Abstract:
A method of powering a refrigeration system. The method includes providing a mover and an alternator, the alternator being coupled to the mover and generating a power signal. The method also includes monitoring at a control a plurality of system parameters and sending a control signal based on the system parameters from the control. The method further includes receiving the power signal and the control signal at an inverter-based device which has a plurality of inverters, converting the power signal into a controlled power signal based on the control signal, and driving a plurality of components of the refrigeration system with the controlled power signal, which are also controlled by the control.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. § 119 to provisional patent application serial no. 60/296,874, filed on Jun. 8, 2001. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    Transport refrigeration units are used to maintain a desired temperature in a conditioned load space inside an enclosure used for carrying cargo, such as in a trailer, truck or other transport container. A transport refrigeration unit can be installed on the exterior of the enclosure, outside of the conditioned load space. A typical cargo container is a truck, and a typical mounting site for the transport temperature control unit is above the truck cabat the front wall of the enclosure.  
           [0003]    Transport refrigeration units generally include an evaporator assembly that transfers heat from the conditioned load space into a refrigerant, and a condenser assembly that transfers heat from the refrigerant to the outside environment. The evaporator assembly typically includes an evaporator coil and an air-moving apparatus (e.g., a fan). The air-moving apparatus draws relatively warm air from the conditioned load space, passes the air over the evaporator coils, which take heat from the air and return the cooler air to the conditioned load space. The condenser assembly typically includes condenser coils and an air-moving apparatus (e.g., a fan), which draws air from the outside environment over the condenser coils and returns the heated air to the outside environment.  
           [0004]    Transport refrigeration units also generally include a refrigerant compressor to pressurize the refrigerant and an expansion valve to depressurize the refrigerant. The evaporator assembly, condenser assembly, compressor and other components in the temperature control unit require a power supply. Conventional transport refrigeration units employ an engine, such as an internal combustion diesel engine, to supply the needed power (for the compressor, fans, valves, etc.). The engine can be separate from the vehicle engine or the vehicle engine itself can be used. If the vehicle engine is used, electrical connections need to be made between the refrigeration unit and the engine&#39;s electrical source, usually an alternator. In addition, some units utilize a compressor driven directly by the vehicle engine, requiring pipe connections from the compressor to the refrigeration unit. The potential for leakage or for electrical problems is increased with the increased distance between the refrigeration unit and the engine. Other units utilize a separate engine mounted near the refrigeration unit. This eliminates the leakage problems but introduces new problems. The engine will require additional maintenance and fuel to operate, increasing the costs of operating the unit. Typical vehicle engine-driven air conditioning systems have included inverter circuit components. However, these systems are power supply driven, where the output frequencies of the inverters are adjusted in response to the power supplied to the unit.  
         SUMMARY OF THE INVENTION  
         [0005]    It is generally desirable to make the transport refrigeration unit as compact and as efficient as possible. Both objectives can be advanced by making the powered components of the unit electrically-powered and independently controlled. By making compressor, condenser fans and evaporator fans electrically-powered and independently-controlled, there is no need for a mover in the transport refrigeration unit. Generally speaking, a mover is a device that uses mechanical or chemical energy to drive another component. In the case of a transport refrigeration unit, a mover could include a diesel engine. Frequently a mover drives another device mechanically, by means such as belts and pulleys. A mover and the mover&#39;s associated apparatus consume considerable space. If the mover is a diesel engine that drives an electric motor for example, the engine and the motor both take up space, as do the belts and pulleys and other mechanical driving systems.  
           [0006]    Components in a transport refrigeration unit that are electrically-powered and independently-controlled can utilize the already existing, and required, vehicle engine as a mover. Using the already existing electrical system allows for the use of efficient self-contained fans and compressors. A condenser fan, for example, can include its own inductive motor, and need not be mechanically driven by a mover. Compressors such as hermetic scroll compressors likewise can include their own electric motors.  
           [0007]    However, using the electricity generated by the vehicle engine may be inefficient due to the wide variations in frequency and voltage that occur during normal operation. For example, as the vehicle engine speed increases or decreases, the frequency of the electric power from the corresponding alternator fluctuates. Therefore it is desirable to control the power supply to the differing components to optimize the efficiency of the cooling unit given the limited amount of power that may be available.  
           [0008]    Therefore the present invention provides a method of powering a refrigeration system is provided. The method includes providing a mover, and providing an alternator, the alternator being coupled to the mover, and generating a power signal. The method further includes monitoring at a control a plurality of system parameters, and sending a control signal based on the system parameters from the control. Furthermore, the method includes receiving the power signal and the control signal at an inverter-based device, the inverter-based device having a plurality of inverters, and converting the power signal into a controlled power signal based on the control signal. The method also includes driving a plurality of components of the refrigeration unit with the controlled power signal, the components also being controlled by the control.  
           [0009]    In another embodiment, a method of power distribution in a temperature controlled transport unit is provided. The method includes providing a primary power signal and converting the primary power signal into a secondary controlled power signal with a plurality of inverters. The inverters are coupled to a control that sends control signals to the inverters. The method further includes driving a plurality of components in the temperature controlled system with the controlled power signal. The components are also controlled by the control.  
           [0010]    In still another embodiment according to the present invention, a power distribution system in a temperature controlled transport system is provided. The system includes a mover operatively coupled to an alternator that generates a power signal. The mover is also operatively coupled to a control that monitors a plurality of system parameters and sends a control signal based on the system parameters. The system also includes an inverter-based device operatively coupled to the control, the inverter based device having a plurality of inverters, receiving the power signal and the control signal, and converting the power signal into a controlled power signal based on the control signal. Furthermore, the system includes a plurality of components, the components being driven with the controlled power signal and being controlled by the control.  
           [0011]    In the preferred embodiments, the present invention utilizes an alternator coupled to the vehicle engine to provide an alternating current (“AC”) power signal. Rectification of the AC power signal creates a direct current (“DC”) power signal that is passed through a DC bus voltage controller and then supplied to a pair of inverters. The DC bus voltage controller controls the variable voltage generated by the alternator. Each inverter converts the DC power signal into a controlled AC power signal for driving the components of the refrigeration unit. One AC power signal can be used to drive the compressor, which requires the largest amount of power. The second AC power signal is then used to power the evaporator fan and condenser fan. This arrangement allows for the motors to be run at different speeds and power levels depending on the amount of cooling required and the amount of power that is available from the engine.  
           [0012]    The alternator is liquid cooled, with the coolant heat rejected using a heat exchanger incorporated in the refrigeration system condenser coil or, alternatively, in the refrigeration system evaporator coil. The increased cooling efficiency resulting from the use of the refrigeration system heat exchanger or evaporator coil, as opposed to using the vehicle radiator heat exchanger, enables the use of a smaller alternator.  
           [0013]    A microprocessor-based control receives and processes a number of input variables and runs a control algorithm to efficiently manage the power supply and electrical load of the system. The control continuously monitors refrigeration system parameters including alternator speed, refrigeration system pressure, watt power transducer values, current power transducer values, refrigeration system suction pressure value, fixed suction pressure value, and condenser and evaporator discharge temperature to determine the current electrical load of the system. The control algorithm continuously establishes: (1) the position of a suction line proportional refrigeration valve; and (2) the inverters&#39; output frequency/voltage. Together, these controlled parameters establish the alternator input power. A predetermined, prime mover speed-dependent utilizable power map is incorporated in the control algorithm. The alternator&#39;s input power consumption is made equivalent to the prime mover speed-dependent utilizable power map.  
           [0014]    The control algorithm also controls the refrigeration unit high side refrigerant pressure at extreme conditions. A high pressure control set point is input into a control algorithm. Rising refrigerant pressure results in changing the compressor speed with Proportional Integral Derivative (“PID”) control to limit the pressure to the set point value. Further required reduction in refrigerant pressure uses a suction line PID controlled refrigerant valve. Other unit performance parameters are continuously monitored to allow for further changes in compressor speed or refrigeration unit suction flow. The control algorithm prevents exceeding the refrigeration pressure set point limit and avoids shut down of the refrigeration unit.  
           [0015]    The control algorithm incorporates a soft start power management function. The rate of application of load power to a vehicle engine influences drivability of the vehicle. Using an established load application rate, measured in watts per second, minimizes the influence on vehicle performance. Determination of an acceptable application rate for each vehicle or a rate for all vehicles, optimizes the performance of a vehicle powered refrigeration product, deriving power from a vehicle.  
           [0016]    Further objects and advantages of the present inventive transport refrigeration system, together with the organization and manner of operation thereof, will become apparent from the following detailed description of the invention when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings. Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    In the drawings:  
         [0018]    [0018]FIG. 1 shows a truck with a transport refrigeration system according to the present invention mounted thereon;  
         [0019]    [0019]FIG. 2 shows a schematic drawing of the mechanical components of the refrigeration system of FIG. 1;  
         [0020]    [0020]FIG. 3 shows a schematic drawing of the electrical components of a first preferred embodiment of the transport refrigeration unit of FIG. 1;  
         [0021]    [0021]FIG. 4 shows a schematic drawing of the electrical components of a second preferred embodiment of the transport refrigeration unit of FIG. 1;  
         [0022]    [0022]FIG. 5 shows a schematic drawing of the electrical components of a third preferred embodiment of the transport refrigeration unit of FIG. 1;  
         [0023]    [0023]FIG. 6 shows a schematic drawing of the electrical components of a fourth preferred embodiment of the transport refrigeration unit of FIG. 1;  
         [0024]    [0024]FIG. 7 shows a schematic drawing of the electrical components of a fifth preferred embodiment of the transport refrigeration unit of FIG. 1; and  
         [0025]    [0025]FIG. 8 shows a schematic drawing of the electrical components of a sixth preferred embodiment of the transport refrigeration unit of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.  
         [0027]    [0027]FIG. 1 depicts a temperature-controlled cargo carrier  10  in the form of a straight truck including a conditioned load space  12  for carrying cargo. A transport refrigeration unit  14  according to the present invention according to the present invention mounts to the front wall  16  of the load space  12 . The truck  10  further includes a cab  18 , which houses an engine  20 , such as a diesel engine, that provides power to move the truck  10  and to operate the transport refrigeration unit  14 .  
         [0028]    [0028]FIG. 2 shows the mechanical components of the transport refrigeration unit  14  that may be used with the truck  10  as shown in FIG. 1. The prime mover  20 , that is, the supplier of energy for the system, is, for example, an engine such as a diesel engine of the truck  10 . U.S. Pat. No. 6,367,269, assigned to the same assignee as the present invention and incorporated by reference herein, fully describes the operation of the transport refrigeration system of FIG. 2.  
         [0029]    [0029]FIG. 2 shows an alternator  204  (component  22  of FIG. 1) of an undermount refrigeration system. The alternator  204 , being moved by the prime mover of FIG. 1, generates a power signal that an inverter  208  further regulates and yielding a controlled power signal. The controlled power signal then drives a condenser fan  212  and a heat exchanger or a condenser coil  216 , which is further controlled by a coolant pump  220 . A suction line proportional refrigeration valve or stepper motor driven valve  224  controls the amount of a refrigerant, such as R404A, that a compressor  228  provides to the condenser coil  216  through a pressure regulator  232 , a receiver/accumulator tank  236 , and a series of valves  237  and transducers  238 . FIG. 2 also shows an evaporator coil  240  controlled by an expansion valve  244  and a refrigerant distributor  248 . Although a preferred embodiment of an undermount refrigeration system is shown in FIG. 2, it is understood that, according to the present invention, a nosemount refrigeration system (as shown in FIG. 1) can also use similar system architecture and operations. The controlled operations of a system according to the present invention and for use in either arrangement is detailed hereinafter.  
         [0030]    As shown in FIGS.  3 - 8 , a belt drive assembly mechanically couples the diesel engine  20  to a synchronous alternator  22 . The belt drive assembly comprises a belt  24  and pulleys  26 ,  28  and couples a power shaft  30  from the engine  20  to the alternator  22 . The alternator  22  is preferably brushless, providing longer wear and greater efficiency, and preferably is an eight-pole alternator, for providing polyphase or three phase alternating current (“AC”) output. Because the alternator  22  is a synchronous generator, the AC outputs from the alternator are proportional to the speed imparted by the engine  20 . The AC output frequency therefore varies generally from a low of 45 Hz to a high of 75 Hz. The alternator  22  is a compact unit with liquid cooling supplied by the transport refrigeration system&#39;s  14  heat exchanger. The coolant heat is rejected using a heat exchanger incorporated in the refrigeration system condenser coil or, alternatively, in the refrigeration system evaporator coil. The increased cooling efficiency of the refrigeration system heat exchanger, as compared to the vehicle radiator heat exchanger, enables the use of a smaller alternator.  
         [0031]    As shown in FIG. 3, the alternator  22  electrically couples to an input diode rectifier  31  for conversion of the three phase AC power signal to a single direct current (“DC”) signal. Because the rectifier  31  is uncontrolled, a DC bus voltage controller  32  is used to control the variable voltage and frequency of the electrical power. In a preferred embodiment, the DC bus voltage controller  32  is an elevator chopper circuit.  
         [0032]    A first and second inverter  34 ,  36 , arranged in parallel, receive DC power from the DC bus voltage controller  32 . In the parallel arrangement, both inverters  34 ,  36  receive the same DC voltage and will draw current in proportion to their load. The DC voltage will vary typically in a range from 500 VDC to 700 VDC depending on the engine speed.  
         [0033]    Each inverter  34 ,  36  uses a microprocessor control  37  to regulate its three phase AC output. Under normal operating conditions, when the alternator  22  produces enough power to operate the electrical components of the truck  10  and the transport refrigeration system  14 , the inverter output will be maintained at 500 VAC and 75 Hz. While any voltage and frequency could be used, most electric motors in the United States are designed to operate at 460 V and 60 Hz thereby making this their most efficient operating input. If the engine  20  is not supplying sufficient power, such as when the truck is idling, one or both of the inverters output voltage or frequency can be reduced to efficiently use the power that is available. Typically, the inverters  34 ,  36  are capable of outputting a three phase AC power supply between 360 V at 45 Hz and 500 V at 75 Hz.  
         [0034]    The principles by which inverters operate are well-known. A typical inverter uses pulse width modulation (“PWM”) to control the voltage and frequency of the AC output. The inverter  34  includes a bank of electronic switches that turn on and off in response to control signals from an inverter controller. By electronic switching, pulses of varying amplitude and duration are produced. The output is a three-phase AC waveform. In many inverters, the AC power is filtered to remove high frequency harmonics, and to produce a sine-wave output.  
         [0035]    FIGS.  4 - 5  show additional preferred embodiments of the present inventive transport refrigeration system. An input inverter bridge with a DC bus voltage control  33  replaces the rectifier  31  and elevator chopper  32  to produce the regulated DC power signal. The input inverter bridge  33  is an inverter with the output terminal coupled to the alternator  22 .  
         [0036]    The inverters  34 ,  36  provide numerous advantages to the system described above. First, the transport refrigeration system  14  is more compact because a second prime mover, independent from engine  20 , need not be included in the system  14 . The electrical operation of the system  14  and the absence of a second prime mover in the system also produce additional benefits such as reduced noise and vibration, and lower emissions.  
         [0037]    Second, the multiple-inverter system allows different components in the system  14  to be powered at different voltages and frequencies. While the system  14  operates most efficiently when all of the electric motors are powered at 460 VAC and 60 Hz, this may not be possible at all engine speeds. If the engine  20  is unable to supply sufficient power to maintain this output, the voltage and frequency to certain components can be reduced to maintain the optimum cooling efficiency at different power levels without having to completely shut down components. For example, the compressor power supply could be maintained at 460 VAC and 60 Hz allowing for full load operation of the compressor  38  while the evaporator and condenser fan motor supply voltage and frequency would be reduced to reduce the total power load of the system. In this way, the refrigeration unit  14  can continue to cool with a diminished power supply.  
         [0038]    Third, the inverters  34 ,  36  isolate the alternator  22  from the transport refrigeration unit  14  and regulate the power, such that the power supplied to the unit  14  is no longer synchronous with the alternator  22 . Fourth, the inverters  34 ,  36  are efficient drivers for fans, blowers and compressors. Fans, blowers and compressors are heavily inductive loads, and if connected directly to a power generator would require considerable power factor correction. Frequency inverters electrically coupled to a power generator, by contrast, absorb virtually no reactive power and require no power factor correction. Inverters also eliminate start-up power surges in fans, blowers and compressors.  
         [0039]    Furthermore, the use of inverters  34 ,  36  enables soft start power management. The rate of application of load power to engine  20  influences the drivability of the truck  10 . Using an established load application rate, measured in watts per second, minimizes the influence on truck performance. Determination of an acceptable application rate for each truck or rated for all trucks, optimizes the performance of vehicle-powered transport refrigeration unit  14 .  
         [0040]    [0040]FIG. 3 shows the first and second inverters  34 ,  36  connected in parallel to the DC power supply from the alternator  22 . While the first and second inverters  34 ,  36  are utilized in parallel in the illustrated embodiment, it is contemplated that other numbers of inverters could be utilized. In the illustrated embodiment, the first inverter  34  is configured to power the compressor  38  and the second inverter  36  is configured to power the condenser fan  40  and the evaporator fan  42 . While this is the preferred arrangement, it is contemplated that the three refrigeration components that require the most electrical power, namely the compressor  38 , condenser fan motor  40 , and the evaporator fan motor  42 , could be connected in any desired manner. For example, three inverters could be utilized, with the first operating the condenser fan, the second operating the compressor, and the third operating the evaporator fan. Alternatively, two inverters could be used with the condenser fan and the compressor being operated by the first inverter and the evaporator fan being operated by a second inverter. Many different arrangements and combinations are possible and will be readily apparent to those of ordinary skill in the art.  
         [0041]    The transport refrigeration unit control  37  is a microprocessor-based control that receives and processes a number of input variables and runs a control algorithm to efficiently manage the power supply and electrical load demands of the system. The control  37  continuously monitors refrigeration unit  14  parameters including alternator speed, refrigeration unit pressure, watt power transducer values, current power transducer, refrigeration system suction pressure value, fixed suction pressure value, and condenser and evaporator discharge temperature to determine the current electrical load of the system. The control algorithm continuously establishes: (1) a position of a suction line proportional refrigeration valve  224  (FIG. 2), which regulates a refrigerant liquid flow rate; and (2) the inverters&#39;  34 ,  36  output frequency/voltage. Together, these controlled parameters establish the alternator  22  input power. A predetermined, prime mover  20  speed-dependent utilizable power map is incorporated in the control algorithm. The alternator&#39;s input power consumption is made equivalent to the prime mover speed-dependent utilizable power map.  
         [0042]    In operation, the control  37  can sense that no cooling is required and shut off the refrigeration components. The control  37  can also determine that a cooling cycle is necessary and that sufficient power is available to run all of the components at their most efficient point. The control  37  will signal the inverters  34 ,  36  to maintain 60 Hz at 460 VAC.  
         [0043]    In another example, the control  37  can sense that the power output of the engine  20  is insufficient to run all the components and that cooling is required. The control  37  can use the available power to operate the compressor  38  at its design point while operating the condenser fan  40  and evaporator fan  42  at a lower voltage or frequency using whatever power may remain. While this would not provide the most cooling, it will provide some cooling given the limited available power output of the truck engine. If the engine  20  suddenly begins outputting more power, the control  37  will sense this and adjust the first and second inverters  34 ,  36  accordingly. In addition, the first and second inverters  34 ,  36  can supply power at a frequency and voltage that is higher than the design point of the electrical components allowing them to operate at a capacity higher than their design point.  
         [0044]    The control algorithm also controls the refrigeration unit high side refrigerant pressure at extreme conditions. A high pressure control set point is input into a control algorithm. Rising refrigerant pressure results in changing the compressor speed with Proportional Integral Derivative (“PID”) control to limit the pressure to the set point value. Further required reduction in refrigerant pressure uses a suction line PID controlled refrigerant valve  224 . Other unit performance parameters are continuously monitored to allow for further changes in compressor speed or refrigeration unit suction flow. The control algorithm prevents exceeding the refrigeration pressure set point limit and shut down of the refrigeration system is avoided.  
         [0045]    Transport refrigeration units, such as the system of the present invention shown in FIG. 3, often require auxiliary power to maintain the load space at a desired temperature when the engine  20  is not operating or when the cab  18  and the load space  12  are separated, such as when the truck parks at a terminal. To facilitate this, the present inventive transport refrigeration unit is adapted to use a standby power supply  46 . The standby supply  46  is three phase 460 VAC 60 Hz power from an electrical grid. An automatic phase correction (“APC”) module  48  employs a first and second switch  49 ,  51  to ensure that the three phase AC power supplied by the electrical grid is in the proper phase. APC module  48  detects the phase sequence of the AC power and opens and/or closes the first and second switches  49 ,  51  as required to maintain the power supplied to the compressor  38 , condenser fan  40  and evaporator fan  42  in the proper phase.  
         [0046]    A first set of contactors  50  switches between the first inverter  34  supply and standby supply  46  to power the compressor  38  and a second set of contactors  52  switches between the second inverter  36  supply and the standby supply  46  to power the condenser fan  40  and evaporator fan  42 . No control of the standby supply  46  or of the various components is necessary as the standby supply  46  is capable of operating all of the components at their design points.  
         [0047]    When it is desired to switch to the standby power supply  46 , the control  37  signals a first, second and third contactor control  54 ,  56 ,  58 , to close a first series of contactors  60 ,  62 ,  64  and connect the standby power supply  46  to the refrigeration unit components, while simultaneously opening a second series of contactors  66 ,  68  and  70  to disconnect the first inverter  34  and the compressor, and disconnect the second inverter  36  and the condenser fan  40  and evaporator fan  42 .  
         [0048]    [0048]FIGS. 4 and 6 show additional preferred embodiments of the present invention where the standby power supply  46  electronically couples to the compressor  38 , condenser fan  40  and evaporator fan  42  via the DC bus controller  33 . When it is desired to switch to the standby power supply  46 , the control  37  signals contactor control  72  to close a first contactor  76 , to connect the standby power supply  46  to the refrigeration unit components, while simultaneously opening a second contactor  74 , disconnecting alternator  22  and the DC bus controller  33 .  
         [0049]    [0049]FIGS. 5, 7, and  8  show additional preferred embodiments of the present invention where an electric heater  78  electrically couples to the first inverter  34 . The embodiments shown in FIGS. 5 and 8 include a contactor control  86  and a first and second contactor  88 ,  90 . When the control  37  determines that a cooling cycle is necessary, the control  37  signals the contactor control  86  to close the first contactor  88  connecting the compressor  38  and first inverter  34 , while simultaneously opening the second contactor  90  to disconnect the electric heater  78  from first inverter  34 . When the control  37  determines that a heating cycle is necessary, the control  37  signals the contactor control  86  to close the second contactor  90 , connecting the electic heater  78  and the first inverter  34 , while simultaneously opening the first contactor  88  to disconnect the compressor  38  from first inverter  34 . Situations requiring a heat cycle may include defrosting of the condenser coil or heating of the load space. If the control  37  determines that neither a cooling cycle nor a heat cycle is required, the control  37  may signal contactor control  86  to open the first and second contactors  88 ,  90  to disconnect the compressor  38  and electric heater  78  and the first inverter  34 .  
         [0050]    [0050]FIG. 7 shows an embodiment of the present invention where the control  37  can signal the contact control  80  to: (1) close the first contactor  82 , to connect the electric heater  78  and first inverter  34 , while simultaneously opening the second contactor  84 , to disconnect the electric heater  78  and the standby power supply  46 ; (2) close the second contactor  84 , to connect the electric heater  78  and the standby power supply  46 , while simultaneously opening the first contactor  82 , to disconnect the electric heater  78  and the first inverter  34 ; or (3) open the first and second contactors  82 ,  84  when the control  37  determines that a heat cycle is not required. Additionally, the control  37  can signal the contactor control  54  to: (1) close the first contactor  60 , to connect the compressor  38  and first inverter  34 , while simultaneously opening the second contactor  66 , to disconnect the compressor  38  and the standby power supply  46 ; (2) close the second contactor  66 , to connect the compressor  38  and the standby power supply  46 , while simultaneously opening the first contactor  60 , to disconnect the compressor  78  and the first inverter  34 ; or (3) open the first and second contacts  60 ,  66  when the control  37  determines that a cooling cycle is not required.  
         [0051]    Various features and advantages of the invention are set forth in the following claims.