Patent Publication Number: US-6217287-B1

Title: Variable capacity compressor having adjustable crankpin throw structure

Description:
RELATED APPLICATIONS 
     This is a continuation of application Ser. No. 09/013,154, filed Jan. 26, 1998 now U.S. Pat. No. 6,099,259 all of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is concerned with variable capacity compressors, vacuum or other pumps or machines, and particularly those reciprocating piston compressors used in refrigeration, air conditioning systems or heat pumps or the like, including machines such as scotch yoke compressors of U.S. Pat. No. 4,838,769, wherein it is desirable to vary the compressor output, i.e., compressor capacity modulation, in accordance with cooling load requirements. Such modulation allows large gains in efficiency while normally providing reduced sound, improved reliability, and improved creature comforts including one or more of reduced air noise, better de-humidification, warmer air in heat pump mode, or the like. 
     The efficiency gains resulting from a compressor with capacity modulation are beneficial in a variety of commercial applications. For example, most residential refrigerators currently utilize a single capacity compressor and cycle the compressor on and off to maintain a certain temperature within the cabinet of the refrigerator. During normal operation, the temperature of the refrigerator increases due to the warmer ambient air surrounding the refrigerator or when the refrigerator door is opened or a load of perishables having a temperature greater than that of the cabinet is introduced to the refrigerator. If the temperature exceeds a preset limit, the compressor is activated to cool the cabinet of the refrigerator. To account for the higher load conditions when the door is opened or perishables are introduced to the cabinet, the cooling capacity of the compressor is necessarily greater than the minimum required to maintain a particular temperature in the ambient conditions. With this design, the compressor undergoes multiple starts and stops to respond to varying load conditions. The high number of starts and stops will shorten the life of the compressor. Additionally, operating the compressor at full capacity during periods of minimal load is inefficient. 
     One approach to achieving modulation of a compressor has been to switch the stroke length, i.e., stroke, of one or more of the reciprocating pistons whereby the volumetric capacity of the cylinder is changed. In these compressors the reciprocating motion of the piston is effected by the orbiting of a crankpin, i.e., crankshaft eccentric, which is attached to the piston by a connecting rod means which has a bearing in which the eccentric is rotatably mounted. 
     A proposed mechanism in the published art for switching stroke is the use of a cam bushing mounted on the crankshaft eccentric, which bushing when rotated on the eccentric will shift the orbit axis of the connecting rod bearing radially and parallelly with respect to the crankshaft rotational axis and thus reduce or enlarge the rod bearing orbit radius. This, in turn, changes the piston stroke accordingly. In such cam action mechanism the piston at the reduced stroke does not attain full or primary stroke top-dead-center (TDC) positioning within the cylinder. This design diminishes compression and permits considerable reexpansion of the only partially compressed refrigerant. The efficiency of the compressor is thus markedly compromised. 
     Certain prior art cam mechanisms are shown and described in U.S. Pat. Nos. 4,479,419; 4,236,874; 4,494,447; 4,245,966; and 4,248,053, the disclosures of which with respect to general compressor construction and also with respect to particular structures of cylinder, piston, crankshaft, crankpin and throw shifting mechanisms are hereby incorporated herein by reference in their entirety. With respect to these patents the crankpin journal is comprised of an inner and one or more outer eccentrically configured journals, the inner journal being the outer face of the crankpin or eccentric, and the outer journal(s) being termed “eccentric cams or rings” in these patents. The outer journals are rotatably mounted or stacked on the inner journal. The bearing of the connecting rod is rotatably mounted on the outer face of the outermost journal. In these patents, all journal and bearing surfaces of the coupling structure or power transmission train of the shiftable throw piston, from the crankshaft to the connecting rod are conventionally circular. 
     Referring particularly to the U.S. Pat. No. 4,245,966 patent, a TDC position of the piston is said to be achieved thru the use of two eccentric rings which are provided with stops to orient the cams, in the hope of achieving the TDC position. This structure is very complex, expensive, and difficult to manufacture and to assemble, in a commercial sense. Further, as stated in this patent at col. 4 lines 32-38, the operability of these two eccentrics to attain TDC is essentially by chance and is just as likely to result in a piston-valve plate crash. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide an improved coupling structure for a crankpin throw shifting mechanism for a single or multi-cylinder compressor wherein the piston always achieves primary TDC position regardless of the degree of stroke change. 
     Another object is to provide improved commercial applications of single or multiple compressors that include the improved coupling structure. These and other objects will become apparent from the description and claims of the invention, presented below. 
     SUMMARY OF THE INVENTION 
     Accordingly, one aspect of the present invention is directed to a unique, simple and reliable coupling structure for functionally connecting a connecting rod bearing and a crankpin. This structure is adapted to change the primary stroke of a piston while always effecting primary top dead center positioning of said piston on its up-stroke regardless of the stroke change. 
     In accordance with another aspect of the present invention, as embodied and broadly described herein, the invention is directed to a two stage reciprocating compressor. The compressor includes a reversible motor for rotating in a forward and a reverse direction and a block with a single cylinder and associated single compression chamber and single piston. A mechanical system is provided between the motor and the single piston for driving the piston at a full stroke between a bottom position and a top dead center position when the motor is operated in the forward direction and for driving the piston at a reduced stroke between an intermediate position and the top dead center position when the motor is operated in the reverse direction. There is further provided a control for selectively operating said motor either in the forward direction at a first preselected, fixed speed or in the reverse direction at a second preselected, fixed speed. 
     According to another aspect, the invention is directed to a refrigerator appliance that includes a two-stage reciprocating compressor that has an electrical motor and a single cylinder with an associated single compression chamber and single piston. The compressor is operable at either at a first stage with a first capacity or at a second stage with a second, reduced capacity. 
     In another aspect, the invention is directed to a heating, ventilating, and air conditioning (“HVAC”) system for conditioning air within an enclosure. The HVAC system includes a two-stage reciprocating compressor that has an electrical motor and a single cylinder with an associated single compression chamber and single piston. The compressor is operable at either at a first stage with a first capacity or at a second stage with a second, reduced capacity. 
     In still another aspect, the invention is directed to a power system for a motordriven component of a heating and/or air conditioning system (“HVAC”). The power system includes an induction motor with a start and a run winding and a circuit for controlling the motor to rotate in a forward direction in a first stage and in a reverse direction in a second stage. The circuit design includes a first terminal for connection to line power, a second terminal for connecting to the line power, a capacitor, and a switching device that places the capacitor in series with the start winding and utilizes the run winding as the main winding when the motor is in the first stage and that places the capacitor in series with the start winding and utilizes the start winding as the main winding when the motor is in the second stage. 
     As explained in more detail below, the present invention provides a structurally simple coupling mechanism which can be manufactured to give any desired compressor capacity shift. The coupling structure of the invention can be applied to give different strokes for two or more pistons of multi-cylinder compressors and provide a wide range of desired variations in compressor capacity without reducing compressor efficiency thru significant volume clearance, i.e., clearance between the piston top and valve plate at TDC. The invention also includes a motor control circuit that can be used to advantage with the disclosed compressor to achieve markedly improved overall efficiency of operation. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be understood further from the drawings herein which are not drawn to scale and in which certain structural portions are exaggerated in dimension for clarity, and from the following description wherein: 
     FIG. 1 is a sectional view of a two-stage reciprocating compressor for a heating, ventilating, and air conditioning (“HVAC”) system, generally illustrating a coupling structure according to the present invention; 
     FIGS. 2 a - 2   b  are perspective views of a mechanical system for linking a reversible motor to a piston in accordance with the present invention; 
     FIG. 3 a  is a cross sectional view of a crankshaft according to the present invention; 
     FIG. 3 b  is an end view of the crankshaft of FIG. 3 a;    
     FIG. 4 a  is a perspective view of an eccentric cam according to the present invention; 
     FIG. 4 b  is a cross sectional view of the eccentric cam of FIG. 4 a;    
     FIG. 4 c  is a second perspective view of the eccentric cam of FIG. 4 a;    
     FIG. 5 a  is a perspective view of a connecting rod according to the present invention; 
     FIG. 5 b  is a front plan view of the connecting rod of FIG. 5 a;    
     FIG. 5 c  is a cross-sectional view of the connecting rod of FIG. 5 a;    
     FIG. 6 a  is a front plan view of a second embodiment of an eccentric cam; 
     FIG. 6 b  is a front plan view of a second embodiment of a connecting rod; 
     FIG. 7 is a partially cross-sectional view of portions of a refrigerant compressor; 
     FIG. 8 is a view of a section of a crankshaft and a crankpin taken along line  2 — 2  in FIG. 7; 
     FIG. 9 is an enlarged view of a segment of FIG. 7 showing a variation in the stop mechanism structure; 
     FIG. 10 is an enlarged view as in FIG. 7 taken along line  4 — 4  of FIG. 11 in the direction of the arrows and showing a variation in the stop mechanism; 
     FIG. 11 is a cross sectional view taken along line  5 — 5  of FIG. 10 in the direction of the arrows and rotated 90° in the plane of the drawing sheet; 
     FIG. 12 is an isolated view of the cam bushing per se of FIG. 11; 
     FIGS. 13 a - 13   e  are a series of front views of a mechanical system according to the present invention, illustrating the operation of a mechanical system in a full stroke mode; 
     FIGS. 14 a - 14   e  are a series of rear views of a mechanical system according to the present invention, illustrating the operation of the mechanical system in a half stroke mode; 
     FIG. 15 a  is a front view of a mechanical system for linking a reversible motor to a piston, illustrating a stabilizing system when the compressor is operating in a full stroke mode; 
     FIG. 15 b  is a rear view of a mechanical system for linking a reversible motor to a piston, illustrating a stabilizing system when the compressor is operating in a half stroke mode; 
     FIG. 16 is a motor control schematic for full capacity compressor operation; 
     FIG. 17 is a motor control schematic for motor reversal and reduced capacity compressor operation; 
     FIG. 18 is a schematic diagram of a refrigeration cycle; 
     FIG. 19 is a schematic diagram of a heating, ventilating, and air conditioning (“HVAC”) system; and 
     FIG. 20 is a perspective view of a refrigerator appliance. 
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     The present invention is directed to improved two stage, reversible reciprocating compressors and the application of such compressors to cooling systems including, but not limited to, both refrigerator appliances and heating, ventilating and air conditioning (“HVAC”) systems. The compressors include a mechanical system that alters the stroke of at least one piston, when the direction of motor rotation is reversed. When the motor is operating in a forward direction, the piston travels through a full stroke within the respective cylinder. When the motor is reversed, the piston travels through a reduced stroke within the cylinder. The mechanical system preferably ensures that the piston reaches the top dead center positioning within the cylinder in both the full stroke and reduced stroke operation modes. In the exemplary embodiments, the mechanical system is illustrated in compressors having a single compression chamber and piston. However, the present invention contemplates that the mechanical system may also be used in compressors having multiple compression chambers and pistons. 
     One exemplary embodiment of a two-stage reciprocating compressor is illustrated in FIG.  1  and is generally designated as reference number  80 . As shown, compressor  80  includes a block  82  formed with a cylinder  9 . Cylinder  9  slidably receives a piston  8  for reciprocal motion within the cylinder. 
     Piston  8  is connected to a rotatable crankshaft  15  that is also mounted within block  82 . A reversible motor  86  selectively rotates crankshaft  15  in either a forward direction or a reverse direction to thereby effect motion of piston  8 . 
     In accordance with the present invention, a mechanical system is provided to connect the piston and the rotatable crankshaft. The mechanical system drives the piston through a full stroke between a bottom position and a top dead center position when the motor is operated in the forward direction. The mechanical system drives the piston through a half stroke between an intermediate position and the top dead center position when the motor is operated in the reverse direction. 
     As illustrated in FIG. 1, mechanical system  84  includes an eccentric crankpin  14 , an eccentric cam  16 , and a connecting rod  27 . As illustrated in FIGS. 3 a  and  3   b , eccentric crankpin  14  is formed as part of crankshaft  15  and has an eccentricity  18 . As illustrated in FIGS. 4 a - 4   c , eccentric cam  16  is includes an opening  101  in which crankpin  14  is rotatably disposed and has an eccentricity  19 . As shown in FIGS. 5 a - 5   c , crankpin  27  includes an opening  92  in which eccentric cam  16  is rotatably disposed. 
     As shown in FIGS. 2 a  and  2   b , connecting rod  27  is connected to piston  8  by a wrist pin  28 . This connection allows connecting rod  27  to pivot with respect to piston  8 . It is contemplated that other, similar connecting devices will be readily apparent to one skilled in the art. 
     The mechanical system also includes a first stop mechanism for restricting the relative rotation of the eccentric cam about the crankpin when the motor is rotating the crankshaft in the forward direction and a second stop mechanism for restricting the relative rotation of the eccentric cam with respect to the connecting rod when the motor is rotating the crankshaft in the reverse direction. Thus, when the motor is running in the forward direction, the eccentric cam is fixed to the crankpin at a first position by the first stop mechanism and the eccentric cam rotates with respect to the connecting rod. When the rotational direction of the motor is reversed, the eccentric cam rotates out of the first position to a second position where the second stop mechanism fixes the cam to the connecting rod. In the preferred embodiment, at the second position the crankpin rotates within the eccentric cam. 
     In one exemplary embodiment and as illustrated in FIGS. 3 a  and  3   b , the first stop mechanism includes a stop  110  positioned on crankshaft  15  adjacent eccentric crankpin  14 . As illustrated in FIGS. 4 a - 4   c , eccentric cam  16  includes a first sloping projection  102  that ends in a face  104 . When crankshaft  15  is rotated in the forward direction stop  110  engages face  104  so that eccentric cam  16  is fixed with respect to eccentric crankpin  14 . When crankshaft  15  is rotated in the reverse direction, stop  110  rides along sloping projection  102 , causing eccentric cam  16  to slide along crankpin  14 , until stop  110  eventually drops over face  104 . Thus, when crankshaft  15  rotates in the reverse direction, eccentric crankpin  14  is free to rotate within eccentric cam  16 . 
     Preferably, the components of the first stop mechanism are disposed on crankshaft  15  and eccentric cam  16  so that when crankshaft  15  is rotated in the first direction and the eccentric cam is fixed with respect to the crankpin, the eccentricity  18  of crankpin  14  aligns with eccentricity  19  of eccentric cam  16 . FIGS. 13 a - 13   e  illustrate the operation of the coupling structure in the full stroke mode. Crankpin  15  is rotated in the first direction as indicated by arrow  114 . As shown in FIG. 13 a , when crankpin  14  is at the bottom of its rotation, the combined eccentricity of cam  16  and crankpin  14  move connecting rod  27  and connected piston to the bottom position. Similarly, as shown in FIG. 13 c , when crankpin  14  is at the top of its rotation, the combined eccentricity of cam  16  and crankpin  14  move connecting rod  27  and connected piston to the top dead center position. 
     As illustrated in FIGS. 4 a - 4   c , the second stop mechanism includes a second sloping projection  106  on eccentric cam  16 , preferably on the opposite side of the eccentric cam from first sloping projection  102 . Second sloping projection  106  ends in face  108 . As shown in FIGS. 5 a - 5   c , connecting rod  27  includes a stop  94  having two support members  96  and  98  that form an L-shape extending away from and over opening  92 . Support member  98  includes two faces  100  and  102 . 
     When crankshaft  15  is rotated in the forward direction, the first stop mechanism fixes eccentric cam  16  to crankpin  14  and the eccentric cam rotates within connecting rod  27 . As eccentric cam  16  rotates within connecting rod  27 , face  102  of stop  94  rides along sloping projection  106 , thereby causing eccentric cam  16  to slide along crankpin  14 . Eventually face  102  of stop  94  moves over face  108  of sloping projection  106 . When the direction of rotation is reversed, the first stop mechanism disengages and crankpin  14  rotates freely within eccentric cam  16 . The eccentric cam will rotate in the reverse direction with respect to connecting rod  27  until face  108  of sloping projection  106  on eccentric cam  16  engages stop  94  on connecting rod  27 . This engagement will restrict the rotation of the eccentric cam with respect to the connecting rod when the crankshaft is rotated in the reverse direction. 
     Preferably, as illustrated in FIGS. 2 a  and  2   b , a spring  88  and a collar  89  are positioned on crankshaft  15 . Spring  88  and collar  89  rotate with crankshaft  15 . Spring  88  acts through collar  89  to bias eccentric cam  16  along crankpin  14 . The action of spring ensures that faces  104  and  108  on eccentric cam  16  will align with and engage stops  110  and  94  on crankshaft  15  and connecting rod  27 , respectively when the rotational direction of crankshaft  15  is switched. It is contemplated that the sizing and tolerances of the components of the mechanical system may be such that spring  88  and collar  89  may be omitted and the acceleration forces generated when the motor is reversed will ensure that the first and second stop mechanisms will still engage the respective stops on the connecting rod and crankshaft. 
     FIGS. 14 a - 14   e  illustrate the operation of the coupling structure in the reduced stroke mode. Crankpin  15  is rotated in the reverse direction as indicated by arrow  115 . It should be noted that FIGS. 14 a - 14   e  depict the opposite side of the coupling structure from FIGS. 13 a - 13   e . Thus, while the figures depict the rotation of the crankpin  15  as counter-clockwise in both sets of figures, the actual direction of the crankpin is in the opposite direction. 
     Preferably, the components of the second stop mechanism are disposed on eccentric cam  16  and connecting rod  27  so that when crankshaft  15  is rotated in the reverse direction the eccentricity  18  of eccentric cam  16  aligns with an axis  23  of connecting rod  27 . Thus, the eccentricity  19  of the crankpin will only align with eccentricityl 8  of the eccentric cam when crankpinl 4  is at the top of its rotation. As shown in FIG. 14 c , this alignment results in the piston reaching the top dead center position when operating in the half stroke mode. As shown in FIGS. 14 a  and  14   e , when crankpin  14  is at the bottom of its rotation, the eccentricity of cam  16  is opposite the eccentricity of crankpin  14 . Thus, the piston only moves to an intermediate position, and not to the bottom position. It should be noted that the stroke length of the reduced stroke operation may be altered by varying the eccentricities  18  and  19  of the eccentric cam and crankpin, respectively. 
     The present invention contemplates that many variations of the first and second stop mechanisms will be readily apparent to one skilled in the art. For example, as illustrated in FIGS. 6 a  and  6   b , eccentric cam  16  may include a projection  120  having a face  122 . Connecting rod  27  may include a sloping projection  123  ending in a stop  124 . When crankshaft  15  is rotated in the forward direction, projection  120  on eccentric cam will ride along and over sloping projection  120  on connecting rod  27 . However, when the direction of crankshaft rotation is reversed, face  122  of eccentric cam will engage stop  124  on connecting rod  27 , thereby preventing the eccentric cam from rotating with respect to the connecting rod. 
     FIGS. 7 and 8 illustrate another exemplary embodiment of the first and second stop mechanisms. This embodiment of the coupling structure is generally designated  12  and is shown in connection with a refrigerator compressor having a piston  8  mounted in a cylinder  9 , and having a reed type discharge valve  21  mounted on a valve plate  10  having a discharge port  11  therethrough. The first stop means  20  comprises cooperating shoulder means such as pin  30  on eccentric cam  16  and shoulder  32  machined into crankpin  14 , and wherein said second stop means  24  comprises cooperating shoulder means such as pin  34  on connecting rod  27  and shoulder  36  machined into eccentric cam  16 . The pins  30  and  34  are continually urged radially inwardly from their sockets  38  by compression springs  40 . 
     As an alternative stop mechanism, as shown in FIG. 9, a leaf-type spring or equivalent structure  42  is affixed by screw  44  or the like in a slot  43  machined into connecting rod  27  and is normally sprung into slot  46  machined into eccentric cam  16 . As eccentric cam  16  orbits counterclockwise, spring  42  is flexed radially outwardly in to slot  43 . It is noted that spring  42  and slot  46  can be dimensioned such that the spring does not strike against the slot floor  48  upon each counterclockwise orbit of the crankpin and eccentric cam and create objectionable clicking sound. Also in this regard, the radius  50  of the exit from slot  46  further reduces or eliminates any noise created by contact of spring  42  with the eccentric cam. Such structure can also be used for the crankpin to eccentric cam junction. 
     Referring to FIGS. 10-12, a further variation of the stop structure is shown as being operable thru a break-down linkage which eliminates unnecessary contact of the stop with a rotating structure. In this embodiment as applied, for example, to the eccentric cam and connecting rod, a stop arm generally designated  52  is affixed to a sleeve  63  rotatably mounted on crankpin  14  within a recess  54  in a face  55  of eccentric cam  16 . Arm  52  is comprised of an inner section  56  affixed to sleeve  53  and an outer stop section  58  providing a stop end  59 . Sections  56  and  58  are pivotally connected by a hinge pin  60 . 
     In the operation the stop mechanism of FIGS. 10-12 with the motor and crankshaft rotating in a clockwise direction for reduced stroke wherein only the crankpin will orbit clockwise, the crankpin will drag eccentric cam  16  also clockwise to engage its recess edge  68  with stop arm  52  and move it and straighten it from its dotted line neutral position  70  to its operative stopping position  72  as shown in FIG. 10 wherein end  59  is set into socket  74 . This action locks the eccentric cam  16  to the connecting rod at the precise position that the eccentricity of eccentric cam  16  is aligned with the stroke axis  23  of the connecting rod to assure TDC. A light spring  76  affixed to the top of one of the sections  56  or  58  and slidable on the other may be used to urge section  58  downwardly (as viewed in the drawing) to assist in its insertion into socket  74 . Other springs such as a torsional spring mounted over an extension of pivot pin  60  may also be used. 
     Reversal of the motor and crankshaft direction to a counterclockwise rotation for full stroke will forcibly rotate eccentric cam  16  to engage its recess edge  78  with arm  52  and break it down easily against the force of spring  76  as indicated by the dotted line positions  70  of arm sections  56  and  58  in FIG.  10 . This action, at precisely said positions  70 , will maintain alignment of the eccentricities of eccentric cam  16  and crankpin  14  in cooperation with the stop means which operatively connects crankpin  14  and eccentric cam  16  for simultaneous orbiting to ensure TDC. 
     It is noted that as crankpin  14  moves alone thru its orbit during reduced stroke the cam eccentricity  19  will be swung back and forth to each side of the piston stroke axis  25 , but as indicated by the approximate dotted lines  23 , the cam eccentricity will remain substantially aligned with the connecting rod axis  23 . 
     It is apparent that the present invention in its broad sense is not limited to the use of any particular type of stop structure and the components of the stops shown herein can be reverse mounted, e.g., the spring  40  and pin  34  can be mounted in the cam bushing and the shoulder  36  cut into the bearing. 
     In the illustrated embodiments, the eccentricities of the eccentric cam and the crankpin are substantially equal whereby the cylinder capacity can be switched from full to substantially one half upon reversing the crankshaft rotation. 
     It is particularly noted that the first and second stop means or stop mechanisms may be positioned at any angular position around the crankpin and eccentric cam, and around the eccentric cam and connecting rod respectively as long as the two eccentricities are aligned for full stroke, and the bushing eccentricity is substantially aligned with the connecting rod stroke axis for the reduced stroke. 
     As shown in FIGS. 15 a  and  15   b , first stop mechanism  130  and second stop mechanism  132  are preferably offset from connecting rod axis  23 . When the crankshaft rotates in the forward direction to achieve the full stroke mode, first stop mechanism has a tendency to become unstable just after the piston passes top dead center. If first stop mechanism  130  is offset as shown in FIG. 15 a , the forces that create the instability will act on eccentric cam  16  to move the eccentric cam into connection with the stop on the crankshaft, thereby removing the instability. 
     When the crankshaft rotates in the reverse direction and causes the piston to move through the half stroke, there is no tendency for the system to become unstable. However, during transients an instability could exist. Thus, second stop mechanism  132  is preferably advanced as shown in FIG. 15 b  to prevent any unstable conditions. 
     In accordance with the present invention, a unique electrical circuit has been developed for controlling the reversible motor and may be employed in a preferred embodiment of the invention as described below in connection with a single cylinder compressor, the circuit being shown schematically in FIGS. 16 and 17. 
     The control schematic of FIG. 16 is equivalent to industry conventional PSC (permanent, split capacitor) wiring schematics using predetermined power supply. Line I runs through the common terminal (C) which leads into the motor protection. After leaving the motor protection, the current flow will split, going through both the start (S) and main, i.e., run (R) windings with M (motor) High contactor closed. This stage will be using the run winding as the main winding and places the run capacitor in series with the start winding, obtaining standard motor rotation with the piston fully active, i.e., full capacity operation. 
     The present unique Control Schematic of FIG. 17 employs a predetermined power supply depending on application. Line one will run through the common terminal (C), which leads to the motor protection. After leaving the motor protection, the current flow separates going through both the original start and original main windings with M low contactor energized. The compressor will now be using the start winding as the main and placing the run capacitor in series with the original main winding. Run capacitor placement in this mode facilitates both motor and mechanical rotation changes and simultaneously reduces motor strength to match the resulting reduced piston stroke, thus maximizing motor efficiency for the reduced load. It is particularly noted that for certain applications the original main winding and start capacitor, in reduced compressor capacity mode, may be taken off-line by a centrifugal switch or the like after the motor attains operational speed. 
     Suitable exemplary solenoid actuated contactors or switches for use as the “switching means” of the present invention are shown and described in the General Electric, Product information brochure GEA-115408 4/87 ISM 1800, entitled “Definite Purpose Controls”, 23 pages, the disclosure of which is hereby incorporated herein by reference in its entirety. 
     As best known at this time for use with a single cylinder compressor described below, the power unit would employ the following structures and operating characteristics: 
     Motor—reversible, squirrel cage induction, PSC,  1 - 3  hp; 
     Protector—Protects against overload in both load modes. Senses both T ° and current; 
     Run Capacitor - - - 35 μF/370 VAC; 
     Speed (rated load) - - - 3550 rpm; 
     Motor Strength - - - 252 oz. ft. Max/90 oz. ft. rated load; 
     Power Supply—Single or three phase of any frequency or voltage, e.g., 230V-60 H z  single phase, or 460V—60 H z  three phase; 
     Switching Mechanism—control circuit which is responsive to load requirements to operate solenoid contactor and place the run capacitor in series with either the start winding or main winding, depending on the load requirements. 
     The compressor would have substantially the following structure and operating characteristics: 
     (a) size (capacity) - - - 3 Ton; 
     (b) number of cylinders - - - One; 
     (c) cylinder displacement at full throw - - - 3.34 in 3 /rev; 
     (d) full stroke length - - - 0.805 in.; 
     (e) normal operating pressure range in full stroke mode - - - 77 to 297 Psig. 
     In accordance with the present invention, the two stage reciprocating compressor and control system described above may be used in a variety of commercial applications utilizing a refrigeration cycle. An exemplary embodiment of a refrigeration cycle is illustrated in FIG.  18  and generally designated as reference number  143 . As shown, refrigeration cycle  143  includes a condenser  148 , an expansion device  146 , an evaporator  152 , and a two-stage reciprocating compressor  150 . A refrigerant is circulated through the refrigeration cycle. As is known in the art, the capacity of compressor  150  directly affects the amount of cooling provided by the refrigerant in the evaporator. When the two stage reciprocating compressor is operated in the full stroke mode, compressor  150  operates at full capacity and provides maximum cooling to the evaporator. When the two stage reciprocating compressor is operated in the reduced stroke mode, the amount of cooling provided to the evaporator is similarly reduced. 
     It is contemplated that the two stage reciprocating compressor of the present invention may be used in a variety of commercial applications. For example, as illustrated in FIG. 19, refrigeration cycle  143  may be used in a heating, ventilating, and air conditioning (“HVAC”) system. The HVAC system is used to condition the air in an enclosure  156 . Air is circulated through the HVAC unit  154  through supply duct  160  and return duct  166  by a blower  164 . Blower  164  passes air over the evaporator of the refrigeration cycle to cool the air before the air enters the room. A temperature sensor  158  is positioned within enclosure  156 . When sensor  158  determines the temperature of enclosure has risen above a preset limit, sensor  158  activates the compressor in either the full stroke mode or the reduced stroke mode depending upon the sensed temperature of the air. Operating the compressor at the appropriate capacity depending upon the current conditions of the room will improve the overall efficiency of the system. It is contemplated that the present invention may be used in other air conditioning systems, such as heat pumps, or the like. 
     The refrigeration cycle may also be used with a refrigerator appliance. As illustrated in FIG. 20, a refrigerator  140  includes at least one insulated cooling compartment  144 . A temperature sensor  142  is positioned inside compartment  144 . Depending on the temperature of compartment  144 , the compressor may be operated in either the full stroke or reduced stroke mode. Preferably, the compressor is continuously operated in the reduced stroke mode until a high cooling demand, such as opening the door or introducing a load of relatively warm perishables, is placed on the refrigerator. When the high demand is sensed by sensor  142  by a rise in the temperature of compartment  144 , the compressor may be switched to full stroke mode to compensate for the increased demand. In this manner, compartment  144  of refrigerator  140  may be kept cool efficiently and reliably. 
     Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.