Abstract:
A brushless electric machine ( 30 ) having a stator ( 31 ) and a rotor ( 32 ) and a main air gap ( 34 ), the rotor ( 32 ) having pairs of rotor pole portions ( 22   b,    22   c,    32   f,    32   l ) disposed at least partly around the axis of rotation ( 32   p ) and facing the main air gap ( 24   b,    24   c,    34 ), at least one stationary winding ( 20   b,    20   c,    33   b ) separated from the rotor ( 22   b,    22   c,    32 ) by a secondary air gap ( 23   b,    23   c,    35 ) so as to induce a rotor-side flux in the rotor ( 22   b,    22   c,    32 ) which controls a resultant flux in the main air gap ( 24   b,    24   c,    34 ). PM material ( 27   b,    27   c ) is disposed in spaces between the rotor pole portions ( 22   b,    22   c,    32   f,    32   l ) to inhibit the rotor-side flux from leaking from said pole portions ( 22   b,    22   c,    32   f,    32   l ) prior to reaching the main air gap ( 24   b,    24   c,    34 ). By selecting the direction of current in the stationary winding ( 20   b,    20   c,    33   b ) both flux enhancement and flux weakening are provided for the main air gap ( 24   b,    24   c,    34 ). The stationary windings ( 31   a,    33   b ) which are used for both primary and secondary excitation allow for easier adaptation to cooling systems as described. A method of non-diffused flux enhancement and flux weakening is also disclosed.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH  
       [0001] 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  
         [0002]    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  
         [0003]    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.  
           [0004]    In an induction motor, operation at higher speeds is provided by field weakening in the constant power speed range. An induction machine is robust and requires only a relatively simple power electronics drive. However, the rotor of an induction machine produces considerable resistance heating as a result of current produced in the rotor during operation. In an electrical vehicle this would provide a significant source of heat that should be cooled.  
           [0005]    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 applications is that the air gap flux produced by the PM rotor is fixed, 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.  
           [0006]    The switched reluctance motor is another type of machine that does not have the copper loss of the induction machine. However, a complicated control and sensor system is required to provide sufficient torque over its range of operation.  
           [0007]    The direct current (DC) brush-type motors have a long history. The maintenance of the brushes that conduct the electric current from the stator to the rotor is a major reason limiting the application development. Hsu, U.S. Pat. No. 5,929,579, issued Jul. 27, 1999, discloses advanced brush and soft commutation technologies.  
           [0008]    Electric and magnetic phenomena are closely related in such machines. As disclosed in Hsu, U.S. patent application No. 09/475,591, filed Dec. 3, 1999, and entitled “Hybrid Secondary Uncluttered Machine,” DC magnetomotive force (mmf) may be transferred from a stator to a rotor without using brushes and without a significant core loss. As further disclosed in Hsu, U.S. Pat. No. 6,057,622, issued May 2, 2000, and entitled “Direct Control of Air Gap Flux”, a stator core section with a winding can be added to the basic stator to reduce flux in the main air gap for field weakening operation. When the magnetic flux is guided by ferrous material, flux leakage can become a major problem. PM elements may be used to “guide” the flux as disclosed in Hsu, “Flux Guides for Permanent Magnet Machines,” PES/IEEE Transactions on Energy Conversions Paper No. PE-007EC, March, 2001. In order to differentiate the flux leakage between the rotor poles and the flux leakages elsewhere in an electric machine, the flux leakage between rotor poles is referred to herein as “flux diffusion.” 
           [0009]    In order to overcome the above problems, including “flux diffusion,” the invention provides a novel machine described below.  
         SUMMARY OF THE INVENTION  
         [0010]    This invention provides a new type of machine for transferring mmf from a stationary winding to a rotor without the use of brushes or rotating windings.  
           [0011]    The invention is incorporated in a brushless electric machine having a stator and a rotor spaced from the stator to define a main air gap. The rotor an axis of rotation and has pairs of rotor pole portions disposed at least partly around the axis of rotation and facing the main air gap, with the pairs of rotor pole portions being spaced from each other to provide spaces. A stationary excitation winding with at least one coil is adapted for receiving direct current from an external source and is positioned next to the rotor so as to induce a rotor-side flux in the rotor which increases a resultant flux in the main air gap when the direct current is of a first polarity and which reduces resultant flux in the main air gap when the direct current is of a second polarity opposite said first polarity. PM material is disposed in spaces between the rotor pole portions to inhibit the rotor-side flux from leaking from said pole portions prior to reaching the main air gap.  
           [0012]    The invention provides stationary windings in the stator and avoids the use of any rotating windings. This allows for cooling systems to be added to cool the areas around the stationary windings.  
           [0013]    The invention is applicable to both AC and DC machines, and to both motors and generators.  
           [0014]    The invention is disclosed in terms of a preferred embodiment in an axial gap configuration, however, the invention is also applicable to radial gap machines.  
           [0015]    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.  
           [0016]    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  
       [0017]    [0017]FIGS. 1 a  and  1   b  are perspective views of a simplified iron core and excitation coil apparatus for illustrating flux diffusion;  
         [0018]    [0018]FIG. 2 is a graph of flux density (B) vs. field intensity (H) for the apparatus of FIGS. 1 a  and  1   b;    
         [0019]    [0019]FIG. 3 a -FIG. 3 f  are schematic diagrams of the apparatus of FIG. 1 b  with various configurations of PM material;  
         [0020]    [0020]FIG. 4 is a graph of flux density (B) vs. field intensity (H) for the arrangements in FIGS. 3 a - 3   f;    
         [0021]    [0021]FIGS. 5 a - 5   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;  
         [0022]    [0022]FIG. 6 is a quarter section view of a machine of the present invention incorporating the operating principles illustrated in FIGS. 5 a - 5   c;    
         [0023]    [0023]FIGS. 7 a - 7   d  are sectional and plan views of parts of a rotor assembly used in the machine of FIG. 6;  
         [0024]    [0024]FIG. 8 is a perspective photographic view of a front side of prototype rotor as seen in FIGS. 7 a - 7   d;    
         [0025]    [0025]FIG. 9 is a perspective photographic view of a reverse side of a prototype rotor as seen in FIGS. 7 a - 7   d.    
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0026]    [0026]FIGS. 1 a  and  1   b  show a test apparatus for illustrating the concept of flux diffusion. An iron core  10   a ,  10   b  has an air gap  11   a ,  11   b , respectively, and a toroidally wound excitation coil  12   a ,  12   b , respectively. In FIG. 1 a  the excitation coil  12   a is located right next to the main air gap  11   a , while in FIG. 1 b , the excitation coil  12   b  is located a considerable distance away from the main air gap  11   b.    
         [0027]    The length of the main air gap  11   a ,  11   b  in both cases of FIG. 1 is 0.11 inches. This relatively large gap is provided to accommodate a Hall effect probe (not shown) for measurement of the air gap flux density.  
         [0028]    Referring to FIG. 2, the lower two curves  13   a ,  13   b  show the variation of main air gap flux density (B g ) with respect to field intensity (H), also referred to as mmf (magnetomotive force), in ampere-turns of the excitation coil, for the two situations illustrated in FIGS. 1 a  and  1   b . The upper curve  13  gives the ampere-turn-drop provided by the main air gap  11   a ,  11   b.    
         [0029]    The lowest curve  13   b  indicates that for a given value of ampere-turns, the excitation coil  12   b  which is located away from the air gap  11   b , produces a lower flux density in the air gap  11   b . This occurs because more “flux diffusion” is experienced along the iron core  10   b  between the coil  12   b  and the air gap  11   b  than when the coil  12   a is closer to the air gap  11   a . The middle curve  13   a  shows that a significant increase of the main air gap flux density when the excitation coil  12   a is located closer to the air gap  11   a . A comparison of the middle curve  13   a  and the uppermost curve  13  shows that when the excitation coil is located close to the air gap, most of the mmf is required for the ampere-turn drop of the air gap. The middle curve  13   a  shows an increasing saturation effect when the flux density becomes greater. It should be noted that when the air gap is smaller, the flux densities of the three curves for a given mmf would be higher than those shown in the figure. The curves would all move closer to the vertical axis.  
         [0030]    [0030]FIGS. 3 a - 3   f  shows possible solutions to the problem of flux diffusion in the case of FIG. 1 b  above. FIG. 3 a  is similar to FIG. 1 b . In FIG. 3 b , permanent magnet (PM) material  17   b , is introduced to partly reduce flux diffusion. Greater amounts of PM material  17   c - 17   f  and  18   e  and  18  are introduced in FIGS. 3 c  to  3   f  to further reduce flux diffusion. The greatest reduction occurs when the core section is completely encased in PM material as seen in FIG. 3 f . In addition, elements  18   e  and  18   f  have been added on the other side of the coils  16   e  and  16   f  as seen in FIGS. 3 e  and  3   f.    
         [0031]    [0031]FIG. 4 shows the magnetization curves (B vs. H) for the apparatus of FIGS. 3 a  to  3   f . The curve corresponding to FIG. 3 a  is labeled “a”; the curve corresponding to FIG. 3 b  is labeled “b” and the remaining curves “c”-“f” correspond to FIGS. 3 c  to  3   f.    
         [0032]    The curves “d” and “e” in FIG. 4 corresponding to the examples in FIGS. 3 d  and  3   e  show little difference by adding PM element  18   e  on the other side of a pole as shown in FIG. 3 e . The most effective arrangement is found by wrapping a pole, on one side of the excitation coil, as completely as possible as shown in FIG. 3 f . For a given mmf of the excitation coil, the main air gap flux density will increase when the gap becomes smaller. This is due to a smaller mmf drop across the main air gap and a lower counter mmf acts on the PM material. Again, the curves with a smaller air gap would move closer to the vertical axis.  
         [0033]    To control flux in the main air gap in a brushless motor, it is advantageous to provide a stationary DC excitation coil that is not a part of the rotor. FIGS. 5 a - 5   c  and FIG. 6 provide a secondary air gap  23   a - 23   c ,  35  between this stationary DC excitation coil  20   a - 20   c ,  32   b  and the rotor  22   a - 22   c ,  32 .  
         [0034]    [0034]FIGS. 5 a - 5   c  illustrate a simplified stator and rotor apparatus showing three states of operation: 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. 5 b  and  5   c , with it being understood that additional material can be added according to FIG. 3 f.    
         [0035]    The main air gap flux density of a PM machine can be increased with an additional excitation coil  20   a - 20   c ,  33   b  as seen in FIGS. 5 a - 5   c  and  6 . These diagrams also illustrate how PM material will discourage flux diffusion.  
         [0036]    [0036]FIG. 5 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.  
         [0037]    [0037]FIG. 5 b  shows that in order to enhance the main air gap flux  25   b , PM material  27   b  is placed between the upper and lower pole pieces  22   b  of the rotor or around a pole as those shown in FIG. 3 f . 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.  
         [0038]    [0038]FIG. 5 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 simple field weakening feature in the main air gap  24   c  of the machine of the present invention.  
         [0039]    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.  
         [0040]    [0040]FIG. 6 shows a quarter section view of a brushless electrical machine  30  according to the present invention. The machine has an armature  31 , a rotor  32 , and a direct-current (DC) excitation stator portion  33 . The first two components  31 ,  32  are separated by a main air gap  34  and the second two components  32 ,  33  are separated by a second air gap  35 . The second air gap is as thin as machine clearance will allow between the two components  32 ,  33 . A gap of 0.011 inches was used in one embodiment. The core portions of the armature  31  and the stator portion  33  as well as the rotor  32  are made of iron, one of many suitable steels or another iron alloy.  
         [0041]    The armature  31  has a set of polyphase windings  31   a  and a magnetic core  31 b. When phase currents energize the polyphase windings  31   a  they produce a rotating magnetic flux wave in the main air gap  34 . The DC excitation portion  33  has a coil  33   b  for receiving direct current from an external source through leads  33   e . This current can be of a first polarity illustrated in FIG. 5 b , or of a second polarity as illustrated in FIG. 5 c.    
         [0042]    Referring to FIGS. 7 a - 7   d ,  8  and  9 , the rotor has a front side (FIG. 8) which bounds the main air gap  34  in FIG. 6. The rotor also has a back side (FIG. 9) which bounds the secondary air gap  35  and the DC excitation stator portion  33  seen in FIG. 6. The rotor  32  is an assembly comprising an outer ring  32   a  and an inner ring  32   b , which is shown in FIG. 7 b  and  7   d , respectively. The outer ring  32   a  has a circular band of metal  32   d  with support flanges  32   e  slanted by a certain angle from radial. Between pairs of supports  32   e , tooth-shaped pole pieces  32   f  of iron, steel or an iron alloy are welded or otherwise attached. The inner ring  32   b  is disk-shaped with a central hole  32   h  for a rotor shaft  35  seen in FIG. 6, with annular land  32   i  on the front side opposite an annular groove  32   j  on the back side and an annular land  32   n  on the back side forming canted or angled surfaces  32   k . Tooth-shaped pole pieces  32  are spaced radially around the inner ring  32   b  provided spaces between them. The pole pieces  32   l  extend at an acute angle relative to radii from the axial center  32   p.    
         [0043]    As seen better in FIG. 8, pole pieces  32   f  on the outer ring  32   a  and pole pieces  32   l  on the inner ring  32   b  have flat sides  32   q ,  32   s  on the front side of the rotor  32  which faces the armature  31  and the main air gap  34  seen in FIG. 6. The flat surfaces  32   q  of pole pieces  32   l  have lateral angled flats  32   r  and these oppose similar flats (shown) on the undersides of pole pieces  32   f . The pole pieces  32   l ,  32   f  are spaced apart when the rotor is assembled.  
         [0044]    PM material is inserted in the spaces  32   c  between the pole pieces  32   l ,  32   f  seen in FIG. 8. The PM material produces the north and south poles on the side that faces the armature  31 . Subsequently, the main air gap  34  (FIG. 6) between the armature  31  and the rotor  32  sees the rotor-side flux interacting with the armature flux induced by the armature coil  31   b . This flux in the main air gap  34  can be either enhanced or weakened by the DC excitation stator section  33  (FIG. 6) that faces the other side of the rotor  32 .  
         [0045]    During the enhancement of air gap flux (previously described in relation to FIG. 5 b ) the PM material in the rotor  32  tends to prevent the diffusion of flux between the rotor pole pieces  32   f ,  32   l  (FIG. 8) More flux is guided to the main air gap  34  (FIG. 6) to interact with the armature flux.  
         [0046]    During field weakening operation (previously described in relation to FIG. 5 b ) a great portion of the main air gap flux is drawn away from the air gap  34  by controlling the DC current of the DC excitation winding  20   c ,  33   b . The dragging torque is greatly reduced by a lower flux density in the main air gap  34  between the armature  31  and the rotor  32 .  
         [0047]    [0047]FIG. 6 also shows that because both the armature winding and the stationary DC excitation winding are stationary, direct cooling of the stationary armature and excitation windings can be used as an option. A coolant can be circulated in conduits around the windings. The coolant is introduced through ports (threaded holes)  31   d ,  31   e  of the upper half of the machine  30  and exits from ports (additional threaded holes) in the lower half of the machine (not shown).  
         [0048]    [0048]FIG. 6 also shows that the rotor  32  is mounted on a shaft  35  which is supported for rotation in bearings  36 . The stationary DC excitation winding  33   b  and core  33   a  are supported on a machine frame  37 .  
         [0049]    Injected PM is used to fill in the gaps  32   c  between the rotor outer ring  32   b  and inner ring  32   a  of the machine  30 . A high residual flux density and a strong coercive force provided by the PM aids the performance of the machine  30 .  
         [0050]    Thus, the invention provides a high strength undiffused brushless machine. The DC flux produced by an excitation coil  33   b  (FIG. 6) is delivered to the rotor  32  through an air gap  35  without the use of brushes. The DC flux in the rotor  32  is guided to the north and south poles that interact with the armature flux in the main air gap  34 . The undiffused arrangement provided by PM elements  27   b ,  27   c  (see FIGS. 5 b ,  5   c ) guides the flux to the main air gap  34  facing the armature. Both the PM elements and the excitation coil  33   b  enhance the air-gap flux density. Consequently, a high air-gap torque for a given armature current can be obtained. By controlling the direction of the current in the additional stator excitation coil  33   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. And, direct cooling of the stationary armature and excitation windings can be used as an option.  
         [0051]    The invention is applicable to both AC synchronous and DC brushless machines and to both motors and generators. Although an axial gap machine is shown in FIG. 6, the invention is also applicable to radial gap machines.  
         [0052]    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.