Abstract:
A variable speed drive is provided for a compressor in a refrigerant system. When a low load situation has been determined by the refrigerant system controls, the variable speed drive moves the compressor to a lower speed mode of operation. In this case, if a speed is so low that it cannot ensure adequate oil lubrication of the compressor elements, then the motor speed is periodically increased to a level that will ensure proper lubrication. In this manner, a variable speed drive compressor can be operated at an extremely low speed to precisely match load demand on a refrigerant system. The invention can be extended beyond refrigerant system applications and to any oil-lubricated compressors whose lubrication is speed dependant.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This application relates to a control for a variable speed compressor motor, wherein the compressor may operate at extremely low average speeds for extended periods of time while still maintaining adequate lubrication. 
         [0002]    Refrigerant systems are utilized in many applications to condition an environment. In particular, air conditioners and heat pumps are employed to cool and/or heat air entering an environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment. 
         [0003]    A feature that has been gaining popularity in improving the efficiency of refrigerant systems is the use of a variable speed drive for the compressor motor. Often, the compressor need not be operated at full speed, such as when the cooling load on the refrigerant system is relatively low. Under such circumstances, it might be desirable to reduce the compressor speed, and thus reduce the overall energy consumption of the refrigerant system. Implementation of a variable speed drive is one of the most efficient techniques to enhance system performance and reduce life-cycle cost of the equipment over a wide spectrum of operating environments and potential applications, especially at part-load conditions. 
         [0004]    However, compelling reliability concerns set a lower limit to the desirable compressor speed reduction. In particular, inadequate lubrication of the compressor elements may present a problem at low operating speeds. This often occurs as the compressor oil delivery relies on an operation of a pump installed within the compressor, where the oil pump delivery head is strongly affected by the operating speed. If the compressor operating speed is reduced below a certain level, the oil pump cannot generate required pressure head to deliver the oil to the components that need to be lubricated within the compressor. This leads to the inadequate lubrication of those components and subsequent compressor damage. The internal components that are affected the most are the ones located farther away from the pump inlet. Thus, compressors are often rated with a minimum speed (typically 45 Hz) required to ensure adequate compressor lubrication. Even compressors that are specifically designed for variable speed operation, and incorporate special futures (such as a special oil pump) to promote lubrication, often cannot operate below 30 Hz. However, the minimum speed limit established by proper lubrication requirements may still need to be reduced well below 30 Hz to achieve efficient operation at part-load. That is, given the minimum speed limitations, much of the energy efficiency that could be potentially provided by the variable speed drive is essentially eliminated. Thus, there is a need to provide a compressor operating at a lower average speed than what can be currently achievable with current designs. 
       SUMMARY OF THE INVENTION 
       [0005]    In the disclosed embodiment of this invention, a compressor is provided with a variable speed drive. When a low load is detected, the compressor is moved to a low speed to maintain adequate conditions in the environment without switching to a start-stop mode of operation. In fact, the compressor could be moved to a speed far below that was typically previously recommended as a minimum speed, due to reliability concerns. The compressor is allowed to operate at this low speed for a certain period of time. The compressor speed is periodically increased to a level that would ensure adequate lubrication of compressor elements. By periodically driving the compressor at a higher speed, the invention guarantees that an overall adequate supply of lubricant is provided to the compressor elements. The invention relies on residual oil that is left in the components that need to be lubricated during the time when the compressor operates at the low speed. The short bursts of high speed deliver the oil to the components that need to be lubricated. By allowing the compressor to operate at low speed and for short periods of time at high speed, the average compressor operating speed of the present invention can be far below the minimum speed established by the prior art techniques. In a preferred embodiment of this invention, pulse width modulation technique is utilized to vary the compressor motor speed. 
         [0006]    Although, for illustrative purposes, this invention is described in relation to refrigerant systems incorporating scroll compressors, it could be applicable to any variable speed oil-lubricated compressor, whose oil delivery mechanism depends on compressor operating speed. These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of a refrigerant system incorporating the present invention. 
           [0008]      FIG. 2  is a brief flowchart of the present invention. 
           [0009]      FIG. 3  is a graph showing the average speed according to the present invention. 
           [0010]      FIG. 4  shows another schematic of a refrigerant system. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0011]    A refrigerant system  19  is illustrated in  FIG. 1  having a scroll compressor  21  incorporating a non-orbiting scroll member  22  and an orbiting scroll member  24 . As is known, shaft  26  is driven by an electric motor  28  to cause the orbiting scroll member  24  to orbit. As shown, a variable speed drive  30  is schematically connected to drive the electric motor  28 . An oil sump  32  and an oil passage  34  in the shaft  26  supply oil to the various moving elements in the compressor  21 , as known. 
         [0012]    A condenser  36  is positioned downstream of the compressor  21 , an expansion device  38  is located downstream of the condenser  36 , and an evaporator  40  is positioned downstream of the expansion device  38 , as known. As is also known, the compressor  21  is driven by the electric motor  28  to compress the refrigerant vapor and to drive it through the refrigerant system  19 . Oil from the oil sump  32  is delivered to the compressor elements to provide proper lubrication of the compressor components such as the crankcase bearing  100 , orbiting scroll bearing  102 , the fixed scroll  22  and the orbiting scroll  24 , while some amount of oil leaves the compressor  21  with the refrigerant and is circulated through the refrigerant system  19 . One of the most typical oil delivery systems of a scroll compressor is also shown in  FIG. 1 , where the oil from the oil sump  32  is picked up by the oil pick up tube  110 , and delivered along the oil passage  34  to various compressor components as described above. Some of the oil can also be delivered through the suction port  120  by a refrigerant entering the compressor. However most of the oil delivery is accomplished by delivering the oil from the oil sump as described above. In the prior art, when a variable speed drive has been implemented in a refrigerant system, the designer has been limited by a minimum operational speed of the shaft  26  (the operational speed of the shaft is very close to the operational frequency) for the compressor  21 . If the speed drops below a certain level for extended period of time, an insufficient amount of oil is delivered through the oil passage into the compressor components that need to be lubricated. Thus, for a low cooling load situation, where a small amount of the compressed refrigerant is needed to be circulated through the system, a minimum speed such as 45 Hz has often been a limiting factor in reducing the amount of the circulating refrigerant to the desired amount while at the same time ensuring adequate lubrication. 
         [0013]      FIG. 1  shows additional features that may be incorporated into the refrigerant system  19 . As an example, an economizer cycle is included and has an economizer heat exchanger  18 . A main liquid line  13  has a tap line  11  tapped off of the main liquid line and passed through an economizer expansion device  115 . The tap line  11  and the main liquid line  13  both pass through the economizer heat exchanger  18 . In fact, and in practice, the refrigerant flow in the tap line is typically in the counterflow direction through the economizer heat exchanger in relation to the flow in the main liquid line  13 . However, to simplify the illustration in this figure, they are shown in the same direction. As is known, the economizer circuit subcools the refrigerant in the main liquid line, and thus enhances performance (capacity and/or efficiency) of the refrigerant system  19 . An economizer injection line  20  is shown extending back to the compressor  21 , and injects an intermediate pressure refrigerant into compression chambers through passages such as passage  23 . The function and structure of the economizer circuit is known, however, its inclusion with the inventive motor control  30  provides a refrigerant system that has even greater flexibility to enhance operation of the refrigerant system  19 . An unloader line  17  includes an unloader valve  200 . The unloader valve  200  is selectively opened to return partially compressed refrigerant from the compression chambers through the passages  23  back to a suction port  120  of the compressor  21 . The unloader function presents a refrigerant system designer with an extra degree of freedom for performance adjustment and optimization. 
         [0014]    Essentially, when a greater capacity is desired, the economizer function may be utilized with the unloader valve shut. Alternatively, if a lower capacity is necessary, the economizer expansion device  115  (or a separate shut-off device) is shut, with the unloader valve  200  opened. In this manner, the amount of compressed refrigerant delivered to the condenser  36  is reduced. Also, if desired to provide another intermediate stage of capacity for the refrigerant system  19 , the economizer function can be combined with the unloader function by opening both the economizer expansion device  115  and the unloader valve  200 . 
         [0015]    These system configurations in combination with the variable speed motor control disclosed below provide greater freedom and flexibility to a refrigerant system designer. 
         [0016]    It should be understood that the motor control  30  includes a program that takes in inputs from various locations within the refrigerant system, and determines when a lower speed for the compressor motor would be desirable. 
         [0017]    A worker of ordinary skill in the art would recognize when a lower speed might be desirable and preferred in comparison to other available options. 
         [0018]    As shown in  FIG. 2 , the controls for the refrigerant system  19  determine the load demand on the refrigerant system  19 , and if the load demand is low, the speed is reduced to an appropriate level. In the reduced compressor speed mode, if a very low speed (e.g. below 45 Hz) is utilized, then the speed is periodically increased (e.g. to the level above 45 Hz) to ensure that adequate lubrication is provided to the compressor elements. 
         [0019]    As shown for example in  FIG. 3 , a pulse width modulation technique can be utilized to periodically increase the compressor speed up to the 50 Hz level from an otherwise low 20 Hz level. As shown in the embodiment of  FIG. 3 , this could result in an average speed of as low as 25 Hz, while still ensuring adequate lubrication of the compressor elements. Of course, the specific frequencies and modulation time intervals are examples only and would depend on the compressor design specifics. The main thrust of this invention is to allow compressor operation at significantly reduced speeds when a low load demand is imposed on the refrigerant system  19  and compressor  21 . The compressor speed can be reduced far below the speed that would be otherwise necessary to ensure proper lubricant circulation, and then the compressor speed is increased periodically. 
         [0020]    As can be appreciated, the average speed can be calculated as follows: 
         [0000]      AVERAGE SPEED=LOW SPEED*(% OF TIME AT LOW SPEED)+High Speed*(% OF TIME AT LOW SPEED). 
         [0021]    In examples, the lower speed is utilized for longer periods than is the higher speed. In the disclosed example, the 20 Hz speed might occur for 20 seconds with the 50 Hz speed only lasting for 5 seconds. Stated another way, the lower speed can be utilized for more than twice as long as the higher speed. Still, adequate lubrication is achieved. Once again, required modulation intervals, and maximum and minimum compressor speed may vary with the compressor design, required average operating speed, and system operating conditions. 
         [0022]    As can also be appreciated from  FIG. 3 , there might be levels of high speed (high 1 , high 2 ) and levels of lower speed (low 1 , low 2 ) that may associated for instance with different operating conditions. Again, the program incorporated into the control  30  would be provided with these varying options. Once again, the various speeds can be selected based upon system considerations. 
         [0023]      FIG. 4  shows another schematic wherein there are multiple independent refrigerant circuits within a refrigerant system each including a compressor  21 , condenser  36 , expansion device  38  and evaporator  40 . The motors for the compressors  21  are provided with variable speed drives  30 . Although these two circuits are shown in a simplified manner, it should be understood that various additional elements such as the economizer and unloader functions can be incorporated into these systems. Obviously enough, the refrigerant system may incorporate more than two independent circuits, and not necessarily each compressor may be provided with a variable speed drive. 
         [0024]    It has to be understood that although this invention is described in relation to the refrigerant systems incorporating scroll compressors, it could be applicable to any variable speed oil-lubricated compressor that utilizes an oil delivery mechanism that is operating speed dependant. 
         [0025]    Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.