Abstract:
A method of controlling the frequency of a compressor in a chiller system is disclosed. The method includes determining a nominal minimum frequency of a compressor, and providing a control algorithm for controlling the period of time and the deviation of the actual operating frequency below the nominal minimal frequency that the compressor may operate without shutting down the chiller system. The time period of operation below nominal minimum frequency may be predetermined as a fixed time period of operation below nominal minimum frequency, or as a variable time period based on the amount by which the frequency deviates below nominal minimum frequency. Also, an absolute minimum operating frequency is provided that results in the control algorithm shutting down the chiller system and the compressor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/754,765, filed Dec. 29, 2005. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to a control algorithm for controlling the capacity of a compressor, and more particularly to a control algorithm for reducing the capacity of a compressor in a chiller system below the nominal minimum capacity of the compressor. 
     BACKGROUND OF THE INVENTION 
     Refrigeration systems or chiller systems typically include a motor driven compressor, condenser, and evaporator in a closed refrigeration loop. Variable speed drives (VSDs) are used in many instances to power the compressor motor. The voltage and frequency of the AC voltage is varied at the output of the VSD to vary the speed of the compressor motor. In large compressors for use in commercial, industrial and large HVAC&amp;R systems in particular, the compressor manufacturer typically sets forth a minimum rotational frequency of the compressor. This limitation is established by the manufacturer primarily to ensure sufficient lubrication for bearings in the compressor in addition to other considerations. The bearings and other moving parts are lubricated by oil circulating within the compressor. As the compressor speed decreases, the amount of oil available for lubrication is reduced. 
     In order to prevent damage to the compressor bearings and other moving parts due to insufficient lubrication, the manufacturers normally set forth a minimum operating rotational frequency for each compressor. This minimum operating rotational frequency, also referred to as the nominal minimum compressor frequency, is normally monitored by the chiller control system. As a precautionary measure, the control system shuts down the compressor in response to sensing the operating frequency drop below the nominal minimum frequency. This limitation can be problematic, for example, where the compressor speed is controllably reduced due to a temporary reduction in cooling demand. In some instances, the compressor frequency may drop below the nominal minimum frequency for only a brief period, or the compressor frequency may be controlled at a frequency below the nominal minimum frequency by a very small margin. In such a case, the risk of overheating the bearings due to a lack of lubrication is generally minimal, and the inconvenience and expense of restarting the chiller system outweighs the potential benefit of the system shutdown. 
     Therefore, there is a need for a control system that controls the actual operating frequency of the compressor below the minimum operating rotational frequency for a short period before turning the compressor off. 
     SUMMARY OF THE INVENTION 
     One embodiment of the present invention is directed to a method of controlling the frequency of a compressor in a chiller system. The method includes the steps of monitoring an operating frequency of a compressor; comparing the operating frequency to a nominal minimum frequency for the compressor; comparing the operating frequency to a predetermined absolute minimum frequency for the compressor in response to the operating frequency being less than the nominal minimum frequency, the predetermined absolute minimum frequency being less than the nominal minimum frequency; operating the compressor for a predetermined time period in response to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than or equal to the predetermined absolute minimum frequency, and shutting down the compressor in response to an expiration of the predetermined time period of the operating frequency being less than the nominal minimum frequency. 
     Another embodiment of the present invention is directed to a chiller system. The chiller system includes a refrigerant circuit having a compressor, a condenser arrangement and an evaporator arrangement connected in a closed refrigerant loop. The chiller system also includes a drive arrangement connected to the compressor to power the compressor. The drive arrangement has a motor, a variable speed drive, and a controller for controlling the frequency of the compressor. 
     The controller is configured to monitor an operating frequency of the compressor and compare the operating frequency to a nominal minimum frequency for the compressor. The controller compares the operating frequency to a predetermined absolute minimum frequency for the compressor in response to the operating frequency being less than the nominal minimum frequency. The predetermined absolute minimum frequency is less than the nominal minimum frequency. The controller operates the compressor for a predetermined time period in response to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than or equal to the predetermined absolute minimum frequency. The controller shuts down the compressor in response to an expiration of the predetermined time period of the operating frequency being less than the nominal minimum frequency. 
     The predetermined period of time of operation below the nominal minimum frequency may be a fixed time period of operation below the nominal minimum frequency. Alternately, the predetermined period of time of operation below the nominal minimum frequency may be a variable time period based on the amount by which the frequency deviates below the nominal minimum frequency. 
     Also, the present invention contemplates an absolute minimum operating frequency, below which a control algorithm shuts downs the chiller system and the compressor. 
     In one aspect of the invention, the control algorithm monitors the time period during which the actual operating frequency falls below nominal minimum frequency. If the time period exceeds a predetermined interval, e.g., 5 or 10 minutes, then the algorithm performs a controlled chiller system shutdown. Generally, the permitted deviation of the actual operating frequency below the nominal minimum frequency is also limited—e.g., 10 Hz or 20% of nominal minimum frequency, and any further decrease in the actual operating frequency supercedes the timing cycle limit, causing the control algorithm to initiate a controlled shutdown of the chiller system. 
     In another aspect of the invention, the control algorithm controls the time period of operation below the nominal minimum frequency, based on the magnitude of the deviation of the actual operating frequency below the nominal minimum frequency. In this embodiment of the invention, the algorithm controls smaller deviations for a longer operating time period below the nominal minimum frequency, while larger deviations below the nominal minimum frequency are controlled for a shorter operating time period. The control algorithm would also aggregate fluctuations in operating frequency for an intermediate time period, which intermediate time period being computed with respect to a fundamental maximum parameter. The maximum parameter may be computed as a function of Hz-seconds, for example. 
     The present invention is also directed to a computer program product embodied on a computer readable medium and executable by a microprocessor for controlling the frequency of a compressor in a chiller system. The computer program product includes computer instructions for executing the steps of: monitoring an operating frequency of a compressor; comparing the operating frequency to a nominal minimum frequency for the compressor; comparing the operating frequency to a predetermined absolute minimum frequency for the compressor, the predetermined absolute minimum frequency being less than the nominal minimum frequency; operating the compressor for a predetermined time period in respect to the operating frequency being less than the nominal minimum frequency and the operating frequency being greater than the predetermined absolute minimum frequency; and shutting down the compressor in response to an expiration of the predetermined time period wherein the operating frequency is less than the predetermined absolute minimum frequency. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, 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  illustrates schematically a general system configuration of the present invention. 
         FIG. 2  illustrates schematically one embodiment of a variable speed drive used in the present invention. 
         FIG. 3  illustrates schematically a refrigeration system that can be used with the present invention. 
         FIG. 4  illustrates a flow chart of one embodiment of the present invention. 
         FIG. 5  illustrates a partial flow chart of a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates generally the system configuration of the present invention. An AC power source  102  supplies AC power to a variable speed drive (VSD)  104 , which in turn, supplies AC power to a motor  106 . In another embodiment of the present invention, the VSD  104  can power more than one motor. The motor  106  is preferably used to drive a corresponding compressor of a refrigeration or chiller system (see generally,  FIG. 3 ). The AC power source  102  provides single phase or multi-phase (e.g., three phase), fixed voltage, and fixed frequency AC power to the VSD  104  from an AC power grid or distribution system that is present at a site. 
     The VSD  104  receives AC power having a particular fixed line voltage and fixed line frequency from the AC power source  102  and provides AC power to the motor  106  at a desired voltage and desired frequency, both of which can be varied to satisfy particular requirements. Preferably, the VSD  104  can provide AC power to the motor  106  having higher voltages and frequencies or lower voltages and frequencies than the fixed voltage and fixed frequency received from the AC power source  102 .  FIG. 2  illustrates schematically some of the components in one embodiment of the VSD  104 . The VSD  104  can have three stages: a converter stage  202 , a DC link stage  204  and an inverter stage  206 . The converter  202  converts the fixed line frequency, fixed line voltage AC power from the AC power source  102  into DC power. The DC link  204  filters the DC power from the converter  202  and provides energy storage components such as capacitors and/or inductors. Finally, the inverter  206  converts the DC power from the DC link  204  into variable frequency, variable voltage AC power for the motor  106 . Since the VSD  104  can provide a variable input voltage and variable input frequency to the motor  106 , the motor can be operated at a variety of different levels in the constant flux or constant volts/Hz mode depending on the particular load of the motor. 
     The converter  202  can be a pulse width modulated boost rectifier to provide a boosted DC voltage to the DC link  204  to obtain an output voltage from the VSD  104  greater than the input voltage of the VSD  104 . Alternately, the converter  202  can be a diode or thyristor rectifier, possibly coupled to a boost DC/DC converter to provide a boosted DC voltage to the DC link  204  in order to obtain an output voltage from the VSD  104  greater than the input voltage of the VSD  104 . Furthermore, it is to be understood that the VSD  104  can incorporate different components from those shown in  FIG. 2  so long as the VSD  104  can provide the motor  106  with appropriate output voltages and frequencies. 
     The motor  106  is preferably an induction motor that is capable of being driven at variable speeds. The induction motor can have any suitable pole arrangement including two poles, four poles or six poles. The induction motor is used to drive a load, preferably a compressor of a refrigeration system as shown in  FIG. 3 . 
     As shown in  FIG. 3 , the HVAC, refrigeration or liquid chiller system  300  includes a compressor  302 , a condenser  304 , an evaporator  306 , and a control panel  308 . The control panel  308  can include a variety of different components such as an analog to digital (A/D) converter, a microprocessor, a non-volatile memory, and an interface board, to control operation of the refrigeration system  300 . The control panel  308  can also be used to control the operation of the VSD  104  and the motor  106 . The conventional refrigeration system  300  includes many other features that are not shown in  FIG. 3 . These features have been purposely omitted to simplify the drawing for ease of illustration. 
     Compressor  302  compresses a refrigerant vapor and delivers the vapor to the condenser  304  through a discharge line. The compressor  302  is preferably a screw compressor, but can be any suitable type of compressor, e.g., centrifugal compressor, reciprocating compressor, etc. The refrigerant vapor delivered by the compressor  302  to the condenser  304  enters into a heat exchange relationship with a fluid, e.g., air or water, and undergoes a phase change to a refrigerant liquid as a result of the heat exchange relationship with the fluid. The condensed liquid refrigerant from condenser  304  flows through an expansion device (not shown) to an evaporator  306 . 
     The evaporator  306  can include connections for a supply line and a return line of a cooling load. A secondary liquid, e.g., water, ethylene, calcium chloride brine or sodium chloride brine, travels into the evaporator  306  via return line and exits the evaporator  306  via supply line. The liquid refrigerant in the evaporator  306  enters into a heat exchange relationship with the secondary liquid to lower the temperature of the secondary liquid. The refrigerant liquid in the evaporator  306  undergoes a phase change to a refrigerant vapor as a result of the heat exchange relationship with the secondary liquid. The vapor refrigerant in the evaporator  306  exits the evaporator  306  and returns to the compressor  302  by a suction line to complete the cycle. It is to be understood that any suitable configuration of condenser  304  and evaporator  306  can be used in the system  300 , provided that the appropriate phase change of the refrigerant in the condenser  304  and evaporator  306  is obtained. 
     The HVAC, refrigeration or liquid chiller system  300  can include many other features that are not shown in  FIG. 3 . These features have been purposely omitted to simplify the drawing for ease of illustration. Furthermore, while  FIG. 3  illustrates the HVAC, refrigeration or liquid chiller system  300  as having one compressor connected in a single refrigerant circuit, it is to be understood that the system  300  can have multiple compressors, powered by a single VSD or multiple VSDs, connected into each of one or more refrigerant circuits. 
     Preferably, the control panel, microprocessor or controller  308  can provide control signals to the VSD  104  to control the operation of the VSD  104  (and possibly motor  106 ) to provide the optimal operational setting for the VSD  104  and motor  106  depending on the particular sensor readings received by the control panel  308 . The control panel  308  can adjust the output voltage and frequency of the VSD  104  to correspond to changing conditions in the refrigeration system, i.e., the control panel  308  can increase or decrease the output voltage and frequency of the VSD  104  in response to increasing or decreasing load conditions on the compressor  302  in order to obtain a desired operating speed of the motor  106  and a desired load output of the compressor  302 . 
     If necessary, the signal(s) input to control panel  308  over a signal cable(s) is converted to a digital signal or word by an A/D converter. The digital signal (either from the A/D converter or from the sensor) is then input into the control algorithm, which is described in more detail in the following paragraphs, to generate an appropriate control signal as discussed below. 
     The control signal is provided to the interface board of the control panel  308  by the microprocessor (not shown), as appropriate, after executing the control algorithm. The interface board (not shown) then provides the control signals to the VSD  104  and the compressor  302 . 
     The microprocessor or control panel  308  uses a control algorithm to determine when to shut down the compressor  302  or begin a timer sequence for low frequency operation. In one embodiment, the control algorithm can be a computer program having a series of instructions executable by the microprocessor. While it is preferred that the control algorithm be embodied in a computer program(s) and executed by the microprocessor, it is to be understood that the control algorithm may be implemented and executed using digital and/or analog hardware by those skilled in the art. If hardware is used to execute the control algorithm, the corresponding configuration of the control panel  308  can be changed to incorporate the necessary components and to remove any components that may no longer be required, e.g. the A/D converter. 
     Generally, the control panel  308  executes a capacity control program that controls the compressor  302  or multiple compressors, if present, during normal operation. If there are multiple compressors in the system, the capacity control program controls the system capacity by turning on or off any compressors that are not needed to satisfy the capacity demand. When only one compressor is operating, if the cooling demand begins to decrease, the reduced capacity control system of the present invention overrides the shutdown controls of the capacity control program in response to the capacity control program having an operating frequency, which corresponds to the decreased demand, below the minimum frequency. 
     Referring next to  FIG. 4 , a flow chart for a preferred embodiment of the control process of the present invention is generally designated as  400 . The system starts at Step  402 , preferably within a short period of time after starting the chiller system. The system then proceeds to Step  404  and loads the minimum frequency, F min , as established by the compressor manufacturer. In this example, F min  is 50 Hz, which is a nominal minimum operating frequency for some screw compressors. The system also has control panel  308  determine the operating frequency F OP  set by the capacity control program. The capacity control program loads and unloads the compressor as necessary. Next, the system proceeds to Step  406 , to determine whether the actual operating frequency, F OP , established by the capacity control program is less than F min . If F OP  is not less than F min , the system resets a timer T (discussed below) and returns to Step  404  to measure the operating frequency again; however, if the actual operating frequency F OP  is below F min , then the system proceeds to Step  407 . In Step  407 , the system overrides the chiller capacity control program to prevent shutdown of the compressor in response to the low operating frequency and starts a low frequency control program. At Step  408 , the system starts timer T to begin counting if timer T has not been previously started. 
     Next, the system then proceeds to Step  410  and determines the frequency deviation ΔF relative to the nominal minimum frequency, F min , i.e., ΔF=F min −F op . Next the system proceeds to Step  412  to determine whether the deviation in the frequency, ΔF, is less than a predetermined frequency difference ΔF max , e.g., 10 Hz, which, in this example, corresponds to an actual operating frequency of about 40 Hz. Alternately, an absolute minimum frequency may be set as a percentage, e.g., about 80%—in a range of percentages from about 60% to about 95% of nominal minimum frequency. If ΔF is greater than or equal to ΔF max  in step  412  then the system proceeds to step  414  where the compressor operating frequency, Fop, is set to F min −ΔF max  and the compressor is prevented from further unloading. The system then proceeds to step  416  to determine if the maximum allowable time has elapsed. If ΔF is less than ΔF max  in step  412 , then the system proceeds to step  416  to determine if the maximum allowable time has elapsed. Once the compressor has been operating at less than F min  for the maximum allowable time T MAX  in step  416 , the compressor enters the shut down sequence and shuts off in step  418 . If the compressor has not been operating for the maximum allowable time in step  416 , the control returns to step  404  to begin again. The maximum allowable time is preferably set between five and ten minutes. 
     In an alternate embodiment, shown by broken line  413 , if ΔF is not less than ΔF max  in step  412  then the compressor enters the shut down sequence and shuts off in step  418 , and the intermediate steps  414 ,  416  are omitted. 
     Referring next to  FIG. 5 , a flow chart of another preferred embodiment is illustrated. The flow chart is generally designated as  500 . The system starts at Step  402 , preferably within a short period of time after starting the chiller system. The system then proceeds to Step  403  and loads the minimum frequency F min  as established by the compressor manufacturer. The system then proceeds to Step  502  to load or determine a maximum accumulated value A max  in Hz-seconds. A max  represents a predetermined maximum threshold parameter corresponding to the frequency deviation in Hz, multiplied by a time period, for tracking and limiting the total amount of operating time at a frequency below F min , as a function of the magnitude of the deviation. In other words, the smaller the frequency deviation, the greater the amount of time that the system may operate at a frequency below F min  and similarly the larger the deviation, the shorter amount of time the system may operate below F min . Next, at Step  504 , the system measures the operating frequency F OP . The system then proceeds to Step  406  to determine if F OP  is less than F min . If F OP  is lower than F min , in Step  406 , the system proceeds to Step  407 . Otherwise, the system resets the accumulator value A to zero at step  508  and returns to Step  504 . 
     In Step  407 , the system overrides the chiller capacity control program to prevent shutdown in response to the low capacity demand and starts a low frequency control program. The system then proceeds to Step  506 . At Step  506 , the accumulator value A is compared to A max ; if A is greater than A max , then the system proceeds to Step  518  and initiates a system shut down sequence. Otherwise, the system proceeds to Step  510  and calculates ΔA, the accumulated parameter in Hz-Seconds. The frequency deviation in the current iteration is defined as ΔF, wherein ΔF is the difference between F OP  and F min . ΔF times the iteration interval is defined as ΔA. This value ΔA represents the incremental time-frequency value since the last sampled value of the previous iteration. Next, the system proceeds to Step  512  to calculate an updated value of A by adding the incremental accumulated value ΔA to the previous value of A. Then the system returns to Step  504  for another iteration. Preferably, the iteration or cycle time is repeated at a constant frequency or period. 
     The low frequency control system of  FIG. 4  or  FIG. 5  may be embodied in a computer program as a standalone system, or may be incorporated into a larger system, e.g., a capacity control program. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.