Abstract:
A variable speed drive (VSD) can be used to vary the voltage-to-frequency ratio (V/f) supplied to a compressor motor of a heating, ventilation, air conditioning or refrigeration (HVAC&amp;R) system to make the motor stronger or weaker to compensate for varying conditions in the HVAC&amp;R system. The VSD and corresponding control system or algorithm can monitor an operating parameter of the HVAC&amp;R system, such as the kW absorbed by the motor, and then raise or lower the V/f of the VSD to obtain the lowest possible power consumption from the motor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Application No. 61/529,437, filed Aug. 31, 2011, entitled VARIABLE SPEED DRIVE CONTROL SYSTEM AND METHOD, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The application generally relates to variable speed drives. The application relates more specifically to controlling the ratio of voltage to frequency output by a variable speed drive or variable frequency drive. 
     In a chiller system or other heating, ventilation, air conditioning or refrigeration (HVAC&amp;R) system where the compressor is coupled with a variable frequency drive (VFD) or variable speed drive (VSD), the compressor motor is typically sized to operate at a particular voltage-to-frequency (V/f) ratio and a particular load point. Because the compressor in the actual system can operate during a variety of conditions, the motor is typically not operating at peak efficiency. 
     Therefore, what is needed is a variable speed drive or variable frequency drive that can vary the ratio of voltage to frequency to compensate for varying load conditions. 
     SUMMARY 
     The present invention is directed to a system having a compressor, a condenser, an expansion device and an evaporator connected in a closed refrigerant circuit. The system includes a motor connected to the compressor to power the compressor and a variable speed drive connected to the motor to power the motor. The variable speed drive is operable to provide a variable voltage to the motor and a variable frequency to the motor. The system also includes a control panel to control operation of the variable speed drive and one or more components of the system and a sensor to measure an operational parameter of the system. The sensor is operable to communicate the measured operational parameter to the control panel. The control panel is operable to execute a control algorithm to determine a voltage-to-frequency ratio to be output by the variable speed drive using the measured operational parameter, and the voltage-to-frequency ratio varies based on the measured operational parameter. 
     The present invention is also directed to a method for controlling a variable speed drive. The method includes measuring an operating parameter of an HVAC&amp;R system and determining a voltage to frequency ratio to be output by a variable speed drive using the measured operational parameter. The variable speed drive powers a compressor motor of the HVAC&amp;R system. The method also includes generating control instructions for the variable speed drive based on the determined voltage to frequency ratio and adjusting the output voltage to frequency ratio provided by the variable speed drive to the compressor motor with the generated control instructions. 
     In the present application, the VFD or VSD can vary the V/f supplied to the motor to make the motor stronger or weaker to compensate for the varying conditions in an HVAC&amp;R system. The VFD or VSD and corresponding controls can monitor the motor&#39;s power consumption (kW) absorbed by the motor and then raise or lower the V/f of the VFD or VSD to obtain the lowest possible power consumption from the motor. 
     One advantage of the present application is lower power consumption by the compressor motor which leads to energy savings. 
     Another advantage of the present application is the ability to correspond the ratio of voltage to frequency provided to the compressor motor based on the load conditions on the compressor. The correspondence of the ratio of voltage to frequency to the load conditions enables the compressor motor to operate at peak efficiency and thereby reduce power consumption. 
     Other features and advantages of the present invention will be apparent from the following, more detailed description of the preferred embodiments, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary embodiment for a heating, ventilation and air conditioning system. 
         FIG. 2  shows an isometric view of an exemplary vapor compression system. 
         FIGS. 3 and 4  schematically show exemplary embodiments of a vapor compression system. 
         FIG. 5  schematically shows an exemplary embodiment of a variable speed drive. 
         FIGS. 6-11  show charts of motor temperature and compressor efficiency versus frequency for different V/f ratios used in an exemplary HVAC&amp;R system. 
         FIG. 12  shows a chart of motor temperature versus frequency for the different V/f ratios from  FIGS. 6-11 . 
         FIG. 13  shows a chart of compressor efficiency versus frequency for the different V/f ratios from  FIGS. 6-11 . 
         FIG. 14  shows a chart of peak sound levels versus frequency for different V/f ratios used in an exemplary HVAC&amp;R system. 
         FIG. 15  shows an enlarged portion of the chart of  FIG. 14 . 
         FIG. 16  shows an exemplary embodiment of a process for adjusting the V/f ratio of a variable speed drive. 
     
    
    
     Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows an exemplary environment for a heating, ventilation and air conditioning (HVAC) system  10  in a building  12  for a typical commercial setting. The system  10  can include a vapor compression system  14  that can supply a chilled liquid which may be used to cool the building  12 . The system  10  can include a boiler  16  to supply heated liquid that may be used to heat the building  12  and an air distribution system which circulates air through the building  12 . The air distribution system can also include an air return duct  18 , an air supply duct  20  and an air handler  22 . The air handler  22  can include a heat exchanger that is connected to the boiler  16  and vapor compression system  14  by conduits  24 . The heat exchanger in the air handler  22  may receive either heated liquid from the boiler  16  or chilled liquid from the vapor compression system  14 , depending on the mode of operation of the system  10 . The system  10  is shown with a separate air handler on each floor of the building  12 , but it is appreciated that the components may be shared between or among floors. 
       FIGS. 2 and 3  show an exemplary vapor compression system  14  that can be used in an HVAC system  10 . The vapor compression system  14  can circulate a refrigerant through a circuit starting with a compressor  32  and including a condenser  34 , expansion valve(s) or device(s)  36 , and an evaporator or liquid chiller  38 . The vapor compression system  14  can also include a control panel  40  that can include an analog to digital (A/D) converter  42 , a microprocessor  44 , a non-volatile memory  46 , and an interface board  48 . Some examples of fluids that may be used as refrigerants in the vapor compression system  14  are hydrofluorocarbon (HFC) based refrigerants, for example, R-410A, R-407, R-134a, hydrofluoro olefin (HFO), “natural” refrigerants like ammonia (NH 3 ), R-717, carbon dioxide (CO 2 ), R-744, or hydrocarbon based refrigerants, water vapor or any other suitable type of refrigerant. In an exemplary embodiment, the vapor compression system  14  may use one or more of each of variable speed drive (VSD)  52 , motor  50 , compressor  32 , condenser  34 , expansion valve  36  and/or evaporator  38  in one or more refrigerant circuits. 
     The motor  50  used with the compressor  32  can be powered by a VSD  52 . The VSD  52  receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source and provides power having a variable voltage and frequency to the motor  50 . The motor  50  can include any type of electric motor that can be powered by a VSD. The motor  50  can be any suitable motor type, for example, a switched reluctance motor, an induction motor, or an electronically commutated permanent magnet motor. 
     The compressor  32  compresses a refrigerant vapor and delivers the vapor to the condenser  34  through a discharge passage. The compressor  32  can be a screw compressor in one exemplary embodiment. However, the compressor  32  can be any suitable type of positive displacement compressor or a centrifugal compressor. The refrigerant vapor delivered by the compressor  32  to the condenser  34  transfers heat to a fluid, for example, water or air. The refrigerant vapor condenses to a refrigerant liquid in the condenser  34  as a result of the heat transfer with the fluid. The liquid refrigerant from the condenser  34  flows through the expansion device  36  to the evaporator  38 . In the exemplary embodiment shown in  FIG. 3 , the condenser  34  is water cooled and includes a tube bundle  54  connected to a cooling tower  56 . 
     The liquid refrigerant delivered to the evaporator  38  absorbs heat from another fluid, which may or may not be the same type of fluid used for the condenser  34 , and undergoes a phase change to a refrigerant vapor. In the exemplary embodiment shown in  FIG. 3 , the evaporator  38  includes a tube bundle having a supply line  60 S and a return line  60 R connected to a cooling load  62 . A process fluid, for example, water, ethylene glycol, calcium chloride brine, sodium chloride brine, or any other suitable liquid, enters the evaporator  38  via the return line  60 R and exits the evaporator  38  via the supply line  60 S. The evaporator  38  lowers the temperature of the process fluid in the tubes. The tube bundle in the evaporator  38  can include a plurality of tubes and a plurality of tube bundles. The vapor refrigerant exits the evaporator  38  and returns to the compressor  32  by a suction line to complete the cycle. 
       FIG. 4 , which is similar to  FIG. 3 , shows the vapor compression system  14  with an intermediate circuit  64  incorporated between the condenser  34  and the expansion device  36 . The intermediate circuit  64  has an inlet line  68  that can be either connected directly to or can be in fluid communication with the condenser  34 . As shown, the inlet line  68  includes an expansion device  66  positioned upstream of an intermediate vessel  70 . The intermediate vessel  70  can be a flash tank, also referred to as a flash intercooler, in an exemplary embodiment. In an alternate exemplary embodiment, the intermediate vessel  70  can be configured as a heat exchanger or a “surface economizer.” In the configuration shown in  FIG. 4 , the intermediate vessel  70  is a flash tank and the expansion device  66  operates to lower the pressure of the liquid received from the condenser  34 . During the expansion process, a portion of the liquid vaporizes. The intermediate vessel  70  may be used to separate the vapor from the liquid received from the expansion device  66  and may also permit further expansion of the liquid. The vapor may be drawn by the compressor  32  from the intermediate vessel  70  through a line  74  to the suction inlet, a port at a pressure intermediate between suction and discharge or an intermediate stage of compression. The liquid that collects in the intermediate vessel  70  is at a lower enthalpy from the expansion process. The liquid from the intermediate vessel  70  flows in a line  72  through a second expansion device  36  to the evaporator  38 . 
     In an exemplary embodiment, a compressor  32  can include a compressor housing that contains the working parts of the compressor  32 . Vapor from the evaporator  38  can be directed to an intake passage of the compressor  32 . The compressor  32  compresses the vapor with a compression mechanism and delivers the compressed vapor to the condenser  34  through a discharge passage. The motor  50  may be connected to the compression mechanism of the compressor  32  by a drive shaft. 
     Vapor flows from the intake passage of a positive displacement compressor  32  and enters a compression pocket of the compression mechanism. The compression pocket is reduced in size by the operation of the compression mechanism to compress the vapor. The compressed vapor can be discharged into the discharge passage. For example, for a screw compressor, the compression pocket is defined between the surfaces of the rotors of the compressor. As the rotors of the compressor engage one another, the compression pockets between the rotors of the compressor, also referred to as lobes, are reduced in size and are axially displaced to a discharge side of the compressor. 
       FIG. 5  shows an exemplary embodiment of a VSD. The VSD  52  receives AC power having a particular fixed line voltage and fixed line frequency from an AC power source and provides AC power to a motor  50  at a desired voltage and desired frequency, both of which can be varied to satisfy particular requirements. The VSD  52  can have three components: a rectifier/converter  222 , a DC link  224  and an inverter  226 . The rectifier/converter  222  converts the fixed frequency, fixed magnitude AC voltage from the AC power source into DC voltage. The DC link  224  filters the DC power from the converter  222  and provides energy storage components such as capacitors and/or inductors. Finally, the inverter  226  converts the DC voltage from the DC link  224  into variable frequency, variable magnitude AC voltage for the motor  50 . 
     In an exemplary embodiment, the rectifier/converter  222  may be a three-phase pulse width modulated boost rectifier having insulated gate bipolar transistors to provide a boosted DC voltage to the DC link  224  to obtain a maximum RMS output voltage from the VSD  52  greater than the input voltage to the VSD  52 . Alternately, the converter  222  may be a passive diode or thyristor rectifier without voltage-boosting capability. 
     The VSD  52  can provide a variable magnitude output voltage and a variable frequency to the motor  50 , to permit effective operation of the motor  50  in response to particular load conditions. The control panel  40  can provide control signals to the VSD  52  to operate the VSD  52  and the motor  50  at appropriate operational settings for the particular sensor readings received by the control panel  40 . For example, the control panel  40  can provide control signals to the VSD  52  to adjust the output voltage and output frequency provided by the VSD  52  in response to changing conditions in the vapor compression system  14 . In one exemplary embodiment, the control panel  40  can provide instructions to increase or decrease the output voltage and output frequency, while maintaining the same V/f ratio, provided by the VSD  52  in response to increasing or decreasing load conditions on the compressor  32 . 
     However, in another exemplary embodiment, the control panel  40  can individually increase or decrease the output voltage and/or the output frequency from the VSD  52  to obtain different V/f ratios from the VSD  52 . In one exemplary embodiment, the control panel can adjust the V/f ratio based on the motor&#39;s power consumption (kW). However, in other embodiments, different operating parameters (e.g., compressor discharge temperature or motor temperature), can be used in addition to or instead of the motor&#39;s power consumption. The control panel can select the appropriate V/f ratio for the VSD from one or more look-up tables based the current or measured operating conditions or parameters of the motor and/or system. The look-up tables can be generated as part of the system start-up process (either at the factory or at the site) and involves operating the system at varying conditions to determine the optimal V/f ratio for particular conditions. In another embodiment, the control panel can determine an operating frequency for the VSD using a capacity control algorithm with the current or measured operating conditions or parameters of the motor and/or system as an input and then select the appropriate voltage corresponding to that operating frequency from the capacity control algorithm that provides maximum efficiency. In yet another embodiment, the control panel can control the VSD to iteratively cycle through various V/f ratios and select the one that provides the best efficiency. In still another embodiment, the V/f ratio can be calculated from a control algorithm, such as a fuzzy logic algorithm, based on the measured operating conditions or parameters of the motor and/or system. 
       FIG. 16  shows an exemplary embodiment of a control process executed by the control panel to vary the V/f ratio of a VSD. The process begins by measuring one or more operating parameters from the HVAC&amp;R system (step  302 ). In one embodiment, the measured operating parameter can be the motor&#39;s power consumption (kW). However, in other embodiments, different operating parameters, e.g., compressor discharge temperature, motor temperature or motor current, can be used in addition to or instead of the motor&#39;s power consumption. Next, a V/f ratio for the VSD is determined from the measured operating parameter (step  304 ). In one embodiment, the determined V/f ratio can be determined from one or more tables that correspond the measured operating parameters to V/f ratios. In other embodiments, one or more control algorithms can be used to determine or calculate the V/f ratio using the measured operating parameter or other preselected parameters. Once the V/f ratio for the VSD has been determined, the control panel can generate control instructions for the VSD to implement the determined V/f ratio (step  306 ). The output of the VSD is then adjusted using the control instructions to provide the determined V/f ratio to the compressor motor (step  308 ). The process then returns to the start to repeat the process. 
     For  FIGS. 6-13 , an HVAC system was operated at different V/f ratios. The HVAC system used R-134a refrigerant, and operated at a condenser temperature of about 100° Fahrenheit (F) and an evaporator temperature of about 40° F. In each of  FIGS. 6-11 , the compressor (adiabatic) efficiency, i.e., the ratio of the theoretical power consumption for the compressor to the actual power consumption (W theo /W actual ), is shown for a range of frequencies and a particular V/f ratio. In addition, a temperature of the compressor motor is shown for the same range of frequencies and particular V/f ratio. 
     In another embodiment, the V/f ratio can be varied for sound attenuation purposes since noise can be generated by vibrations within the motor. As shown in  FIGS. 14 and 15 , different V/f ratios produce different peak noise levels in the compressor and an optimum V/f can be selected to reduce noise levels in the compressor. In  FIGS. 14 and 15 , the “Poly” lines represent trend data for the corresponding voltage identified. The optimum V/f can be selected in a manner similar to that previously described for motor/system efficiency and can be dependent upon the selected motor and the applied load. 
     It is important to note that the construction and arrangement of the present application as shown in the various exemplary embodiments is illustrative only. Although only a few embodiments have been described in detail in this application, those who review this application will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters (e.g., temperatures, pressures, etc.), mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described in the application. For example, elements shown as integrally formed may be constructed of multiple parts or elements, the position of elements may be reversed or otherwise varied, and the nature or number of discrete elements or positions may be altered or varied. Accordingly, all such modifications are intended to be included within the scope of the present application. The order or sequence of any process or method steps may be varied or re-sequenced according to alternative embodiments. In the claims, any means-plus-function clause is intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. Other substitutions, modifications, changes and omissions may be made in the design, operating conditions and arrangement of the exemplary embodiments without departing from the scope of the present application. Accordingly, the present application is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims. 
     Furthermore, in an effort to provide a concise description of the exemplary embodiments, all features of an actual implementation may not have been described (i.e., those unrelated to the presently contemplated best mode of carrying out the invention, or those unrelated to enabling the invention). It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation specific decisions may be made. Such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure, without undue experimentation.