Abstract:
An process and method for monitoring and limiting high voltage conditions in a transport refrigeration unit is disclosed. The system provides a microprocessor control which monitors the load, generator temperature and engine speed and compares it to an algorithm or map programmed into a controller for the unit. The map or algorithm preferably predicts voltage based upon load (amperage), generator temperature and engine speed. If the result of the monitored features predicts a voltage above preselected acceptable levels which are programmed into the controller, then the controller will drop the engine into low speed.

Description:
FIELD OF THE INVENTION 
     The field of the present invention relates to control systems for transport refrigeration systems. More specifically, the present invention is directed towards monitoring the generator current of a transport refrigeration system and, using a function derived from previous testing of operating conditions versus voltage, forcing the unit into low speed whenever high generator voltage conditions are predicted to occur. 
     DESCRIPTION OF THE PRIOR ART 
     A transport refrigeration system used to control enclosed areas, such as the box used on trucks, trailers, containers, or similar intermodal units, functions by absorbing heat from the enclosed area and releasing heat outside of the box into the environment. To accomplish this, a typical transport refrigeration unit requires a highly pressurized refrigerant is introduced into a low pressure environment such as an evaporator coil. The refrigerant is pressurized by flowing through a compressor, which can be powered by a generator run off of a diesel engine. 
     Refrigeration systems, including particularly refrigeration transport systems, require operation under a wide variety of ambient temperatures and operating loads. Excessive voltage generated by certain operating conditions is believed to be a significant factor in causing component failures on the transport refrigeration system. Currently available transport refrigeration systems include a speed solenoid for dropping or limiting engine speed to a preselected maximum rate. Unfortunately, the engine speed control systems that applicant is currently aware of involved manual selected activation of the speed solenoid or similar engine speed control. 
     The applicants have found that, in order to operate under acceptable voltage conditions, it is desirable to automatically monitor and control the voltage conditions of the system based upon the engine speed, generator temperature and current draw of the system. 
     SUMMARY OF THE INVENTION 
     The control method and process of this invention provides a refrigeration unit for a transport system having a controller for predicting and preventing high voltage conditions on the unit. The system includes sensors for monitoring engine speed, current draw and generator temperature (which reflects the ambient load). The data received from these sensors is loaded into the system controller, which utilizes a map, or more preferably an algorithm to predict voltage based upon those variable. If the algorithm or map predicts a voltage above preselected acceptable voltage levels programmed into the controller, then the system will drop the engine into low speed. Most preferably, this function would be accomplished by a digital signal generated by the controller to de-energize the speed solenoid connected to the engine, thus limiting engine speed. 
     Accordingly, one object of the present invention is to provide a microprocessor control for the regulation of transport refrigeration unit voltage levels. 
     It is another object of the present invention to provide a method or process for limiting or eliminating transport refrigeration unit component failure rates by automatically controlling voltage limits on the unit. 
     These and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, and as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic of the transport refrigeration system of the present invention. 
     FIG. 2 shows a block schematic of a first preferred embodiment of a controller of the present invention; and 
     FIG. 2 a  shows a block schematic of a second preferred embodiment of a controller of the present invention. 
     FIG. 3 shows a sample prediction curve or operating parameter model showing the interrelationship between generator temperature versus no load voltage at high engine speed for the preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention that is the subject of the present application is one of a series of applications dealing with transport refrigeration system design and control, the other copending applications including: “Superheat Control for Optimum Capacity Under Power Limitation and Using a Suction Modulation Valve” (U.S. patent application Ser. No. 09/277,508); “Economy Mode For Transport Refrigeration Units” (U.S. Pat. No. 6,044,651); “Compressor Operating Envelope Management” (U.S. patent application Ser. No. 09/277,473); “High Engine Coolant Temperature Control” (U.S. patent application Ser. No. 09/277,472); “Generator Power Management” (U.S. patent application Ser. No. 09/277,509); and “Electronic Expansion Valve Control Without Pressure Sensor Reading” (U.S. patent application Ser. No. 09/277,333) all of which are assigned to the assignees of the present invention and which are hereby incorporated herein by reference. These inventions are most preferably designed for use in transportation refrigeration systems of the type described in copending applications entitled: “Transport Refrigeration Unit With Non-Synchronous Generator Power System;” Electrically Powered Trailer Refrigeration Unit With Integrally Mounted Diesel Driven Permanent Magnet Generator;” and “Transport Refrigeration Unit With Synchronous Generator Power System,” each of which were invented by Robert Chopko, Kenneth Barrett, and James Wilson, and each of which were likewise assigned to the assignees of the present invention. The teachings and disclosures of these applications are likewise incorporated herein by reference. 
     FIG. 1 illustrates a schematic representation of the transport refrigeration system  100  of the present invention. The refrigerant (which, in its most preferred embodiment is R404A) is used to cool the box air (i.e., the air within the container or trailer or truck) of the refrigeration transport system  100 . is first compressed by a compressor  116 , which is driven by a motor  118 , which is most preferably an integrated electric drive motor driven by a synchronous generator (not shown) operating at low speed (most preferably 45 Hz) or high speed (most preferably 65 Hz). Another preferred embodiment of the present invention, however, provides for motor  118  to be a diesel engine, most preferably a four cylinder, 2200 cc displacement diesel engine which preferably operates at a high speed (about 1950 RPM) or at low speed (about 1350 RPM). The motor or engine  118  most preferably drives a 6 cyliinder compressor  116  having a displacement of 600 cc, the compressor  116  further having two unloaders, each for selectively unloading a pair of cylinders under selective operating conditions. In the compressor, the (preferably vapor state) refrigerant is compressed to a higher temperature and pressure. The refrigerant then moves to the air-cooled condenser  114 , which includes a plurality of condenser coil fins and tubes  122 , which receiver air, typically blown by a condenser fan (not shown). By removing latent heat through this step, the refrigerant condenses to a high pressure/high temperature liquid and flow to a receiver  132  that provides storage for excess liquid refrigerant during low temperature operation. From the receiver  132 , the refrigerant flows through subcooler unit  140 , then to a filter-drier  124  which keeps the refrigerant clean and dry, and then to a heat exchanger  142 , which increases the refrigerant subcooling. 
     Finally, the refrigerant flows to an electronic expansion valve  144  (the “EXV”). As the liquid refrigerant passes through the orifice of the EXV, at least some of it vaporizes. The refrigerant then flows through the tubes or coils  126  of the evaporator  112 , which absorbs heat from the return air (i.e., air returning from the box) and in so doing, vaporizes the remaining liquid refrigerant. The return air is preferably drawn or pushed across the tubes or coils  126  by at least one evaporator fan (not shown). The refrigerant vapor is then drawn from the exhanger  112  through a suction modulation valve (or “SMV”) back into the compressor. 
     Many of the points in the transport refrigeration system are monitored and controlled by a controller  150 . As shown in FIGS. 2 and 2A Controller  150  preferably includes a microprocessor  154  and its associated memory  156 . The memory  156  of controller  150  can contain operator or owner preselected, desired values for various operating parameters within the system, including, but not limited to temperature set point for various locations within the system  100  or the box, pressure limits, current limits, engine speed limits, and any variety of other desired operating parameters or limits with the system  100 . Controller  150  most preferably includes a microprocessor board  160  that contains microprocessor  154  and memory  156 , an input/output (I/O) board  162 , which contains an analog to digital converter  156  which receives temperature inputs and pressure inputs from various points in the system, AC current inputs, DC current inputs, voltage inputs and humidity level inputs. In addition, I/O board  162  includes drive circuits or field effect transistors (“FETs”) and relays which receive signals or current from the controller  150  and in turn control various external or peripheral devices in the system  100 , such as SMV  130 , EXV  144  and the speed of engine  118  through a solenoid (not shown). 
     Among the specific sensors and transducers most preferably monitored by controller  150  includes: the return air temperature (RAT) sensor which inputs into the processor  154  a variable resistor value according to the evaporator return air temperature; the ambient air temperature (AAT) which inputs into microprocessor  154  a variable resistor value according to the ambient air temperature read in front of the condenser  114 ; the compressor suction temperature (CST) sensor; which inputs to the microprocessor a variable resistor value according to the compressor suction temperature; the compressor discharge temperature (CDT) sensor, which inputs to microprocessor  154  a resistor value according to the compressor discharge temperature inside the cylinder head of compressor  116 ; the evaporator outlet temperature (EVOT) sensor, which inputs to microprocessor  154  a variable resistor value according to the outlet temperature of evaporator  112 ; the generator temperature (GENT) sensor, which inputs to microprocessor  154  a resistor value according to the generator temperature; the engine coolant temperature (ENCT) sensor, which inputs to microprocessor  154  a variable resistor value according to the engine coolant temperature of engine  118 ; the compressor suction pressure (CSP) transducer, which inputs to microprocessor  154  a variable voltage according to the compressor suction value of compressor  116 ; the compressor discharge pressure (CDP) transducer, which inputs to microprocessor  154  a variable voltage according to the compressor discharge value of compressor  116 ; the evaporator outlet pressure (EVOP) transducer which inputs to microprocessor  154  a variable voltage according to the evaporator outlet pressure or evaporator  112 ; the engine oil pressure switch (ENOPS), which inputs to microprocessor  154  an engine oil pressure value from engine  118 ; direct current and/or alternating current sensors (CT 1  and CT 2 , respectively), which input to microprocessor  154  a variable voltage values corresponding to the current drawn by the system  100  and an engine RPM (ENRPM) transducer, which inputs to microprocessor  154  a variable frequency according to the engine RPM of engine  118 . 
     The present invention preferably involves use of an algorithm by controller  150 . The system current (e.g. CT 2 ) and GENT values are input into controller  150  and are used by the processor  154  in implementing the algorithm to predict the voltage for the system  100 . A sample prediction curve or model showing the interrelationship between generator temperature versus no load voltage at high engine speed for the preferred embodiment of the present invention is shown in FIG.  3 . This predicted voltage is then compared to a voltage limit mapped or stored in memory  156  (those of skill in the art will appreciate that the specific voltage level limits involved will vary based upon system components and operating conditions). If the predicted voltage is higher than the voltage limit stored in memory (i.e., if high or variable voltage conditions are predicted above the preselected voltage limit stored in memory  156 ), then controller  150  issues a control signal forcing engine  118  into low speed. Alternatively, or in addition to the system current and generator temperature operating parameters, the algorithm employed by the processor  154  of the present invention can employ the value received for the speed of the engine  118  or ENRPM in calculating the predicted voltage. 
     It will be appreciated by those skilled in the art that various changes, additions, omissions, and modifications can be made to the illustrated embodiments without departing from the spirit of the present invention. All such modifications and changes are intended to be covered by the following claims.