Abstract:
An electric machine ( 10 ) has a disk-shaped rotor ( 24 ) disposed in an operating space between two opposing stator assemblies ( 11, 12 ) to provide two axial air gaps ( 15, 16 ). The rotor ( 24 ) has a hub ( 28 ) and an outer ring ( 26 ) of non-magnetic material and is further provided with a plurality of permanent magnetic elements ( 25 ) for coupling flux that is induced by the magnetic field of the stator assemblies ( 11, 12 ). The permanent magnetic elements ( 25 ) are spaced apart and reluctance poles ( 27 ) are positioned in spaces between the magnetic elements ( 25 ) to couple additional flux induced by the magnetic field of the stator assemblies ( 11, 12 ). Various constructions and shapes ( 40 - 45 ) for the PM magnetic elements ( 25 ) are disclosed, and including PM covers ( 60 ) of ferromagnetic material for enhancing q-axis flux in the air gaps ( 15, 16 ) and for reducing harmonics where toothed stators are used. Methods of providing increased torque using the the various rotor constructions are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation-in-part of U.S. patent application Ser. No. 10/018,751, filed Dec. 21, 2004, and now copending. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0002]     This invention was made with Government support under Contract No. DE-AC05-00OR22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy. The Government has certain rights in this invention. 
     
    
     TECHNICAL FIELD  
       [0003]     The field of the invention is brushless machines, including both ac and dc machines, including both motors and generators, and including induction machines, permanent magnet (PM) machines and switched reluctance machines.  
       DESCRIPTION OF THE BACKGROUND ART  
       [0004]     There are various radial-gap PM-reluctance motors available in the market. However, there is no axial-gap PM reluctance motor seen in the market. The rotor structure of an axial-gap motor is a thin disk, which is very different from the rotor of the radial-gap motor.  
         [0005]     It is commonly known that a permanent magnet (PM) electric machine has the properties of high efficiency and high power density. By introducing a reluctance path to a PM motor the total torque that includes the PM synchronous torque and the reluctance torque can be increased.  
         [0006]     U.S. Pat. No. 4,996,457 issued Feb. 26, 1991, disclosed a high speed permanent magnet (PM) axial gap machine with multiple stators. This machine employed one rotor sandwiched between two stators. This early machine consisted of two supporting non-magnetic annuli, each having an even number of embedded equi-angularly spaced cylindrical magnets held in place against opposite sides of a ferromagnetic flux return plate. In U.S. Pat. No. 5,117,141 issued on May 26, 1992 equi-angularly spaced cylindrical magnets were embedded in a single non-magnetic disk to enable a single rotor to be used by two stators and to allow flux to pass through both stators and the rotor. This allowed more efficient use of magnetic material, which is the most expensive component of a PM motor.  
         [0007]     Permanent magnet motors produce a back-emf and torque that depend upon the amount of magnetic material; however, after the back-emf reaches the level of the supply voltage it becomes difficult to drive the motor.  
         [0008]     It is desired to make such an axial gap PM machine that will provide increased torque for the same amount of applied energy.  
         [0009]     Naito et al., U.S. Pat. Pub. 2004/0135453, discloses an axial gap PM machine with an iron rotor having reluctance poles. This machine is intended for use as a starter motor and generator in a motor vehicle. This machine has a stator on only one side as it is primarily for starting and providing power for accessories in a vehicle. Such a machine is not deemed suitable for application as a traction drive motor in an electric or hybrid vehicle.  
         [0010]     It would be desirable to provide a PM and reluctance pole machine that operates not only for startup and a low-level power supply, but also as a machine that is suitable for operating through the full speed range of a vehicle. Such a machine would have a higher power rating and other characteristics which are different from the prior art vehicle generators.  
         [0011]     The single-sided stator in the prior art machine provides a magnetic pull on only one side. This does not provide sufficient axial balance under loading conditions encountered by a vehicle traction motor. The single stator design would result in a heavy axial load under such circumstances.  
         [0012]     The prior art starter motor/generator with reluctance poles has a high rotor inertia due to the heavy rotor iron disc which is used for a flux return yoke path.  
       SUMMARY OF THE INVENTION  
       [0013]     This invention teaches a method for improving a conventional axial-gap PM machine to become a high strength, high power PM reluctance machine.  
         [0014]     The prior art PM machine is characterized by a rotor disk having a plurality of PM wedges disposed radially on the disk. This invention proposes to reduce the angle and width of the PM pole elements and alternate them with smaller ferromagnetic reluctance pole elements. The reluctance torque provided through the reluctance poles adds more torque than the PM elements. Less PM material will allow the motor to reach higher speeds.  
         [0015]     The invention is incorporated in a brushless electric machine, comprising at least one stator assembly for receiving ac electrical power to provide a magnetic field, a rotor disposed within the magnetic field of the stator assembly and spaced from the stator to define a primary air gap relative to an axis of rotation for the rotor. The rotor is disk-shaped and is spaced from the stator assembly along an axis of rotation for the rotor to form a first axial air gap. The rotor is provided with a plurality of permanent magnetic elements for coupling flux that is induced by the magnetic field of the stator assembly. The permanent magnetic elements are spaced apart and reluctance poles are positioned in spaces between the magnetic elements to couple additional flux induced by the magnetic field of the stator assembly.  
         [0016]     A further aspect of the invention is that with stator assemblies on both sides of the rotor, axial loading on the rotor is reduced. In a further aspect, by providing a rotor hub and outer ring of non-magnetic material, a flux return path for flux through the reluctance poles does not pass through supporting parts of the rotor, and the reduction in iron reduces rotor inertia. In still a further aspect, PM covers or outer layers are provided for the PM elements to enhance q-axis (torque-producing) flux through the reluctance poles and to shield the PM elements from harmonics caused by the stator assemblies having a toothed configuration.  
         [0017]     The invention is also practiced in a method of increasing available torque in a brushless electrical PM machine, the method comprising providing a rotor with PM poles spaced apart and with reluctance poles situated in the spaces between the PM poles, inducing a flux in a rotor disposed between two stator assemblies, the flux being conducted through two air gaps spaced along an axis of rotation of the rotor, and positioning the rotor with the reluctance poles offset from the stator teeth such that the flux is twisted as it is conducted from one tooth on one of the stator assemblies through one of the reluctance poles in the rotor and into another one of the two stator assemblies.  
         [0018]     The invention also provides various advantageous configurations for the rotor, the PM poles and the reluctance poles.  
         [0019]     The invention is also applicable to both ac and dc machines and to both motors and generators.  
         [0020]     Other objects and advantages of the invention, besides those discussed above, will be apparent to those of ordinary skill in the art from the description of the preferred embodiments which follows. In the description reference is made to the accompanying drawings, which form a part hereof, and which illustrate examples of the invention. Such examples, however are not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]      FIG. 1  is a sectional view of a basic configuration of an axial gap PM machine applicable to both the prior art and to the present invention;  
         [0022]      FIG. 2  is a plan view of a rotor for the axial machine of the prior art;  
         [0023]      FIG. 3  is a plan view of a rotor for the axial gap machine of the present invention;  
         [0024]      FIG. 4  is a sectional view of several reluctance pole configurations for the rotor according to the present invention;  
         [0025]      FIG. 5  is a detail sectional view of the embodiment in  FIGS. 1 and 3  taken in a plane indicated by line  5 - 5  in  FIG. 1 ; and  
         [0026]      FIG. 6  is a detail view of several rotor PM poles and reluctance poles of the present invention, showing a skew angle;  
         [0027]      FIG. 7  is a plan view of a second embodiment of a rotor for an axial gap machine of the present invention;  
         [0028]      FIG. 8  is a detail plan view of a rotor in an embodiment according to  FIGS. 1 and 3  with the addition of PM covers;  
         [0029]      FIG. 9  is an edge view of the rotor of  FIG. 8  with an outer ring removed;  
         [0030]      FIG. 10  is an edge view of the rotor of  FIG. 8  with an outer ring removed, showing q-axis flux paths in the machine;  
         [0031]      FIG. 11  is an enlarged detail edge view of the rotor of  FIG. 8  with an outer ring removed, showing tooth harmonic flux paths; and  
         [0032]      FIG. 12  is graph of flux vs. time in the air gap seen in  FIG. 11 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]      FIG. 1  is a sectional view of a basic configuration of an axial gap PM machine  10  applicable both to the prior art and to the present invention.  FIG. 2  is a plan view of a rotor for the axial machine of the prior art that illustrates portions of permanent magnet (PM) material  22  of north (N) and south (S) polarity embedded in a non-magnetic disk  23  to form a rotor  14 . This embodiment provides eighteen such PM poles. A hub  17  is provided and this is securely attached the rotor shaft  18  with the aid of collar pieces  17   a  seen in  FIG. 1 . The rotor shaft  18  is carried in bearings  19 ,  20  in a machine housing  21 . The rotor  14  is positioned between two stator assemblies  11 ,  12  which are spaced apart to provide a vertical operating space for the rotor  14 . Each stator assembly includes a core  11   a ,  12   a  and a winding of multiple turns of wire  11   b ,  12   b . When the rotor  14  is positioned in that space, two axial air gaps  15 ,  16  are provided, one between each face of the rotor  14  and a respective stator assembly  11 ,  12 . The stator assemblies  11 ,  12  are ring-shaped, so that both portions on each side of the rotor  14  seen in  FIG. 1  are connected, although this connection is not shown in  FIG. 1 . There are other various known configurations for axial-gap PM motors. For example, a PM rotor having a flux return path on one side of the rotor can be used with one stator assembly. The present invention is applicable to all of these known configurations.  
         [0034]      FIG. 3  shows an example of a rotor  24  for an axial gap machine of the present invention. Portions of permanent magnet (PM) material  25  of north (N) and south (S) polarity are embedded in a non-magnetic disk  29  for a machine rotor. The portions of PM material  25  are angularly spaced and alternated with reluctance pole elements  27 . The other aspects of construction are similar to the general configuration described above in relation to  FIG. 1 . A hub  17  is provided to attach the rotor shaft to the rotor  24 . A disk  28  is disposed around the hub  17  to help hold the pole pieces  25 ,  27  in place together with an outer ring  26 . The hub and disk  28  can be one piece of aluminum of different thicknesses. The rotor shaft  18  is carried in bearings  19 ,  20  in a machine housing  21 . The rotor  24  is positioned between two stator assemblies  12  which provide the vertical operating space for the rotor. When the rotor  24  is positioned in that space, the two axial air gaps  15 ,  16  are provided, one between each face of the rotor  14  and a respective stator assembly  11 ,  12 . The stator cores  11   b ,  12   b  are ring-shaped, so that both portions on each side of the rotor are connected, although this connection is not shown in the sectional view.  
         [0035]      FIG. 4  shows examples of constructions and shapes for the reluctance poles. A first reluctance pole configuration  40  includes laminations and two sections of different width or thickness. A second reluctance pole configuration  41  includes laminations but has a uniform width or thickness. A third configuration  42  is solid with uniform width or thickness. A fourth, trapezoidal configuration  43  is solid with a tapered width or thickness extending radially from wider at an outer end to narrower at an inner end. A fifth configuration  44  is elliptical in shape. A sixth configuration  45  is circular in shape, and a more illustrative example is seen in  FIG. 7 . In  FIG. 7 , round PM pole portions  62  and round reluctance pole portions  63  are embedded in a disc  61  of non-magnetic material to form a rotor  60  for mounting on a rotor shaft. The reluctance poles in these examples are made of ferromagnetic material, preferably iron or an iron alloy.  
         [0036]     The present invention can be applied to many axial-gap PM machines. As shown in  FIG. 5 , instead of a circular magnet shape which generates a sinusoidal back emf wave shape, a wedge shape for the PM portions  25  generates a back emf wave  55  of trapezoidal shape. As shown in  FIG. 6  the PM poles  25  and reluctance poles  27  can be skewed at an angle  50  in relation to normal radii from a center of the rotor. Normally this angle  50  is chosen as one stator slot angle (example: for a 54-stator-slot motor the angle is 360/54=6.67 degrees). This angular offset will reduce reluctance torque ripple. An even number of wedges  25  with alternating axial magnetic polarity are alternated with reluctance poles  27  to form an annulus of magnetic material. Fourth, the PM magnets  25 , instead of being embedded in nonmagnetic material for support, are held in place by radial interference between an outer ring  26  and an aluminum hub  28  as seen in  FIG. 3 . A titanium ring  26  (or other non-magnetic high-strength material) may be used with a hub  28  of aluminum to provide and interference fit that is aided by these pieces having a different rate of thermal expansion in response as the operating temperature of the machine is increased up to a steady-state value. An axial-gap PM motor without reluctance poles has been built and tested to deliver 30 kW. Prior to electrical testing it maintained its integrity to 6600 rpm. The reluctance poles can be introduced for this motor and for all of the aforementioned rotors.  
         [0037]     As further seen in  FIG. 5 , conductors A, B and C for three phases of electricity are shown with a dot signifying a direction of current out of the plane of the figure and an “X” signifying a direction of current into the plane of the paper. The instantaneous current in phase A must equal the sum of the currents in phases B and C. The incoming current is signified by the large wires for phase A. The return currents in wires—C and B are each equal to one-half the incoming current. These three phase currents produce a stator MMF wave (magnetomotive force)  55  that is illustrated at the bottom of the figure. The flux produced by this MMF wave  55  seeks the shortest path and creates the twisted flux  56  of stator MMF as shown in upper portion of  FIG. 5 . This twisted flux  56  produces the reluctance torque. The PM pole elements  25  interacting with the currents in the conductors also create a PM synchronous torque. The resultant torque is normally greater than either the PM synchronous torque or the reluctance torque alone. The above explanation for the PM and reluctance torques can also be applied to a six-step current conduction, with only two phases conducting at a given time, as for brushless DC motors.  
         [0038]     The same techniques used to design the 30 kW axial-gap PM motor can be used to design a non-round-pole axial-gap PM reluctance motor. For example, the different thermal coefficients of expansion of the aluminum hub  28  (13.6×E-6/° F.) and a titanium ring  26  (5.3×E-6/° F.) seen in  FIG. 3  assures that, for the same dimensions as the 30 kW PM motor (R A1 =3.5 in. and R Ti =5 in.), the gap between the aluminum hub  28  and titanium ring  26  will decrease as temperature rises, to increase the assembly interference on the magnets  25 , which is necessary for stable operation as temperature increases. There will be a difference in the thermal coefficient of expansion of the ferromagnetic reluctance poles  27  (7×E-6/° F.) alternated between the PM elements  25 , whose radial coefficient is negative, because the radial direction is transverse to the magnetic field (−0.2×E-6/° F.). Each ferromagnetic reluctance pole wedge  27  will push out on the outer ring  26 , which will cause the material between to move inward like an inflexible neutral membrane. This, in turn, will further increase the original interference on the PM elements  25  for stable operation as temperature increases.  
         [0039]     The non-magnetic rotor hub  28  and outer ring  26  prevents flux from flowing through a body of the rotor  24  and concentrates flux in the reluctance pole pieces  27 . The hub  28  and ring  26  also withstand centrifugal stresses in a high-power machine and provide a lower inertia machine than one having a rotor with additional iron for a flux return path.  
         [0040]      FIGS. 8 and 9  show details of the rotor  24  with three PM elements  25 , one of which has an outer layer or cover  60  of soft ferromagnetic material, but non-PM material. In  FIG. 9 , the outer ring  26  has been removed for a better view. Two of the elements  25  are shown in  FIG. 8  without this PM cover  60 , but in  FIG. 9 , all three elements  25  have this layer or cover  60  which extends over all surfaces of the PM elements  25 , the top portion being removed to expose the PM material in  FIG. 9 . The PM cover  60  is made from a soft ferromagnetic material which can be radially or circumferentially laminated, molded or formed using other known techniques. Between the elements  25  are positioned reluctance pole elements  27  of iron or an iron alloy. In  FIG. 9 a  portion of the rotor  24  is shown positioned between the two wound stators  11 ,  12  to provide two axial air gaps  15 ,  16 .  
         [0041]     Referring to  FIG. 10 , the PM covers  60  result in enhancement of conduction of the q-axis (torque-producing) flux  61  through the reluctance poles  27  by allowing flow through the covers  60  to reinforce flow through the reluctance poles  60 . Such flux does not flow through the PM elements  25 . In addition, as seen in  FIGS. 11 and 12 , a harmonic flux  62  produced by the teeth  64  of the stators  11 ,  12 , (only one being shown by example) is short-circuited in the PM covers  60 , so that only fundamental frequency flux is produced in the air gaps  15 ,  16 .  
         [0042]     The machine of the present invention is applicable for the hybrid electric vehicle application, but is not limited to this application.  
         [0043]     The invention provides higher power density through the use of reluctance poles in an axial gap machine. The invention also provides multiple configurations for the rotor of an axial gap machine. The constructions described herein are compact. The use of a single PM rotor with two stator assemblies doubles the inductance. The reluctance poles then further increase the phase inductance. This compensates for the normally low inductance characteristic of PM motors. The use of PM elements located near the slot openings in the stator reduces the cogging torque and rotor surface losses.  
         [0044]     The invention is applicable to both AC and DC brushless machines. It is also applicable to both motors and generators.  
         [0045]     This has been a description of the preferred embodiments of the invention. The present invention is intended to encompass additional embodiments including modifications to the details described above which would nevertheless come within the scope of the following claims.