Abstract:
A new rotor for a positive displacement compressor is provided which includes a non-rotating rotor shaft, a rotatable rotor body having end surfaces and surrounding the rotor shaft, and a mechanism for rotating the rotor body around the shaft. The rotor may be either a male rotor or a female rotor, and a positive displacement compressor is also provided which includes the new male rotor, the new female rotor, and an internal drive mechanism. Such positive displacement compressor includes radial induction bearings, and the internal drive mechanism replaces an external drive mechanism.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a rotor and a positive displacement compressor, and more particularly to a compressor which includes two rotors.  
         [0002]     Positive displacement compressors including helical screw compressors as well as roots compressors are well known in the art. A helical compressor employs one male rotor axially aligned with and in combination with one female rotor. Usually the male rotor is the drive rotor driving the female rotor. In a compressor, one male rotor is commonly combined with one female rotor. Such a compressor is usually referred to as a twin helical screw rotor compressor.  
         [0003]     An example of a known twin helical screw rotor compressor or screw compressor is shown in  FIGS. 1 and 2  and is described briefly below.  
         [0004]     A compressor  100  includes two mutually engaging screw rotors, of which a first rotor  101  is a male rotor and a second rotor  102  is a female rotor. The rotors  101 ,  102  are rotatably mounted in a working chamber that is delimited by a first end wall  103 , a second end wall  104  and a barrel wall  105  that extends between the end walls  103 ,  104 . As can be seen from  FIG. 2 , the barrel wall  105  has a form that corresponds generally to the form of two mutually intersecting cylinders. The compressor has an inlet port  108  at the first end wall  103  and an outlet port  109  at the second end wall  104 .  
         [0005]     The male rotor  101  has a rotor body  22  that includes a plurality of lobes  106  and intermediate grooves  111  which extend in a helical line along the rotor  22 . Similarly, the female rotor  102  has a rotor body  23  that includes a plurality of lobes  107  and intermediate grooves  112  that extend in a helical line along the rotor  23 . The major part of each lobe  107  on the male rotor  101  is located outwardly of the circle of contact with the female rotor  102 , whereas the major part of each lobe  107  on the female rotor  102  is located inwardly of said circle of contact. The female rotor  102  will normally have more lobes than the male rotor  101 . A typical combination is one in which the male rotor  101  has four lobes and the female rotor  102  six lobes, as shown in  FIG. 2 .  
         [0006]     The gas to be compressed, normally air, is delivered to the working space of the compressor through an inlet port  108  and is then compressed in V-shaped working chambers defined between the rotors and the chamber walls. Each chamber moves to the right in  FIG. 1 , as the rotors  101 ,  102  rotate. The volume of a working chamber decreases continuously during the latter part of its cycle, after communication with the inlet port  108  has been cut off. The gas is therewith compressed and leaves the compressor through an outlet port  109 . The ratio of outlet pressure to inlet pressure is determined by the built-in volumetric relationship between the volume of a working chamber immediately after its communication with the inlet port  108  has been cut-off and its volume when it commences communication with the outlet port  109 .  
         [0007]     The male rotor  101  in  FIG. 1  has a shaft  21  around which the rotor body  22  is disposed. The male rotor body  22  has a first end surface  4  which lies in close proximity to the first end wall  103 , and a second end surface  3  which lies in close proximity to the second end wall  104 . The lobes  106  of the male rotor body  22  have crowns  5 , shown linearly in  FIG. 1 . The female rotor  102  in  FIG. 1  has a shaft  26  around which the rotor body  23  is disposed. The female rotor body  23  includes a first end surface  27  which lies in close proximity to the first end wall  103 , and a second end surface  28  which lies in close proximity to the second end wall  104 . The lobes  107  of the female rotor body  23  have crowns  15 , shown linearly in  FIG. 1 .  
         [0008]     Such a compressor is coupled to an electrical motor or combustion engine in order to rotate the male rotor  101  and is for example used to enhance the performance of an engine in a vehicle such as an automobile or to enhance the performance of a fuel cell due a higher inlet pressure of air or oxygen. As a consequence thereof, the engine or fuel cell can be produced having a lower volume and mass. Also, there is a need for reducing the volume and weight of such a compressor for supercharging the engine or fuel cell.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the invention to provide a small compressor.  
         [0010]     According to the present invention, a new rotor for a positive displacement compressor is provided which includes a non-rotating rotor shaft; a rotatable rotor body having end surfaces and surrounding the rotor shaft; and means for rotating the rotor body around the non-rotating rotor shaft.  
         [0011]     In a preferred arrangement, the means for rotating the rotor body around the non-rotating rotor shaft comprises rotor magnets on the rotor body, wherein the magnets are arranged in a circle centered on an axis of the rotor body and facing the shaft; and an electrical stator provided on the shaft in registration with the rotor magnets. When the stator is electrically energized, the rotor body is caused to rotate about the non-rotating rotor shaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a longitudinal sectional view of a Prior Art helical rotary screw compressor;  
         [0013]      FIG. 2  is a sectional view taken on line II-II in  FIG. 1 ;  
         [0014]      FIG. 3  is an end view showing two cooperating rotors of the present invention;  
         [0015]      FIG. 4  is a sectional view of the rotors shown in  FIG. 3  taken along line IV-IV;  
         [0016]      FIG. 5  is a sectional view corresponding to the view in  FIG. 4  including a compressor housing; and  
         [0017]      FIG. 6  is a sectional view of a radial induction bearing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     In  FIGS. 3 and 4  two cooperating rotors for a screw rotor compressor according to the invention are illustrated. The left rotor  1  is a male rotor and the right rotor  21  is a female rotor.  
         [0019]     The male rotor  1  comprises a rotor body  3  having the shape of a cylindrical shell. The outer surface of the rotor body  3  has helically extending lobes  6  separated by intermediate grooves  7 . As is apparent from  FIG. 3 , the male rotor body  3  has six lobes  6  and the same number of intermediate grooves  7 . The inner surface  10  of the cylindrical shell of the male rotor body  3  is cylindrical. Further, the male rotor body  3  has parallel planar end surfaces  4  and  5 . Inside of the male rotor body  3  a male rotor shaft  2  is arranged. The two end parts  2 A and  2 B of the shaft  2  extend beyond the end surfaces  4 ,  5 , respectively, of the male rotor body  3 .  
         [0020]     Two bearings  8 ,  9  are disposed in a spaced relationship between the shaft  2  and the inner surface  10  of the male rotor body  3 . Between the bearings  8 ,  9  on the inner surface  10  of the male rotor body  3 , permanent magnets  11  in the form of rods or bars are placed in parallel such that they form a cylindrical shell ring. The magnets  11  are secured to the rotor body  3 . The magnets  11  may be arranged in an insert to be placed inside the rotor body  3  and fixed (i.e., bonded) to the inner surface  10  of the rotor body  3  or may be bonded separately to this surface  10  as is known in the art.  
         [0021]     The male rotor shaft  2  is provided with an axially extending blind bore  13  opening in one of the planar ends thereof. Further, a bore  13 A in the cylindrical surface of the shaft  2  is connected to (in communication with) the axial bore  13 .  
         [0022]     Electrical windings  12  are wound on the shaft  2  of the rotor  1  between the bearings  8 ,  9  in registration with the cylindrical shell of magnets  11 . These windings  12  function as stator leads. End parts  14  of the windings  12  are passed through the bore  13 A from the cylindrical surface of the shaft  2  and through the axial bore  13  to the outside of the shaft  2  and are connected to an electrical power source  80 . The stator leads  13  when energized will cooperate with the ring shaped shell of permanent magnets  11  in the same way as in an electrical motor. In this case, contrary to a conventional electrical motor, the outer part will rotate while the central part acts as a stator.  
         [0023]     Similarly, as also shown in  FIG. 4 , the right rotor  21  is a female screw rotor. The female rotor  21  comprises a female rotor body  23  having the shape of a cylindrical shell. The outer surface of the female rotor body  23  has helically extending lobes  26  separated by intermediate grooves  27 . As is apparent from  FIG. 3 , the female rotor body  23  has eight lobes  26  and the same number of intermediate grooves  27 . The inner surface  30  of the cylindrical shell of the female rotor body  23  is cylindrical. Further, the female rotor body  23  has parallel planar end surfaces  24  and  25 . Inside of the female rotor body  23  a female rotor shaft  22  is arranged. The two end parts  22 A and  22 B of the female rotor shaft  22  extend beyond the end surfaces  24 ,  25 , respectively, of the female rotor body  23 .  
         [0024]     Two bearings  28 ,  29  are disposed in a spaced relationship between the female rotor shaft  22  and the inner surface  30  of the female rotor body  23 . Between the bearings  28 ,  29  on the inner surface  30  of the female rotor body  23  permanent magnets  31  in the form of rods or bars are placed in parallel such that they form a cylindrical shell ring. The magnets  31  are secured to the female rotor body  23 . The magnets  31  may be arranged in an insert to be placed inside the female rotor body  23  and be fixed (i.e., bonded) to the inner surface  30  of the female rotor body  23  or may be bonded separately to this surface  30  as is known in the art.  
         [0025]     The female rotor shaft  22  is provided with an axially extending blind bore  33  opening in one of the planar ends thereof. Further, a bore  33 A in the cylindrical surface of the shaft  22  connects to (communicates with) the axial bore  33 .  
         [0026]     Electrical windings  32  are wound on the shaft  22  of the female rotor  21  between the bearings  28 ,  29  in registration with the cylindrical shell of magnets  31 . The windings  32  function as stator leads. End parts  34  of the windings  32  are passed through the bore  33 A from the cylindrical surface of the female rotor shaft  22  and through the axial bore  33  to the outside of the female rotor shaft  22  and are connected to an electrical power source  80 . The stator leads  33  when energized will cooperate with the ring shaped shell of permanent magnets  31  in the same way as in an electrical motor. In this case, contrary to a conventional electrical motor, the outer part will rotate while the central part acts as a stator.  
         [0027]      FIG. 5  shows a sectional view of a compressor which includes the two rotors  1  and  21  of  FIG. 4 . The compressor housing  40  comprises two parallel end walls  41  and  42  and a barrel wall  43  between the end walls  41 ,  42 . The inside of the barrel wall  43  has the shape of two intersecting cylinders corresponding to the diameters of the two rotors  1  and  21 . The end parts of the rotor shaft  2 ,  22  are protruding into the end walls  41 ,  42  of the compressor housing  40 . The configuration of the compressor housing corresponds to that shown in  FIGS. 1 and 2  having an inlet port  44  and an outlet port  45 .  
         [0028]      FIG. 6  is an enlarged sectional view (not to scale) of a part of a rotor shaft, and shows a radial induction bearing  60  between a shaft  61  and a rotor body  70  shown as a cylindrical shell. The shaft  61  is part of the bearing by serving as an inner stator-mounting rod. Two ring shaped axial magnets  62 ,  63  are fixed around the mounting rod  61  in a spaced relationship. The magnets  62 ,  63  have opposing magnetic directions (polarities). In the space between the magnets a spacer ring  64  is arranged with an iron washer  65  provided around the outside of the spacer ring  64 . Also, second and third spacer rings  66 ,  67  bearing against the magnets  62 ,  63 , respectively, are provided. Similarly, outside the spacer rings  66 ,  67  end plates  68 ,  69 , respectively, are arranged. Such radial induction bearings are known in the art.  
         [0029]     Reverting to  FIG. 5 , the operation of the compressor will now be described. In order to start the rotation of the compressor rotors, the windings  12 ,  32  have to be energized. Upon energizing the windings, the rotor bodies  3 ,  23  will start to rotate. In this case both rotors are energized and separately driven by electrical power. Since the male rotor  1  has 6 lobes and the female rotor  21  has 8 lobes, the rotational speeds of these rotors must differ. If the male rotor  1  has a rotational speed of N rpm the rotational speed of the female rotor  21  is 0.75N rpm. Generally, if the compressor male rotor has X lobes and the compressor female rotor has Y lobes, the rotational speed of the female rotor is (X/Y)*N rpm, where N is the rotational speed of the male rotor.  
         [0030]     As seen from the foregoing, according to the present invention, a rotor for a positive displacement compressor comprises a non-rotating shaft around which a rotatable rotor body is mounted. The rotatable rotor body has a central bore in which the shaft is inserted. The rotor body has a shape which essentially corresponds to an elongated shell having planar parallel end walls. An outer surface of the rotor body comprises lobes and intermediate grooves helically extending between the end walls. There are typically at least 2 but less than 10 lobes and intermediate grooves. The lobes and grooves extend helically from the outer surface of the rotor body. The helical twist is preferably in a range of 5-90° for a roots compressor and 150-330° for a helical screw compressor. End parts of the rotor shaft extend beyond the rotor body and serve as trunnions. Two bearings are arranged in a space between the rotor shaft and the rotor body in a spaced relationship. The bearings allow the rotor body to be rotated relative to the shaft.  
         [0031]     Further, on an inner part of the rotor body facing the shaft, magnets are arranged in a circle around the shaft. Metal wires are wound on the shaft as electrical windings which are arranged such that, when fed with electrical power, the windings cooperate with the magnets in the rotor body. This cooperation results in rotation of the rotor body around the rotor shaft.  
         [0032]     The positive displacement compressor according to the invention thus comprises a housing including a first and a second end wall, a barrel wall having an inner shape substantially corresponding to two intersecting cylinders between the end walls, an inlet port and an outlet port for fluid. Two cooperating rotors are mounted in parallel in the two intersecting cylinders. One of the rotors is a male rotor and the other one is a female rotor. The two end portions of the non-rotating shafts of the respective rotors are mounted in the end walls of the rotor housing.  
         [0033]     Each of the rotors comprises a rotor body which has a central bore and is arranged rotatably around the non-rotating shaft, and bearings mounted on the shaft. The bearings are arranged in a spaced relationship near each end of the respective rotor body. Each rotor body is provided with magnets, preferably permanent magnets, arranged in a circle and facing the shaft. On the periphery of the shaft there are windings for cooperation with the magnets on the rotor body. Both the windings and the magnets are arranged between the bearings. The windings have leads which are connectable to a power source. When the windings are energized, the respective rotors are caused to rotate, like a motor.  
         [0034]     Each rotor body is provided with lobes and intermediate grooves on the outer surface. The lobes and grooves are preferably arranged as a helix. The male rotor body has usually two or more lobes. The number of lobes of the female rotor body is usually but not necessarily greater than the number of lobes of the male rotor body. Such a difference in the number of lobes requires that the two rotors rotate with different revolution units of time.  
         [0035]     Additional advantages and modifications will occur to those readily skilled in the art. For example, although according to the present invention each of the rotors  1 ,  21  is driven by its own source of energy, in known compressors a motor drives one rotor while the other rotor is driven indirectly by the driven rotor or by means of synchronizing gears. Additionally, the bearings in the present compressor may be conventional bearings, such as roller bearings, or radial induction bearings. Preferably, at least one of the bearings  8 ,  9  and  28 ,  29 , respectively, is a radial induction bearing. Various additional modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.