Abstract:
A distributed coil stator ( 102 ) for external rotor electric motors includes a core having a cylindrical surface bounded by a first end surface and a second end surface, wherein a first set of openings ( 110 ) aligned in a first circular path ( 111 ) extends within the core from the first end surface to the second end surface, a second set of openings ( 114 ) aligned in a second circular path ( 115 ) positioned concentrically within the first circular path extends within the core from the first end surface to the second end surface, and a third set of openings ( 118 ) aligned in a third circular path ( 119 ) positioned concentrically within the second circular path extends within the core from the first end surface to the second end surface.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of priority to U.S. provisional patent application Ser. No. 60/834,664, filed Aug. 1, 2006, which is incorporated by reference herein. 

   TECHNICAL FIELD 
   This invention relates to electric motors, and specifically to a distributed coil stator for external rotor electric motors. 
   BACKGROUND OF INVENTION 
   Electric motors have existed for many years. Today, some electric motors are designed with a rotor disposed radially outward of the stator. These motors are commonly referred to as “inside-out” or “external rotor” electric motors and are typically of the three-phase induction type. External rotor electric motors have many applications, typically commercial or industrial, such as for fans, pumps, machine drives, etc. Furthermore, with the advent of single-to-three phase electronic converters, three-phrase external rotor motors can also be used in residential applications and other environments limited to single-phase electric service. 
   Typically, the stator of an external rotor electric motor includes coils (or windings) for each phase that are inserted through slots of the same size and shape positioned along the periphery of the stator at a single radius. As a result, the coils are bundled together and cross-over or overlap each other resulting in lengthy coil wire usage, high heat generation requiring additional copper and metal materials for dissipation, and high magnetic losses causing low electro-mechanical efficiency. Additionally, the typical stator design limits the motor speed options, which is a function of the number of slots that can be positioned on the periphery of the rotor. 
   Accordingly, it is seen that a need exists for a stator for external rotor electric motors that avoids coil wire bundling and cross-over and offers more flexibility for the number of slots to overcome the foregoing consequences. It is to the provision of such therefore that the present invention is primarily directed. 
   SUMMARY OF INVENTION 
   The invention, in accordance with exemplary embodiments described herein, provides a distributed coil stator for external rotor electric motors. A general exemplary embodiment includes a core having a cylindrical surface bounded by a first end surface and a second end surface. The core includes a first set of openings aligned in a first circular path extending from the first end surface to the second end surface, a second set of openings aligned in a second circular path positioned concentrically within the first circular path extending from the first end surface to the second end surface, and a third set of openings aligned in a third circular path positioned concentrically within the second circular path extending from the first end surface to the second end surface. 
   A general exemplary method includes configuring the shape or size of at least one of the first, second, and third set of openings in the core dependent on the number of openings needed to obtain a desired number of magnetic poles corresponding to a desired motor speed. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is an exploded view that includes the distributed coil stator for external rotor motors shown in  FIG. 1 . 
       FIG. 2  is a top view of the distributed coil stator for external rotor motors shown in  FIG. 1 . 
       FIG. 3  is another top view of the distributed coil stator for external rotor motors shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION 
   With reference to the drawings, there is shown a distributed coil stator  102  for external rotor motors in a preferred form of the invention. A rotor  104  is disposed radially outward of the stator  102  with a gap (or “air gap”)  103  in between. In another sense, the stator  102  is disposed concentrically within the rotor  104 . The stator  102  includes securing means  106 , such as one or more shafts, bearings, bolts and corresponding openings, washers and plates, etc. for securing components of the stator  102  together and securing the stator  102  for operation. The rotor  104  includes similar securing means  108  for securing components of the rotor  104  together, securing the rotor  104  for operation, and securing other parts to the rotor  104  such as one or more fan blades, pump impellers, drive gears or belts, etc. Such components typically include a plurality of sheets of metallic material (such as iron) or “laminations” that are secured together to form the stator  102  and/or the rotor  104  respectively. 
   The stator  102  is generally cylindrical in shape with a top end surface and a bottom end surface. The stator  102  also includes various slots  110 ,  114 ,  118  positioned along concentric cross-sectional circular paths  111 ,  115 ,  119  and extending longitudinally along the stator  102 . The slots  110 ,  114 ,  118  typically also include openings  116  (e.g., linear) extending radially inward from the cylindrical exterior surface to a wider opening along the circular path  111 ,  115 ,  119 , which can facilitate inserting and supporting coil wiring. Each set or circular path  111 ,  115 ,  119  of slots  110 ,  114 ,  118  may have different characteristics such as different sizes (e.g., dimensions) and/or shapes. These characteristics are varied in order to vary the number of slots disposed in the stator  102  (which corresponds to the number of magnetic poles of the stator  102  and therefore to the revolving speed of the rotor  104  during operation) to provide various revolving speed options using a stator  102  of the same diameter. Furthermore, these characteristics are varied in order to produce an equivalent amount of magnetic flux among each set of slots  110 ,  114 ,  118  to insure desirable (e.g., proper, efficient, etc.) motor operation. 
   Coils (or windings)  120 ,  124 ,  128  of an insulated electrical conductor extend through the slots  110 ,  114 ,  118  respectively. Since the slots  110 ,  114 ,  118  are separated along the circular paths  111 ,  115 ,  119 , the coils  120 ,  124 ,  128  extending through them are distributed and are not bundled or overlapping about the periphery of the stator  102 . Each set of coils  120 ,  124 ,  128  may have different characteristics such as a different number of wraps or “turns” through the slots  110 ,  114 ,  118  respectively and/or a different conductor size (e.g., wire gauge). These characteristics are also varied in order to produce an equivalent amount of magnetic flux among each set of slots  110 ,  114 ,  118  to insure desirable motor operation. Each set of coils  120 ,  124 ,  128  ends or terminates at an external connection of conductors or leads  129  that extend from the stator  102  and can be connected to a source of three-phase power such as a three-phase power system or supply. 
   Each circular path  111 ,  115 ,  119  of slots  110 ,  114 ,  118  and the respective coils  120 ,  124 ,  128  extending through them corresponds to one phase of a three-phase power source connection to the stator  102  via the leads  129  as part of an external rotor motor. For example, the outermost circular path  111  of slots  110  and respective coils  120  can correspond to a first phase (e.g., phase “1” or “A”) of a three-phase power source connection. Similarly, the middle circular path  115  of slots  114  and respective coils  124  can correspond to a second phase (e.g., phase “2” or “B”) of a three-phase power source connection. And finally, the innermost circular path  119  of slots  118  and respective coils  128  can correspond to a third phase (e.g., phase “3” or “C”) of a three-phase power source connection. The corresponding phases may be different depending on the connection of the leads  129  to a three-phase power source. 
   The rotor  104  includes slots  130  positioned along a circular path (e.g., its periphery) and extending longitudinally along the rotor  104 . Rotor bars  131 , typically metallic conductors such as die-cast aluminum bars, extend through the slots  130 . These rotor bars  131  are typically connected together at the ends (i.e., short-circuited) by one or more “end-rings” (not shown), which are also typically metallic conductors such as die-cast aluminum bars. 
   In operation, an external rotor motor including the above described stator  102  and rotor  104  typically operates on the principles of a three-phase alternating current (“AC”) induction motor. That is, three-phase power is supplied to the coils  120 ,  124 ,  128  of the stator  102  via the leads  129 . The flow of current through the coils  120 ,  124 ,  128  induces magnetic flux in the stator  102 . The magnetic flux crosses the air gap  103  and interacts with the shorted rotor bars  131  to create a force that moves the rotor  104  about its rotational axis for example as indicated by the rotation mark  140 . The rotation  140  of the rotor  104  can be reversed by altering the polarity of the three-phase power supplied to the coils  120 ,  124 ,  128 , for example by switching two of the three connections at the leads  129 . 
   In contrast to an internal rotor motor that includes a traditional “peripheral coil” stator, a motor that includes the distributed coil stator  102  offers various benefits such as the following. Less coil wire (which is typically copper) is needed since there is no bundling or cross-over of the coils at the periphery of the stator (for example, where a traditional stator might include 150-turns of coil wire through the slots for each phase, a distributed coil stator with comparable operating capabilities might include 120-turns of coil wire through the outermost circular path of slots, 100-turns of coil wire through the middle circular path of slots, and 80-turns of coil wire though the innermost circular path of slots). Less copper and steel are needed in components of the motor to dissipate heat produced during operation of the motor since the coils are distributed over a larger surface area of the stator and do not bundle or cross-over. Higher electro-mechanical operation efficiency is obtained since the distributed coil stator design decreases magnetic flux losses by avoiding coil cross-over. More speed options can be designed into the same diameter stator since characteristics of the slots (such as the size and/or shape) can be varied (e.g., for each phase) to vary the number of slots that are disposed in the stator and thereby the number of magnetic poles induced in the stator. 
   Because of the different flux paths, each coil will offer different reactance and hence the current flow will vary in each of these coils. This situation not desirable, such imbalance will produce flux that is not equal in strength as produced in convention motor where the slots are of similar shape and size. This flux imbalance can cause noise and vibration in the motor. However this issue can be resolved easily by designing each coil such that they produce equal flux by selecting correct number of turns in each coil and proper gauge of wire. Therefore winding of each coil, unlike conventional motor will have different number of turns in order to create flux that is similar in strength in each of the phase coils when passing through the air gap. 
   For typical conventional motor we may have 150 turns in each of the coils  120 ,  124 , and  128 . While in case of three layer slots winding of the my proposed design we may have 120 turns in coil  120 , 100 turns in coil  124  and 80 turns in coil  128 . Coils  124  and  128  will have less resistance and will allow more current to produce more flux to overcome the losses caused by in the longer iron path. 
   It is thus seen that a distributed coil stator for external rotor electric motors is now provided to reduce coil wire usage by avoiding bundling and overlap, lower heat generation to decrease copper and metal materials needed for dissipation, decrease magnetic losses resulting in higher electro-mechanical efficiency, and offer more motor speed options using the same stator diameter. It should be understood that the foregoing descriptions merely relate to exemplary, illustrative embodiments of the invention. Furthermore, various elements of the described exemplary embodiments may be known in the art or recognized by one of ordinary skill in the art based on the disclosure herein. Therefore, it should also be understood that various modifications may be made to exemplary embodiments described herein that are within the spirit and scope of the invention as set forth in the following claims.