Abstract:
Cooling is provided to the power electronics of a compressor speed control system by way of refrigerant that is routed from a refrigeration system, through the power electronics, and then back to the refrigeration system. The amount of refrigerant flowing to the power electronics is automatically regulated to that needed to cool the electronics since both are substantially proportional to compressor speed. The housing for the power electronics is mounted directly to the side of the compressor, and the compressor is resiliently mounted to a support to thereby provide shock protection to the power electronics.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to refrigeration systems and, more particularly, to transport refrigeration systems with compressor speed controls. 
         [0002]    For the transport of goods that are required to be kept cold or frozen, vehicles such as trucks or trailers or refrigerated containers are provided with a refrigeration system which interfaces with the cargo space to cool the cargo down to a predetermined temperature. The refrigeration system includes a compressor which is driven by an electric motor, with the most common type being a hermetic compressor with the motor being disposed within the compressor housing. 
         [0003]    In the usual transport refrigeration system, the duty cycle of the compressor will vary substantially depending on various factors such as the ambient temperature, the type and volume of cargo, the desired temperature for the cargo space, and the frequency and length of time that the cargo space is opened for loading or unloading. The compressor must be designed to operate at sufficient capacity and speed to provide a cooling capability that is necessary to satisfy the most adverse conditions (such as pulldown) that are anticipated. However, during a majority of the operating time, the compressor can be operating at less than full capacity and at times may be completely shut off. For purposes of efficiency, it is therefore become common to provide a control system for varying the speed of the compressor so as to thereby maximize the efficiency while at the same time meeting the demands of the cooling system. 
         [0004]    One way in which the speed control is accomplished is by way of a power electronics unit which is used to selectively vary the power to the drive motor, and in particular, by varying the current, voltage and/or frequency thereto. When using such a unit with its various electronic components, it has been recognized that even the most robust power electronic systems are subject to malfunction and/or failure unless they are protected from certain unfavorable conditions. Firstly, it is recognized that the inverter must be protected against overheating. This is often accomplished by the use of heat sinks and by providing fans to circulate air through the electronic components to provide the necessary cooling thereof. In this regard, it is recognized that, generally, the size of the power electronics package can be reduced as the cooling capabilities are increased. 
         [0005]    The second condition against which one would preferably protect a power electronics unit is that of mechanical shock that can be transferred to the electronic components by jarring movements of the type that may occur in moving vehicles. This can be accomplished by providing resilient structure between the inverter apparatus and the structure to which it is mounted. 
       SUMMARY OF THE INVENTION 
       [0006]    Briefly, in accordance with one aspect of the invention, the power electronics package is cooled by way of refrigerant that is being returned to the suction inlet of the compressor, with the suction gas being routed to flow first through the power electronics package and then to the suction port of the compressor. In this way, the electronic components are more effectively cooled than by the mere circulation of air therethrough, and thereby allowing for the use of a smaller power electronics package. 
         [0007]    In accordance with another aspect of the invention, the speed of the compressor, as controlled by the electronics package, is generally proportional to both the degree of heat generated by the electronic components and the amount of refrigerant that is circulated by the compressor, thereby providing an inherent balanced arrangement to obtain efficient operation with a smaller electronics package. 
         [0008]    In accordance with yet another aspect of the invention, a power electronics unit is mounted directly to the side of a hermetic compressor, with the compressor itself being mounted on shock mounts. In this way, the power electronics unit derives the benefit of the compressor mounting system without the need for its own resilient mounting system. 
         [0009]    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 
         [0010]      FIG. 1  is a schematic illustration of a transport refrigeration system in accordance with one embodiment of the present invention. 
           [0011]      FIG. 2  is a schematic illustration of a power electronics unit as mounted to a compressor in accordance with one embodiment of the present invention. 
           [0012]      FIG. 3  is a schematic illustration of a power electronics cooling arrangement in accordance with one aspect of the invention. 
           [0013]      FIG. 4  is a schematic illustration thereof in accordance with an alternative embodiment thereof. 
           [0014]      FIG. 5  is a graphic illustration of a power dissipation de-rating curve in accordance with one aspect of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0015]    Referring to  FIG. 1 , the invention is shown generally at  10  wherein a power electronics package  11  is supportably attached to a compressor  12 , with the details thereof to be described more fully hereinafter. 
         [0016]    The compressor  12  is a hermetic compressor with the motor enclosed in its casing and may be a reciprocating compressor, a rotary compressor or a scroll compressor. It is operatively connected within a refrigeration system that includes, in serial flow relationship, a condenser coil  13 , an expansion device  14 , and an evaporator coil  16 . It preferably also includes a receiver  18 , a filter/dryer  19 , an economizer heat exchanger  21  and a liquid injection valve  22 . 
         [0017]    The evaporator coil  16  is so positioned within the cargo space  17  as to provide cooling thereto, and one or more fans  23  are provided to circulate the air from the cargo space over the evaporator coil  16 . Similarly, the condenser coil  13  is so positioned that its fan  24  is operable to circulate ambient air thereover for purposes of condensing the refrigerant gases within the condenser coil  13 . 
         [0018]    In operation, the refrigerant gas passes from discharge service connection  15  of the compressor  12  along line  26  to the condenser coil  13  with the condensed refrigerant then passing along line  27  to the receiver  18  where liquid refrigerant can be temporarily stored. The liquid refrigerant then passes along line  28  to the filter dryer  19  which acts to remove any impurities from the refrigerant. The refrigerant then passes along line  29  to the economizer heat exchanger  21  and from there along line  31  to the expansion device  14 . The expanded refrigerant passes to the evaporator  16  for purposes of cooling the cargo space, and then along line  32  to a suction service connection  33  through the power electronics package  11  and to the compressor  12 . 
         [0019]    In an economized mode of operation, the frozen range and pull down capacity of the unit is increased by subcooling the liquid refrigerant entering the evaporator expansion valve such that overall efficiency is increased because the gas leaving the economizer enters the compressor at a higher pressure, therefore requiring less energy to compress it to the required condensing conditions. 
         [0020]    Liquid refrigerant for use in economizer circuit is taken from the main liquid line as it leave the filter dryer  19  with the flow being activated when the controller energizes the economizer celluloid valve  20 . A liquid refrigerant flows through the economizer expansion valve  25  the economizer heat exchanger  21  and the line  30  to the economizer service connection  35 . 
         [0021]    During unloaded operation, the economizer solenoid valve  20  is closed and the unloading solenoid valve  40  is opened such that a portion of the mid-staged compressed gas is bypassed to decrease compressor capacity. 
         [0022]    It should be understood that the power electronics package  11  can be any electronic system that is provided for the purpose of varying the speed of the compressor  12 , and the compressor can be of any type of rotary or reciprocating compressor that is driven by an ac or a dc motor. For example, it can be an ac induction motor with an inverter to vary its speed. Alternatively, the speed control can be provided by other apparatus such as a PWM (pulse width modulation) unit or even a variable resistance power electronics package. 
         [0023]    Referring now to  FIG. 2 , the compressor  12  and the mounted power electronics package  11  is shown in greater detail. The compressor drive motor M is, of course, operably disposed within the compressor  12 , which is mounted in a vertical position by way of a base  39  being attached to a pair of resilient shock mounts  41  by bolts  42 . In this way, the compressor  12  is protected against any shock that may otherwise be transferred thereto by way of jarring motions or sudden movements of the vehicle, for example. That is, the shock is absorbed by the shock mounts  41  with the compressor  12  being relatively isolated from such shocks. 
         [0024]    The power electronics package  11  includes a power wiring terminal block housing  43  which contains the power electronics  44 . As will be seen, the power wiring terminal block housing  43  is rigidly secured to a side  46  of the compressor  12  by a plurality of bolts  47 . The resilient mounting that is normally required for the power electronics package  11  is not required since, because of the direct connection to the compressor  12 , the power electronics package  11  derives the benefit of the shock mounts  41  for the compressor. Thus, the power electronics package  11  is protected from shocks by way of the shock mounts  41 . 
         [0025]    An electrical power input is made to the power electronics  44  by way of electrical line  48 , and the power electronics  44  is electrically connected to the motor M by way of electrical line  49 , preferably by way of a fusite member  50 . 
         [0026]    A control device C is electrically interconnected between the power electronics  44  and the motor M so as to selectively vary the power from the power electronics  44  to control the speed of the motor M in a desired manner, with certain operational parameters and sensed conditions being provided to the control C by way of various inputs indicated at numeral  52 . 
         [0027]    Even more important than the resilient mounting benefit is that of using the refrigerant system to cool the electrical components within the inverter power electronics  44  by way of circulating the returning refrigerant gas therethrough. That is, at one side  53  of the housing  43 , provision is made to introduce the flow of suction gas as shown at  54  in such a way as to cause it to flow through the housing  43  and, in doing so to cool the power electronics  44 . The refrigerant gas then flows out the other side  56 , with the flow stream  57  then passing to the suction inlet of the compressor  12 . In this way, the electronic components can be more efficiently cooled than by way of the usual method of circulating air thereover, and will thus allow for the reduction in size and weight of the power electronic package  11 . Further, it will allow operation of the system in a more harsh environment such as at higher ambient temperatures and higher shock loads. 
         [0028]    Considering now in greater detail as to how the refrigerant is applied to cool the electronic components, reference is made to  FIGS. 3 and 4  where two alternatives are shown. In each case, the power electronics package  11  is divided into sections, a power electronics section  58  and a refrigeration section  59 , with the two sections being divided by an intermediate wall or heat sink  61 . Within the power electronics section  58  are located the power electronics and the power switching semiconductors such as, for example, insulated gate bipolar transistors (IGBTs). The power switching semiconductors that require cooling are mounted to the heat sink  61  as shown. The heat sink consists of a highly thermally conductive metal material. 
         [0029]    In the refrigeration section  59 , there are a plurality of heat transfer elements that are integrally connected to the heat sink  61  and whose geometry are designed to maximize the heat transfer effect from the heat sink  61  to the low temperature refrigerant that flows through this section. In  FIG. 3 , for example, the heat transfer elements comprise a plurality of wavy fins  62 , wherein in  FIG. 4 , the heat transfer elements comprise a plurality of staggered perforated plates  63 . In operation, the low temperature refrigerant flows into the inlet  64 , across the heat transfer elements  62  or  63  and out of the outlet  66  where it passes to the suction of the compressor. The cooling effect of the low temperature refrigerant will keep the power switching semiconductors below a specified power semiconductor case temperature. Maximizing the power semiconductor case temperature will allow less power dissipation de-rating of the power semiconductor and thereby allow a smaller power semiconductor package for the same amount of power dissipation. The effect of the cooling will therefore minimize the size of the power switching semiconductor. 
         [0030]    Referring to  FIG. 5 , the power semiconductor power dissipation de-rating curve is shown for a typical power switching power semiconductor to indicate that as the case temperature is decreased, the power dissipation multiplier is proportionally increased. 
         [0031]    It should be recognized that since the power switching semiconductors are part of the compressor speed control there is an inherent relationship between the amount of cooling that is required and the amount of cooling that is provided. That is, when the compressor is operating at full speed the power switching semiconductors will be operating at maximum capacity and maximum generation of heat. At the same time however, since the compressor is operating at full speed the amount of refrigerant being circulated through the system is at a maximum flow rate, and therefore the maximum cooling effect is provided to the heat sink  61 . On the other hand, when the compressor is operating at lower speeds, the heat loss from the power switching semiconductors will be lower as will be the rate of refrigerant flow through the system. In this way, the amount of cooling that occurs is automatically adjusted with changes in compressor motor speed.