Abstract:
A method includes the steps of 1) providing a ring-shaped permanent magnet having radially inner and outer surfaces and opposite first and second side surfaces; 2) overmolding a casing material about the magnet to yield a magnet assembly, the casing material comprising a material that contracts as it cools from a molten state and that includes a portion located along the radially outer surface and the first side surface; and 3) mounting the assembly about a rotor shaft.

Description:
[0001]    This is a division of U.S. application Ser. No. 10/614,371, filed Jul. 7, 2003, hereby incorporated herein by reference, which is a continuation of U.S. application Ser. No. 09/923,484, filed Aug. 6, 2001, now U.S. Pat. No. 6,664,689. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This application relates to permanent magnet motors.  
         BACKGROUND  
         [0003]    A motor comprises a multi-pole ring-shaped magnet that rotates relative to a stator. The stator comprises multiple coils toroidally-wound about a ring-shaped core and sequentially disposed along the circumference of the core.  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0004]    [0004]FIG. 1 is a sectional perspective view of a motor comprising a first embodiment of the invention;  
         [0005]    [0005]FIG. 2 is an expanded sectional perspective view of a core of the motor;  
         [0006]    [0006]FIG. 3 is a perspective view of a magnet of the motor;  
         [0007]    [0007]FIG. 4 is a sectional side view of the core and magnets of the motor;  
         [0008]    [0008]FIG. 5 is a sectional side view of the core, the magnets and backplates of the motor;  
         [0009]    [0009]FIG. 6 is a sectional perspective view a disk of the motor;  
         [0010]    [0010]FIG. 7 is a sectional side view of a mold for making the disk of the motor;  
         [0011]    [0011]FIG. 8 is a sectional side view of the disk after being molded but before receiving a mounting hole;  
         [0012]    [0012]FIG. 9 is a sectional side view of the disk after receiving a mounting hole;  
         [0013]    [0013]FIG. 10 is a sectional side view of a rotor comprising a second embodiment;  
         [0014]    [0014]FIG. 11 is a sectional side view of the rotor comprising a third embodiment;  
         [0015]    [0015]FIG. 12 is a sectional side view of a the rotor comprising a variation of the third embodiment;  
         [0016]    [0016]FIG. 13 is a side view of the core and of coils shown in FIG. 1;  
         [0017]    [0017]FIG. 14 is an expanded partial view of parts shown in FIG. 13;  
         [0018]    [0018]FIG. 15 is a view taken on line  15 - 15  of FIG. 14;  
         [0019]    [0019]FIG. 16 is a view taken on line  16 - 16  of FIG. 14;  
         [0020]    [0020]FIG. 17 is a perspective view of a bracket shown in FIG. 1;  
         [0021]    [0021]FIG. 18 is a sectional side view of the rotor comprising a fourth embodiment;  
         [0022]    [0022]FIG. 19 is a sectional side view of the rotor comprising a variation of the fourth embodiment;  
         [0023]    [0023]FIG. 20 is a sectional side view of the rotor comprising another variation of the fourth embodiment;  
         [0024]    [0024]FIG. 21 is a view taken on line  21 - 21  of FIG. 20;  
         [0025]    [0025]FIG. 22 is a sectional side view of the motor comprising a fifth embodiment;  
         [0026]    [0026]FIG. 23 is a nearsighted sectional side view of the motor comprising a sixth embodiment;  
         [0027]    [0027]FIG. 24 is a nearsighted sectional side view of the motor comprising a variation of the sixth embodiment;  
         [0028]    [0028]FIG. 25 is a perspective view of the motor comprising a seventh embodiment;  
         [0029]    [0029]FIG. 26 is a perspective view of the motor comprising an eighth embodiment; and  
         [0030]    [0030]FIG. 27 is a perspective view of the motor comprising a variation of the eighth embodiment.  
     
    
     DETAILED DESCRIPTION  
       [0031]    An embodiment of the present invention is shown in FIG. 1. The embodiment is a brushless motor  10 . A stator  12  of the motor  10  has coils  14  toroidally wound on a ring-shaped core  16 . A rotor  18  of the motor  10  has two magnet assemblies  20  that are disposed on either side of the core  16  and mounted on a rotatable shaft  22 . A stationary plastic housing  24  of the motor  10  encases the stator  12  and the rotor  18 . The coils  14  are powered by a motor controller (not shown) that activates the coils  14  in a sequence that rotates the rotor  18  relative to the stator  12 .  
         [0032]    The shaft  22  is of steel and is centered on an axis of rotation  26 . The shaft  22  is received by two bearings  27  that are mounted on the housing  24 . The bearings  27  are low-friction sleeves configured to enable the shaft  22  to rotate about the axis  26 .  
         [0033]    As shown in FIG. 2, the core  16  is centered on the axis  26 . The core  16  is bounded by two flat side surfaces  30 , a cylindrical radially-inner surface  32  and a cylindrical radially-outer surface  34 . The core  16  has an inner diameter ID c , an outer diameter OD c  and a thickness T c . The cross-sectional profile of the core  16  is rectangular, and can be square. The profile is centered on an annular axis  36 , which runs lengthwise through the core  16 . The core  16  has a central section  38  that is magnetically permeable and resistant to eddy currents that would circulate along the skin of the side surfaces  30  of the core  16 . To achieve this, the central section  38  is formed of a tightly spirally wound steel tape.  
         [0034]    The core  16  also has two annular overlapping sections  42 ,  43  extending respectively along the radially-inner surface  30  and the radially-outer surface  34 . Like the central section  38 , the overlapping sections  42 ,  43  are magnetically permeable. The overlapping sections  42 ,  43  are resistant to eddy currents that would circulate along the skin of any of the surfaces  30 ,  32 ,  34  of the core  16 . To achieve this, the overlapping sections  42 ,  43  are formed of turns of a magnetically permeable wire  44 . Alternatively, the overlapping sections  42 ,  43  can be formed of compressed iron-based powder. The ring is coated on all sides with enamel (not shown).  
         [0035]    As shown in FIG. 1, the two magnet assemblies  20  are alike, centered on the axis  26 , and facing each other from opposite sides of the core  16 . Each magnet assembly  20  includes a ring-shaped magnet  46 . As shown in FIG. 3, each magnet  46  is a permanently magnetized multi-pole magnet. Each magnet  46  has two north-south poles  48  interspersed between two south-north poles  50 . The poles  48 ,  50  are symmetrically disposed about the axis  26  and aligned axially. Boundaries  52  between neighboring poles  48 ,  50  extend radially and are spaced 90° apart. Each pole  48 ,  50  has a generally trapezoid shape with two straight side edges  52 , an arcuate radially-inner edge  56  and an arcuate radially-outer edge  58 . In this embodiment, each magnet  46  is formed of C8 ferrite that is molded as a single-piece structure.  
         [0036]    [0036]FIG. 4 shows the orientation of the magnets  46  relative to the core  16  when the motor  10  (FIG. 1) is assembled. Each magnet  46  has flat axially-inner and axially-outer surfaces  60 ,  62 . The axially-inner surfaces  60  of the magnets  46  are adjacent to and face the side surfaces  30  of the core  16 . The inner diameter ID m  of the magnets is smaller than the inner diameter ID c  of the core. Thus, each magnet  46  has an inner overhang  64 , which is the section of the magnet  46  that extends radially inward from the core ID c . The radial length L1 of the inner overhang is approximately equal to the spacing S m  between the axially-inner surfaces  60  of the magnets  46 . Similarly, the outer diameter OD m  of the magnets is larger than the outer diameter OD c  of the core  16 . Thus, each magnet  46  has an outer overhang  65 , which is the section of the magnet  46  that extends radially outward from the core outer diameter OD c . The radial length L o  of the outer overhang  65  is also approximately equal to the spacing S m  between the axially-inner surfaces  60  of the magnets  46 . The flux gap G f  is configured as small as possible to maximize flux.  
         [0037]    As shown in FIG. 5, the magnets  46  are parallel to each other and aligned north-facing-north and south-facing-south. A profile of flux lines  66  is shown. The inner and outer overhangs  64 ,  65  provide flux through the radially-inner and radially-outer surfaces  32 ,  34  of the core  16 . The portions of the magnets  46  that face the core  16  are attracted axially inward toward the core  16 . The overhangs  64 ,  65  repel each other axially outward. These attractive and repulsive forces substantially cancel each other out. The overhangs  64 ,  65  thus reduce stresses in the magnet assemblies  20  caused by the attractive force.  
         [0038]    The magnet assemblies  20  (FIG. 1) further have identical ring-shaped backplates  68  formed of magnetically permeable material, such as steel. The backplates  68  have the same inner diameter ID m  and outer diameter OD m  as the magnets  46 . The backplates  68  are centered on the axis  26  and affixed to the axially-outer surfaces  62  of the magnets  46  to form two magnet/backplate assemblies  70 .  
         [0039]    Each magnet assembly  20  (FIG. 1) further includes a disk  72 , shown in FIG. 6. Each disk  72  has a central bore  74  configured to tightly receive the shaft  22  (FIG. 1) in an interference fit. Each disk  72  also has an annular pocket  76  to securely retain the magnet/backplate assembly  70  (FIG. 5). The disks  72  are formed of a non-magnetically permeable material that contracts as it hardens from a fluid state. The material can be zinc or a fiber-reinforced thermoset plastic.  
         [0040]    The process for producing the magnet assembly  20  (FIG. 1) comprises the following steps. The magnet  46  is glued to the backplate  68  to form the magnet/backplate assembly  70  shown in FIG. 5. The magnet/backplate assembly  70  is placed within a mold  78  or die cast cavity, as shown in FIG. 7. Within the mold  78 , the plastic that will form the disk  72  flows into crevices of the magnet/backplate assembly  70  and adheres to the magnet/backplate assembly  70 . As the plastic hardens, the plastic contracts about the magnet/backplate assembly  70  to securely hold the magnet/backplate assembly  70  within the disk pocket  76 . This yields a rotor blank  80  shown in FIG. 8, comprising the magnet  46 , the backplate  68  and the disk  72 . The rotor blank  80  is molded without a mounting hole. Next, the rotor blank  80  is mounted on a balance tester (not shown). The balance tester indicates the center of gravity of the rotor blank  80 . A mounting hole  82  is then drilled into the rotor blank  80  at the center of gravity indicated by the balance tester, to yield the magnet assembly  20  shown in FIG. 9. Alternatively, the balance tester is an apparatus that indicates both an initial drilling location and a drilling angle for the rotor blank  80 . Then the mounting hole  82  is drilled at the initial drilling location and at the drilling angle to yield the magnet assembly  20  shown in FIG. 9.  
         [0041]    In a second embodiment of the invention, shown in FIG. 10, the rotor  18  further comprises a radially-inner magnet  84  centered on the axis  26  and underlying the radially-inner surface  32  of the core  16 . The core  16  is thus surrounded on three sides by the magnets  46 ,  84 , which can be formed together as a one-piece structure. The radially-inner magnet  84  extends axially between the two ring magnets  46  and rotates in unison with the ring magnets  46 . The radially-inner magnet  84  has the same sequence of magnetic poles as the ring magnets  46 , with north of the radially-inner magnet  84  adjacent north of the ring magnets  46  and south of the radially-inner magnet  84  adjacent south of the ring magnets  46 . The radially-inner magnet  84  has an annular backplate  86  adhering to the radially inner surface  88  of the radially-inner magnet  84 . The backplate  86  can be an iron-based ring surrounding the shaft  22 , as shown in FIG. 10, or can be a portion of the shaft  26  itself.  
         [0042]    In a third embodiment, shown in FIG. 11, the rotor  18  comprises a radially-outer magnet  89  centered on the axis  26  and overlying the radially-outer surface  34  of the core  16 . The core  16  is thus surrounded on three sides by the magnets  46 ,  89 , which can be formed together as a one-piece structure. The radially-outer magnet  89  extends axially between the two ring magnets  46  and rotates in unison with the two magnets  46 . The radially-outer magnet  89  has the same sequence of magnetic poles as the ring magnets  46 , with north of the radially-outer magnet  89  adjacent north of the ring magnets  46  and south of the radially-outer magnet  89  adjacent south of the ring magnets  46 . The radially-outer magnet  89  has an annular backplate  90  adhering to a radially outer surface  92  of the annular magnet. The backplate  90  can be a steel ring. The ring magnets  46  and the radially-outer magnet  89  are attached to the shaft  22  through a nonmagnetic disk  93 . The core  16  is held in place by a nonmagnetic bracket  94  that extends between the shaft  22  and the core  16 . Alternatively, as shown in FIG. 12, the core  16  can be held in place by a different nonmagnetic bracket  95  that extends radially-outward from the core  16  through an annular opening  96  in the radially-outer magnet  89 .  
         [0043]    The coils  14  are shown in FIG. 13. The coils  14  are formed of insulated electrically-conductive wire, typically copper, toroidally-wound around the core  16 . Coils  14  that are connected so as to be electrically activated and deactivated in unison are considered to share a single “phase”. This embodiment has three phases, designated A, B and C. The coils  14  are sequentially positioned along the length of the core  16  in a sequence A, B, C, A′, B′, C′, A, B, C, A′, B′, C′, etc. The prime after a letter indicates reverse winding. In this embodiment, the four A coils  14 , including primed and unprimed, are in series with each other. Similarly, the four B coils  14  are in series, and the four C coils  14  are in series. Each coil  14  is generally centered on a radially-projecting coil centerline  100 .  
         [0044]    In FIG. 13, the core  16  is shown overlying one of the magnets  46  to illustrate how the geometry of the coils  14  is related to the geometry of the magnet poles  48 ,  50 . The angular spacing (90° in this embodiment) between coil centerlines  100  of a single phase equals the angular spacing (90°) between pole centerlines  102 . Similarly, the angular spacing (90° in this embodiment) between coil boundaries  104  of the same single phase equals the angular spacing (90°) between pole boundaries  52 .  
         [0045]    The coils  14  in this embodiment are alike. The structure of the coils  14  is illustrated in FIGS. 14-16, with reference to one of the coils  14 . In this embodiment, each coil  14  comprises one layer of turns  110 . The turns are closely packed and substantially parallel to the coil centerline  100 . Each turn of the coil  14  comprises two radially-extending legs  112 , a radially-inner end turn  114 , and a radially-outer end turn  116 . The coil  14  has a bundle thickness T b . The wire has a rectangular, preferably square, profile. This yields lower resistive loss than a similar coil using round wire.  
         [0046]    Spaces  118  between adjacent coils  14  are filled by brackets  120 , such as one shown in FIG. 17. Each bracket  120  comprises two identical generally triangular flanges  122  having rounded edges  124  that are connected by a bowed rectangular flange  126 . The flange thickness T f  of the triangular flanges  122  and the rectangular flange  126  approximately equals the bundle thickness T b  (FIG. 15) of the coils  14 . The bracket  120  also has a mounting flange  128  extending perpendicularly from the rectangular flange  122 . A chain of the brackets  120  can be molded as one piece, with neighboring brackets  120  held together by a thin plastic web.  
         [0047]    In FIGS. 14 and 16, the brackets  120  are shown mounted on the core  16 . The triangular flanges  122  of each bracket abuts the side surfaces  30  of the core  16 , and the rectangular flange  124  abuts the radially-outer surface  34  of the core  16 . The mounting flange  128  extends to any suitable section of the housing  24  (FIG. 1) to mount the bracket  120 , and thus the core  16 , to the housing  24 . By filling in the coil-free spaces  118 , the brackets  120  provide a smooth flat side surface defined jointly by the radially-extending legs  112  of the coils  14  and the side flanges  122  of the brackets  120 . The brackets  120  also thus provide a smooth cylindrical surface defined jointly by the radially-outer turns  116  of the coils  14  and the rectangular flanges  126  of the brackets  120 . The brackets  120  serve three functions. They reduce wind turbulence during rotation of the rotor  18 ; they impart and maintain proper positioning of the coil turns  110 ; and they connect the core  16  to the housing  24 .  
         [0048]    The brackets  120  are formed of a non-magnetically permeable material, so as not to affect the magnet flux. In a variation of this embodiment, the trangular flanges  122  and/or the rectangular flange  126  are formed of a magnetically permeability, low eddy current loss material, such as compressed powdered iron, so as to effectively narrow the flux gap G f  (FIG. 4).  
         [0049]    Referring to FIG. 1, the stator  12  and rotor  18  are installed in the housing  24 . The coils  14  are connected to a brushless motor controller (not shown) to be activated in a manner known in the art. For each phase, the controller can apply forward current, reverse current, or no current. In operation, the controller applies current to the phases in a sequence that continuously imparts torque to turn the magnet assemblies  20  in a desired direction. The controller can decode the rotor position from signals from Hall effect switches or can infer the rotor position based on current drawn by each phase.  
         [0050]    In the first embodiment, the core  16  has a rectangular profile with planar side surfaces  30 , as shown in FIG. 2. In contrast, in a fourth embodiment, the side surfaces  30  of the core  16  are bowed outward. One example of bowed sides is shown in FIG. 18. The profile is generally lenticular and is thickest at a location about 50-75% of the way from the radially-inner surface  32  to the radially-outer surface  34 . This profile renders the core  16  thickest were the flux is strongest and thinnest where the flux is weakest. This profile also reduces the length of the end turns  114 ,  116  (FIG. 15), which are the legs of the coil  14  that contribute the least torque per resistive loss.  
         [0051]    The surfaces  60  of the magnets  46  that face the outwardly-bowed surfaces  30  of the core  16  are bowed inward to yield a gap thickness T g  that is uniform along a significant portion of the periphery of the core profile. Like the second embodiment (FIG. 10), this fourth embodiment includes a radially-inner magnet  84 . However, unlike the second embodiment, the radially-inner magnet  84  is one-piece with the ring magnets  46 . Like the third embodiment (FIG. 12), this fourth embodiment includes a radially-outer magnet  89 , comprising two sections  132  on either side of an opening  134 . As illustrated in this embodiment, the magnets  46 ,  84 ,  89  can be magnetized such that each flux line  66  is generally perpendicular to the section of the core surface  30  and/or the magnet surface  60  that the flux line  66  intersects.  
         [0052]    Another example of bowed sides is shown in FIG. 19. The profile of the core  16  is oval. In this variation, the profiles of both the axially-inner and axially-outer surfaces of the magnet match the profile of the core  16 . In the examples of FIGS. 18-19, the core  10  is surrounded on four sides by magnets  46 ,  84 ,  89 .  
         [0053]    Another feature is shown in FIGS. 20 and 21. The core  16  has an annular cavity  136  that extends circumferentially through the core  16 . The cavity  136  serves as an internal cooling channel. An inlet  140  and an outlet  142  extend radially outward from opposite ends of the cooling channel  136 . Tubes  144  attached to the inlet  140  and the outlet  142  enable cooling fluid to be pumped through the cooling channel  136 .  
         [0054]    The first embodiment (FIG. 4) comprises one core  16  disposed between two backplated magnets  46 . This is considered a single-stage motor. A fifth embodiment, shown in FIG. 22, includes a second stage  150 , comprising a second core  152  and a non-backplated rotating magnet  154 , and also a third stage  160 , comprising a third core  162  and another non-backplated magnet  164 . The second and third stages  150 ,  160  are interposed between the first core  16  and one of the backplated magnets  46 . Thus, the cores  16 ,  152 ,  162  are interspersed between the magnets  46 ,  154 ,  164 . The second and third cores  152 ,  162  are mounted to the housing  24  (FIG. 1) in any suitable manner, and the non-backplated magnets  154 ,  164  are mounted to the shaft  22  in any suitable manner. The added stages  150 ,  160  provide additional motor torque. Any number of such stages is possible. The motor  10  thus is stackable. This stackability enables a manufacturer to build a motor of any desired torque with an inventory of a single-size of magnets and cores.  
         [0055]    The first embodiment, shown in FIG. 4 includes two magnets  46  spaced axially from the core  16 . In contrast, in a sixth embodiment, shown in FIG. 23, the two ring-shaped magnets  46  are spaced radially from the core  16 —specifically, radially inward and radially outward from the core  16 . Also, alternatively, as shown in FIG. 24, the motor  10  can have only one ring-shaped magnet  46 . In the embodiments of FIGS. 23 and 24, the magnets  46  axially-overhang the core  16 , similar to the magnets  46  radially overhanging the core  16  in embodiment of FIG. 4.  
         [0056]    In the previous embodiments (FIGS. 1-24), the core  16  and the magnets  46 ,  84 ,  89  are each arcuate and form an endless ring. However, in a seventh embodiment shown in FIG. 25, the core  16  can be arcuate and have ends  170 , thus forming an incomplete ring. Similarly, the magnets  46  can be arcuate and have ends  172 . This is most suitable in an embodiment in which the magnets  46 ,  84 ,  89  have a limited range of rotational motion about the axis  22 . In this embodiment, the profiles of the magnets  46  overhang the profile of the core  16 .  
         [0057]    In an eighth embodiment shown in FIG. 26, the magnets  46  do not rotate but rather move linearly relative to the core  16 . The coils (not shown) are wound about the core  16  and sequentially disposed along the length of the core  16 . In this embodiment, the magnets  46  overhang the core  16 . A variation of this embodiment, shown in FIG. 27, the core  16  is surrounded on three sides by a magnet  174  that also overhangs the core  16 . In both embodiments (FIGS. 26 and 27), the magnets  46 ,  174  have a backplate  68 .  
         [0058]    An “electrical machine” herein is any device that has both stationary and moving parts and that can convert electrical power into mechanical motion, or vice versa. An electrical machine can be, for example, a generator, a motor, an actuator, or a motion sensor. The present invention can apply to any of such machines. Although, in the embodiments described above, the core  16  (FIG. 1) is part of the stator  12  and the magnet assemblies  20  are part of the rotor  18 , the opposite is also within the scope of the invention.  
         [0059]    In each of the embodiments, several magnets described as though they are separate structures can equivalently be formed together as a one-piece structure. Conversely, in each of the embodiments, each magnet described as though it were a one-piece structure can equivalently be formed of separate parts combined together. Consequently, for example, in the claims, “surrounded on three sides by a magnet” is equivalent to “surrounded on three sides by magnets.” 
         [0060]    The “lengthwise direction” or “along the length” herein can refer to either a linear path, an open arcuate path, or a closed circumferential path such as the annular axis  36  (FIG. 2). The term “elongated” herein can characterize the shape of a closed ring, as well as a straight structure, by defining the structure as having a profile that extends uniformly along a circumference.  
         [0061]    The embodiments described above are chosen to be included herein only due to their being good examples of or best modes of practicing the invention. The scopes of the claims are therefore not intended to be limited by these embodiments.