Patent Publication Number: US-2023155430-A1

Title: Electric motor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to co-pending U.S. Provisional Patent Application No. 62/870,125 filed on Jul. 3, 2019, and co-pending U.S. Provisional Patent Application No. 62/857,337 filed on Jun. 5, 2019, the entire contents of both of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electric motors, and more particularly to stators for electric motors. 
     BACKGROUND OF THE INVENTION 
     A stator includes a plurality of teeth that each respectively retain a stator coil. A variety of methods can be used to wind the stator coils around the teeth. 
     SUMMARY OF THE INVENTION 
     The invention provides, in one aspect, stator comprising a back stator portion, a first tooth extending radially inward from the back portion, a second tooth extending radially inward from the back portion, a first flange extending away from the first tooth, a second flange extending away from the second tooth and toward the first flange, and insulation. The insulation includes a back insulation portion covering the back stator portion, a first tooth portion covering the first tooth, a second tooth portion covering the second tooth, and a first flange portion covering the first flange. The first flange portion has a first face that is in facing relationship with the back insulation portion. The insulation further includes a second flange portion covering the second flange. The second flange portion has a second face that is in facing relationship with the back insulation portion. The first face and the second face together substantially define a boundary plane, such that a cross-sectional slot area is defined between the back insulation portion, the first tooth portion, the second tooth portion, and the boundary plane. The stator further comprises a plurality of conductive wires arranged between the first and second teeth in the cross-sectional slot area, the plurality of conductive wires defining a cross-sectional winding area within the cross-sectional slot area. A ratio of the cross-sectional winding area to the cross-sectional slot area is greater than or equal to 0.45. 
     The invention provides, in another aspect, a power tool comprising an output member, a brushless, direct current electric motor having a rotor configured to provide torque to the output member and a stator. The stator comprises a back stator portion, a first tooth extending radially inward from the back portion, a second tooth extending radially inward from the back portion, a first flange extending away from the first tooth, a second flange extending away from the second tooth and toward the first flange, and insulation. The insulation includes a back insulation portion covering the back stator portion, a first tooth portion covering the first tooth, a second tooth portion covering the second tooth, and a first flange portion covering the first flange. The first flange portion has a first face that is in facing relationship with the back insulation portion. The insulation further includes a second flange portion covering the second flange. The second flange portion has a second face that is in facing relationship with the back insulation portion. The first face and the second face together substantially define a boundary plane, such that a cross-sectional slot area is defined between the back insulation portion, the first tooth portion, the second tooth portion, and the boundary plane. The stator further comprises a plurality of conductive wires arranged between the first and second teeth in the cross-sectional slot area, the plurality of conductive wires defining a cross-sectional winding area within the cross-sectional slot area. The power tool further comprises a battery configured to provide power to the motor and a motor drive circuit configured to control operation of the motor. A ratio of the cross-sectional winding area to the cross-sectional slot area is greater than or equal to 0.45. 
     The invention provides, in yet another aspect, a method of forming a stator. The method comprises forming a first stator segment with first and second teeth, applying a first layer of insulation around the first tooth, applying a second layer of insulation around the second tooth, winding a first stator coil around the first layer of insulation, winding a second stator coil around the second layer of insulation, electrically connecting the first and second stator coils, forming a second stator segment with third and fourth teeth, applying a third layer of insulation around the third tooth, applying a fourth layer of insulation around the fourth tooth, winding a third stator coil around the third layer of insulation, winding a fourth stator coil around the fourth layer of insulation, electrically connecting the third and fourth stator coils, forming a third stator segment with fifth and sixth teeth, applying a fifth layer of insulation around the fifth tooth, applying a sixth layer of insulation around the sixth tooth, winding a fifth stator coil around the fifth layer of insulation, winding a sixth stator coil around the sixth layer of insulation, electrically connecting the fifth and sixth stator coils, axially coupling the first stator segment to the second stator segment, and axially coupling the third stator segment to the first stator segment. 
     The invention provides, in yet another aspect, a method of forming a stator. The method comprises forming a first stator segment with first, second, and third teeth, applying a first layer of insulation around the first tooth, applying a second layer of insulation around the second tooth, applying a third layer of insulation around the third tooth, winding a first stator coil around the first layer of insulation, winding a second stator coil around the second layer of insulation, winding a third stator coil around the third layer of insulation, forming a second stator segment with fourth, fifth, and sixth teeth, applying a fourth layer of insulation around the fourth tooth, applying a fifth layer of insulation around the fifth tooth, applying a sixth layer of insulation around the sixth tooth, winding a fourth stator coil around the fourth layer of insulation, winding a fifth stator coil around the fifth layer of insulation, winding a sixth stator coil around the sixth layer of insulation, and axially coupling the first stator segment to the second stator segment. 
     The invention provides, in yet another aspect, a stator comprising a back portion and a tooth having a spoke portion extending radially inward from the back portion and a flange extending transverse to the spoke portion, such that a slot is defined between the flange and the back portion. The stator further comprises a stator coil wound around the spoke portion of the tooth and within the slot in progressive turns including a first turn, a plurality of intermediate turns, and a final turn. The final turn of the stator coil has a greater cross sectional length than a first turn of the stator coil. 
     The invention provides, in yet another aspect, a method of applying a stator coil around a stator tooth. The method comprises using a 3D printer to print a plurality of alternating layers of insulation and electrically conductive metal around the tooth, coupling a first electrical connector to a first of the layers of electrically conductive metal, and coupling a second electrical connector to a final of the layers of electrically conductive metal. 
     The invention provides, in yet another aspect, a method of applying a stator coil around a stator tooth. The method comprises applying a full turn of insulation around the tooth, applying a first turn of electrically conductive metal around the full turn of insulation, and applying a second half turn of electrically conductive metal around the full turn of insulation, such that the first and second half turns of electrically conductive metal together form a full turn of electrically conductive metal around the full turn of insulation. The method further comprises electrically connecting the first half turn of electrically conductive metal to the second half turn of electrically conductive metal. 
     Other features and aspects of the invention will become apparent by consideration of the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is an exploded, schematic view of an electric motor. 
         FIG.  2    is a perspective view of an embodiment of a stator of the electric motor of  FIG.  1   . 
         FIG.  3    is a perspective view of a first stator segment of the stator of  FIG.  2   . 
         FIG.  4    is a perspective view of a second stator segment of the stator of  FIG.  2   . 
         FIG.  5    is a perspective view of a third stator segment of the stator of  FIG.  2   . 
         FIG.  6    is a perspective view of the first stator segment of  FIG.  3    with insulation added. 
         FIG.  7    is a perspective view of the third stator segment of  FIG.  5    with insulation added. 
         FIG.  8    is a perspective view of the second stator segment of  FIG.  4    with insulation added. 
         FIG.  9    is a perspective view of the first stator segment of  FIG.  3    with insulation and first and second stator coils added. 
         FIG.  10    is a perspective view of the second stator segment of  FIG.  4    with insulation and third and fourth stator coils added. 
         FIG.  11    is a perspective view of the third stator segment of  FIG.  5    with insulation and fifth and sixth stator coils added. 
         FIG.  12    is a plan view of the first, segment, and third stator segments of  FIGS.  9 - 11    axially coupled together. 
         FIG.  13    is a perspective view of the stator of  FIG.  2   . 
         FIG.  14    is a plan view of a die stamping cross section. 
         FIG.  15    is a perspective view of a partial stator segment resulting from a die stamping process using the die stamping cross section of  FIG.  14   . 
         FIG.  16    is a perspective view of a pair of tooth portions resulting from a die stamping process using the die stamping cross section of  FIG.  14   . 
         FIG.  17    is a perspective view of another embodiment of a stator of the electric motor of  FIG.  1   . 
         FIG.  18    is a perspective view of a first stator segment of the stator of  FIG.  17   . 
         FIG.  19    is a perspective view of a second stator segment of the stator of  FIG.  17   . 
         FIG.  20    is a perspective view of the first stator segment of  FIG.  18    with insulation and first, second, and third stator coils added. 
         FIG.  21    is a perspective view of the second stator segment of  FIG.  19    with insulation and fourth, fifth, and sixth coils added. 
         FIG.  22    is an enlarged plan view of a stator filled with a round wire. 
         FIG.  23    is an enlarged plan view of a stator filled with a flat wire. 
         FIG.  24    is an enlarged plan view of a stator filled with a variable cross-section conductor. 
         FIG.  25    is a schematic view of one method of applying a stator coil to a stator tooth. 
         FIG.  26    is a schematic view of another method of applying a stator coil to a stator tooth. 
         FIG.  27    is a plan view of a power tool including the electric motor of  FIG.  1   . 
         FIG.  28    is a block diagram of the power tool of  FIG.  27   . 
         FIG.  29    is a block diagram of a motor drive circuit of the power tool of  FIG.  27   . 
         FIG.  30    is a block diagram of the motor drive circuit of  FIG.  29    during braking of a motor of the power tool of  FIG.  27   . 
         FIG.  31    is a perspective view of the first stator segment of  FIG.  3    with insulation, first and second stator coils, and a motor braking coil added. 
         FIG.  33    is a perspective view of a motor incorporated in the power tool of  FIG.  27    according to another embodiment of the invention. 
         FIG.  34    is a perspective view of a rotor of the motor of  FIG.  33   . 
         FIG.  35    is a perspective view of a rotor end cap of the rotor of  FIG.  34   . 
         FIG.  36    is a rear perspective view of the rotor of  FIG.  34   . 
         FIG.  37    is a perspective view of a position sensor board assembly of the motor of  FIG.  33   . 
         FIG.  38    is a cross section of the motor of  FIG.  33   . 
         FIG.  39    is an enlarged cross-sectional view of a stator of the motor of  FIG.  33   , with the stator coils removed. 
         FIG.  40    is an enlarged cross-sectional view of a stator of the motor of  FIG.  33   , with the stator coils included. 
         FIG.  41    is a bar graph showing a relationship between amperage used by the motor of  FIG.  33   , slot-fill ratio of the motor, and time for the motor to reach a critical temperature. 
         FIG.  42    is a bar graph showing a relationship between amperage used by the motor of  FIG.  33   , slot-fill ratio of the motor, and airflow through the power tool of  FIG.  27   . 
         FIG.  43    is a bar graph showing a relationship between amperage used by the motor of  FIG.  33   , slot-fill ratio of the motor, and power output of the power tool of  FIG.  27   . 
         FIG.  44    is a bar graph showing a relationship between amperage used by the motor of  FIG.  33   , slot-fill ratio of the motor, and power output of the power tool of  FIG.  27    to airflow through the power tool. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
     DETAILED DESCRIPTION 
       FIG.  1    schematically illustrates a brushless direct current (DC) motor  10  for, e.g., a power tool. The electric motor  10  includes a stator  14  and a rotor  18 . In operation, current is passed through windings in the stator  14  to produce a magnetic field that causes the rotor  18  to rotate, as is well known in the art. As described in more detail below, this invention includes a plurality different stators and ways of winding the stator  14 . 
       FIG.  2    illustrates an embodiment of the stator including a three-part stator  14   a . The stator  14   a  is a three-phase stator. The stator  14   a  includes a first stator segment  22  ( FIG.  3   ), a second stator segment  26  ( FIG.  4   ), and a third stator segment  30  ( FIG.  5   ). The first stator segment  22  ( FIG.  3   ) includes a first annular back portion  34  and a first tooth  38  and a second tooth  42  that each extend radially inward from the first back portion  34  and axially away from the first back portion  34  in a first direction. The first and second teeth  38 ,  42  each have a first height H 1 . The second stator segment  26  ( FIG.  4   ) includes a second annular back portion  46  and a third tooth  50  and a fourth tooth  54  that each extend radially inward from the second back portion  46  and axially away from the second back portion  46  in the first direction and a second direction that is opposite the first direction. Like the first and second teeth  38 ,  42 , the third and fourth teeth  50 ,  54  also each have the first height H 1 . 
     The third stator segment  30  ( FIG.  5   ) includes a third annular back portion  58  and a fifth tooth  62  and a sixth tooth  66  that each extend radially inward from the third back portion  58  and axially away from the third back portion  58  in the second direction. Like the first, second, third, and fourth teeth  38 ,  42 ,  50 ,  54  the fifth and sixth teeth  62 ,  66  also each have the first height H 1 . In some embodiments, each of the back portions  34 ,  46 ,  58  can be produced in the same progressive stamping die process. In some embodiments, the teeth  38 ,  42 ,  50 ,  54 ,  62 ,  66  are fixed to the respective back portions  34 ,  46 ,  58  with staking during the progressive stamping die process. 
     Each of the teeth  38 ,  42 ,  50 ,  54 ,  62 ,  66  respectively have outer flanges  38   a ,  42   a ,  50   a ,  54   a ,  62   a ,  66   a , inner flanges  38   b ,  42   b ,  50   b ,  54   b ,  62   b ,  66   b , and spoke portions  38   c ,  42   c ,  50   c ,  54   c ,  62   c ,  66   c  coupling the inner flanges  38   b ,  42   b ,  50   b ,  54   b ,  62   b ,  66   b  to the outer flanges  38   a ,  42   a ,  50   a ,  54   a ,  62   a ,  66   a , such that slots  38   d ,  42   d ,  50   d ,  54   d ,  62   d ,  66   d  are respectively defined between the outer flanges  38   a ,  42   a ,  50   a ,  54   a ,  62   a ,  66   a  and inner flanges  38   b ,  42   b ,  50   b ,  54   b ,  62   b ,  66   b  on both sides of the spokes  38   c ,  42   c ,  50   c ,  54   c ,  62   c ,  66   c.    
     After each of the first, second, and third stator segments  22 ,  26 ,  30  have been separately formed, the stator  14   a  can be assembled in the following manner. As shown in  FIG.  6   , first and second layers of insulation  38   e ,  42   e  are respectively applied about the first and second teeth  38 ,  42  of the first stator segment  22 . Specifically, the first and second layers of insulation  38   e ,  42   e  respectively cover portions of the outer flanges  38   a ,  42   a , the inner flanges  38   b ,  42   b  and the spoke portions  38   c ,  42   c  of the first and second teeth  38 ,  42 . Also, the first and second layers of insulation  38   e ,  42   e  respectively include first and second outer bookends  38   f ,  42   f  that extend axially from the top and bottom portions of the outer flanges  38   a ,  42   a . The first and second layers of insulation  38   e ,  42   e  also respectively include first and second inner bookends  38   g ,  42   g  that extend axially from the top and bottom portions of the inner flanges  38   b ,  42   b . In addition, a first insulation end cap  70  is applied to a first axial end  74  of the first back portion  34 . In some embodiments, the first insulation end cap  70  is formed with the first and second layers of insulation  38   e ,  42   e , e.g. via insert molding. 
     As show in  FIG.  7   , in a manner similar to the first and second layers of insulation  38   e ,  42   e  and the first insulation end cap  70  being applied to the first stator segment  22 , fifth and sixth layers of insulation  62   e ,  66   e  are respectively applied about the fifth and sixth teeth  62 ,  66  of the third stator segment  30 , and a second insulation end cap  78  is applied to a second axial end  80  of the third back portion  58 . In some embodiments, the second insulation end cap  78  is formed with the fifth and sixth layers of insulation  62   e ,  66   e . As shown in  FIG.  8   , in a manner similar to the first and second layers of insulation  38   d ,  42   d  being applied to the first and second teeth  38 ,  42  of the first stator segment  22 , third and fourth layers of insulation  50   e ,  54   e  are respectively applied about the third and fourth teeth  50 ,  54  of the second stator segment  26 , but no end cap is formed on the second stator segment  26 . 
     Next, as shown in  FIG.  9   , a first stator coil  38   h  is wound around the insulated first tooth  38  or more specifically, the insulated spoke portion  38   c  of the first tooth  38 . In some embodiments, the first stator coil  38   h  is a winding of electrically conductive metal, such as a copper winding. Because the only other tooth on the first stator segment  22  is the second tooth  42 , which is 180 degrees away from the first tooth  38 , the first stator coil  38   h  is advantageously able to fill or substantially fill the slots  38   d  on the first tooth  38 . In other words, because there are no teeth immediately adjacent the first tooth  38  of the first stator segment  22 , during the winding process of the first stator coil  38   h , there is no other structure inhibiting the first stator coil  38   h  from being wound about the first tooth  38  until the first stator coil  38   h  is flush with or even wound past the outer flange  38   a  in a circumferential direction. The first outer bookend  38   f  and the first inner bookend  38   g  radially secures the portions of the first stator coil  38   h  that respectively extend above and below the first tooth  38 , such that the first stator coil  38   h  can be wound to a second total height H 2  that is longer than the first height H 1 . 
     Likewise, as shown in  FIG.  9   , a second stator coil  42   h  is wound around the insulated second tooth  42  in the same manner that the first stator coil  38   h  was wound around the first tooth  38 . A first crossover wire  90  is used to electrically connect the first and second stator coils  38   h ,  42   h . In some embodiments, the first crossover wire  90  may simply be a continuation of the first stator coil  38   h  that is continued over to the second tooth  42  to begin the second stator coil  42   h  winding process. The first stator coil  38   h  has a first end wire portion  38   i  and the second stator coil  42   h  has a second end wire portion  42   i . The first and second end wire portions  38   i ,  42   i  are electrically connected to a power source, such that the first and second stator coils  38   h ,  42   h  can transmit current to form a first phase of the three phase stator  14   a.    
     As shown in  FIG.  10   , in a manner similar to the first and second stator coils  38   h ,  42   h  being wound around the first and second teeth  38 ,  42  of the first stator segment  22 , third and fourth stator coils  50   h ,  54   h  are respectively wound around the insulated third and fourth teeth  50 ,  54  of the second stator segment  26 . A second crossover wire  110  is used to electrically connect the third and fourth stator coils  50   h ,  54   h . In some embodiments, the second crossover wire  110  may simply be a continuation of the third stator coil  50   h  that is continued over to the fourth tooth  54  to begin the fourth stator coil  54   h  winding process. The third stator coil  50   h  has a third end wire portion  50   i  and the fourth stator coil  54   h  has a fourth end wire portion  54   i . The third and fourth end wire portions  50   i ,  54   i  are electrically connected to a power source, such that the third and fourth stator coils  50   h ,  54   h  can transmit current to form a second phase of the three phase stator  14   a.    
     As shown in  FIG.  11   , in a manner similar to the first and second stator coils  38   h ,  42   h  being wound around the first and second teeth  38 ,  42  of the first stator segment  22 , fifth and sixth stator coils  62   h ,  66   h  are respectively wound around the insulated fifth and sixth teeth  62 ,  66  of the third stator segment  30 . A third crossover wire  130  is used to electrically connect the fifth and sixth stator coils  62   h ,  66   h . In some embodiments, the third crossover wire  130  may simply be a continuation of the fifth stator coil  62   h  that is continued over to the sixth tooth  62  to begin the sixth stator coil  66   h  winding process. The fifth stator coil  62   h  has a fifth end wire portion  62   i  and the sixth stator coil  66   h  has a sixth end wire portion  66   i . The fifth and sixth end wire portions  62   i ,  66   i  are electrically connected to a power source, such that the fifth and sixth stator coils  62   h ,  66   h  can transmit current to form a third phase of the three phase stator  14   a.    
     Next, as shown in  FIG.  12   , the first stator segment  22  is axially coupled to the second stator segment  26  in a manner such that the first and second teeth  38 ,  42  are circumferentially offset from the third and fourth teeth  50 ,  54 . Similarly, the third stator segment  30  is axially coupled to the second stator segment  26  on a side of the second stator segment  26  that is opposite the first stator segment  22 , such that the fifth and sixth teeth  62 ,  66  are circumferentially offset from the first, second, third, and fourth teeth  38 ,  42 ,  50 ,  54 . In some embodiments, the first and third stator segments  22 ,  30  are axially coupled to the second stator segment  26  via welding of the respective first, second, and third back portions  34 ,  46 ,  58 . However, in other embodiments, the back portions  34 ,  46 ,  58  have mating portions  140 , such as reciprocal recess and protrusion arrangements, that permit the back portions  34 ,  46 ,  58  to be snap fit together. As shown in  FIG.  12   , when stacked together, the first, second, and third back portions  34 ,  46 ,  58  collectively have the first height H 1 . 
     As shown in  FIG.  13   , the crossover wires  90 ,  110 ,  130  are routed adjacent to the third back portion  58 . In some embodiments, the second insulation end cap  78  may include hooks  142  and/or notches  146  to support the crossover wires  90 ,  110 ,  130  as they are routed to be adjacent to the third back portion  58 . The stator  14   a  is then assembled and ready to be implemented as part of the motor  10 . 
     As described above, in some embodiments, the teeth  38 ,  42 ,  50 ,  54 ,  62 ,  66  are fixed to their respective back portions  34 ,  46 ,  58  with staking during the progressive stamping die process. In other embodiments, a stamping cross section  148  can include a first tooth portion  150  and a fourth tooth portion  154  included as part of the first back portion  34 , as well as second, third, fifth, and sixth tooth portions  158 ,  162 ,  166 ,  170  nested within an inner diameter  174  of the first back portion  34 , as shown in  FIG.  14   . In other embodiments, the second, third, fifth, and sixth tooth portions  158 ,  162 ,  166 ,  170  can be arranged outside an outer diameter  176  of the first back portion  34 . 
     Thus, after stamping, the first back portion  34  includes the first and fourth tooth portions  150 ,  154 , as shown in  FIG.  15   , and the second, third, fifth, and sixth tooth portions  158 ,  162 ,  166 ,  170  have been separately stamped out, with the second, and fifth tooth portions  158 ,  166  shown in  FIG.  16   . Then, the second and third tooth portions  158 ,  162  can be coupled to the first tooth portion  150  via welding, staking, insert molding, bolting or other methods to form the first tooth  38 . Likewise, the fifth and sixth tooth portions  166 ,  170  can be coupled to the third tooth portion  154  via welding, staking, insert molding, bolting or other methods to form the second tooth  42 . By nesting the second, third, fifth, and sixth tooth portions  158 ,  162 ,  166 ,  170  inside the inner diameter  174  or arranging them outside the outer diameter  176 , material respectively inside or outside the inner and outer diameters  174 ,  176  of the first back portion  34  that would otherwise be scrapped after the stamping process is instead used to form the first and second teeth  38 ,  42 . 
       FIG.  17    illustrates an embodiment of the stator including a two-part stator  14   b . The stator  14   b  is also a three-phase stator. The stator  14   b  includes a first stator segment  180  ( FIG.  18   ) and a second stator segment  184  ( FIG.  19   ). The first stator segment  180  includes a first annular back portion  188  and first, second, and third teeth  192 ,  196 ,  200  that each extend radially inward from the first back portion  188  and axially away from the first back portion  188  in a first direction. The first, second, and third teeth  192 ,  196 ,  200  each have a height H 3 . The second stator segment  184  includes a second annular back portion  204  and fourth, fifth, and sixth teeth  208 ,  212 ,  216  that each extend radially inward from the second back portion  204  and axially away from the second back portion  204  in a second direction that is opposite the first direction. Like the first, second, and third teeth  192 ,  196 ,  200 , the fourth, fifth, and sixth teeth  208 ,  212 ,  216  also each have the height H 3 . In some embodiments, each of the back portions  188 ,  204  can be produced in the same progressive stamping die process. In some embodiments, the teeth  192 ,  196 ,  200 ,  208 ,  212 ,  216  are fixed to the respective back portions  188 ,  204  with staking during the progressive stamping die process. 
     Each of the teeth  192 ,  196 ,  200 ,  208 ,  212 ,  216  respectively have outer flanges  192   a ,  196   a ,  200   a ,  208   a ,  212   a ,  216   a  proximate the respective back portions  34 ,  46 ,  58 , inner flanges  192   b ,  196   b ,  200   b ,  208   b ,  212   b ,  216   b , and spoke portions  192   c ,  196   c ,  200   c ,  208   c ,  212   c ,  216   c  respectively coupling the inner flanges  192   b ,  196   b ,  200   b ,  208   b ,  212   b ,  216   b  to the outer flanges  192   a ,  196   a ,  200   a ,  208   a ,  212   a ,  216   a , such that slots  192   d ,  196   d ,  200   d ,  208   d ,  212   d ,  216   d  are respectively defined between the outer flanges  192   a ,  196   a ,  200   a ,  208   a ,  212   a ,  216   a  and inner flanges  192   b ,  196   b ,  200   b ,  208   b ,  212   b ,  216   b  on both sides of the spokes  38   c ,  42   c ,  50   c ,  54   c ,  62   c ,  66   c.    
     After each of the stator segments  180 ,  184  have been separately formed, the stator  14   b  can be assembled in the following manner. First, as shown in  FIG.  20   , first, second, and third layers of insulation  192   e ,  196   e ,  200   e  are respectively applied about the first, second, and third teeth  192 ,  196 ,  200  of the first stator segment  180 . Specifically, the layers of insulation  192   e ,  196   e ,  200   e  respectively cover portions of the outer flanges  192   a ,  196   a ,  200   a , the inner flanges  192   b ,  196   b ,  200   b  and the spoke portions  192   c ,  196   c ,  200   c  of the corresponding tooth  192 ,  196 ,  200 . Also, the first, second, and third layers of insulation  192   e ,  196   e ,  200   e  respectively include first, second, and third outer bookends  192   f ,  196   f ,  200   f  that extend axially from the top and bottom portions of the outer flanges  192   a ,  196   a ,  200   a . The first, second, and third layers of insulation  192   e ,  196   e ,  200   e  also respectively include first, second, and third inner bookends  192   g ,  196   g ,  200   g  that extend axially from the top and bottom portions of the inner flanges  192   b ,  196   b ,  200   b . Also, a first insulation end cap  220  is applied to a first axial end  224  of the first back portion  188 . In some embodiments, the first insulation end cap  220  is formed with the first, second, and third layers of insulation  192   e ,  196   e ,  200   e , e.g. via insert molding. As shown in  FIG.  21   , in a manner similar to the first, second, and third layers of insulation  192   e ,  196   e ,  200   e  and the first insulation end cap  220  being applied to the first stator segment  180 , fourth, fifth and sixth layers of insulation  208   e ,  212   e ,  216   e  are respectively applied about the fourth, fifth and sixth teeth  208 ,  212 ,  216  of the second stator segment  184 , and a second insulation end cap  228  is applied to a second axial end  232  of the second back portion  204 . 
     Next, as shown in  FIG.  20   , a first stator coil  192   h  is wound around the insulated first tooth  192  or more specifically, the insulated spoke portion  192   c  of the first tooth  192 . In some embodiments, the first stator coil  192   h  is a winding of electrically conductive metal, such as a copper winding. Because the only other teeth on the first stator segment  180  are the second and third teeth  196 ,  200 , which are each 120 degrees away from the first tooth  192 , the first stator coil  192   h  is advantageously able to fill or substantially fill the slots  192   d  on the first tooth  192 . In other words, because there are no teeth immediately adjacent the first tooth  192  of the first stator segment  180 , during the winding process of the first stator coil  192   h , there is no additional structure to inhibit the first stator coil  192   h  from being wound about the first tooth  192  until the first stator coil  192   h  is flush with or even wound past the outer flange  192   a  in a circumferential direction. The first outer bookend  192   f  and the first inner bookend  192   g  radially secures the portions of the first stator coil  192   h  that respectively extend above and below the first tooth  192 , such that the first stator coil  192   h  can be wound to a total height H 4  that is longer than the height H 3  ( FIG.  17   ). 
     With continued reference to  FIG.  20   , in a manner similar to the first stator coil  192   h  being wound around the insulated first tooth  192 , second and third stator coils  196   h ,  200   h  are wound around the second and third insulated teeth  196 ,  200  of the first stator segment  180 . Also, as shown in  FIG.  21   , in a manner similar to the first stator coil  192   h  being wound around the insulated first tooth  192 , fourth, fifth, and sixth stator coils  208   h ,  212   h ,  216   h  are wound around the insulated fourth, fifth, and sixth teeth  208 ,  212 ,  216  of the second stator segment  184 . 
     Next, as shown in  FIG.  17   , the first stator segment  180  is axially coupled to the second stator segment  184  in a manner such that the first, second, and third teeth  192 ,  196 ,  200  are respectively opposite the fourth, fifth and sixth teeth  208 ,  212 ,  216 . In some embodiments, the first stator segment  180  is axially coupled to the second stator segment  184  via welding of the respective first and second back portions  188 ,  204 . First, second, and third crossover wires are then used to respectively electrically connect the first and fourth stator coils  192   h ,  208   h , the second and fifth stator coils  196   h ,  212   h , and the third and sixth stator coils  200   h ,  216   h.    
     Each of the stator coils  192   h ,  196   h ,  200   h ,  208   h ,  212   h ,  216   h  respectively has pairs of wire portions  192   i ,  196   i ,  200   i ,  208   i ,  212   i ,  216   i . The first, second, third, fourth, fifth, and sixth end wire portions  192   i ,  196   i ,  200   i ,  208   i ,  212   i ,  216   i  may be electrically connected to each other, with another component, or directly to a power source, such that the first and fourth stator coils  192   h ,  208   h  can transmit current to form a first phase of the three phase stator  14   b , the second and fifth stator coils  196   h ,  212   h  can transmit current to form a second phase of the three phase stator  14   b , and the third and sixth stator coils  200   h ,  216   h  can transmit current to form a third phase of the three phase stator  14   b . The stator  14   b  is then assembled and ready to be implemented as part of the motor  10 . 
       FIG.  22    illustrates a top-down view of a stator  284  having a back portion  288 , a first tooth  292  having a spoke portion  296  and a flange  300  extending transverse to the spoke portion  296 , such that a slot  302  is defined between the flange  300  and the back portion  288 . A radial distance defined between the flange  300  and the back portion  288  increases along the slot  302  in a direction away from the spoke portion  296 . A stator coil  304  is formed around the first tooth  292  by winding a round wire  308  around the spoke portion  296 . However, as shown by  FIG.  22   , the winding of the round wire  308  does not substantially fill the slot  302 .  FIG.  23    illustrates a top-down view of the stator  284 , except in the embodiment of  FIG.  23   , the stator coil  304  is formed around the first tooth  292  by winding a flat wire  312  around the spoke portion  296 . However, as shown by  FIG.  23   , the winding of the flat wire  312  does not substantially fill the slot  302 . 
       FIG.  24    illustrates a top-down view of the stator  284 , except in the embodiment of  FIG.  24   , the stator coil  304  is formed around the first tooth  292  by winding a variable-cross-section conductor  316  around the spoke portion  296 . Specifically, the stator coil  304  includes a first turn 320, a plurality of intermediate turns 324, and a final turn 328 of the variable-cross-section conductor  316 . As shown in  FIG.  24   , the final turn 328 of the variable cross-section conductor  316  has a cross-sectional length LF that is greater than the cross-sectional length L 1  of the first turn 320. Because the cross-sectional length of the variable cross-section conductor  316  generally increases in a direction away from the spoke portion  296 , the stator coil  304  can substantially or completely fill the slot  302 . In some embodiments, the variable cross-section conductor  316  is applied via a 3D printer. 
       FIG.  25    schematically illustrates a method of applying a stator coil  332  around a tooth  336  of a stator using a 3D printer  340  with a first extrusion head  344  for applying insulation and a second extrusion head  348  for applying an electrically conductive metal, such as copper.  FIG.  25    illustrates a schematic cross-sectional view of a spoke portion  352  of the tooth  336 , such that the internal flange is not visible. To apply the stator coil  332  around the tooth  336 , the first extrusion head  344  first prints a first layer of insulation  356  around the spoke portion  352 . The second extrusion head  348  then prints a first layer of electrically conductive metal  360  around the first layer of insulation  356 . The first extrusion head  344  then prints a second layer of insulation  364  around the first layer of electrically conductive metal  360 . The second extrusion head  348  then prints a second layer of electrically conductive metal  368  around the second layer of insulation  364 , which for purposes of this simplified example, is a final layer of electrically conductive metal  372 . The first extrusion head  344  then prints a third layer of insulation  376  around the second layer of electrically conductive metal  368 . A first end wire connection  380  is then coupled to the first layer of electrically conductive metal  360  and a second end wire connection  384  is coupled to the final layer of electrically conductive metal  372 . 
     While this simplified example only include two layers of electrically conductive metal, an actual application of the method of  FIG.  25    would include a greater number of alternating layers of insulation and electrically conductive metal printed by the first and second extrusion heads  344 ,  348 . In some embodiments, the alternating layers of insulation and electrically conductive metal are printed sequentially by the first and second extrusion heads  344 ,  348 . In some embodiments, the alternating layers of insulation and electrically conductive metal are printed substantially simultaneously by the first and second extrusion heads  344 ,  348 . In other words, as the first extrusion head  344  is applying the first layer of insulation  356 , the second extrusion head  348  is applying the first layer of electrically conductive metal  360 . And then as the first extrusion head  344  is applying the second layer of insulation  364 , the second extrusion head  348  is applying the second layer of electrically conductive metal  368 . By using the 3D printer  340  to print alternating layers of insulation and electrically conductive metal, the slots of the teeth of a stator can be completely or substantially filled. 
       FIG.  26    schematically illustrates another method of applying the stator coil  332  around the tooth  336  of a stator.  FIG.  26    illustrates the schematic cross-sectional view of the spoke portion  352  of the tooth  336 , such that the internal flange is not visible. First, a first full-turn layer of insulation  372  is applied around the spoke portion  352 . Next, a first half turn of electrically conductive metal  376  is coupled to the first layer of insulation  372 . The first half turn of electrically conductive metal  376 , as well as all subsequent half turns of electrically conductive metal, can be formed using any suitable manufacturing process, such as extrusion, casting, machining, or 3D printing. Next, a second half turn of electrically conductive metal  380  is coupled to the first layer of insulation  372 , such that there is now a full turn of electrically conductive metal around the first layer of insulation  372 . Next, a first electrical connection  384  is made between the first and second half turns of electrically conductive metal  376 ,  380 . The first electrical connection  384 , as well as all subsequent electrical connections between half turns of electrically conductive metal, can be made using any suitable electrical connection method, such as brazing, welding, fusing, or screwing. 
     Next, a second full-turn layer of insulation  388  is applied over the first and second half turns of electrically conductive metal  376 ,  380 . Next, a third half turn of electrically conductive metal  392  is applied to the second layer of insulation  388  and a second electrical connection  396  is made between the second and third half turns of electrically conductive metal  380 ,  392 . Next, a fourth half turn of electrically conductive metal  400  is applied to the second layer of insulation  388  and a third electrical connection  404  is made between the third and fourth half turns of electrically conductive metal  392 ,  400 , such that the third and fourth half turns of electrically conductive metal  392 ,  400  comprise a final turn of electrically conductive metal  406  in this simplified example. Next, a third full-turn layer of insulation  408  is applied around the third and fourth half turns of electrically conductive metal  392 ,  400 . A first end wire connection  412  is then coupled to the first half turn of electrically conductive metal  376  and a second end wire connection  416  is coupled to the final turn of electrically conductive metal  406 . While this simplified example only include two full turns of electrically conductive metal made up of half turns of electrically conductive metal, an actual application of the method of  FIG.  26    would include a greater number of alternating turns of insulation and electrically conductive metal. By using the method of  FIG.  26    to apply alternating layers of insulation and electrically conductive metal, the slots of the teeth of a stator can be completely or substantially filled. 
       FIG.  27    illustrates a power tool  500  incorporating the BLDC motor  10  with stator  14   a . In a brushless motor power tool, such as power tool  500 , switching elements are selectively enabled and disabled by control signals from a controller to selectively apply power from a power source such as a battery pack  501 , to drive the motor  10 . In some embodiments, the battery pack  501  is a nominal 18V, 6 Amp-hour battery pack. 
     In some embodiments, the power tool  500  is a brushless hammer drill having a housing  502  with a handle portion  504  and motor housing portion  506 . The power tool  500  further includes an output driver  507  (illustrated as a chuck), torque setting dial  508 , forward/reverse selector  510 , trigger  512 , battery interface  514 , and light  516 . Although  FIG.  27    illustrates a hammer drill, in some embodiments, the motors described herein are incorporated into other types of power tools including drill-drivers, impact drivers, impact wrenches, angle grinders, circular saws, reciprocating saws, string trimmers, leaf blowers, vacuums, and the like. 
       FIG.  28    illustrates a simplified block diagram of the brushless power tool  500 , which includes the battery pack  501 , a motor drive circuit  524 , the motor  10 , Hall sensors  528 , a motor controller  530 , user input  532 , and other components  533  (battery pack fuel gauge, work lights (LEDs), current/voltage sensors, etc.). The battery pack  501  provides DC power to the various components of the power tool  500  and may be a power tool battery pack that is rechargeable and uses, for instance, lithium ion cell technology. In some instances, the battery pack  501  may receive AC power (e.g., 120V/60 Hz) from a tool plug that is coupled to a standard wall outlet, and then filter, condition, and rectify the received power to output DC power. Each Hall sensor  528  outputs motor feedback information, such as an indication (e.g., a pulse) when a magnet of the rotor  18  rotates across the face of that Hall sensor  528 . Based on the motor feedback information from the Hall sensors  528 , the motor controller  530  can determine the position, velocity, and acceleration of the rotor  18 . The motor controller  530  also receives user controls from user input  532 , such as by depressing the trigger  512  or shifting the forward/reverse selector  510 . In response to the motor feedback information and user controls, the motor controller  530  transmits control signals to the motor drive circuit  524  to drive the motor  10 , as explained in further detail with respect to  FIG.  29   . Although not shown, the motor controller  530  and other components of the power tool  500  are electrically coupled to the battery pack  501  such that the battery pack  501  provides power thereto. 
       FIG.  29    illustrates a simplified block diagram of the motor drive circuit  524 . The motor drive circuit  524  includes a plurality of high side power switching elements  540  (for example, Field Effect Transistors (FETs)), a plurality of low side power switching elements  544  (for example, FETs), a motor braking switch  548  (for example, motor braking FET  548 ), and a braking resistor  552  (also referred to as a braking coil  552 ). The motor controller  530  provides the control signals to control the high side FETs  540  and the low side FETs  544  to drive the motor  10  based on the motor feedback information and user controls, as noted above. For example, in response to detecting a pull of the trigger  512  and the input from forward/reverse selector  510 , the motor controller  530  provides the control signals to selectively enable and disable the FETs  540  and  544  (e.g., sequentially, in pairs) resulting in power from the battery pack  501  to be selectively applied to stator coils of the motor  10  to cause rotation of the rotor  18 . 
     More particularly, to drive the motor  10 , the motor controller  530  enables a first high side FET  540  and first low side FET  544  pair (e.g., by providing a voltage at a gate terminal of the FETs) for a first period of time. In response to determining that the rotor  18  of the motor  10  has rotated based on a pulse from the Hall sensors  528 , the motor controller  530  disables the first FET pair, and enables a second high side FET  540  and a second low side FET  544 . In response to determining that the rotor of the motor  10  has rotated based on pulse(s) from the Hall sensors  528 , the motor controller  530  disables the second FET pair, and enables a third high side FET  540  and a third low side FET  544 . In response to determining that the rotor of the motor  10  has rotated based on further pulse(s) from the Hall sensors  528 , the motor controller  530  disables the third FET pair and returns to enable the first high side FET  540  and the third low side FET  544 . This sequence of cyclically enabling pairs of high side FET  540  and a low side FET  544  repeats to drive the motor  10 . Further, in some embodiments, the control signals include pulse width modulated (PWM) signals having a duty cycle that is set in proportion to the amount of trigger pull of the trigger  512 , to thereby control the speed or torque of the motor  10 . 
     To stop the motor  10 , the motor controller  530  shorts the low side FETs  544  (i.e., enables the low side FETs  544  and disables the high side FETs  540 ) to allow the back EMF to flow through the motor coils of the motor  10 . The back EMF provides a braking force on the magnets of the rotor  18 . For power tools  500  in which it may be desirable to have a faster stopping of the motor  10  (e.g., saws, grinders, and the like), an additional resistance is used to brake the motor  10 . As illustrated in  FIG.  30   , the motor controller  526  controls the braking FET  548  to close thereby connecting the braking resistor  552  to the current path of the motor  10 . The braking resistance  552  absorbs the excess current and brings the motor  10  to a faster stop in comparison to a motor drive circuit  524  without the braking resistor  552 . In the example illustrated, the high side FETs  540  are also closed to allow the back EMF to flow from the motor  10  through the high side FETs  540 , the braking resistance  552  and to the ground or the negative terminal. 
     In some embodiments, during assembly of stator  14   a , the motor braking coil  552  can be added. For example,  FIG.  31    illustrates the motor braking resistance coil  552  being wound around the first and second stator coils  38   h ,  42   h  (not shown), after the first and second stator coils  38   h ,  42   h  have been wound around the insulated first and second teeth  38 ,  42  as shown in  FIG.  9   . A single coil is used as the motor braking coil  552  and is, for example, first wound around the stator coil  38   h , then wound around the second stator coil  42   h . Because the only other tooth on the first stator segment  22  is the second tooth  42 , which is 180 degrees away from the first tooth  38 , the first stator coil  38   h  and motor braking coil  552  are advantageously able to fill or substantially fill the slots  38   d  on the first tooth  38 . In other words, because there are no teeth immediately adjacent the first tooth  38  of the first stator segment  22 , during the winding process of the first stator coil  38   h  and motor braking coil  552 , there is no other structure inhibiting the first stator coil  38   h  and motor braking coil  552  from being wound about the first tooth  38  until the motor braking coil  552  is flush with or even wound past the outer flange  38   a  in a circumferential direction. The first outer bookend  38   f  and the first inner bookend  38   g  radially secures the portions of the first stator coil  38   h  and motor braking coil  552  that respectively extend above and below the first tooth  38 , such that the first stator coil  38   h  and motor braking coil  552  can be wound to a second total height H 2  that is longer than the first height H 1 . Likewise, these principles apply to the winding of the motor braking coil  552  around the second tooth  42 . Then, the rest of the stator  14   a  can be assembled as described above and shown in  FIGS.  10 - 13   , except that in this embodiment, the stator  14   a  includes the braking coil  552 . 
     In some embodiments, the motor braking coil  552  may be wound such that ends  572  and  576  of the motor braking coil  552  are provided on the same side of the motor  10 . For example, the first-sixth end wire portions  38   i ,  42   i ,  50   i ,  54   i ,  62   i ,  66   i  of the stator coils  38   h ,  42   h ,  50   h ,  54   h ,  62   h ,  66   h  are provided on one end of the motor  10  and the ends  572  and  576  of the motor braking coil  552   b  are provided on the opposite end of the motor  10 . The ends  572  and  576  of the motor braking coil  552  are connected between the battery pack  501  and the braking FET  548  and are illustrated in  FIG.  31   . 
     In the embodiment illustrated in  FIG.  32   , the motor braking coil  552  is wound around the stator  14   a  of the motor  10 . After the stator  14   a  has been assembled, the motor braking coil  552  is wound around the stator  14   a  between the first and second insulation end caps  70 ,  78 . In the embodiment illustrated in  FIG.  32   , the motor braking coil  552  is wound around the second annular back portion  46 . In other examples, the motor braking coil  552  may also be wound around another large object of the power tool  500 . The motor braking coil  552  is, for example, a similar coil that is used for the stator coils  38   h ,  42   h ,  50   h ,  54   h ,  62   h ,  66   h . A coil of appropriate length may be cut from the material used for stator coils  38   h ,  42   h ,  50   h ,  54   h ,  62   h ,  66   h  to use as the motor braking coil  552 . The ends  572  and  576  of the motor braking coil  552  are connected between the power source  122  and the braking FET  548  and are illustrated in  FIG.  32   . The embodiment of  FIG.  32    also illustrates a plurality of mounting ears, such as radially-outward extending bosses  578  on the first annular back portion  34 , that can be used to secure the stator  14   a  within a power tool by, e.g. putting a fastener through the bosses  578 . In some embodiments, the bosses  578  are injection molded onto the stator  14   a.    
     Compared to an over the shelf resistor, the motor braking coil  552  distributes the braking resistance over a larger area. The motor braking coil  552  therefore generates less heat than an over the shelf resistor. The heat due to the current flowing in the motor braking coil  552  is generated over a larger surface area allowing for easier dissipation. Additionally, because the motor braking coil  552  is made from the same coil used for the stator coils  38   h ,  42   h ,  50   h ,  54   h ,  62   h ,  66   h  and no additional heat sink is required, the motor braking coil  552  results in reduced cost of the power tool  500 . 
       FIGS.  33 - 38    illustrate a motor  1000  according to some embodiments of the invention. Like the motor  10 , the motor  1000  may be incorporated into the tool  500  of  FIG.  28    and is an example of the motor  10  in the block diagram of  FIG.  28   . Additionally, like the motor  10 , the motor  1000  may also be incorporated into other types of power tools, as described above. The motor  1000  includes the stator  1100 , a rotor  1200  that drives a shaft  1300 , a fan  1400 , and a position sensor board assembly  1500 . The stator coils are not shown in  FIG.  33   , but are described in greater detail below. The stator  1100  also includes a stator frame  1535  including a front stator end cap  1542  and a rear stator end cap  1544 . The front stator end cap  1542  and the rear stator end cap  1544  may be integrally formed as a single piece (i.e., the stator frame  1535 ) or, alternatively, may be two separate pieces that together form the stator frame  1535 . The stator frame  1535  may be formed by an injection molding process, for example, by injecting a resin material into a mold including a stator lamination stack  1550  ( FIG.  38   ). Accordingly, the stator frame  1535  may be a monolithic structure formed of hardened resin. 
     The position sensor board assembly  1500  is provided on a front side of the motor  1000  and the fan  1400  is provided on a rear side of the motor  1000 . In some embodiments, the stator coils of motor  1000  may be routed on the rear side of the stator  1100 . For example, the stator coils may be routed with the help of the tabs  1110  positioned on the rear stator end cap  1544  of the stator  1100 . In some embodiments, the stator  1100  could be assembled and the stator coils wound around the teeth of stator  1100  in the same manner as stator  14   a  or stator  14   b . In some embodiments, the stator coils could be wound around the teeth of the stator  1100  in the same manner as described with stator  284 . In some embodiments, the stator coils could be wound around the teeth of stator  1100  in the same manner as described with stator coil  332 , using either the method schematically shown in  FIG.  25    or  FIG.  26   . 
       FIG.  34    is a perspective view of the rotor  1200  and the fan  1400 . The rotor  1200  includes a rotor core  1210  and a rotor frame  1220 . The rotor core  1210  is made of rotor laminations, which form a rotor stack, and that define a central aperture ( FIG.  38   ) to receive the shaft  1300  and magnet receiving apertures to receive rotor magnets ( FIG.  38   ).  FIG.  35    is a perspective view of the rotor frame  1220  and the fan  1400 . The rotor frame  1220  includes a first face portion  1222  (also referred to as a face plate) provided on a front side of the rotor  1200 , a magnet housing portion  1224 , and an end portion  1226  provided on a rear side of the rotor  1200  opposite the front side. The end portion  1226  includes a second face portion  1225  and the fan  1400 . The first face portion  1222  and the second face portion  1225  house the rotor core  1210  having the rotor laminations. That is, the first face portion  1222  and the second face portion  1225  enclose the rotor lamination stack (i.e., the rotor core  1210 ) between them. The first face portion  1222  and the second face portion  1225  also retain the rotor magnets in the magnet receiving apertures of the rotor core  1210 . The rotor  1200 , in contrast to the rotors  202  and  320  above, does not include a separate rotor enclosure with additional end caps. Accordingly, the first face portion  1222  and the second face portion  1225  may be referred to as rotor end caps of the rotor  1200 . In contrast to the fixed (non-rotating) configuration of the rotor end caps in the rotors  202  and  320 , the first face portion  1220  and the second face portion  1225  rotate with the rotor core  1210 . The magnet housing portion  1224  houses the permanent magnets inserted into the rotor core  1210 . The magnet housing portion  1224  also extends through the magnet apertures of the rotor core  1210 , as shown in  FIG.  38   . 
     As can be seen from  FIGS.  34  and  35   , the rotor frame  1220  is integrally formed with the fan  1400 . The rotor frame  1220  and the fan  1400  may be integrally formed during an injection molding process. During the injection molding process, the rotor core  1210  and the rotor magnets may be placed in a die while a plastic or resin material is injected into the die to form the rotor frame  1220  and the fan  1400 . Accordingly, the rotor frame  1220  may be a monolithic structure formed of hardened resin. In some embodiments, for example, rather than injection molding to integrally form the fan  1400  with the rotor frame  1220 , the fan  1400  may be press-fit onto the rotor frame  1220 . 
     In some embodiments, the first face portion  1222  may include apertures  1228 . The apertures may be provided to balance the motor  1000 . The number and placement of the apertures  1228  may be determined during the injection molding process based on the weight and size imbalances of the rotor  1200 . In some embodiments, other motor balancing techniques may also be used. In one example, the mold may be modified or calibrated such that certain portions of the fan  1400  or the rotor frame  1220  may be removed to balance the motor  1000 . For a particular die, a sample rotor  1200  may be formed using injection molding as described. The sample rotor  1200  may be tested to detect imbalances. Based on the imbalances, projections or posts may be placed in the die to occupy space in the die and prevent injected resin material from forming in that location, resulting in the apertures  1228 . In another example, the apertures  1228  may be formed by scraping away or otherwise removing material from the resin-formed portions of the rotor  1200  after the injection molding process. In some embodiments, the plastic or resin material may be injected into the magnet receiving apertures such that the plastic or resin material pushes the rotor magnets frontward and outward within the magnet receiving apertures for even distribution, reducing imbalances. 
     Returning to  FIG.  33   , the motor  1000  also includes a front bearing  1600  that rotatably couples the shaft  1300  to the gear case  900 . As such, the front bearing  1600  fixes the motor  1000  to the gear case  900 . Referring to  FIG.  36   , the motor  1000  also includes a rear bearing  1700  provided in a rear opening of the fan  1400 . The outer race of the rear bearing  1700  may be positioned within a recess located at the rear of the power tool housing  102  to secure the motor  1000  within the power tool  500 . 
       FIG.  37    illustrates the position sensor board assembly  1500 . The position sensor board assembly  1500  includes an annular portion  1510  (also referred to as an annular board portion) with legs  1520  extending radially outward from the annular portion  1510 . The position sensor board assembly  1500  includes the Hall sensors  528  (or other position sensors) ( FIG.  28   ) to detect one or more of the rotational position, velocity, and acceleration of the motor  1000 . Returning to  FIG.  33   , fasteners  1530  extend through holes  1525  in the legs  1520  into fastener mounts  1532  ( FIG.  33   ) of a stator frame  1535  of the stator  1100  to fix the position sensor board assembly  1500  to the stator  1100 . The legs  1520  are circumferentially positioned on the annular portion  1510  to align with gaps between adjacent stator end cap teeth  1540  such that the legs  1520  extend through the gaps between adjacent stator end cap teeth  1540 . This alignment, and the annular portion  1510  having a diameter that is less than inner diameter of the stator frame  1535 , enables the position sensor board assembly  1500  to be positioned closer to the rotor  1200  and within a stator envelope  1565  ( FIG.  38   ). This positioning enables the Hall sensors  128  to be closer to the rotor magnets and reduces the axial length of the motor  1000 . 
       FIG.  38    is a cross-sectional view of the motor  1000 . In the illustrated embodiment, the stator  1100  includes a stator lamination stack  1550  having a predetermined number of stator laminations  710 . The stator laminations  710  together define a stack length  1560  in the axial direction extending between axial ends of the stator lamination stack  1550 .  FIG.  38    also illustrates an outer diameter  1562  of the stator lamination stack  1550 . In the embodiment illustrated in  FIGS.  33 - 38   , the outer diameter  1562  is 50 mm, but in other embodiments, the outer diameter  1562  could be greater or smaller. 
       FIG.  38    also illustrates the stator envelope  1565  of the motor  1000 , which extends between the axial ends of the stator frame  1535  (i.e., between the axial end faces of the front stator end cap  1542  and the rear stator end cap  1544 ). In some embodiments, the position sensor board assembly  1500 , the first face portion  1222 , and the second face portion  1225  are within the stator envelope  1565 , while the end portion  1226  is partially within and partially outside the stator envelope  1565 . The front bearing  1600  and the rear bearing  1700  may be located outside the stator envelope  1565 . 
     In addition, the front bearing  1600  and the rear bearing  1700  define a bearing-to-bearing length  1570  in the axial direction between the axial ends of the front bearing  1600  and the rear bearing  1700 . Further, the position sensor board assembly  1500  and the rear bearing  1700  define a bearing-to-board length  1575  in the axial direction between the axial ends of the position sensor board assembly  1500  and the rear bearing  1700 . A bearing-to-board length, such as the bearing-to-board length  1575 , describes the distance between a bearing and position sensor board assembly that are located on axially opposite sides of a motor. In some embodiments, the stator  1100  (including the stator frame  1535 ), the rotor  1200 , the rotor frame  1220 , the fan  1400 , the position sensor board assembly  1500 , the front bearing  1600 , and the rear bearing  1700  may be located entirely within the bearing-to-bearing length  1570 . In some embodiments, the stator  1100  (including the stator frame  1535 ), the rotor  1200 , the rotor frame  1220 , the fan  1400 , the position sensor board assembly  1500 , and the rear bearing  1700  may be entirely within the bearing-to-board length  1575 , while the front bearing  1600  may be (either partially or entirely) outside the bearing-to-board length  1575 . 
     In some embodiments, the bearing-to-bearing length  1570  is 51.5 millimeters and the bearing-to-board length  1575  is 44.5 millimeters. However, these lengths vary based on the stack length  1560 . The stack length  1560  may vary for each motor  1000  based on the desired motor characteristics. For example, the stack length  1560  may vary between about 10 millimeters and 45 millimeters based on the output requirements of the motor  1000 . In some embodiments, a difference between the bearing-to-bearing length  1570  and the stack length  1560  is 27.5 millimeters or less than 27.5 millimeters. In some embodiments, the difference between the bearing-to-bearing length  1570  and the stack length  1560  is less than 26.5 millimeters, less than 28.5 millimeters, less than 29.5 millimeters, less than 30.5 millimeters, between 25.5 millimeters and 30.5 millimeters, between 25.5 millimeters and 27.5 millimeters, between 27.5 millimeters and 30.5 millimeters, between 26.5 millimeters and 28.5 millimeters, or another range between 25.5 millimeters and 30.5 millimeters. This difference may be adjusted in the range, for example, by reducing the axial thickness of one or both of the bearings  1600 ,  1700  or of the end portion  1226 . In the context of a measurement range herein, such as, “between 25.5 millimeters and 28.5 millimeters,” the term “between” is intended to include values that are greater than or equal to the lower endpoint and that are less than or equal to the upper endpoint. Accordingly, as an example, 25.5 millimeters is considered to be between 25.5 millimeters and 28.5 millimeters. In some embodiments, a difference between the bearing-to-board length  1575  and the stack length  1560  is 20.5 millimeters or less than 20.5 millimeters. In some embodiments, a difference between the bearing-to-board length  1575  and the stack length  1560  is less than 19.5 millimeters, less than 21.5 millimeters, less than 22.5 millimeters, less than 23.5 millimeters, between 18.5 millimeters and 23.5 millimeters, between 20.5 millimeters and 23.5 millimeters, between 19.5 millimeters and 21.5 millimeters, or another range between 18.5 millimeters and 23.5 millimeters. This difference may be adjusted in the range, for example, by reducing the axial thickness of the bearing  1700  or of the end portion  1226 . 
     In some embodiments, the bearing-to-bearing length  1570  may be in a range of 30 millimeters to 60 millimeters, depending on the stack length  1560 , an axial fan length of the fan  1400 , and a board thickness (in the axial direction) of the position sensor board assembly  1500 . Here, a difference between the bearing-to-bearing length  1570  and a sum of the stack length  1560 , the axial fan length of the fan  1400 , and the board thickness of the position sensor board assembly  1500  is 15 millimeters or less than 15 millimeters. In some embodiments, this difference is less than 14 millimeters, less than 16 millimeters, less than 17 millimeters, less than 18 millimeters, less than 19 millimeters, less than 20 millimeters, between 13 millimeters and 15 millimeters, between 15 millimeters and 20 millimeters, between 14 millimeters and 16 millimeters, between 13 millimeters and 18 millimeters, or another range between 13 millimeters and 20 millimeters. This difference may be adjusted in the range, for example, by reducing the axial thickness of one or both of the bearings  1600 ,  1700 . Thus, the configuration of the embodiments provides an axially compact motor design. 
     As shown in  FIG.  39   , the stator  1100  includes a back stator portion  1705 . A plurality of teeth are arranged around the inner circumference of the back stator portion  1705  and extend radially inward from the back stator portion  1705 . As with stators  14   a  and  14   b , in some embodiments, the stator  1100  includes a total of six radially-inward extending teeth. For purposes of illustration, only two of the plurality of teeth, a first tooth  1710  and an adjacent second tooth  1715 , are shown in  FIG.  39   . The first tooth  1710  includes a first flange  1720  extending away from the first tooth  1710  and the second tooth  1715  includes a second flange  1725  extending away from the second tooth  1715  and toward the first flange  1720 . 
     As with stators  14   a  and  14   b , insulation  1730  covers portions of the first and second teeth  1710 ,  1715 , as well as the back stator portion  1705 . Specifically, the insulation  1730  includes a back insulation portion  1735  covering the back stator portion  1705 , a first tooth portion  1740  covering the first tooth  1710 , a second tooth portion  1745  covering the second tooth  1715 , a first flange portion  1750  covering the first flange  1720 , and a second flange portion  1755  covering the second flange  1725 . The first flange portion  1750  has a first face  1760  that is in facing relationship with the back insulation portion  1735  and the second flange portion  1755  has a second face  1765  that is in facing relationship with the back insulation portion  1735 . The first face  1760  and the second face  1765  together substantially define a boundary plane P, such that a cross-sectional slot area SA is defined between the back insulation portion  1735 , the first tooth portion  1740 , the second tooth portion  1745 , and the boundary plane P. 
     As shown in  FIG.  40   , during assembly of the stator  1100 , a first plurality of conductive (e.g. copper) wires  1770 , making up a first stator coil  1775 , are wrapped around the first tooth  1710  and thus arranged between the first and second teeth  1710 ,  1715  within the cross-sectional slot area SA. Similarly, during assembly of the stator  1100 , a second plurality of conductive (e.g. copper) wires  1780 , making up a second stator coil  1785 , are wrapped around the second tooth  1715  and thus arranged between the first and second teeth  1710 ,  1715  within the cross-sectional slot area SA. As described above, in some embodiments, the stator  1100  could be assembled and the stator coils wound around the teeth of stator  1100  in the same manner as stator  14   a  or stator  14   b . In some embodiments, the stator coils could be wound around the teeth of the stator  1100  in the same manner as described with stator  284 . In some embodiments, the stator coils could be wound around the teeth of stator  1100  in the same manner as described with stator coil  332 , using either the method schematically shown in  FIG.  25    or  FIG.  26   . 
     The sum of the cross-sectional areas of each of the individual copper wires  1770 ,  1780  within the cross-sectional slot area SA collectively define a total cross-sectional winding area WA, such that a slot-fill ratio (WA/SA) of cross-sectional winding area WA to cross-sectional slot area SA is defined. In some embodiments, the slot-fill ratio is 0.30 or greater. In some embodiments, the slot-fill ratio is 0.37 or greater. In some embodiments, the slot-fill ratio is 0.45 or greater. In some embodiments, the slot-fill ratio is 0.57 or greater. While  FIG.  40    only shows the wires  1770 ,  1775  arranged between the first and second teeth  1710 ,  1715 , wires of different stator coils would also arranged between each and every adjacent pair of teeth on the stator  1100 , such that the slot-fill ratio would be substantially similar or identical between every pair of adjacent teeth on the stator  1100 . 
     The below four tables illustrate results from tests to determine performance characteristics of the motor  1000  and power tool  500  when implementing slot-fill ratios of 0.30, 0.37, 0.45 and 0.57 at different amperage levels  40  A,  60  A,  80  A,  100  A drawn by the motor  1000 . A single test was run for each combination of amperages and slot-fill ratios. In each of these tests, the stator lamination stack  1550  has a stack length  1560  of 24 mm and an outer diameter  1562  of 50 mm. Also, in each of these tests, the battery pack  501  used with power tool  500  was a nominal 18V, 6 Amp-hour battery pack. 
     Each table lists the slot-fill ratio for the slots between each pair of adjacent teeth in the stator  1100 . Each table also lists the time in continuous seconds of run time that it took for one of the motor drive circuit  524  or the stator coils of the motor  1000 , to reach a critical temperature at which the motor controller  530  would shut down the motor  1000  to prevent damage thereto. Each table also lists the power output of the power tool  500  in Watts, the airflow through the power tool  500  in cubic feet per minute (CFM), and power out in Watts per CFM, which measures the rate at which the power tool  500  can perform work per rate of airflow to keep the power tool  500  cool. 
     Table 1 illustrates when the motor  1000  is drawing  40  A of current, and compares performance characteristics when the stator  1100  of the motor  1000  has a slot-fill ratio of 0.30, 0.37, 0.45 and 0.57 between each pair of adjacent teeth. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Motor 1000 drawing 40 A of current 
               
            
           
           
               
               
               
               
               
            
               
                 Slot-Fill Ratio 
                 0.30 
                 0.37 
                 0.45 
                 0.57 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Time to Critical Temp (Seconds) 
                 536 
                 484 
                 424 
                 511 
               
               
                 Power Out (Watts) 
                 541 
                 525 
                 550 
                 537 
               
               
                 Airflow through tool 500 (CFM) 
                 4.9 
                 4.4 
                 4 
                 3.8 
               
               
                 Power Out/CFM (Watts/CFM) 
                 110.4 
                 119.3 
                 137.5 
                 141.3 
               
               
                   
               
            
           
         
       
     
     Table 2 illustrates when the motor  1000  is drawing  60  A of current, and compares performance characteristics when the stator  1100  of the motor  1000  has a slot-fill ratio of 0.30, 0.37, 0.45 and 0.57 between each pair of adjacent teeth. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Motor 1000 drawing 60 A of current 
               
            
           
           
               
               
               
               
               
            
               
                 Slot-Fill Ratio 
                 0.30 
                 0.37 
                 0.45 
                 0.57 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Time to Critical Temp (Seconds) 
                 27 
                 79 
                 76 
                 91 
               
               
                 Power Out (Watts) 
                 728 
                 722 
                 755 
                 755 
               
               
                 Airflow through tool 500 (CFM) 
                 4.2 
                 3.9 
                 3.3 
                 3.1 
               
               
                 Power Out/CFM (Watts/CFM) 
                 173.33 
                 185.13 
                 228.79 
                 243.55 
               
               
                   
               
            
           
         
       
     
     Table 3 illustrates when the motor  1000  is drawing  80  A of current, and compares performance characteristics when the stator  1100  of the motor  1000  has a slot-fill ratio of 0.30, 0.37, 0.45 and 0.57 between each pair of adjacent teeth. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Motor 1000 drawing 80 A of current 
               
            
           
           
               
               
               
               
               
            
               
                 Slot-Fill Ratio 
                 0.30 
                 0.37 
                 0.45 
                 0.57 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Time to Critical Temp (Seconds) 
                 15 
                 27 
                 37 
                 43 
               
               
                 Power Out (Watts) 
                 827 
                 832 
                 874 
                 900 
               
               
                 Airflow through tool 500 (CFM) 
                 3.5 
                 3.1 
                 2.9 
                 2.6 
               
               
                 Power Out/CFM (Watts/CFM) 
                 236.3 
                 268.4 
                 301.4 
                 346.2 
               
               
                   
               
            
           
         
       
     
     Table 4 illustrates when the motor  1000  is drawing  100  A of current, and compares performance characteristics when the stator  1100  of the motor  1000  has a slot-fill ratio of 0.30, 0.37, 0.45 and 0.57 between each pair of adjacent teeth. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Motor 1000 drawing 100 A of current 
               
            
           
           
               
               
               
               
               
            
               
                 Slot-Fill Ratio 
                 0.30 
                 0.37 
                 0.45 
                 0.57 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Time to Critical Temp (Seconds) 
                 8 
                 15 
                 19 
                 26 
               
               
                 Power Out (Watts) 
                 847 
                 871 
                 907 
                 970 
               
               
                 Airflow through tool 500 (CFM) 
                 3.0 
                 2.7 
                 2.5 
                 2.0 
               
               
                 Power Out/CFM (Watts/CFM) 
                 282.3 
                 322.6 
                 362.8 
                 485.0 
               
               
                   
               
            
           
         
       
     
       FIG.  41    provides a bar graph representation of the time to reach the critical temperature, with variations of the stator  1100  having a 0.30, 0.37, 0.45, and 0.57 slot-fill ratio respectively at  40  A,  60  A,  80  A and  100  A loads. For loads of  60  A,  80  A, and  100  A, implementing a 0.57 slot-fill ratio results in a higher time to reach the critical temperature than when implementing a 0.30, 0.37, or 0.45 slot-fill ratio. This is because the increased amount of copper filling the slot between the first and second teeth  1710 ,  1715  improves the heatsinking ability of the stator coils. Thus, with a 0.57 slot-fill ratio, more thermal energy from the motor drive circuit  524  and the motor  1000  itself can be absorbed by the copper than with 0.30, 0.37, and 0.45 slot-fill ratios, resulting in the motor  1000  and motor drive circuit  524  taking a longer time to reach their respective critical temperature. Indeed, in some tests, it was also shown the battery pack  501  ran at a cooler temperature when utilizing the a 0.57 slot-fill ratio than when utilizing the 0.30, 0.37, and 0.45 slot-fill ratios, indicating that the additional copper filling the slots between the stator teeth was also absorbing heat from the battery pack  501 . 
     In the aforementioned tests, the motor drive circuit  524  was arranged remote from the motor  1000 . However, in embodiments where the motor drive circuit  524  is arranged proximate or on the motor  1000 , the increase in time to critical temp could be even greater, as the coils would have an increased ability to absorb thermal energy from the motor drive circuit  524  due to their increased proximity. Further, in embodiments in which the motor  1000  is arranged in totally enclosed system, in which air outside an enclosure for the motor  1000  is not permitted to flow through the motor  1000 , using a higher slot-fill ratio to heatsink the motor drive circuit  524  could increase the time to critical temperature even greater than when motor  1000  is used in an open system. Since only one test was run for each of the slot-fill ratios at  40  A, and because the time to reach critical temperature was much higher at  40  A than at  60  A,  80  A or  100  A, it is likely that variance can explain the relational difference between the slot-fill ratio and time to reach critical temperature at  40  A. 
     The longer time to reach the critical temperature due to utilizing a higher slot-fill ratio is especially advantageous at higher loads such as  60 A,  80  A and  100  A, because the increased time duration over which the power tool  500  can operate can mean the difference between an operator completing a difficult, high load operation or the motor  1000  (such as finishing a difficult fastener) and the motor drive circuit  524  hitting the critical temperature and shutting down, thus requiring the operator to wait for the power tool  500  to cool down before continuing the operation. 
       FIG.  42    provides a bar graph representation of the airflow through the power tool  500 , with the variations of the stator  1100  using a 0.30, 0.37, 0.45, and 0.57 slot-fill ratio respectively at  40  A,  60  A,  80  A and  100  A. As shown in  FIG.  42   , the airflow through the power tool  500  decreases as the slot-fill ratio increases, which is a logical result of there being more copper filling each slot between adjacent teeth on the stator  1100 . Specifically, with more copper filling each slot, there is less room for air to flow therethough, resulting in a lower CFM. 
       FIG.  43    provides a bar graph representation of power generated by the power tool  500 , with the variations of the stator  1100  using a 0.30, 0.37, 0.45, and 0.57 slot-fill ratio respectively at  40  A,  60  A,  80  A and  100  A. In each of the tests, the power out was measured using a dynamometer that could measure the energy output of, e.g. the output driver  507 . As shown in  FIG.  43   , in the higher load  80  A and  100  A tests, there was a positive relationship between the slot-fill ratio and power. In these tests, the increased copper at the higher slot-fill ratios reduced the impedance of the stator coils of stator  1100 , resulting in a smaller amount of energy being lost, such that the motor  1000  became more efficient at communicating the power from the battery pack  501  to the output driver  507 . Though there is not a straight positive relationship between the slot-fill ratio and power for the  40  A and  60  A tests, it is likely that this was due to variance. 
       FIG.  44    illustrates the power out in Watts per CFM, which measures the rate at which the power tool  500  can perform work per rate of airflow to keep the power tool  500  cool, with the variations of the power tool  500  using a 0.30, 0.37, 0.45, and 0.57 slot-fill ratio respectively at  40  A,  60  A,  80  A and  100  A. In the above-described tests, utilizing a higher slot-fill ratio both increases power generated by the motor  1000  and results in a lower CFM because of the additional volume occupied by the copper. However, because the higher slot-fill ratio results in increased heatsinking for the motor drive circuit  524 , less air is required to keep the motor drive circuit  524  cool, such that the lower CFM does not negatively affect performance of the motor  1000 . Thus, as the slot-fill ratio increases, the capability of the power tool  500  to produce more power for longer periods of time without reaching the critical temperature is increased. 
     Various features of the invention are set forth in the following claims.