Abstract:
The present invention is a brushless electric motor that can be used in high performance applications, such as model airplanes. The rotor assembly, which includes the rotor shaft, encloses a winding core, which is part of a fixed stator assembly. Conducting wire in the armature portion of the stator assembly is wrapped around a set of spokes, extending radially from an inner hub. The spokes are formed from a stack of flat metal laminations, the laminations oriented perpendicular to the rotation axis. The spokes are wrapped with a single layer of copper wire, which is connected to the energy source when the motor is operational. These spokes are long relative to the diameter of the inner hub, leaving V-shaped slots through the winding core. Openings in both end bells allow air to freely flow into the core, cooling the single layer of conducting wire. Using a wedge to force the wire against the spokes during varnishing results in close contact between the wire and the conducting material in the winding core. When the motor is operational, this contact causes the winding core to act as a heat sink, allowing a relatively heavy motor to sustain high power without damage.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/130,912, filed Jun. 4, 2008, having inventor Leslie Hoffman and entitled “Brushless Motor Apparatus,” and hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to electric motors. More specifically, the invention relates electronically commutated brushless motors. 
       BACKGROUND OF THE INVENTION 
       [0003]    Brushless DC motors are widely used in hobby applications such as model aviation. The advent of lithium secondary batteries and low “on” resistance power integrated circuits have permitted rapid advances in model aviation. The high power to weight ratios now available with electrically commutated motors powered by lithium batteries have permitted sustained flights of model helicopters and high speed ducted fan powered model jets. Traditional brushless motor designs for model aviation are derived from the CD/DVD-ROM drive industries. Despite recent advances, there is a continuing need for better brushless motors. More power handling ability and lighter weight than can be achieved by the older motor designs are advantageous in the application of modern model aircraft. 
       SUMMARY OF THE INVENTION 
       [0004]    The motor of the present invention achieves very high performance and exhibits a very wide efficient operating range. The permanent magnet structures are carried on a rotor that overhangs and surrounds the inner stator. The rotor spins outside the stator structures in a configuration commonly called “outrunner”. 
         [0005]    The rotor is an insert injection molded piece with features to locate permanent magnets. The stator includes the armature structure and is very compact. The armature laminations form radial spokes, about which relatively thick (low gauge) wire is wound in a single layer. In contrast to conventional practice in which the armature is filled with as much conducting material as possible, radial slots between the spokes allow air to circulate across the wire. The inner hub from which the spokes radiate has a small diameter. Because of the combination of these two aspects, the ratio of open space to solid material in the winding core is low compared to conventional practice. 
         [0006]    After the wire is wound around the spokes, a wedge is used to hold the wire tightly against the spokes while the wire is varnished into place. Because of the substantial contact consequently achieved once the varnish cures between the wire and the armature laminations, the armature laminations serve as a sink for heat generated in the wire when the motor is operational. 
         [0007]    Several other features also contribute to improving heat transfer away from the wire coils, thereby improving performance. The end bells enclosing the winding core at both ends are open. The rotor end bell has deep blades, effectively pushing the air like a fan as the shaft rotates. The mounting end bell includes skeletal struts that reduce resistance to air flow, both axially and radially. The openings through the winding core and the end bells combine to facilitate axial air flow to cool the conducting material in the single layer of wire. 
         [0008]    The rotating components are supported by a very low profile collection of bearings. The overall design structure and construction techniques permit sustained dissipation of heat and remarkable efficiency at all power settings in a lightweight assembly. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  depicts a first embodiment of the motor. 
           [0010]      FIG. 2  depicts a single lamination of the wiring core viewed in the Z-direction. 
           [0011]      FIG. 3  depicts the armature assembly portion of the stator assembly in isolation. 
           [0012]      FIG. 4  illustrates an embodiment of a winding wedge. 
           [0013]      FIG. 5  depicts the mounting end bell, including struts, in isolation, viewed toward the positive Z-direction. 
           [0014]      FIG. 6  shows the rotor assembly in isolation. 
           [0015]      FIG. 7  shows the rotor assembly, including rotor struts in isolation, viewed toward the positive Z-direction. 
           [0016]      FIG. 8  shows an alternate embodiment of the motor. 
       
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0017]    The figures and associated text illustrate exemplary embodiments, which are not intended to be comprehensive of the scope of the invention. A person of ordinary skill in the art will recognize many embodiments of the inventive concept that are not explicitly detailed here. 
         [0018]      FIG. 1  depicts a first embodiment of the motor  100 , shown in cross-section. To orient the figures, a cylindrical coordinate system (R, θ, Z) is used. The cross-section is in a plane parallel to the Z-axis. As shown by the inset  103  in  FIG. 1 , the Z-axis points along the axis of the shaft  115  of the motor  100 , toward the end where the shaft  115  would engage the load when the motor  100  is operational. The R-direction is radially outward from the axis of the shaft  115 . θ (see  FIG. 2 ) is an angle about the Z-axis, in a plane perpendicular to the Z-axis. 
         [0019]    The motor  100  includes a rotor assembly  105  and a stator assembly  150 . The rotor assembly  105  is made up of a rotor cylinder  110  and the power delivery shaft  115 , insert-molded into a rotor end bell  120  (also known as a rotor end “cap”). The rotor end bell  120  includes a plurality of blades  127 , typified by blade  127   a , for structural support. A roll pin  122  through the shaft  115  is encapsulated by the rotor end bell  120 , ensuring the coupling of the rotor end bell  120  to the shaft  115  as the shaft  115  rotates. While the rotor end bell  120  is being molded, the mold holds the rotor cylinder  110  and the power delivery shaft  115  in proper axial and radial relationship. 
         [0020]    The rotor end bell  120  is fabricated from structural thermoplastic. Any of a number of thermoplastics may be used, but RYTON R-4-200BL, a composite of resin and fiberglass, is one thermoplastic found to have the advantages of high strength and the ability to withstand high temperatures. All the thermoplastic parts in the motor may be fabricated from RYTON R-4-200BL or similar material. 
         [0021]    The stator assembly  150  includes the armature assembly  155 , which is supported, at least in part, by a bushing support  160 . The bushing support  160  is in turn supported by the mounting end bell  165  (also known as a “stator end bell/cap”), which is also a molded structural thermoplastic part. These elements are stationary when the motor is operating. The framework of the mounting end bell  165  includes a plurality of struts  167 , typified by  167   a , describing openings  168 . The stator assembly  150  includes a mounting flange  175 , which has mounting holes  170 , typified by mounting hole  170   a , to mount the motor  100  for use. In the embodiment shown, bearings  180 ,  185  and  190  facilitate smooth rotation of the shaft  115 . Bearing  185  is enclosed by a bushing  195  within the bushing support  160 . Other than the bearings and the bushing  195 , the remainder of the shaft enclosure  196  may be fabricated from structural thermoplastic. 
         [0022]    The armature assembly  155  is formed from a stack  330  of laminations  200 , each lamination  200  oriented perpendicular to the Z-axis in an assembled motor  100 . A representative lamination  200  is shown in  FIG. 2 . For reference, coordinate axes  290  are shown, but for clarity, they are offset from their correct position; namely, the point  215 ×should be regarded as coinciding with the center  215  of the lamination  200 . The Z-axis is perpendicular to the paper. 
         [0023]    The lamination  200  is stamped from thin, flat, metal, such as steel, that has a high magnetic permeability. The lamination  200  may have small indentations (not shown) in its surface, for alignment with adjacent laminations in the stack  330 . At the center of the lamination  200  is a circular shaft hole  205 , through which the shaft  115  of the rotor assembly  105  passes. (It should be noted that not all laminations  200  have the same shaft hole  205  size. The radius of the shaft hole  205  may be enlarged in some laminations  200  relative to others, to serve as a counterbore  320  to hold bushing  195  or bearing  180 . In some laminations  200 , the shaft hole  205  in the lamination  200  may be modified by a key  325 , a groove used for alignment. Such a counterbore  320  and a key  325  are shown in  FIG. 3 .) Immediately surrounding the shaft hole  205  is a ring of material, forming the remainder of the central hub  210 . The outer radius RH  221  of the central hub  210  is the distance from the center  215  of the lamination  200  to the point labeled  220 . Extending from the central hub  210  are a set of spokes  225 , typified by  225   a  and  225   b . The spokes  225  are spaced at equal intervals of angle θ around the central hub  210 . For a three-phase motor  100 , the number of spokes  225  will be a multiple of three. The armature  155  shown has six spokes  225 . 
         [0024]    Each spoke  225  terminates in a spoke cap  230 , typified by spoke cap  230   a . The outermost edges of the spoke caps  230  lie on a circle  235 , about which the rotor cylinder  110  rotates (see  FIG. 7 ). The radius RR  241  of this circle  235  is the distance from the center  215  of the lamination  200  to point  240  in  FIG. 2 . The ratio of RH/RR is preferably less than 50%, which is true for the embodiment shown. The difference RR-RH is the length L of each spoke  225 . In the embodiment shown, the ratio of the width W of each spoke  225  to L is less than 60%, and the ratio of the thickness (along a radius) of a spoke cap  230  to L is less than 30%. The combination of these dimensions results in large slots  245  between the spokes  225 , typified by slot  245   a , with a ratio of area of empty space to solid material outside the central hub  210  greater than 50%. As will be described in relation to  FIG. 3 , the laminations  200  are wrapped with wire  250  to form the winding core  340 .  FIG. 2  illustrates the thickness of the single layer of wire wrapping  250  (shown in cross-section) on a pair of selected typical adjacent spokes  225 , and the somewhat reduced size of a typical slot  255  after wiring. 
         [0025]      FIG. 3  depicts the armature assembly  155  portion of the stator assembly  150  in isolation. The laminations  200  are pressed together into a stack  330 , coated, and wrapped with wire  250  to form windings  300 . The set of windings  300  form the winding core  340  of the armature assembly  155 . When the motor  100  is in operation, electricity passing through the winding core  340  from a power source exerts magnetic forces on magnets  620  contained in the rotor assembly  105 , causing the shaft  115  to rotate the load. 
         [0026]    The wire  250  that wraps each spoke  225  in the winding core  340  is conducting wire  250 , typically copper or copper-based. Because wire heat is proportional to wire resistance, which is, in turn, inversely proportional to area, according to conventional reasoning, the wire should be wrapped with a plurality of layers of thin gauge copper wire. Accordingly, the available core volume in an armature  155  is typically filled with the maximum practical amount of conductor volume, so that no valuable armature core is wasted as air space. 
         [0027]    The inventor has found that the conventional approach is prone to thermal runaway. At high power density, as power continues to increase, temperature increases, which causes power to increase, and so forth. Eventually, the winding core will burn up. 
         [0028]    Applicant has realized that the conventional approach of using multiple layers of copper wire prevents the inner layers from exposure to cooling air. In contrast, the spokes  225  in the winding core  340  shown in  FIG. 3  are very tightly wrapped with a single layer of conducting wire  250  having a relatively thick gauge. The resulting open structure in the armature allows a large amount of air to pass over and cool each conductor. 
         [0029]    The winding may be performed by a machine that translates as the wire, under tension, is looped around a spoke, so that adjacent wire loops are positioned radially closely adjacent to each other. After winding, a winding wedge  400  is inserted between adjacent spokes to keep the wire  250  flush against the spokes  225 . An example of a wedge  400  is shown in  FIG. 4 , although a practitioner of ordinary skill in the art will realize that many alternative configurations would provide essentially identical functionality. The illustrated wedge  400  is made from a flexible material, such as rubber or silicone rubber. The wedge  400  has a narrow end  410  and a wide end  420 . A wedge  400  is inserted between each adjacent pair of spokes  225  after winding is completed. The narrow end  410  of the wedge  400  is first inserted, or threaded, between the spokes  225 , and then the wide end  420  is forced between the spokes  225  by pulling on the narrow end  410 . At that point, the wide end  420  will fill the slot  245 , and assume a wedge shape. Once all wedges  400  are in place, the wire  250  is varnished to the stack  330 , and the varnish is allowed to cure. Finally, the wedges  400  are removed. 
         [0030]    The resulting ratio of wire  250  area to slot  245  area in cross-section through the winding core  340  is optimized for lowest resistance-induced energy loss to heat, at peak power output. Returning to  FIG. 2 , we see how the thickness of a single layer wire  250  compares with the area of the slots  245  between the spokes  225 . The percentage of area between RH and RO that is open to axial air flow should, after winding, still exceed 20%, and preferably exceed 30%. 
         [0031]    A person of ordinary skill in the art will recognize that alternative geometries are possible that can also achieve such a high ratio of space to solid material area in the axial direction through the winding core  340 . For example, the spokes  225  may be different from those of the winding core  340  shown in  FIGS. 2 and 3  in shape and/or in number. There might be additional cross-sectional material, oriented in other directions, such as concentrically. However, any motor is regarded as being within the scope of the invention described herein that achieves axial space to solid material ratios consistent with those specified above. 
         [0032]    In addition to increasing the space for air flow through the armature  155 , the above-described winding/wedging/varnishing process achieves close contact between the wire  250  and the laminations  200 . Thus, the entire winding core  340  becomes a sink for heat produced when electricity flows through the wire. This effect of increasing the heat capacity of the winding core  340  also contributes to preventing thermal runaway. 
         [0033]    A small counterbore  320  in the armature assembly  155  seats bushing  195  and bearing  185  (see  FIG. 1 ), which support the power delivery shaft  115 . The bushing  195  ensures stable alignment of the rotor assembly  105  around the stator assembly  150 . A second bearing  190 , distal from the rotor cylinder  110 , further supports the shaft  115  for rotational movement of the shaft  115  with respect to the mounting end bell  165 . The mounting end bell  165  is insert-molded with bearings  185  and  180  in place. The bearings are typically fabricated from steel, although other materials can be used. As described above with respect to the rotor end bell  120 , the mounting end bell  165  may be molded of any suitable plastic, such as RYTON PPS. 
         [0034]      FIG. 5  depicts the mounting end bell  165  of the armature assembly  155 . In this view looking in the positive Z-direction, the narrowness of the struts  167 , typified by struts  167   b  and  167   c , can be appreciated. In the embodiment shown in the figure, the struts  167  are in scale proportionately to the rest of the mounting end bell  165 . A side view of the mounting end bell  165  and struts  167  is provided by  FIG. 1 . Clearly, the struts  167 , typified in  FIG. 1  by strut  167   a  are skeletal, allowing air to freely ventilate the armature assembly  155 . When the rotor assembly  105  is spinning, the openings  168  formed by elongation of the struts  167  in the Z-direction, as seen in  FIG. 1 , cause air to be pulled through the stator slots  245  somewhat like a centrifugal blower. 
         [0035]      FIG. 5  illustrates the projection of the struts  167  of the mounting end bell  165  onto a plane that is oriented perpendicular to the shaft  115  axis. The axial spaces  500 , typified by space  500   a  between adjacent struts  167   b  and  167   c  further facilitate air flow past the windings  300  of the armature assembly  155 . The outer circle  510  of the mounting end bell  165  aligns with the outside of the rotor cylinder  110 . 
         [0036]    Just as the winding core  340  cross-section can vary in geometry without departing from the inventive concept, so can the detailed geometry of the mounting end bell  165 . Essentially, the mounting end bell  165  should be rigid and substantially permeable to air flowing at least axially. The axial ratio of space to solid material in the mounting end bell  165  should be at least 25%, and preferably should be greater than 30%. Like the armature  155 , the mounting end bell  165  has a central hub  520 , bounded by a circle. In a projection of the type shown in the figure, the projected (i.e., axial) ratio of space to solid material in the mounting end bell  165  outside the central hub  520  should be at least 30% and preferably should be greater than 40%. Because of their curving skeletal shape, the struts  167  of the mounting end bell  165  shown in  FIGS. 1 and 5  offer the advantage of allowing radial, as well as axial, air movement. The radial air flow can either entrain air into axial flow in the negative Z-direction, or evacuate air moving axially in the positive Z-direction, in either case contributing to flow of air through the armature  155 . 
         [0037]    The mounting end bell  165  might have struts  167  having shapes different from those shown in  FIGS. 1 and 5 . It might have concentric spaces, some combination of concentric and radial structures, or any other structure achieving the above ratios, all within the scope of the invention so long as the space to solid matter ratios permit substantial air flow into and out from the winding core  340 . 
         [0038]      FIG. 6  shows the rotor assembly  105  in isolation. The rotor end bell  120 , as previously described, engages a steel rotor cylinder  110  and a steel shaft  115 . The insert-molding process allows close control over the dimensions of the rotor assembly  105 . The molding process produces magnet alignment features  610 , or spacing fingers, typified by  610   a  and  610   b , around the interior of the rotor cylinder  110 . Strong rare-earth magnets  620 , typified by magnet  620   a , are spaced between the features  610 . During assembly, the magnets  620  exert attracting/repelling force on each other, tending to inhibit their precise alignment. The alignment features  610  ensure that magnets  620  are properly seated. The features  610  also serve to keep the magnets  620  from drifting in position when the motor  100  is operating, particularly when the structure upon which the motor  100  is mounted undergoes significant acceleration. 
         [0039]      FIG. 1  shows the rotor assembly  105  viewed from the side. The shaft  115 , rotor cylinder  110 , and rotor end bell  120  are visible. A plurality of radial blades  127 , typified by blade  127   a , provide a framework for the rotor end bell  120 .  FIG. 7  shows the rotor assembly  105  looking in the positive Z-direction. The relative narrowness of the blades  127  is clearly apparent from the large portion of the winding core  340  that is visible through openings  700 , typified by opening  700   a , between adjacent blades  127   b  and blade  127   c . This configuration leaves the rotor end bell  120  largely open to flow of air parallel to the shaft  115 . 
         [0040]    In the embodiment shown, when viewed from the side as in  FIG. 1 , the blades  127  are wedge-shaped, each describing a solid in a portion of plane parallel to the shaft  115 . Four blades  127  are included in the embodiment shown in  FIG. 7 , spaced at intervals in θ of 90 degrees. Although the number and shape of these blades  127  may vary within the scope of the invention, the particular blade-like shape in the embodiment shown in the figure has the advantage of augmenting air flow through the interior of the winding core  340 . 
         [0041]    Again, other geometries of the blades  127  are possible within the scope of the invention. The blades  127  may differ in shape and/or number, or have some other geometry entirely. For example, while each blade  127  shown in  FIG. 1  and  FIG. 7  is essentially flat in a plane parallel to the axis of the shaft  115  (i.e., an R-θ plane), another embodiment might use blades that are contoured into a shape like a house fan blade or boat propeller. In such cases, the blades might be flat, or might be curved and thin, with an essentially uniform thickness over some portion of the blade. On the other hand, the blades might be fashioned to more closely resemble skeletal struts  167 , like those used in the mounting end bell  165 . In any case, the blades  127  should provide structural integrity and should be largely permeable to axial air flow and, optionally, radial flow. In the axial direction, the rotor end bell  120  should have a ratio of space to solid material of at least 40%, and preferably greater than 50%. 
         [0042]    The motor  100  may be mounted in or on a superstructure, such as a model airplane or helicopter, such that air will impinge upon the motor as a result of the motion of the superstructure. In such situations, the permeability of the motor  100  to relative air velocity parallel to the axis not only will cool the components of the motor  100 , but also may reduce resistance of the superstructure to forward motion. The motor  100  might drive a ducted fan or a propeller. 
         [0043]      FIG. 8  shows a motor  800  with an alternate stator end bell  810  construction. In this alternative embodiment, the stator end bell  810  is shorter and more compact than the version of  FIG. 1 . It supports shaft  815  via bearings  830  and has no intermediate bushing. Further, rotor end bell  820  has a lower profile than the rotor end bell  120  of  FIG. 1 . In other respects, the motor  800  is analogous to that of  FIG. 1-7 , with a stator assembly  850  disposed generally inside a rotor assembly  805 . The stator assembly  850  includes an armature assembly  855  having steel laminations in the form of spokes, each spoke wound in wire forming a winding core. The rotor assembly  805  further includes a rotor end bell  820  attached to a rotor cylinder  890 . 
         [0044]    In some embodiments, a motor  100  may be constructed, using structures and methods described herein, that weighs at least 120 grams and can produce an average of at least 1600 watts of power over an interval of at least 150 seconds, with the temperature of the wire  250  not exceeding 140 Celsius. 
         [0045]    Of course, many variations of the above method are possible within the scope of the invention. For example, the respective structures of the mounting end bell and the rotor end bell can vary considerably while still allowing substantial axial air flow consistent with the inventive concept. The present invention is, therefore, not limited to all the above details, as modifications and variations may be made without departing from the intent or scope of the invention. Consequently, the invention should be limited only by the following claims and equivalent constructions.