Abstract:
A method and apparatus in which a stator ( 11 ) and a rotor ( 12 ) define a primary air gap ( 20 ) for receiving AC flux and at least one source ( 23, 40 ), and preferably two sources ( 23, 24, 40 ) of DC excitation are positioned for inducing DC flux at opposite ends of the rotor ( 12 ). Portions of PM material ( 17, 17   a ) are provided as boundaries separating PM rotor pole portions from each other and from reluctance poles. The PM poles ( 18 ) and the reluctance poles ( 19 ) can be formed with poles of one polarity having enlarged flux paths in relation to flux paths for pole portions of an opposite polarity, the enlarged flux paths communicating with a core of the rotor ( 12 ) so as to increase reluctance torque produced by the electric machine. Reluctance torque is increased by providing asymmetrical pole faces. The DC excitation can also use asymmetric poles and asymmetric excitation sources. Several embodiments are disclosed with additional variations.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The benefit of priority based on U.S. Provisional Patent Application No. 60/752,695, filed Dec. 21, 2005, is claimed herein. The benefit of priority based on U.S. Provisional Patent Application No. 60/806,968, filed Jul. 11, 2006, is also claimed herein. The disclosure of these documents is incorporated herein by reference. 
     
    
     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 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.  
         [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 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.  
         [0006]     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.  
         [0007]     Rosenberg et al., U.S. Pat. No. 3,411,027, illustrates a permanent magnet (PM) machine with field excitation. There is no intent to shield the flux path for the field induced flux to prevent flux leakage. Therefore, significant flux leakage would occur resulting in a reduction of power density. In addition, Rosenberg et al. does not teach any additional reluctance poles or reluctance flux paths for producing reluctance torque in such a PM motor.  
         [0008]     Koharagi et al. U.S. Pat. No. 6,441,525, discloses permanent magnet (PM) barriers arranged in a V-shape and a U-shape. This patent also teaches the additional reluctance flux paths of a rotor, but without using a shunt field excitation.  
         [0009]     It is known in the art that the reluctance torque is produced by the difference between the d-axis inductance, L d , and the q-axis inductance, L q . As this difference increases, the reluctance torque increases. The flux passing through the d-axis poles in Koharagi is concentrated, but is also reduced, by the V-shaped and U-shaped PM barriers. This means that the q-axis inductance, L q , produces a flux having a less restrictive flux path to pass through along the q-axis, compared with the flux produced by the d-axis inductance, L d . The above d-axis and q-axis flux paths are restricted to two dimensions and without the additional excitation flux paths disclosed herein.  
         [0010]     Tajima et al., U.S. Patent Pub. No. U.S. 2005/0200223 utilizes PMs arranged in a V-shape to reduce the core loss under certain operating conditions. There are several drawbacks with this approach. Because the back emf produced by the strong PMs is proportional to the speed, a boost converter is needed to raise the voltage fed to the motor at high speed. The core loss at high speed is also high, due to the strong PMs, and this is true even when the motor is disconnected from the power supply. This publication does not disclose the use of field excitation to improve performance.  
         [0011]     Tajima also discloses symmetrical concavities per pole formed on the air gap face of the magnetic pole pieces of the rotor iron core. These are provided for the purpose of reducing core losses otherwise inherent in the design.  
         [0012]     Hsu, U.S. Pat. No. 6,972,504, issued Dec. 6, 2005, discloses a PM machine with reluctance poles and DC excitation coils positioned at opposite ends of the rotor. Prior designs have been largely symmetrical in their configuration of the PM poles and reluctance poles.  
         [0013]     The present invention is intended to improve reluctance torque and power density in a PM machine, while still providing a compact configuration.  
       SUMMARY OF THE INVENTION  
       [0014]     This invention provides a high density PM machine in which reluctance poles are added to permanent magnets (PM&#39;s) in a machine rotor to allow enhanced field control. The PM pole portions of one polarity provide enlarged flux paths in relation to flux paths for pole portions of an opposite polarity, with the enlarged flux paths communicating with a core of the rotor so as to increase torque produced by the electric machine.  
         [0015]     In another embodiment, the reluctance poles can be provided with asymmetric pole faces.  
         [0016]     In another embodiment, DC excitation is provided by asymmetric PMs disposed at the ends of the rotor making it unnecessary to use the stationary DC excitation coil assemblies. The PM elements rotate with the rotor.  
         [0017]     The invention provides increased power and torque without increasing the size of the machine.  
         [0018]     The invention is applicable to both AC synchronous machines and DC brushless machines, and to both motors and generators.  
         [0019]     The invention is provides a compact electric machine structure for application to electric or hybrid vehicles.  
         [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 longitudinal section view of a brushless PM machine with reluctance poles;  
         [0022]      FIG. 1   a  is a transverse sectional view of a rotor assembly seen in  FIG. 1 ;  
         [0023]      FIG. 2  is a longitudinal section view of a second embodiment of a brushless PM machine with reluctance poles;  
         [0024]      FIG. 2   a  is a transverse sectional view of a rotor assembly seen in  FIG. 2 ;  
         [0025]      FIG. 3  is a longitudinal section view of a third embodiment of a brushless PM machine with reluctance poles;  
         [0026]      FIGS. 3   a  and  3   b  are transverse sectional views of a rotor assembly seen in  FIG. 3 ;  
         [0027]      FIG. 4  is a longitudinal section view of a fourth embodiment of a brushless PM machine with reluctance poles w without field excitation coils;  
         [0028]      FIG. 4   a  is a transverse sectional view of a rotor assembly taken in a plane indicated by line  4   a - 4   a  in  FIG. 4 ;  
         [0029]      FIG. 5  shows a modification to the rotor assembly in  FIGS. 1-4 ;  
         [0030]      FIGS. 6   a  and  6   b  are schematic views of one alternative for utilization of excitation coils that can be used in the present invention;  
         [0031]      FIGS. 7   a  and  7   b  are schematic views of another alternative for utilization of excitation coils that can be used in the present invention;  
         [0032]      FIGS. 8-12  show modifications to the rotor assembly of FIGS.  1  to  4  to vary the radial air gap; and  
         [0033]      FIG. 13  shows a modification to the rotor assembly to improve performance while maintaining a uniform air gap.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]      FIG. 1  shows a longitudinal section view of a radial gap PM machine  10  of the present invention having a ring-shaped stator  11  mounted in a housing assembly  14 . The stator has a plurality of stator coils arranged in a manner which is known in the art to produce an AC flux in a radial air gap  20  disposed between the stator  11  and a rotor  12 . The rotor  12  is mounted for rotation with a primary drive shaft  15  and a short internal drive shaft  16  that are in turn mounted on bearings  15   a,    16   a  in the housing assembly  14 . A shaft encoder  33  and a pump  34  for lubricant for the motor  10  are situated inside a passageway  35  in which in internal shaft  16  is positioned. A housing cover  36  closes the passageway  33 .  
         [0035]     The rotor  12  is an assembly that has a hub  12   a  with a plurality of laminations  12   b  of ferromagnetic material stacked on the hub, keyed to the hub at location  12   c  and clamped by non-magnetic metal end pieces  12   d  as further described in U.S. Pat. No. 6,972,504, cited above. The stacked laminations reduce the occurrence of eddy currents resulting from the flux which travels through in an axial direction through the rotor assembly  12 .  
         [0036]     Referring to  FIG. 1   a,  PM pole pieces  18  as described in U.S. Pat. No. No. 6,972,504, cited above, are disposed in longitudinal grooves and retain the PM magnetic elements  17  in place in still deeper grooves with the assistance of adhesives. The PM magnetic elements  17  can be pre-formed pieces or the injected type. Between pieces of PM material  17 , an epoxy material can be used to fill gaps. PM end pieces (not shown) are separate pieces attached to the ends of the rotor assembly  12  to hold the PM pole pieces  18  and magnets  17  in position. The PM material separates the north (N) PM poles and south (S) south PM poles  18  from the rotor hub  12   a  and from reluctance poles  19  disposed between the north (N) PM poles and south (S) south PM poles.  
         [0037]      FIG. 5  shows a modification of the rotor  12  to provide a second set of PM elements  17   a  set deeper into the rotor  12  and parallel to the first set of PM elements  17 . The spaces between the sets of PM elements  17 ,  17   a  become N polarity reluctance flux paths  19  for AC flux entering and leaving the rotor through a q-axis N pole. The d-axis reluctance flux paths are superimposed on the PM N polarity poles. The PM poles are non-identical with S pole portions having enlarged flux paths in relation to flux paths for the N pole portions, the enlarged flux paths communicating with a core  12   a  of the rotor  12  so as to increase torque produced by the electric machine.  
         [0038]     Referring to  FIG. 1 , the rotor  11  rotates with a main drive shaft  15  around an axis of rotation  15   b.  The stator  11  is disposed around the rotor  12  and has a laminated core and coils as seen in a conventional AC machine. The rotor  12  is separated from the stator  17  by the 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 coils  23 ,  24  by air gaps  21  and  22 , respectively. These air gaps  21 ,  22  are oriented axially relative to the axis  15   a  of the rotor  11 . DC flux will be produced in these air gaps  21 ,  22  by the stationary DC excitation coils  23  and  24 . Flux collector rings  25  are disposed between the axial air gaps  21 ,  22  and the DC excitation coils  23  and  24  to smooth the DC flux component and reduce the possible occurrence of eddy currents.  
         [0039]     Referring to  FIGS. 1, 5 ,  6  and  7 , the DC flux  26  produced by the excitation coils  23 ,  24  is conducted into the rotor  11  from one set of the PM side poles of N polarity, and then turns to flow radially outward across the main air gap  20  into the stator  11 , then loops and returns radially inward and is conducted axially outward through adjacent poles of S polarity at the other end of the rotor  11 . The DC flux  26  produced by the excitation coils  23 ,  24  does not pass through the reluctance poles  19 .  FIG. 1  illustrates a flux path  26  and flux return path  26   a  for only one of the pole pairs.  FIG. 6a  illustrates flux paths  26   b,    26   c  for two adjacent poles pairs. The other pole pairs would have flux paths of the same pattern. The DC flux return path  26   a  shown in  FIG. 1  is using a return path through the motor housing  14 . In that case the motor housing  24  is made of a ferromagnetic material. It is also possible to use the rotor  12  for the return path as shown in  FIG. 6  as previously disclosed in U.S. Pat. No. No. 6,972,504, cited above. It is also possible to use the stator frame for the DC flux return path as shown in  FIG. 7  by re-positioning the excitation sources  23   b,    24   b  opposite both the stator core  11   a  and the rotor  12 .  
         [0040]     Referring to  FIGS. 1 and 1   a,  the PM material  14  together with the excitation current going through the excitation coils  23  and  24  produces the north (N) and south (S) poles on the exterior of rotor  12  ( FIG. 1   a ) that faces the stator  11  and the radial air gap  20  ( FIG. 1 ). 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 . Subsequently, the radial air gap  20  receives the rotor flux from the rotor  12 , which interacts with the primary flux induced by the coils in the stator  11  to produce a torque.  
         [0041]      FIGS. 1 and 1   a  illustrate an embodiment in which the first stationary excitation coil  23  and said second excitation coil  24  are electrically operated in series. Referring next to  FIGS. 2 and 2   a,  an embodiment is illustrated in which the first stationary excitation coil  23   a  and the second excitation coil  24   a  are electrically operated in parallel to produce two DC flux paths  26   e,    26   f.  In  FIGS. 2 and 2   a,  parts which are the same as in  FIGS. 1 and 1   a  have been given the same number.  
         [0042]     In order to increase the area of the excitation coil flux axial path,  FIG. 2   a  shows that by making every other PM pole (the N poles) without the essential portions of PM material  27  and as an asymmetrical pole, the region of the flux path can be increased. This will ease the magnetic saturation and result in more effective control of the air-gap flux by the excitation coils. The torque can be increased by introducing additional sets of auxiliary PM flux guides  28 . The number of set of essential flux guides  27  is half of the total number of PM poles. The shapes of the auxiliary PM flux guides  28  can be changed from a V-shape to different patterns, such as a flat and thin rectangular shape, or even a reversed V-shape with or without PM elements inside the reversed V-shape grooves. The reluctance poles for AC flux are superimposed on the PM poles as illustrated by the reluctance poles  19   a  in  FIG. 2   a.    
         [0043]      FIG. 3  shows another option for the location of a DC excitation coil  23   q.  In this embodiment, the stator  11  and the rotor  12  are comprised of two laminated stacks  11   f,    11   g  and  12   f,    12   g,  respectively. A ring-shaped excitation coil  23   q  is located at the axial center of the stator  11 , between two stator stacks  11   f,    11   g.  The excitation coil flux paths  29  are shown in the  FIG. 3  as they travel in a loop from one stator stack  11   g,  across the primary air gap  20  to a corresponding rotor stack  12   g,  through the central opening in the coil  23   b  to the other rotor stack  12   f  and then back across the primary air gap  20  to the other stator stack  11   f  and then through the motor housing  14 , which is again made of a ferromagnetic material.  
         [0044]     Referring to  FIGS. 3   a  and  3   b,  because the excitation-coil flux directions are different in the two rotor stacks,  12   f,    12   g,  the polarities of the PM return poles  30   a,    30   b  (N vs. S) without the essential PM elements are arranged to be opposite of each other. The return poles  30   a,    30   b  are non-identical to the other PM poles  31   a,    31   b  and are enlarged to provide a greater region for the DC flux to travel as compared with the other PM poles  31   a,    31   b.    
         [0045]     Referring next to  FIG. 4 , an embodiment is shown in which the excitation coils have been replaced by permanent magnetic (PMs) exciting elements  40  mounted on the ends of the rotor  12  to rotate with the rotor  12 . These PM exciting elements  40  are held in place by a first inner diameter ring piece  41  of non-magnetic material and a ring of ferromagnetic material  42  to complete a flux path through the PM. An outer diameter ring piece  43  of non-magnetic material assists in holding the ring of ferromagnetic material in place. The rotor  12  in this embodiment has been modified so that the N and S PM poles defined by essential PM elements  44  in  FIG. 4   a  are asymmetrical in relation to a radial axis  45  through a center of the N and S PM poles.  
         [0046]     The asymmetrical air gap of a pole and the asymmetrical interior PM locations increases reluctance torque. The asymmetrical PM elements  40  make more room for the q-axis flux to flow in the rotor lamination, which results in less q-axis magnetic saturation. This asymmetrical-pole technology can be used for the interior-permanent-magnet-reluctance motors with or without field excitation.  
         [0047]      FIG. 8  shows a modified rotor  12 ′ in which the reluctance pole faces  18   a  facing the primary air gap  20  are asymmetrical in relation to a radial axis of symmetry  18   b  from an axis of rotation  15   b  for the rotor  12 ′ to cause a thickness of the primary air gap  20  to vary across the PM pole faces. This increases the air gap asymmetrically across the pole faces  18   a.  This further enhances the forward rotation torque but will reduce the backward rotation torque. The difference between the forward and backward torque difference can be controlled through design.  
         [0048]      FIG. 9  shows a modification of the rotor in which the PM pole faces  19   b  facing the primary air gap are depressed relative to the pole faces  19   a  for the reluctance poles  19  causing a thickness of the primary air gap to vary across the PM pole faces and the reluctance pole faces.  
         [0049]      FIG. 10  shows a modified rotor  12 ′″ in which the d-axis reluctance pole faces  19 e facing the primary air gap  20  project further into the primary air gap  20  and the pole faces  19   f  for the q-axis reluctance poles are recessed relative to the d-axis reluctance pole faces  19   e  causing a thickness of the primary air gap to vary across the reluctance pole faces.  FIG. 10  also shows PM elements  17   b  defining the q-axis reluctance poles as being disposed along radiuses parallel to side pieces  17  for the d-axis poles to provide q-axis poles of different sized flux conduction regions than for the d-axis reluctance poles.  
         [0050]      FIG. 11  shows a modification of  FIG. 10  in which the q-axis defining PM elements in  FIG. 10  have been eliminated while retaining the difference in radius between the d-axis reluctance pole faces  19   e  and q-axis reluctance pole faces  19   f.    
         [0051]      FIG. 12  shows that the depressed or recessed reluctance pole faces  19   f  for the q-axis can be disposed asymmetrically relative to a radial axis of symmetry  19   g  for each q-axis pole while retaining the difference in radius between the d-axis reluctance pole faces  19   e  and q-axis reluctance pole faces  19   f.    
         [0052]      FIG. 13  shows a modification to  FIG. 11  in which a thin bridge  19   h  is provided over the q-axis poles faces  19   f  of  FIG. 11 . This reduces the rotor surface variation for noise and hydraulic friction reasons.  
         [0053]     The invention has been disclosed in terms of a motor which can be an AC synchronous motor or a DC brushless motor according to the type of control as known in the art. The invention is applicable to both motors and generators.  
         [0054]     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.