Abstract:
The invention concerns a linear compressor ( 1 ), particularly a refrigerant compressor, with a compression unit ( 3 ) having a cylinder ( 8 ) and a piston ( 16 ) reciprocating in the cylinder ( 8 ), and a linear motor ( 4 ) having an outer stator ( 18 ), an inner stator ( 20 ) and an armature ( 22 ) located in a gap ( 21 ) formed between the outer stator ( 18 ) and the inner stator ( 20 ), the armature ( 22 ) being connected to the piston ( 16 ) via a piston rod ( 28 ). It is endeavoured to design such a linear compressor in a simple manner with the smallest possible dimensions. For this purpose, it is ensured that the armature ( 22 ) is connected to the piston rod ( 28 ) inside the axial length of the inner stator ( 20 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Applicant hereby claims foreign priority benefits under U.S.C. § 119 from German Patent Application No. 10 2005 038 785.3 filed on Aug. 17, 2005, the contents of which are incorporated by reference herein. This Application relates to German Patent Applications No. 10 2005 038 783.7 (Attorney Docket No. 6495-0168); No. 10 2005 038 784.5 (Attorney Docket No. 6495-0169); No. 10 2005 038 781.0 (Attorney Docket No. 6495-0172); No. 10 2005 038 780.2 (Attorney Docket No. 6495-0173), filed on the same date herewith.  
       FIELD OF THE INVENTION  
       [0002]     The invention concerns a linear compressor, particularly a refrigerant compressor, with a compression unit having a cylinder and a piston reciprocating in the cylinder, and a linear motor having an outer stator, an inner stator and an armature located in a gap formed between the outer stator and the inner stator, the armature being connected to the piston via a piston rod.  
       BACKGROUND OF THE INVENTION  
       [0003]     Such a linear compressor is, for example, known from U.S. Pat. No. 6,565,332 B2. Here, the cylinder is arranged radially inside the inner stator. The armature being movable between inner stator and outer stator has permanent magnets and is connected to the piston rod by way of a radial flange arrangement transferring the movement of the armature to the piston. The flange arrangement is located axially between the motor and a resonance spring arrangement.  
         [0004]     A similar linear compressor is known from U.S. Pat. No. 6,793,470 B2. Here, the armature is not connected to the piston via a piston rod but via some kind of cylinder pipe having approximately the same outer diameter as the piston. Also here the piston moves inside the inner stator, which causes that the stator and thus the linear compressor must have a relatively large diameter. Usually, such a linear motor will be operated in a lying state. This means that, due to the large diameter of the linear motor, the linear motor will have a relatively large height. When the linear compressor is used as refrigerant compressor in a domestic cooling appliance, for example a refrigerator or a freezer, the height of the linear compressor reduces the space available for the cooling chamber.  
         [0005]     U.S. Pat. No. 5,772,410 shows another linear compressor, in which the cylinder is located in an axial extension of the motor. Here, the inner stator is located inside the piston and the armature is connected directly to the side of the piston facing away from the cylinder head. This enables a relatively compact design, however, requires a relatively expensive manufacturing and mutual alignment of the individual parts, as only one end of the armature and of the inner stator can be fixed on the piston or on a carrier, respectively.  
       BRIEF SUMMARY OF THE INVENTION  
       [0006]     The invention is based on the task of providing a linear compressor with a simple design and the smallest possible dimensions.  
         [0007]     With a linear compressor as mentioned in the introduction, this task is solved in that the armature is connected to the piston rod inside the axial length of the inner stator.  
         [0008]     Thus, a connection between the armature and the piston rod is guided through the inner stator. An arrangement located axially outside the motor, via which the armature can be connected to the piston rod, is no longer required. The result is a relatively short design, which reduces the space required for the compressor. This permits, for example, an increase of the space available for the cooling chamber in a domestic cooling appliance with specified dimensions.  
         [0009]     Preferably, the compression unit is located axially outside the linear motor. Thus, the diameter of the motor and also the height of a horizontally located compressor can be kept small. Usually, sufficient space is available for a domestic cooling appliance in the width direction. The compressor can thus have a relatively large axial length, when the consequence of this is that its diameter and thus its height is reduced.  
         [0010]     Preferably, the piston rod is guided through the entire axial length of the linear motor, a resonance spring arrangement being located on a side of the linear motor facing away from the compression unit. The resonance spring arrangement has a resonance frequency, which is adapted to the operation frequency of the linear motor. The operation frequency is determined by the frequency of the supplied current. When the linear compressor is operated in the resonance range of the resonance spring arrangement, it will require less energy. As the piston rod can now be used to connect the resonance spring arrangement to the piston, the resonance spring arrangement can be located on the side of the linear motor lying opposite the compression unit. This decoupling simplifies the design. The resonance spring arrangement will not conflict with the movement of the piston.  
         [0011]     Preferably, the piston rod is guided by the piston and the resonance spring arrangement. Thus, the piston rod has no contact points with the linear motor. Thus, the piston rod can also hold the armature so that it is guided practically touch-free through the gap between inner stator and outer stator.  
         [0012]     Preferably, the cylinder is displaceable in relation to the outer stator during mounting. In connection with a piston compressor, it has certain significance that the compression chamber formed by the piston and the cylinder has a minimal value in the upper dead point of the piston. During mounting, the piston can now be positioned in its upper dead point, and then the cylinder can be displaced in relation to the outer stator, and thus also in relation to the piston, so that the compression chamber assumes the smallest permissible value. For example, the cylinder can be displaced in a pipe-shaped intermediary piece. When the desired position has been reached, the cylinder is fixed in the intermediary piece, for example by welding, soldering or gluing.  
         [0013]     Preferably, the armature and the piston rod are connected to each other via at least one connecting element, which is guided through at least two slots in the inner stator. Thus, the armature and the piston rod are connected to each other in two places in the circumferential direction, which increases the stability of the unit formed by the piston rod and the armature. The slots in the inner stator are of minor importance for the magnetic behaviour of the whole stator. In the inner stator, the magnetic field mainly extends in the axial direction, but not in the circumferential direction, so that the magnetic field is not much disturbed by the axially extending slots.  
         [0014]     It is preferred that the slots are made to be through over the axial length of the inner stator. This means that the inner stator can be made up of several segments. This simplifies the manufacturing.  
         [0015]     Preferably, the connecting element has an inner ring, which is connected to the piston rod. Thus, the inner ring secures the radial positioning of the connecting element in relation to the piston rod.  
         [0016]     It is preferred that the inner ring bears under a predetermined pressure on a circumferential bearing surface of the piston rod. Thus, the inner ring also secures the axial positioning of the connecting element in relation to the piston rod and thus the position of the piston rod and the armature in relation to each other.  
         [0017]     Preferably, the connecting element has an outer ring, which is connected to the armature. With this embodiment, the armature is held annularly, so that also its radial position in relation to the piston rod is secured.  
         [0018]     Preferably, the inner ring and the outer ring are connected to each other by means of at least two radially extending arms. These arms can be guided through the slots in the inner stator. They preferably extend radially.  
         [0019]     Preferably, the armature is formed by a cylinder pipe shaped permanent magnet arrangement, whose longitudinal axis coincides with the axis of the piston rod. The permanent magnets forming the permanent magnet arrangement are radially magnetised, that is, all their radial outsides have the same magnetical polarity. In a simple manner, this permanent magnet arrangement can be kept together by the outer ring.  
         [0020]     Preferably, the piston rod is connected to the armature in at least two positions, which have a distance in the axial direction. This ensures that the armature cannot tilt in relation to piston rod and vice versa. The allocation of armature and stator can be maintained very accurately, which gives favourable operation parameters. In particular, the gap, which is formed between the inner stator and the outer stator, can be set at a minimum width, as the risk that the armature will touch the stator is practically non-existent.  
         [0021]     Preferably, the axial and radial positions of the armature in relation to the piston rod are controlled by the connecting element. The armature and the piston rod thus form a unit, in which all elements have a fixed allocation in relation to each other.  
         [0022]     Preferably, the armature is suspended axially between two connecting elements. This embodiment has two advantages. Firstly, the armature is held in the axial direction. Secondly, it is sufficient to let two stops of the piston rod act upon the connecting elements from the radial outside. The armature is held by means of the mounting of the connecting elements on the piston rod and the tightening of the connecting elements on the piston rod.  
         [0023]     Preferably, at least one connecting element has a projection located radially inside the armature, the armature bearing from the radial outside on said projection. This projection also fixes the armature radially.  
         [0024]     It is preferred that the projection is located in the area of the arms. Here, the connecting element has its largest resisting force, so that even larger forces will not make the armature displace radially in relation to the connecting element.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     In the following, the invention is described on the basis of a preferred embodiment in connection with the drawings, showing:  
         [0026]      FIG. 1  is a schematical, longitudinal section through a linear compressor;  
         [0027]      FIG. 2  is a top view on a component group comprising piston, piston rod and armature; and  
         [0028]      FIG. 3  is a section III-III according to  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0029]      FIG. 1  shows a linear compressor, which is located in a hermetically closed case  2 .  
         [0030]     The linear compressor  1  has a compression section  3 , a drive section  4  and a resonance spring arrangement  5 . The unit formed by the compression section  3 , the drive section  4  and the resonance spring arrangement  5  is suspended in the case  2  by means of two plane annular springs  6 ,  7 , each being formed as a spiral with one winding. The annular springs  6 ,  7  are fixed on the drive section  4 .  
         [0031]     The compression section  3  has a cylinder  8 , whose one front side is covered by a cylinder head  9 . Cylinder  8  and cylinder head  9  are joined in a capsule  10  in a cartridge-like manner. A suction muffler  11  and a pressure muffler  12  are fixed on the cylinder head  9 . The suction muffler  11  is connected to a suction opening  13  and the pressure muffler  12  is connected to a pressure opening  14  in the cylinder head.  
         [0032]     The capsule  10  is inserted in an intermediary ring  15 , which is connected to the drive section  4 . During mounting, the capsule  10  and thus the cylinder  8  can be displaced within certain limits in the axial direction of the cylinder in relation to the intermediary ring  15 . When, during mounting, a predetermined position of the cylinder in relation to the drive section  4  has been reached, the capsule  10  is fixed in the intermediary ring, for example by welding, soldering or gluing.  
         [0033]     In the cylinder  8  is located a piston  16 , which borders a compression chamber  17  together with the cylinder  8  and the cylinder head  9 . Before the cylinder with the capsule  10  is fixed in the intermediary piece  15 , the piston  16  is moved to its upper dead point, and the cylinder  8  is displaced, until the compression chamber  17  has reached its smallest permissible extension.  
         [0034]     The drive section  4  has a linear motor. The linear motor has an outer stator  18  with a recess  19  for a winding, not shown in detail, and an inner stator  20 . Between the outer stator  18  and the inner stator  20  is an annular gap  21 , in which an armature  22  is movable. The armature carries permanent magnets  23 , which are connected to each other by means of two outer rings  24 ,  25 . The outer rings  24 ,  25  can, for example, be made of a plastics material. The outer rings  24 ,  25  are connected to inner rings  26 ,  27  via arms shown in  FIGS. 2 and 3  and guided through slots in the inner stator  20 .  
         [0035]     The inner rings  26 ,  27  are connected with a piston rod  28 , which again is connected to the piston  16 .  
         [0036]     The outer stator  18  and the inner stator  20  are connected to each other via motor covers  29 ,  30 , which are fastened against each other by means of screw bolts  31 . The screw bolts extend in parallel to the movement direction of the piston rod  28 .  
         [0037]     The intermediary ring  15  is connected to the cylinder-side motor cover  30 , for example by soldering, gluing or welding.  
         [0038]     The resonance spring arrangement  5 , which is located at an end of the drive section  4 , which lies opposite the compression section  3 , has a spring pack  32  of several plate springs  33 . The spring pack  32  is connected in a central area  34  to the piston rod  28 . An outer section  35  of the spring pack  32  is connected via bolts  36  to a stop housing  37 , which forms a stop for the spring pack  32 .  
         [0039]     At the end projecting from the spring pack  32 , the piston rod  28  is connected to an oil pump arrangement  38 , which immerses in an oil sump, not shown in detail, which forms in the bottom part of the case  2 .  
         [0040]     When the winding located in the recess  19  is energised, the armature  22  moves in one direction and takes along the piston rod  28  in this direction. If the direction of the current is reversed, the armature  22  with the piston rod  28  moves in the opposite direction, thus also moving the piston  16  in the opposite direction. This periodically increases and reduces the volume of the compression chamber  17 . The resonance spring arrangement  5  is adapted to the frequency of the current, so that the movable part of the linear compressor  1 , which is formed by the armature  22 , the piston rod  28 , the piston  16 , the oil pump arrangement  38  and the moving part of the resonance spring arrangement  5  oscillates in resonance.  
         [0041]      FIGS. 2 and 3  now show the allocation of the piston rod  28  and the armature  22  with further details.  
         [0042]     In the area of the piston-side end, the piston rod  28  has a circumferential projection  41 , on which the inner ring  27  bears from the side lying opposite the piston  16 . On the opposite side of the projection  41  a connecting element  42  bears, with which the piston rod  28  is connected to the piston  16 .  
         [0043]     The axial fixing of the inner ring  27  in relation to the piston rod  28  also ensures axial fixing of the outer ring  25 . It forms an axial stop for the permanent magnets  23 .  
         [0044]     The second inner ring  26  is also pushed onto the piston rod  28 . Thus, the second outer ring  24  comes to rest on the opposite axial front side of the permanent magnets  23 , which are then fixed between the two outer rings  24 ,  25 . The second inner ring  26  is fixed by an end piece  43  screwed onto the piston rod  28 . The end piece  43  can also be connected to the piston rod  28  in other ways. The fact that in the axial direction the two inner rings  26 ,  27  are pressed against each other causes that the permanent magnets  23  are fixed between the outer rings  24 ,  25  in the axial direction.  
         [0045]     An outer ring  24 ,  25 , an inner ring  26 ,  27  and in the present embodiment three arms  40  form a connecting element  39 . Of course, the number of arms  40  can also be chosen to be different. However, at least two arms are available. The connecting elements  39  can be made of a plastics material.  
         [0046]     Each arm  40  has an axial projection  44 , on which the permanent magnets  23  bear from the outside. As the permanent magnets  23  form a hollow cylinder, the projections  44  are sufficient to fix the permanent magnets  23  reliably between the two connecting elements  39  in the radial direction. Thus, the permanent magnets  23  are fixed in relation to the piston rod  28  in two positions, which have an axial distance to each other, so that the permanent magnets  23  cannot tilt in relation to the piston rod  28 . On the contrary, they can be guided in parallel with the piston rod  28  with a relatively high accuracy. The piston rod is merely supported via the piston in the compression section  3  and in the spring pack  32  in the resonance spring arrangement  5 . Thus, it can be guided through the inner stator in a touch-free manner. Accordingly, the armature  22  can also be supported in the annular gap  21  in a touch-free manner, which reduces frictional losses. The piston rod  28  can also be guided through the motor covers  29 ,  30  in a touch-free manner.  
         [0047]     The compression unit with the cylinder  8  and the piston  16  is located axially outside the linear motor, that is, the driving section  4 . Accordingly, the compression unit  3  must not be considered with regard to the diameter. The linear motor can therefore be built with a relatively small diameter. The piston rod is guided through the total axial length of the linear motor. At one end it is provided with the oil pump arrangement  38 , so that the oil used for lubricating the compression section  3  can also be used to dissipate a certain heat amount from the drive section  4 .  
         [0048]     The slots required for the arms  40  in the inner stator  20  are of minor importance with regard to the magnetical properties of the stator. In the inner stator  20 , the magnetic field is directed mainly axially. Merely in the axial end, the magnetic field is deflected to the radial direction. However, the magnetic field has practically no component in the circumferential direction, so that the air gaps formed by the slots are not significantly disturbing.  
         [0049]     Guiding the armature  22 , together with the piston rod  28  and the connected oil pump arrangement  38 , by means of the arms  40  in the inner stator  20  prevents that the armature can rotate in the circumferential direction. This secures that the oil pump arrangement is always immersed in the oil sump.  
         [0050]     While the present invention has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this invention may be made without departing from the spirit and scope of the present invention.