Abstract:
A rotor assembly for a brushless motor is disclosed. In an exemplary embodiment of the invention, the rotor assembly includes a core having a central opening for insertion of a rotor shaft therein. A plurality of rotor magnets disposed upon a periphery of the core, wherein a space is defined between one of the plurality of rotor magnets and another of the plurality of rotor magnets. A portion of said core occupies said space, thereby defining a salient pole therewithin.

Description:
BACKGROUND  
         [0001]    Brushless motors presently in existence generally include a rotor assembly having one or more rotor magnets disposed on the periphery thereupon. The rotor magnets, when positioned upon the periphery of a rotor, may cover the entire outer surface of the rotor. Alternatively, a plurality of rotor magnets may have gaps or spaces located between each individual magnet, which gaps are either typically filled with a non-magnetic material or are left unfilled. In either case, the torque produced by a motor having such a rotor assembly is linearly proportional to the current applied. Thus, the motor torque constant K, (torque per unit current) will not vary over a given range of operating speeds.  
           [0002]    However, in certain applications using brushless motors, such as electric power steering systems, it may desirable to have a relatively high applied torque at low motor speeds and a relatively low applied torque at high motor speeds.  
         SUMMARY  
         [0003]    The problems and disadvantages of the prior art are overcome and alleviated by a rotor assembly for a brushless motor. In an exemplary embodiment of the invention, the rotor assembly includes a core having a central opening for insertion of a rotor shaft therein. A plurality of rotor magnets disposed upon a periphery of the core, wherein a space is defined between one of the plurality of rotor magnets and another of the plurality of rotor magnets. A portion of said core occupies said space, thereby defining a salient pole therewithin.  
           [0004]    In a preferred embodiment, the brushless motor has a total output torque having a first torque component and a second torque component. The first torque component is proportional to the applied current to the brushless motor and the second torque component is proportional to the square of the applied current to the brushless motor. Furthermore, the first torque component is generated as a result of the interaction between the plurality of rotor magnets and the magnetomotive force generated in a stator of the brushless motor. The first torque component is maximized when the angle between the stator magnetomotive force and a pole axis defined by a pair of the plurality of rotor magnets is about 90 degrees. In contrast, the second torque component is generated as a result of the interaction between the plurality of salient poles and the magnetomotive force generated in the stator of the brushless motor. The second torque component is maximized when the angle between said stator magnetomotive force and a pole axis defined by a pair of said plurality of rotor magnets is about 135 degrees. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    The present invention will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:  
         [0006]    [0006]FIG. 1 is a schematic diagram of an electric power steering system using a polyphase brushless motor in accordance with an embodiment of the invention;  
         [0007]    [0007]FIG. 2 is a cross sectional view of an existing rotor configuration for a brushless motor;  
         [0008]    [0008]FIG. 3 is a cross sectional view of another existing rotor configuration for a brushless motor;  
         [0009]    [0009]FIG. 4 is a cross sectional view of another existing rotor configuration for a brushless motor;  
         [0010]    [0010]FIG. 5 is a cross sectional view of a rotor assembly for a brushless motor, in accordance with an embodiment of the invention;  
         [0011]    [0011]FIG. 6 is an alternative embodiment of the rotor assembly of FIG. 5; and  
         [0012]    [0012]FIG. 7 is a graph illustrating the torque versus angle characteristics for the rotor assembly shown in FIG. 5. 
     
    
     DETAILED DESCRIPTION  
       [0013]    Referring initially to FIG. 1, a motor vehicle  10  is provided with an electric power steering system  12 . Electric power steering system  12  may include a conventional rack and pinion steering mechanism  14  having a toothed rack  15  and a pinion gear (not shown) under a gear housing  16 . As steering wheel  18  is turned, an upper steering shaft  20  turns a lower shaft  22  through a universal joint  24 . Lower steering shaft  22  turns the pinion gear. The rotation of the pinion gear moves the rack  15 , which then moves tie rods  28  (only one shown). In turn, tie rods  28  move steering knuckles  30  (only one shown) to turn wheels  32 .  
         [0014]    An electric power assist is provided through a controller  34  and a power assist actuator comprising a motor  36 . Controller  34  receives electric power from a vehicle electric power source  38  through a line  40 . The controller  34  also receives a signal representative of the vehicle velocity on line  41 , as well as steering pinion gear angle from a rotational position sensor  42  on line  44 . As steering wheel  18  is turned, a torque sensor  46  senses the torque applied to steering wheel  18  by the vehicle operator and provides an operator torque signal to controller  34  on line  48 . In addition, as the rotor of motor  36  turns, rotor position signals for each phase are generated within motor  36  and provided over bus  50  to controller  34 . In response to vehicle velocity, operator torque, steering pinion gear angle and rotor position signals received, the controller  34  derives desired motor phase currents. The motor phase currents are provided to motor  36  through a bus  52  to motor  36 , which thereby provides torque assist to steering shaft  20  through worm  54  and worm gear  56 .  
         [0015]    Referring now to FIG. 2, an existing motor  36  features a rotor assembly  60  having a plurality of rotor magnets  62  circumferentially mounted upon a core  64 . A rotor shaft  65  is inserted through an opening in core  64 . The core  64  is circular in shape and may comprise a plurality of lamina of soft iron, steel or other magnetic material. In the embodiment shown, the rotor magnets  62  completely cover the outer surface of the core  64 . Alternatively, FIG. 3 illustrates the rotor assembly  60  wherein the rotor magnets  62  do not entirely cover the outer surface of core  64 . In this case, a space  66  is defined in between each pair of adjacent magnets  62 , which space  66  is either left unfilled or is filled with non-magnetic material  68 , such as a plastic mold filler, shown in FIG. 4.  
         [0016]    In each of the existing rotor assembly  60  configurations shown in FIGS.  2 - 4 , the output torque of the motor  36  is directly proportional to the motor current. Furthermore, the output torque is maximized when the angle between the rotor pole axis  69  (shown by way of example in FIG. 4) for a given pair of rotor magnets  62  and the magnetomotive force generated in the stator (not shown) is at 90 electrical degrees with respect to one another. In terms of the torque produced per unit current, or torque constant K τ , this value remains a constant over the range of motor operating speeds.  
         [0017]    Therefore, in accordance with an embodiment of the invention, a rotor assembly  80  for a brushless motor is shown in FIG. 5. For ease of description, like elements appearing in the prior Figures are shown with the same reference numerals and component designations. In addition to the elements previously described, a space  66  is defined between each pair of adjacent magnets  62 ; however each space  66  is partially filled or occupied by a protruding portion of the core, thereby defining a salient pole  82  within each space. The sailent poles  82 , being comprised of the same soft magnetic material as the core  64 , are magnetically attracted to an energized stator coil (not shown). Thus, the salient poles  82  provide another component of torque in a comparable fashion to the rotor poles of a switched reluctance motor. More specifically, the magnetic interaction between the energized coils around the poles of the stator and the salient poles  82  produces a torque.  
         [0018]    [0018]FIG. 6 is an alternative embodiment of the rotor assembly  80  shown in FIG. 5. In the embodiment shown in FIG. 6, the salient poles  82  are dimensioned such the entire space  66  between each pair of adjacent rotor magnets  62  are filled with salient pole material.  
         [0019]    Thus configured, rotor assembly  80  therefore provides both a first torque component τ 1  and a second torque component τ 2 . The first torque component τ 1 , being generated by the interaction between the stator and the rotor magnets  62  is directly proportional to the applied current. The second torque component τ 2 , generated as described above, is not linearly proportional to the applied motor current, but proportional to the square of the motor current. As a result, the second torque component τ 2  assists in providing an overall greater torque at lower speeds where the motor current is initially higher. In addition, since the second torque component τ 2  is also proportional to twice the angle between the rotor pole axis for a given pair of rotor magnets and the magnetomotive force generated in the stator, τ 2  is maximized at intervals of 45 electrical degrees.  
         [0020]    Finally, FIG. 7 illustrates the relationship between the per unit torque of the first and second torque components τ 1, τ 2  versus the angle between the stator magnetomotive force. Curve  90  represents the per unit torque of the first torque component τ 1 , generated from the interaction between the stator mmf and the rotor magnets. As can be seen from the graph, τ 1  is maximized at a 90 degree angle. On the other hand, curve  92  represents the per unit torque of the second torque component τ 2 , generated from the interaction between the stator mmf and the salient poles. During the first half cycle of curve  92 , it is seen that the second torque component τ 2  opposes the first torque component τ 1 , with maximum opposition occurring at an angle of 45 degrees. During the second half cycle, the second torque component τ 2  assists the first torque component τ 1 , with maximum assistance at an angle of 135 degrees.  
         [0021]    Curve  94  represents the total output torque resulting from τ 1  and τ 2 . It can be seen that the maximum total output torque for motor  36 , therefore, will be between 90 and 135 degrees. At relatively small motor currents, the second torque component τ 2  will have less of an effect and, thus, the maximum total torque output will occur close to 90 degrees. At higher motor currents, the second torque component will have a greater effect on total torque output. Thus, the angle at which maximum torque occurs will also increase. Over a range of operating speeds, the second torque component τ 2  contribution to the total torque will be about 0-30% of the first torque component τ 1  contribution.  
         [0022]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.