Abstract:
A rotor for a brushless permanent magnet generator/motor comprises a retention slot extending into a rotor flange for receiving a root of a permanent magnet. The retention slot comprises a base extending axially into the rotor flange, a pair of side walls extending radially from the base, and a pair of lugs projecting from the side wall to engage the root to provide radial and tangential retention of the permanent magnet. In other embodiments, the permanent magnet is further restrained in the axial direction by a spring pre-loaded axial retention ring.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to rotors for permanent magnet (PM) motors and generators. More particularly, the present invention relates to retention systems for rotor magnets in brushless PM motors and generators. 
         [0002]    Brushless PM motors convert electrical energy to kinetic energy by exploiting the electromagnetic relationship between a magnet and an electric field. Conversely, brushless PM generators use electromagnetic relationships to convert kinetic energy to electrical energy. In a typical brushless PM motor, electric current is passed through stationary windings of conductive wires to generate an alternating magnetic field to push and/or pull a magnetic rotor. The magnetic rotor is coupled to a shaft to produce rotational shaft power. In a typical brushless PM generator, a mechanically rotating shaft rotates a magnetic rotor to push electrical current through a stationary coil. The electrical current is then available to provide electric power. Thus, brushless PM motors and generators comprise two main concentrically aligned components: a stator, comprising wire windings, and a rotor, comprising permanent magnets. Brushless PM motors and generators can be configured in a conventional design, with the stator surrounding the rotor, or in an inside out design, with the rotor surrounding the stator. In either case, the rotor is subjected to extremely high rotational speeds, which places significant mechanical loading on the magnets. 
         [0003]    A rotor of a brushless PM motor or generator must meet multiple requirements in order to efficiently convert electromagnetic power to or from rotational shaft power. First, the rotor must include magnets that are able to convert electromagnetic force to or from mechanical force. Second, the magnets need to be magnetically coupled in order to produce a magnetic flux path between adjacent magnets. Third, the magnets must be connected to a shaft in such a manner to transmit the torque necessary for inputting or outputting the mechanical power. 
         [0004]    For both conventional and inside out brushless PM motor and generator designs, various prior art systems for retaining the magnets with respect to the rotor have been developed. For example, in inside out brushless PM motor and generator designs, the rotor comprises a disk having a central bore for receiving a shaft and an outer diameter flange for receiving the permanent magnets. The permanent magnets are circumferentially arranged around the inner diameter face of the flange such that they will face the wire windings when coupled with the stator. Conventional methods for securing magnets to rotors have relied upon adhesives that immobilize the magnets on the outer flange. Adhesive provides a strong bond that also permits magnetic flux between the magnets. However, adhesive rigidly bonds each magnet to the flange, thus subjecting the magnet to the strain imparted to the rotor during high-speed rotational operation. Thus, the magnets become load-bearing members subject to centrifugal stresses that potentially exceed their stress limitations. Additionally, the bonded magnets become permanently attached to the disk, making it difficult or otherwise infeasible to repair or replace them, wholly or individually, or the rotor disk. Adhesive is also susceptible to failure due to extreme temperatures, aging and chemical exposure. Therefore, there is a need for an improved system for retaining brushless PM motor and generator rotor magnets. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention is directed toward a rotor for a brushless permanent magnet generator/motor. The rotor comprises a retention slot extending into a rotor flange for receiving a root of a permanent magnet. The retention slot comprises a base extending axially into the rotor flange, a pair of side walls extending radially from the base, and a pair of lugs projecting from the side wall to engage the root to provide radial and tangential retention of the permanent magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  shows a cross sectional view of an inside out brushless permanent magnet motor having a rotor and a stator. 
           [0007]      FIG. 2  shows a perspective view of the rotor of  FIG. 1  having a magnet retention system of the present invention. 
           [0008]      FIG. 3  shows a perspective view of a broach slot connection of the magnet retention system of  FIG. 2 . 
           [0009]      FIG. 4  shows a front view of the connection between the broach slot of  FIG. 3  and a permanent magnet tang. 
           [0010]      FIG. 5  shows a cross sectional view of the magnet retention system of  FIG. 2  showing inner and outer axial retention means. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  shows a cross sectional view of inside out brushless permanent magnet (PM) motor  10  in which the magnet retention system of the present invention is used. Although the invention is described hereinafter with respect to an inside out brushless PM motor, the invention is universally applicable to brushless PM generators and motors in both conventional and inside out configurations. Brushless PM motor  10  includes rotor  12  and stator  14 , which are situated inside housing  16 . Housing  16  comprises first housing half  16 A and second housing half  16 B, which are secured together using fasteners  18  to form a hollow annular disk having central bore  20 , through which extends centerline CL. 
         [0012]    Stator  14  comprises a plurality of wire windings wrapped around armature  22  to form circular hoop  24 . Input voltage and current is supplied to armature  22  through conduits  26  such that hoop  24  produces an electromagnetic field. Armature  22  is secured to first housing half  16 A through, for example, threaded fasteners  28  such that hoop  24  is maintained stationary with respect to housing  16 . Armature  22  is circumferentially disposed around bore  20  such that space is provided within housing  16  between the outermost extent of housing  16  and the outermost extent of hoop  24 . Armature  22  also allows space between hoop  24  and second housing half  16 B such that bore  20  is connected with the space between hoop  24  and the outer extent of housing  16 . Rotor  12  is situated within the open space of housing  16  such that it extends from central bore  20  past hoop  24  to interact with the electromagnetic field. 
         [0013]    Rotor  12  is comprised of hub  30 , disk  32 , outer flange  34 , a plurality of permanent magnets  36 , and outer retention ring  38 . Hub  30  is inserted into central bore  20  of housing  16  between housing half  16 A and housing half  16 B. Hub  30  is supported by rolling element bearings  40 A and  40 B such that rotor  14  is rotatable with respect to housing  16 . Hub  30  includes shaft bore  42 , which is concentric with centerline CL, for receiving an output shaft or some other output means. Disk  32  extends radially from hub  30  (concentrically with centerline CL) beyond hoop  24  such that the innermost side of flange  34  faces hoop  24 . Permanent magnets  36  are secured to the innermost side of flange  34  using an attachment slot of the present invention and retention ring  38  such that the magnets interact with the electromagnetic field generated by hoop  24 . Hoop  24  utilizes the electrical power supplied by conduits  26  and, in conjunction with switching devices and other electrical components, produces an alternating electromagnetic field that exerts pushing and pulling forces on magnets  36 . As such, magnets  36  are subjected to rotational torque such that rotor  12  rotates on hub  30  about centerline CL. The torque is transmitted through flange  34  and disk  32  to hub  30 , which is connectable with an output shaft at bore  42  such that the electrical power input from conduits  26  is converted to rotational shaft power. In order to transmit the rotational torque from magnets  36  to flange  34  in a manner that permits non-destructive removal of magnets  36 , magnets  36  are joined with flange  34  through slotted attachments of the magnet retention system of the present invention. 
         [0014]      FIG. 2  shows a perspective view of rotor  12  of  FIG. 1  having the magnet retention system of the present invention. Rotor  12  includes hub  30 , disk  32 , outer flange  34 , a plurality of permanent magnets  36 , outer retention ring  38  and center bore  42 . Hub  30  and flange  34  extend axially outward from disk  32  in the same direction such that hoop  24  of stator  14  can be inserted between flange  34  and hub  30  within housing  16  in a compact manner. Hub  30  is disposed at the center of rotor  12  and includes bore  42  such that rotational torque applied to rotor  12  can be transmitted to a shaft or some other output means. Rotational torque is transmitted to hub  30  through disk  32  from flange  34 . Flange  34  receives input torque from the plurality of permanent magnets  36 , disposed about the inner circumference of flange  34 . Rotational torque is imparted to the permanent magnets through an alternating electromagnetic field generated by hoop  24 . In order to prevent excessive stress from being generated in the permanent magnets during torque transmission, a slotted magnet retention system is used to transmit the torque from the permanent magnets to flange  34 . 
         [0015]    Flange  34  extends circumferentially around the outermost diameter edge of disk  32  and provides a platform onto which permanent magnets  36  can be mounted such that they face hoop  24  of stator  14 . In the embodiment shown, rotor  12  includes twenty-eight permanent magnets that are displaced at regular intervals along flange  34 . Permanent magnets  36  are mounted to flange  34  through correspondingly shaped retention features to restrain the magnets radially and tangentially. For example, for each magnet  36 , flange  34  includes a broach slot into which a magnet tang having a matching profile is inserted. Once inserted, the broach slot/tang interface restricts circumferential and radial movement of the permanent magnets within flange  34 . The broach slot/tang interface also permits efficient torque transmission from the permanent magnets to flange  34  without over stressing the magnets. In other embodiments, flange  34  includes a tang for receiving a correspondingly shaped broach slot on each magnet  36 . A shoulder positioned at the inner end of the broach slot and retention ring  38  positioned at the outer end of the broach slot restrict axial movement of the permanent magnets. Retention ring  38  is secured to flange  34  using fasteners (such as fastener  44 ) such that ring  38  is repeatably attached and removed from rotor  12 . Accordingly, permanent magnets  36  are attached to flange  34  in stress-free manner, and can be removed from flange  34  without causing damage to either rotor  12  or the magnets. 
         [0016]      FIG. 3  shows a front projection view of the magnet retention system of  FIG. 2  showing the connection between magnet retention flange  34  and permanent magnets  36 . Rotor  12  includes magnet retention flange  34 , to which magnets  36  are connected through a slotted interface. Flange  34  includes broach slots  46  for receiving magnet tangs  48  of magnets. 36 . Tangs  48  are inserted into slots  46  such that magnets  36  are restrained from moving in both the tangential and radial directions. However, broach slots  46  are also configured such that a magnetic flux path is maintained from magnet to magnet. 
         [0017]    Permanent magnets  36  include flux lines F 1 , F 2 , F 3 , F 4  and F 5 , which extend from north pole N to south pole S. Each flux line emits from pole N and returns at pole S. Adjacent flux lines repulse each other such that the flux lines form an outer flux boundary circumscribing each magnet. Each magnet includes a plurality of magnetic flux lines that interact with neighboring magnets and rotor  12 . For example, flux line F 1  extends from the center of each magnet at pole N and bends around the magnet toward pole S. Flux line F 1  extends out of the magnet toward centerline CL ( FIG. 1 ) of rotor  12 . Thus, flux line F 1  will interact with stator  14  of motor  10 . Similarly, each magnet includes a plurality of other flux lines, such as flux lines F 2  and F 3 , that extend from pole N to pole S at various angles to interact with stator  14 . Flux lines F 4  and F 5  extend in the plane of each magnet to interact with permanent magnets on either side of each magnet. Thus, each magnet is positioned on flange  34  of rotor  12  such that it is remains in magnetic contact with hoop  24  when assembled with stator  14 . Additionally, each broach slot is disposed along flange  34  such that each magnet is in magnetic contact with two adjacent magnets to complete a magnet flux path around the circumference of rotor  12 . Consequently, during operation of motor  10 , rotor  12  is able to maintain the requisite electromagnetic interaction with hoop  24  of stator  14  to maintain rotational torque transmission to hub  30 . 
         [0018]    As mentioned above, tangs  48  are inserted into slots  46  such that magnets  36  are restrained from moving in both the tangential and radial directions. An inner retention shoulder prevents axial movement of magnets  36  toward disk  32 , and retention ring  38  (shown in  FIG. 2 ) is secured to flange  34  at bore  50  to restrain outward axial movement of magnets  36 . Flange  34  is configured to provide radial retention to each magnet through the interaction of tangs  48  with slots  46 . 
         [0019]      FIG. 4  shows tang  48  of magnet  36  inserted into broach slot  46  as shown in  FIG. 3 . Magnet retention flange  34  includes first flange face  52 , into which broach slot  36  extends perpendicularly. Rotor  12  rotates about centerline CL of motor  10  such that magnet retention flange  34  would rotate in the plane of  FIG. 3 . Broach slot  46  includes base  54 , first side wall  56 , second side wall  58 , first side tooth  60  and second side tooth  62 . Broach slot  46  extends axially (parallel with centerline CL) into magnet retention flange  34  of rotor  12  such that it opens towards centerline CL. Broach slot  46  extends axially into first face  52  to form base  54 . First side wall  56  and second side wall  58  extend radially (perpendicular with centerline CL) from base  54 . First side tooth  60  and second side tooth  62  extend in the tangential or circumferential direction from first side wall  56  and second side wall  58 , respectively. First side tooth  60  and second side tooth  62  overhang base  54  to narrow a segment of broach slot  46  that is radially displaced from base  54 . 
         [0020]    Magnet  36  includes tang  48 , which generally refers to the portion of magnet  36  situated below first side tooth  60  and second side tooth  62 . Magnet  36  also includes first tang tooth  64  and second tang tooth  66  that extend in the tangential or circumferential direction out from tang  48 . Tang  48  of magnet  36 , including first tang tooth  64  and second tang tooth  66 , is shaped to match the shape of broach slot  46 . In the embodiment shown, broach slot  46  comprises a T-shaped slot and side walls  56  and  58  comprise rounded walls. Correspondingly, teeth  64  and  66  comprise rounded nubs having a profile matching that of side walls  56  and  58 . In other embodiments of the present invention, broach slot  46  and tang  48  comprise different shapes. Teeth  64  and  66  can comprise any shape for interlocking with the side walls  56  and  58  and teeth  60  and  62  of slot  46 . For example, slot  46  and tang  48  can comprises a fir tree type configuration as is commonly used in gas turbine engines to radially retain blades in a rotor disk. 
         [0021]    Magnet  36  is inserted into broach slot  46 , such that first side tooth  60  overhangs first tang tooth  64 , and second side tooth  62  overhangs second tang tooth  66  to prevent magnet  36  from breaking loose during operation of motor  10 . Although tang  48  is shaped to match the shape of broach slot  46 , a small amount of slop or play is permitted in the interaction between tang  48  and slot  46  to avoid producing stress concentrations in magnet  36  during operation of motor  10 . (The space shown between tang  48  and slot  46  in  FIG. 3  is exaggerated for illustrative purposes.) During operation of motor  10 , rotor  12  is subjected to rotational forces by the electromagnetic field generated by stator  14 . Typically, motor  12  rotates at high enough speeds such that rotor  12  is subjected to considerable centrifugal force. In an inside out configuration motor, such as that of motor  10 , the centrifugal force tends to open up slot  46 , causing teeth  60  and  62  to grow apart. As such, during operation of motor  10 , magnet  36  is forced outward while teeth  60  and  62  are forced apart. 
         [0022]    Side teeth  60  and  62  overhang tang teeth  64  and  66  by a sufficient length to avoid the possibility of magnet  36  radially dislodging from slot  46 . While motor  10  operates at speed, magnet  36  is held in place by frictional force generated between magnet  36  and rotor flange  34 . This frictional force is proportional to the centrifugal force acting on rotating magnets  36 . In one embodiment, first side tooth  60  overhangs first tang tooth  64  by a length greater than about the distance first side tooth  60  grows away from second side tooth  62  during operation of motor  10 . Likewise, second side tooth  62  overhangs second tang tooth  66  by a length greater than about the distance first side tooth  60  grows away from second side tooth  62  during operation of motor  10 . Additionally, to avoid fracturing of magnet  36 , slot  46  is slightly oversized such that any twisting or bending of rotor  12  during operation of motor  10  does not transmit stress to magnet  36 . Magnet  36  is permitted free movement within slot  46  such that teeth  60  and  62  are prevented from pulling on teeth  64  and  66  as broach slot  46  tends to open up during operation. Magnet  36  is thus ultimately restrained from exiting slot  46 , however, is not rigidly attached to rotor  12 . Thus, unnecessary production of tensile or compressive stress in magnet  36  is avoided. 
         [0023]    Additionally, tang  48  is configured to transmit torque to flange  34  of rotor  12 . Magnet  36  is subjected to rotational forces by the alternating electromagnetic field generated by stator  14 . The force of the applied field is transmitted from magnet  36  through flange  34  and to hub  30  such that useful rotational output can be obtained. Teeth  64  and  66  interact with wall  56  and  58 , respectively, to efficiently transmit torque from magnet  36  to flange  34 . As described above, teeth  64  and  66  comprise rounded projections such that they mesh with rounded walls  60  and  62 . Thus, the tangential component of the rotational force applied to magnet  36  is effectively transmitted to broach slot  46 . Thus, little energy is lost in the transmission of torque from magnet  36  to broach slot  46 , and rotor  12  is efficiently rotated. 
         [0024]    Thus, tang  48  is loosely fit within broach slot to restrain movement of magnet  36  in the radial direction  46 , to prevent stress build up in magnet  36 , and to transmit torque to flange  34 . Rotor  12  is fitted with other restraints for preventing axial withdrawal of magnet  36  from slot  46  during operation of motor  10 . Ring  38  (shown in phantom in  FIG. 4 ), which is secured to bore  50  of flange  34  by fastener  44 , provides a barrier strip spanning the distance between teeth  60  and  62  to prevent axial dislodgment of magnet  36 . 
         [0025]      FIG. 5  shows a cross sectional view of the magnet retention system of the present invention as taken along section  5 - 5  of  FIG. 2 , showing inner and outer axial retention means. Rotor  12  includes magnet retention flange  34 , which extends parallel to centerline CL from disk  32  at first side  68 , which includes inner axial retention shoulder  70 . Flange  34  extends from first side  68  to second side  70 , to which outer axial retention ring  38  is attached. Flange  34  also includes fastener  44 , fastener bore  50 , base  54  of broach slot  46 , stress relief notch  74  and bushing  76 . Magnet  36  is secured between shoulder  70  and ring  38  to restrain axial movement of magnet  36 , e.g. in the direction of centerline CL. 
         [0026]    Permanent magnet  36  is inserted into broach slot  46  at second side  70  such that it extends along base  54 . Magnet  36  is inserted into slot  46  such that it contacts inner retention shoulder  70 . Inner retention shoulder  70  extends around the circumference of flange  34  between first side  68  and second side face  70 . Inner retention shoulder comprises a lip or a barrier to prevent magnet  36  from sliding through broach slot  46  once inserted. Stress relief notch  74  is provided between shoulder  70  and base  54  to prevent stress concentration from occurring in flange  34  during high speed rotation of rotor  12 . 
         [0027]    After magnet  36  is inserted into broach slot  46  to meet shoulder  70 , outer retention ring  38  is affixed to second face  72  to prevent magnet  36  from retreating out of slot  46 . Outer retention ring  38  covers face  72  and extends over a lower portion of magnet  36 . Ring  38  is secured to flange  34  by threaded fastener  44 , which extends through an opening in ring  38  and into bore  50 . Bore  50  includes self-locking helical coil insert  76 , into which fastener  44  is threaded. Ring  38  includes notch  78  that enables the inner diameter of ring  38  to act as a spring-like element. Thus, when ring  38  is fastened to flange  34 , the inner diameter of ring  38  pre-loads magnet  36  such that it is biased against shoulder  70 . Notch  78  also permits ring  38  to span any variance in the alignment of second face  72  and magnet  36  such that ring  38  will be pulled flush with magnet  36 . Thus, ring  38  is prevented from applying pressure directly to the edge of magnet  36 , where stress concentration can readily produce fracture of magnet  36 . 
         [0028]    As explained above, magnet  36  is maintained within flange  34  such that it is able to magnetically interact with adjacent magnets and the coil winding of stator  14  during operation of motor  12 . Shoulder  70 , ring  38 , and broach slot  46  immobilize magnet  36  within flange  34  without overstressing magnet  36 . Thus, the incidence of fracture of magnet  36  is greatly reduced with the retention system of the present invention. However, due to extreme operating conditions of motor  10 , rough handling of rotor  12 , or other such similar occurrences, it is possible that magnet  26 B may become damaged. The magnet retention system of the present invention, as described above, provides a means for removing the permanent magnets from rotor  12 . Magnets can be removed from flange  34  by simply removing ring  38  after unthreading fastener  44 . Individual magnets can be removed without damaging adjacent or other magnets, or without damaging rotor  12 . Individual magnets are therefore easily removed such as might be required for replacement if the magnet becomes damaged or demagnetized, or simply for maintenance or cleaning. Thus, the present invention provides a magnet retention system that reduces the incidence of magnet breakage and provides improved maintenance capabilities. 
         [0029]    Although the invention has been described with respect to flange  34  having broach slots and magnets  36  having tangs, other configurations of the magnet retention system of the present invention can also be used. For example, in another embodiment, magnets  36  can be configured with broach slots, and flange  34  can be configured with mating tangs. In other embodiments, magnets  36  and flange  34  include mating male and female components that have correspondingly shaped geometric profiles to restrain radial and tangential movement of magnets  36  with respect to rotor  12 . 
         [0030]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.