Abstract:
A method of operating a variable flux electric machine includes passing an electrical current through a plurality of wound poles of a wound field to generate a first flux, rotating an armature at a first crank point having a first speed in response to the first flux, shorting at least one of the plurality of wound poles, generating a second flux through a permanent magnet (PM) field having a plurality of PM poles, and rotating the armature at a second crank point having a second speed in response to the second flux, the second speed being greater than the first speed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Divisional Application of U.S. application Ser. No. 13/481,024 filed May 25, 2012 which claims priority to U.S. application Ser. No. 13/466,525 filed May 8, 2012, the disclosure of which is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Exemplary embodiments pertain to the art of electric machines and, more particularly to a variable flux electric machine having dual fields. 
         [0003]    Electric machines are employed in a wide range of applications. For example, vehicles that employ internal combustion engines generally include an electric machine in the form of a starter motor. The starter motor is selectively activated to initiate operation of the internal combustion engine. The electric starter motor includes an armature that rotates in response to a magnetic motive force established between armature windings and a stationary field. The armature is coupled to a pinion gear that is configured to engage with a ring gear on the internal combustion engine. A solenoid drives the pinion gear into the ring gear to start the internal combustion engine. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    Disclosed is a method of operating a variable flux electric machine. The method includes passing an electrical current through a plurality of wound poles of a wound field to generate a first flux, rotating an armature at a first crank point having a first speed in response to the first flux, shorting at least one of the plurality of wound poles, generating a second flux through a permanent magnet (PM) field having a plurality of PM poles, and rotating the armature at a second crank point having a second speed in response to the second flux, the second speed being greater than the first speed. 
         [0005]    Also disclosed is a method of operating a variable flux electric machine. The method includes rotating an armature at a first speed in response to a first flux provided by a plurality of wound poles, and rotating the armature at a second speed in response to a second flux provided by a permanent magnet (PM) field, the second speed being greater than the first speed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The following descriptions should not be considered limiting in any way. With reference to the accompanying drawings, like elements are numbered alike: 
           [0007]      FIG. 1  depicts a partial cross-sectional side view of a variable flux electric starter motor in accordance with an exemplary embodiment; 
           [0008]      FIG. 2  depicts a partial cross-sectional end view of the variable flux electric starter motor of  FIG. 1 ; 
           [0009]      FIG. 3  depicts a Torque-Speed (T-S) Graph illustrating T-S curves for a wound field, a permanent magnet (PM) field, and a shunted PM field; 
           [0010]      FIG. 4  depicts a schematic diagram illustrating an electrical connection of first and second wound poles of the variable flux electric starter motor of  FIG. 1 ; and 
           [0011]      FIG. 5  depicts a block diagram illustrating electrical connections of first and second wound poles of the variable flux electric starter motor of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0012]    A detailed description of one or more embodiments of the disclosed apparatus and method are presented herein by way of exemplification and not limitation with reference to the Figures. 
         [0013]    A variable flux electric starter motor in accordance with an exemplary embodiment is indicated generally at  2  in  FIG. 1 . Starter motor  2  includes a frame  4  having an outer wall  6 . Outer wall  6  includes a first end  8  that extends to a second end  9 . Outer wall  6  defines an interior portion  10 . In the exemplary aspect shown, starter motor  2  includes a pinion housing  12  arranged at first end  8 . Pinion housing  12  surrounds, in part, a pinion gear  14  rotatably mounted to a pinion gear shaft  16 . An end plate  18  is mounted at second end  9 . End plate  18  includes a recessed portion  19 . Starter motor  2  is also shown to include a field assembly  24  mounted to an inner surface (not separately labeled) of outer wall  6  and a rotor or armature assembly  30 . 
         [0014]    Armature assembly  30  includes an armature core  31  supported upon an armature shaft  32 . Armature core  31  is spaced from field assembly  24  by an air gap (not separately labeled). Armature shaft  32  includes a first end portion  34  that extends to a second end portion  36 . First end portion  34  is supported by a bearing  37  provided within a recess (not separately labeled) of pinion gear shaft  16  while second end portion  36  is supported by a bearing  38  arranged within recessed portion  19 . First end portion  34  of armature shaft  32  is operably coupled to pinion gear  14  through a gear assembly  40 . Armature assembly  30  is also shown to include a commutator  44  that is coupled to a brush assembly  46 , thus starter motor  2  is a brushed direct current (DC) starter. Brush assembly  46  delivers electrical current to armature windings  47  through commutator  44 . The electrical current flowing through armature windings  47  interact with field assembly  24  to set up a magnetic motive force (MMF). The MMF sets up a flux within the air gap between armature core  31  and field assembly  24 . The flux interacts with current flowing through armature core  31  causing armature assembly  30  to rotate within frame  4 . The rotation of armature assembly  30  is translated to pinion gear  14  through gear assembly  40 . A solenoid  48  shifts pinion gear  14  along pinion gear shaft  16  into engagement with a ring gear (not shown) that is typically provided on a fly wheel (also not shown) of a motor  2 . 
         [0015]    In accordance with an exemplary embodiment, field assembly  24  includes a first or wound field  70  and a second or permanent magnet (PM) field  74  as shown in  FIG. 2 . In this manner, starter motor  2  includes a selectively activated mixed field having properties derived from wound field  70  or from PM field  74 . Wound field  70  includes a first wound pole  76  and a second wound pole  77 . First wound pole  76  includes a first pole shoe  79  mounted to an inner surface (not separately labeled) of outer wall  6 . Similarly, second wound pole  77  includes a second pole shoe  80  mounted to the inner surface (also not separately labeled) of outer wall  6  substantially directly opposite to first pole shoe  79 . A first plurality of windings  83  is provided at first pole shoe  79  and a second plurality of windings  84  is provided at second pole shoe  80 . 
         [0016]    As shown in  FIG. 4 , first plurality of windings  83  is electrically connected in parallel with second plurality of windings  84 . As will be discussed more fully below, first and second wound poles  76  and  77  are configured to produce a first flux when starter motor  2  is operated. PM field  74  includes first and second permanent magnets  88  and  89  mounted to the inner surface (not separately labeled) of outer wall  6 . First permanent magnet  88  is positioned generally opposite to second permanent magnet  89 . First permanent magnet  88  defines a first PM pole  91  and second permanent magnet defines a second PM pole  92 . First and second PM poles  91  and  92  are configured to establish a second flux when starter motor  2  is operated. A first shunt  94  is positioned adjacent to first PM pole  91  and a second shunt  95  is positioned adjacent second PM pole  92 . First and second shunts  94  and  95  condition the second flux established by PM field  74 . 
         [0017]    Wound field  70  produces a generally curvilinear Torque-Speed (T-S) curve such as shown at  97  in  FIG. 3 . PM field  74  is known to produce a generally linear curve. T-S curve  97  includes a sweeping tail portion  98  that extends beyond a design speed threshold  99  for starter motor  2 . In accordance with an exemplary embodiment, wound field  70  cooperates with PM field  74  to eliminate sweeping tail portion  98  and produce a more linearized T-S curve  96 . In this manner, PM field  74  more closely matches an upper portion of T-S curve  97  produced by wound field  70 . As will be discussed more fully below, PM field  74  is selectively enabled to overcome wound field  70  to allow pinion gear  14  to rotate at a higher crank point than would be produced if powered by wound field  70 . With this arrangement, PM field  74  produces a T-S curve such as shown at  100 . 
         [0018]    In further accordance with an exemplary embodiment, a relay  105  is coupled across first and second windings  83  and  84 . A controller  110  is coupled to, and selectively activates, relay  105  to operate starter motor  2  at higher crank points. Controller  110  generally takes the form of an electronic control unit (ECU) provided in a motor vehicle. However, it should be understood, that controller  110  can take on a variety of forms. At this point it should be understood that relay  105  may be mounted remotely from starter motor  2  or, alternatively may be arranged within or mounted to frame  4  or integrated into solenoid  48 . The particular starter motor described herein is configured to be employed in connection with start/stop operations. More specifically, in addition to traditional use of starting a cold motor, starter motor  2  may be employed to start a warm motor such as following motor shut down at a traffic light, while an electric motor is in use, and the like. 
         [0019]    During cold starts, higher pinion torque and lower pinion speeds are desirable. The higher pinion torque is generally more adept at turning over a cold motor. Accordingly, during cold start situations relay  105  is open thereby enabling electrical current to flow through windings  83  and  84  to produce the first flux (not separately labeled) that establishes a first crank point  110 . In this manner, wound field  70  may be designed to produce a cold crank target that possesses relatively high torque at relatively lower speeds. It should be understood that PM field  74  also contributes to the first flux but is dominated by wound field  70  during cold start situations. 
         [0020]    During warm starts, when it is desirable to start the motor in a short time period, controller  110  activates relay  105  to cause windings  83  and  84  to be shorted. At this point it should be understood that while being shorted, some current will continue to flow. The amount of current flow is determined by resistance of relay  105  and resistance of windings  83  and  84 . In this manner, PM field  74  dominates wound field  70  to produce a second flux that achieves a warm crank target that has a second crank point  120  having lower torque and higher speeds than the cold crank target ( FIG. 3 ). The particular cold crank target and warm crank target can vary depending on the particular vehicle, operating conditions, environmental conditions and the like. The PM field  74 , coupled with shunts  94  and  95  provides the desired higher pinion speeds that lead to quicker motor starting without exceeding a maximum pinion speed of the starter motor  2 . 
         [0021]    The exemplary embodiments provide a single starter motor that produces variable flux achieved through a selective application of mixed fields. That is, the starter motor possesses both the operational characteristics of a wound field and a PM field. The particular field active at any one time depends on the desired starting conditions as determined by, for example controller. It should also be understood that while shown and described as a four pole configuration, the number of poles in the starter motor may vary. For example, the exemplary embodiments may be incorporated into a starter motor having as few as two poles or as many as eight or more poles. 
         [0022]    While the invention has been described with reference to an exemplary embodiment or embodiments, 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 claims.