Abstract:
A fault tolerant synchronous permanent magnet machine is disclosed that reduces rotor losses by limiting eddy-current losses in the retaining sleeve. The machine limits eddy-current loss by any one or combination of axially segmenting the retaining sleeve, providing a highly electrically conductive non-magnetic shield to the retaining sleeve, and by configuring stator teeth width, stator teeth tip width, and slot distribution of the stator.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to permanent magnet machines and more particularly to a permanent magnet machine with reduced eddy-current losses in the retaining sleeve. 
   BACKGROUND OF THE INVENTION 
   High-speed high-efficiency fault-tolerant surface permanent magnet (PM) machines are used in a variety of applications including automotive and aerospace applications, and have been found particularly effective in safety critical applications such as fuel pumps in aerospace applications. 
   Conventional PM synchronous electric machines employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. Alternatively, the machine may be designed so that the rotor surrounds the stator. For high-speed operation, a retaining sleeve is usually wrapped around the magnets as needed to keep the magnets in place. The retaining sleeve may be shrink fit upon the magnets to ensure a non-slip fit. Usually the retaining sleeve is made of one whole metallic piece for structural integrity. When the coils formed on the stator are energized, a magnetic flux is induced by the current through the coils, creating electromagnetic forces between the stator and the rotor. These electromagnetic forces contain tangential and/or circumferential forces that cause the rotor to rotate. 
   In order to achieve inherent fault-tolerance in these surface PM machines, there has to be complete electromagnetic, thermal, and physical isolation between the coils of the various phases. This is achieved by using fractional-slot concentrated windings where each coil is wound around a single stator tooth and each stator slot is occupied by one side of a coil. Since slots formed between the teeth and the permanent magnets on the rotor are spaced from each other, the magnetic flux passing through a tooth will pass through the neighboring tooth in the next moment as the rotor rotates. 
   This winding configuration generates significant space harmonics when the windings are excited. These harmonics generate asynchronous rotating fields in the air gap that can generate significant losses in the rotor and hence significantly reduce the machine efficiency and exacerbate heat removal. Additionally, because the retaining ring is formed of a single metallic piece for mainly mechanical reasons, it provides an easy path for the eddy currents generated by the asynchronous rotating fields in the air gap generated by the concentrated winding configuration. This significantly aggravates the rotor losses and can reduce the machine efficiency to unacceptable limits and/or cause overheating that may lead to permanent demagnetization of the magnets. These harmonics may also lead to other problems including rotor dynamic imbalance. 
   In order to minimize the sleeve losses and to achieve acceptable efficiency, it has been proposed to use winding configurations such as the distributed lap or distributed wave windings that have less harmonic contents but at the same time have a strong coupling among the different coils and phases. However, these winding configurations result in compromising the fault-tolerance 
   Therefore, a need exists for a fault-tolerant, high-speed, high efficiency PM machine that operates with reduced eddy-current losses and improved operating efficiency. 
   Other features and advantages of the present invention will be apparent from the following more detailed description of a preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate by way of example, the principles of the invention. 
   SUMMARY OF THE INVENTION 
   According to a first embodiment of the invention, a fault-tolerant synchronous permanent magnet machine is disclosed that comprises a stator having an axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said teeth having two side faces terminating at a distal head, each of said distal heads having a head end that provides a plurality of head ends spaced around said interior cylindrical surface, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and disposed between the permanent magnets and the interior cylindrical surface of the stator, and a gap between the retaining sleeve and the interior cylindrical surface of the stator, wherein the retaining sleeve is axially segmented. 
   According to a second embodiment of the invention, a fault-tolerant synchronous permanent magnet machine is disclosed that comprises a stator having an axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said distal heads having a head end that provides a plurality of head ends spaced around said interior cylindrical surface, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and an outer radial surface and disposed between the permanent magnets and the interior cylindrical surface of the stator, a shield disposed upon the outer radial surface of the retaining sleeve, and a gap between the shield and the interior cylindrical surface of the stator. 
   According to a third embodiment of the invention, a fault-tolerant synchronous permanent magnet machine is disclosed that comprises a stator having an axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said distal heads have a head end that provides a plurality of head ends that circumferentially span said interior cylindrical surface and spaced to create slot openings to the slots, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and an outer radial surface and disposed between the permanent magnets and the interior cylindrical surface of the stator, and a gap between the retaining sleeve and the interior cylindrical surface of the stator, wherein the slots between the plurality of teeth of the stator are irregularly circumferentially spaced. 
   According to a fourth embodiment of the invention, a method of reducing eddy-current losses in a permanent magnet machine is disclosed that comprises providing a permanent magnet machine comprising a stator having and axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said teeth having two side faces terminating at a distal head, each of said distal heads having a head end that provides a plurality of head ends spaced around said interior cylindrical surface, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and an outer radial surface and disposed between the permanent magnets and the interior cylindrical surface of the stator, and a gap between the retaining sleeve and the interior cylindrical surface of the stator; and axially segmenting the retaining sleeve. 
   According to a fifth embodiment of the invention, a method of reducing eddy-current losses in a permanent magnet machine is disclosed that comprises providing a permanent magnet machine comprising a stator having and axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said teeth having two side faces terminating at a distal head, each of said distal heads having a head end that provides a plurality of head ends spaced around said interior cylindrical surface, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and an outer radial surface and disposed between the permanent magnets and the interior cylindrical surface of the stator, and a gap between the retaining sleeve and the interior cylindrical surface of the stator; and providing a copper shield, for example, upon the outer radial surface of the retaining sleeve. 
   According to a sixth embodiment of the invention, a method of reducing eddy-current losses in a permanent magnet machine is disclosed that comprises providing a permanent magnet machine comprising a stator having and axial length and a plurality of teeth spaced to define slots therebetween and circumferentially aligned to define an interior cylindrical surface for the stator, each of said teeth having two side faces terminating at a distal head, each of said distal heads have a head end that provides a plurality of head ends that circumferentially span said interior cylindrical surface and spaced to create slot openings to the slots, a rotor having an axial length and comprising a rotor core fixed to a rotatable shaft for rotation therewith and a plurality of permanent magnets mounted on said rotor core at a plurality of locations circumferentially spaced around said rotor core facing the interior cylindrical surface of the stator, a retaining sleeve having an axial length and an outer radial surface and disposed between the permanent magnets and the interior cylindrical surface of the stator, and a gap between the retaining sleeve and the interior cylindrical surface of the stator; and forming the slots between the plurality of teeth of the stator such that that slots are irregularly circumferentially spaced. 
   Further aspects of the method and apparatus are disclosed herein. The features as discussed above as well as other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a prior art PM machine. 
       FIG. 2  is a partial sectional view of an embodiment of the current invention. 
       FIG. 3  is a partial sectional view of a second embodiment of the current invention. 
       FIG. 4  is an illustration of the third embodiment of the current invention 
       FIG. 5  is an illustration of a first variation of the third embodiment of the current invention. 
       FIG. 6  is an illustration of a second variation of the third embodiment of the current invention. 
       FIG. 7  is a partial cross sectional view of the second variation of the third embodiment of the current invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Disclosed herein is a fault-tolerant synchronous permanent magnet machine and method for providing a fault-tolerant synchronous permanent magnet (PM) machine. The disclosed PM machine has reduced rotor loss because of improvements in directing and reducing eddy-currents in the rotor. 
   A prior art PM machine  10  is shown at  FIG. 1 . As can be seen in  FIG. 1 , the prior art PM machine  10  contains a plurality of magnets  20  provided in a radial arrangement upon a back iron  25  that is disposed around a shaft (not shown). The back iron  25  is also known as a yolk. The magnets  20  are surrounded by a retaining sleeve  30 . A stator  40  surrounds the retaining sleeve  30  and is separated from the magnets  20  by a gap  50 . The stator  40  that has of a plurality of radial disposed teeth  60  that form stator slots  70 . The teeth  60  are wound with coils  80  that substantially fill the stator slots  70  as is conventional in the art. 
   Referring now to  FIG. 2 , an embodiment of the invention showing the segmented retaining sleeve  200  is shown. As is shown in  FIG. 2 , a shaft  205  was surrounded by a cylindrical rotor  210 . The rotor  210  had magnets  220  placed thereupon. A segmented retaining sleeve  230  surrounded the magnets  220 . 
   The inventors have found, that by segmenting the retaining sleeve  230 , a greater than 14 percent reduction in rotor loss can be achieved. This improvement results from the effect the segmenting has on the level of electromagnetic coupling that results in reduced eddy-currents. 
   In particular, the inventors have unexpectedly discovered that when the retaining ring is segmented into four or more segments, a significant improvement in eddy-current loss may be realized. For a particular machine, a retaining sleeve was segmented into four sections that resulted in an eddy-current loss reduction of approximately 14%. Another retaining sleeve was segmented into six sections and resulted in an eddy-current loss reduction of approximately 40%. A retaining sleeve segmented into eight sections resulted in an eddy-current loss reduction of 56%. However, segmenting a retaining sleeve into two sections resulted in an increase in eddy-current loss of 8%. 
   In a second embodiment of the invention, the inventors have discovered that placing an electrically conductive, non-magnetic material shield upon the retaining sleeve resulted in a reduction in eddy-current loss. The electrically conductive, non-magnetic material shield may be made of materials, non-limiting examples of which include copper and copper alloys. In a particular embodiment, the shield is made of copper and copper alloys. The shield may be placed upon a segmented or non-segmented retaining sleeve. 
   In this embodiment, as shown in  FIG. 3 , a shaft  305  is surrounded by a cylindrical rotor back iron  310 . The rotor  310  has magnets  320  placed thereupon. A segmented retaining sleeve  330  surrounds the magnets  320 . A copper shield  335  is placed on the segmented retaining sleeve  330 . A copper sleeve of about 2 mm thick resulted in an eddy-current loss reduction of about 75%. 
   In a third embodiment of the invention, the stator structure is modified to reduce eddy-current loss. A standard slot distribution  401  is shown in  FIG. 4 .  FIG. 4  shows a series of stator teeth  402  formed of teeth body sections  400  and teeth head sections  410 . The teeth body sections  400  and teeth head sections are of equal width. The spacing of the stator teeth  402  forms stator slots  420 . The spacing between the stator head sections  410  forms stator slot openings  430 . This stator structure provides for a regular distribution of both stator slots  420  and stator slot openings  430 . 
   In a first variation of this embodiment, an irregular slot opening distribution  501  is shown in  FIG. 5 . In  FIG. 5 , stator teeth  502  are shown with teeth body sections  500  and teeth head sections  510 . The teeth body sections  500  are of equal width, but the teeth head sections  510  are of irregular width. The spacing of the stator teeth  502  forms stator slots  520 . The spacing between the stator head sections  510  forms stator slot openings  530 . This stator structure provides for a regular distribution of stator slots  520  but an irregular distribution of stator openings  530 . The inventors have found that eddy-current loss can be reduced by up to 10% by forming a stator according to this variation. 
   In a second variation of this embodiment, an irregular slot opening distribution and an irregular slot distribution  601 , is shown in  FIG. 6 . In  FIG. 6 , the stator teeth  602  are shown with teeth body sections  600  and teeth head sections  610 . The teeth body sections  600  and the teeth head sections  610  are of unequal width. The spacing of the stator teeth  602  forms stator slots  620 . The spacing between the stator head sections  610  forms stator slot openings  630 . This stator structure provides for an irregular distribution of stator slots  620  and stator openings  630 . The inventors have found that eddy-current loss can be reduced by up to 30% by forming a stator according to this variation. 
     FIG. 7  illustrates the second variation of the third embodiment in a partial cross sectional view of a PM machine  701  having windings A, B and C, with polarity as shown. As shown in  FIG. 7 , the PM machine is formed with stator teeth  702  formed of a stator teeth body sections  700  and a stator teeth head sections  710 . The width of both the stator body sections  700  and the stator head sections  710  is irregular in partially forming the PM machine  701 . This stator structure provides for an irregular distribution of stator slots  720  and stator slot openings  730 .  FIG. 7  illustrates the variation of the third embodiment that provides for a irregular distribution of slots  620 , but an irregular distribution of slot openings  630 . 
   Additionally, the inventors have found that it is possible to combine the embodiments of their discoveries to further decrease eddy-current loss. While the combination of the embodiments may not provide a direct additional savings to eddy-current loss, a combined benefit is still realized. As such, a PM machine may be provided with a shield upon the retaining sleeve that may be segmented or non-segmented. Furthermore, it is possible to combine the variations of stator teeth and slot configurations with the segmented sleeve and shield embodiments to further reduce eddy-current loss. 
   While the invention has been described with reference to preferred embodiments, 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.