Abstract:
A pole piece structure is provided to be used as part of an active rotor structure for an electrical rotary machine, wherein the active rotor structure is circular comprising pole pieces and permanent magnets magnetized in the circumferential direction of the rotor structure, wherein the rotor pole piece structure is a single structure with at least two individually spaced pole pieces connected by an integration bridge. Further provided is a method of manufacturing a rotor pole piece structure and a rotor for an electrical machine with a rotor pole piece structure. The assembly of magnets to the pole pieces is significantly enhanced using these rotor pole piece structures as described.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rotor for an electrical rotary machine. The invention further relates to a method of manufacturing a rotor for an electrical rotary machine with a rotor pole piece structure. The assembly of magnets to the pole pieces is significantly enhanced using these rotor pole piece structures according to the present invention. 
     BACKGROUND OF THE INVENTION 
     During the years, electric machine designs evolved from modulated pole machines, claw pole machines, Lundell machines and transverse flux machines (TFM) have become more and more interesting. Electric machines using the principles of these machines were disclosed as early as about 1910 by Alexandersson and Fessenden. One of the most important reasons for the increasing interest is that the design enables a very high torque output in relation to, for instance, induction machines, switched reluctance machines and even permanent magnet brushless machines. Further, such machines are advantageous in that the coil is often easy to manufacture. However, one of the drawbacks of the design is that they are typically relatively expensive to manufacture and that they experience a high leakage flux which causes a low power factor and a need for more magnet material. The low power factor requires an up-sized power electronic circuit (or power supply when the machine is used synchronously) that also increases the volume, weight and cost of the total drive. 
     The modulated pole electric machine stator is basically characterised by the use of a central single winding that will magnetically feed multiple teeth formed by the soft magnetic core structure. The soft magnetic core is then formed around the winding, while for other common electrical machine structures the winding is formed around the tooth core section. Examples of the modulated pole machine topology are sometimes recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM-machines. The modulated pole machine with buried magnets is further characterised by an active rotor structure including a plurality of permanent magnets being separated by pole sections. The active rotor structure is built up from an even number of segments, whereas half the numbers of segments are made of soft magnetic material and the other half number of segments are made from permanent magnet material. The permanent magnets are arranged so that the magnetization direction of the permanent magnets is substantially circumferential, i.e. the north and south pole, respectively, is pointing in a substantially circumferential direction. 
     The state of the art method is to produce the rather large number of rotor segments, typically 10-50 individual pieces, as individual components. The assembly of these segments is further complicated by the opposing polarisation direction of the permanent magnet segments that will tend to repell the pole sections from each other during the assembly. The magnetic functionality of the soft magnetic pole sections in the desired machine structure is fully three-dimensional and it is required that the soft magnetic pole section is able to efficiently carry magnetic flux with high magnetic permeability in all three space directions. A traditional design using laminated steel sheets will not show the required high permeability in the direction perpendicular to the plane of the steel sheets and it is here beneficial to use a soft magnetic structure and material which shows a higher magnetic flux isotropy than a state of the art laminated steel sheet structure. 
     SUMMARY 
     Embodiments of the present invention provide a rotor solving one or more of the above-mentioned problems. 
     This is obtained by a rotor structure for an electrical rotary machine, wherein said rotor structure is tubular with a circular cross-section comprising rotor pole sections and permanent magnets magnetised in the circumferential direction of said rotor structure where two neighbouring permanent magnets are separated in the circumferential direction by a rotor pole section. Embodiments of the rotor structure comprise one or more rotor pole piece structures, each in the form of a single structure with at least respective parts of two individually spaced rotor pole sections connected by an integration bridge. 
     In some embodiments, all of the connected and integrated rotor pole sections show equal magnetic polarity after assembly to the magnetically active permanent magnets and there will then not be any additional magnetic leakage due to the integration bridge in-between the pole sections of the same integrated structure. 
     As the permanent magnets generate the rotor magnetic field, and since the permanent magnets are magnetically coupled directly to respective ones of the rotor pole sections, the integration bridge merely serves as a mechanical connection of the rotor pole sections with each other, so as to provide a stable arrangement of the rotor pole sections with well-defined distances between them, thereby facilitating fast and accurate arrangement of the permanent magnets in slots formed between the rotor pole sections. The integration bridge is not required and normally not even desired to carry magnetic flux, in particularly when the permanent magnets are the only magnetic source of the rotor structure, e.g. in a coilless rotor structure. Hence, the integration bridge may have a shape and a size large enough so as to provide sufficient mechanical strength to the pole piece structure to allow arrangement of the permanent magnets between the rotor pole sections. Furthermore, the integration bridge may have a shape and a size small enough so as to minimize any magnetic flux leakage. 
     The integration bridge between the pole sections may be physically distanced from the rotor pole sections, in particular the rotor pole sections of the respective other pole piece structure of opposite polarity, so that leakage in between the pole piece structures of opposite polarity will be minimised; the reduction of effective flux crossing the air-gap and potentially reducing torque of the machine is thereby minimised. 
     In an embodiment the rotor pole piece structures can be assembled to form a rotor by first positioning and fixing the rotor pole piece structures and secondly by slipping the permanent magnets into slots formed between the pole sections of the rotor pole piece structure. The assembly of magnets to the pole sections is significantly enhanced because of the geometric pre-positioning of the soft magnetic pole sections in the integrated rotor pole piece structure. In an embodiment the permanent magnets could be locked by gluing or by mechanical interlocking created by integrated geometrical features of the pole piece structure. 
     After the assembly has been performed, the rotor is ready for usage and therefore the integration bridge could be removed, e.g. cut off by machining, which would reduce the weight of the complete rotor. In an embodiment the rotor could be formed by using one rotor pole piece structure for half the pole sections and then add the permanent magnets and additional pole sections, respectively, to form the rotor. 
     In an embodiment said structure is made from soft magnetic material such as soft magnetic powder. By making the pole sections from soft magnetic powder the manufacturing of the rotor may be simplified and magnetic flux concentration, utilizing the advantage of effective three-dimensional flux paths, may be more efficient. 
     In an embodiment the structure is circular comprising a circular/ring-shaped integration bridge connecting individually spaced rotor pole sections. Thereby only two structures are necessary to form a rotor which further enhances the assembling of the rotor. 
     In an embodiment said structure is a circular subsection comprising an integration bridge in the form of a ring segment connecting at least two pole sections. Thereby only smaller and less complicated elements have to be manufactured, thus reducing the manufacturing complexity. 
     Depending on the manufacturing process of the structures and the required flux requirements, the integration bridge could either be placed with a radial distance to said pole sections or with an axial distance to said pole sections. Having an integration bridge with a radial distance results in a shorter, but wider rotor than having an integration bridge with an axial distance. The rotor could be used for both axial and radial rotary machines. 
     In an embodiment the pole sections have a width smaller than the distance between said pole sections. Thereby the pole piece structure can be used together with a similar pole piece structure to generate at least part of a rotor, leaving space to the permanent magnet after the assembly. 
     In some embodiments, each of the at least two rotor pole pieces is divided into respective first and second subsections, the respective subsections of each rotor pole section being part of respective rotor pole piece structures, i.e. the first subsection of a first one of the two rotor pole sections is connected via a first integration bridge to the first subsection of the second one of the two rotor pole sections, and the second subsection of the first rotor pole section is connected via a second integration bridge to the second subsection of the second rotor pole section. Each rotor pole piece structure provides one or more slots for at receiving at least a part of one of the permanent magnets, the slot being defined at least in part by the subsections that are connected to each other by the integration bridge. 
     Hence, in this embodiment, the integration bridge connects and integrates subsections of rotor pole sections that, after assembly with the permanent magnets, have different magnetic polarity. Even though such an arrangement results in a certain amount of magnetic leakage, this embodiment provides slots for receiving the permanent magnets where the size of the slot in at least the circumferential direction is defined by the dimensions of the individual integrated rotor pole piece structures rather than defined by an assembly of different pole piece structures. Therefore, the parts of the rotor structure can be assembled with an increased precision. 
     In some embodiments, a rotor pole piece structure comprises two pole subsections extending in axial direction along respective lateral sides of the permanent magnet, the subsections being connected with each other at each end by respective integration bridges so as to surround the permanent magnet in axial and circumferential direction, while leaving an opening extending in radial direction through the pole piece structure for receiving the permanent magnet. The lateral outward side faces of the pole piece structure abut to corresponding side faces of other, like pole piece structures to form the rotor. In one embodiment, the side faces are parallel with the axial direction, while in another embodiment, the lateral side faces of a rotor pole piece are arranged at an angle relative to the axial direction so as to form a wedge-shaped pole piece structure. Hence one of the ends, seen in axial direction, of the wedge-shaped pole piece structure is wider in the circumferential direction than the other end. Such wedge shaped structures may be arranged with alternating orientation, i.e. a wide end of a first pole piece structure is arranged adjacent to the narrow ends of the respective neighbouring wedge-shaped pole piece structures. It is an advantage of this embodiment, that the assembly of the rotor is further facilitated, as the individual pole piece structures can be aligned relative to each other by applying axial pressure. In particular, axial alignment of the rotor pole piece structures ensures accurate inter-magnet distances in the circumferential direction. Furthermore, the permanent magnets may then be slid into the slots of the respective pole piece structures. 
     The invention further relates to a method of manufacturing a rotor as disclosed herein. The rotor pole piece structure may be shaped in one piece by compaction of soft magnetic powder in a tool. 
     The soft magnetic powder could be a substantially pure water atomised iron powder or a sponge iron powder having irregular shaped particles which have been coated with an electrical insulation. In this context the term “sub-stantially pure” means that the powder should be substantially free from inclusions and that the amount of the impurities O, C and N should be kept at a minimum. The average particle sizes are generally below 300 μm and above 10 μm. 
     However, any soft magnetic metal powder or metal alloy powder may be used as long as the soft magnetic properties are sufficient and that the powder is suitable for die compaction. 
     The electrical insulation of the powder particles may be made of an inorganic material. Especially suitable are the type of insulation disclosed in U.S. Pat. No. 6,348,265 (which is hereby incorporated by reference), which concerns particles of a base powder consisting of essentially pure iron having an insulating oxygen- and phosphorus-containing barrier. Powders having insulated particles are available as Somaloy®500, Somaloy®550 or Somaloy®700 available from Höganas AB, Sweden. 
     Thereby the integrated pole sections are efficiently made in the same operation by use of a powder forming method where the forming is made in a single compaction tool set up. 
     An embodiment of the method described herein comprises the steps of
         combining a first and a second pole piece structure each with individually spaced rotor pole sections connected by an integration bridge, wherein the structures are combined forming slots between their pole sections,   arranging at least one permanent magnet between said first and second pole piece structure.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, preferred embodiments of the invention will be described referring to the figures, where 
         FIG. 1  is a schematic, exploded, perspective view of a state of the art radial modulated pole electric machine and prior art rotor/stator design, 
         FIG. 2  is a cross-sectional view of the radial modulated pole electric machine in  FIG. 1 , 
         FIG. 3   a  illustrates a rotor for an electrical machine according to the present invention, 
         FIG. 3   b  illustrates a rotor for an electrical machine according to the present invention, 
         FIGS. 4   a ,  4   b  and  4   c  illustrate an embodiment of an assembly principle of assembling rotor pole piece structures to form a rotor, 
         FIG. 5  illustrates an embodiment of a rotor pole piece structure with an axially displaced integration bridge, 
         FIG. 6  illustrates an embodiment of a rotor pole piece structure with a radially displaced integration bridge, 
         FIG. 7  illustrates another embodiment of an assembly principle of assembling rotor pole piece structures to form a rotor; 
         FIGS. 8   a ,  8   b ,  8   c  and  8   d  illustrate  8  illustrates yet another embodiment of a rotor; 
         FIGS. 9   a ,  9   b ,  9   c  and  9   d  illustrate yet another embodiments of a rotor. 
         FIG. 10  illustrates an embodiment of a rotor pole piece structure with a radially displaced integration bridge wherein the connection parts have a predetermined breaking point for disconnecting the integration bridge from rotor pole pieces. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     This invention is in the field of a modulated pole electric machine  100  of which one example is shown in  FIG. 1  in a schematic, exploded, perspective view. The modulated pole electric machine stator  10  is basically characterised by the use of a central single winding  20  that will magnetically feed multiple teeth  102  formed by the soft magnetic core structure. The stator core is then formed around the winding  20  while for other common electrical machine structures the winding is formed around the individual tooth core section. Examples of the modulated pole machine topology are sometimes recognised as e.g. Claw-pole-, Crow-feet-, Lundell- or TFM-machines. More particularly the shown modulated pole electric machine  100  comprises two stator core sections  14 ,  16  each including a plurality of teeth  102  and being substantially circular, a coil  20  arranged between the first and second circular stator core sections, and a rotor  30  including a plurality of permanent magnets  22 . Further, the stator core sections  14 ,  16 , the coil  20  and the rotor  30  are encircling a common geometric axis  103 , and the plurality of teeth of the two stator core sections  14 ,  16  are arranged to protrude towards the rotor  30  for forming a closed circuit flux path. The machine in  FIG. 1  is of the radial type as the stator teeth protrudes in a radial direction towards the rotor in this case with the stator surrounding the rotor. However, the stator could equally well be placed interiorly with respect to the rotor which type is also illustrated in some of the following figures. The scope of invention as presented in the following is not restricted to any specific type of modulated pole electric machine and can equally well be applied to machines of both the axial and the radial type and for both interiorly and exteriorly placed stators relative to the rotor. Similarly, the invention is not restricted to single phase machines but can equally well be applied to multi phase machines. 
     The active rotor structure  30  is built up from an even number of segments  22 ,  24  whereas half the numbers of segments also called rotor pole sections  24  are made of soft magnetic material and the other half of number of segments of permanent magnet material  22 . The state of art method is to produce these segments as individual components. Often the number of segments can be rather large typically of order 10-50 individual sections. The permanent magnets  22  are arranged so that the magnetization directions of the permanent magnets are substantially circumferential, i.e. the north and the south pole, respectively, is facing in a substantially circumferential direction. Further, every second permanent magnet  22 , counted circumferentially is arranged having its magnetization direction in the opposite direction in relation to the other permanent magnets. The magnetic functionality of the soft magnetic pole sections  24  in the desired machine structure is fully three dimensional and it is required that the soft magnetic pole section  24  is able to efficiently carry varying magnetic flux with high magnetic permeability in all three space directions. A traditional design using laminated steel sheets will not show the required high permeability in the direction perpendicular to the plane of the steel sheets and its here beneficial to use a soft magnetic structure and material that shows a higher magnetic flux isotropy than a state of art laminated steel sheet structure. 
       FIG. 2  shows the same radial modulated pole electric machine as from  FIG. 1  but in a cross-sectional view of the assembled machine showing more clearly how the stator teeth  102  extend towards the rotor and how the stator teeth of the two stator core sections  14 ,  16  are rotationally displaced in relation to each other. 
     In  FIG. 3   a  the rotor  301  for an electrical machine according to an embodiment of the present invention is illustrated. The rotor comprises a first and a second rotor pole piece structure  303 ,  305  each with half of the rotor pole sections of the rotor, and in slots between neighbouring rotor pole sections from the first and the second rotor pole piece structure, respectively, permanent magnets  307  are positioned. 
     The pole sections of the rotor pole piece structures are connected by an integration bridge. The integration bridge may have the form of a ring-shaped element or in the form of a ring-segment. In this embodiment, each of the rotor pole piece structures connects pole pieces of the same respective polarity. 
     In  FIG. 3   a  each rotor pole piece structure is one integrated element, whereby two structures along with the number of permanent magnets are necessary to form a rotor, but in alternative embodiments each structure could be formed by a number of interconnected rotor pole piece structures. 
       FIG. 3   b  illustrates an embodiment where a number of interconnected rotor pole piece structures are used to form the rotor. Four interconnected rotor piece structures  309 ,  311 ,  313 ,  315  now constitute the rotor pole piece structure  303  illustrated in  FIG. 3   a.    
     In  FIGS. 4   a ,  4   b  and  4   c  the assembly principle of assembling rotor pole piece structures to form a rotor is illustrated. 
     In  FIG. 4   a  the rotor pole piece structure or part of a rotor pole piece structure  401  is illustrated; the rotor pole piece structure comprises pole sections  403 ,  405  connected by an integration bridge  407 . The pole sections of a similar rotor pole piece structure  409  are added by insertion according to the illustrated arrows. In  FIG. 4   b  the pole piece structures have been positioned and fixed, where the distance between the rotor pole sections and the dimension of the rotor pole sections ensure that a slot  411  is present between the pole sections. In  FIG. 4   c  permanent magnets  413  have been slipped into the slots formed between the pole sections. 
     The integration bridge between the pole sections is physically distanced to the pole sections so that leakage in between the pole piece structures of opposite polarity will be minimised; the reduction of effective flux crossing the air-gap and potentially reducing torque of the machine is thereby minimised. In the example of  FIG. 4 , the integration bridge  407  is axially displaced from the rotor pole sections of the other pole piece structure  409 , leaving an air gap  414  preventing leakage. To this end the rotor pole sections  405  comprise an active part  415  that is in direct magnetic connection with one of the permanent magnets  413 , and a connection part  416  that is not directly in contact with the permanent magnet  413  and that is connected to the integration bridge  407 . 
     In  FIG. 5  an embodiment of a rotor pole piece structure with an axially displaced integration bridge is illustrated. In this embodiment, integration bridge  509  is a ring-shaped structure having two lateral, ring-shaped side faces and radially inward and outward circumferential surfaces. The rotor pole sections  503  are connected to and axially extend from one of the lateral side faces. 
     In  FIG. 6  an embodiment of a rotor pole piece structure with a radially displaced integration bridge is illustrated. In this embodiment, integration bridge  609  is a ring-shaped structure having two lateral, ring-shaped side faces and radially inward and outward circumferential surfaces. The rotor pole sections  603  are connected to, and axially extend from one of the circumferential surfaces, in this example from the radially inward surface. The rotor pole sections  603  have an L-shape structure including an axial part  615  that, when assembled in the rotor, are in contact with the permanent magnets, and a connection part  616  radially extending from the axial part and being connected to the integration bridge  609 . It will be appreciated that the connection part  616  may extend from the axial part  615  at an angle different from 90 deg.  FIG. 10  shows an embodiment with a predetermined breaking point for disconnecting the integration bridge from the rotor pole sections. 
       FIG. 7  illustrates another embodiment of an assembly principle of assembling rotor pole piece structures to form a rotor. In this embodiment rotor pole piece structures  701  and  709  are arranged with permanent magnets  713  to form a rotor. The rotor pole piece structures  701  and  709  are similar to the rotor pole piece structures described in connection with  FIGS. 4-6 . However, in this embodiment the axial length of the pole pieces  703  and  705  is shorter than the axial length of the permanent magnets  713  such that the combined length of the rotor pole pieces  705  and  703  is the same or larger than the axial length of the permanent magnets. Furthermore, in the embodiment of  FIG. 7 , the circumferential distance between pole sections of the same rotor pole is substantially equal to the width of the permanent magnets  713 . In the assembled rotor, the rotor pole piece structures  701  and  709  are arranged such that the distal end faces  720  of the pole sections  705  of rotor pole piece structure  701  abut the corresponding end faces of the pole sections  703  of rotor pole piece structure  709  so as to form slots for the permanent magnets  703 . 
       FIGS. 8   a ,  8   b ,  8   c  and  8   d  illustrate yet another embodiment of a rotor.  FIG. 8   a  shows a top view, i.e. seen in radial direction of an embodiment of a rotor pole piece structure, generally designated  801 .  FIG. 8   b  shows a cross sectional view of the rotor pole piece structure  801 ,  FIG. 8   c  shows a top view, seen in the radial direction, of a part of a rotor including four like pole piece structures  801 , and  FIG. 8   d  shows a cross sectional view of the part of the rotor. 
     The rotor of this embodiment is arranged of a number of pole piece structures  801  and a like number of permanent magnets  813 . Each pole piece structure  801  comprises a subsection  802  of the pole section that separates two permanent magnets in the circumferential direction. To this end, the pole piece structure comprises two elongated side parts  802  that extend parallel with each other in axial direction. At their end parts, the side parts  802  are connected to each other by integration bridges  803  leaving a slot  804  between side parts  802  for receiving the permanent magnet  813 . Hence, in this embodiment, the surfaces  820  at which neighbouring pole piece structures abut each other are all parallel with the axial direction. 
       FIGS. 9   a ,  9   b ,  9   c  and  9   d  illustrate yet another embodiment of a rotor.  FIG. 9   a  shows a top view, i.e. seen in radial direction of an embodiment of a rotor pole piece structure, generally designated  901 .  FIG. 9   b  shows a cross sectional view of the rotor pole piece structure  901 ,  FIG. 9   c  shows a top view, seen in the radial direction, of a part of a rotor including four like pole piece structures  901 , and  FIG. 9   d  shows a cross sectional view of the part of the rotor. 
     The rotor of this embodiment is arranged of a number of pole piece structures  901  and a like number of permanent magnets  913  in a similar manner as the rotor of  FIGS. 8   a ,  8   b ,  8   c  and  8   d . As in  FIGS. 8   a ,  8   b ,  8   c  and  8   d , each pole piece structure  901  of the embodiment of  FIGS. 9   a ,  9   b ,  9   c  and  9   d  comprise two elongated side parts  902  having respective inwardly opposing side faces that extend parallel with each other in axial direction. At their end parts, the side parts  902  are connected to each other by integration bridges  903   a  and  903   b  leaving a slot  904  between side parts  902  for receiving the permanent magnet  913 . However, while the pole piece structures  801  of  FIGS. 8   a ,  8   b ,  8   c  and  8   d  have the same width in circumferential direction over their entire axial length, the width of the pole piece structures  901  gradually changes from a wide end  903   b  to a narrow end  903   a , thus resulting in a wedge-shaped structure. As illustrated in  FIGS. 9   c  and  9   d , the wedge-shaped structures  901  are arranged with alternating orientation to form a tubular rotor structure. Hence, in this embodiment, the surfaces  920  at which neighbouring pole piece structures abut are arranged at an angle relative to the axial direction.