Abstract:
A hybrid electrical machine containing surface mounted magnets which includes a magnetically permeable cylindrically shaped stator assembly having at least one stator winding formed about a plurality of stator teeth, a rotor assembly concentrically disposed within the stator assembly, including a magnetically permeable rotor backiron, a rotational drive mechanism coupled to the rotor backiron, and a plurality of protruding rotor poles, each including a magnetically permeable pole support assembly, a winding provided around the pole support assembly, and a radially magnetized permanent magnet assembly disposed about the pole support assembly.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present U.S. patent application is related to and claims the priority benefit of U.S. Provisional Patent Application Ser. No. 61/969,894, filed Mar. 25, 2014, the contents of which are hereby incorporated by reference in their entirety into the present disclosure. 
    
    
     STATEMENT REGARDING GOVERNMENT FUNDING 
     This invention was made with government support under N00014-12-1-1020 awarded by the Office of Naval Research. The government has certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to electric machines and particularly to hybrid electrical machines with permanent magnets and field windings. 
     BACKGROUND 
     This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art. 
     A typical approach to power generation includes use of a wound rotor synchronous machine (WRSM) which can be used for voltage regulation. The same machine may be used as a motor, containing increased flexibility associated to a secondary winding which may be used to regulate output torque. Another type of machine is a permanent magnet synchronous machine (PMSM), which typically offers lower loss and higher power densities. 
     Referring to  FIG. 1 , a cross-sectional schematic of an example topology for a hybrid machine is shown. The hybrid machine  1  includes a stator assembly  2  and a rotor assembly  3 . The stator assembly  2  includes a stator backiron  4  while the rotor assembly  3  includes a rotor backiron  5 . The stator assembly  2  further includes a winding  6 , configured to provide a magnetomotive force applicable on the rotor assembly  3 . The rotor assembly  3  includes a winding  7  and one or more permanent magnets  8  which together generate a magnetomotive force applicable on the stator assembly  3  which together with the magnetomotive force generated by the stator assembly cause rotation of the rotor assembly  3 . The stator assembly  2  and the rotor assembly  3  are radially separated by an air gap  12 . The flux produced from the rotor assembly  3  may be approximately modeled using a series circuit configuration depicted in  FIG. 1  in a cutout spanning the rotor assembly  3  and the stator assembly  2 . Based on the model, a combination of two sources identified as F fd  and F pm , referring to field windings and permanent magnet magnetomotive force, coupled to each other in series, are connected to a further two reluctances              pm  and            g , referring to the permanent magnet and air gap, connected to the two supplies also in series. The magnetomotive force sources in the magnetic circuit, dissipated across the reluctances, produce a flux in the air-gap (Φ g ), similar to current in an electric circuit comprised of electromotive force sources (i.e. voltage) and resistances. The series combination is terminated at each end at a ground, representing the stator backiron  4  and the rotor backiron  5 .
     Placement of magnets and field windings within the rotor is a significant factor in the performance metrics of the hybrid machine, one of which is the torque producing capability relative to the machine size or mass and for a given loss. While beneficial when compared to the mass vs. loss characteristics of the WRSM, there are several limitations inherent in the topology shown in  FIG. 1 . 
     Given prior machine configurations, the torques, powers, regulation capability, and the achievable torque density associated therewith, there is unmet need for developing machines with improved torque density, power regulation capability, and lower mass for a given power loss compared to machines of prior art. 
     SUMMARY 
     A hybrid electrical machine which includes surface mounted magnets includes a magnetically permeable cylindrically shaped stator assembly having at least one stator winding formed about a plurality of stator teeth, a rotor assembly concentrically disposed within the stator assembly, including a magnetically permeable rotor backiron, a rotational drive mechanism coupled to the rotor backiron, and a plurality of protruding rotor poles, each including a magnetically permeable pole support assembly, a winding provided around the pole support assembly, and a radially magnetized permanent magnet assembly disposed about the pole support assembly. 
     An energy conversion system is also disclosed. The system includes a mechanical arrangement configured to i) provide mechanical power in a power generation mode, and ii) receive mechanical power in a motoring mode, and a hybrid electrical machine which includes a magnetically permeable cylindrically shaped stator assembly having at least one stator winding formed about a plurality of stator teeth, a rotor assembly concentrically disposed within the stator assembly, including a magnetically permeable rotor backiron, a rotational drive mechanism coupled to the rotor backiron, and a plurality of protruding rotor poles, each including a magnetically permeable pole support assembly, a winding provided around the pole support assembly, and a radially magnetized permanent magnet assembly disposed about the pole support assembly. The mechanical arrangement is coupled to the rotational drive mechanism. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Some of the figures shown herein may include dimensions. Further, some of the figures shown herein may have been created from scaled drawings or from photographs that are scalable. It is understood that such dimensions or the relative scaling within a figure are by way of example, and not to be construed as limiting. 
         FIG. 1  is a cross sectional view of a typical four-pole hybrid machine configuration found in the prior art. 
         FIG. 2 a    is a cross sectional schematic view of a hybrid machine with a stator and rotor, according to the present disclosure, wherein the permanent magnets are placed on the rotor between two pole body and pole shoe halves. 
         FIG. 2 b    is an approximate model schematic, representing the approximate flux producing capability of the rotor, in the hybrid machine of  FIG. 2   a.    
         FIG. 3  is a cross sectional schematic view of another embodiment of hybrid machine according to the present disclosure. 
         FIG. 4  is a cross sectional schematic view of another embodiment of hybrid machine according to the present disclosure. 
         FIG. 5  is a cross sectional schematic view of another hybrid machine. 
         FIGS. 6 a -6 d    are a collection of plots of flux density vs. electrical angle relative to the rotor (φ r ) for the hybrid machine of  FIG. 2   a.    
         FIG. 7  is a block diagram of the components used in a power system to deliver power to a mechanical load in a motoring mode, or an electrical load in generating mode. 
         FIG. 8  is a plot of the loss vs. mass, for the hybrid machines according to the present disclosure and traditional PMSM and WRSM machines found in the prior art. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the principles of the present disclosure, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of this disclosure is thereby intended. 
     A cross sectional schematic representation of one embodiment of a hybrid surface magnet (HSM) machine  100  according to the present disclosure is depicted in  FIG. 2 a   . The HSM  100  includes a stator assembly  102  and a rotor assembly  103 . The stator assembly  102  includes a stator backiron  104  while the rotor assembly  103  includes a rotor backiron  105 . The stator assembly  102  further includes a distributed winding, configured to provide a magnetomotive force (MMF) applicable on the rotor assembly  103 . The rotor assembly  103  is formed in the shape of a cylindrical body, a rotational drive mechanism, in this case a shaft  125  positioned at the center of the rotor assembly  103 , coupled to the rotor backiron  105  by an inert rotor material  126  and configured to rotate along with the rotor backiron  105 , and a plurality of outwardly protruded magnetically permeable pole assemblies  121  which include field windings  107  and one or more radially magnetized permanent magnets  108  which together generate a magnetomotive force applicable on the stator assembly  103  and which together with the magnetomotive force generated by the stator assembly  102  cause rotation of the rotor assembly  103 . The stator assembly  102  and the rotor assembly  103  are radially separated by an air gap  112 . Each of the pole assemblies includes pole bodies  120  and pole shoes  122  surrounding the one or more permanent magnets  108  with spacers  127  between them. 
     The HSM  100  includes a conventional distributed winding based stator assembly  102 . The stator assembly  102  includes a plurality of teeth  106  inwardly protruded towards the center of the machine  100  along the radial direction, formed at predetermined intervals represented by slots  111  along the circumferential direction. The stator assembly  102  is configured to have a winding that can be placed in the stator slots  111 . 
     The flux from the rotor assembly  103  may be approximately modeled using a parallel circuit configuration depicted in  FIG. 2 b   . Based on the model, a source identified as F fd , referring to the magnetomotive force associated with the field windings  107  is in series coupling with a parallel network which includes another source identified as F pm , referring to the magnetomotive force associated with the one or more permanent magnets  108 , and further two reluctances              pm  and            g , referring to the permanent magnet and air gap. The magnetomotive force sources in the magnetic circuit, dissipated across the reluctances, produce a flux (Φ g ), similar to current in an electric circuit comprised of electromotive force sources (i.e. voltage) and resistances. The source identified as F fd  terminates at a ground associated with the rotor backiron  105  while the parallel network is terminated at a ground associated with the stator backiron  104 . The percentage of the rotor pole  121  facing the air-gap  112  is represented by the factor α. The flux crossing the air-gap (φ g ) is related to torque density.
     The permanent magnets  108  allow the rotor assembly  103  to produce either voltage or torque with minimal losses as compared to those associated with excitation of the field winding  107 . At higher loads in generating mode, the field windings  107  are excited to account for the resulting voltage drops and regulate the output. At higher loads in motoring mode, the field windings  107  are excited to produce additional flux, resulting in additional torque. At higher speeds (in both motoring and generating modes), the field windings  107  are excited in the reverse direction to provide field weakening capability, allowing for power regulation. The shape of the stator and rotor steel, as well as the placement of the MMF sources in the topologies explored allows the air-gap flux distribution to be manipulated, wherein the electromechanical energy conversion process occurs. Adequate placement of steel, magnets, spacers, and windings allows the flux to be distributed such that the flux linking the stator winding is maximized. 
     Referring to  FIG. 3 , a cross sectional schematic representation of another embodiment of the HSM  200  is depicted. The HSM  200  includes a stator assembly  202  and a rotor assembly  203 . The stator assembly  202  includes a stator backiron  204  while the rotor assembly  203  includes a rotor backiron  205 . The stator assembly  202  further includes a winding, configured to provide a magnetomotive force applicable on the rotor assembly  203 . The rotor assembly  203  is formed in the shape of a cylindrical body, a rotational drive mechanism, in this case a shaft  225  positioned at the center of the rotor assembly  203 , coupled to the rotor backiron  205  by an inert rotor material  226  and configured to rotate along with the rotor backiron  205 , and a plurality of outwardly protruded magnetically permeable pole assemblies  221  which include field windings  207  and one or more radially magnetized permanent magnets  208  which together generate a magnetomotive force applicable on the stator assembly  203  and which together with the magnetomotive force generated by the stator assembly  202  cause rotation of the rotor assembly  203 . The stator assembly  202  and the rotor assembly  203  are radially separated by an air gap  212 . Each of the pole assemblies includes pole bodies  220  and pole shoes  222  surrounding the one or more permanent magnets  208  with spacers  227  between them. 
     Each of the plurality of poles assemblies  221  contains a T-shaped magnetically permeable pole support assembly, which includes a pole body  220  and a pole shoe  222 , a radially magnetized permanent magnet assembly, which includes permanent magnets  208  and spacers  227 , and field winding  207  made of conductive materials. The permanent magnets  208  are placed about the T-shaped pole support assembly, separated via the spacers  227 . The permanent magnets  208  are magnetized in the radial direction. The field windings  207  is coiled around the pole support assembly. 
     The permanent magnets  208  allow the rotor assembly  203  to produce either voltage or torque with minimal losses as compared to those associated with excitation of the field winding  207 . At higher loads in generating mode, the field windings  207  are excited to account for the resulting voltage drops and regulate the output. At higher loads in motoring mode, the field windings  207  are excited to produce additional flux, resulting in additional torque. At higher speeds (in both motoring and generating modes), the field windings  207  are excited in the reverse direction to provide field weakening capability, allowing for power regulation. The shape of the stator and rotor steel, as well as the placement of the MMF sources in the topologies explored allows the air-gap flux distribution to be manipulated, wherein the electromechanical energy conversion process occurs. Adequate placement of steel, magnets, spacers, and windings allows the flux to be distributed such that the flux linking the stator winding is maximized. 
     Referring to  FIG. 4 , a cross sectional schematic representation of another embodiment of the HSM  300  is depicted. The HSM  300  includes a stator assembly  302  and a rotor assembly  303 . The stator assembly  302  includes a stator backiron  304  while the rotor assembly  303  includes a rotor backiron  305 . The stator assembly  302  further includes a winding, configured to provide a magnetomotive force applicable on the rotor assembly  303 . The rotor assembly  303  is formed in the shape of a cylindrical body, a rotational drive mechanism, in this case a shaft  325  positioned at the center of the rotor assembly  303 , coupled to the rotor backiron  305  and configured to rotate along with the rotor backiron  305 , and a plurality of outwardly protruded magnetically permeable pole assemblies  321  which include field windings  307  and one or more radially magnetized permanent magnets  308  which together generate a magnetomotive force applicable on the stator assembly  303  and which together with the magnetomotive force generated by the stator assembly  302  cause rotation of the rotor assembly  303 . The stator assembly  302  and the rotor assembly  303  are radially separated by an air gap  312 . Each of the pole assemblies includes pole bodies  320  and pole shoes  322  surrounding the one or more permanent magnets  308  with spacers  327  between them. 
     Each of the plurality of poles assemblies  321  contains a T-shaped magnetically permeable pole support assembly, which includes a pole body  320 , a pole tip  323 , and a pole shoe  322 , a radially magnetized permanent magnet assembly, which includes permanent magnets  308  and spacers  327 , and field winding  307  made of conductive materials. The permanent magnets  308  are placed about the T-shaped pole support assembly, separated via the spacers  327 . The permanent magnets  308  are magnetized in the radial direction. The field winding  307  is coiled around the pole support assembly. 
     The permanent magnets  308  allow the rotor assembly  303  to produce either voltage or torque with minimal losses as compared to those associated with excitation of the field winding  307 . At higher loads in generating mode, the field windings  307  are excited to account for the resulting voltage drops and regulate the output. At higher loads in motoring mode, the field windings  307  are excited to produce additional flux, resulting in additional torque. At higher speeds (in both motoring and generating modes), the field windings  307  are excited in the reverse direction to provide field weakening capability, allowing for power regulation. The shape of the stator and rotor steel, as well as the placement of the MMF sources in the topologies explored allows the air-gap flux distribution to be manipulated, wherein the electromechanical energy conversion process occurs. Adequate placement of steel, magnets, spacers, and windings allows the flux to be distributed such that the flux linking the stator winding is maximized. 
       FIG. 5  is a schematic view of yet another hybrid machine topology that was explored, containing a stator and two rotors mechanically coupled via a common shaft, wherein a first rotor contains a backiron section, a pole shoe, pole body, and field winding, and a second rotor contains permanent magnets on the surface of the rotor backiron. The topology is used, among others, to compare the relative benefits of the hybrid machine presented in this disclosure. 
       FIGS. 6 a -6 d    are a collection of plots of flux density vs. electrical angle relative to the rotor (φ r ) for the hybrid machine according to one embodiment of the present disclosure (see  FIG. 2 a   ). The plotted flux densities are obtained using an analytical model (AM) and a finite element model (FEM), and include profiles resulting due to excitation of the a) q-axis, b) d-axis, c) field winding, and d) magnet. Typically, in the analysis of electric machinery, the fringing flux, resulting in the curved regions of a)-c), are ignored. For the hybrid machine HSM  100  of the present disclosure, ignoring the flux in these regions results in generally underestimating the associated flux linkages, and particularly that resulting from the field winding, used to regulate the output voltage in generation mode, or allowing for an additional parameter to regulate output torque in motoring mode. As a result, the benefits of the embodiment of  FIG. 2 a   , containing a magnet, with a lower relative permeability than that of the rotor steel, in the center of the pole assembly  121 , are not immediately obvious. 
     The hybrid machines HSM  100 ,  200 , and  300  may be operated as part of an energy conversion system shown in  FIG. 7 . Referring to  FIG. 7 , the system includes a mechanical arrangement that i) provides electrical power in generation mode or ii) receives electrical power in motoring mode, a hybrid electrical machine, a power converter, a controller, a position sensor (or estimator), and an electrical power i) load in generation mode or ii) source in motoring mode. The rotor position of the machine, the voltage and/or current at terminal t 1 , and/or the voltage and/or current at terminal t 2  are detected and used as a feedback to the controller to determine the appropriate field winding voltage and/or current. A position estimator can also be used to estimate position of the rotor and use the estimated position of the rotor as a feedback signal in a similar manner. 
     Referring to  FIG. 7 , the system operating in power generation mode includes an electrical load, which is one of a single-phase alternating power load, a multi-phase alternating power load, a direct current power load, and other loads known to a person having ordinary skill in the art. 
     Referring to  FIG. 7 , the system operating in motoring mode includes an electrical power source, which is one of a single-phase alternating power source, a multi-phase alternating power source, a direct current power source, and other sources known to a person having ordinary skill in the art. 
     Referring to  FIG. 7 , the power converter may be operating in AC/AC mode or AC/DC mode, with one AC side coupled to the electrical connection of the hybrid electrical machine. The converter is comprised of a plurality of switching devices, which may be of various types, including passive switches (e.g. diodes), semi-active switches (e.g. thyristors), and active switches (e.g. IGBTs). If semi-active or active switches are used, a controller is configured to provide the required control signals. 
     In  FIG. 8 , the Pareto-optimal fronts, providing the mass vs. loss characteristic, known to a person of ordinary skill in the art, of several hybrid machine topologies, as well as of a WRSM and PMSM, are shown. Each machine topology is optimized to minimize loss and mass for a given output power. The computation of this characteristic is repeated twice for each machine to check for consistency. From the Pareto-optimal front comparison in  FIG. 8 , the HSM  100  topology (referred in the figure as parallel-inner-split) is shown to have a better tradeoff than the WRSM and comparable to that of a PMSM. 
     Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. Therefore, the following claims are not to be limited to the specific embodiments illustrated and described above. The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.