Abstract:
A brushless disk DC motor that exhibits high power density and light weight and is capable of power regeneration and reverse operation employs a flat circular non-ferrous stator plate having a plurality of electromagnets mounted in a ring pattern on an inner face thereof. Permanent magnets are mounted in equal numbers in inner and outer ring patterns on the outer and inner cylindrical surfaces, respectively, of a pair of steel rotors of different diameter that rotate in concert. The stator plate and the pair of rotors are axially aligned such that the inner and outer rings of permanent magnets rotate adjacent to and inside and outside, respectively, the ring of electromagnets. The electromagnets utilize tape-wound amorphous metal cores to minimize eddy currents and resultant iron losses and to permit the use of heavier gauge copper windings to minimize resistive power losses. A greater number of poles in the form of permanent magnets can be accommodated, the number being limited only by the diameter of the rotor, thus providing increased power and torque over prior art brushless DC motors having a limited number of poles. The present motor exhibits up to 200% more starting torque, thus eliminating the need for a gear box or clutch in electric vehicle applications.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     This invention relates generally to DC motors and, more particularly, to a highly efficient, sealed, brushless, disk DC motor having inner and outer rings of permanent magnets mounted on cylindrical surfaces of outer and inner rotors, respectively, and having a ring of electromagnets mounted on a stator such that inwardly facing surfaces of the permanent magnets rotate in close proximity to adjacent opposite 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 on the order of 0.3 horsepower/pound. Thus, they are heavy and unresponsive, as a typical 40-horsepower motor weighs 130 pounds. They are not sealed, since air flow through the motor is required to cool the rotor, and they cannot run in reverse. Their speed is difficult to control under varying load conditions, and they have no power regeneration capability. 
     Three-phase AC induction motors are typically powered by a DC battery pack coupled to an 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. 
     Representative of prior art brushless DC motors are the High Power Density Brushless DC Motor described in U.S. Pat. No. 4,187,441; the Toroidally Wound Brushless DC Motor described in U.S. Pat. No. 4,547,713; and the Brushless Motors taught in U.S. Pat. Nos. 4,801,830; 4,982,125; 5,637,945; 5,689,147; and 5,747,910. 
     In accordance with the illustrated preferred embodiment of the present invention, a brushless DC motor employs a flat circular non-ferrous stator plate having a plurality of electromagnets mounted in a ring pattern on an inner face thereof. A plurality of permanent magnets are mounted in equal numbers in inner and outer ring patterns on the outer and inner cylindrical surfaces, respectively, of a pair of steel rotors of different diameter that rotate in concert. The stator plate and the pair of rotors are axially and diametrically aligned such that the inner ring of permanent magnets rotates in close proximity to and inside the ring of electromagnets, and the outer ring of permanent magnets rotates in close proximity to and outside the ring of electromagnets. The electromagnets utilize tape-wound amorphous metal cores to minimize eddy currents and resultant heat losses and to permit the use of heavier gauge copper windings to minimize resistive power losses and attendant heat. The DC motor of the present invention is capable of accommodating a larger number of poles, in the form of permanent magnets, than prior art brushless DC motors, the number being limited only by the diameter of the rotor, thus providing significantly increased power and torque. The precise location of the rotor is monitored through the use of three Hall effect sensors, and conventional three-phase pulse width modulation (PWM) control circuitry is employed as a source of operating power and to control the speed of the motor. The present motor exhibits higher power density and lighter weight than prior art brushless DC motors. It can be driven to 200% of its current rating 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, the present motor exhibits up to 200% more starting torque than conventional brushless DC 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 can be machine wound, the present motor can be manufactured at a cost saving over conventional AC induction motors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view, from the rotor end, of the brushless disc DC motor of the present invention. 
         FIG. 2  is an exploded view, from the stator end, of the brushless disc DC motor of  FIG. 1 . 
         FIG. 3  is a cross-sectional exploded view, from the stator end, of the brushless disc DC motor of the present invention. 
         FIG. 4  is a diametrical cross-sectional elevation view of the brushless disc DC motor of the present invention, illustrating the motor components of  FIGS. 1-3  in assembled form. 
         FIG. 5  is a cross-sectional plan view of the assembled motor components of  FIGS. 1-3 , taken along the section line  5 - 5  of  FIG. 4 . 
         FIG. 6  is a simplified rendition of the cross-sectional diagram of  FIG. 5 , illustrating the wiring interconnections between the stator electromagnets required to form three sets of windings for three-phase commutation. 
         FIG. 7  is a diagram illustrating a representative one of the tape-wound amorphous metal cores employed in each of the stator electromagnets of the brushless disc DC motor of  FIGS. 1-6 . 
         FIG. 8  is a simplified cross-sectional diagram of a representative encapsulated stator electromagnet utilizing one of the tape-wound amorphous metal cores of  FIG. 7 , illustrating how the encapsulated stator electromagnets are mounted to the stator plate of the motor of  FIGS. 1-6  in order to provide a strong mechanical connection and good heat conduction therebetween. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now generally to  FIGS. 1-6 , there is shown a motor  100 , in accordance with the present invention, that includes a stator assembly  10 , a rotor assembly  20 , a cover  30 , and a bearing  80 , all axially aligned. Stator assembly  10  includes a stator plate  12  fabricated as a flat disc of non-ferrous material such as aluminum or copper having a central circular opening  14  therein. A cylindrical stator hub  13  has a stator end of reduced outer diameter that fits into central opening  14  in stator plate  12  such that an inner face  15  of the stator end of stator hub  13  is flush with an outer surface  11  of stator plate  12 . Stator hub  13  is bolted in that position to stator plate  12  by means of a plurality of bevel-headed bolts  19  inserted through countersunk bolt holes  17  in stator plate  12  and into threaded  12  bolt holes  16  in an outer step face  21  of the stator end of stator hub  13 . Threaded bolt holes  18  provided in the inner face  15  of the stator end of stator hub  13  facilitate mounting of motor  100  to a vehicle structure when employed in a mobile application or to a rigid frame structure when employed in a stationary application, such as in a manufacturing facility, for example. 
     Stator assembly  10  includes a plurality of electromagnet assemblies  50  that are attached in a circular pattern to an inner surface  52  of stator plate  12 . Each of the electromagnet assemblies  50  includes a tape-wound amorphous metal core  60  having a rectangular central cavity  62 , as illustrated in  FIG. 7 . The tape-wound fabrication of each of the amorphous metal cores  60  results in rounded core corners that facilitate the addition of heavy gauge copper windings  64  on the cores  60  to minimize heat-producing resistive power loss therein that follow a heat path from cores  60  through windings  64  to stator plate  12 . The construction of cores  60  is advantageous over conventional motors and generators whose laminated magnetic steel stators generate excessive heat that is difficult to dissipate. Minimizing heat generation results in increased efficiency and is a critical feature of motor  100 , since reducing heat losses by one-half allows power output to be doubled. In addition to facilitating the reduction of heat losses in copper windings  64 , the use of amorphous metal cores  60  minimizes eddy currents and the resultant “iron losses” of prior art motors that employ laminated steel cores. Amorphous metal cores  60  allow operation at higher frequencies than is possible using conventional laminated steel cores and, hence, faster response by motor  100 . 
     The free ends of each of the copper windings  64  exit the electromagnet assemblies  50  for routing and interconnection in accordance with a desired configuration. Each amorphous metal core  60 , with copper winding  64  in place, is potted or encapsulated in a volume of a heat-conducting epoxy  66 , as illustrated in  FIG. 8 , using conventional potting methods and materials. The width of each of the electromagnet assemblies  50  is sufficiently greater than the width of each of the wound metal cores  60  so as to provide space for retaining four metal inserts  68  in the epoxy volume outside each of the wound metal cores  60 . The inserts  68  open to a mounting surface of each of the electromagnet assemblies  50 . Each of the electromagnet assemblies  50  is mounted to the inner surface  52  of stator plate  12  by means of four fasteners, such as screws  61  that are inserted through countersunk openings in the outer surface  11  of stator plate  12 , and into the four inserts  68 . Alternatively, the plurality of electromagnet assemblies  50  may be potted together as a single unit attached to the inner surface  52  of stator plate  12 , with the free ends of each of the copper windings  64  exiting the outer surface  11  of stator plate  12 . The free ends of the copper windings  64  may then be interconnected to configure the plurality of electromagnet assemblies  50  in series or in parallel, or as a wye or a delta, for example. The interconnection of copper windings  64  may be made by means of a circular buss attached to the outer surface  11  of stator plate  12 , for example. Potting the electromagnet assemblies  50 , whether individually or collectively, facilitates their attachment to stator plate  12  and mechanically strengthens the amorphous metal cores  60  to relieve stresses that may otherwise cause magnetorestriction. 
     Rotor assembly  20  includes a rotor  22  having concentric inner and outer cylindrical ferrous rotor bowls  23 ,  24 , respectively, and a circular drive hub  25 . Rotor bowls  23 ,  24  are inherently well balanced if fabricated of spun steel. Alternatively, they may be stamped from a steel plate, but this method of fabrication requires additional manufacturing operations to balance rotor assembly  20  to ensure that motor  100  runs smoothly and quietly at speeds of several thousand RPM. Fabrication of rotor bowls  23 ,  24  using spun steel techniques permits them to be made larger to accommodate permanent magnets in greater numbers and of higher strength, as described below, to thereby maximize the power output of motor  100 . Drive hub  25  preferably includes a central splined drive shaft receiver  26  which will accept an external mating shaft at either end thereof. The protruding end of the external drive shaft may be shaped to permit its connection to any device. The external drive shaft may either be driven by motor  100 , or it may be driven by an external device in those applications in which motor  100  is employed as a generator. Several of the motors  100  may be employed to form a bank of motors by stacking them on a single external splined drive shaft to thereby achieve nearly any desired level of power and torque. This arrangement also permits several smaller motors  100  to be employed in place of one large motor, thus allowing more flexibility in vehicle application design. Inner cylindrical rotor bowl  23  is formed to include an inwardly-facing open end and an outwardly-facing closed end having flat inner and outer surfaces. Outer cylindrical rotor bowl  24 , of larger diameter than inner rotor bowl  23 , but of equal length, is similarly formed to include an inwardly-facing open end and an outwardly-facing closed end having flat inner and outer surfaces. A central opening is provided in the closed ends of both cylindrical rotor bowls  23 ,  24  to allow access to drive shaft receiver  26 . Drive hub  25  includes a circular mounting flange  27  with four equally-spaced mounting holes  31  therein. Bolts  28  are inserted through mounting holes correspondingly located in the closed ends of both of the inner and outer rotor bowls  23 ,  24  and through mounting holes  31  in drive hub  25 . Nuts  29 , which are integrated with drive hub  25 , securely connect the inner and outer rotor bowls  23 ,  24  and drive hub  25  to each other. 
     A plurality of outwardly-facing permanent magnets  72  are mounted on the outer cylindrical surface of inner rotor bowl  23 . A plurality of inwardly-facing permanent magnets  74  are mounted on the inner cylindrical surface of outer rotor bowl  24 . Mounting of permanent magnets  72 ,  74  to rotor bowls  23 ,  24  may be accomplished by means of screws and/or an adhesive, for example. Each of the permanent magnets  72 ,  74  preferably consists of a high-strength, rare-earth compound magnet consisting of Nd—Fe—B or Nd—Dy—Co—Fe—B, for high temperature operation, for example. Each of the permanent magnets  72 ,  74  is also preferably shaped to include curved inner and outer surfaces having a curvature matching the curvature of the cylindrical walls of inner and outer rotor bowls  23 ,  24 . Permanent magnets  72 ,  74  are typically ½″ thick, 2″ long, and of sufficient width to subtend a 20° arc. Rare earth permanent magnets  72 ,  74  can be readily formed to most desired shapes. The use of permanent magnets  72 ,  74 , formed to have curved inner and outer surfaces, allows them to lie flat on the cylindrical surfaces of inner and outer rotor bowls  23 ,  24  for added strength. In addition, the curved permanent magnets  72 ,  74  allow for a minimum gap between permanent magnets  72 ,  74  and the respective steel rotor bowls  23 ,  24  to which they are attached, thus providing improved magnetic coupling and increased power. Alternatively, off-the-shelf permanent magnets having flat inner and outer surfaces may be employed, in which case each of the permanent magnets  74  would consist of two flat magnets of the same polarity. Whether curved or flat, the permanent magnets  72 ,  74  are magnetized such that one of the inner and outer surfaces thereof is magnetic N and the opposite surface is magnetic S. The permanent magnets  72 ,  74  are mounted in an orientation that provides alternating polarity around the cylindrical surfaces of rotor bowls  23 ,  24  to which they are mounted, as illustrated in  FIGS. 5 and 6 . Thus, every one of permanent magnets  72  that is magnetic N is mounted opposite one of permanent magnets  74  that is magnetic S and vice versa. The cylindrical walls of inner and outer rotor bowls  23 ,  24  act as flux rings to complete the paths of the magnetic fields produced by permanent magnets  72 ,  74  and to thereby contain those magnetic fields, thus maximizing the power output of motor  100 . 
     Stator assembly  10  and rotor assembly  20  are joined, during assembly of motor  100 , by a double-row, angular contact bearing  80 . Motor  100  is sealed against outside contaminants by means of a non-ferrous cylindrical cover  30  that is shaped like rotor bowls  22 ,  24  to have an outwardly-facing flat closed end and an inwardly-facing open end. Cylindrical cover  30  includes a central opening in its outwardly-facing closed end that is aligned with the central openings in rotor bowls  22 ,  24 . When motor  100  is assembled, cover  30  fits over rotor bowl  24  such that the central opening in cover  30  engages an outwardly-protruding cylindrical extension  76  of drive hub  25  and such that the inner cylindrical surface of cover  30  fits over the peripheral edge of stator plate  12 . Cover  30  is secured in place by means of a plurality of screws or bolts  78  inserted through holes provided in the cylindrical surface of cover  30  adjacent the open end thereof and into corresponding holes in the peripheral edge of stator plate  12 . This arrangement provides contaminant sealing by means of cover  30  at both the central opening therein and at the open end thereof. 
     Motor  100  is constructed such that rotor bowl  24  has a large diameter/length ratio, thus providing a larger surface area for radiant cooling. Radiant cooling may be further enhanced with the addition of pin-finned heat sinks attached to the outer surface  11  of stator plate  12 . Motor  100  may also be liquid-cooled if required in high power applications. Water cooling, for example, may be accomplished by nickel-plating the aluminum stator plate  12  and soldering a single turn of shaped copper tubing to the outer surface  11  thereof. A conventional liquid coolant may be circulated through this copper tubing to a heat exchanger. Alternatively, an entire coolant sleeve may be attached to outer surface  11  of stator plate  12 . In very high power applications, stator plate  12  and cover  30  may be fabricated of copper, and permanent magnets  72 ,  74  may be fabricated of high-temperature N45HS material. 
     Referring now to  FIG. 5  and the wiring diagram of  FIG. 6 , it can be seen that twelve stator electromagnet assemblies  50  are wired together in three sets or phases, Aa, Bb, Cc, of four each, by means of a plurality of interconnections  51 . Two of the electromagnet assemblies  50  forming the set, Aa, for example, are adjacent to each other and are spatially displaced 180° from the other two electromagnet assemblies in the set Aa. The two adjacent electromagnet assemblies of a particular set are wound in opposite directions, so they exhibit reverse polarity when energized. The order of energization of the twelve electromagnet assemblies  50  is AabBCcaABbcC. 
     Motor  100  is commutated by employing three Hall effect sensors  80  that are mounted in a standard 120° configuration. Sensorless control may also be employed instead to control motor  100 . Motor  100  may be energized by any standard three-phase brushless controller that is properly suited to match the output power capability of motor  100 . 
     The illustrated preferred embodiment of motor  100  that utilizes twelve electromagnet assemblies  50 , fourteen permanent magnets  72 , and fourteen permanent magnets  74 , eliminates a locked position of the rotor assembly  20  by insuring that at all times two-thirds of the electromagnet assemblies  50  are in a position to be energized and drive the rotor assembly  20  forward. Depending on the desired performance characteristics of motor  100 , other numbers of electromagnet assemblies  50  and permanent magnets  72 ,  74  and ratios thereof are possible, such as 9/12, 9/6, 12/16, 12/10, and 12/8, for example. 
     An external conventional electronic controller is connected to motor  100  to sense the state of each of the three Hall effect sensors  80  and to supply operating power to the electromagnet assemblies  50 . Motor  100  has been found to run optimally using a Controller capable of supplying 96-144 volts of operating power, but can run at lower voltages such as 12, 24, 36, 48, 60 or 72 volts. In contrast, prior art AC induction motors typically require higher operating voltages. Any of a number of off-the-shelf pulse width modulation (PWM) controllers, such as those manufactured by Kelly Controls, LLC, for example, may be employed to control motor  100 . Such a controller measures speed and current drawn by motor  100  and includes three PWM channels to vary the current as the load varies. 
     The motor structure described in detail above permits motor  100  to be optimized for use in various applications, such as driving utility terrain vehicles, automobiles, trucks, and buses. It may also be utilized as a generator in wind turbine and motor/generator set applications. Optimization for different applications is accomplished by varying the Kv constant (RPM/voltage) within the range of less than 1.0 to more than 50, by varying the ratio of the number of electromagnet assemblies  50  to permanent magnets  72 ,  74 , by varying the number of turns of copper windings  64  on each of the electromagnet assemblies  50 , or by varying the wiring interconnection configuration of the electromagnet assemblies  50 . For example, a thirteen-inch diameter embodiment of motor  100  having twelve electromagnet assemblies  50 , fourteen permanent magnets  72 , and fourteen permanent magnets  74 , as illustrated, was tested and developed a constant torque of approximately 400 ft. lbs. and a constant power output of 10 kilowatts (13.41 horsepower).