Abstract:
A radial flux permanent magnet AC motor/generator employs a flat circular stator plate having a plurality of separately-formed electromagnets mounted in a ring pattern on a top surface thereof. A circular flux ring fabricated of powdered metal is mounted to the stator plate outside the ring of electromagnets. A plurality of permanent magnets are mounted in a ring pattern on the outer cylindrical surface of a steel rotor. The stator plate and rotor are axially and diametrically aligned such that the ring of permanent magnets rotates in close proximity to and inside the ring of electromagnets. The electromagnets utilize powder metal cores shaped to have rounded corners and flat sides that permit the use of heavier gauge windings and eliminate the air gaps that exist between the core and windings of prior art electromagnets.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates generally to permanent magnet motors and generators and, more particularly, to a highly efficient sealed AC radial flux motor/generator having a ring of permanent magnets mounted on the cylindrical surface of an inner rotor and having a ring of electromagnets mounted on an outer stator such that outwardly facing surfaces of the permanent magnets rotate in close proximity to the inwardly facing surfaces of the ring of electromagnets. 
     Variable speed electric motors of different types have been employed in a variety of applications over the years, most recently in electric vehicles. One such motor is the series DC motor, which employs brushes and wound coils. These motors suffer from low peak power densities. Thus, they are heavy and unresponsive; they are not sealed since air flow through the motor is required to cool the rotor; and they cannot run in reverse. Furthermore, their speed is difficult to control under varying load conditions, and they have no power regeneration capability. 
     Three-phase asynchronous AC induction motors are typically powered by a DC battery pack coupled to a DC/AC inverter and associated pulse width modulation circuitry to achieve variable speed control. These motors are characterized by heavy weight, low torque, and power factor losses. Power electronics are employed to receive a DC voltage supplied by a battery pack and produce an AC voltage whose waveform frequency is modulated so that the motor&#39;s speed may be varied. These motors offer good speed control, are brushless, can be sealed, can run in reverse, and provide power regeneration. However, AC induction motors exhibit low starting torque, which means they usually require a gear box, have power factor losses, are typically more expensive than series DC motors, and the controller required to vary the motor&#39;s speed is large, complex and expensive as it must handle the slippage between the input current and the rotational speed necessary for such an asynchronous motor to operate. 
     Brushless DC motors, a more recent development, are lighter, more powerful, produce more torque, and are typically more efficient than comparably sized AC induction motors. These motors use permanent magnets mounted on the rotor, obviating the need for brushes as no power need be supplied to the rotor. Power to the electromagnets mounted on the stator is typically controlled by power electronics that sense the position of the rotor permanent magnets with Hall sensors and use six-step commutation and pulse width modulation to control the electromagnets. These motors offer good speed control, power regeneration, are brushless, are sealed to protect the internal components from dirt and foreign particles, are reversible, are lighter in weight, are more powerful, are more efficient than series DC motors, and exhibit higher torque than AC induction motors. 
     The most recent development in motor art is the permanent magnet AC motor which is virtually identical to the brushless DC motor except that it uses more precise sinusoidal control than six-step commutation, with the result that the motor delivers more torque, especially at the higher speeds at which it is capable of operating. Sinusoidal controllers typically require encoders or resolvers, rather than Hall sensors, to more accurately signal the position of the rotor electromagnets so that power can be applied to them as a smooth sinusoidal waveform rather than as a stepped waveform. Thus, the essential difference between a brushless DC motor and a permanent magnet AC motor lies not in the motor itself but in how it is driven, either sinusoidally or by step commutation, and in the required sensing. As a broad class, brushless DC (BLDC) motors and permanent magnet AC (ACPM) motors are appropriately referred to simply as permanent magnet motors. Unlike asynchronous AC induction motors, there is no slippage between the input current and the motor speed and so an ACPM motor controller is lighter, less complex, and less expensive than the controllers used for AC induction motors. 
     A permanent magnet motor, whether a BLDC or an ACPM motor, is usually one of two types. The first type is a radial flux motor in which the magnetic flux field in the air gap between the rotating permanent magnets and the stationary electromagnets is produced in a radial direction from the center of the motor to the outside thereof, and vice versa. The second type is an axial flux motor in which the magnetic flux field in the air gap between the rotating permanent magnets and the stationary electromagnets is produced in an axial direction parallel to the axis of rotation of the rotor. Both types of motors offer good magnetic structures, but radial flux motors have a mechanical advantage in that the attractive and repulsive forces between the permanent magnets and electromagnets are carried by the rotor and stator flux rings in tension or compression, while those forces are carried in axial flux motors by the rotor and stator plates in bending, thus requiring a heavier structure and heavier bearings. 
     Permanent magnet AC and DC motors are known in the prior art. U.S. Pat. No. 5,689,147 to Kaneda et al. is directed to a brushless motor having a toothed stator core. U.S. Pat. No. 5,903,082 to Caamano is directed to an axial flux motor/generator having a laminated amorphous metal core. U.S. Pat. No. 5,918,360 to Forbes et al. is directed to a radial flux motor having individually-wound laminated electromagnetic cores. U.S. Pat. No. 6,700,279 to Uchiyama et al. is directed to a brushless motor in which the electromagnet cores are teeth on the stator flux ring, U.S. Pat. No. 6,384,496 to Pyntikov et al. is directed to a multiple-magnetic-path motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,617,746 to Maslov et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,717,323 to Soghomonian et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 6,727,629 to Soghomonian et al. is directed to a radial flux motor in which electromagnets are arranged in pairs rather than individually. U.S. Pat. No. 7,541,711 to Adaniya et al. is directed to a radial flux motor in which the electromagnet coils are wound on sharp-cornered stator teeth. U.S. Pat. No. 7,898,134 to Shaw is directed is directed to a radial flux motor that utilizes a double rotor rather than a stator flux ring. 
     Representative of the prior art cited above is the motor/generator  1000  illustrated in  FIG. 1  of the present drawings that includes a stator flux ring  1100 , a plurality of teeth  1200  formed to protrude inwardly from stator flux ring  1100  serving as the cores of stator electromagnets like electromagnet  1401  of  FIG. 2 , and a like plurality of shoes  1300  positioned at the inward ends of the teeth  1200  for the purpose of optimally diffusing the magnetic flux field produced by the stator electromagnets  1401 . Stator flux ring  1100 , teeth  1200 , and shoes  1300  are typically formed as a single laminated component using steel plates stamped from a stack of steel plates. The use of laminated steel plates minimizes eddy currents and consequent iron losses in the finished component and also provides the mechanical strength required to resist the rotational forces applied at the shoes  1300 . Stator flux ring  1100  is conventionally mounted to a stator plate or directly to the motor case so that the shoes  1300 , electromagnets  1401 , and stator flux ring  1100  are able to transfer those rotational forces to the external body to which the motor is mounted. 
     Each of the teeth  1200  is typically wound with insulated wire  1400  to form the electromagnet  1401  shown in cross-section in  FIG. 2 . As illustrated, the windings  1400  contact the square corners  1202  of the teeth  1200 , thus leaving a considerable air gap  1450  between the windings  1400  and the laminated core material that forms each of the teeth  1200 . Electromagnets  1401  can be formed with an air core only, but in that case would produce a much lower magnetic force. Ferrous cores serve to greatly increase the magnetic force produced by any electromagnet. Thus, the magnetic force produced by prior art electromagnet  1401  is limited due to the presence of the air gap  1450 . Because the physical size of electromagnets  1401  is limited in any given motor design, the size of copper wire  1400  is also limited, thus increasing the resistive losses in the windings. In addition, it is difficult to mechanize the winding of teeth  1200  in the presence of shoes  1300 , so winding is typically accomplished manually, at much higher cost. 
     The theory of operation of the prior art motor/generator  1000  of  FIG. 1  is well understood by those skilled in the art. The magnetic flux field produced by each of the electromagnets  1401  is selectively switched on and off by an electronic controller, based on the relative position of the rotor with respect to the stator. This magnetic flux field extends through the shoes  1300  and the air gap  1800  and interacts with the magnet flux field produced by the permanent magnets  1600 , which are either mechanically or adhesively mounted on the rotor flux ring  1700 , thereby producing rotational motion of the rotor body  1750  and of a drive shaft to which it may be connected. The space  1850  between the finish cover  1990  and the stator  1100  can be filled with a heat conducting epoxy or left empty. 
     The present application is directed to a radial flux permanent magnet AC motor/generator having a magnetic and mechanical structure that is unlike any of the representative prior art motors described above. 
     In accordance with the illustrated preferred embodiment of the present invention, a radial flux permanent magnet AC motor/generator employs a flat circular stator plate having a plurality of electromagnets mounted in a ring pattern on an inner face thereof. A circular flux ring fabricated of powder metal is permanently mounted to the stator plate outside the ring of electromagnets. Permanent magnets are mounted in a ring pattern on the outer cylindrical surface of a steel rotor. The stator plate and the rotor are axially and diametrically aligned such that the ring of permanent magnets rotates in close proximity to and inside the ring of electromagnets and such that the magnetic flux field across the air gap between the permanent magnets and electromagnets is in the radial direction. The electromagnets utilize powder metal cores which minimizes eddy currents and resultant heat losses, to permit the use of heavier gauge copper windings to minimize resistive power losses and attendant heat, and to allow the use of higher frequency drivers to further increase the magnetic force produced by the electromagnets. 
     The radial flux permanent magnet AC motor of the present invention utilizes electromagnets whose cores are not simply tooth extensions formed along the inner edge of a laminated flux ring, in accordance with the prior art. Instead, the electromagnets of the present invention are separately formed units that can be more easily wound, using automatic machines, and that are made of powder metal that can be shaped to have rounded corners and flat sides that permit the use of heavier gauge windings and facilitate heat removal. Further, by so shaping the electromagnet cores, the usual air gaps between a core and the windings thereon are eliminated, resulting in the production of a stronger magnetic field. 
     The electromagnets are anchored to resist the rotational forces produced by the motor through the use of a structural epoxy material filling the space between each of the electromagnets and the stator surface on which the electromagnets are positioned. The epoxy material in turn transfers the rotational forces to non-metallic anchor plates that are mounted to the stator midway between each of the electromagnets. This is unlike prior art motors in which the electromagnet cores are tooth extensions of the flux ring and rotational force is transferred through the core to the flux ring. 
     The precise location of the rotor is monitored through the use of a conventional encoder or resolver, and conventional sinusoidal control circuitry is employed to direct operating power and to control the speed of the motor. 
     The present motor exhibits higher power density and lighter weight than prior art permanent magnet AC motors or brushless DC motors. It is also more efficient, resulting in low heat generation, the small amount of which may be quickly conducted away by means of a liquid coolant circulated through tubes imbedded in the stator and positioned under the electromagnets. Alternatively, heat may be conducted to an outer surface of the motor that is air-cooled or by connecting the motor to a large heat sink such as the body of a vehicle using metallic heat conducting fasteners or a heat strap. 
     The present motor may be driven to 200% of its sustainable continuous current to thereby double the output horsepower for short periods of time. It is reversible, is capable of power regeneration, and offers good speed control. By employing a large diameter rotor and many electromagnet pole pieces, and by increasing the power of the electromagnets through the use of heavier gauge windings and more flux magnifying material in the cores, the present motor exhibits up to 200% more starting torque than prior art brushless DC and permanent magnet AC motors, thus eliminating the need for a gear box or clutch in electric vehicle applications. Since the rotor components are individually assembled and the electromagnet coils are machine wound, the present motor can be manufactured at a cost saving over prior art motors. 
     In summary, the AC motor/generator of the present invention 1) utilizes electromagnet cores fabricated of a high-performance powder metal material to increase their performance over prior art laminated steel cores; 2) utilizes electromagnets wound with heavy gauge copper wire to minimize resistive losses; 3) eliminates any air core from the electromagnets thereof to thereby increase the magnetic force produced by the electromagnets; 4) utilizes electromagnets having a flat surface that is interfaced to the stator surface to provide improved heat conduction from the electromagnets to the stator; 5) utilizes separately formed cores and shoes in order to facilitate machine winding of the electromagnets; 6) utilizes electromagnetic cores that are separate from the flux ring and shoes to further enable machine winding of the electromagnets; 7) utilizes a structural epoxy material in combination with non-metallic anchor plates to anchor the electromagnets to the stator; and 8) utilizes wedges to hold the permanent magnets to the rotor surface to provide resistance to rotational forces, thereby eliminating the prior art use of screws that penetrate the permanent magnets, displacing some of the magnetic material thereof and reducing their magnetic field strength. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diametrical cross-sectional plan view of a prior art permanent magnet motor, showing both the stator and rotor assemblies. 
         FIG. 2  is a cross-sectional elevation view through of one of the electromagnets of the prior art motor of  FIG. 1 . 
         FIG. 3  is a diametrical cross-sectional plan view of the motor/generator of the present invention, showing both the stator and rotor assemblies 
         FIG. 3A  is an isometric view showing the use of wedges for anchoring the permanent magnets to the rotor. 
         FIG. 4  is diametrical cross-sectional plan view through the stator plate of the motor/generator of the present invention, showing coolant tubes embedded in the stator for conveying a liquid coolant that serves to conduct heat away from the motor. 
         FIG. 5  is a diametrical cross-sectional elevation view of the motor/generator of  FIG. 3 . 
         FIG. 6  is a cross-sectional elevation view of one of the stator electromagnets employed in the motor/generator of  FIGS. 3 and 5 . 
         FIG. 7  is an isometric view of one of the electromagnets employed in the motor of the present invention, showing an associated shoe that is positioned adjacent to both the inner end of the electromagnet and the outer cylindrical surface of the rotor assembly, as illustrated in  FIGS. 3 and 5 . 
         FIG. 8  is a diametrical cross-sectional elevation view of the motor/generator of  FIG. 3 , showing heat-conducting tabs that are employed to conduct heat away from a non-metallic stator plate. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now generally to  FIGS. 3-8 , there is shown the stator and rotor assemblies of a motor/generator  100  in accordance with the present invention. Referring more specifically to  FIG. 3 , there is shown a stator flux ring  10 , constructed of powder metal, a plurality of separate powder metal cores  20  wound as individual electromagnets and positioned immediately adjacent to and in contact with the stator flux ring  10 , and a like plurality of shoes  30  also constructed of powder metal to diffuse the magnetic flux field produced by each of the electromagnets. The powder metal stator flux ring  10 , powder metal cores  20 , and powder metal shoes  30  are not fabricated as a single component, as in the case of prior art motor  1000  of  FIG. 1 , but as separate components to facilitate their manufacture using molds and presses and to facilitate machine winding of the cores  20  to form electromagnets. The stator flux ring  10  provides a path for the magnetic flux field between the electromagnets. 
     The use of powder metal for fabricating the stator flux ring  10 , electromagnet cores  20 , and shoes  30 , rather than laminated steel plates, as in the prior art, provides the advantages of allowing the cores  20  to be machine wound, optimally shaped for winding with heavy copper wire, and for improved heat dissipation. The use of powder metal in fabricating these components also reduces eddy currents in the flux ring  10 , cores  20 , and shoes  30  at power frequencies higher than the usual 60 Hz, above which laminated steel plates suffer significant and increasing eddy currents and losses. When direct-connected to the drive wheels of an electric vehicle, motor  100  must run at electrical power frequencies up to 400 Hz. 
     Referring now to  FIG. 6 , there is shown a cross-sectional elevation view of one of the stator electromagnets employed in the motor/generator of  FIGS. 3 and 5 . Flux ring  10 , core  20 , and shoe  30  are fabricated of a commercially available powder metal such as AnchorLam or Somaloy and is shaped to have rounded corners  22 , as illustrated, and an otherwise flat outer surface to facilitate good contact between the first layer of windings  40  and the outer surface of the core  20 . The resulting flat bottom surface  26  of each of the wound electromagnets provides an improved interface for heat conduction to an aluminum stator plate  90 . The rounded corners  22  of electromagnet cores  20  permit them to be tightly wound with a heavier insulated wire that is preferably rectangular in cross section for easy bending. The result is that the first layer of winding  40  contacts the core  20  at all points on its outer peripheral surface, thereby eliminating any of the air voids  1450  that are typically present in prior art electromagnets like that of  FIG. 2 . Thus, wire equivalent to 5-gauge square wire can be wound onto a small core whose overall height and width dimensions are as small as 1.75 inch by 1.15 inch, respectively. The air void  1450  in the prior art electromagnet  1401  of  FIG. 2  is the result of the laminated steel core material  1200  being punched from a stack of steel plates to have sharp corners  1202  that prevent the first layer of windings  1400  from being in close and complete contact with the top, bottom, and side surfaces of the core  1400 . By maximizing the cross-sectional area of electromagnet core  20 , as illustrated in  FIG. 6 , the magnetic force produced by the electromagnet is increased. 
     The depth of the windings  40  of the electromagnet of  FIG. 6  is clearly greater that the depth of the windings  1400  of the  FIG. 2  prior art electromagnet  1401  having the same overall dimensions. Mobile applications that employ electric motors, such as electric vehicles, have limited space for fitting a motor, thereby restricting the dimensions of the electromagnets themselves. By increasing the depth and weight of the windings  40  of the electromagnets employed in the present motor, reduced copper losses are achieved for a given current or much higher currents are obtainable, resulting in higher torque output, for the same gauge wire. Alternatively, more turns can be wound onto the electromagnet core  20  in a given space, thus increasing the ampere turns parameter and, hence, the magnetic force produced by the electromagnet. 
     The electromagnets of  FIG. 6  are not anchored as being extensions of the stator flux ring  10  that is fastened to the aluminum stator plate  90 , as is the case in prior art motors. Instead, the stator flux ring  10  of  FIG. 3  is formed to include wide teeth  12  that protrude into the space between the electromagnets and against which the electromagnets bear. Anchor plates  53 , constructed of a commercially available non-metallic engineered polymer such as PEEK, are positioned between the electromagnets and are mechanically attached to the aluminum stator plate  90  by stainless steel anchor screws  55 . The space between the electromagnets and each of the anchor plates  53  and wide teeth  12  is filled with a structural epoxy potting material  50 , against which the electromagnets bear when subjected to the rotational forces produced during motor operation. Those forces are first transmitted to the anchor plates  53 , then to anchor bolts  55 , then to the stator plate  90 , and finally to the external structure to which motor  100  is mounted. 
     The shoes  30  are made of powder metal, like electromagnet cores  20  and flux ring  10 . As may be seen in  FIG. 7 , the shoes  30  are slanted in order to reduce cogging torque as the permanent magnets  60 , mounted on the outer cylindrical surface of the rotor flux ring  70 , move from one of the stator electromagnets to the next. Shoes  30  cannot be mechanically attached to the electromagnet cores  20  whose powder metal composition is soft and brittle. Instead, shoes  30  are anchored by stainless steel screws  33  located at the distal acute-angled corners of each of the shoes  30  that is positioned in a magnetic null zone midway between the electromagnets. Shoes  30  are potted into a structural epoxy material  50  that fills the space between the electromagnets. 
     Heat that is generated primarily in the windings  40  of the electromagnets must be conducted away and dissipated. Each of the electromagnets is positioned on a separate sill pad  45  that overlies stator plate  90 . Each of the sill pads  45  provides electrical isolation of the flat bottom surface  26  of an associated one of the electromagnets from stator plate  90 . In addition, since each of the sill pads  45  has a high dielectric coefficient, they can made sufficiently thin to conduct heat to stator plate  90 . A coolant such as a water/glycol mixture is circulated through a copper tube  92 , imbedded in a groove provided in stator plate  90 , as shown in  FIGS. 4 and 5 , by way of inlet and outlet ports  94 ,  96  to an external heat exchanger. The routing of tube  92  within stator plate  90 , as illustrated in  FIG. 4 , to include two parallel circular paths, one outgoing and one return, connected by a U-turn at the far end, ensures equal average temperature of the coolant in the two tubes beneath each electromagnet. 
     The electromagnets of the present motor/generator are potted using a structural epoxy  50  that is also heat conducting in order to transfer heat from the windings  40  to the aluminum stator plate  90  or to the stator flux ring  10 , then through a heat conductive epoxy  95  located between the stator flux ring  10  and the motor/generator case  99  for external dissipation. Additional heat generated by both iron and hysteresis losses within the electromagnet cores  20  is likewise transferred away from motor/generator  100 . 
     In accordance with an alternative embodiment, stator plate  90  may be fabricated of a commercially available moldable engineered polymer material such as Ultem or PEEK. Use of such a non-metallic material eliminates eddy currents in stator plate  90  and results in a lighter weight and less expensive motor/generator  100 . If this alternative is chosen, the imbedded liquid coolant tube  92  of  FIGS. 4 and 5  may be eliminated, and heat-conducting tabs  98  can be inserted into the stator plate  90  directly beneath the sill pads  45  on which the electromagnets bear, as illustrated in  FIG. 8 . Heat-conducting tabs  98  are fabricated of a material such as copper or aluminum and are directly connected to a large external heat sink such as a vehicle body, or they are connected to the heat sink by way of a woven copper or aluminum heat-conducting strap. Use of the above-described alternative structure eliminates the need for an expensive liquid coolant and heat exchanger system and permits motor/generator  100  to be positioned in a closed location without the cooling benefit of outside air flow. 
     The rotor assembly of motor/generator  100  is sized to provide an air gap  80  between the inner face of each of the electromagnet shoes  30  and the outer face of each of the permanent magnets  60 . This air gap is minimized to be approximately 0.100″. With additional reference to  FIG. 3A , permanent magnets  60  are affixed to a rotor flux ring  70  which provides a magnetic flux path between the permanent magnets  60 . The permanent magnets  60  are held in place on the rotor flux ring  70  by a series of aluminum wedges  63 , rather than by the screws used in prior art motors. The permanent magnets  60  are shaped to have a wide base so that they can be captured by the wedges  63  to prevent the permanent magnets  60  from flying off the rotor flux ring  70  at high rotational speeds. In order to facilitate construction of motor/generator  100 , wedges  63  are first positioned loosely. Each of the permanent magnets  60  is then slid into place with the wedges  63  acting as a positioning guide. The wedges are then tightened by means of wedge screws  65  that are inserted into rotor flux ring  70 . Rare earth permanent magnets  60  like those used in the present invention are characteristically brittle. By employing wedges  63  to carry the rotational forces produced during operation of motor/generator  100 , the prior art use of screws through each of the permanent magnets  60  that reduces the magnetic material and subjects it to high bearing forces is eliminated. 
     Permanent magnets  60  are preferably either samarium cobalt (SmCo) or neodymium iron boron (NdFeB). If NdFeB permanent magnets are used, they are electrically insulated from the wedges  63  and rotor flux ring  70  by means of a dielectric material  67  such as capton tape. Dielectric material  67  serves to prevent eddy currents generated in NdFeB permanent magnets  60  from flowing into rotor flux ring  70 , rotor body  75 , and then across the motor bearing that could cause it to pit and fail over time. 
     An output shaft may be conventionally attached to rotor body  75  to transfer the rotational energy produced by motor/generator  100  to an external load.