Patent Publication Number: US-9897104-B2

Title: Compressor

Description:
REFERENCE TO RELATED APPLICATION 
     This application claims priority of United Kingdom Application No. 1308091.6, filed May 3, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a compressor. 
     BACKGROUND OF THE INVENTION 
     Efforts are continually being made to design compressors of smaller size. A smaller compressor may be achieved by employing a smaller impeller. However, a smaller impeller is required to rotate at higher speeds in order to achieve the same mass flow rates. Higher rotational speeds typically reduce the lifespan of the bearings, which are often the first components of the compressor to fail. Consequently, efforts to design a smaller compressor are often beset with lifespan problems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a compressor comprising a frame, a rotor assembly, and a heat sink assembly, wherein the rotor assembly comprises a shaft to which an impeller, a bearing assembly and a rotor core are secured, the bearing assembly comprises a pair bearings, the heat sink assembly comprises a sleeve to which one or more heat sinks are secured, the sleeve is secured the bearings, and the heat sinks are secured to the frame. 
     The rotor assembly is thus secured to the frame by the heat sink assembly. The heat sink assembly provides the additional benefit of carrying heat away from the bearing assembly. As a result, the lifespan of the bearings and thus the compressor is prolonged. 
     The heat sink assembly may comprise a heat sink having a plurality of legs, each leg extending radially from the sleeve. The compressor may be configured such that air flows through the interior of the compressor. By employing a heat sink having legs, the air flowing through the interior of the compressor is free to flow between the legs so as to improve cooling of the heat sink and thus the bearings. 
     The legs may be spaced evenly around the sleeve. This then has the advantage that vibration of the rotor assembly is distributed evenly among the legs. As a result, vibration and the inherent noise that it produces is reduced. Additionally, heat may be transferred more evenly from the heat sink to the surrounding air. 
     The width of each leg may taper in a direction away from the sleeve. The temperature of each leg, and thus the rate of heat transfer, decreases as one moves away from the sleeve. Accordingly, by tapering the width of the legs, the mass of the heat sink may be reduced without adversely affecting cooling. As a result, a lighter and cheaper compressor may be realised. 
     The heat sink assembly may comprise a heat sink that is generally disc shaped. A disc-shaped heat sink has the advantage of providing a relatively large surface area over which heat may be transferred to the surrounding air. 
     The heat sink may be located immediately beneath the impeller. Moreover, the heat sink may have a diameter that is greater than that of the impeller. The portion of the heat sink extending radially beyond the impeller may then be secured to the frame. This then has the benefit that the heat sink and the frame are better able to oppose axial thrust generated by the impeller. 
     The heat sink may project into the underside of the impeller. This then has the benefit of reducing the size of the cavity beneath the impeller. As a result, windage and/or other parasitic losses may be reduced. 
     The frame may comprise an aperture having a diameter greater than that of the impeller and smaller than that of the heat sink. This then aids the assembly of the compressor. For example, the rotor assembly may be balanced as a complete unit, i.e. with the impeller, bearings and rotor core secured to the shaft. The heat assembly may then be secured to the rotor assembly. The rotor-heat sink assembly may then be inserted into the frame such that the impeller passes through the aperture. The heat sink, being larger than the aperture, then abuts and is secured to the frame at the aperture. 
     The heat sink assembly may comprise a first heat sink and a second heat sink spaced along the sleeve from the first heat sink. This then has the advantage that movement of the rotor assembly relative to the frame is opposed at two planes that are spaced axially. As a result, vibration of the rotor assembly and the inherent noise that it produces is reduced. Additionally, in having two heat sinks, improved cooling of the bearing assembly may be achieved. 
     The first heat sink may be proximal the impeller and may be generally disc shaped. Additionally or alternatively, the second heat sink may be proximal the rotor core and may comprise a plurality of legs that extend radially from the sleeve. The advantages of these two heat sinks have been described above. 
     The heat sink assembly may be formed of a metal. Metals typically have a relatively high structural strength and high thermal conductivity. Consequently, the heat sink assembly is able to provide relatively good opposition to movement of the rotor assembly, thereby reducing vibration and noise, as well as provide relatively good cooling of the bearing assembly. 
     The heat sink assembly may be formed of a material having a coefficient of thermal expansion that substantially matches that of the shaft. Consequently, uneven thermal expansion of the heat sink assembly and the shaft, which might otherwise lead to adverse changes in the loading of the bearing assembly, may be avoided. 
     The present also provides a compressor comprising a frame, a rotor assembly, and a heat sink assembly, wherein the rotor assembly comprises a shaft to which an impeller, a bearing assembly and a rotor core are secured, the heat sink assembly is secured to the bearing assembly and comprises a heat sink, the heat sink is located beneath the impeller, is generally disc shaped, has a diameter greater than that of the impeller, and is secured to the frame. 
     The rotor assembly is thus secured to the frame by the heat sink assembly. The heat sink assembly provides the additional benefit of carrying heat away from the bearing assembly. As a result, the lifespan of the bearings and thus the compressor is prolonged. 
     Being disc shaped, the heat sink provides a relatively large surface area over which heat may be transferred to the surrounding air. The heat sink is located immediately beneath the impeller. Moreover, the heat sink has a diameter that is greater than that of the impeller. The portion of the heat sink extending radially beyond the impeller may then be secured to the frame. This then has the benefit that the heat sink and the frame are better able to oppose axial thrust generated by the impeller. 
     The heat sink may project into the underside of the impeller. This then has the benefit of reducing the size of the cavity beneath the impeller. As a result, windage and/or other parasitic losses may be reduced. 
     The frame may comprise an aperture having a diameter greater than that of the impeller and smaller than that of the heat sink. This then aids the assembly of the compressor. For example, the rotor assembly may be balanced as a complete unit, i.e. with the impeller, bearings and rotor core secured to the shaft. The heat assembly may then be secured to the rotor assembly. The rotor-heat sink assembly may then be inserted into the frame such that the impeller passes through the aperture. The heat sink, being larger than the aperture, then abuts and is secured to the frame at the aperture. 
     The present invention further provides a compressor comprising a frame, a rotor assembly, and a heat sink assembly, wherein the rotor assembly comprises a shaft to which an impeller, a bearing assembly and a rotor core are secured, the heat sink assembly is secured to the bearing assembly and comprises a heat sink, the heat sink comprises a plurality of legs that extend radially outward, and each of the legs is secured to the frame. 
     The rotor assembly is thus secured to the frame by the heat sink assembly. The heat sink assembly provides the additional benefit of carrying heat away from the bearing assembly. As a result, the lifespan of the bearings and thus the compressor is prolonged. The compressor may be configured such that air flows through the interior of the compressor. By employing a heat sink having legs, the air flowing through the interior of the compressor is free to flow between the legs so as to improve cooling of the heat sink and thus the bearing assembly. 
     The legs may be spaced evenly around the sleeve. This then has the advantage that vibration of the rotor assembly is distributed evenly among the legs. As a result, vibration and the inherent noise that it produces is reduced. Additionally, heat may be transferred more evenly from the heat sink to the surrounding air. 
     The width of each leg may taper in a direction away from the sleeve. The temperature of each leg, and thus the rate of heat transfer, decreases as one moves away from the sleeve. Accordingly, by tapering the width of the legs, the mass of the heat sink may be reduced without adversely affecting cooling. As a result, a lighter and cheaper compressor may be realised. 
     The present invention still further provides a compressor comprising a frame, a rotor assembly, and a heat sink assembly, wherein the rotor assembly comprises a shaft to which an impeller, a bearing assembly and a rotor core are secured, the heat sink assembly comprises a sleeve to which a first heat sink and a second heat sink are secured, the sleeve is secured to the bearing assembly, each of the heat sinks is secured to the frame, the first heat sink is proximal the impeller and is generally disc shaped, and the second heat sink is proximal the rotor core and comprises a plurality of legs that extend radially from the sleeve. 
     The rotor assembly is thus secured to the frame by the heat sink assembly. The heat sink assembly provides the additional benefit of carrying heat away from the bearing assembly. As a result, the lifespan of the bearings and thus the compressor is prolonged. By providing a first heat sink that is generally disc shaped, a relatively large surface area is provided over which heat may be transferred to the surrounding air. By employing a second heat sink having legs, air flowing through the interior of the compressor is free to flow between the legs so as to improve cooling of the heat sink and thus the bearing assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is an axonometric view of a compressor in accordance with the present invention; 
         FIG. 2  is an exploded view of the compressor; 
         FIG. 3  is a first axonometric view of the frame of the compressor; 
         FIG. 4  is a second axonometric view of the frame of the compressor; 
         FIG. 5  is an axonometric section through the shroud of the compressor; 
         FIG. 6  is an axonometric view of the rotor assembly of the compressor; 
         FIG. 7  is a side view of the heat sink assembly of the compressor; 
         FIG. 8  is a first axonometric view of the heat sink assembly; 
         FIG. 9  is a second axonometric view of the heat sink assembly; 
         FIG. 10  is an axonometric view of the stator assembly of the compressor; 
         FIG. 11  is an axonometric view of a subassembly of the compressor; 
         FIG. 12  is an axonometric view of a product incorporating the compressor; 
         FIG. 13  is a section through part of the product housing the compressor; and 
         FIG. 14  is the same section as that of  FIG. 13  highlighting the path taken by air flowing through the product. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The compressor  1  of  FIGS. 1 to 11  comprises a frame  2 , a shroud  3 , a rotor assembly  4 , a heat sink assembly  5 , a stator assembly  6 , and a circuit assembly  7 . 
     The frame  2  is generally cylindrical in shape and comprises a side wall  20 , an end wall  21 , a plurality of inlet apertures  22  located around the side wall  20 , a plurality of pockets  23  and a plurality of channels  24  located on the inside of the side wall  20 , a central aperture  25  located in the end wall  21 , and a plurality of diffuser vanes  26  located around the end wall  21 . The pockets  23  and the channels  24  take the form of recesses that extend axially along the inside of the side wall  20 . The recesses are open at one end (distal the end wall  21 ) and closed at the opposite end (proximal the end wall  21 ). The end wall  21  is located at one end of the side wall  20  and resembles an annulus around which the diffuser vanes  26  are located. The opposite end of the side wall  20  is open and terminates with a plurality of prongs  28 . 
     The shroud  3  comprises an inlet  30 , a flared inner section  31 , a planar outer section  32  and a plurality of holes  33  that extend through the outer section  32 . The inner section  31  covers the impeller  41  of the rotor assembly  4 , and the outer section  32  covers the end wall  11  of the frame  2 . Each of the diffuser vanes  26  includes a projection that extends through a respective hole  33  in the shroud  3 . A ring of adhesive  34  then secures the shroud  3  to the vanes  26  and seals the holes  33 . The shroud  3  and the end wall  21  thus define a diffuser  35  that surrounds the impeller  41 . 
     The rotor assembly  4  comprises a shaft  40  to which an impeller  41 , a bearing assembly  42 , and a rotor core  43  are secured. The bearing assembly  42  is located between the impeller  41  and the rotor core  43  and comprises a pair of bearings  44 , 45  and a spring  46 . The spring  46  is located between the two bearings  44 , 45  and applies a preload to each of the bearings  44 , 45 . 
     The heat sink assembly  5  comprises a cylindrical sleeve  50 , a first heat sink  51  secured to the sleeve  50  at one end, and a second heat sink  52  secured to the sleeve  50  at the opposite end. The first heat sink  51  is generally disc shaped and comprises a raised, dome-shaped centre  53  and a flat outer flange  54 . The second heat sink  52  resembles the rowel of a spur and comprises a central hub  55  from which a plurality of legs  56  extend radially outward. The legs  56  are spaced evenly around the hub  55 . That is to say that the angle between adjacent legs  56  is the same for all legs  56  of the heat sink  52 . In the present embodiment, the heat sink  52  has six legs  56  spaced apart by 60 degrees. The width of each leg  56  tapers (i.e. decreases gradually) in a direction away from the hub  55 . 
     The heat sink assembly  5  is secured to the rotor assembly  4 . More specifically, the sleeve  50  surrounds both bearings  44 , 45  and is secured to each of the bearings  44 , 45  by an adhesive. The underside of the impeller  41  is recessed, which helps reduce the mass of the impeller  41 . The heat sink assembly  5  is then secured to the rotor assembly  4  such that the dome-shaped centre  53  of the first heat sink  51  projects into the underside of the impeller  41 . This then reduces the size of the cavity beneath the impeller  41 . As a result, windage and/or other parasitic losses are reduced. 
     The stator assembly  6  comprises a pair of stator cores  60 , 61 , each stator core comprising a bobbin  62  around which electrical windings  63  are wound and a pair of terminal connectors  64  connected to the windings  63 . The stator assembly  6  is secured to the heat sink assembly  5 . Each bobbin  62  is secured to two legs  56  of the second heat sink  52  by an adhesive. The glue points of the bobbins  62  do not align perfectly with the legs  56  of the heat sink  52 . Accordingly, each of the four legs  56  to which the stator assembly  6  is secured includes a small bump  57  which serves as an anchor for the adhesive between the bobbin  62  and the heat sink  52 . 
     The subassembly  8  comprising the rotor assembly  4 , the heat sink assembly  5  and the stator assembly  6  is secured within the frame  2 . The outer flange  54  of the first heat sink  51  is secured to the end wall  21  of the frame  2  by a ring of adhesive. Each of the legs  56  of the second heat sink  52  is secured within a respective pocket  23  by beads of adhesive. Finally, the corners of the stator cores  60 , 61  are secured to the frame  2  by adhesive located within the channels  24 . The subassembly  8  is therefore secured to the frame  2  around the outer flange  54  of the first heat sink  51 , at the ends of the legs  56  of the second heat sink  52 , and at the corners of the stator cores  60 , 61 . 
     The circuit assembly  7  comprises a circuit board  70  on which electronic components  71  for controlling the operation of the compressor  1  are mounted. The circuit assembly  7  is secured to the frame  2  and to the stator assembly  6 . More specifically, the circuit board  60  is secured to the prongs  28  of the frame  2  by an adhesive, and the terminal connectors  64  of the stator assembly  6  are soldered to the circuit board  70 . 
     A method of assembling the compressor  1  will now be described. 
     The heat sink assembly  5  is first secured to the rotor assembly  4 . This is achieved by applying a ring of adhesive around the bearing  44  proximal the impeller  41 , applying a ring of activator around the inside of the sleeve  50  at the end adjacent the first heat sink  51 , and applying a further ring of adhesive around the inside of the sleeve  50  at the end adjacent the second heat sink  52 . The rotor assembly  4  is then inserted into the sleeve  50  until the sleeve  50  surrounds both bearings  44 , 45 . The activator within the sleeve  50  causes the adhesive around the bearing  44  adjacent the impeller  41  to cure. UV light is then used to cure the adhesive around the bearing  45  adjacent the rotor core  43 . The net result is that the sleeve  50  is adhered to both bearings  44 , 45 . 
     The stator assembly  6  is then secured to the heat sink assembly  5 . This is achieved by mounting the stator assembly  6  within one part of a jig, and mounting the rotor-heat sink assembly  4 , 5  in another part of the jig. The jig ensures relative alignment between the rotor assembly  4  and the stator assembly  6 , and more specifically between the rotor core  43  and the stator cores  60 , 61 . Two small beads of adhesive are then applied to each of the bobbins  62 , and the two parts of the jig are brought together such that the bobbins  62  contacts the legs  56  of the second heat sink  52 . The adhesive is then cured using UV light. 
     The subassembly  8  comprising the rotor assembly  4 , the heat sink assembly  5  and the stator assembly  6  is then secured to the frame  2 . The subassembly  8  is mounted within one part of a jig and the frame  2  is mounted in another part. The jig ensures relative alignment between the rotor assembly  4  and the frame  2 , and more specifically between the impeller  41  and the end wall  21  on which the diffuser vanes  26  are located. A ring of heat-curable adhesive is then applied to the inner surface of the end wall  11  of the frame  2 . Beads of heat-curable adhesive are also applied to each of the pockets  23  of the frame  2 . The two parts of the jig are then brought together, causing the subassembly  8  to be inserted into the frame  2  via the open end. The outer diameter of the first heat sink  51  is greater than that of the impeller  41 , and thus the outer flange  54  of the heat sink  51  extends radially beyond the impeller  41 . The diameter of the central aperture  25  in the end wall  21  of the frame  2  is greater than that of the impeller  41  but smaller than that of the first heat sink  51 . As the two parts of the jig are brought together, the impeller  41  passes through the central aperture  25 . The outer flange  54  of the first heat sink  51  then contacts the ring of adhesive formed around the end wall  21 . Additionally, each of the legs  56  of the second heat sink  52  slot into a respective pocket  23 . UV-curable adhesive is then applied over the two legs  56  of the heat sink  52  that are not secured to the stator assembly  6 . These two beads of adhesive are then cured to temporarily hold the subassembly  8  to the frame  2 . Further heat-curable adhesive is then injected into the channels  24  of the frame  2 , which act to secure the corners of stator cores  60 , 61  to the frame  2 . The frame  2  and the subassembly  8  are then removed from the jig and placed in an oven to cure the heat-curable adhesive. 
     The shroud  3  is then secured to the frame  2 . Again, the shroud  3  is mounted in one part of a jig and the frame  2  and subassembly  8  are mounted in another part of the jig. The jig ensures relative alignment between the shroud  3  and the rotor assembly  4 , and more specifically between the shroud  4  and the impeller  41 . The jig also ensures relative alignment between the holes  33  in the shroud  3  and the diffuser vanes  26  of the frame  2 . The two parts of the jig are then brought together causing the shroud  3  to cover the impeller  41  and the end wall  21  of the frame  2 . The outer section  32  of the shroud  3  contacts and rests on top of the diffuser vanes  26 , and each projection protrudes through a respective hole  33 . A ring of adhesive  34  is then applied around the shroud  3 , which acts to secure the shroud  3  to the projections as well as to seal the holes  33 . The adhesive is then allowed to cure in air. 
     Finally, the circuit assembly  7  is secured to the frame  2  and to the stator assembly  6 . The circuit assembly  7  is mounted in one part of a jig and the shroud  3 , frame  2  and subassembly  8  are mounted in another part of the jig. A few beads of adhesive are applied at points around the perimeter of the circuit board  70 . The two parts of the jig are then brought together such that the terminal connectors  64  pass through holes in the circuit board  70 , and the circuit board  70  contact the prongs  28  of the frame  2 . The adhesive is then cured, and the terminal connectors  64  are soldered to the circuit board  70 . The completed compressor  1  is then removed from the jig. 
     There are a couple of advantages associated with this method of assembly. 
     First, the rotor assembly  4  may be balanced as a complete unit before securing the rotor assembly  4  within the frame  2 . This is made possible because the rotor assembly  4  is secured to the frame  2  by the heat sink assembly  5 . Moreover, the first heat sink  51  has an outer diameter greater than that of the impeller  41 , and the aperture  25  in the end wall  21  of the frame  2  has a diameter greater than the impeller  41  but smaller than the first heat sink  51 . This then enables the rotor assembly  4  to be inserted and then secured with the frame  2  as a complete unit. With conventional compressors, it is often necessary to assemble the various components of the rotor assembly within the frame. Accordingly, whilst the individual components may be balanced, the completed rotor assembly is generally not. 
     Second, the rotor assembly  4  may be better aligned with the stator assembly  6 , the diffuser  35 , and the shroud  3 . With a conventional compressor, the rotor assembly and the stator assembly are typically secured to the frame as separate assemblies. However, once the rotor assembly has been secured within the frame, it is generally difficult to secure the stator assembly within the frame whilst simultaneously aligning the stator assembly relative to the rotor assembly. As a result of the tolerances in the alignment of the rotor assembly and the stator assembly, a larger air gap is required between the rotor core and the stator cores in order to ensure that, at the tolerance limit, the rotor core is free to rotate without contacting the stator cores. However, a larger air gap has the disadvantage of increasing the magnetic reluctance. With the assembly method described above, the stator assembly  6  is first aligned relative to the rotor assembly  4  and then secured to the heat sink assembly  5 . The subassembly  8  comprising the rotor assembly  4 , the heat sink assembly  5  and the stator assembly  6  is then secured to the frame  2 , during which time the rotor assembly  4  is aligned relative to the end wall  21  and the diffuser vanes  26 . Since the heat sink assembly  5  is secured to both the rotor assembly  4  and the stator assembly  6 , the heat sink assembly  5  maintains the relative alignment between the rotor assembly  4  and the stator assembly  6 . Consequently, when the rotor assembly  4  is aligned relative to the frame  2 , the alignment with the stator assembly  6  is maintained. A smaller air gap may therefore be employed between the rotor core  43  and the stator cores  60 , 61 . 
     Operation of the compressor  1  will now be described with reference to the product  100  illustrated in  FIGS. 12 to 14 , which in this particular example is a handheld vacuum cleaner. 
     The product  100  comprises a housing  101  within which the compressor  1  is mounted by means of an axial mount  110  and a radial mount  120 . Each of the mounts  110 , 120  is formed of an elastomeric material and acts to isolate the housing  101  from vibration generated by the compressor  1 . The axial mount  110  is similar in shape to that of the shroud  3 , and is secured to the top of the shroud  3 . The radial mount  120  comprises a sleeve  121 , a lip seal  122  located at one end of the sleeve  121 , and a plurality of axial ribs  123  that extend along and are spaced around the sleeve  121 . The radial mount  120  is secured around the frame  2  of the compressor  1 . More specifically, the sleeve  121  surrounds the side wall  20  of the frame  2  such that the lip seal  122  is located below the inlet apertures  22  in the side wall  2 . 
     The housing  101  comprises a front section  102  and a rear section  103 , which together define a generally cylindrical recess  104  within which the compressor  1  is mounted. The front section  102  includes an inlet  105  through which air is admitted to compressor  1 , and the rear section  103  comprises a plurality of exhaust apertures  106  through which air from the compressor  1  is exhausted. The axial mount  110  abuts an end wall  107  of the front section  102  to create a seal between the compressor  1  and the inlet  105 . Additionally, the radial mount  120  abuts a side wall  108  of the front section  102  such that the lip seal  122  creates a seal between the compressor  1  and the side wall  108 . 
     During operation, air enters the compressor  1  via the shroud inlet  30 . The air is centrifuged outwards by the impeller  41  and flows through the diffuser  35  defined between the frame  2  and the shroud  3 . The air then exits the compressor  1  via an annular opening  36  defined by the axial gap between the frame  2  and the shroud  3  at the periphery. On exiting the compressor  1 , the air re-enters the compressor  1  via the inlet apertures  22  in the side wall  20  of the frame  2 . The air then flows through the interior of the compressor  1 , whereupon the air acts to cool the heat sink assembly  5 . The air flows radially over the first heat sink  51  and flows axially over the sleeve  50  and the second heat sink  52 . The legs  56  of the second heat sink  52  extend directly into the path taken by the air flowing through the compressor  1 . As a result, cooling of the second heat sink  52  is particular effective. After passing through the legs  56  of the heat sink  52 , the air flows over and cools the stator assembly  6 . Finally, the air is redirected in a radial direction by the circuit assembly  7 , whereupon the air exits the compressor  1  via the gaps  72  between the circuit board  70  and the side wall  20  of the frame  2 . In flowing over the circuit assembly  7 , the air cools the electrical components  71  of the circuit assembly  7 . In particular, the circuit assembly  7  comprises power switches that are used to control the flow of current through the windings  63  of the stator assembly  6 . Owing to the magnitude of the currents that are carried by the switches, the switches tend to generate relatively high levels of heat. 
     The heat sink assembly  5  provides at least three useful functions. 
     First, the heat sink assembly  5  supports the rotor assembly  4  within the frame  2 . In this regard, it is to be noted that the rotor assembly  2  is not secured to the frame  2  by any other means. The provision of the heat sink assembly  5  enables the rotor assembly  2  to be balanced as a complete unit before being secured to the frame  2 . Moreover, the heat sink assembly  5  simplifies the assembly of the compressor  1  whilst providing relatively good support to the rotor assembly  4 . In this regard, it is to be noted that the rotor assembly  4  comprises a bearing assembly  42  located between the impeller  41  and the rotor core  43 . This has the advantage that a relatively short axial length may be achieved for the rotor assembly  4 . Moreover, the bearing assembly  42  comprises two spaced-apart bearings  44 , 45 . This then has the further advantage of increasing the stiffness of the rotor assembly  4  in comparison to, say, two bearings located at opposite ends of the shaft. If the heat sink assembly  5  were omitted and the rotor assembly  4  were secured directly to the frame  2 , it would then be necessary to secure each of the bearings  44 , 45  to the frame  2 . It might then prove difficult or indeed impossible to insert the rotor assembly  4  into the frame  2  as a complete unit. 
     The heat sink assembly  5  comprises two heat sinks  51 , 52  that are each secured to the frame  2 . The heat sinks  51 , 52  are spaced axially and thus radial movement of the rotor assembly  4  relative to the frame  2  is opposed within two planes that are spaced axially. As a result, vibration of the rotor assembly  4  and the inherent noise that results are reduced. The legs  56  of the second heat sink  52  are spaced evenly around the sleeve  50 . 
     Consequently, vibration of the rotor assembly  5  is evenly distributed among the legs  56 . This then avoids excessive vibration occurring in a particular direction. The first heat sink  51  is secured to the inside of the end wall  21  of the frame  2 , and the second heat sink  52  is secured within the pockets  23  of the frame  2 . Accordingly, in addition to opposing radial movement, the heat sink assembly  5  opposes axial thrust generated by the impeller  41 . 
     Second, the heat sink assembly  5  carries heat away from the bearing assembly  42 . As a result, the lifespan of the bearing assembly  42  and thus the compressor  1  is prolonged. The first heat sink  51  is disc shaped and thus provides a relatively large surface area over which heat may be transferred to the surrounding air. The second heat sink  52 , on the other hand, comprises a plurality of legs  56 . This then enables air to flow between the legs  56  of the heat sink  52 . In the present embodiment, the legs  56  extend radially into the path of the air flowing axially through the compressor  1 . As a result, relatively good heat transfer is achieved between the second heat sink  52  and the surrounding air. The legs  56  of the heat sink  52  create a restriction in the flow path. The size of the restriction influences the rate at which heat transfers from the heat sink assembly  5  to the air, as well as the performance of the compressor  1  (e.g. mass flow rate and/or efficiency). The number, size and arrangement of the legs  56  are therefore chosen so as to maximise cooling without adversely affecting the performance of the compressor  1 . The legs  56  are spaced evenly around the sleeve  50 , which helps ensure that heat is transferred more evenly from the heat sink  52  to the surrounding air. Additionally, the width of each leg  56  tapers in a direction away from the sleeve  50 . The temperature of each leg  56  and thus the rate of heat transfer decreases as one moves away from the sleeve  50 . By tapering the width of the legs  56 , the mass of the heat sink  52  may be reduced without adversely affecting cooling of the bearing assembly  42 . As a result, a lighter and cheaper compressor  1  may be realised. 
     Third, the heat sink assembly  5  maintains the alignment between the rotor assembly  4  and the stator assembly  6  when securing the subassembly  8  to the frame  2 . As a result, the rotor assembly  4  may be aligned within the frame  2  whilst maintaining the alignment with the stator assembly  6 . Relatively good alignment may therefore be achieved between the rotor assembly  4  and the stator assembly  6 , and between the rotor assembly  4  and the diffuser  35  and shroud  3 . 
     The heat sink assembly  5  is made of steel and was selected following a balance of different requirements: structural strength, thermal conductivity, thermal expansivity and cost. Since the heat sink assembly  5  is used to secure the rotor assembly  4  within the frame  2 , the structural strength of the heat sink assembly  5  is important for minimising vibration of the rotor assembly  4 . The thermal conductivity of the heat sink assembly  5  is clearly important for carrying heat away from the bearing assembly  42 . The bearings  44 , 45  are secured to the shaft  40  and the sleeve  50  of the heat sink assembly  5 . Consequently, uneven thermal expansion of the shaft  40  and the sleeve  50  may cause the inner race of each bearing  44 , 45  to move relative to the outer race. This in turn may lead to adverse changes in the preload of the bearings  44 , 45 . Accordingly, the thermal expansivity of the heat sink assembly  5  may play an important role in determining the lifespan of the bearing assembly  42 . For this reason, it is advantageous to form the heat sink assembly  5  from a material having a coefficient of thermal expansion closely matching that of the shaft  40 . Whilst steel was employed in the present embodiment, other materials may be used that fulfil the particular design requirements of the compressor  1 . 
     Whilst a particular embodiment has thus far been described, various modifications may be made, both to the compressor and its method of assembly, without departing from the scope of the invention as defined by the claims. For example, in the embodiment described above, the heat sink assembly is described as providing three useful functions. Conceivably, the compressor may comprise a heat sink assembly that provides only one or two of these functions. For example, rather than securing the stator assembly to the heat sink assembly, the stator assembly may be secured to the frame after the rotor-heat sink assembly has been secured to the frame. As a further example, the compressor described above is configured such that air is drawn through the interior of the compressor and over the heat sink assembly. Nevertheless, the heat sink assembly may be employed in a compressor for which air is not drawn through the interior and over the heat sink assembly. Moreover, whilst the heat sink assembly described above comprises two heat sinks, one or more of the aforementioned advantages may be achieved through the use of a single heat sink.