Abstract:
A method and apparatus in which a rotor ( 11 ) and a stator ( 17 ) define a radial air gap ( 20 ) for receiving AC flux and at least one, and preferably two, DC excitation assemblies ( 23, 24 ) are positioned at opposite ends of the rotor ( 20 ) to define secondary air gaps ( 21, 22 ). Portions of PM material ( 14   a   , 14   b ) are provided as boundaries separating the rotor pole portions ( 12   a   , 12   b ) of opposite polarity from other portions of the rotor ( 11 ) and from each other to define PM poles ( 12   a   , 12   b ) for conveying the DC flux to or from the primary air gap ( 20 ) and for inhibiting flux from leaking from the pole portions prior to reaching the primary air gap ( 20 ). The portions of PM material ( 14   a   , 14   b ) are spaced from each other so as to include reluctance poles ( 15 ) of ferromagnetic material between the PM poles ( 12   a   , 12   b ) to interact with the AC flux in the primary-air gap ( 20 ).

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of U.S. patent application Ser. No. 10/848,450 filed May 18, 2004. The benefit of priority based on U.S. Provisional Patent Application No. 60/607,105, filed Sep. 3, 2004, is also claimed herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with Government support under Contract No. DE-AC05-000R22725 awarded to UT-Battelle, LLC, by the U.S. Department of Energy. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     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 
     There are three major types of brushless electric machines available for the electric vehicle (HV) and hybrid electric vehicle (HEV) drive systems. These are the induction machine, the PM machine, and the switched-reluctance machine. 
     Permanent magnet (PM) machines have been recognized for having a high power density characteristic. A PM rotor does not generate copper losses. One drawback of the PM motor for the above-mentioned application is that the air gap flux produced by the PM rotor is limited, and therefore, a sophisticated approach is required for high speed, field weakening operation. Another constraint is that inductance is low, which means that current ripple must be controlled. 
     It is understood by those skilled in the art that a PM electric machine has the property of high efficiency and high power density, however, the air gap flux density of a PM machine is limited by the PM material, which is normally about 0.8 Teslas and below. A PM machine cannot operate at an air gap flux density as high as that of a switched reluctance machine. When the PM motor needs a weaker field with a reasonably good current waveform for high-speed operation, a sophisticated power electronics inverter is required. 
     When considering a radial gap configuration for undiffused, high strength operation, several problems have to be overcome. It is desirable to provide a compact design with a shape similar to a conventional radial gap machine. 
     It would also be beneficial to further enhance the control of the field above that which is available with known PM rotor constructions. This would increase the motor torque. It is also an objective to accomplish this while retaining the compactness of the machine. 
     The enhanced field weakening can reduce the field strength at high speed to lower the back emf produced in the winding. Therefore, for a specified DC link voltage, the speed range of the machine can be increased over that it otherwise would be. This will meet the compactness objective and allow simplification of the drive system requirements. 
     The present invention continues the ability to enhance and weaken flux in the primary air gap, while improving the construction of the rotor. 
     SUMMARY OF THE INVENTION 
     This invention provides a high strength PM machine and method for brushless undiffused operation in which reluctance poles are added to permanent magnets (PM&#39;s) in a machine rotor to allow enhanced field control. 
     The invention is incorporated in a method and apparatus in which a rotor and a stator define a radial air gap for receiving AC flux and at least one and preferably two DC excitation assemblies are positioned at opposite ends of the rotor to define secondary air gaps. Portions of PM material are provided as boundaries separating the rotor pole portions of opposite polarity from an interior of the rotor and from each other to define PM poles for conveying the DC flux to or from the primary air gap and for inhibiting flux from leaking from said pole portions prior to reaching the primary air gap. The portions of PM material are spaced from each other so as to leave reluctance poles of ferromagnetic material between the PM poles to interact with the AC flux in the primary air gap. 
     In a further aspect of the invention, the flux path through the reluctance poles can be tapered in the direction of the flux paths through the rotor to reduce the size and weight of ferromagnetic material in the rotor. This also allows for two DC flux paths from opposite ends as well as for return paths for the DC flux. 
     The invention provide increased power and torque without increasing the size of the machine. 
     The invention is applicable to both AC and DC machines, and to both motors and generators. 
     The invention is provides a compact electric machine structure for application to electric or hybrid vehicles. 
     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 
         FIG. 1  is a longitudinal section view of a brushless PM machine with reluctance poles; 
         FIGS. 2 and 3  are end views of the rotor incorporated in the assembly in  FIG. 1 ; 
         FIGS. 4   a  and  4   b  are diagrams illustrating how the portion of the rotor carrying the flux through reluctance poles can be tapered and reduced to the portion actually carrying the flux; 
         FIG. 5  is a longitudinal section view of a brushless PM machine having a rotor with reluctance poles and a tapered flux path according to the present invention; 
         FIG. 6  is an end view of the rotor seen in  FIG. 5 ; 
         FIGS. 7–11  are transverse sectional views through the rotor of  FIG. 5  taken in the planes indicated by the dashed lines in  FIG. 5 ; and 
         FIGS. 12 and 13  are longitudinal section and end views of a brushless PM machine of the present invention having a tapered flux portion and showing the flow of flux through the rotor and adjoining air gaps. 
         FIG. 14  shows that the externally excited DC flux return path can go through the stator instead of the rotor if the frame (or portion of the frame) is made of magnetically conducting material. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The principle of a high strength, undiffused brushless machine has been previously disclosed in the Hsu, U.S. Pat. No. 6,573,634, issued Jun. 3, 2003, Hsu, U.S. patent application Ser. No. 10/688,586 filed Sep. 23, 2003, and Hsu U.S. patent application Ser. No. 10/848,450 filed May 18, 2004, the disclosures of which are hereby incorporated by reference. 
     For a conventional PM machine the air-gap flux density is about 0.6 to 0.8 Teslas and cannot be weakened without the aid of some sophisticated power electronics. Both the stationary excitation coil and the PM material in the rotor maximize rotor flux in the PM machine of the present invention. It can produce two to three times the air gap flux density of a conventional PM machine. Because the torque produced by an electric machine is directly proportional to the air gap flux density, a higher torque, more powerful machine is provided with only small additions to size and weight. 
       FIG. 1  shows a longitudinal section view of a radial gap, high strength undiffused machine  10  with eight PM poles  12   a ,  12   b  in a rotor assembly  11 .  FIGS. 2 and 3  each show the eight PM poles  12  bounded by eight sets of permanent magnets  14 . Reluctance poles are provided by the portions of the rotor  15  positioned in between these sets of permanent magnets  14 . The reluctance poles  15  allow the flux  16  produced by a stator  17  to go through these reluctance poles  15  easier than the path going through the PM poles  12   a ,  12   b.    
     The rotor assembly  11  is preferably made as described in the disclosures cited above, namely, the rotor has a hub  11   a  and a plurality of laminations  11   b  of ferromagnetic material are mounted and stacked on the hub  11   a  and clamped by non-magnetic metal end pieces  11   c . The rotor laminations  11   b  and end pieces  11   c  have keyed projections lid for insertion in keyways in the rotor hub  11   a . The stacked laminations  11   c  reduce the occurrence of eddy currents resulting from the flux which travels through in an axial direction through the rotor assembly  11 . 
     PM pole pieces  12   a  (N),  12   b  (S) are disposed in longitudinal grooves and retain the PM magnetic material  14  in place in still deeper grooves with the assistance of adhesives. The PM magnetic material  14  can be pre-formed pieces or the injected type. Between pieces of PM material  14 , an epoxy material can be used to fill gaps. PM pole faces (not shown) are separate pieces attached to the ends of the rotor assembly  11  to hold the PM pole pieces  12   a ,  12   b  and magnets  14  in position. 
     It is also possible add two end rings of a soft magnetic material to the ends of the stack of laminations  11   a  before adding the clamping pieces  11   c . The end rings provide smoothing for flux in a circumferential direction around an axis of rotation  19   a . The pole faces can also made of a soft magnetic material, such as steel. They can be attached to the thin steel end rings by rivets, screws, welds, or any feasible means. The thin steel rings hold the pole pieces in place against centrifugal force. Alternatively, end pole faces can be held by rivets. 
     The machine  10  has two DC excitation assemblies  23  and  24  at opposite ends of the rotor assembly  11 . The DC excitation assemblies  23 ,  24  each include a stationary, ring-shaped excitation core  23   b ,  24   b  and a multi-turn coil  23   a ,  24   a  for receiving direct current from an external source. This DC current can be of a first polarity or of a second opposite polarity. The cores  23   b ,  24   b  encircle the rotor shaft  11  and are mounted to a machine housing  37 . The cores can be made of iron, steel, another iron alloy or a compressed powder ferromagnetic material. A stationary toroidal excitation coil  23   a ,  24   a  fits in an annular recess in each excitation core  23   b ,  24   b.    
     The rotor assembly  11  rotates with a main drive shaft  19  around an axis of rotation  19   a . The stator  17  is disposed around the rotor  11  and has a laminated core  17   a  and windings  17   b  as seen in a conventional AC machine. The rotor assembly  11  is separated from the stator  17  by a radial air gap  20 , which is also referred to herein as the primary air gap. AC flux is produced in this air gap  20  by the stator field. The rotor assembly  11  is separated from the DC excitation assemblies  23  and  24  by air gaps  21  and  22 , respectively. These air gaps  21 ,  22  are oriented axially relative to the axis  19   a  of the rotor  11 . DC flux will be produced in these air gaps  21 ,  22  by the DC excitation assemblies  21  and  22 . Flux collector rings  25  are disposed between the axial air gaps  21 ,  22  and the DC excitation assemblies  23  and  24  to smooth the DC flux component and reduce the possible occurrence of eddy currents. 
     The drive shaft  19  is supported by bearings  31  and  32 . The cores  23   b ,  24   b  for the excitation assemblies form brackets for these bearings  31 ,  32 . The bearing brackets conduct DC magnetic flux. If needed, the ceramic bearings or insulated bearings (i.e., an electrically insulating material is used to isolate the rotor outer ring to the bearing housing) can be used. A short internal shaft  30  is also coupled to the rotor  11 . A shaft encoder  33  and a pump  34  for lubricant for the motor  10  are situated inside a passageway  35  through the core  24 . A housing cover  36  closes the passageway  33 . 
     Referring to  FIG. 2 , the DC flux  16  produced by the excitation assemblies  23 ,  24  is conducted into the rotor from one set of the PM side poles  12   a  of N polarity, and then turns to flow radially outward across the main air gap  20  into the stator core  17   a , then loops and returns radially inward and is conducted axially outward through adjacent poles  12   b  of S polarity at the other end of the rotor  11  ( FIG. 3 ). The DC flux  16  produced by the excitation coils does not pass through the reluctance poles  15 .  FIG. 1  illustrates a flux path  16  for only one of the pole pairs. The other pole pairs would have flux paths of the same pattern. The DC flux return path  16  shown in  FIG. 1  is using the rotor  11  for its return path. Normally, a return path is located in the rotor  11  is more compact than a return path through the aluminum motor housing  37 . This is because the diameter of the rotor  11  is smaller than that of a stator frame for conducting the DC return flux. However, it is possible to use the stator frame for its DC flux return path.  FIG. 14  shows that the externally excited DC flux return path can go through the stator instead of the rotor if the frame (or portion of the frame) is made of magnetically conducting material. 
     Referring to  FIGS. 2 and 3 , the PM material  14  together with the excitation current going through the excitation coils  23   a  and  24   a  produce the north (N) and south (S) poles on the exterior of rotor  11  that faces the stator  17  and the radial air gap  20 . This rotor flux in the radial air gap  20  can be either enhanced or weakened according to the polarity of the DC excitation in the excitation assemblies  23 ,  24  that face the ends the rotor  11 . Subsequently, the radial air gap  20  receives the rotor flux from the rotor  11 , which interacts with the primary flux induced by the stator windings  17   b  to produce a torque. 
     Referring to  FIGS. 4   a  and  4   b , the DC flux in an axial direction turns to the radial direction (i.e. a 90-degree turn). Assuming the depth (i.e. the distance going into the paper) of the paths shown in  FIGS. 4   a  and  4   b  is a constant,  FIG. 4   a  shows that the DC flux component  16   e  entering the bottom of the pole piece material  12  makes the 90-degree turn first, followed by successive flux components  16   b – 16   d , until the component at the top  16   a  turns upward last. This provides a tapered flux path  16  in which a portion of the pole piece material  12  in the rotor  11  is not utilized.  FIG. 4   b  shows that a material-saving flux path can be provided a tapered-shape of the pole piece material  12 . As the depth of the path changes, the contour of the tapered path is not a straight line, in order to maintain a cross sectional area that is inversely proportional to the distance down the path. 
       FIG. 5  shows a modification to the rotor  11 . This provides a pole piece  12   a  tapered in a direction parallel to axis  19   a . The tapered pole piece  12   a  means that the DC flux going into the first side poles sees a gradually smaller cross sectional area. At the middle section of the rotor  11 , the cross-sectional area of the pole piece  12   a  is nearly zero. The tapered flux path is separated from other parts of the rotor by sets of PM material  14   a  seen in  FIG. 6 . Second sets of PM material  14   b  are spaced from the first sets of PM material  14   a  to define reluctance poles  15 . 
     The cross section of this flux path is seen in the sectional views of the rotor at the axial locations shown in  FIGS. 7–11 . As seen in  FIGS. 7–11 , the spacing between the sets of PM magnets  14   a ,  14   b  defines eight N-S PM poles  12   a ,  12   b  and eight reluctance poles  15 , pairs of these poles  15  being connected through a narrow cross sectional area  15   a  seen in  FIG. 7 . This cross sectional area  15   b ,  15   c  then becomes progressively wider in  FIGS. 8 and 9 . This cross sectional area then becomes progressively narrower  15   d ,  15   e  in  FIGS. 10 and 11 . This provides a flux path  18   a ,  18   b  shown in  FIG. 13  for two of the reluctance poles  15 . 
       FIG. 12  illustrates two parallel DC flux paths  16   f ,  16   g  for the PM poles  12 . Unlike the series DC flux path (see  FIG. 1 ) that has the flux going into the side poles at one end of the rotor  11  and coming out from the other end of the rotor  11 , the parallel DC flux paths  16   f ,  16   g  illustrated here have flux entering the rotor from both sides through the secondary air gaps  21 ,  22 . From there, the flux turns ninety degrees to cross the primary air gap  20  and then return across the primary air gap to the core assemblies  23 ,  24  across the secondary air gaps  21 ,  22  (the return path being represented by the dashed line in  FIG. 12 ). 
       FIG. 12  also illustrates two additional retaining pieces each having a central ring-shaped portion  11   f  and four radially extending flanges  11   e  for holding the rotor assembly  11  together. 
     As seen in  FIG. 13 , the invention provides a reluctance pole flux path  18   a ,  18   b  between the reluctance poles  15  of the brushless machine  10 . In addition,  FIGS. 12 and 13  show that the return path for the DC flux  16   f  enters a south (S) polarity return pole  12   b  situated between two of the second sets of PM magnets  14   b , is conducted into the laminations  11   b , and then is conducted through gaps in the PM material  14   a ,  14   b  to reach the cooperating N pole  12   a . The north-south polarity of the pieces of magnetic material  14   a  around the N poles is such that the N-polarity material faces the N poles and the S-polarity material faces away from the N poles. The north-south polarity of the pieces of magnetic material  14   b  around the S poles is such that the S-polarity material faces the S poles and the N-polarity material faces away from the S poles. The DC flux paths  16   f ,  16   g  are generally of the same configuration (symmetrical) and of equal strength in this embodiment but could be asymmetrical and of unequal strength in alternative embodiments. 
     By controlling energization of the core assemblies  23 ,  24 , field weakening can be used to reduce the DC field strength at high speed to lower the back emf produced in the winding. Therefore, under a given DC link voltage the speed range of the machine can be increased. This again meets the compactness objective by simplifying the drive system requirement. 
     The invention is applicable to both AC synchronous and DC brushless machines and to both motors and generators. 
     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.