Abstract:
A current sensor is installed in a generator set to measure the current being delivered from the generator set to a refrigeration unit. A control is responsive to the current transformer to reduce the speed of the drive engine to a lower speed when the current being delivered to the refrigeration unit is determined to be below a predetermined higher level. Provision is made to override the system when the sensed current is determined to be transient caused or in the event that ambient temperature is determined to be above a predetermined temperature threshold.

Description:
TECHNICAL FIELD 
     This invention relates generally to transport refrigeration systems and, more particularly, to speed control of a motor/generator therefor. 
     BACKGROUND OF THE INVENTION 
     An all electric mobile refrigeration unit or refrigerated container uses an auxiliary generator set to power the unit when traveling by rail or road. That is, whereas, when such a unit is being transported on board ship it is provided electrical power by way of the ships power, but when the container is being transported by rail car or by truck, no such electrical power is available. Accordingly, electrical power is provided by way of a motor/generator set during such period. 
     Unlike a truck/trailer refrigeration system, which is integrated and includes an overall control system for controlling all of the various components including the refrigeration system and the generator, the engine/generator set in a refrigerated container is a stand alone unit which does not communicate with the refrigeration system. This problem is exacerbated by the fact that various types of refrigeration units will be electrically powered by such an engine/generator set, with each such refrigeration system having its own unique operating characteristics. Accordingly, heretofore, there has been no unifying control system for communicating between the refrigeration system and the engine/generator set. 
     For this reason, in order to ensure that sufficient power is being delivered to the refrigeration system, the engine/generator set has been operated at a single, relatively high, speed at all times, even though the refrigeration unit may be operating under light load conditions or even in an off condition if the load requirements have been met. In this regard, the applicants have made studies which indicate that such a unit is typically lightly loaded for a majority of the time (i.e. up to 70% or more). However, a relatively large engine has been required to provide the power at high load conditions such as for pull down. The unit is therefore oversized for lighter load conditions, thereby resulting in inefficient fuel use. 
     DISCLOSURE OF THE INVENTION 
     According to one aspect of the invention, provision is made to sense the level of current being delivered by the generator to the refrigeration system, and if the current is below a predetermined high level threshold, then the speed of the generator drive engine is reduced to a lower level. 
     In accordance with another aspect of the invention, a timing function is included in order to eliminate short cycling of the system as may be caused by transients. 
     By yet another aspect of the invention, provision is made to measure the ambient temperature and, if it exceeds a predetermined level, the system is prevented from switching into a lower speed. 
     In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the spirit and scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the container refrigeration unit and its associated generator set with the present invention incorporated therein. 
         FIGS. 2A and 2B  are block diagrams illustrating the method of control in accordance with the present invention. 
         FIG. 3  is a graphic illustration of the current levels during start up operation. 
         FIG. 4  is a graphic illustration of the current levels during transitional and transient operation. 
         FIG. 5  is a graphic illustration of the current levels during defrost mode of operation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The electrical interconnection between a generator set  11  and a container refrigeration unit  12  is shown in  FIG. 1 . Such a three wire connection is standard in the industry, and, rather than a single generator set being primarily associated with a single container unit, the various generator sets and container units are customarily interchanged, such that a single generator set will commonly be used with various types and brands of container refrigeration units. For these and other reasons, there has been no standardized communication between the standard generator set  11  and container refrigeration unit  12 . Accordingly, the generator set  11  has, heretofore, had no knowledge of the operating condition of the container refrigeration unit  12 . 
     The generator set  11  includes a generator  13  and a driving mechanism  14 , which may be any of various types, such as a diesel engine, an electric motor or a turbine, for example. The electrical output of the generator is provided along lines  16 ,  17  and  18  which are electrically connected into the container refrigeration unit  12 . 
     The container refrigeration unit  12  has incorporated therein a standard refrigeration circuit which includes, in serial flow relationship, a compressor, a condenser, an expansion device, and an evaporator (not shown). The evaporator fluidly communicates with the air in the container and operates to cool the space within the container to a desired temperature level for the preservation of its cargo. 
     Although this invention is being described in terms of a standard refrigeration circuit, it should be understood that it may equally be applicable to other types of cooling systems. 
     Within the container refrigeration unit, the compressor, as well as the fans for the condenser and the evaporator, are powered by electrical motors. Thus, when the generator set  11  is electrically connected to the container refrigeration unit  12 , the power from the line  16 ,  17  and  18  is electrically connected to the compressor motor  19 , the condenser fan motor  21  and the evaporator fan motors  22  and  23 . The amount of power being used by the motors  19 - 23  depends on the operating mode of the container refrigeration unit  12  which, in turn, depends on various factors such as the ambient temperature, the amount of cargo in the container, the desired temperature or set point within the container, and other factors. Since, in the past, there was no way for the generator set  11  to know what the demand was in the container refrigeration unit  12 , it was necessary to assume that it was operating at its maximum capacity. Accordingly, the driving mechanism  14  needed to be operated at a high speed in order to ensure that the generator  13  would be providing electrical power sufficient to operate the container refrigeration unit  12  at maximum capacity. 
     In order to provide an indication of the electrical load on the container refrigeration unit  12  (i.e. the amount of electrical power being used by the motors  19 - 23 ), a current sensing device  24 , such as a current transformer is provided on one of the wires  17  to sense the amount of current being delivered from the generator set  11  to the container refrigeration unit  12 . A representative signal is then sent along line  26  to a controller  27  which is powered by a voltage source  28 , typically a 12 volt dc battery. The controller  27  then responsively sends an appropriate signal i.e. either a high speed output along line  29  or a low speed output along line  31  to an engine control  32 , which then provides an input to the driving mechanism  14  to operate either at a high speed or at a lower speed in a manner as to be described hereinafter. 
     It should also be mentioned that, in addition to the current level signal on line  26 , the controller  27  also receives a signal along line  35  from a temperature transducer  33  indicating the ambient temperature. 
     Referring now to  FIGS. 2A-2C , there is shown a flow chart of the logic contained within the controller  27 . Again, the controller  27  operates in response to the sensed current along line  26 , and to the ambient temperature signal received along line  35 , to send either a high or low speed signal to the engine control  32 . Timing functions are also added to eliminate the effect of transients which could cause frequent cycling. 
     In block  34  the power from the voltage source  28  is turned on to thereby initialize all logic. Any time the control power is turned off, all logic will be reset. In block  36 , the controller  27  will send a high speed output to the engine control such that the driving mechanism  14  will initially be started at high speed. 
     In block  37 , the control  27  will sense, by the use of comparators or the like, whether the current being sensed by the current sensing device  24  is below a low limit or above a high limit. The low limit threshold is simply to determine whether the container refrigeration unit  12  is operating in a normal range. For example, if the fan motors  21 - 23  have been started but the compressor motor  19  has not yet been started, there will be very little electrical power being drawn from the generator set  11 , and the control logic will therefore not proceed. 
     The high limit referred to in block  37  is the established threshold which determines whether the controller  27  will provide a high speed output along line  29  or a low speed output along line  31 . Thus, if the sensed current is below the high limit threshold (as will be more fully explained hereinafter), then the logic will proceed toward the change in the engine control  32  to adjust the engine speed to a lower speed. Before this occurs, it is necessary to determine whether the ambient temperature is above a predetermined level, which would indicate that the outdoor temperature is too hot to allow the system to operate at a low speed. Thus, if the temperature is over that threshold, such as, for example, 85° F., then the logic is directed back to block  36  which will cause the engine to continue to operate at high speed. In this regard, it should be recognized that higher ambient temperatures cause higher compressor head pressures, which, in turn, result in higher current delivery. 
     If the ambient temperature does not exceed the established high limit threshold, then the logic proceeds to block  39  where a timer is started for purposes of determining whether the present sensed condition is provided by a transient or whether it is a steady state condition. 
     After the timer is started in block  39 , the control  27  continues to query whether the sensed current is within the prescribed window as shown in block  41 . Further, the sensed ambient temperature continues to be provided to the control  27  as shown in block  44 . 
     If, in block  41 , it is determined by the controller that the sensed current is now outside the established window, or in block  44  that the ambient temperature now exceeds the predetermined upper limit threshold, then the logic passes to block  42  to ensure that the timer is not reset due to transients. After passing to block  43 , it is then determined whether the transient timer has timed out and if so, it is determined that it was not a transient current, and the low speed timer is reset. If, on the other hand, the transient timer is not timed out, then it is necessary to keep checking to see if the current goes into low speed range. 
     At block  46 , a query is made as to whether the low speed timer has reached a predetermined threshold of time. In this regard, if the conservation of fuel is a priority, and other factors, such as a history of predominately low demand operation, are present, then a relatively short period of time such as 30 minutes may be established as the low speed time threshold. On the other, if, for various reasons, it is expected that the system will be operating at a high demand level for a greater period of time, then a higher threshold of time, such as 3 hours will be established as the low speed time threshold. 
     If the low speed time threshold has not been reached, the system cycles back to block  41 . If the established time period has elapsed, then the controller  27  sends a low speed output signal along line  31  as indicated in block  47 . The engine control  32  will then change the speed of the engine  14  to a lower speed, at which it will continue to run so long as the sensed current at the current sensing device  24  remains within the established window as indicated at block  48  and the ambient temperature is not determined to exceed the predetermined threshold as indicated at block  49 . If either the sensed current is determined to be outside of the window, or the temperature exceeds the predetermined level, the logic proceeds to block  51  to set a low speed transient time delay to ensure that the timer is not reset due to transients. In block  52 , if the transient timer has not timed out, then it is determined that the indication at block  48  or  49  was caused by a transient, and the system remains in low speed operation. If, on the other hand, the transient timer has timed out, that would be an indication that the signal is not caused by a transient and the system would then go back to high speed operation and the low speed timer would be reset. 
     From the discussion above, it should be recognized that these are three separate timers which are included in the system. In block  39 , a first timer is set at a time in which it is desired to switch from high speed to low speed. This will typically be in a range from 30 minutes to 3 hours. In block  42 , a second timer is set to establish a high speed transient time delay to ensure that the sensed current which was determined to be outside the window in block  41  was not caused by a transient. In block  51 , a third timer is set to establish a low speed transient time delay to ensure that the sensing of the current to be outside of the window in block  48  was not caused by transient operation. In each of the latter two timers, a time of 3 to 5 minutes would be typical. 
     Considering now the relationship between the current being delivered by the generator set  11  and the operational mode of the container refrigeration unit  12 , in the table set forth below, various operational conditions are shown with an indication of the typical current draw that is required to maintain that operational condition. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 CURRENT 
                 MODE OF OPERATION 
               
               
                   
                   
               
             
             
               
                   
                 26 A 
                 Pull-down 
               
               
                   
                   
                 High ambient operation 
               
               
                   
                   
                 Current Limit: 21 A, 23 A 
               
               
                   
                 20 A 
                 Perishable (steady state) 
               
               
                   
                   
                 Frozen (steady state) 
               
               
                   
                   
                 Current Limit 15 A, 17 A, 18 A, 19 A 
               
               
                   
                 14 A 
                 Defrost 
               
               
                   
                   
                 Frozen Economy 
               
               
                   
                   
                 Current Limit 13.5 A 
               
               
                   
                   
               
             
          
         
       
     
     Considering now the above relationships, the changing between high and low speed operations by way of the controller  27  during typical operation cycles are shown in  FIGS. 3-5 . 
     In  FIG. 3 , a typical start up operation is shown wherein at time=0, the system is started at high speed with the current draw initially being 0 and then ramping up as the various motors  19 - 23  are brought into play. The start up may occur in a pull-down situation where the temperature condition in the container is relatively high. Alternatively, it could be a situation where the system was temporarily shut down because the set temperature had been met. In any case, the typical current draw for start up is shown to be about 23 amps and, after the low speed timer has timed out, the control  27  sends the low speed output to the engine control  32  and the engine control  32  acts to slow down the drive mechanism  14 . The result is that the sensed current is decreased down to below the 20 amp level as shown by the line A. It will remain at that level until conditions change so as to cause the controller  27  to increase the speed of the engine. 
     In  FIG. 4 , the current is decreased from a high speed level down to a low speed level as indicated by the line B. The downward and upward spikes are an indication of transients which would indicate a need to go to a high speed if the sensed current drops below the window as indicated at C, D and E. However, since the time that elapsed did not reach the established threshold, it was determined that they were transient caused, and so the control allowed continued low speed operation. 
     In  FIG. 5 , the system is shown as running in the low speed range until it is caused to operate in a defrost mode. It then switched to high speed operation and remained there until defrost was complete, at which time the algorithm caused it to switch back to low speed operation. 
     Referring again to  FIG. 1 , there is shown a disabling unit  30  which is connected to the control  27  for disabling the logic described above. Such a unit may be by way of a manual switch or an electrical control to disable the above described function such that it only operates when the system is operating in high speed. 
     Although the present invention has been described in terms of use with a transport refrigeration system, it should be understood that it is equally applicable to other types of refrigeration systems such as stationary refrigeration systems of the type found in supermarkets and the like, as well as comfort systems such as air conditioning and heat pump systems. Further, although described in terms of a single speed charge step, it should be understood that multiple, sequential steps may be taken between desirable limits such as 1800 RPM (60 Hz)-1700 RPM-1600 RPM-1500 RPM (50 Hz). 
     While the present invention has been particularly shown and described with reference to a preferred embodiment as illustrated in the drawings, 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.