Abstract:
An armature for use in an electric traction motor comprises a rotor having a central portion and a peripheral portion with the peripheral portion having a plurality of cavities. Permanent magnets solidified from liquid magnetic material are disposed in the cavities to form poles of the rotor with at least a portion of the cavities having directly abutting permanent magnets comprised of at least first and second magnetic materials of different properties injected in liquid or mobile form into single cavities. In interface regions between the at least first and second magnetic materials the first and second magnetic materials are intermingled to form a transition zone. A method of making the armature includes injecting the magnetic materials simultaneously as well as injecting a subsequent magnetic material when the initial magnetic material is still fluidly mobile.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to interior permanent magnet rotors with multiple properties and methods for making same. More particularly, the present invention relates to such rotors configured for use in electric motors used in electric or hybrid vehicles, or in electric motors used for other purposes.  
       BACKGROUND OF THE INVENTION  
       [0002]     The type and strength of magnetic material in different regions of a rotor used in electric motors may be varied. For example in U.S. Pat. No. 6,703,746, NdFeB is used to configure in the entire outer barrier of the rotor where it may be easily magnetized. However, in this patent high energy magnetic material in the middle section or the inner regions of the rotor are not exposed to a magnetizing field strong enough to fully magnetize high energy magnetic material. Consequently, low energy magnetic material is placed in areas of the rotor that are difficult to magnetize because low energy magnetic materials require a smaller magnitude magnetizing field than high energy magnetic materials. Accordingly, low energy magnetic material in the inner region may be fully magnetized. Low energy magnets in the inner region do not contribute to the air gap flux. However, the low energy magnets ensure bridge saturation, which is important to ensure high saliency corresponding to better performance. A non-magnetized high energy magnet in the inner region may contribute to a waste of valuable magnetic material as well as to inadequate bridge saturation. Inadequate bridge saturation can lower the rotor saliency and motor performance.  
         [0003]     In U.S. Pat. No. 6,703,746, the high energy and low energy magnetic materials are injected in liquid form into separate cavities within the rotor with the high energy magnetic material being injected into outer cavities adjacent the periphery of the rotor and low energy magnetic material being injected into inner cavities closer to the axis of the rotor. When the liquid magnetic material solidifies, it is magnetized by the stator or other source of magnetization. In the arrangement of U.S. Pat. No. 6,703,746, each magnetic material in a cavity is consistent with an unmixed interface therebetween.  
         [0004]     The design of an IPM motor is a series of tradeoffs to meet system objectives while minimizing unwanted side effects of the design. Torque ripple and losses within a rotor are two such unwanted side effects. Eddy current losses within a magnet in the motor contribute to the heating of the rotor. Segmenting sintered magnets along the axial length of the rotor is often employed to minimize the eddy current losses and thus rotor heating. Torque ripple can be minimized by careful shaping of the rotor magnets (thinner towards rotor surface) and/or skewing either the rotor or stator to smooth out air gap flux along the length of the motor. Skewing reduces the average torque production and complicates the manufacturing of the motor. Eddy current losses may be minimized by creating an additional mechanical bridge in the rotor so that a small portion of the cavity near the rotor surface can be left unfilled with magnet material. Since magnet material nearest the surface of the rotor contributes significantly to torque ripple and AC flux contributes to eddy currents, such techniques reduce both torque ripple and eddy current losses. Magnetic material nearer the surface of the rotor is easier to magnetize when the entire rotor is magnetized as one assembly, therefore placing stronger magnets near the rotor surface and weaker magnets deeper inside the rotor is a cost savings measure. There is a need, however, to make such arrangements of magnetic material as efficient as possible.  
       SUMMARY OF THE INVENTION  
       [0005]     An armature for use in an electric traction motor comprises a rotor having a central portion and a peripheral portion with the peripheral portion having a plurality of cavities. Permanent magnets solidified from liquid magnetic material are disposed in the cavities to form poles of the rotor. At least a portion of the cavities have directly abutting permanent magnets comprised of at least first and second magnetic materials of different properties that are injected while both are in liquid or at least mobile form, which magnetic materials together solidify in each cavity. This results in an interface region between the first and second magnetic materials in which the first and second materials mix with one another along a common interface region within each cavity.  
         [0006]     In a further aspect, there is at least a third magnetic material in each cavity of the rotor.  
         [0007]     In still a further aspect, the magnetic materials are Nd FeB and ferrite.  
         [0008]     In still a further aspect, the selected cavities have a substantially radial component with respect to the rotor, the magnetic properties of the variable property permanent magnetic varying in a radial direction with respect to axis the rotor.  
         [0009]     In a further aspect, the selective cavities have an axial extent with respect to the rotor and the magnetic properties of the variable property permanent magnetic vary in an axial direction with respect to the rotor.  
         [0010]     A method of fabricating a rotor to form an armature for an electric traction motor comprises first providing the rotor with cavities adjacent its periphery. First and second magnetic materials having different properties are injected while both are simultaneously in liquid form into first and second portions of the cavity. When the magnetic materials are allowed to solidify, the cavity has both the first and second magnetic materials concentrated in separate regions thereof with an interface region with a mixture of the first and second materials. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Various other features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood when considered in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the several views, and wherein:  
         [0012]      FIG. 1  is a diagrammatic cross-sectional drawing of a permanent magnetic motor and control system;  
         [0013]      FIG. 2  is a cross-section of a multi-layer interior or buried magnetic motor geometry;  
         [0014]      FIG. 3  is a cross-section of a multi-layer interior or buried magnetic motor with bottom barriers filled with low energy magnetic material and upper barriers filled with high energy magnetic material;  
         [0015]      FIG. 4  is a view similar to  FIGS. 1-3 , but showing single cavities some of which have at least two magnetic materials therein, and in accordance with the principles of the present invention the two magnetic materials have been injected while simultaneously in liquid form to produce an interface region in which the two magnetic materials intermix;  
         [0016]      FIG. 5  is a view of a single cavity in a rotor similar to that of  FIGS. 1-3  wherein a third material is injected into a cavity, the materials being separated by two interface regions with intermixtures of magnetic materials therein;  
         [0017]      FIG. 6  is a view of a single cavity in a rotor similar to  FIG. 4  wherein injected magnetic materials vary over axial length of the rotor with an interface region of mixed material therebetween, and  
         [0018]      FIG. 7  is a view of a single cavity in a rotor similar to  FIG. 4  wherein injected magnetic material is varied near the peripheral surface of the rotor.  
     
    
     DETAILED DESCRIPTION  
       [0019]      FIG. 1  is a diagrammatic drawing of a permanent magnet motor  10  having a wound stator  12  and a permanent magnet rotor  14 . The rotor  14  is formed around a longitudinal axis  15  and a peripheral surface  16 . A power supply and inverter  17  communicate and control the speed and torque of the motor  10  in response to feedback including, but not limited to, an encoder, resolver, tachometer, proximity switch and tooth set, and back electromotive force (emf) detection. The motor  10  may be characterized as a brushless DC motor with square wave or sinewave excitation provided by the power supply and inverter  17 .  
         [0020]      FIG. 2  is a cross-section of a prior art multi-layer or barrier buried magnet rotor geometry. Permanent magnets  20  are defined by regions  26  of magnetic material layers or barriers  24  that are difficult to fully magnetize because of a relatively long distance from the peripheral surface  16  of the rotor  14 . The surface of the magnetic material layers  24  are magnetized by a magnetizing fixture or by the wound stator  12  during a post-magnetization process. The post-magnetization process in one embodiment of the present invention includes positioning a magnetizing fixture around the rotor  14  to magnetize the magnetic material in the rotor. Magnetizing fixtures are similar to the stator  12  and contain windings which are used for the magnetization process. In alternate embodiments of the present invention, the stator  12  may itself be used to magnetize the rotor  14  instead of using a magnetizing fixture. The magnetizing fixture includes enough iron to prevent it from becoming saturated. Windings in the magnetizing fixture are placed such that the magnetic field is guided along a desired magnetization direction.  
         [0021]      FIG. 3  is a cross-section of a rotor similar to  FIG. 2  in which magnetic powder mixed with plastic is injected into the rotor  14  cavities under high temperature and pressure, allowing the material to bond and form to a solid magnet inside the rotor  14  cavity upon curing. This process is desirable for large scale production. As mentioned earlier, post-magnetization of high energy magnetic material is currently only practical if the magnetic material is buried near the rotor surface.  
         [0022]     Magnetic material  24 , depending on its composition, requires varying magnetic field strengths to become fully magnetized. The high energy magnets  20  which are preferred for variable speed motor drive applications due to their higher demagnetization strength, require very high magnetic fields to saturate the magnetic material  24  to become fully magnetized. The magnetic field is produced by the flow of current in the winding of the stator  12  or in a magnetizing fixture. Usually, a very high current burst is needed for a very short period of time to magnetize the rotor  14 . If the stator  12  lacks sufficient iron, it may become saturated during this process, preventing the generated magnetic field from penetrating into the rotor  14 .  
         [0023]     As described previously, multi-layer or barrier geometry for an IPM rotor improves the saliency of the rotor  14 . Accordingly, the geometry of the rotor  14  shown in  FIG. 2  has the advantage of having relatively high saliency, improving the machine torque density and lowering the magnetic material volume requirements for a specific torque or wattage motor rating. Lower magnetic material volume requirements reduce the motor cost and also alleviate the problems associated with high flux PM machines, such as short circuit and open circuit fault problems, and spin losses (eddy current induced losses) due to the presence of the permanent magnetic field.  
         [0024]      FIG. 3  is a cross section of a multi-layer or barrier buried magnet motor  10  with bottom barriers of the magnets  20  disposed closer to the axis  15  of the rotor  14  filled with low energy magnetic material  40  and upper barriers of the magnets filled with high energy magnetic material  42 . In the present invention, high energy magnetic material is removed from areas of the rotor  14 , such as regions  26  in  FIG. 2 , where it is difficult to magnetize the high energy magnetic material with a low energy magnetic material. The high energy magnetic material  42  may comprise a material requiring a magnetizing field more than 2000 kA/m to become magnetized. The low energy magnetic material  40  may comprise a material requiring a magnetizing field less than 2000 kA/m. Low coercivity of the low energy magnetic material  40  allows easier magnetization. In the preferred embodiment of the present invention, the high energy magnetic material  42  is NdFeB and the low energy magnetic material  40  is ferrite, but any other high energy or low energy magnetic material is considered within the scope of the present invention.  
         [0025]     The low energy magnetic material  40  placed nearer to center of the rotor  14  can be fully magnetized by the magnetizing fixture because of its lower magnetizing field. The main performance contribution of the magnetic material  40  is to saturate the bridges  22  between barriers  24  and therefore ensure the saliency of the rotor  14 . These bridges  22  also ensure the mechanical strength of the rotor  14 . The mechanical strength of the low magnetic material  40  that is placed near the center of the rotor is sufficient to fulfill this function.  
         [0026]     Referring now to  FIG. 4  where a portion  14 A of a rotor  14  is shown, the use of multiple property injection molded magnets  50  as discussed in the prior art of  FIGS. 1-3 , offers the designer additional degrees of freedom to optimize machine design. One area of concern are mechanical rotor bridges  52 . Since rotor bridges  52  provide leakage paths, it is desirable that the rotor iron comprising the bridges is magnetically saturated so that leakage flux is minimized. Strong magnets in the vicinity of the rotor bridges  52  increase the localized saturation of the bridges and help achieve this goal. However, the magnet strength to saturate the bridges  52  may be too strong to meet the system objectives if used in the entire cavity.  
         [0027]     In  FIG. 4  the magnets  50  are multiple property magnets injected in liquid form or mobile (i.e., flowable plastic) form into rotor cavity segments  53  and  54  so as to control the localized saturation of the rotor bridges  52 , while the magnets  54  are single property injection magnets. In  FIG. 4 , magnet portions  60  and  62  having different properties (i.e., different magnet strength, temperature stability, etc.) are injected into rotor cavity segments while both magnetic materials are in liquid form. Since the magnet portions  60  and  62  are injected while both portions are in liquid form, some mixing occurs at the interface regions  63 . This results in the magnets  50  having a smooth transition of magnetic properties along the radial extent of the magnets. Consequently, when the magnetic portions  60  and  62  are injected while both portions a still in a liquid or mobile form, or are injected simultaneously, the magnetic portions are in the cavity  53  and  54  segments intermingle in the interface region  63 .  
         [0028]     Preferably, the first magnetic material  60  is a relatively high energy material, such as Nd Fe B, and the second material  62  is a relatively low energy material, such as ferrite, with the materials in the interface region being a mixture of NdFeB and ferrite. By varying the strength and/or thermal properties of magnetic cavities in a radial direction away from the periphery  16  and toward the rotor axis  15  (see  FIGS. 1 and 2 ), magnetic saturation is controlled in the radial steel sections of which the rotor is configured affording the designer increased flexibility in configuring the rotor  14 .  
         [0029]      FIG. 5  illustrates another embodiment of the invention wherein a cavity  70  has a third magnetic material  72  injected between a first magnetic material  60 ′ and second magnetic material  62 ′. Disposed between the first magnetic material  60 ′ and the third magnetic material  72  is a second interface region  74  and disposed between the second magnetic material  62 ′ and the third magnetic material  62 ′ and the third magnetic material  74  is a third interface region  76 . The second and third interfaces regions  74  and  76  are two in number, whereas the first interface region  63  of  FIG. 4  is one in number. As with the first interface region  63  of  FIG. 4 , the third magnetic material  72  is injected into the cavity  70  while the first and second materials  60 ′ and  62 ′ are in liquid or mobile (i.e., flowable plastic) form and are mobile enough to intermingle with the third material  72 . Consequently, the second and third interface regions  74  and  76  provide a smooth transition zone between the three magnetic materials in the cavity  70 . In  FIG. 5 , the transition of magnetic material  60 ′ to the second magnetic material  62 ′ is made smoother by the third magnetic material  72  and smoother still by the interface regions  74  and  76 .  
         [0030]     Referring now to  FIG. 6 , the magnetic properties of the magnetic materials  80  and  82  are in another embodiment varied along the axial length of a rotor  14 ′ as is shown by the axial cavity  84 . Between the magnetic materials  82  and  82 ″ is an interface region  86  where the magnetic materials  80  and  82  intermingle to smooth transition between the materials. This arrangement skews the rotor  14  electromagnetically so that it is not necessary to physically arrange rotor laminations.  
         [0031]     In  FIG. 7 , the magnetic properties are varied near the peripheral surface  16  of the rotor  14  by mixing some magnetic material  60 ″ with magnetic material  62 ″. This is still an interface region  63 ″ between magnetic material  62 ″ and the mixed region adjacent the periphery  16  of the rotor  14 . This arrangement decreases torque ripple associated with the slotting effect between the rotor and stator  12 , while simultaneously insuring desirable bridge saturation deeper within the rotor. By controlling saturation on a localized basis, reluctance torque availability is increased resulting in an increased motor torque and power density.  
         [0032]     While this invention has been described in terms of some specific embodiments, it will be appreciated that other forms can readily be adapted by one skilled in the art. Accordingly, the scope of this invention is to be considered limited only by the following claims.  
         [0033]     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing form the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.