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Capacity control of a compressorRelated Patent Categories: Pumps, Motor Driven, Electric Or Magnetic Motor, Reciprocating Rigid Pumping Member, Reciprocating Motor, Unitary Pump And Motor Working MemberCapacity control of a compressor description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070286751, Capacity control of a compressor. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to compressors, and more particularly, to the capacity control of linear compressors. [0003] 2. Description of the Related Art [0004] Compressors can include a piston which is reciprocated within a cylinder bore to compress refrigerant, for example, in the cylinder bore. The compressor can further include a spring, or springs, which bias the piston into position. In some linear compressors, the piston is positioned intermediate two springs which hold the piston in a substantially stationary position until the piston is moved by an electromagnetic armature or motor, for example. The piston and springs comprise a spring-mass system having a natural, or resonant, frequency, as known in the art. If the piston is driven, via the armature or motor, at the natural frequency of the spring-mass system, the spring-mass system will resonate. Driving the piston of the compressor at, or very close to, the natural frequency of the system allows the compressor to operate more efficiently. In effect, when the spring-mass system is driven at, or close to, its natural frequency, the driving force has less inertial forces in the system to overcome. [0005] In view of the above, previous compressors were typically operated at the natural frequency of their spring-mass systems. To increase or decrease the capacity of these compressors, the displacement, or stroke, of the piston was adjusted to change the output of the compressor. For example, if a greater capacity was needed, the stroke of the piston was increased to draw in, compress, and discharge a larger quantity of refrigerant per stroke. To increase the stroke of the piston, the magnitude of the current flowing through the armature was increased, thereby causing a greater displacement between the piston and the armature. However, modulating the capacity of the compressor in this way has some limitations. For example, increasing the magnitude of the current flowing through the armature can increase the resistance losses in the armature windings, thereby reducing the efficiency of the compressor. Further, large displacements of the piston draws large quantities of refrigerant into the cylinder bore which may bog down or overpower the compressor. [0006] Previously, as discussed above, it was desirable to operate linear compressors at the natural frequency of their spring-mass system. However, owing to changes in the parameters of the refrigerant in the cylinder bore, the natural frequency of the spring-mass system can change throughout the operation of the compressor. More specifically, when the refrigerant is compressed by the piston in the cylinder bore, the refrigerant gas acts as an elastic spring force against the piston. The magnitude of this elastic force depends on, among other things, the fluid being compressed and its density, pressure, and temperature. As known in the art, the magnitude of the spring force from the refrigerant gas affects the natural frequency of the spring-mass system, and, when the parameters of the refrigerant change, the natural frequency of the spring-mass system typically changes as well. In order to determine the natural frequency of the spring-mass system at any instant during the operation of the compressor, a parameter, or parameters, of the refrigerant and/or refrigeration system can be monitored. For example, it was known to monitor temperature of the refrigerant and/or the voltage drop across the armature driving the piston of the compressor. In view of the information obtained from monitoring these parameters, the frequency of the driving force acting on the piston was altered to match the instantaneous natural frequency of the system. [0007] In effect, some previous compressors actively monitored the natural frequency of the spring-mass system and corrected the frequency of the driving force to match the natural frequency of the system. However, when these compressors were required to produce a greater output of compressed refrigerant, their output was limited to that generated at the natural frequency of the compressor. As a result, as discussed above, these compressors were sometimes unable to keep up with the demands of the refrigeration system. To accommodate a potentially greater demand, a compressor having a larger capacity was typically used. However, these larger-capacity compressors are typically more expensive and may become less efficient when lower demands of the compressor are required. What is needed is an improvement over the foregoing. SUMMARY OF THE INVENTION [0008] The present invention includes a linear compressor that is operated at a frequency greater than the natural frequency of the spring-mass system of the compressor. Operating the compressor at such a frequency can increase the output of the compressor. In one embodiment, the linear compressor includes a cylinder block having a cylinder bore, a piston positioned within the cylinder bore, first and second springs for positioning the piston where the piston and the first and second springs comprise a spring-mass system having a natural frequency, and an armature operably engaged with the piston to drive the piston at a frequency greater than the natural frequency of the spring-mass system. [0009] In another embodiment, the linear compressor includes a controller which monitors the instantaneous natural frequency of the spring-mass system and modulates the frequency of the current passing through the armature. As discussed above, the natural frequency of the spring-mass system can change as a result of fluctuations in the temperature and/or pressure of the refrigerant in the cylinder bore. In this embodiment, a parameter of the refrigerant in the refrigerant circuit, such as the pressure and/or temperature of the refrigerant, for example, or the electrical power transmitted to the armature, such as the voltage and/or current, for example, is monitored by the controller. In view of the information obtained from monitoring these parameters, the controller can determine the instantaneous natural frequency of the spring-mass system and evaluate whether the frequency of the current being transmitted to the armature is greater than the instantaneous natural frequency of the system. If necessary, the controller can increase the frequency of the current such that it exceeds the natural frequency of the spring-mass system, or, even if the driving frequency is already greater than the natural frequency, it can increase the driving frequency to increase the output of the compressor to meet the demands of the refrigeration system. BRIEF DESCRIPTION OF THE DRAWINGS [0010] The above-mentioned and other features and objects of this invention will become more apparent and the invention itself will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: [0011] FIG. 1 is a schematic of a typical refrigeration circuit including a compressor and a controller for operating the compressor; [0012] FIG. 2 is a partial cut-away view of a linear compressor in accordance with an embodiment of the present invention; [0013] FIG. 3 is a cross-sectional perspective view of a first alternative embodiment linear compressor; [0014] FIG. 4 is an exploded cross-sectional view of a second alternative embodiment linear compressor; [0015] FIG. 5 is a schematic representing the spring-mass system of the compressor of FIG. 2; and [0016] FIG. 6 is a graph charting the cooling capacity of a linear compressor with respect to the current flowing through the armature of the compressor. [0017] Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplifications set out herein illustrate embodiments of the invention, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise form disclosed. DETAILED DESCRIPTION [0018] Referring to FIG. 1, typical refrigeration system 10 includes, in serial order, compressor 12, condenser 14, expansion device 16, and evaporator 18 connected in series by fluid conduits. As is well known in the art, compressor 12 draws a refrigerant or working fluid through compressor inlet 11, compresses the refrigerant, and expels the compressed refrigerant through compressor outlet 13. The refrigerant expelled from compressor 12 is communicated into condenser 14 where thermal energy of the refrigerant is dissipated. Subsequently, the cooled, compressed refrigerant is communicated to expansion device 16 where it is decompressed. The cooled, low-pressure refrigerant is then communicated to evaporator 18 where the refrigerant in evaporator 18 draws heat from an environment surrounding the evaporator. Subsequently, the refrigerant exits evaporator 18 and is communicated to compressor 12 and the cycle described above is repeated. [0019] Referring to FIG. 2, compressor 12, in the present embodiment, is a dual-cylinder linear compressor having two axially-driven compressor mechanisms 48 mounted therein. Compressor 12 further includes housing 42 having interior cavity 44 and end caps 46 on opposite ends thereof which also define cavity 44. Generally, in operation, refrigerant is drawn into compressor 12 through suction inlet 11 and suction manifold 45, compressed by compressor mechanisms 48, and is then discharged into discharge muffler chamber 51 through discharge valves 55. Referring to FIG. 4, which illustrates an alternative embodiment of a linear compressor, each compressor mechanism 48 can include gasket 61, suction valve 59, valve plate 53, and discharge valve 55 for controlling the flow of suction refrigerant into, and the flow of discharge refrigerant out of, the compression cylinder of compressor mechanism 48. Thereafter, the compressed refrigerant is discharged from compressor 12 through discharge outlet 13. [0020] Each compressor mechanism 48 includes a cylinder block 50 having cylinder bore 52 therein, a piston 54 positioned within cylinder bore 52, an armature 56 mounted to one end of piston 54, and a permanent magnet 58 positioned within end cap 46. In operation, piston 54 is reciprocatingly driven within cylinder bore 52 by the interaction of armature 56 and permanent magnet 58. More particularly, armature 56 is energized by an electrical source which conducts electricity to armature 56 through terminal cluster 60 and spring 62 positioned intermediate cylinder block 50 and armature 56. Armature 56 includes a series of copper windings, or coils, which are, in this embodiment, arranged in a cylindrical configuration. The cylindrical configuration of armature 56 is sized and configured to fit in gap 66 defined between permanent magnet 58 and end cap 46 so that relative movement of armature 56 therebetween is possible. Owing to a magnetic field created by permanent magnet 58, armature 56, when energized, is motivated to move axially along axis 64. Continue reading about Capacity control of a compressor... 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