensor signal corresponding to a compressor saturated discharge pressure, and mapping the compressor saturated discharge pressure to the compressor saturated discharge temperature. In some embodiments, the method includes receiving a sensor signal corresponding to a compressor saturated suction pressure, and mapping the compressor saturated suction pressure to the compressor saturated suction temperature. In some embodiments, determining the operational condition of the compressor includes determining if the ratio between the actual input power and the mapped input power exceeds a predetermined ratio. In some embodiments, a compressor health message is transmitted. In some embodiments, a compressor health message is issued in response to the exceeding. In some embodiments, the predetermined ratio may be about 1.1:1 and/or about 1.3:1. 
     In some embodiments, determining the operational condition of the compressor includes determining if the ratio between the actual input power and the mapped input power is less than a predetermined ratio. In some embodiments, the method includes issuing an alert in response to a determination that the mapped input power is less than a predetermined ratio. In some embodiments, the predetermined ratio may be about 0.7:1 and/or about 0.85:1. 
     In some embodiments, the compressor health message includes information such as, without limitation, one, some, or all of a timestamp, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, and/or information indicative of the compressor saturated suction temperature. 
     In some embodiments, the method includes storing, in response to the exceeding, historical information including at least one of a timestamp, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, and/or information indicative of the compressor saturated suction temperature in a memory device. In some embodiments, the method includes retrieving, from the memory device, the historical information. 
     In some embodiments, the method includes calculating the mapped input power in accordance with the equation:
 
 P   mapped   =a+b*T   css   +c*T   csd   +d*T   css   2   +e*T   css   *T   csd   +f*T   csd   2   +g*T   css   3   +h*T   csd   *T   css   2   +i*T   css   *T   csd   2   +j*T   csd   3  
 
     where T css  represents saturated suction temperature and T csd  represents saturated discharge temperature. 
     In another aspect, the present disclosure is directed to an HVAC compressor prognostics system. The disclosed system includes a compressor; an electric motor configured to drive the compressor; and a prognostic-diagnostic unit configured for receiving information indicative of motor input power, compressor speed, compressor saturated discharge temperature, and compressor saturated suction temperature, the prognostic-diagnostic unit further configured to determine an operational condition of the compressor from the received information. 
     In some embodiments, the prognostic-diagnostic unit is further configured to receive a signal indicative of motor input power from an inverter drive coupled to the electric motor. In some embodiments, the prognostic-diagnostic unit is further configured to receive a signal indicative of compressor speed from an inverter drive coupled to the electric motor. In some embodiments, the prognostic-diagnostic unit is configured for receiving a signal indicative of compressor saturated discharge pressure, and the prognostic-diagnostic unit is further configured to map the compressor saturated discharge pressure to compressor saturated discharge temperature. 
     In some embodiments, the prognostic-diagnostic unit is configured for receiving a signal indicative of compressor saturated suction pressure, and the prognostic-diagnostic unit is further configured to map the compressor saturated suction pressure to compressor saturated suction temperature. In some embodiments, the prognostic-diagnostic unit is configured for receiving signals indicative of the input current of the electric motor and of the input voltage of the electric motor, and the prognostic-diagnostic unit is further configured to calculate the input power of the electric motor from the signals. In some embodiments, the prognostic-diagnostic unit is configured to determine a mapped input power from the received compressor speed, the compressor saturated discharge temperature, and the compressor saturated suction temperature. 
     In some embodiments, the prognostic-diagnostic unit is configured for calculating the mapped input power in accordance with the equation:
 
 P   mapped   =a+b*T   css   +c*T   csd   +d*T   css   2   +e*T   css   *T   csd   +f*T   csd   2   +g*T   css   3   +h*T   csd   *T   css   2   +i*T   css   *T   csd   2   +j*T   csd   3  
 
     In some embodiments, the prognostic-diagnostic unit is further configured for determining a ratio between motor input power and mapped input power. In some embodiments, the prognostic-diagnostic unit is further configured to transmit a fault signal if the ratio between the motor input power and the mapped input power exceeds a predetermined ratio. In some embodiments, the prognostic-diagnostic unit may be configured to transmit a second fault signal if the ratio between the motor input power and the mapped input power exceeds a second predetermined ratio. In some embodiments, the prognostic-diagnostic unit includes a memory device, and the prognostic-diagnostic unit is configured to store historical information including at least one of a timestamp, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, and/or information indicative of the compressor saturated suction temperature in the memory device if the ratio between the motor input power and the mapped input power exceeds a predetermined ratio. In some embodiments, the prognostic-diagnostic may transmit historical information from the memory device. 
     In yet another aspect, the present disclosure is directed to an HVAC compressor prognostic-diagnostic unit. In an embodiment, the HVAC compressor prognostic-diagnostic unit includes a mapping unit configured to receive a saturated suction pressure signal, a saturated discharge pressure signal, and a motor speed signal. The mapping unit is further configured to compute a mapped power from the saturated suction pressure signal, a saturated discharge pressure signal, and a motor speed signal. HVAC compressor prognostic-diagnostic unit includes a power determination unit configured to compute a motor input power. The HVAC compressor prognostic-diagnostic unit includes a comparison unit operatively coupled to the mapping unit and the power determination unit. The comparison unit is configured to receive the mapped power and the motor input power, and to compare the motor input power to the mapped motor input power to determine an operational condition of the compressor. The HVAC compressor prognostic-diagnostic unit includes a memory unit operatively coupled to the comparison unit and configured to stored the operational condition of the compressor, and an output unit operatively coupled to the comparison unit and/or the memory unit and configured to transmit the operational condition of the compressor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various embodiments of the disclosed system and method are described herein with reference to the drawings wherein: 
         FIG. 1  is a schematic diagram of an HVAC system incorporating a compressor prognostics system in accordance with an embodiment of the present disclosure; 
         FIG. 2  is a schematic diagram of an HVAC system incorporating a compressor prognostics system in accordance with another embodiment of the present disclosure; 
         FIG. 3  depicts a flowchart illustrating a method of assessing compressor health in accordance with an embodiment of the present disclosure; and 
         FIG. 4  shows a thermostat configured to receive and display compressor health in accordance with an embodiment of the present disclosure. 
     
    
    
     The various aspects of the present disclosure mentioned above are described in further detail with reference to the aforementioned figures and the following detailed description of exemplary embodiments. 
     DETAILED DESCRIPTION 
     Particular illustrative embodiments of the present disclosure are described hereinbelow with reference to the accompanying drawings; however, the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known functions or constructions and repetitive matter are not described in detail to avoid obscuring the present disclosure in unnecessary or redundant detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. In this description, as well as in the drawings, like-referenced numbers represent elements which may perform the same, similar, or equivalent functions. The word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments. The word “example” may be used interchangeably with the term “exemplary.” 
     The present disclosure is described herein in terms of functional block components and various processing steps. It should be appreciated that such functional blocks configured to perform the specified functions may be embodied in mechanical devices, electromechanical devices, analog circuitry, digital circuitry, and/or instructions executable on a processor. For example, the present disclosure may employ various discrete components, integrated circuit components (e.g., memory elements, processing elements, logic elements, look-up tables, and the like) which may carry out a variety of functions, whether independently, in cooperation with one or more other components, and/or under the control of one or more processors or other control devices. It should be appreciated that the particular implementations described herein are illustrative of the disclosure and its best mode and are not intended to otherwise limit the scope of the present disclosure in any way. 
     Embodiments of the present disclosure are directed to a prognostic-diagnostic unit (PDU) that compares expected compressor power consumption, based on current operating conditions of the HVAC system, to actual compressor power consumption. If the actual power consumption is within a predefined range of the expected power consumption, the HVAC system is determined to be operating properly, e.g., operating without any immediate or expected faults. Conversely, if the actual power consumption deviates from the expected power consumption by more than a predetermined amount, a fault is indicated. In this case, an appropriate action is taken, such as displaying a fault alert, transmitting a fault message to another device, storing information relating to the fault for later retrieval to assist troubleshooting, and so forth. 
     The relationship between the expected and actual power consumption, such as whether the actual power consumption is higher, lower, trending higher, or trending lower than the expected power consumption may provide additional detail as to the nature of an impending fault condition. 
     In one non-limiting example, a common compressor failure is caused by increased internal friction of compression elements (rotors, vanes, pistons, scrolls, etc.) or shaft bearings. Such increased internal friction causes input power to be converted to heat rather than perform useful compression work, which overheats the compressor and accelerates its failure. In this instance, input power will be greater than the expected power since some of the input power is being used to overcome increased friction rather than for performing work. The PDU will detect this imbalance and flag a fault condition indicating possible compressor failure. 
     In another non-limiting example, a mechanical failure within a compressor may cause input power to be less than expected. In this instance, the PDU will detect this imbalance and flag a fault condition indicating possible compressor failure. 
     In more detail, and with reference to  FIG. 1 , an example embodiment of a prognostic-diagnostic unit (PDU)  110  in accordance with the present disclosure is shown. PDU  110  is operatively associated with HVAC system  100  that includes an outdoor unit  102  and an indoor unit  104 . Outdoor unit  102  includes an electric motor  106  operatively engaged in rotational communication with compressor  108 ; and an electric motor  112  configured to drive fan  114 . Outdoor unit  102  includes a variable frequency inverter drive unit (VSD)  116  configured to drive electric motor  106 , and thus compressor  108 , at variable speed. VSD  116  receives power from utility  115 . VSD  116  is operatively coupled to a thermostat  122  or other suitable setpoint controller which activates and deactivates VSD  116  and motor  106  as required to maintain a desired setpoint temperature. A communications display assembly (CDA)  117  is operatively associated with VSD  116  and is configured to receive and display data relating to HVAC system  100 , including operational, configuration, and diagnostic data. Outdoor unit  102  includes a second VSD unit (not explicitly shown) that drives motor  112  and fan  114  at variable speed to circulate outdoor air through condenser  128 . Indoor unit  104 , sometimes referred to as an air handler, includes an electric motor  118  that drives blower  120  to circulate indoor air through evaporator  132 . 
     In use, outdoor unit  102  and indoor unit  104  interoperate to perform a vapor compression refrigeration cycle. Circulating refrigerant, such as R- 410 A, enters compressor  108  through inlet  124  as saturated vapor. Compressor  108  increases the pressure and temperature of the refrigerant resulting in superheated vapor which exits compressor  108  via outlet  126 . Superheated vapor flows through condenser  128 , where the superheated refrigerant vapor is cooled and condensed into saturated liquid form, moving heat from the refrigerant into outside air driven through condenser  128  by fan  114 . The refrigerant then passes through expansion valve  130  where it undergoes an abrupt reduction in pressure causing adiabatic flash evaporation of a portion of the liquid refrigerant, which, in turn, lowers the temperature of the liquid and vapor refrigerant mixture. The cold liquid and vapor mixture flows through the evaporator  132 . Blower  120  circulates warm indoor air through evaporator  132 . Heat from the indoor air is transferred into the refrigerant, causing it to expand and vaporize, cooling the indoor air. Warm refrigerant vapor exiting evaporator  132  returns to compressor  108 , and the vapor compression refrigeration cycle continues. 
     It should be understood that, while the present example embodiment illustrates HVAC system  100  configured as an air conditioning system, HVAC system  100  may be configured as a heat pump system. 
     A suction sensor  134  is operatively associated with compressor inlet  124  and configured to sense a property of the saturated refrigerant vapor as it enters compressor  108 . In the example embodiment shown in  FIG. 1 , suction sensor  134  is configured to sense the pressure of the refrigerant as it enters compressor  108 . In some embodiments, suction sensor  134  is configured to sense refrigerant temperature. A discharge sensor  136  is operatively associated with compressor outlet  126  and configured to sense a property of the superheated saturated refrigerant vapor as it exits compressor  108 . In the example embodiment shown in  FIG. 1 , discharge sensor  136  is configured to sense refrigerant pressure as it exits compressor  108 . In some embodiments, discharge sensor  136  is configured to sense exiting refrigerant temperature. Since refrigerant pressure at inlet  124  and/or outlet  126  is directly related to refrigerant temperature, the sensed pressure values are readily converted to temperature, or may be used in place of temperature, by employing a conversion coefficient or conversion formula known in the art. 
     The example embodiment illustrated by PDU  110  includes a mapping unit  138  that is configured to receive a saturated suction pressure signal received from suction sensor  134 , a saturated discharge pressure signal received from discharge sensor  136 , and a motor speed signal received from VSD  116 . The received signals operate to provide mapping unit  138  with terms used to calculate mapped input power. In the present embodiment, the saturated suction pressure signal operates to provide saturated suction temperature T css , the saturated discharge pressure signal operates to provide saturated discharge temperature T csd , and the motor speed signal operates to provide compressor speed S c . 
     In embodiments where mapping unit  138  receives an analog signal from suction sensor  134 , discharge sensor  136 , and/or VSD  116 , mapping unit  138  converts the analog signal received into a corresponding digital signal, and stores the resulting digital signal indicative of the respective measured or calculated refrigerant suction temperature, refrigerant discharge temperature, and/or compressor speed. 
     Mapping unit  138  calculates the mapped input power using equation (1) presented below where constants a through j are empirical coefficients:
 
 P   mapped =( a+b*T   css   +c*T   csd   +d*T   css   2   +e*T   css   *T   csd   +f*T   csd   2   +g*T   css   3   +h*T   csd   *T   css   2   +i*T   css   *T   csd   2   +j*T   csd   3 )  (1)
 
     In some embodiments, mapping unit  138  calculates the mapped input power using T css  and T csd  as indices into a pre-programmed data structure (array, database, etc.) in which the expected relationships between saturated suction pressure, saturated discharge pressure, and discrete compressor speeds to the expected (mapped) input power is encoded. The mapped input power is transmitted to comparison unit  140 . 
     To determine empirical coefficients a through j and/or to pre-program the data structure, outdoor unit  102  and/or subcomponents thereof (e.g., motor  106  and compressor  108 ) are evaluated using the calorimetric method under test conditions which simulate the operating conditions under which the system is intended to operate, as will be familiar to the skilled practitioner. In embodiments, multiple sets of coefficients are created, each set for a specific compressor speed. During use, the speed-specific data sets may be interpolated to determine P mapped  for compressor speeds falling between the particular speeds at which the coefficient data sets were generated. 
     Embodiments of the present disclosure may be advantageously utilized with HVAC systems having a wide range of capacities, therefore it is envisioned that coefficients a through k, and/or the pre-programming of the data structure are determined in connection with a particular configuration (e.g., production model) of outdoor unit  102  with which the disclosed prognostic system is utilized. 
     PDU  110  includes a comparison unit  140  that is configured to receive an input power signal that represents the actual input power P input  supplied to motor  106  from VSD  116  and the mapped power signal transmitted by mapping unit  138 . In some embodiments where an analog input power signal is received from VSD  116 , comparison unit  140  is configured to convert the analog input power signal into a corresponding digital signal. Comparison unit  140  is additionally configured to receive an ambient temperature signal from ambient temperature sensor  135 . Comparison unit  140  then compares the actual input power reported by VSD  116  to the mapped (expected) input power computed by mapping unit  138  to determine the operational condition of the compressor. In the present embodiment, comparison unit  140  determines if the ratio P input :P mapped  (e.g., the ratio between actual input power and mapped input power) exceeds a predetermined ratio. For example, if the P input :P mapped  is less than 1.1:1, it is determined that compressor  108  is operating normally. If, however P input :P mapped  is 1.1:1 or greater (e.g., actual input power is greater than 1.1 times mapped input power), it is concluded that compressor  108  is operating in a pre-fault condition, and in response, comparison unit  140  transmits a fault signal. In embodiments, the amount by which the actual input power exceeds the mapped input power may be quantified into a fault severity signal indicative of compressor health whereby as the ratio of actual to mapped compressor power increases, the severity of the fault signal increases. Quantifying fault severity in this manner may assist in evaluating the probability and timing of a compressor failure, in addition to providing diagnostic information to a technician. Fault severity may also be employed to assess the urgency of needed repairs, which enables service providers to better allocate and prioritize resources (e.g., technician time and replacement parts) to those HVAC systems most in need of attention. The fault severity signal may be included within the fault signal. 
     PDU  110  includes a memory unit  142  and output unit  144  in operative communication with comparison unit  140 . Comparison unit  140  is configured such that, when a fault is detected, information relating to the fault is transmitted to memory unit  142 , which stores the fault information for later retrieval. Fault information transmitted to comparison unit and/or stored in memory unit  142  may include any, some, or all of, without limitation, the date and time of the fault occurrence, actual input power, mapped power, compressor speed, saturated suction pressure, saturated suction temperature, saturated discharge pressure, saturated discharge temperature, fault severity, ambient temperature, and so forth. In some embodiments, historical information may be stored in the absence of a fault to maintain a record of the operational parameters received by PDU  110  corresponding to normal or acceptable operating conditions. Such information may be stored and/or transmitted on a periodic basis to confirm that compressor  108  is in good operating condition. 
     Information relating to the health of compressor  108  (which may indicate the presence or absence of a fault) may additionally or alternatively be transmitted via output unit  144 . In the example embodiment shown in  FIG. 1 , output unit  144  is in operative communication with VSD  116 . VSD  116  is configured for receiving fault signals from PDU  110 , which may be displayed on CDA  117  in response to user inputs received thereat from a user, typically an HVAC technician or facilities engineer. In embodiments, VSD  116  and PDU  110  may be configured for bidirectional communications whereby a technician inputs a request for historical fault information into CDA  117  which is transmitted to PDU  110 . PDU  110  responds to the request by retrieving stored fault information from memory unit  142 , which, in turn, is transmitted to VSD  116  for display on CDU  117 . In some embodiments, PDU  110  is configured to transmit predetermined mapping information to VSD  116  for display on CDU  117 . In these embodiments, a technician has the option to compare measurements manually obtained from the technician&#39;s test equipment to expected values for the system under test provided by PDU  110  and displayed on CDU  117 . In this manner, the fault indications provided by PDU  110  may be confirmed by the technician, and/or may facilitate troubleshooting of other malfunctions which may otherwise elude detection. 
     Output unit  144  may include a wired or wireless communications interface, and may include the capability to communicate using any suitable communication protocol, including without limitation Ethernet, RS-485, CANBus, 802.11WiFi, 802.15.4 personal area networks (Z-Wave®, ZigBee®), and so forth. 
       FIG. 2  illustrates a PDU  210  that is operatively associated with an HVAC system  200  in accordance with another embodiment of the present disclosure. In this embodiment, PDU  210  and thermostat  222  are configured to communicate fault signals to a homeowner and/or or a service provider. HVAC system  200  of  FIG. 2  is substantially similar to the  FIG. 1  example, and includes outdoor unit  202  having an electric motor  206  that drives compressor  208 . HVAC system  200  includes a control unit  216  that is in operative communication with thermostat  222  to receive system control commands (e.g., compressor on/off, compressor speed, etc.) transmitted by thermostat  222  in response to environmental conditions within the building and setpoint temperature. Thermostat  222  is additionally configured for communication with a remote database  230  via network  232 , such as the public internet. Thermostat  222  includes a communications interface  223  that is configured to communicate with PDU  210  via a corresponding communications interface  245  operatively coupled to output unit  244 . Thermostat  222  includes a user interface  221  having a display assembly and a user input assembly, such as, without limitation, an LCD display panel, a speaker or other audio output device, one or more buttons, rotary controls and/or switches, and/or a touchscreen device. 
     Utility power  215  is delivered to control unit  216 , which selectively provides input power to motor  206  in response to control signals received from thermostat  222 . In some embodiments where motor  206  is a variable speed motor, control unit  216  includes a variable speed inverter drive that is configured for driving motor  206  at a variable speed. In some embodiments, where motor  206  is a single speed motor, control unit  216  may contain an electromagnetic relay, solid state relay, or other suitable switching device configured to selectively deliver power to motor  206 . 
     Current sensor  235  is coupled in series between control unit  216  and motor  206 , and is configured to provide a current sensor signal indicative of the input current of motor  206  to a power determination unit  239  included in PDU  210 . Voltage sensor  237  is coupled in parallel with motor  206 , and is configured to provide a voltage sensor signal indicative of the input voltage to motor  206  to power determination unit  239 . Power determination unit  239  is configured to compute the actual input power to motor  206  from the current sensor signal and the voltage sensor signal. 
     A motor speed sensor  233  is operatively associated with motor  206  and configured to provide a speed sensor signal to mapping unit  238 . Motor speed sensor  233  may include any suitable rotational speed sensing device, such as an optical tachometer, magnetic or hall-effect tachometer, and/or may determine motor speed via back EMF measurement. A suction sensor  234  is operatively associated with compressor inlet  224  and configured to sense a property of the saturated refrigerant vapor as it enters compressor  208 . A discharge sensor  236  is operatively associated with compressor outlet  226  and configured to sense a property of the superheated saturated refrigerant vapor as it exits compressor  208 . 
     Mapping unit  238  is configured to receive a saturated suction pressure signal received from suction sensor  234 , a saturated discharge pressure signal received from discharge sensor  236 , and a speed sensor signal received from speed sensor  233 . The received signals operate to provide mapping unit  238  with terms used to calculate mapped input power. In the present embodiment, the saturated suction pressure signal operates to provide saturated suction temperature T css , the saturated discharge pressure signal operates to provide saturated discharge temperature T csd , and the speed sensor signal operates to provide compressor speed S c . 
     Comparison unit  240  is configure to compare the actual input power computed by power determination unit  239  to the mapped (expected) input power computed by mapping unit  238  to determine if a pre-fault or fault compressor condition is indicated as described hereinabove. In response to a determination that a pre-fault or fault condition exists, comparison unit  240  transmits fault information to memory unit  242 , which stores the fault information for later retrieval. Fault information may additionally or alternatively be transmitted via output unit  244  and communications interface  245 . In this embodiment, fault information is wirelessly transmitted to thermostat  222 . In some embodiments, thermostat  222  is configured to display a fault message on user interface  221  in response to receipt of fault information from PDU  210 . The fault message may include a visual indication, such as a textual message and/or an icon, and/or an alert sound. In some embodiments, thermostat  222  transmits the fault information to database  230 . In some embodiments, the user is presented with a choice of whether to transmit the fault information to a service provider. In response to an affirmative choice, thermostat  222  transmits the fault information to a database  230  that is accessible by the service provider. The service provider may choose to take action, including without limitation, contacting the homeowner to schedule a service call, soliciting additional diagnostic information from HVAC system  200 , and/or causing thermostat  222  to display an informational message to the user. In some embodiments, thermostat  222  provides an interactive service appointment scheduler that enables the service provider and user to collaboratively schedule a service call. 
     Turning now to  FIG. 3 , a method  300  of determining an operational condition of an HVAC compressor is illustrated wherein the health of the compressor is characterized into three “health” zones. The method  300  begins in step  305  where a motor input power P input  is received. P input  indicates the actual input power delivered to an electric motor that drives the subject HVAC compressor. P input  may be obtained from a data port provided on a variable speed inverter drive unit (VSD) and/or derived from one or more sensors electrically associated with the motor and configured to sense an electrical property of the motor, such as current, voltage, Watts, VA, etc. A power factor correction may be applied to a measured property or a power value derived therefrom. In step  310 , the compressor speed (e.g., the rotational speed of the compressor shaft), the saturated suction temperature, and the saturated discharge temperature are obtained. In some embodiments, the saturated suction pressure and/or the saturated discharge pressure may be obtained and converted to saturated suction temperature and/or saturated discharge temperature, respectively. 
     In step  315 , the mapped (expected) input power P mapped  is determined. In some embodiments, P mapped  is computed using formula (1) discussed above. In some embodiments, P mapped  is determined using a preprogrammed lookup tabled stored in a memory device. In steps  320  and  330  the ratio of P input :P mapped  is evaluated. In step  320 , if P input :P mapped  is less than 1.1:1, the compressor is determined to be operating normally, and therefore the health of the compressor is said to be in the “green zone” (step  325 ). In some embodiments, in step  325  a “healthy compressor” (“green zone”) message is caused to be displayed on a thermostat or other device having a suitable user interface that is operatively associated with the compressor. The method iterates to step  305  where the process repeats. Additionally or alternatively, health data may be transmitted to indicate that the compressor is operating normally. 
     If, in step  320 , P input :P mapped  is not less than 1.1:1, then in step  330 , the ratio of P input :P mapped  is evaluated to determine whether P input :P mapped  is less than 1.3:1. If P input :P mapped  is less than 1.3:1 (e.g., P input :P mapped  is at least 1.1:1 but no more than 1.3:1) the compressor is determined to be in a level  1  pre-fault condition, and therefore the health of the compressor is said to be in the “yellow zone.” In step  335 , compressor health data is stored in a memory unit. Compressor health data may include, but is not limited to, any, some, or all of a timestamp indicating the date and time of the detected fault, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, information indicative of the compressor saturated suction temperature, and/or information indicative of an environmental condition (indoor temperature, indoor humidity, outdoor temperature, outdoor humidity, etc.). In some embodiments, a “yellow zone” health message may be caused to be displayed on a user interface, as described above. In step  345 , compressor health data is transmitted to a device, such as without limitation, a thermostat, a diagnostic device, and/or a service provider database, and the process iterates to step  305 . 
     If, in step  330 , it is determined that the ratio of P input :P mapped  is 1.3:1 or greater, then the compressor is determined to be in a level  2  pre-fault condition, and the health of the compressor is said to be in the “red zone.” In step  340 , the compressor health data is stored in a memory unit as described above. In some embodiments, a “red zone” health message may be caused to be displayed on a user interface, as described above, and the method proceeds with transmission step  345 , and iterates to step  305 . 
     It should be appreciated that embodiments of the present disclosure may employ greater or fewer health characterization zones than the example embodiment described herein, and may employ ratio thresholds other than 1.1:1 and 1.3:1 to demarcate the various health zones. 
     For example, embodiments of the present disclosure may additionally or alternatively be configured to detect a malfunction(s) which causes P input  to be undesirably less than P mapped . In these embodiments, the “red zone” may additionally or alternatively be defined as when P input :P mapped  is determined to be 0.7:1 or less. The “yellow zone” may additionally or alternatively be defined as when P input :P mapped  is determined to be between 0.7:1 and 0.85:1. 
     It should be appreciated that aspects of the present disclosure may embodied in a user device, such as handheld diagnostic device, and/or a software application executable on a computing device such as a smart phone, tablet computer, or notebook computer. In these embodiments, a technician has the ability to input into the user device the motor input power, compressor speed, saturated suction temperature, and saturated discharge temperature. In embodiments, a camera included within the user device may be employed as an optical tachometer to facilitate measurement of compressor speed. A software application that includes instructions for performing the methods described herein executes on a processor included in the user device to receive the measured parameters and to generate a diagnostic health message. 
       FIG. 4  illustrates an example embodiment of a device  400  that is configured to display compressor health. In the present embodiment, device  400  is depicted as a wall-mounted thermostat having a touchscreen user interface  410 , however device  400  may include any device having a suitable user interface display, for example, CDA  117  and/or VDU  116 . Device  400  receives health data that has been transmitted as described above, e.g., from PDU  110 , from PDU  220 , and/or from a device which performs method  300 . In response to receiving the health data, device  400  displays a health message  420  on user interface  420 . As can be seen in  FIG. 4 , health message  420  indicates the HVAC compressor is operating normally, in the “green zone.” Health messages which indicate operating conditions, e.g., “yellow zone” messages, “red zone” message and/or messages containing additional information such as compressor speed, suction and discharge temperatures and pressures, input power, and so forth may additionally or alternative be displayed in response to received health data. In some embodiments, device  400  includes a speaker or other audio output device that is configured to issue an alert sound when compressor health is in a fault zone (e.g., yellow or red). In these embodiments, the alert sound is repeated on a periodic basis until acknowledged or canceled by a user. 
     ASPECTS 
     It is noted that any of aspects 1-11, any of aspects 12-27, and/or aspect 28 may be combined with each other in any combination. 
     Aspect 1. A method for determining an operational condition of a compressor operatively coupled to an electric motor, comprising receiving information indicative of an actual input power of the electric motor; receiving information indicative of a compressor speed, a compressor saturated suction temperature, and a compressor saturated discharge temperature; determining a mapped input power from the received compressor speed, the compressor saturated suction temperature, and the compressor saturated discharge temperature; and determining the operational condition of the compressor from the actual input power and the mapped input power. 
     Aspect 2. The method in accordance with aspect 1, wherein receiving information indicative of an actual input power includes receiving the information from an inverter drive coupled to the electric motor. 
     Aspect 3. The method in accordance with any of aspects 1-2, wherein receiving information indicative of a compressor speed includes receiving the information from an inverter drive coupled to the electric motor. 
     Aspect 4. The method in accordance with any of aspects 1-3, further comprising receiving a sensor signal corresponding to a compressor saturated discharge pressure; and mapping the compressor saturated discharge pressure to the compressor saturated discharge temperature. 
     Aspect 5. The method in accordance with any of aspects 1-4, further comprising receiving a sensor signal corresponding to a compressor saturated suction pressure; and mapping the compressor saturated suction pressure to the compressor saturated suction temperature. 
     Aspect 6. The method in accordance with any of aspects 1-5, wherein determining the operational condition of the compressor includes determining if the ratio between the actual input power and the mapped input power exceeds a predetermined ratio. 
     Aspect 7. The method in accordance with any of aspects 1-6, further comprising issuing an alert in response to the exceeding. 
     Aspect 8. The method in accordance with any of aspects 1-7, wherein the predetermined ratio is selected from the group consisting of about 1.1:1 and about 1.3:1. 
     Aspect 9. The method in accordance with any of aspects 1-8, wherein determining the operational condition of the compressor includes determining if the ratio between the actual input power and the mapped input power is less than a predetermined ratio. 
     Aspect 10. The method in accordance with any of aspects 1-9, further comprising issuing an alert in response to a determination that the mapped input power is less than a predetermined ratio. 
     Aspect 11. The method in accordance with any of aspects 1-10, wherein the predetermined ratio is selected from the group consisting of about 0.7:1 and about 0.85:1. 
     Aspect 12. The method in accordance with any of aspects 1-11, further comprising storing, in response to the exceeding, historical information including at least one of a timestamp, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, and/or information indicative of the compressor saturated suction temperature in a memory device. 
     Aspect 13. The method in accordance with any of aspects 1-12, further comprising retrieving, from the memory device, the historical information. 
     Aspect 14. The method in accordance with any of aspects 1-13, wherein determining a mapped input power includes calculating the mapped input power in accordance with the equation:
 
 P   mapped   =a+b*T   css   +c*T   csd   +d*T   css   2   +e*T   css   *T   csd   +f*T   csd   2   +g*T   css   3   +h*T   csd   *T   css   2   +i*T   css   *T   csd   2   +j*T   csd   3  
 
     Aspect 15. An HVAC compressor prognostics system, comprising a compressor; an electric motor configured to drive the compressor; and a prognostic-diagnostic unit configured for receiving information indicative of motor input power, compressor speed, compressor saturated discharge temperature, and compressor saturated suction temperature, the prognostic-diagnostic unit further configured to determine an operational condition of the compressor from the received information. 
     Aspect 16. The HVAC compressor prognostics system in accordance with aspect 15, wherein the prognostic-diagnostic unit is further configured to receive a signal indicative of motor input power from an inverter drive coupled to the electric motor. 
     Aspect 17. The HVAC compressor prognostics system in accordance with any of aspects 15-16, wherein the prognostic-diagnostic unit is further configured to receive a signal indicative of compressor speed from an inverter drive coupled to the electric motor. 
     Aspect 18. The HVAC compressor prognostics system in accordance with any of aspects 15-17, wherein the prognostic-diagnostic unit is configured for receiving a signal indicative of compressor saturated discharge pressure and wherein the prognostic-diagnostic unit is further configured to map the compressor saturated discharge pressure to compressor saturated discharge temperature. 
     Aspect 19. The HVAC compressor prognostics system in accordance with any of aspects 15-18, wherein the prognostic-diagnostic unit is configured for receiving a signal indicative of compressor saturated suction pressure and wherein the prognostic-diagnostic unit is further configured to map the compressor saturated suction pressure to compressor saturated suction temperature. 
     Aspect 20. The HVAC compressor prognostics system in accordance with any of aspects 15-19 wherein the prognostic-diagnostic unit is further configured for receiving signals indicative of the input current of the electric motor and of the input voltage of the electric motor, and wherein the prognostic-diagnostic unit is further configured to calculate the input power of the electric motor from the signals. 
     Aspect 21. The HVAC compressor prognostics system in accordance with any of aspects 15-20 wherein the prognostic-diagnostic unit is further configured to determine a mapped input power from the received compressor speed, the compressor saturated discharge temperature, and the compressor saturated suction temperature. 
     Aspect 22. The HVAC compressor prognostics system in accordance with any of aspects 15-21 wherein the prognostic-diagnostic unit is further configured for calculating the mapped input power in accordance with the equation:
 
 P   mapped   =a+b*T   css   +c*T   csd   +d*T   css   2   +e*T   css   *T   csd   +f*T   csd   +g*T   css   3   +h*T   csd   *T   css   2   +i*T   css   *T   csd   2   +j*T   csd   3  
 
     Aspect 23. The HVAC compressor prognostics system in accordance with any of aspects 15-22, wherein the prognostic-diagnostic unit is further configured for determining a ratio between motor input power and mapped input power. 
     Aspect 24. The HVAC compressor prognostics system in accordance with any of aspects 15-23, wherein the prognostic-diagnostic unit is further configured to transmit a fault signal if the ratio between the motor input power and the mapped input power exceeds a predetermined ratio. 
     Aspect 25. The HVAC compressor prognostics system in accordance with any of aspects 15-24, wherein the prognostic-diagnostic unit is further configured to transmit a second fault signal if the ratio between the motor input power and the mapped input power exceeds a second predetermined ratio. 
     Aspect 26. The HVAC compressor prognostics system in accordance with any of aspects 15-24, the prognostic-diagnostic unit further comprising a memory device, wherein the prognostic-diagnostic unit is further configured to store historical information including at least one of a timestamp, information indicative of the actual input power of the electric motor, information indicative of the compressor speed, information indicative of the compressor saturated discharge temperature, and/or information indicative of the compressor saturated suction temperature in the memory device if the ratio between the motor input power and the mapped input power exceeds a predetermined ratio. 
     Aspect 27. The HVAC compressor prognostics system in accordance with any of aspects 15-25, wherein the prognostic-diagnostic unit is further configured to transmit historical information from the memory device. 
     Aspect 28. An HVAC compressor prognostic-diagnostic unit, comprising a mapping unit configured to receive a saturated suction pressure signal, a saturated discharge pressure signal, and a motor speed signal, the mapping unit further configured to compute a mapped power from the saturated suction pressure signal, a saturated discharge pressure signal, and a motor speed signal; a power determination unit configured to compute a motor input power; a comparison unit operatively coupled to the mapping unit and the power determination unit and configured to receive the mapped power and the motor input power, the comparison unit further configured to compare the motor input power to the mapped motor input power to determine an operational condition of the compressor; a memory unit operatively coupled to the comparison unit and configured to store the operational condition of the compressor; and an output unit operatively coupled to the comparison unit and/or the memory unit and configured to transmit the operational condition of the compressor. 
     Particular embodiments of the present disclosure have been described herein, however, it is to be understood that the disclosed embodiments are merely examples of the disclosure, which may be embodied in various forms. Well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in any appropriately detailed structure.