Abstract:
A permanent magnet (PM) machine includes a rotor and a stator assembly. The rotor includes a plurality of permanent magnets disposed about an axis of rotation. The stator assembly includes a stator body, a plurality of coil sides and a plurality of sintered iron magnetic wedges. The stator body includes a plurality of stator teeth defining a plurality of stator slots, each stator slot having an inside position and an outside position, such that each of the plurality of stator slots includes a first plurality of inside positions, and a first plurality of outside positions. The first plurality of coil sides are disposed in each of the first plurality of inside positions and the first plurality of outside positions. The first plurality of coil sides correspond to a first power phase. The first plurality of coil sides are electrically coupled to one another by a first plurality of end-coils. The plurality of sintered iron magnetic wedges are disposed at the openings of at least one stator slot of the plurality of stator slots.

Description:
BACKGROUND 
       [0001]    The subject matter disclosed herein relates to electric machines and, specifically, permanent magnet (PM) machines. 
         [0002]    PM machines are used in various applications (e.g., aviation, propulsion motor for passenger vehicles, military ground vehicles, etc.) to convert between electrical power and mechanical power. Conventional PM synchronous electric machines employ permanent magnets as the magnetic poles of a rotor, around which a stator is disposed. The stator has a plurality of teeth that face the rotor. Alternatively, the machine may be designed so that the rotor surrounds the stator. For high-speed operation, a retaining sleeve is usually wrapped around the magnets as needed to keep the magnets in place. The retaining sleeve may be shrink fit upon the magnets to ensure a non-slip fit. Usually the retaining sleeve is made of one whole metallic piece for structural integrity. When the coils formed on the stator are energized, a magnetic flux is induced by the voltage, creating electromagnetic forces between the stator and the rotor. These electromagnetic forces contain tangential and/or circumferential forces that cause the rotor to rotate. When a PM machine is operating in the generating mode and experiences a fault (e.g., a short circuit due to winding defects or defective components), it may not be possible to quickly stop the PM machine because it is externally driven by the mechanical system. A fault-tolerant PM machine may be capable of sustaining a fault condition indefinitely. However, typical approaches to increasing fault tolerance may negatively impact the torque density of the PM machine. As such, it would be beneficial to improve the fault tolerance of a PM machine without sacrificing torque density. 
       BRIEF DESCRIPTION 
       [0003]    Certain embodiments commensurate in scope with the original claims are summarized below. These embodiments are not intended to limit the scope of the claims, but rather these embodiments are intended only to provide a brief summary of possible forms of the claims. Indeed, the claims may encompass a variety of forms that may be similar to or different from the embodiments set forth below. 
         [0004]    In one embodiment, a permanent magnet (PM) machine includes a rotor and a stator assembly. The rotor includes a plurality of permanent magnets disposed about an axis of rotation. The stator assembly includes a stator body, a plurality of coil sides and a plurality of sintered iron magnetic wedges. The stator body includes a plurality of stator teeth defining a plurality of stator slots, each stator slot having an inside position and an outside position, such that each of the plurality of stator slots includes a first plurality of inside positions, and a first plurality of outside positions. The first plurality of coil sides are disposed in each of the first plurality of inside positions and the first plurality of outside positions. The first plurality of coil sides correspond to a first power phase. The first plurality of coil sides are electrically coupled to one another by a first plurality of end-coils. The plurality of sintered iron magnetic wedges are disposed at the openings of at least one stator slot of the plurality of stator slots. 
         [0005]    In another embodiment, a permanent magnet (PM) machine includes a rotor and a stator assembly. The rotor includes a rotor hub, and a plurality of permanent magnets disposed about the rotor hub. The stator assembly includes a stator body, first, second, and third pluralities of coil sides, and a plurality of sintered iron magnetic wedges. The stator body includes a plurality of stator teeth defining a plurality of stator slots, each stator slot having an inside position and an outside position, such that the plurality of stator slots includes a first plurality of inside positions, a second plurality of inside positions, a third plurality of inside positions, a first plurality of outside positions, a second plurality of outside positions, and a third plurality of outside positions. The first plurality of coil sides are disposed in each of the first plurality of inside positions and the first plurality of outside positions, wherein the first plurality of coil sides correspond to a first power phase, wherein the first plurality of coil sides are electrically coupled to one another by a first plurality of end coils, and wherein the first plurality of coil sides are separated from one another by two stator teeth. The second plurality of coil sides are disposed in each of the second plurality of inside positions and the second plurality of outside positions, wherein the second plurality of coil sides correspond to a second power phase, wherein the second plurality of coil sides are electrically coupled to one another by a second plurality of end coils, and wherein the second plurality of coil sides are separated from one another by two stator teeth. The third plurality of coil sides are disposed in each of the third plurality of inside positions and the third plurality of outside positions, wherein the third plurality of coil sides correspond to a third power phase, wherein the third plurality of coil sides are electrically coupled to one another by a third plurality of end coils, and wherein the third plurality of coil sides are separated from one another by two stator teeth. The plurality of sintered iron magnetic wedges are disposed at the openings of at least one stator slot of the plurality of stator slots. 
         [0006]    In a third embodiment, a permanent magnet (PM) machine includes a rotor and a stator assembly. The rotor includes a rotor hub and a set of 10*N permanent magnets disposed about the rotor hub. The stator assembly includes a stator body, three sets of coil sides, and a plurality of sintered iron magnetic wedges. The stator body includes 24*N stator teeth, wherein the stator teeth define 24*N stator slots, each stator slot having an inside position and an outside position, such that the 24*N stator slots include a first set of 8*N inside positions, a second set of 8*N inside positions, a third set of 8*N inside positions, a first set of 8*N outside positions, a second set of 8*N outside positions, and a third set of 8*N outside positions. The first set of 16*N coil sides disposed in each of the first set of 8*N inside positions and the first set of 8*N outside positions, wherein the first set of 16*N coil sides correspond to a first power phase. The first set of 16*N coil sides are electrically coupled to one another by a first set of end coils, and the first set of 16*N coil sides are separated from one another by two stator teeth. The second set of 16*N coil sides are disposed in each of the second set of 8*N inside positions and the second set of 8*N outside positions, wherein the second set of 16*N coil sides correspond to a second power phase. The second set of 16*N coil sides are electrically coupled to one another by a second set of end coils, and the second set of 16*N coil sides are separated from one another by two stator teeth. The third set of 16*N coil sides are disposed in each of the third set of 8*N inside positions and the third set of 8*N outside positions, wherein the third set of 16*N coil sides correspond to a third power phase. The third set of 16*N coil sides are electrically coupled to one another by a third set of end coils, wherein the third set of 16*N coil sides are separated from one another by two stator teeth. The plurality of sintered iron magnetic wedges are disposed at the openings of each of the plurality of stator slots. Wherein N is a positive integer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0008]      FIG. 1  is a cut-away view of one embodiment of a typical PM machine; 
           [0009]      FIG. 2  is a section view of one embodiment of a rotor in accordance with aspects of the present disclosure; 
           [0010]      FIG. 3  is a cross-sectional view of one embodiment of a 24-slot, 10-pole, 3-phase PM machine in accordance with aspects of the present disclosure; 
           [0011]      FIG. 4  is a diagram of one embodiment of a 2-tooth concentrated winding topology for a 24-slot, 10-pole, 3-phase PM machine in accordance with aspects of the present disclosure; and 
           [0012]      FIG. 5  is a plot of the MMF space harmonics produced by a 24-slot, 10-pole, 3-phase PM machine with a 2-tooth concentrated winding topology in accordance with aspects of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    One or more specific embodiments will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
         [0014]    When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Furthermore, any numerical examples in the following discussion are intended to be non-limiting, and thus additional numerical values, ranges, and percentages are within the scope of the disclosed embodiments. 
         [0015]    Permanent magnet (PM) machines may be used to convert between electrical power and mechanical power. Typically, a rotor rotates within a stator, though it is also possible for the stator to be interior to the rotor. The rotor may include a plurality of magnets disposed circumferentially about a shaft. The stator may include one or more coil sides, which may be connected to a load. By rotating within the stator, the rotating magnets on the rotor induce a voltage in the coils. In other embodiments, the rotor may include coil sides and the stator may include a plurality of magnets. When a PM machine experiences a fault (e.g., a short circuit due to winding defects or defective components), the magnetic flux of the PM machine cannot be turned off, as with some other electric machines. Thus, the flux of the magnets may continue to add energy (e.g., heat) to the faulted winding by linking the short circuited winding or turn. Accordingly, a fault tolerant PM machine may be able to sustain a fault (e.g., a three-phase short circuit) condition indefinitely if the heat produced from winding resistive losses is less than or equal to the heat produced during rated operation. However, typical techniques for increasing fault tolerance can negatively impact the torque density of the PM machine. The techniques describe herein utilize sintered iron magnetic wedges and two-tooth concentrated winding topology in order to increase fault tolerance without sacrificing torque density. 
         [0016]      FIG. 1  is a cut-away view of one embodiment of a typical PM machine  10 . The PM machine  10  includes a rotor  12  that rotates within a stator  14 . The rotor  12  may include a plurality of permanent magnets  16 . The stator may include coil sides  18 . As the rotor  12  rotates within the stator  14 , a voltage is created by way of magnetic induction, thus converting mechanical energy into electrical energy and vice-versa. It should be understood, however, that in some embodiments the placement of the magnets  16  and coil sides  18  may be reversed. That is, in some embodiments the coil sides  18  may be a part of the rotor  12  and the magnets  16  may be part of the stator  14 . Additionally, coil sides  18  may not be actual coils of wire. For example, coil sides  18  may be vertical strips of a conductor (e.g., copper), stranded Litz wire, carbon nano-tube conductors, form-wound coils, or any other configuration that allows for voltage to be induced by the rotating permanent magnets. 
         [0017]      FIG. 2  is a cross-section view of one embodiment of a rotor  12 . In the present embodiment, the rotor  12  includes a shaft  30 . Surrounding the shaft  30  is a rotor hub  32 . The rotor hub  32  may be laminated magnetic steel, a solid machined forging of magnetic steel, or some other magnetic material. In the embodiment shown in  FIG. 2 , the rotor  12  includes a rotor hub  32  having the cross-sectional shape of a 10-sided regular polygon. In other embodiments, the cross-sectional shape of the rotor hub  32  may be circular, triangular, square, pentagonal, hexagonal, octagonal, or a regular or irregular polygon having any number of sides. For example, in some embodiments, the rotor hub  32  may have the cross-sectional shape of a polygon with 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or more sides. In some embodiments, the shaft  30  and rotor hub  32  may be a single part. As shown in  FIG. 2 , a plurality of permanent magnets  18  (or “poles”), are disposed about the rotor hub  32 . In the embodiment shown in  FIG. 2 , the number of poles  18  is equal to the number of sides of the cross-sectional shape of the rotor hub  32 . However, in other embodiments the number of poles may be more or less than there are number of sides of the cross-sectional shape of the rotor hub  32 . For example, the rotor hub  32  may be circular in shape with 10 poles  18  disposed about the rotor hub  32 . In other embodiments, the cross-sectional shape of the rotor hub  32  may be a 20-sided polygon, with 10 poles  16  disposed about the rotor hub  32 . The rotor  12  shown in  FIG. 2  also includes a retaining sleeve  34  surrounding the permanent magnets  16 . It should be understood, however, that some embodiments of the rotor  12  may not include a retaining sleeve  34 . Though a surface permanent magnet (SPM) configuration is shown in  FIG. 2 , wherein the permanent magnets  16  are disposed about the rotor hub  32 , some embodiments may use an interior permanent magnet (IPM) configuration. That is, in some embodiments, the permanent magnets  16  may be disposed within the rotor hub  32  or the shaft  16 . 
         [0018]      FIG. 3  is a cross-sectional view of one embodiment of a 24-slot, 10-pole, 3-phase PM machine  10  having a rotor  12  and a stator  14 . It should be understood that for the sake of clarity, the winding topology is not shown in  FIG. 3 . The winding topology will be discussed in detail with regard to  FIG. 4 . The stator  14  may include a stator body  40  having a plurality of stator teeth  42  that define a plurality of stator slots  44 . The embodiment shown in  FIG. 3  includes 24 teeth  42  and 24 slots  44 . Because the embodiment shown in  FIG. 3  includes 10 magnetic poles  16  and 24 stator slots  44 , it is referred to as a “24-slot, 10-pole PM machine.” It should be understood, however, that in some embodiments the stator body  40  may have any number of slots  44  and teeth  42  such that the PM machine  10  has a fractional number of stator slots-per-pole (i.e., a “fractional” PM machine  10 ). For example, the stator body  40  may have 3, 6, 8, 9, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 26, 27, 28, 30, 32, 33, 36, 38, 40, or any other number of teeth  42  and slots  44  as long as the number of slots is evenly divisible by the number of poles. Alternatively, some embodiments may multiple the number of elements by an integer, N. For example, one embodiment of the PM machine  10  may have 24*N slots and 10*N poles, wherein N is any positive integer. In other embodiments, the number of slots may be evenly divisible by the number of poles (i.e., an “integral” PM machine  10 ). For example, in one embodiment, the PM machine  10  may have 4 magnetic poles  16  and 24 slots  44 , or any other number of slots and poles, such that the number of slots is evenly divisible by the number of poles. 
         [0019]    The stator slots  44  shown in  FIG. 3  are open (i.e., the width of each opening is as wide as the slot  44 ). Accordingly, unlike stators in some PM machines, the teeth  42  of the stator  14  shown in  FIG. 3  do not have tangs. The open stator slots  44  allow for coil sides  18  to be partially or fully formed ahead of time and then dropped into the stator slots  44 . This may result in cost savings in manufacturing the stator  14 . End coils may connect coil sides  18  and wrap around the stator teeth  42  in a concentrated two-tooth winding topology that will be described in more detail with regard to  FIG. 4 . As is shown in  FIG. 3 , the stator slots  44  may be divided into two or more sections to allow for multiple phases of coil sides  18 , separated by an insulator, to occupy a single stator slot  44 . 
         [0020]    Magnetic wedges  46  may be placed at the openings of the stator slots  44  to keep the coil sides  18  in place. Typically, the leakage inductance of a stator  14  may be tuned by adjusting the design or the stator tooth  42  tangs. Though the present embodiment lacks stator tooth  42  tangs, the leakage inductance may be tuned by adjusting the design of the magnetic wedges  46 . In some embodiments, the magnetic wedges  46  may be made of a sintered powdered iron material mixed with fillers. Using a sintered powdered iron material mixed with fillers results in a magnetic wedge with good relative magnetic permeability, but low electrical conductivity. Additionally, use of a sintered powdered iron material allows magnetic wedges  46  to be designed with a wide range of relative magnetic permeability properties by varying the amount of iron. In contrast, magnetic wedges made with other materials and/or processes, such as Vetroferrite®, may have limited ranges of relative permeability due to the use of non-magnetic materials as fillers. For example, a magnetic wedge  46  may have a relative permeability (μ r ) of 1, 3, 5, 8, 10, 14, 20, 25, 30, 38, 48, 60, 72, 85, 100, or any other number greater than, less than, or between the listed values, wherein the relative permeability of air is 1. 
         [0021]    In typical stator  14  designs, the use of open stator slots  44  lowers the net flux-linkage of the stator winding and the leakage inductance, which in turn reduces the torque density and increases the short circuit current of the machine. However, the use of magnetic wedges  46  increases the torque density of the machine while also increasing the leakage inductance of the machine design when compared to a similar design without magnetic wedges. In some embodiments, the magnetic wedges  46  may be coated to prevent dusting or erosion due to vibration during operation. The coating may be metal, thermoset, thermoplastic, a composite, or any other material used to prevent erosion of the magnetic wedges  46 . It should be understood, however, that in some embodiments the magnetic wedges  46  may not be coated. 
         [0022]      FIG. 4  is a diagram of one embodiment of a 2-tooth concentrated winding topology for a 24-slot, 10-pole, 3-phase PM machine  10 . Though fully concentrated winding topologies typically span a single stator tooth  42 , in the 2-tooth concentrated winding topology shown in  FIG. 4 , the end coils  74  connecting the coil sides  18  span 2 stator teeth  42 . As previously discussed, the stator slots  44  may be divided into two sections, as shown in  FIG. 4 , allowing 2 coil sides  18 , which may be of different phases, to share a stator slot  44 . In such an embodiment, an insulator may be used between the coil sides  18  in order to avoid contact between end coils  74 . For example, in some embodiments, the coil sides  18  may include stranded Litz wire compacted into a rectangular cross-section and coated with an insulator. In other embodiments, the divider may be an insulator. For example, the open stator slots  44  with an insulating divider may allow individual coil sides  18  to be formed outside of the stator  14 , dropped into the stator slots  44 , and then brazed. This “drop in” coil configuration facilitates the use of rectangular vertical strips of copper for winding rectangular-shaped turns, which may limit the peak current during turn-to-turn short circuit faults. The various coil sides  18  may be connected using end coils  74  as shown in  FIG. 4 . 
         [0023]    In general, the winding topology has two separate winding patterns that are shifted with respect to one another and then connected in series. In  FIG. 4 , the 3 phases are represented by A, B, and C. As shown in  FIG. 4 , an end coil  74  attaches to the positive coil side  70  of one phase (e.g., A+), spans 2 teeth (i.e., “2-tooth throw”), and then connects to the negative coil side  72  of the same phase (e.g., A−). The end coils  74  are then connected in series or parallel. 
         [0024]    As shown in  FIG. 4 , the outside end coils  74  may be paired such that both end coils  74  of a pair go the same direction. The directions of the end coil  74  pairs alternate as one moves around the exterior of the stator  14  between clockwise and counterclockwise. Additionally, each of the pairs of end coils  74  along the exterior of the stator include end coils  74  of two different phases. Accordingly, as one moves around the outside of the stator  14  in a clockwise direction, the phase pattern of the coil sides  70  is A+, A+, C+, A−, C−, C−, B−, C+, B+, B+, A+, B−. The pattern then repeats, but with opposite polarities, A−, A−, C−, A+, C+, C+, B+, C−, B−, B−, A−, B+. 
         [0025]    The inside end coils  74  may be paired such that both end coils  74  of a pair go in opposite directions. The pairs alternate as one moves around the interior of the stator  14  between inside-out, and outside-in. As with the outside end coils  74 , each of the pairs of end coils  74  along the interior of the stator include end coils  74  of two different phases. Accordingly, as one moves around the inside of the stator  14  in a clockwise direction, the phase pattern of the coil sides  70  is C−, B−, C+, A−, B+, A+, B−, C+, A−, C−, A+, B−. As with the outside coil sides  70 , the pattern repeats, but with opposite polarities, C+, B+, C−, A+, B−, A−, B+, C−, A+, C+, A−, B+. 
         [0026]    The 2-tooth concentrated winding topology shown in  FIG. 4  provides a compromise between fully-concentrated winding and distributed winding. Specifically, the winding topology limits the electromagnetic losses of the rotor by minimizing the magneto-motive force (MMF) space harmonics that would otherwise be produced by a fully concentrated winding. The concentrated coils also limit electromagnetic coupling between phase end coils  74  to limit short circuit currents. However, some phase-to-phase isolation is preserved due to the short pitch angle of the concentrated winding and careful shaping of the end-regions of the phase end coils  74 . 
         [0027]    It should be understood, however, that the disclosed techniques are not limited to the specific winding pattern shown in  FIG. 4 . Embodiments having stators  14  with greater than or less than 24 stator slots  44  may have slightly different winding patterns to accommodate different numbers of stator slots  44  and stator teeth  42 , or different numbers of poles  16  and or phases. In such embodiments, a given end coil  74  will still connect a to positive coil side  70  to a negative coil side  72 , and may or may not cross two stator teeth  42 . 
         [0028]      FIG. 5  is a plot  100  of the MMF space harmonics produced by a 24-slot, 10-pole, 3-phase PM machine  10  with the 2-tooth concentrated winding topology shown in  FIG. 4 . The x-axis  102  represents the various harmonics and the y-axis  104  represents the MMF space harmonic magnitude produced by the stator winding  10 . In general, a single harmonic produces torque. The other harmonics are considered parasitic. Generally, it is desirable to have a high torque-producing MMF space harmonic, but minimal parasitic harmonics. Note that in the plot  100  of  FIG. 5  the fifth harmonic  106  is the torque producing harmonic and the remaining parasitic MMF space harmonics are low. Accordingly, the 24 slot, 10 pole PM machine  10  with the winding topology shown in  FIG. 4  is a fault tolerant PM machine that also limits the parasitic MMF space harmonics. 
         [0029]    The torque values and steady-state 3-phase short circuit currents of several embodiments of a 24-slot, 10 pole fractional PM machine  10  are set out below in Table 1. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Linear 
               
               
                   
                   
                 Non-Linear 
                 Non-Linear 
                 Linear 
                 Perme- 
               
               
                 Current 
                 Semi- 
                 Permeability 
                 Permeability 
                 Permeability 
                 ability 
               
               
                 (A rms ) 
                 Closed 
                 Wedge 
                 Wedge 
                 Wedge 
                 Wedge 
               
               
                   
                 Slot 
                 (μ r  = 60) 
                 (μ r  = 14) 
                 (μ r  = 3) 
                 (μ r  = 1) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Torque (Nm) 
               
             
          
           
               
                 270 
                 125.27 
                 121.70 
                 121.23 
                 119.18 
                 113.85 
               
               
                 600 
                 249.93 
                 252.81 
                 262.31 
                 262.66 
                 252.91 
               
             
          
           
               
                 Steady-State 3-Phase Short Circuit Current (A rms ) 
               
             
          
           
               
                   
                 290 
                 250 
                 320 
                 370 
                 450 
               
               
                   
               
             
          
         
       
     
         [0030]    The “semi-closed slot” PM machine refers to a PM machine that does not use magnetic wedges  46 . In such a design, a PM machine has a stator body in which the stator teeth  42  have tangs and the stator slots  44  are semi-closed. The remaining embodiments referred to in Table 1 have a stator body with open stator slots and magnetic wedges  46  of varying relative magnetic permeability. As compared to the semi-closed slot design, the torque production at a current of 270 A rms  is only slightly lower in designs with open stator slots  44  and magnetic wedges  46  than the semi-closed slot design. At 600 A rms  the torque production of the designs with open stator slots  44  and magnetic wedges  46  are improved as compared to a semi-closed slot design. As is shown in Table 1, as the relative magnetic permeability (μ r ) of the magnetic wedge  46  approaches a value of 60, the steady-state 3-phase short circuit current falls. Using a magnetic wedge having a relative magnetic permeability of 60, the steady-state 3-phase short circuit current of the PM machine is 40 A rms  lower than the semi-closed slot design. 
         [0031]    Similarly, the torque values and steady-state 3-phase short circuit currents of several embodiments of a 24-slot, 4-pole integral PM machine  10  are set out below in Table 2. Though the 24-slot, 4-pole integral PM machine  10  has a 5-tooth throw winding topology, it should be understood that other winding topologies may be possible. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
             
               
               
               
               
               
               
             
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Linear 
               
               
                   
                   
                 Non-Linear 
                 Non-Linear 
                 Linear 
                 Perme- 
               
               
                   
                 Semi- 
                 Permeability 
                 Permeability 
                 Permeability 
                 ability 
               
               
                 Current 
                 Closed 
                 Wedge 
                 Wedge 
                 Wedge 
                 Wedge 
               
               
                 (A rms ) 
                 Slot 
                 (μ r  = 60) 
                 (μ r  = 14) 
                 (μ r  = 3) 
                 (μ r  = 1) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Torque (Nm) 
               
             
          
           
               
                 270 
                 96.79 
                 95.52 
                 95.08 
                 94.68 
                 92.34 
               
               
                 600 
                 213.81 
                 210.58 
                 210.56 
                 210.28 
                 205.26 
               
             
          
           
               
                 Steady-State 3-Phase Short Circuit Current (A rms ) 
               
             
          
           
               
                   
                 740 
                 760 
                 830 
                 820 
                 960 
               
               
                   
               
             
          
         
       
     
         [0032]    As discussed with regard to Table 1, the “semi-closed slot” PM machine in Table 2 refers to a PM machine that having a stator body in which the stator teeth  42  have tangs and the stator slots  44  are semi-closed rather than sintered iron magnetic wedges. The remaining embodiments referred to in Table 2 have a stator body with open stator slots and sintered magnetic wedges  46  of varying relative magnetic permeability. As compared to the semi-closed slot design, the torque production at currents of 270 A rms  and 600 A rms  is only slightly lower in designs with open stator slots  44  and magnetic wedges  46  than the semi-closed slot design. As is shown in Table 2, as the relative magnetic permeability (μ r ) of the magnetic wedge  46  approaches a value of 60, the steady-state 3-phase short circuit current falls. Using a magnetic wedge having a relative magnetic permeability of 60, the steady-state 3-phase short circuit current of the PM machine is similar to the semi-closed slot design. 
         [0033]    The use of sintered powdered iron magnetic wedges  46  with the 2-tooth concentrated winding topology discussed with regard to  FIG. 4  results in a fault-tolerant PM machine  10  that does not sacrifice torque density. The disclosed techniques allow the designer of a PM machine  10  to tune the leakage inductance of the winding, thereby limiting the short circuit current of the machine, without sacrificing torque density. Additionally, the disclosed techniques may reduce torque ripple. The resulting PM machine may be capable of sustaining a 3-phase short circuit indefinitely. A stator body  40  having open slots  44  also allows for the PM machine&#39;s  10  coil sides  18  to be manufactured outside of the stator  14  and then inserted into the stator slots  44 . The use of sintered powdered iron magnetic wedges allows the designer more control in tuning the leakage inductance as compared to other magnetic wedges (e.g., Vetroferrite®) or semi-closed slot designs. Coating the sintered powdered iron magnetic wedges  46  may prevent erosion or dusting due to vibration. 
         [0034]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.