Abstract:
A radial gap brushless electric machine ( 30 ) having a stator ( 31 ) and a rotor ( 32 ) and a main air gap ( 34 ) also has at least one stationary excitation coil ( 35   a,    36   a ) separated from the rotor ( 32 ) by a secondary air gap ( 35   e,    35   f,    36   e,    36   f ) so as to induce a secondary flux in the rotor ( 32 ) which controls a resultant flux in the main air gap ( 34 ). Permanent magnetic (PM) material ( 38 ) is disposed in spaces between the rotor pole portions ( 39 ) to inhibit the second flux from leaking from the pole portions ( 39 ) prior to reaching the main air gap ( 34 ). By selecting the direction of current in the stationary excitation coil ( 35   a,    36   a ) both flux enhancement and flux weakening are provided for the main air gap ( 34 ). A method of non-diffused flux enhancement and flux weakening for a radial gap machine is also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The benefit of priority based on U.S. Provisional Patent Application No. 60/472,544, filed May 22, 2003, is claimed herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     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 
     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 to have a weaker field with a reasonably good current waveform for high-speed operation, a sophisticated power electronics inverter is required. 
     Hsu, U.S. Pat. No. 6,573,634, issued Jun. 3, 2003, and entitled “Method and Machine for High Strength Undiffused Brushless Operation” discloses and claims an axial gap PM machine for higher strength, undiffused operation. 
     In many applications, a radial gap machine is preferred. 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 is also not apparent how to arrange the PM material so as to control diffusion between poles of opposite polarity. It is also not apparent how to design the auxiliary field coils so as to complete a magnetic circuit through the rotor. 
     In order to overcome the above problems, the invention provides a novel machine described below. 
     SUMMARY OF THE INVENTION 
     This invention provides a radial gap high strength PM machine and method for undiffused operation. 
     The invention is incorporated in a brushless electric machine with a stator and with a rotor spaced from the stator to define a radial air gap relative to an axis of rotation for the rotor. The rotor has pairs of rotor pole portions of opposite polarity with extensions projecting toward an axially disposed secondary air gap. At least one, and preferably two, stationary excitation coils are provided for receiving direct current from an external source. These coils are positioned across the secondary air gaps, so as to induce a secondary component of flux in the rotor which increases a resultant flux in the radial air gap when the direct current is of a first polarity and which reduces resultant flux in the radial air gap when said direct current is of a second polarity opposite the first polarity. PM material with a suitable polarity is disposed between the rotor pole portions for conveying the secondary component of flux to or from the radial air gap and for inhibiting the secondary flux from leaking from said pole portions prior to reaching the radial air gap. 
     The invention provides stationary auxiliary field windings and avoids the use of any rotating windings. 
     The invention is applicable to both AC and DC machines, and to both motors and generators. 
     The invention is also practiced in a method of controlling flux in a brushless electrical machine having a stator with a stationary, primary excitation winding and a rotor separated by a main air gap, with the rotor having a portion facing the main air gap. The method comprises inducing a first flux in the rotor from the stator across the main air gap; passing a direct current through a stationary coil; positioning said stationary coil so as to induce a second flux in the rotor from a position separated from the main air gap by at least a portion of the rotor; and providing portions of PM material at least partly around said portions of the rotor separating the coil from the main air gap so as to prevent leakage of the second flux induced in the rotor before reaching the main air gap. 
     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 
         FIGS. 1   a – 1   c  are schematic diagrams of a simplified stator and rotor apparatus showing three states of operation: a) diffused flux, b) enhanced air main gap flux with the addition of PM material and c) reversed excitation for field weakening operation; 
         FIG. 2  is a longitudinal section view of a machine of the present invention incorporating the operating principles illustrated in  FIGS. 1   a – 1   c;    
         FIG. 3  is transverse sectional view taken in a plane indicated by line  3 — 3  in  FIG. 2 ; 
         FIG. 4  is a side elevational view of the rotor seen in  FIG. 2 ; 
         FIG. 5  is a transverse sectional view of the rotor taken in a plane indicated by line  5 — 5  in  FIG. 4 ; 
         FIG. 5   a  is a detail view of a portion in  FIG. 5 ; 
         FIG. 6  is a detail view of another portion of the rotor assembly seen in  FIG. 4 ; 
         FIG. 6   a  is a detail of further aspect of a pole piece portion of the parts seen in  FIG. 6 ; 
         FIGS. 7–10  are end views and section views of two cores used in the auxiliary winding assemblies seen in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1   a – 1   c  illustrate a simplified stator and rotor apparatus showing three states of operation for a motor according to the present invention: a) diffused flux, b) enhancement of main air gap flux with the addition of PM material and c) reversed excitation for reducing flux in the air gap in a field weakening operation. It should be noted that only a portion of the desired PM material has been represented in  FIGS. 1   b  and  1   c , with it being understood that additional material can be added according to the following description. 
     The main air gap flux density of a PM machine can be increased or weakened with an additional excitation coil  20   a – 20   c , as seen in  FIGS. 1   a – 1   c . These diagrams also illustrate how PM material will inhibit flux diffusion. 
       FIG. 1   a  shows the flux components  25   a  traveling through the iron core  22   a  of the rotor, the iron core of the stator  26   a , the main air gap  24   a  on the left-hand side, and the excitation coil  20   a  supported on an additional stator iron core  21   a  providing a secondary air gap  23   a  on the right hand side of the rotor  22   a . When the current flows in the excitation coil  20   a , magnetic fluxes are produced in the iron cores  21   a ,  22   a ,  26   a . The main air gap flux  25   a  is not the total flux produced by the coil  20   a . A significant portion of the flux is shown as the diffused flux  28   a  which passes between pole portions  22   a  of the rotor core. 
       FIG. 1   b  shows that in order to enhance the main air gap flux  25   b , PM material  27   b  with an N-S polarity as shown, is placed between the upper and lower pole pieces  22   b  of the rotor. The PM material  22   b  in the rotor produces flux in the main air gap  24   b  and also inhibits magnetic flux diffusion between the poles  22   b . Thus, it enhances the usable main air gap flux density. 
       FIG. 1   c  shows that by reversing the direction or polarity of the current in the excitation coil  20   c , the main air gap flux is weakened by removing the component provided by coil  20   c . This provides a field weakening feature in the main air gap  24   c  of the machine of the present invention. 
     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. 2  shows a side view of an end excitation, radial gap, high strength undiffused machine  30 . The overall shape is similar to a conventional induction machine. The stator laminated core  31   a  and windings  31   b  are identical to those of a conventional AC machine. The rotor  32  of this end excitation, radial gap, machine  30  is preferably made of solid steel with the option of having slits  32   s  along the axial direction for reducing the slot harmonics losses. In other embodiments, the core portions  31   a ,  32   a  of the stator  31  and the rotor  32  can be made of iron, one of many suitable steels or another iron alloy. The stator and rotor  31 ,  32  are separated by a radial air are separated by a radial air gap  34 , which is a radial distance from an axis of rotation  33   a  for the rotor  32 . When phase currents energize the polyphase windings  31   b , they produce a rotating magnetic flux wave in the main air gap  34 . 
     At each end of the rotor  32  is a secondary DC excitation assembly  35 ,  36  including a stationary, ring-shaped excitation core  35   b ,  36   b  and a multi-coil winding  35   a ,  36   a  for receiving direct current from an external source. This current can be of a first polarity illustrated in  FIG. 1   b , or of a second polarity as illustrated in  FIG. 1   c . The rings  35   b ,  36   b  encircle the rotor shaft  33  and have two projecting portions  35   c ,  35   d ,  36   c ,  36   d  to provide air gaps  35   e ,  35   f ,  36   e  and  36   f . A stationary toroidal excitation coil  35   a ,  35   b  fits in an annular recess in each excitation core  35   b ,  36   b . The cores  35   a ,  36   a  are mounted to a machine housing  45  using bolts  44  represented by centerlines in  FIG. 2 . 
     Referring to  FIGS. 2 and 4 , steel pole pieces  32   c ,  32   d  extend from the rotor  32  on opposite ends and are fastened to steel rings  32   e ,  32   f  at the ends by pins or by other suitable fasteners. As seen in  FIG. 2 , the magnetic flux in the steel pole pieces  32   c ,  32   d  is axially conducted to the steel rings  32   e ,  32   f  and passes through to the stationary excitation cores  35   b ,  36   b , through air gaps  35   e ,  36   e . The rotating steel rings  32   e ,  32   f  conduct the flux back to the steel rotor body  32   a  by crossing another set of end gaps  35   f ,  36   f . These end magnetic paths through the rotating excitation rings  32   e ,  32   f  are controlled by the current in the stationary toroidal excitation coils  35   a ,  36   a  located inside the stationary excitation cores  35   b ,  36   b.    
     Referring to  FIGS. 3 and 5 , permanent magnets (PM)  38  having N and S polarity as shown, are sandwiched between the steel pole pieces  39  and the steel rotor body  32   a . The PMs can be the preformed pieces or the injected type. The rotor  32  has a body portion  32   a  that is cylindrical except for longitudinally extending grooves  32   b  ( FIG. 4 ), wherein PM material  38  is positioned in the grooves  32   b . Pole pieces  39  are positioned in the grooves  32   b  over the PM material  38  to form a cylindrical rotor  32  with poles of alternating north (N) and south (S) polarity separated by PM material  38  ( FIGS. 5 ,  5   a ). Between pieces of PM material  38 , an epoxy material  40  ( FIG. 3 ) can be used to fill gaps. The pole pieces  39  are held in place by non-magnetic stainless steel screws  41  ( FIG. 3 ). 
     Referring to  FIG. 5 , the PM material  38  produces the north and south poles on the side of the exterior of rotor  32  that faces the stator  31  and the radial air gap  34  ( FIG. 2 ). Subsequently, the radial air gap  34  ( FIG. 2 ) receives the secondary flux from the rotor  32 , which interacts with the primary flux induced by the stator windings  31   b  to produce a resultant flux. This resultant flux in the radial air gap  34  can be either enhanced or weakened by the DC excitation in the excitation assemblies  35 ,  36  ( FIG. 2 ) that face the ends the rotor  32 . 
     During the enhancement of air gap flux (previously described in relation to  FIG. 1   b ) the PM material  38  in the rotor  32  tends to prevent the diffusion of flux between the rotor poles ( FIGS. 3 ,  5 ) More flux is guided to the main air gap  34  ( FIG. 2 ) to interact with the stator-induced flux. 
     During field weakening operation (previously described in relation to  FIG. 1   c ) a great portion of the main air gap flux is drawn away from the air gap  34  by controlling the DC current in the DC excitation winding  35 ,  36 . The dragging torque is greatly reduced by a lower flux density in the main air gap  34  between the stator  31  and the rotor  32 . 
       FIG. 2  also shows that the rotor  32  is mounted on a shaft  33  which is supported for rotation in bearings  43  around axis of rotation  33   a . The stator  31 , the rotor  32  and the excitation assemblies  35 ,  36  are enclosed in motor housing  45 , which is supported on supports  46 . It is noted that  FIG. 2  sectional view shows that two of the north poles are provided at the top and bottom of the rotor  32 .  FIG. 2  could be considered an offset sectional view of a six-pole machine shown in  FIGS. 3–6  or would also be illustrative of machines with four and higher numbers of poles according to the invention. 
       FIGS. 4 ,  5 ,  5   a ,  6  and  6   a  show details of the rotor pole extensions  32   c ,  32   d  and the rotor end rings  32   e ,  32   f  for a six-pole machine. The extensions  32   c ,  32   d  are made of ferromagnetic steel material. The extensions  32   d ,  32   d ′,  32   d ″ ( FIG. 5 ), which correspond to the north poles, are spaced 120 degrees apart and there are three such extensions,  32   d ,  32   d ′, and  32   d ″. The extensions project beyond the PM material  38  as seen in  FIG. 2 . Alternating with the rotor extensions, as seen in  FIG. 5 , are non-magnetic stainless steel mounting blocks  32   h  which are welded to the rotor body  32   a  and to the rotor end rings  32   e ,  32   f . Fasteners (not shown) may also be inserted through the rings  32   e ,  32   f , into the blocks  32   h . The pins (not shown) for fastening the rings  32   e ,  32   f  to the pole pieces  39  are ferromagnetic steel materials which are inserted with a force fit into holes (not shown) in the rings  32   e ,  32   f  and holes  32   g  ( FIG. 5   a ) in the pole piece extensions  32   d.    
       FIGS. 6 and 6   a  show a detail wherein stepped flanges  48  can be provided on the end rings  32   f  to mate with stepped ends of the pole piece extensions  32   d  to make a sturdier connection for withstanding rotational forces during motor operation. As seen in  FIG. 6   a , the pole pieces  32   d  can be made of a plurality of thin pieces  48  held together to reduce core loss due to stator slot harmonics. The rotor pole extensions  32   c ,  32   d  can also be of a skewed configuration, of a type known in the art, to align with offset stator slots, to counteract harmonic torque reduction. 
     The excitation cores  35   b ,  36   b  can be made from different types of material, such as solid steel, or thin pieces of steel  50  ( FIGS. 7 and 8 ). These cores  35   b ,  36   b  will not be subjected to torque, so many types of suitable bonding materials may be utilized to bond the thin pieces of steel  50 . Another variation involves forming an excitation core  47  from a compressed powder having ferromagnetic properties ( FIGS. 9 and 10 ). This aids in reducing losses in the core  47 . The powdered cores  47  have features  47   c ,  47   d  corresponding to like features on the cores  36   b.    
     The invention provides a high strength undiffused brushless machine. The DC flux produced by the excitation coils  35   b ,  36   b  ( FIG. 2 ) flows to or from the north and south poles of the rotor  32  through the air gaps  35   e ,  36   e ,  35   f ,  36   f  without the use of brushes. The DC flux in the rotor  32  is guided to the north and south pole portion on the circumference of the rotor  32  to interact with the armature flux in the main air gap  34 . The undiffused arrangement provided by PM elements  38  guides the flux to the main air gap  34  facing the stator. Both the PM elements and the excitation coils  35   b ,  36   b  enhance the air-gap flux density. Consequently, a high air-gap torque for a given stator current can be obtained. By controlling the direction of the current in the additional stator excitation coils  35   b ,  36   b , the main air-gap flux can be weakened, when desired. This motor requires only a simple power electronics drive of a type known in the art, which lowers the overall cost of a system using this machine. 
     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.