Abstract:
A motor comprising, a rotor having a shaped pole operative to minimize cogging torque and electro motive Force (EMF) harmonics, a stator having teeth members, the teeth members including, end portions, wherein the end portions partially define slot openings having a first dimension, and a notch opening defined by the end portions having a second dimension.

Description:
BACKGROUND 
       [0001]    Brushless motors often fail to rotate smoothly. Motor structures and phase commutations result in periodic disturbances in a motor, also called torque ripples. The torque ripples may degrade the performance of the motor because of vibrations and noise. Torque ripples also affect a speed of the motor. 
         [0002]    Torque ripples may be caused, in part, by cogging torque. Cogging torque is a result of the interaction between slots in a stator and permanent magnets on a rotor. 
         [0003]    An interaction between the motor current and the back-EMF of the motor current can also cause a torque ripple. Torque ripples may be measured as a harmonic resonance of varying orders. A motor system that minimizes torque ripples is desired. 
       SUMMARY 
       [0004]    The above described and other features are exemplified by the following Figures and Description which includes an exemplary embodiment of a motor comprising, a rotor having a shaped pole operative to minimize cogging torque and electro motive Force (EMF) harmonics, a stator having teeth members, the teeth members including, end portions, wherein the end portions partially define slot openings having a first dimension, and a notch opening defined by the end portions having a second dimension. 
         [0005]    An alternate exemplary embodiment including a motor comprising, a rotor having a shaped pole partially defined by a middle portion dimension and an end portion dimension, a stator having teeth members, the teeth members including, end portions, wherein the end portions partially define slot openings having a first dimension, and a notch opening defined by the end portions having a second dimension. 
         [0006]    Another alternate exemplary embodiment including a motor comprising, a rotor having a shaped pole partially defined by a middle portion dimension and an end portion dimension, and a stator having teeth members, the teeth members including end portions, wherein the end portions partially define slot openings having a first dimension, wherein the end portion dimension is 0.3-0.9 times the middle portion dimension, a normalized slot depth dimension is 0.01-0.06 from a end surface of the teeth members to a beveled portion of the teeth members, and a slot angle is 30 to 50 degrees defined by the beveled portion and a slot surface of the teeth members. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Referring now to the Figures wherein like elements are numbered alike: 
           [0008]      FIG. 1  illustrates a side partially cut-away view of an exemplary embodiment of a motor. 
           [0009]      FIG. 2  illustrates a side view of a shaped pole of an exemplary embodiment of the motor of  FIG. 1 . 
           [0010]      FIG. 3  further illustrates a side view of a shaped pole of an exemplary embodiment of the motor of  FIG. 1 . 
           [0011]      FIG. 4  illustrates graphs representing cogging torque and cogging harmonics for a motor having shaped poles and a motor having unshaped poles. 
           [0012]      FIG. 5  illustrates a side view of a portion of an exemplary embodiment of a stator of the motor of  FIG. 1 . 
           [0013]      FIG. 6  illustrates graphs representing cogging torque and cogging harmonics for a motor having unshaped poles and a stator with one notch. 
           [0014]      FIG. 7  illustrates a side view of a portion of an alternate exemplary embodiment of a stator of the motor of  FIG. 1 . 
           [0015]      FIG. 8  illustrates graphs representing cogging torque and cogging harmonics for a motor having unshaped poles and a stator with two notches. 
           [0016]      FIG. 9  illustrates a graph comparing the cogging torque for an exemplary motor having a combination shaped poles and a stator with one notch and two notches. 
           [0017]      FIG. 10  illustrates a graph comparing the cogging harmonics for an exemplary motor having shaped poles and a stator with one notch having a variety of dimensions in millimeters. 
           [0018]      FIG. 11  illustrates a graph comparing the cogging harmonics for an exemplary motor having shaped poles and a stator with two notches having a variety of dimensions. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Torque ripple in brushless motors may be caused in part, by cogging torque and back-electromotive Force (EMF) harmonics. Shaping the magnetic poles of a rotor to optimize the amount of cogging torque and back-EMF harmonics reduces torque ripple. Stators also include teeth that are separated with slots. The teeth may include notches in end portions of the teeth. Incorporating notches having widths different than widths of the slots and optimizing the design to reduce cogging torque and back-EMF harmonics may reduce the torque ripple in a motor. By combining shaped poles and notches having widths different than widths of the slots the overall torque ripple in a motor is reduced. 
         [0020]      FIG. 1  illustrates a side partially cut-away view of an exemplary embodiment of a motor  100  having a rotor  101  with shaped poles  104 . The motor  100  also includes a stator  104  having teeth members  105  and stator windings  107 . The spaces between the teeth members are slots  109 . 
         [0021]    The motor  100  has six poles  103  and nine slots  109 . The poles  103  have been optimized using a Finite Element (FE) Analysis software to reduce the cogging torque and back-EMF harmonics in the motor.  FIGS. 2 and 3  further illustrate the shape of the poles  103 . 
         [0022]      FIG. 2  illustrates a side view of an exemplary embodiment of the pole  103 . The pole  103  has a thickness (Lm) at a middle portion of the pole  103 , and a thickness (mLm) at end portions of the pole  103 .  FIG. 3  further illustrates the dimensions of the pole  103 . 
         [0023]    Referring to  FIG. 3 , the pole  103  has an inner arc  301 , an outer arc  303 , and end portions  305 . The inner arc  301  has a center point C 1  at [0,0] and a radius (R 1 ). The outer arc  303  has a center point C 2  (Y,0) and a radius (R 2 ). The end portions  305  are defined by points A′ and A. A radius C 1 A′=R 1 . The distance AA′=mLm where (m) is the ratio of the magnet thickness at the two ends of the magnet to the thickness at the center of the magnet (Lm) in  FIG. 3 . An angle C 1 A′,C 1 B=Polearcangle/2 and an angle C 1 A,C 1 B=theta/2 and is less than or equal to Polarcangle/2. The points are defined as follows:
   A: [(R 1 ) cos(polearcangle/2)+m*Lm, ((R 1 ) sin(Polearcangle/2)]; 0&lt;m&lt;1.0   B: [R 1 +Lm, 0]   C: mirror image of A   A′: [(R 1 ) cos(polarcangle/2), ((R 1 ) sin(polarcangle/2)]   C 1 : [0, 0]   C 2 : [Y, 0];   where Y is distance C 1 C 2 .   
 
         [0031]      FIGS. 2 and 3  illustrate one exemplary embodiment of a pole  103  optimized to reduce torque ripples. Other motor configurations having different numbers of poles and gaps may use different shaped poles to reduce torque ripples. The shapes for optimized poles in other motor configurations may be found by experimentation and simulations. 
         [0032]      FIG. 4  includes graphs comparing the cogging torque and cogging harmonics for an exemplary motor having unshaped rotor magnets and shaped poles. The graph  4   a  shows the improved cogging torque of a rotor having pole shaped magnets verses a rotor having unshaped magnets. Graph  4   b  illustrates the improved 18th, 36th, and 54th order harmonics of the rotor having pole shaped magnets. The improvement in 18th order harmonics is 89%, and the improvement in 36th order harmonics is 66.7%. 
         [0033]    Though pole shaping may reduce torque ripples, the use of notches in stator teeth further reduces the effects of cogging torque and back-EMF harmonics. 
         [0034]      FIG. 5  illustrates a side view of a portion of a stator  400 . The stator  400  includes a first stator tooth  401  having an end portion  403 . A second stator tooth  402  is adjacent to the first stator tooth  401 . The closest distance between the stator teeth  401  and  402  is a gap  407 . The gap  407  allows stator windings (not shown) to be wound around the stator teeth  401  and  402 . The gap has a dimension “d” defined by the stator teeth as shown. The end portion  403  of the stator tooth  402  defines a notch  405 . The notch  405  is circular shaped and has a dimension “a” as shown. A slot depth (Sd) dimension is also shown. The Sd dimension is defined by a distance from an end surface  411  of the end portion  403  to a beveled portion  409  of the end portion  403 . A slot angle (Sa) is defined by the angle between the beveled portion  409  of the end portion  403  to a line  415  that is perpendicular to a slot surface  413  of the stator teeth. 
         [0035]    The stator  400  includes a number of other stator teeth similar to the stator teeth  401  and  402 . Each of the stator teeth in the stator  400  has a notch  405  and a gap  407 . The dimension “d” may be increased or decreased, though it has a minimum dimension that is limited by the size of the stator windings. The dimension “a” of the notch may also be increased or decreased. The dimensions “a” and “d” may be optimized through experimentation and simulation to determine dimensions that reduce the cogging torque and the back-EMF harmonics of the motor. 
         [0036]    The following table describes exemplary embodiments of motors having improved cogging harmonics. The table below includes three types of motors, similar to the motor illustrated in  FIG. 1  having a stator similar to  FIG. 4 , and a pole similar to the pole  103  of  FIGS. 2 and 3 . The motors include a 27 slot/6 pole motor, a 12 slot/10 pole motor, and a 9 slot/6 pole motor. 
         [0000]                                                                                              Motor Type                        9 slot/   9 slot/           27 slot/6 pole   12 slot/10 pole   6 pole   6 pole                        Number of   0   1   1   2       Notches       m   0.3-0.9   0.5-0.9   0.4-0.6   0.4-0.6       a to b ratio       0.9-1.1   0.3-1.7   0.3-1.7            Normalized   .01-.06       Slot Depth       (Sd/(Rl + Lm)       Slot Angle (Sa)   30-50 deg.                    
The number of notches indicates the number of notches in each type of motor similar to the notches  405  of  FIG. 4 . The m values represent a range of m values for the pole (as shown in  FIG. 2 ). The a to b ratios represent a range of ratios for the a dimension to the b dimension (shown in  FIG. 4 ). The slot depth (Sd) (shown in  FIG. 4 ) represents a range of dimensions of the slot depth as a ratio of the (R 1 +Lm) dimension. The slot angle (Sa) represents a range of angles for the Sa angle (shown in  FIG. 4 ). For example, the 12 slot/10 pole motor has improved cogging harmonics with: one notch in the stator teeth of the motor stator; the ratio of the notch opening  405  to the slot opening  407  dimensions—the a to b ratio, is between 0.9 and 1.1; and the m value (0.5-0.9) multiplied by the Lm dimension results in the mLm dimension of the end portions  305  of the pole  103 . For all motors included are a range of Slot depth (Sd) dimensions and (Sa) dimensions (shown in  FIG. 5 ).
 
         [0037]      FIG. 6  includes graphs comparing the cogging torque and cogging harmonics for an exemplary motor having unshaped poles and a stator with one notch. Graph  6   a  illustrates the improved cogging torque of a stator with one optimized notch versus a stator having one regular notch. Graph  6   b  illustrates the improved cogging harmonics resultant from the optimized notch. The 18 th  order harmonics are improved 35.48% while the 36 th  order harmonics are improved 41.05%. 
         [0038]      FIG. 7  illustrates an alternate embodiment of a stator. Stator  500  includes a stator tooth  501  having an end portion  503 . The end portion  503  is similar to the end portion  405  (of  FIG. 5 ) and includes two notches  505  each having a dimension “c”. The notches  505  in the illustrated embodiment are U-shaped. A gap  507  having a dimension “d” is similar to the gap  407  of  FIG. 5 . The dimension “c” may be increased or decreased and optimized with the dimension “d” of the gap  507  to reduce the cogging torque and the back EMF-harmonics of the motor. 
         [0039]      FIG. 8  includes graphs comparing the cogging torque and cogging harmonics for an exemplary motor having unshaped poles and a stator with two notches. Graph  8   a  shows improved cogging torque of a stator having two notches versus a stator having no notches. Graph  6   b  illustrates the improved cogging harmonics of the motor having the optimized two notches. The 18 th  order harmonics are improved 57.49% and the 36 th  order harmonics are improved 61.39%. 
         [0040]    Simulations have shown that embodiments optimized with stator teeth having one notch generally reduce torque ripples, back-EMF harmonics, and cogging torque when the notch dimension “a” is greater than the slot dimension “b,” while embodiments with stator teeth having two notches are optimized when the notch dimension “c” is less than the slot dimension “d.” However, some motors having different numbers of poles and slots may not have the same properties resulting in different relationships between the notch dimensions and slot dimensions for optimization. 
         [0041]    The benefits of shaped poles and notches having different dimensions than slot dimensions are enhanced when combined in a motor using both shaped poles and optimized dimensions of the slots and notches. Simulated results of a motor having 9 slots and six poles show a further reduction in cogging torque and back-EMF harmonics when the two optimized configurations are combined. 
         [0042]      FIG. 9  includes a graph comparing the cogging torque for an exemplary motor having a combination shaped poles and a stator with one notch and two notches. In the illustrated graph, the cogging torque is considerably less with the combination of shaped poles and notches than a motor having unshaped poles and no notches. 
         [0043]      FIG. 10  includes a graph comparing the cogging harmonics for an exemplary motor having shaped poles and a stator with one notch having a variety of dimensions in millimeters. The harmonics of the motors having shaped poles and a stator with one notch where the notch is between 4.0 mm and 3.0 mm are particularly improved. 
         [0044]      FIG. 11  includes a graph comparing the cogging harmonics for an exemplary motor having shaped poles and a stator with two notches having a variety of dimensions. The harmonics and cogging torques of the motors having shaped poles and a stator with two notches where the notches are optimized are improved. 
         [0045]    While the invention has been described with reference to exemplary embodiments, it will be understood by those of ordinary skill in the pertinent art that various changes may be made and equivalents may be substituted for the elements thereof without departing from the scope of the present disclosure. In addition, numerous modifications may be made to adapt the teachings of the disclosure to a particular object or situation without departing from the essential scope thereof. Therefore, it is intended that the Claims not be limited to the particular embodiments disclosed as the currently preferred best modes contemplated for carrying out the teachings herein, but that the Claims shall cover all embodiments falling within the true scope and spirit of the disclosure.