Abstract:
A method and apparatus for fabricating a rotor for an electric traction motor including forming cavities in the rotor, injecting magnetic material in a portion of the cavities, configuring an electrical winding in a portion of the cavities, and post-magnetizing the magnetic material.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates generally to electric or hybrid electric vehicle propulsion systems. More specifically, the present invention relates to the design of electric traction motors or machines for use in electric or hybrid vehicles.  
         BACKGROUND OF THE INVENTION  
         [0002]    In today&#39;s automotive market, there exists a variety of electric propulsion or drive technologies used to power vehicles. The technologies include electric traction motors such as DC motors, AC induction motors, switched reluctance motors, synchronous reluctance motors, brushless DC motors and corresponding power electronics. Brushless DC motors are of particular interest for use as traction motors in an electric vehicle because of their superior performance characteristics, as compared to DC motors and AC induction motors. Brushless DC motors typically operate with a permanent magnet rotor. A permanent magnet rotor may be configured as a surface mount or interior or buried permanent magnet rotor. An interior permanent magnet (IPM) motor or machine has performance attributes, when compared to DC motors and AC induction motors, that include relatively high efficiency, relatively high torque, relatively high power densities, and a long constant power operating range which make an IPM machine attractive for vehicle propulsion applications.  
           [0003]    Permanent magnets buried inside a rotor for a brushless DC motor exhibit high reluctance directly along the magnetic axis or the d-axis due to the low permeability of the permanent magnets, while along the q-axis, between the magnetic poles or magnet barriers of an IPM rotor, there exists no magnetic barrier and reluctivity to magnetic flux is very low. This variation of the reluctance around the rotor creates saliency in the rotor structure of an IPM machine. Therefore, the IPM rotors have reluctance torque in addition to the permanent magnet torque generated by the magnets buried inside the rotor. Reluctance in the d-axis can be created by one magnet such as found in a single barrier rotor design.  
           [0004]    A single magnet of the one barrier rotor design can also be split into several layers creating a multi-barrier design. The multi-barrier design reduces leakage and improves the rotor saliency. Accordingly, motors having multi-barrier rotors have numerous performance advantages over a single barrier rotor design, including relatively high overall efficiency, extended high speed constant power operating range, and improved power factor. Improved saliency of the multi-barrier rotor helps to lower the amount of magnets or magnetic material in an IPM machine, as compared to a single barrier IPM machine or surface mounted permanent magnet machine, by reducing dependency on magnetic torque. The amount of magnetic material needed to generate a specific torque and wattage rating depends on the level of saliency of the rotor. The higher the rotor saliency, the lower the amount of magnetic material usage for the same overall machine performance. Electric motors having a multi-barrier rotor design, as compared to single barrier design, generate higher rotor saliency.  
           [0005]    Magnets in an IPM machine can be pre-magnetized and then inserted inside the rotor. This magnet insertion is a complex and relatively costly step that adds manufacturing steps to the assembly of the IPM machine.  
           [0006]    Post-magnetization of inserted magnetic material is possible if the magnets are inserted near the rotor surface. For post-magnetization, magnetic material may be preformed outside of the rotor, inserted into the rotor, and then magnetized. This is usually the case with sintered magnets, which require a certain orientation. A further type of magnetic material used that may be used in an IPM rotor is bonded magnets, which are usually mixed with a plastic, such as PPS, and may also be preformed outside of the rotor and then inserted into the rotor. However, generally bonded magnetic material is injected into the rotor cavities under high temperature and pressure.  
           [0007]    Electric motors having multi-layer buried magnets in their rotors, as shown in FIG. 2, exhibit excellent performance characteristics for vehicle propulsion application. The problems associated with post-magnetizing such a rotor geometry would result in a large amount of magnetic material buried deep within the rotor that may only partially magnetize or not magnetize at all, resulting in a waste of material. Post-magnetization works efficiently for magnetic material buried or located near the surface of the rotor. For magnetic material buried relatively deep in the rotor, post-magnetization is difficult due to the weakening of the magnetizing field.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention includes a method and apparatus for the design of an IPM machine rotor. The present invention removes magnetic material from the regions of the rotor which cannot be effectively or strongly magnetized during the post-magnetization process and inserts magnetizing coils. The outer barriers of the rotor of FIG. 2 are relatively easy to magnetize. However, the middle section of the inner regions of the rotor may not be exposed to a magnetic field strong enough to fully magnetize these regions. In the present invention, magnetic material is removed from these middle section areas and magnetizing coils are inserted in the empty areas for the magnetization process. The inserted magnetizing coils will enhance the magnetizing field produced by the stator or other magnetizing fixture, thus improving the rotor magnetization. Keeping the middle section areas void of any magnetic material does not change the rotor saliency or the reluctance torque, provided that the remaining areas are filled with magnet material that is fully magnetized. Specifically, the bridges between voids and filled areas are saturated by the magnetic material so as to ensure saliency. The magnetizing coils that are inserted in the void areas for the magnetizing process will enhance the field in this region and help magnetize the magnetic material that may not be fully magnetized by the stator fixture. The magnetizing coils in the preferred embodiment are removed from the rotor following the post magnetization process. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a diagrammatic cross-sectional drawing of a Starr permanent magnet motor and controls;  
         [0010]    [0010]FIG. 2 is a cross-section of a multi-layer interior or buried magnet motor geometry;  
         [0011]    [0011]FIG. 3 is a cross-section of a multi-layer interior or buried magnet motor with an empty bottom barrier;  
         [0012]    [0012]FIG. 4 is a cross-section of a multi-layer interior or buried magnet motor with an empty extended bottom barrier;  
         [0013]    [0013]FIG. 5 is a cross-section of a multi-layer rotor geometry with a magnetizing auxiliary winding in the empty barriers of the rotor; and  
         [0014]    [0014]FIG. 6 is a detailed partial view of FIG. 5. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    U.S. Ser. No. 09/952,319, assigned to the assignee of this invention, includes a detailed description of multi-layer motor geometry and is hereby incorporated by reference in its entirety.  
         [0016]    [0016]FIG. 1 is a diagrammatic drawing of a permanent magnet motor  10  having a wound stator  12  and permanent magnet rotor  14 . A power supply and inverter  16  commutate 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 electro-motive force (emf) detection. The motor may be characterized as a brushless DC motor with square wave or sinewave excitation provided by the power supply and inverter  16 .  
         [0017]    [0017]FIG. 2 is a cross-section of a multi-layer/barrier buried magnet rotor geometry. Regions  26  of the magnetic material layers or barriers  24  will be difficult to fully magnetize because of the distance from the rotor  14  surface. The magnetic material layers  24  surface may be magnetized by a magnetizing fixture or 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  14 . Magnetizing fixtures similar to the stator  12  contain windings that are used for the magnetization process. The magnetizing fixture includes enough iron to prevent it from becoming very saturated. Windings in the magnetizing fixture are placed such that the magnetic field is guided along a desired magnetization direction.  
         [0018]    In a preferred embodiment of the present invention, magnetic powder mixed with plastic may be injected into rotor cavities/barriers  24  under high temperatures and pressure, allowing the material to bond and form inside the rotor  14  cavity. This process is desirable for large scale production. As detailed previously, post-magnetization of the magnetic material is currently only practical if the magnetic material is buried near the rotor surface.  
         [0019]    Magnetic material, depending on its composition, requires varying magnetic field strengths to become fully magnetized. The high energy magnets 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 to become fully magnetized. The magnetic field is produced by the flow of current in the stator  12  winding 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 .  
         [0020]    As described previously, multi-layer or barrier geometry for an IPM rotor improves the rotor  14  saliency. Accordingly, the rotor  14  geometry of 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. Lower magnetic material volume requirements reduce the total 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 magnet field. Multi-barrier rotor geometries also have the advantage of favorable torque speed profile, with extended constant power range, for vehicle propulsion application. This multi-layer design may have magnetic material in all the layers  24  as shown in FIG. 2 or it may have magnetic material in one or more layers while the remaining layers are empty. The particular design depends on the magnet flux requirements, the type of magnetic material, and the saliency requirement. Despite all these favorable attributes, multi-layer designs are difficult to produce due to the difficulty of magnetizing all the magnet layers  24 , especially the regions  26  of the layers  24 .  
         [0021]    [0021]FIGS. 3 and 4 illustrate the removal of magnetic material from the central region or bottom barriers  28  of a multi-layer rotor  14 . The present invention removes magnetic material from areas of the rotor  14 , such as the regions  26  seen in FIG. 2, where it is difficult to magnetize the magnetic material. FIG. 3 illustrates rectangular bottom barriers  28  and FIG. 4 illustrates extended bottom barriers  28 . Accordingly, the bridge  22  between the empty bottom barriers  28  and the filled barriers  29  has also been modified as seen in FIG. 4. The barrier geometry of FIG. 4 is more easily magnetized as compared to the barrier geometry of FIG. 3. However, the bridge  22  seen in FIG. 4 is subjected to more mechanical stress due to centrifugal force generated by the rotation of the rotor  14 . Less magnetic material is used in the barrier geometry of FIG. 4 as compared to the barrier geometry of FIG. 3. Thus, to maintain the same airgap flux (i.e., torque) of the barrier geometry of FIG. 3, the magnetic strength of the filled barriers  29  of FIG. 4 must be larger than the magnetic strength of the filled barriers  29  of FIG. 3. The concentration of magnetic powder in the filled barriers  29  would be higher in the geometry of FIG. 4, as compared to the geometry of FIG. 3. Therefore, the geometry of FIG. 4 will be subjected to higher mechanical pressure and stress during the injection of moldable magnets into the filled barriers  29  during the fabrication of the rotor  14 .  
         [0022]    The torque ripple in the barrier geometry of FIG. 4 is also higher, as compared to the barrier geometry of FIG. 3. Therefore, although the barrier geometry of FIG. 4 may have a better chance to become fully magnetized, the barrier geometry of FIG. 3 is generally preferable. The present invention optimizes the performance of the barrier geometry of FIG. 3 with the inclusion of an auxiliary winding or windings for the enhancement of the magnetic field used to magnetize the filled barriers  29 .  
         [0023]    An auxiliary magnetizing coil or winding  32 , as seen in FIGS. 5 and 6, is inserted into the empty barriers  28  for the magnetizing process for the barrier geometry of FIG. 3. The auxiliary magnetizing winding  32  will enhance the magnetizing field produced by the magnetization fixture  12 , improving the magnetization of the inner part of the magnet barriers  29 , which cannot be fully magnetized by the magnetization fixture. The auxiliary winding  32  is inserted in each empty barrier  28  one per rotor pole. Therefore, adjacent coils (from pole to pole) have their polarity reversed. The auxiliary windings  32  in the preferred embodiment are removed after the post-magnetization process.  
         [0024]    [0024]FIG. 6 shows the details of the auxiliary winding. Each auxiliary magnetizing  32  winding comprises one complete coil having a one way and return side, and the coil  32  is wound around a core so as to provide a return path for the flux. The whole assembly is inserted in the empty cavity  28  for the magnetizing process. The auxiliary winding  32  is connected to a current source similar to the stator magnetizing fixture source, which provides the current burst for a short time. After the post-magnetization process, the magnetizing windings  32  are removed.  
         [0025]    The auxiliary windings  32  can incorporate much less Ampere-turns than the stator magnetization fixture because of their smaller area. However, it has a significant effect on the magnetization of magnetic material within the rotor  14  due to its proximity to magnetic material. Moreover, the saturation in the deep part of the rotor  14  is less than near the rotor periphery. Therefore, the ampere-turn requirement is further reduced.  
         [0026]    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.