Abstract:
An axial gap electric dynamo machine has a horizontally disposed rotor disk that is stabilized at its periphery by a plurality of permanent magnets connected to a ferromagnetic bearing plate that provides an opposing or repulsive force against the rotor magnets. In some preferred embodiments, the bearing plate magnets are configured in a dual band to further enhance the magnetic field that supports the periphery of the spinning rotor.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to the U.S. Provisional patent application of filed on Feb. 10, 2008, having application Ser. No. 61/027,465, which is incorporated herein by reference. 
    
    
     BACKGROUND OF INVENTION 
     The present invention relates to axial gap dynamo electric machines and more particularly, in improvements of the bearings thereof. 
     Axial gap dynamo electric machines deploy stators and rotators that are generally in the shape of parallel and adjacent planar discs, with one of more rotators attached to an axle that passes though the center of each disk. 
     The stators comprises multiple windings that generally wrap across the radial direction of the disc. A Lorenz force is generated by the interaction with magnets arranged along the periphery of the rotor disc. A more detailed description of this technology can be found in the U.S. Pat. Nos. 4,567,391; 4,578, 610; 5,982,069; and 5,744,896, all of which are incorporated herein by reference. 
     Axial gap EDM&#39;s are ideally suitable for Vertical Axis Wind Turbine (VAWT) designs. VAWT offers a number of advantages over conventional Horizontal Axis Wind Turbine (HAWT), such as lower maintenance costs and increased durability and reliability. VAWT installations are believed to present a significantly lower hazard to migrating birds as HAWT systems, as well as require a lower cost and less obtrusive support tower due the axial symmetry of the generator and turbine blades. One such VWAT is disclosed in U.S. Pat. No. 5,531,567, which is incorporated herein by reference. A magnetically levitated VWAT is disclosed in U.S. Pat. No. 7,303,369, which is also incorporated herein by reference. 
     While VAWT systems are also more economically viable in remote locations than 100+ kW HAWT systems, there is an ongoing need to improve the efficiency of such machines as well as lower their capital cost so reduce the cost of electrical power derived from this renewable energy resource, and make small to medium size facilities more economically viable for say small communities or even the individual homeowner. 
     Accordingly, it is a general object of the invention to improve the quality and economic viability of large scale axial gap electro-dynamo machines (EDM) for use as generators and motors. 
     It is a more specific object of the invention to provide an axial gap EDM with efficient magnetic bearings to minimize the need for bearing replacement. 
     It is a further objective of the invention to provide the above benefits in a cost effective manner and not unduly complicate or compromise the overall design and function of the EDM. 
     SUMMARY OF INVENTION 
     In the present invention, the first object is achieved by providing an axial gap dynamo electric machine, the machine comprising: an axle, a stator disk having at least one electrically energizable planar coil array for generating a Lorenz force disposed co-axially about said axle, a rotor disk in rotary co-axle connection to said axle and having at the periphery thereof an array of permanent magnets with each magnetic having an alternating orientation of the poles with respect to the adjacent magnets in the array, a bearing plate disposed immediately below and coplanar with said rotor disk, the bearing plate having two or more circular array of permanent magnets wherein the circular array of the bearing plate coincides with the circular array of permanent magnets about the periphery of said rotor disk to levitate said rotor disk on said axle. 
     A second aspect of the invention is characterized by an axial gap dynamo electric machine, the machine comprising: an axle, a stator disk having at least one electrically energizable planar coil array for generating a Lorenz force disposed co-axially about said axle, a rotor disk in rotary co-axle connection to said axle and having at the periphery thereof an array of permanent magnets with each magnetic having an alternating orientation of the poles with respect to the adjacent magnets in the array, a bearing plate disposed immediately below and coplanar with said rotor disk, the bearing plate having two or more circular array of permanent magnets wherein the circular array of the bearing plate coincides with the circular array of permanent magnets about the periphery of said rotor disk wherein the magnets of the circular arrays of the bearing disk are oriented with the opposite polarity. 
     The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a first embodiment of the invention. 
         FIG. 2  is a plan view showing an embodiment of a rotor structure of  FIG. 1 . 
         FIG. 3  is a plan view showing an embodiment of the magnetic bearing plate of  FIG. 1 . 
         FIG. 4  is a cross-sectional view of another embodiment of the invention. 
         FIG. 5A  is a plan view showing an alternative embodiment of the rotor structure of  FIG. 1  or  FIG. 4 , whereas  FIG. 5B  is a enlarged view of a portion thereof. 
         FIG. 6  is a cut away perspective view of another alternative embodiment of the invention. 
         FIG. 7A-D  illustrate in cross-sectional elevation another alternative embodiment for the placements of the magnetic with respect to the bearing plates as well as the result of FEM calculation of magnetic field lines. 
         FIG. 8  is a graph showing the repulsive force generated by the alternative placements of the magnets with respect to the bearing plates for  FIG. 7A-C  as a function of the magnet strength. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIGS. 1 through 8 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved Axial gap dynamo electric machine with magnetic bearing, generally denominated  100  herein. 
     The coordinate system for  FIG. 1 through 8  is non-orthogonal and circular, with the x-direction being the long axis of rotor axle  110 , r-direction being the radial direction of the stator disk  120  and rotor disk  130 , and the t-direction being tangential to the stator disk  120  and rotor disk  130 . 
       FIG. 1  illustrates that primary components of the axial gap EDM  100 , which has an axle  110  coupled to at least one rotor disk  130 . The rotor disk  130  has at least one, but preferably two rows  131  and  132  of permanent magnets  135  disposed at the periphery. The rotor disk  130  is connected in rotary engagement to axle  110 . Each of the permanent magnets  131  is disposed with an alternating orientation of its poles with respect to the adjacent magnets in the row it will be appreciated by one familiar with the construction of motors that the stator disk  120  is generated supported or attached to the motor housing and the axle  110  is confined for free rotation of the axle by rotary type bearings that are also attached or coupled to the motor housing. As the motor housing is generally conventional in the art, it is omitted from the Figures for simplicity of illustration. Preferably, at least one central bearing member  136  supports at least one of the axle  110  or the rotator  130  to maintain the planar orientation of the stator disk  120  and the rotor disk  130 . 
     The size, location and configuration of the bearing supports for the axial and attached rotor will depend on the number or rotors and stators, as well as the diameter of the axle  110 . However, to the extent that the rotor disk  130  has a large diameter and is heavy it is advantageous to provide a bearing support distal from axle  110 , and thus minimize the requirements for central bearings like  136 , and the load thereon. Thus, below rotor disk  130  is a bearing plate  140  having at least one, but preferably two circular rows of magnets  145 . As the speed and centrifugal forces are greatest at the outer extent of the rotor  130 , proximal to the circular rows of magnets  145 , the levitation of the rotor plate  140  is frictionless, other than air resistance, and minimizes the load on the axle and central bearings member(s)  136 . 
     Preferably, the bearing plate  140  is disk shaped and is disposed below rotor plate  130 . It should further be appreciated that the disk shaped bearing plate  140  is preferably a ferromagnetic material, such as iron or steel. The magnets  145  of the bearing plate are preferably arranged in a dual circular track near the periphery of the bearing plate  140 , having an outer row  141  and an inner row  142  of magnets  145 . The individual magnets that comprise each ring or row  141  and  145  are oriented such that the poles are opposed to that of the magnet in the adjacent ring. 
     The magnets  141  and  142  of the bearing plate  140  can be tangential arc segments attached to the surface of the bearing plate  140 , as shown in  FIG. 3A . 
     The magnets  135  of rotor  130  can also be tangential arc segments attached to the surface of the rotor, as shown in  FIG. 6 , but are more preferably at least partially embedded in the rotor plate  140  (as shown in  FIGS. 1 and 4 ) for improved mechanical stability and to reduce the gap between the magnets of the rotor tracks or rings  131  and  132  and the magnetic bearing tracks or rings  141  and  142 . Ideally, when the bearing plate  140  and at least one rotor plate  130  are adjacent the gap between there associated magnets is less than the gap between the other portions of the bearing plate  140  and rotor disk  130  that support these magnets. When embedded in the rotor  130 , magnets  135  can be round as shown in  FIG. 2 , but more preferably are trapezoidal shaped with round edges as shown in  FIG. 5   
     Accordingly, as shown in  FIG. 7D , it is also preferred that polarity of the pairs of magnets arranged about the periphery of the stationery bearing plate  140  alternate. That is, in outer rings  132  (on the rotor  130 ) and  142  (on the bearing plate  140 ) the magnets  135  and  145  respectively are arranged to have their N poles face each other across the gap between the inner rings  131  and  142 , while the S poles should face each other across the gap between the rotor plates  130  and the bearing plate  140 . It has been discovered that this arrangement results in an increased levitation force compared to arranging the magnetic in each track with the same polarity. 
     The resulting magnetic levitation of the rotor  130 , using passive magnets  145  has several advantages. The system is generally mechanically stabilized with a bushing type bearing  136  on axle  1   10 . This stabilization can be accomplished in a number of ways including ball and roller bearings. This general concept can be applied to large diameter motors and alternators where the magnetic bearing system disclosed herein can be substituted for expensive and speed limiting mechanical bearing systems. Ball bearings have upper speed limits that go down as the diameter of the bearings increase, with the expense of the bearings increases roughly as the square of the radius. In addition to cost advantages, the magnetic bearing system disclosed herein has very low friction. It is expected that the disclosed magnetic bearing system will have a practically infinite life due to no direct contact and mechanic wear. 
     Further, although a single bearing  136  may still be required, it will be easier to replace during maintenance. 
     The magnets  145  that are supporting the rotor  130  are also the motor magnets, as shown in the configuration of  FIG. 4 , wherein the stator  120  is now disposed between the rotor and the bearing plate  140 . Thus in a sense, magnets  145  are doing double duty, and hence reducing the cost and weight of the device  100 . It should be appreciated that various other patterns of magnets can be used. 
     Another preferred approach is to have the ends of rectangular, trapezoidal or arc segment magnets  135  be extended inward or outward depending on the polarity, so the ends of the magnets are the supporting element, while the middle or main body of the magnet is active as the motor or alternator. As shown in the rotor  130  in  FIG. 5 , narrower edge of the trapezoidal magnet is oriented toward the center of the disk or axle, increasing the packing density. These magnets  135  are longer in the radial direction so that they can be arranged in partially overlapping rows  131  and  132 . The polarity is the same in each row, that is the outer row  132  has the N pole up, while the inner row  131  has the S pole up. This results in the overlapping portions of the rows (region  133 ) providing adjacent magnets of opposite polarity (i.e. N-S-N-S if straddling the middle point of the rows) for interaction with the stator coils. Portions of each magnet also extend beyond the magnet of opposite polarity in the adjacent row, that is the magnets in row  132  have a portion  132   a  extending in the outward radial direction beyond row  131 , while the magnets in row  131  have a portion  131   b  extending in the inward radial direction, beyond the magnet of row  132 . The outward extending portions  132   a  and inward extending portions  131   b  are repelled by the magnets of row  142  and  141  respectively of magnetic bearing  140  to provide the magnetic bearing function. Thus the motor stator windings  120  will react with the magnets in portion  133  where they overlap, while the bearing magnets  141 / 142  interact with the ends  131   b / 132   a  of the rotor  130  magnets where they do not overlap. 
     As shown in  FIG. 4 , the permanent magnets  135  are also preferably at least partially embedded in the rotor disk  130 . Further, it is also preferable that the magnets  145  of the bearing disk  140  are slightly embedded therein. Accordingly, annular grooves  146  are provided in bearing plate  140  to at least partially embed the magnets  145 . 
     It has been discovered and established by the Finite Element Modeling (FEM) that the groove design and placement of the magnets  145  can greatly increase the net repulsive force that levitates the rotor disk  130  and any other structure attached to the axle  110 , such as turbine blades of a generator or a windmill. The results of deploying single and double track of magnets  145  on the bearing plate  140  using various groove configuration is shown in  FIG. 7A-C  in which the geometric design of the magnets and groove is shown in heavy black lines superimposed over the calculated magnetic field lines. The resultant net repulsive force in N/m for these configurations are plotted in  FIG. 8  as a function of magnet field strength in Tesla. The magnets  135  and  145  are preferably of a Nb—Fe—B alloy to provide high field strength. The most efficient design is shown in  FIG. 7C  in which the bearing plate  140  has a stepped groove pattern  147  with a bottom groove  149  that fits the outer dimension of the magnetic and a upper surrounding groove  148  that is wider than bottom groove in which the bottom groove is disposed within the upper surrounding groove. 
     The FEM calculations where performed on a rotor plate  140  wherein the groove  146  for holding the outer rows  142  of bearing magnets had an outer diameter of about 0.82 m and the groove for inner row  141  had an inner diameter of about 0.70 m, for a total track length of 4.8 m.  FIG. 7A  shows the geometry and resulting magnetic field lines when a single magnet track  141  and  131  is used in the bearing plate  140  and rotor  130 . Hence these magnets fit in the groove with a width of 120 mm, wherein each magnet has a height 30 mm and for a total mid-point circumference of 4.8 m. The depth of the groove for holding the magnets in  FIGS. 7A and 7B  is only several mm. In all cases ( FIG. 7A-C ) the air gap between the face of the magnets  135  and  145  is 3 mm. 
     In  FIG. 7B  the same calculations are performed but the single magnets of the same total volume is split into two dual rails or tracks in which the magnets in each of the dual tracks or rings have a height 30 mm and a width 60 mm. 
     In  FIG. 7C , the magnets  145  on the rotor disk  140  are now disposed in a stepped groove  147  with the bottom step  149  that secure the magnets have a depth of about several mm, but potentially up to about a third of the magnet height. The second step  148 , on both side of the lower bottom step  149  has a height that extends from just below the top of the magnet to the edge of the bottom step  149 . It is desirable that the width of the bottom step is at least greater than the gap between the rotor  130  and the bearing plate  140 , with the top of the bearing plate magnets  145  being at or slighting above the top of the bearing plate  140 . 
     As shown in  FIG. 8 , the force (in N/m), normalized to a 1 m length of track, is plotted as a function of the magnet strength, Br, in Tesla. Thus, for the same mass of magnetic material, that is splitting the wider magnet of  FIG. 7A  into the dual track configuration of  FIG. 7B , produces a surprising large increase of repulsive force of 187%. The placement of these magnets into the steps as shown in  FIG. 7C  results in an every greater increase of 283% with respect to the configuration of the single track of  FIG. 7A , as well as impressive increase of 33% over the double track arrangement in  FIG. 7B . It should be appreciated that the placement of magnet  145  is a single stepped groove  147  is also expected to improve the performance of a single track bearing magnet of  FIG. 7A . 
     Given the relatively high cost of magnets, which is largely proportional to weight, the added machining and assembly costs to create the groove or steeped groove is insignificant, as the % increase in force translated almost directly to a cost savings. Further improvements in efficiency can also be expected with 3 or more row or tracks of bearing magnets  145 . However, absent the discovery herein of the much higher efficiency gained by splitting the magnets into dual tracks, one of ordinary skill in the art would not be motivated to do so, as creating two tracks versus one wider track requires more manufacturing and assembly steps which would increase the manufacturing cost. 
     In should now be appreciated that the embodiments of the invention disclosed herein permits the construction of large scale axial gap electric dynamo machines for use as generators and motors, in particular for wind power generation of electricity. Such magnetic bearings permit all types of axial gap dynamoelectric machine to operate at higher speeds with improved reliability and lifetime, while minimizing equipment maintenance. However, the higher efficiently of the magnetic bearing system described herein it is anticipated that the various embodiment need not be limited to EDM&#39;s, but may be used in other rotary devices that operate at low or high speed and require ultra low friction for reliability or the mechanical stability of the bearing system described herein. 
     While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.