Abstract:
A transport refrigeration system is provided with a control apparatus including an inverter and a microprocessor, with the microprocessor receiving signals representative of sensed values of the compressor discharge temperature and pressure, as well as the suction pressure, and controlling the inverter to responsively provide a selective level of electrical voltage and frequency to the compressor in order to maintain a desired compressor envelope.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates generally to transport refrigeration systems and, more particularly, to control of a compressor motor drive in a vapor compression system therefor. 
       BACKGROUND OF THE INVENTION 
       [0002]    Transport refrigeration systems are commonly used in refrigerated trucks, truck trailers and containers in order to preserve perishable cargo during transport from one location to another. Such a system includes the necessary components for a vapor compression cycle, including a compressor which necessarily includes some type of drive means. For containers and truck trailers, this function has generally been provided by a dedicated internal combustion engine with its speed being selectively varied in order to maintain a desired compressor envelope. In refrigerated trucks and vans, however, the compressor has generally been located within the main engine drive compartment, with the compressor then being driven by direct connection to the main engine drive. Conduits then serve to provide the closed circuit refrigerant flow to the condenser and evaporator units of the system. 
         [0003]    With such a so-called direct drive arrangement, the speed of the compressor is dependent on the speed of the main engine drive. Thus, when the truck is proceeding at higher speeds down the highway, the compressor is driven at a high speed so as to obtain a high discharge pressure. However, when the engine is idling, for example, then the compressor will be driven at a relatively slow speed, and the discharge pressure and temperature will be relatively low. In order to protect the compressor envelope, it is therefore necessary to employ the selective use of switches in order to vary the speed of the variable speed condenser and evaporator fans or to modulate valves or stop the unit. 
       DISCLOSURE OF THE INVENTION 
       [0004]    In accordance with one aspect of the invention, electrical power is provided from the generator to an inverter which is, in turn, controlled by a microprocessor receiving pressure and temperature sensed conditions from the compressor in order to regulate the power being provided to the compressor so as to maintain a desired compressor envelope. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a simplified schematic illustration of the present invention as incorporated into a transport refrigeration system. 
           [0006]      FIG. 2  is a more detailed schematic illustration thereof. 
           [0007]      FIG. 3  is a graphic illustration of a compressor envelope. 
           [0008]      FIG. 4  is a control logic flow diagram of the manner in which the compressor envelope is controlled in accordance with the present invention. 
           [0009]      FIGS. 5A-5C  show graphic illustrations of the critical parameters during pull-down conditions with the present invention incorporated in the system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0010]    The invention is shown generally at  10  in  FIG. 1  to include a generator  11 , which is driven by the power of the vehicle engine, an inverter  12 , which receives unregulated voltage from the generator  11 , a vapor compression system  13 , which receives regulated power regulated (i.e voltage and frequency) from the inverter  12 , a box  14  which receives cooled air from the vapor compression system  13 , and a controller  16 , which receives box temperature measurements (i.e. return air temperatures, RAT) from the box  14  along line  17 , and pressure and temperature measurements from the vapor compression system  13  along lines  18  in order to control the inverter  12  by way of line  19 . The controller  16  also sends cooling demand signals to the vapor compression system  13  by way of line  21 . A more detailed illustration of the system is shown in  FIG. 2 . 
         [0011]    Considering first the vehicle itself and the environment surrounding that vehicle, there is included a drive engine  22 , a battery  23 , a stand-by power source  24  and a box with a door  26  that is opened from time to time. Both air and heat are transferred from and to the box to ambient  27 , primarily when the door is open. This heat transfer, of course, will greatly affect the operation of the vapor compression system  13  and therefore the control thereof. The control of the door openings and the speed of the engine  21  is determined by the drive cycle  28 , which is controlled by the operator. 
         [0012]    As mentioned hereinabove, the engine  22  drives a generator  11  which provides unregulated voltage and current to the inverter  12  for powering the vapor compression system  13 . The inverter  12  also provides power to a heater  29  that may be required under certain ambient conditions. 
         [0013]    The vapor compression system  13  includes, in serial flow relationship, a compressor  32 , a condenser  33 , a thermal expansion valve  34  and an evaporator  36 . An oil separator  37  may be provided downstream of the compressor  32 , and a receiver  38  may be provided downstream of the condenser  33 . Also, a control valve  39  may be provided between the receiver  38  and the TXV  34 . The condenser  33  includes a condenser fan  41 , and the evaporator  36  includes an evaporator  42 , with each of these fans being independently driven at selectively variable speeds by a dc motor. 
         [0014]    Control of the system is by way of a microcontroller  43  which receives the various inputs as indicated and then which, responsively, sends signals to the inverter  12  in order to modulate the power (i.e. voltage, frequency and/or current) being provided to the compressor  32  along line  40 . In particular, the inputs to the microcontroller  43  include the discharge temperature t d  and pressure P d  and the suction pressure P s  of the compressor  32  as indicated schematically at line  18 . Also passing through microcontroller  43  is the return air temperature, RAT along line  17 . The various conditions of the system as maintained by the microcontroller  43  are shown in a display  44  for the convenience of the operator. 
         [0015]    Shown in  FIG. 3  is a graphic illustration of the saturated discharge temperature as a function of the saturated suction temperature. The saturated suction temperature is equivalent to the suction pressure, and the saturated discharge temperature is equivalent to the discharge pressure, with the two parameters being the critical parameters that define the envelope of a variable speed compressor. That is, in order to protect the compressor and the system operation, it is desirable to maintain the saturated suction temperature between −40° C. and 2° C. Similarly, it is desirable to maintain the saturated discharge temperature between 10° C. and 66° C. This is accomplished by varying the power being provided by the inverter  12  to the compressor  32  in response to four sensed variables, return air temperature (RAT), discharge pressure P d , discharge temperature t d  and suction pressure P s . This is accomplished as shown in  FIG. 4 . 
         [0016]    As shown, there are four different control modules: 1) the RAT control  48 , the T d  control  49 , the P s  control  51  and the P d  control  52 . Each of the controllers  48 - 52  controls its own designated variable, with only one of the four controllers is acting at one time, maintaining its variable of interest at a desired set point value. The microcontroller  43  monitors the four sensed conditions and switches control from one controller to the other as specified by the switching logic as indicated. The purpose, of course, is to maintain the desired compressor envelope during all operating conditions. 
         [0017]    The condition under which a vapor compression system is under the greatest demand is a condition known as pull-down. This is the process of restoring the operating temperature of a refrigerated space after the introduction of an extraordinary heat load. This would occur, for example, when a new load of unrefrigerated cargo is placed in a truck such that the temperature in the box is caused to increase to a level well above the desired set point. Under these conditions it is desirable to reduce the temperature in the box to the set point temperature as quickly as is reasonably possible. 
         [0018]    In  FIGS. 5A-5C , it will be seen that where continuous control of the compressor envelope is provided as described hereinabove, the variability of these various parameters is substantially lessened. In  FIG. 5A , it will be seen that there is up to a 3.5° C. better control during about 1.5 hours periods of operation as compared with the non-controlled system. In  FIG. 5B , it will be seen that the envelope routing control reduces cycling substantially in the later periods of operation, thereby having a direct impact on reliability. The same is true with respect to compressor speed as shown in  FIG. 5C . 
         [0019]    While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.