Abstract:
An electrical machine apparatus having magnetic gearing embedded therein includes a moveable rotor having a first magnetic field associated therewith, a stator configured with a plurality of stationary stator windings therein, and a magnetic flux modulator interposed between the moveable rotor and the stator windings. The magnetic flux modulator is configured to transmit torque between the first magnetic field associated with said moveable rotor and a second magnetic field excited by the plurality of stationary stator windings.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present disclosure relates generally to electrical machinery such as motors and generators and, more particularly, to an electrical machine apparatus having high torque density magnetic gearing integrated therein.  
         [0002]     Electrical machines (e.g., motors, generators) typically deliver more power at high speeds than at low speeds. In order to adapt a high-speed, rotating electrical machine to a high-torque, lower speed mechanical component (e.g., a prime mover in the case of a generator and a load in the case of a motor), mechanical gear boxes are extensively used as the cost of having a high-speed electrical machine coupled with corresponding mechanical gearing for speed/torque conversion is lower than that for a low-speed electrical machine. As is well known, certain disadvantages are inherent with mechanical gearing such as, for example, acoustic noise, vibration, reliability and the need for lubrication and maintenance, to name a few.  
         [0003]     The concept of magnetic gears has also long been in existence, and can potentially offer significant benefits with respect to their mechanical counterparts, primarily as a result of the lack of physical contact between an input shaft and an output shaft. For the most part, magnetic gears have traditionally received relatively little attention in the industry due to the complexity of the proposed designs, as well as the limited torque density such gears can provide. For instance, a magnetic gear assembly arranged in a spur configuration results in only a small fraction of the permanent magnets located on the gears actually contributing to torque conversion at any given time.  
         [0004]     More recently, however, a planetary-like magnetic gear arrangement using rare-earth permanent magnets has been proposed, which results in a favorable torque transmission density capability between an outer rotor and an inner rotor. In such a configuration, each of the permanent magnets affixed to the outer rotor and the inner rotor contribute to torque transmission. In view of the existence of a magnetic gearing capability providing comparable transmitted torque density with respect to conventional mechanical gearbox arrangements, it is therefore desirable to be able to provide more functionally integrated electromechanical energy conversion systems that take advantage of this type of non-contact gearing arrangement.  
       BRIEF DESCRIPTION OF THE INVENTION  
       [0005]     The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by an electrical machine apparatus having magnetic gearing embedded therein. In an exemplary embodiment, the machine includes a moveable rotor having a first magnetic field associated therewith, a stator configured with a plurality of stationary stator windings therein, and a magnetic flux modulator interposed between the moveable rotor and the stator windings. The magnetic flux modulator is configured to transmit torque between the first magnetic field associated with said moveable rotor and a second magnetic field excited by the plurality of stationary stator windings.  
         [0006]     In another embodiment, a wind turbine generator system includes a wind driven turbine, a generator coupled to the turbine, and a tower connected to the generator. The generator further includes a permanent magnet rotor coupled to the turbine, the rotor having a first magnetic field associated therewith, a stator configured with a plurality of stationary stator windings therein, and a magnetic flux modulator interposed between the moveable rotor and said stator windings. The magnetic flux modulator is configured to transmit torque between the first magnetic field associated with the moveable rotor and a second magnetic field excited by the plurality of stationary stator windings.  
         [0007]     In another embodiment, a ship propulsion system includes a propulsion motor configured to rotate a shaft and a propeller coupled to the shaft. The propulsion motor further includes a permanent magnet rotor coupled to the shaft, the rotor having a first magnetic field associated therewith, a stator configured with a plurality of stationary stator windings therein, and a magnetic flux modulator interposed between the moveable rotor and the stator windings. The magnetic flux modulator is configured to transmit torque between the first magnetic field associated with the moveable rotor and a second magnetic field excited by the plurality of stationary stator windings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:  
         [0009]      FIG. 1  is a schematic block diagram of a conventional electromechanical energy conversion system employing a mechanical gearbox;  
         [0010]      FIG. 2  is a cross sectional view of a recently proposed magnetic gear of a planetary type configuration;  
         [0011]      FIG. 3  is a cross sectional view of a previously proposed electromechanical energy conversion system employing a magnetic gear of the type shown in  FIG. 2 ;  
         [0012]      FIG. 4  is a cross sectional view of an electrical machine apparatus having high torque density magnetic gearing integrated therein, in accordance with an embodiment of the invention;  
         [0013]      FIG. 5  is a side cross sectional view of the electrical machine apparatus of  FIG. 4 ;  
         [0014]      FIG. 6  is side cross sectional view of an alternative embodiment of the electrical machine apparatus of  FIG. 5 ;  
         [0015]      FIG. 7  is side cross sectional view of an alternative embodiment of the electrical machine apparatus of  FIGS. 4 and 5 ;  
         [0016]      FIG. 8  is a side view of a linear electrical machine apparatus having high torque density magnetic gearing integrated therein, in accordance with still another embodiment of the invention;  
         [0017]      FIG. 9  is a side cross sectional view of a wind turbine generator system utilizing high torque density magnetic gearing integrated therein, in accordance with another embodiment of the invention; and  
         [0018]      FIG. 10  is a side cross sectional view of a ship propulsion system and motor utilizing high torque density magnetic gearing integrated therein, in accordance with another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]     Disclosed herein a novel electrical machine apparatus having high torque density magnetic gearing integrated therein. As opposed to previously proposed electrical machinery with integrated magnetic gearing therein, the present embodiments provide an even further level of integration by eliminating a second moving part (i.e., rotor), and instead replaces the same with stator armature windings. In a generator configuration, the stator armature windings are excited by electromotive force or, alternatively in a motor configuration, through an external voltage. Consequently, the present design provides the advantages mentioned above with respect to mechanical gearboxes (e.g., contact-less operation, higher reliability, lower maintenance), as well as reduction in mass due to the elimination of a second rotor (or more generally, the elimination of a second moving component).  
         [0020]     Referring initially to  FIG. 1 , a schematic block diagram of a conventional electromechanical energy conversion system  100  employing a mechanical gearbox  102  is illustrated. A mechanical load/prime mover  104  is coupled to a low speed rotor shaft  106 , which is in turn coupled to the mechanical gearbox  102 . In a generator configuration (mechanical to electrical energy conversion), the rotational speed of rotor shaft  106  driven by the prime mover  104  is converted to a high speed rotation of high speed rotor shaft  108  by the mechanical gearbox  102 , given by the gear ratio 1:X of the gearbox  102 . The high speed rotor shaft  108  then drives the rotor of the generator  110 . By way of example, if the low speed shaft  106  turns at 60 rpm and the gear ratio of the gearbox  102  is 1:30, then the high speed shaft  108  is driven at 1800 rpm.  
         [0021]     Conversely, in a motor configuration (electrical to mechanical energy conversion), the motor  110  is powered by an electrical power source (not shown) to turn the rotor at a high speed (e.g., 1800 rpm). The gearbox  102  converts the high speed rotation of shaft  108  to a low speed rotation of shaft  106  (e.g., 60 rpm) to drive the mechanical load  104 . In either configuration, the gearbox  102  is subject to noise, vibration, and the need for maintenance as outlined above.  
         [0022]      FIG. 2  is a cross sectional view of a recently proposed magnetic gear  200  of a planetary type configuration. A high speed rotor shaft  202  of reduced diameter is supported within and coaxial with a cylindrical low speed rotor shaft  204  of larger diameter. The low speed rotor shaft  204  has a relatively large number (P ls ) of permanent magnet pole-pairs  206  formed on the inner surface thereof. Individual permanent magnets of the pole-pairs  206  are oriented such that the north and south poles are aligned perpendicularly to a common axis of rotation  208  with respect to the high speed rotor shaft  202 . The orientation of the magnets of the pole-pairs  206  alternates, wherein one magnet of the pair has its north pole directed towards the common axis  208  and the adjacent magnet has its south pole directed towards the common axis  208 .  
         [0023]     The high speed rotor shaft  202  has a smaller number (P hs ) of permanent magnetic pole-pairs  210  attached to the outer surface thereof. As with the pole-pairs  206  of the low speed rotor shaft  204 , the magnets of the pole-pairs  210  are oriented so that the north and south poles of flux are aligned perpendicularly to the common axis of rotation  208 . A relatively large number (N s ) of stationary soft iron pole-pieces  212  are located between the exterior of the high speed rotor shaft  202  and the interior of the low speed rotor shaft  204  between the magnets of the pole pairs  206  and  210 . The stationary pole-pieces  212  are located at a fixed distance from (are evenly distributed about) the common axis  208 .  
         [0024]     The magnetic gear  200  operates by the locking of one shaft&#39;s magnetic field onto a space harmonic of the magnetic field created by the other shaft, facilitated by modulation of the fields by the stationary pole-pieces  212 . The gear ratio, G, is given in the simplest case by G=P ls ÷P hs  when N s =P ls +/−P hs . In the example depicted, there are 4 pole pairs  210  on the high speed shaft  202  and 22 pole pairs  206  on the low speed shaft  204 . Accordingly, the low speed rotor shaft  204 , when driven at a low speed causes the high speed rotor shaft  202  to rotate at a high speed, thereby transmitting torque from one shaft to the other at a fixed gear ratio of 22÷4=5.5:1.  
         [0025]     In view of the magnetic gear design of  FIG. 2 , various drive systems have been proposed to incorporate magnetic gearing with high torque transmission density. For instance,  FIG. 3  is a cross sectional view of a previously proposed electromechanical energy conversion system  300  employing a magnetic gear of the type shown in  FIG. 2 . As is shown, a low speed shaft  302  has a plurality of magnetic pole pairs  304  on an inner surface thereof, while a high speed shaft  306  has a plurality of magnetic pole pairs  308  on an outer surface thereof. The magnetic gearing is facilitated through stationary iron pole pieces  310  as described above. Where the system  300  is configured in a generator mode, the low speed shaft  302  is driven by a prime mover (not shown) and the magnetic gearing converts a low rotational speed of the low speed shaft  302  to a high speed rotation of the high speed shaft  306 .  
         [0026]     In addition, an electric machine  312  includes a housing  314  that receives the high speed shaft  306  therein, supported by bearings  316 . A rotor  318  is rotated by the high speed shaft  306  in a generator mode, the rotor  318  including a plurality of magnetic pole pairs  320  disposed on an outer surface thereof. The rotating magnetic fields generated by the high speed rotor  318  induce a voltage in the stator coils of stator  322 . Conversely, where electric machine  312  is configured as a motor, an electrical power source (not shown) coupled to the stator windings causes rotation of the rotor  318  and high speed shaft  306 . The magnetic gearing translates the high speed rotation to a low speed rotation of low speed shaft  302  coupled to a motor load (not shown).  
         [0027]     An additional measure of integration for system  300  has also been implemented, by which the high speed shaft is eliminated. Instead, a high speed rotor is directly magnetically geared to a low speed rotor as described above. The electric machine housing (such as  314  in  FIG. 3 ) is eliminated, and the stator windings of the machine are incorporated into the confines of the low speed rotor (shaft). However, even with this additional integration, such an arrangement still provides for two moving components (i.e., a low speed rotor and a high speed rotor).  
         [0028]     Therefore, in accordance with an embodiment of the invention,  FIG. 4  is cross sectional view of an electrical machine apparatus  400  having high torque density magnetic gearing integrated therein, in accordance with an embodiment of the invention. As is shown, the apparatus  400  includes a moveable rotor  402  and a stator  404 . In the embodiment depicted, the rotor  402  is outwardly disposed with respect to the stator  404 , and thus has a plurality of permanent magnets  406  of alternating orientation formed on an inner surface thereof. However, whereas the previously discussed magnetic gearing arrangements provide for a second rotor with permanent magnets, the present arrangement replaces the second rotor with stationary stator windings  408 . In addition, a plurality of stationary iron pole-pieces  410  are disposed within the air gap  412  present between the rotor magnets  406  and the stator windings  408 .  
         [0029]     Depending upon the machine requirements, the pole-pieces  410  may be mounted to the stator frame  404  (e.g., by stamping them from the same lamination sheet as the stator material) or may be separately mounted. In addition, an air gap  414  may be present between the stator frame  404  and the pole-pieces  410  (as shown in  FIG. 5 ) or, alternatively, a non-magnetic material  416  may be inserted between the stator frame  404  and the pole-pieces  410  (as shown in  FIG. 6 ).  
         [0030]     In any case, the stationary pole-pieces  410  facilitate torque transmission between the magnetic field excited by the permanent magnet rotor  402  and the magnetic field excited by the stationary windings  408 . In the specific example of  FIG. 4 , the machine  400  includes are 33 permanent magnet pole pairs on the rotor  402 , 4 winding pole pairs, and 37 pole-pieces  410 . Thus, the “gear” ratio from the low speed side (rotor  402  in this case) to the high speed side (stator  404  in this case) is 1:8.25. Advantageously, the torque density provided by the present configuration allows for a significant reduction in machine size, resulting in a cost and mass savings.  
         [0031]     As indicated above, an outer rotor/inner stator is one possible configuration for the electrical machine apparatus with integrated magnetic gearing. On the other hand,  FIG. 7  is side cross sectional view of an alternative embodiment of an electrical machine apparatus  700  having an inner permanent magnet rotor  702  and an outer stator  704 . In this example, the permanent magnets  706  are formed on an outer surface of the rotor  702 . Again, in the integrated magnetic gearing arrangement, a second rotor is instead replaced with stationary stator windings  708 , with a plurality of stationary iron pole-pieces  710  disposed between the rotor magnets  706  and the stator windings  708 . As with the earlier embodiments, there may be an air gap  712  between pole-pieces  710  and the stator  704  (as shown in  FIG. 7 ), a non-magnetic material (not shown) disposed therebetween, or the pole-pieces  710  can be directly attached to the stator  704 . The embodiment of  FIG. 7  may be used, for example, in high shaft speed applications where the number of rotor pole pairs is less than the number of stator pole pairs.  
         [0032]     In addition to rotating machines, it will further be appreciated that the presently disclosed magnetic gearing concept may also be applied to the area of linear electric machines (i.e., motors or generators). Linear generators have been proposed as suitable energy conversion devices for ocean wave energy plants, linear motors for electromagnetic valves for internal combustion engines and compressor valves, or for general high force density transportation purposes, such as machine tools for example. As opposed to a rotor that spins about an axis of rotation, the rotor of a linear electric machine moves laterally back and forth around a center of rotation at an infinite distance. The electromagnetic flux in the air gap of a linear machine is the same as for rotational machinery.  FIG. 8  is a side view of a linear electrical machine apparatus  800  having high torque density magnetic gearing integrated therein, in accordance with still another embodiment of the invention.  
         [0033]     As in the case of a rotary machine, the linear electrical machine  800  includes a linearly movable rotor  802  and a stator  804 . In this embodiment, the permanent magnets  806  are formed on the inner surface of the rotor  802  facing the stator  804 . Again, in the integrated magnetic gearing arrangement, a second rotor is instead replaced with stationary three-phase stator windings  808 , and a plurality of stationary iron pole-pieces  810  disposed between the rotor magnets  806  and the stator windings  808  serving as a flux modulator. The linear machine  800  may include an air gap  812  between pole-pieces  810  and the stator  804  (as shown in  FIG. 8 ), a non-magnetic material (not shown) disposed therebetween, or the pole-pieces  810  can also be directly attached to the stator  704 .  
         [0034]     In the various embodiments depicted above, the rotors of the electrical machines are implemented with permanent magnet rotors. However, it is also contemplated that the integrated magnetic gearing may also be accomplished through the use of rotors having wound field, squirrel cage, or switched reluctance poles. In other words, the rotor&#39;s magnetic field may be implemented through DC powered electromagnets, in lieu of permanent magnets. Furthermore, with regard to the stationary pole-pieces that serve as flux modulation devices, the shape of such pieces may be embodied by other insert shapes in addition to square inserts, such as oval or trapezoidal shapes for example.  
         [0035]     Although the winding configurations specifically illustrated in  FIGS. 4 and 8  depict three-phase windings, it should also be understood that a different number of phases may be used as well.  
         [0036]     Finally,  FIGS. 9 and 10  illustrate exemplary applications for one or more of the electrical machine embodiments described herein. For instance,  FIG. 9  is a side cross sectional view of a wind turbine generator system  900  utilizing high torque density magnetic gearing integrated therein, in accordance with another embodiment of the invention. The system  900  includes a wind driven turbine  902 , wind turbine generator  904 , and tower  906 . As is known in the art, the blades of the turbine  902  are attached to a main shaft and bearing assembly  908  through a rotor blade hub  910 . A main frame  911  of nacelle  912  is attached to the main shaft assembly  908  and sits atop the tower  906 . A nacelle cover  914  protects the components inside the nacelle  912 , while a rotor hub cover  916  protects the rotor blade hub  910 .  
         [0037]     Unlike a conventional wind turbine generator system with mechanical gearing, the turbine generator  904  incorporates the above described magnetic gearing by providing the flux modulating stationary pole pieces  918  between the permanent magnet rotor  920  and the stator windings  922 . In an exemplary embodiment, the generator  904  includes 88 rotor pole pairs, 8 stator pole pairs and 96 iron pole-pieces  918 , yielding an 11:1 gear ratio. Other gear ratios, however, are also contemplated.  
         [0038]     Cooling of the turbine generator  904  may be accomplished by circulation of air  923  through holes  924  within the stator frame  926 , passing through the air gap between the permanent magnets  928  of the rotor  920  and the stator windings  922 , and out through holes  930  in the rotor  920 . The cooling air path could also be run in the reverse direction.  
         [0039]     As opposed to other wind turbine systems with direct drive generators operating at low speeds, the integrated turbine generator  904  is implementable at a significantly reduced diameter (and therefore reduced mass) to lower costs. For example, existing wind turbine generators operating at around the 4.5 MW range are about 10 meters in diameter. Such a size presents formidable transportation and assembly challenges, both at the factories and the turbine site. In order to enable cost-effective, land-based transportation, the generators should be preferably limited to about 4 meters or less in diameter, or be modular in construction. Alternative cooling means commonly used by conventional electric machines are also applicable to the disclosed embodiments.  
         [0040]     Referring to  FIG. 10 , a motor application for the integrated magnetic gearing machine is illustrated. In particular,  FIG. 10  illustrates a ship propulsion system  1000  including an inboard propulsion motor  1002  used to drive an outboard propeller  1004  through rotation of a shaft  1006  secured by a mounting and bearing assembly  1008 . Again, the propulsion motor  1002  provides direct drive at a reduced size through the use of the high torque transmission density pole pieces  1010  positioned between the permanent magnets  1012  of the rotor  1014  and the windings  1016  of the stator  1018 . Although not specifically shown in  FIG. 10 , the propulsion motor  1002  may utilize a cooling configuration similar to that shown in  FIG. 9 .  
         [0041]     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.