CROSS-REFERENCE TO RELATED APPLICATION 
     The present disclosure relates to subject matter contained in priority Korean Application No. 10-2011-0090317, filed on Sep. 6, 2011, which is herein expressly incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates to a reciprocating motor and a reciprocating compressor having a reciprocating motor. 
     2. Background of the Invention 
     Generally, a reciprocating compressor serves to intake, compress, and discharge a refrigerant as a piston linearly reciprocates within a cylinder. The reciprocating compressor may be classified into a connection type reciprocating compressor or a vibration type reciprocating compressor according to the method employed to drive the piston. 
     In the connection type reciprocating compressor, the piston is connected to a rotating shaft associated with a rotation motor by a connection rod, which causes the piston to reciprocate within the cylinder, thereby compressing the refrigerant. On the other hand, in the vibration type reciprocating compressor, the piston is connected to a mover associated with a reciprocating motor, which vibrates the piston while the piston reciprocates within the cylinder, thereby compressing the refrigerant. The present invention relates to the vibration type reciprocating compressor, and the term “reciprocating compressor” will hereinafter refer to the vibration type reciprocating compressor. 
     A conventional reciprocating compressor comprises a reciprocating motor including an outer stator and an inner stator, and a mover reciprocating between the inner stator and the outer stator. At least one air gap is provided between the inner stator and the outer stator to cause the mover to reciprocate. 
     In recent years, a so-called 1-air gap type reciprocating motor (hereinafter, referred to as “reciprocating motor”) having an air gap between the inner stator and the outer stator is known.  FIGS. 1 to 4  are views showing a conventional 1-air gap type reciprocating motor. 
     As shown therein, the conventional reciprocating motor includes a stator  1  and a mover  5  reciprocally inserted into the stator  1 . 
     The stator  1  includes an inner stator  2  and an outer stator  3  coupled to an outer circumferential surface of the inner stator  2 . 
     The inner stator  2  is formed as a cylindrical shape by laminating a plurality of rectangular stator core sheets in a radial direction. 
     The outer stator  3  includes a plurality of stator blocks  3   a  formed by laminating a plurality of stator cores in a circular arc shape, the stator cores taking a cap-like shape to insert coils therein, and the plurality of stator blocks  3   a  being radially arranged in a circumferential direction on the outer circumferential surface of the inner stator  2 . 
     A magnetic path connecting portion  1   a  is formed at a side of the stator  1  in a reciprocating direction to interconnect the outer circumferential surface of the inner stator  2  and an inner circumferential surface of the outer stator  3 . An air gap portion  1   b  is formed on the opposite side of the magnetic path connecting portion  1   a  to insert the mover  5  therein. 
     The mover  5  includes a magnet holder  6  having a cylindrical shape and provided reciprocally with respect to the air gap portion  1   b  of the stator  1  and a plurality of magnets  7  coupled to an outer circumferential surface of the magnet holder  6  and forming induced magnetism with a coil  4 . In the drawings, unexplained reference numeral  3   b  denotes a coil receiving slot. 
     However, the above-mentioned conventional reciprocating motor has the problem that it is difficult to manufacture the inner stator  2  and excessive expenses are required because the stator cores of the inner stator  2  have to be radially laminated. 
     Moreover, while the inner stator  2  is formed by radially laminating the stator cores sheet by sheet, the stator blocks  3   a  of the outer stator  3  have an arc shape whose inner and outer circumferential surfaces have the same length by laminating the stator cores sheet by sheet. Therefore, as shown in  FIG. 4 , the outer diameter curvature of the inner stator  2  and the inner diameter curvature of the outer stator  3  are different from each other, and this generates a gap (t) in the magnetic path connecting portion  1   a  between the outer circumferential surface of the inner stator  2  and the inner circumferential surface of the outer stator  3 , thereby bringing about a degradation in motor performance caused by magnetic leakage. 
     In addition, the circumferential length of the stator blocks  3   a  is extended as both ends of the inner circumferential surface of the stator blocks  3   a  constituting the outer stator  3  are radially arranged so as to be in contact with each other. This may increase the use of the magnet  7 , and therefore lead to an increase in manufacturing costs, when compared to the efficiency of the motor. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a reciprocating motor and a reciprocating compressor having a reciprocating motor which make the manufacture of a stator easier and therefore reduce manufacturing costs. 
     Another object of the present invention is to provide a reciprocating motor and a reciprocating compressor which improve motor performance by preventing magnetic leakage of a stator. 
     Yet another object of the present invention is to provide a reciprocating motor and a reciprocating compressor which can reduce manufacturing costs, when compared to the efficiency of the motor, by reducing the use of magnets. 
     To achieve these and other advantages and in accordance with the purpose of this specification, as embodied and broadly described herein, there is provided a reciprocating motor comprising: a stator; and a mover reciprocating relative to the stator, wherein the stator comprises an inner stator positioned inside the mover and an outer stator positioned outside the mover, both being integrally formed. 
     Furthermore, there is provided a reciprocating motor comprising: a stator; and a mover reciprocating relative to the stator, wherein the stator is formed such that the inner and outer circumferential surfaces have the same curvature. 
     Furthermore, there is provided a reciprocating compressor comprising: a cylinder having a compression space; a piston inserted into the compression space of the cylinder and reciprocating relative to the cylinder; a reciprocating motor having a mover coupled to the cylinder or piston to reciprocate with the cylinder or piston; and resonant springs elastically supporting the mover of the reciprocating motor, the reciprocating motor comprising the above-described components. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and together with the description serve to explain the principles of the invention. 
       In the drawings: 
         FIG. 1  is an exploded perspective view showing a conventional reciprocating motor; 
         FIG. 2  is a top plan view showing the assembled state of the reciprocating motor of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view, viewed from a side, showing a part of the reciprocating motor of  FIG. 2 ; 
         FIG. 4  is a cross-sectional view taken along line “I-I” of  FIG. 3 ; 
         FIG. 5  is a cross-sectional view showing a reciprocating compressor according to the present invention; 
         FIG. 6  is an exploded perspective view showing a reciprocating motor in the reciprocating compressor of  FIG. 5 ; 
         FIG. 7  is a top plan view showing the assembled state of the reciprocating motor of  FIG. 1 ; 
         FIG. 8  is a cross-sectional view, viewed from a side, a part of the reciprocating motor of  FIG. 7 ; 
         FIG. 9  is a cross-sectional view taken along line “II-II” of  FIG. 8 ; 
         FIG. 10  is a cross-sectional view showing another embodiment of a stator in the reciprocating motor of  FIG. 8 ; and 
         FIG. 11  is a cross-sectional view showing an embodiment of a gas bearing in the reciprocating compressor of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a reciprocating motor and a reciprocating compressor according to the present invention will be described in detail with reference to an embodiment illustrated in the accompanying drawings. 
     As shown in  FIG. 5 , in the reciprocating compressor  15  according to this embodiment, a frame  20  is installed within a sealed casing  10 , a reciprocating motor  30  and a cylinder  41  are fixed to the frame  20 , and a piston  42  coupled to a mover  32  of the reciprocating motor  30  is inserted into the cylinder  41  to reciprocate. 
     A compression space S 1  is formed in the cylinder  41 , a suction path F is formed in the piston  42 , a suction valve  43  for opening and closing the suction path F is installed at a distal end of the suction path F, and a discharge valve  44  for opening and closing the compression space S 1  of the cylinder  41  is installed at a front end surface of the cylinder  41 . 
     In the drawings, unexplained reference numeral  11  denotes an inner space of the casing,  12  denotes a suction pipe, and  13  denotes a discharge pipe. 
     In the aforementioned reciprocating compressor according this embodiment, when power is applied to a coil  35  of the reciprocating motor  30 , the mover  32  of the reciprocating motor  30  reciprocates. Then, the piston  42  coupled to the mover  32  sucks and compresses a refrigerant gas while linearly reciprocating within the cylinder  41 , and discharges it. 
     More specifically, when the piston  42  moves backwards, the refrigerant gas in the sealed casing  10  is sucked into the compression space S 1  through the suction path F of the piston  42 , and when the piston  42  moves forwards, the suction path F is closed and the refrigerant gas in the compression space S 1  is compressed. Also, when the piston  42  further moves forwards, the discharge valve  44  is opened to discharge the refrigerant gas compressed in the compression space S 1  and move it to the outside refrigeration cycle. 
     As shown in  FIGS. 6 to 8 , the reciprocating motor  30  comprises a stator  31  having a coil  35  and an air gap formed at only one side of the coil  35  and a mover  32  inserted into the air gap of the stator  31  and having a magnet  325  that linearly moves in the motion direction. 
     The stator  31  includes a plurality of stator blocks  311  and a plurality of pole blocks  315  respectively coupled to sides of the stator blocks  311  and forming an air gap portion  31   a  along with the stator blocks  311 . 
     The stator blocks  311  and the pole blocks  315  include a plurality of thin stator cores laminated sheet by sheet in a circular arc shape when axially projected. 
     The stator blocks  311  are formed in the shape of recesses when axially projected, and the pole blocks  315  are formed in a rectangular shape when axially projected. 
     The stator block (or each of the stator core sheets constituting the stator blocks)  311  may include a first magnetic path  312  positioned inside the mover  32  to form the inner stator and a second magnetic path  313  extending integrally from an axial side of the first magnetic path  312 , i.e., the opposite end of the air portion  31   a , and positioned outside the mover  32  to form the outer stator. 
     While the first magnetic path  312  is formed in a rectangular shape, the second magnetic path  313  is formed in a stepwise manner and extends from the first magnetic path  312 . 
     A coil receiving slot  31   b  opened in an axial direction, i.e., the direction of the air gap portion, is formed on inner wall surfaces of the first and second magnetic paths  312  and  313 , and the pole block  315  is coupled to an axial cross-section of the second magnetic path  313  which constitutes the coil receiving slot  31   b  so as to open an axial open surface of the coil receiving slot  31   b.    
     Also, a coupling groove  311   b  and a coupling protrusion  315   b  may be formed on a coupling surface of the stator block  311  and a coupling surface of the pole block  315 , which connect the stator block  311  and the pole block  315  to form a magnetic path connecting portion (not shown), to firmly couple the stator block  311  and the pole block  315  and maintain a given curvature. Although not shown, the stator block  311  and the pole block  315  may be coupled in a stepwise manner. 
     The coupling surface  311   a  of the stator block  311  and the coupling surface  315   a  of the pole block  315 , except the coupling groove  311   b  and the coupling protrusion  315   b , are formed to be flat, thereby preventing an air gap between the stator block  311  and the pole block  315 . This prevents magnetic leakage between the stator block  311  and the pole block  315 , thereby leading to an increase in motor performance. 
     A first pole portion  311   c  having an increasing cross-sectional area is formed at a distal end of the second magnetic path  313  of the stator block  311 , i.e., a distal end of the air gap portion  31   a , and a second pole portion  315   c  having an increasing cross-sectional area is formed at a distal end of the pole block  315 , corresponding to the first pole portion  311   c  of the stator block  311 . 
     As shown in  FIG. 7 , when the stator block  311  is axially projected, the curvatures R 1  and R 2  of the inner and outer circumferential surfaces thereof, the curvature R 3  of the first pole portion  311   c  of the air gap portion  31   a , and the curvature R 4  of the second pole portion  315   c  may be equal. Also, the circular arc length L 1  of the inner circumferential surface of the stator block  311 , the circular arc length L 2  of the outer circumferential surface thereof, the circular arc length L 3  of the first pole portion  311   c , and the circular arc length L 4  of the second pole portion  315   c  may be equal. 
     The mover  32  may include a magnet holder  321  having a cylindrical shape and a plurality of magnets  325  attached onto an outer circumferential surface of the magnet holder  321  in a circumferential direction to form a magnetic flux together with the coil  35 . 
     The magnetic holder  321  may be formed of a non-magnetic substance in order to prevent flux leakage; however, it is not limited thereto. The outer circumferential surface of the magnetic holder  321  may be formed in a circular shape so that the magnets  325  are in line contact therewith and adhered thereto. Also, a magnet mounting groove (not shown) may be formed in a strip shape on the outer circumferential surface of the magnet holder  321  so as to insert the magnets  325  therein and support them in the motion direction. 
     The magnets  325  may be formed in a hexahedral shape and adhered one by one to the outer circumferential surface of the magnet holder  321 . In the case of attaching the magnets  325  one by one, supporting members (not shown), such as fixing rings or a tape made up of a composite material, may be surrounded and fixed around outer circumferential surfaces of the magnets  325 . 
     Although the magnets  325  may be serially adhered in a circumferential direction to the outer circumferential surface of the magnet holder  321 , it is preferable that the magnets  325  are adhered at predetermined intervals, i.e., between the stator blocks in a circumferential direction to the outer circumferential surface of the magnet holder  321  to minimize the use of the magnets, because the stator  31  comprises a plurality of stator blocks  311  and the plurality of stator blocks  311  are arranged at predetermined intervals in the circumferential direction. In this case, the magnets  325  are preferably formed to have a length corresponding to the air gap length of the magnetic holder  321 , i.e., the circumferential length of the air gap. 
     Preferably, the magnet  325  may be configured such that its length in a motion direction is not shorter than a length of the air gap portion  31   a  in the motion direction, more particularly, longer than the length of the air gap portion  31   a  in the motion direction. At its initial position or during its operation, the magnet  325  may be disposed such that at least one end thereof is located inside the air gap portion  31   a , in order to ensure a stable reciprocating motion. 
     Moreover, though only one magnet  325  may be disposed in the motion direction, a plurality of magnets  325  may be disposed in a motion direction in some cases. In addition, the magnets may be disposed in a motion direction so that an N pole and an S pole correspond to each other. 
     Another example of the stator in the reciprocating motor according to this embodiment will be described below. 
     That is, the coil receiving slot is formed such that the coil of the stator is placed outside the mover in the foregoing embodiment, whereas the coil receiving slot  31   b  is formed such that the coil  35  is placed inside the mover  32  as shown in  FIG. 10  in this embodiment. In this case, too, the basic configuration of the stator and the operational effects thereof are similar to those of the foregoing embodiment, except that the stator  31  of this embodiment allows reduced use of coils because the coil receiving slot  31   b  is disposed inside the mover  32  and therefore the diameter of the coil is reduced as much. 
     In the reciprocating compressor according to this embodiment, resonant springs  51  and  52  may be installed at both sides of the piston  42  in the motion direction of the piston  42  in order to induce a resonant movement of the piston  42 , as shown in  FIG. 5 . 
     Although the resonant springs  51  and  52  may be formed as plate springs, the plate springs have a small lateral displacement but a large longitudinal displacement. Therefore, if the compressor is installed stood in a motion direction of the piston, a stroke of the piston may not be properly performed because the piston has to reciprocate in an up-and-down direction when the piston hangs vertically downward. Moreover, when the plate springs are used, the plate springs and the piston have to be connected by a connecting bar made of soft material or by at least one link (preferably two links) on the midway of the connecting bar, in order to maintain the forward movement of the piston, which may increase material costs and the number of assembly processes. 
     Taking this into consideration, this embodiment is devised to reduce material costs and the number of assembly processes by varying the configuration of the compressor by using coil springs as the resonant springs, and avoiding the use of a connecting bar or link. 
     If the first resonant spring  51  and the second resonant spring  52  are compressed coil springs, it is preferable that the resonant springs are arranged to engage each other so as to offset a side force or torsion moment generated when the resonant springs  51  and  52  are expanded. In the drawings, unexplained reference numeral  53  denotes a spring stopper. 
     In the above-stated reciprocating compressor, it is required to reduce a frictional loss between the cylinder and the piston to improve the performance of the compressor. 
     To this end, an oil-lubricated type reciprocating compressor for supplying oil contained within the casing  10  between the cylinder  41  and the piston  42 , or a gas-lubricated type reciprocating compressor for supplying a part of compressed gas discharged from the cylinder  41  between the inner circumferential surface of the cylinder  41  and the outer circumferential surface of the piston  42  to lubricate between the cylinder and the piston  42  by a gas force may be applied. In this embodiment, the gas-lubricated type reciprocating compressor will be discussed. 
     The gas-lubricated type (hereinafter, gas bearing) compressor according to these embodiments may have a plurality of fine through holes, and have an oxide film layer which is formed on the inner circumferential surface of the cylinder or on the outer circumferential surface of the piston and makes it easier to regulate the distribution of the fine through holes. 
     For example as shown in  FIG. 11 , the oxide film layer  412  may be formed on an inner circumferential surface of a cylinder body  411  (or on an outer circumferential surface of a piston body  421 ) to have a plurality of fine through holes. In this case, compressed gas guided to the fine through holes through gas flow paths  401  is uniformly supplied between the cylinder  41  and the piston  42  through the fine through holes to form a gas bearing. 
     The oxide film layer  412  may be formed by anodizing or micro arc oxidation (MAO). 
     A front end surface  411   a  of the cylinder body  411  protrudes to a predetermined height to form a protruding portion  411   b , and a discharge cover  46  is inserted and coupled to an outer circumferential surface of the protrusion  411   b.    
     A starting end of the gas flow path  401  is preferably formed at a greater distance than the radius Ds of the discharge valve  45  relative to the center of the discharge valve  45  so that it is positioned out of the attachment/detachment range of the discharge valve  45  which is selectively attached to and detached from the front end surface  411   a  of the cylinder body  411 . 
     An annular filter  47  may be installed on the front end of the gas flow path  401 , i.e., the front end surface  411   a  of the cylinder body  411  so as to prevent impurities from entering the gas flow path  401 . 
     Although at least one gas diffusion groove (not shown) may be further formed on the outer circumferential surface of the piston  42 , a high-pressure compressed gas may be uniformly distributed over the bearing area between the cylinder  41  and the piston  42 , without forming a gas diffusion groove on the outer circumferential surface of the piston  42 , because the oxide film layer  412  has a porous structure. 
     In the case that the a porous layer is formed of the oxide film layer, the porous layer is easily formed on the inner circumferential surface of the cylinder body, and the reliability of the compressor is improved because of high abrasion resistance and high rub resistance resulting from an increase in the strength of a bearing surface formed of an oxide film layer. 
     Although not shown, a porous material member may be coupled to the outer circumferential surface of the piston to form gas flow paths in the cylinder and uniformly distributing and supplying a high-pressure compressed gas guided through the gas flow paths between the cylinder and the piston, or a gas guide member with gas through holes may be coupled to the outer circumferential surface of the piston to form gas flow paths in the cylinder and uniformly distributing and supplying a high-pressure compressed gas guided through the gas flow paths between the cylinder and the piston, thereby uniformly distributing the high-pressure compressed gas between the cylinder and the piston.
A reciprocating motor and reciprocating compressor having a reciprocating motor are provided. Making of the reciprocating motor is made easier, and therefore, manufacturing costs are reduced by configuring the stator such that an inner stator positioned inside a mover and an outer stator positioned outside of the mover are integrally formed, or by making inner and outer circumferential surfaces of the stator have a same curvature. Magnetic leakage is prevented as no gap is formed between the inner and outer stators, thereby improving performance of the reciprocating motor. The use of magnets may be reduced by omitting magnets between stator blocks, and therefore, manufacturing costs may be reduced, when compared to efficiency of the reciprocating motor.