Patent Publication Number: US-7211920-B2

Title: Field assemblies having pole pieces with axial lengths less than an axial length of a back iron portion and methods of making same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of application Ser. No. 10/934,334, titled, Field Assemblies and Methods of Making Same, filed, Sep. 3, 2004 now U.S. Pat. No. 7,078,843, which claims the benefit of U.S. Provisional Application Nos. 60/500384, filed on Sep. 5, 2003, and 60/546243 filed on Feb. 20, 2004. The disclosures of the above applications are incorporated herein by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to dynamoelectric machines, and more particularly, to fields for dynamoelectric machines and methods of making them. 
   BACKGROUND OF THE INVENTION 
   Dynamoelectric machines are machines that generate electric power or use electric power. Common types of dynamoelectric machines are alternators, generators, and electric motors. 
   Electric motors are used in a wide variety of applications involving power tools such as drills, saws, sanding and grinding devices, and yard tools such as edgers and trimmers, just to name a few such tools. These devices all make use of electric motors having an armature and a field, such as a stator. 
     FIG. 1  shows a typical prior art stator  100  for an electric motor. Stator  100  is formed from a lamination stack  102  around which a plurality of windings of magnet wires  104  are wound to form field coils  114 . Lamination stack  102  is formed by stacking together an appropriate number of individual laminations  108  and welding them together. The individual laminations  108  are typically made by stamping them from steel. To do so, loose laminations  108  are loaded in a stacker. The stacker picks up the appropriate number of laminations  108  and places them in a fixture where they are welded together. The laminations  108  are formed with slots so the resulting lamination stack  102  has slots  110  therein in which the magnet wires  104  are wound. Magnet wires, as that term is commonly understood, are wires of the type conventionally used to wind coils in electric machines, such as armatures and stators. Prior to winding the magnet wires  104 , insulating sleeves or insulating slot liners (not shown), such as vulcanized fiber, are placed in the slots  110  and end rings  112  placed on the lamination stack  102 . End rings  112  are illustratively made of plastic and formed to include coil forms  116 . Field coils  114  are then wound by winding the magnet wires  104  in the slots  110 . After the field coils  114  are wound, the end of the magnet wires  104  are appropriately terminated, such as to terminals  118  in a terminal post  120 . The magnet wires  104  are then bonded together, such as by the application of heat when bondable magnet wires are used. Bondable magnet wires are magnet wires layered with a heat activated thermoplastic or thermoset polymer adhesive. One type of bondable magnet wires commonly used is wire available under the trade name BONDEZE from Phelps Dodge of Fort Wayne, Ind. Alternatively, the magnet wires  104  may be bonded by a trickle resin process described below. Where the stator  100  will be used in an application that exposes it to a particularly abrasive environment, such as a grinder, an epoxy coating is applied to the field coils  114  for abrasion protection. 
   There are a number of problem areas in the process just described. First of all, it is a capital intensive process. To tool a line to make a stator for a fractional horsepower motor that has a six second cycle time typically requires an investment in the three to five million dollar range. The insulating slot liners must be positioned correctly to meet U.L. (Underwriters Laboratories) requirements and kept positioned properly. In the existing process, the paper slot liners can move when the stator moves to the next station in the process. 
   The end ring limits slot fill. Slot fill is the amount of magnet wires that can be placed in the slots. The greater the slot fill, the higher the magnetic field generated by the stator. However, increasing the amount of magnet wires placed in the slots can cause the end ring to deform. The end ring can be thickened to reinforce it, but this reduces the slot volume available for the magnet wires. 
   In the manufacturing process for the stator described above, once the magnet wires have been wound in the slots and the ends of the magnet wires terminated, the magnet wires are bonded if bondable wire is being used and a “trickle” resin is applied over the magnet wires if trickle resin is being used. The process of applying the trickle resin is a somewhat difficult process to manage to obtain consistent results. It also has a number of drawbacks, not the least of which is the cost and difficulty of performing it with reliable, consistent results. 
   Initially, the trickle process requires the use of a relatively large and expensive oven to carefully preheat the partially assembled stators to relatively precise temperatures before the trickle resin can be applied. The temperature of the trickle resin also needs to be carefully controlled to achieve satisfactory flow of the resin through the slots in the lamination stack. It has proven to be extremely difficult to achieve consistent, complete flow of the trickle resin through the slots in the lamination stack. As such, it is difficult to achieve good flow between the magnet wires with the trickle resin. A cooling period must then be allowed during which air is typically forced over the stators to cool them before the next manufacturing step is taken. Further complicating the manufacturing process is that the trickle resin typically has a short shelf life, and therefore must be used within a relatively short period of time. This requires that batches of the trickle resin be mixed frequently with any that isn&#39;t used within its shelf life wasted. 
   The end result is that stators must often be designed for the process as opposed to optimum performance and cost. 
   SUMMARY OF THE INVENTION 
   In an aspect of the invention, a field assembly, such as a stator, for a dynamoelectric machine made in accordance with an aspect of the invention has field coils that are wound to a net shape. Lead wires are brought out from the ends of each field coil. The field coils are insulated. The field coils are assembled with stator core pieces, such as pole pieces and return path pieces, into the stator. The stator core pieces are formed prior to being assembled with the field coils. In an aspect of the invention, the pole pieces and return path pieces are separately formed and then assembled together with the field coils, which have also been separately formed. 
   In an aspect of the invention the pole pieces have an axial length shorter than an axial length of the return path pieces. In an aspect of the invention, axially opposed ends of the pole pieces decrease in width. 
   In an aspect of the invention, separately formed pole pieces are mated with a continuous back iron portion by dovetail features with radial outer ends of the necks of the pole pieces having a dovetail feature and the back iron portion having corresponding recesses in which the dovetail features are received. Separately formed field coils are placed over the necks of the pole pieces. The dovetail features of the necks of the pole pieces are then inserted into the corresponding recesses of the back iron portion and the pole pieces slid into place. The pole pieces are then secured to the back iron portion. 
   In an aspect of the invention, the field coils are wound from turns of bondable magnet wire that are heated after a field coil is wound into the predetermined shape to bond the turns of magnet wire together. 
   In an aspect of the invention, the field coils are insulated by encapsulating them in an encapsulation material. In an aspect of the invention, the field coils are encapsulated by molding plastic around them. In an aspect of the invention, the plastic is a thermally conductive plastic. In an aspect of the invention, the field coils are encapsulated by coating them with an elastomeric material, such as liquid silicon rubber, such as by molding the rubber around them. 
   In an aspect of the invention, the field coils are insulated by coating them with a trickle resin or an epoxy coating. In an aspect of the invention, the field coils are insulated by wrapping insulating tape around them. In an aspect of the invention, the field coils are insulated with an insulating sleeve or insulating slot liner. In an aspect of the invention, the field coils are insulated by wrapping a slot liner made of a layer of insulation material around the portions that are disposed between the pole pieces and return path pieces, the layer of insulation material having a B-stage thermoset adhesive or a thermoplastic adhesive on one or both surfaces. 
   In an aspect of the invention, the stator core pieces include pole pieces having pole tips and a field coil is placed on each pole piece with the field coil extending beyond edges of pole tips of the pole piece. In an aspect of the invention, a motor using a stator so formed has increased power compared to a motor using a conventionally formed stator of the same diameter. In an aspect of the invention, a hand-held power tool has such a motor. In an aspect of the invention, the thickness of the field coils is decreased compared to a motor having a conventional stator, allowing for the use of a larger diameter armature with a resulting increase in motor power. In an aspect of the invention, the field coils are compressed when they are formed. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a perspective view of a prior art stator; 
       FIG. 2  is a flow chart of a method for forming a stator in accordance with an aspect of the invention; 
       FIG. 3  is an exploded assembly view of a stator formed in accordance with the method of  FIG. 2 ; 
       FIG. 3A  is a perspective view of a slot liner; 
       FIG. 3B  is a top view of an electric motor made using the stator of  FIG. 3 ; 
       FIG. 3C  is an exploded assembly view of a variation of the stator of  FIG. 3  in accordance with an aspect of the invention; 
       FIG. 3D  is a perspective view of assembled pole pieces and return path pieces of the stator of  FIG. 3C ; 
       FIG. 3E  is a perspective view of a portion of a field assembly in accordance with a variation of stator of  FIG. 3C ; 
       FIG. 3F  is an exploded assembly view of a variation of the stator of  FIG. 3  in accordance with an aspect of the invention; 
       FIG. 3G  is a end view of a pole piece shown in  FIG. 3F ; 
       FIG. 3H  is a perspective view of a portion of a field assembly in accordance with a variation of the stator of  FIG. 3F ; 
       FIG. 3I  is an end view of a variation of the stators of  FIGS. 3A–3H ; 
       FIGS. 4A–4C  are perspective views of a stator being assembled in accordance with an aspect of this invention; 
       FIGS. 5A–5E  are side section views of stator return path and pole pieces with mating features in accordance with an aspect of this invention; 
       FIG. 5F  is a side section view of a pole piece and field coil with portions of the pole piece staked over the field coil; 
       FIGS. 6A–6C  are perspective views of a mold used to encapsulate a field coil in accordance with an aspect of the invention, a coil prior to molding and a field coil after molding; 
       FIGS. 7A and 7B  are side section views of a variation of the stator of  FIG. 4  in accordance with an aspect of the invention; 
       FIG. 8  is a cross-section of a power tool having a stator in accordance with an aspect of the invention; 
       FIG. 9  is a front perspective view of an insulating sleeve for insulating field coils of a stator in accordance with an embodiment of the invention; 
       FIG. 10  is a rear perspective view of the insulating sleeve of  FIG. 9 ; 
       FIG. 11  is a perspective view of a field coil/insulating sleeve assembly using the insulating sleeves of  FIGS. 9 and 10 ; 
       FIG. 12  is a perspective view of the field coil/insulating sleeve assembly of  FIG. 11  assembled on a pole piece; 
       FIG. 13  is a perspective view of an insulating slot liner in accordance with an embodiment of the invention; 
       FIG. 14  is a side view of the insulating slot liner of  FIG. 13 ; 
       FIG. 15  is a cross-sectional view of a stator in accordance with an embodiment of the invention in which field coils are insulated by the insulating slot liner of  FIGS. 13 and 14 ; 
       FIGS. 16A and 16B  are side and front view of an insulating slot liner in accordance with an embodiment of the invention that is a variation of the insulating slot liner of  FIGS. 13 and 14 ; 
       FIG. 17  is four pole stator formed in accordance with an embodiment of the invention; 
       FIG. 18  is a perspective view of a variation of the insulating sleeve of  FIG. 9 ; 
       FIG. 19  is a side section view of stator core pieces having a coating of insulation; 
       FIG. 20  is an isometric view of an insulating slot liner made of a layer of insulation material with a B-stage thermoset adhesive or a thermoplastic adhesive thereon; 
       FIG. 21  is a side view of a field coil insulated with the insulating slot liner of  FIG. 20 ; and 
       FIG. 22  is a perspective view of a field having the field coil of  FIG. 21 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. 
   Referring to  FIGS. 2 and 3 , a process for making a field assembly, stator  300  in this instance, in accordance with an aspect of the invention is shown. At step  210 , a coil, such as coil  614  ( FIG. 6B ), for field coils  304  of stator  300  is wound to a predetermined shape, preferably net shape, by winding magnet wires  303  to the predetermined shape. “Net shape” means the final shape of the field coils  304  in an assembled stator  300 . At step  212 , the magnet wires  303  are bonded together. The magnet wires  303  are preferably bondable magnet wires, such as BONDEZE wires, having a layer of heat activated thermoplastic or thermoset adhesive thereon and heat is applied to the formed coil  614  to activate the adhesive on the magnet wires  303  to bond them together. It should be understood that the magnet wires can be bonded when the coil is still in the winding tooling or after it has been removed from the tooling. An advantage of bonding the wires when the coil is still in the winding tooling is that it assures that the coil maintains its shape when it is removed from the tooling. The coils may also be compressed during bonding. The bonded coil  614  is then tested at  213 . 
   Field coils  304  have coil ends  305  with lead wires  302  extending therefrom which are brought out at step  214  from the formed coil  614 . Lead wires  302  can be brought out using different alternatives. Coil ends  305  may illustratively be terminated at terminals  307  and lead wires  302  attached to the terminals  307 . Lead wires  302  can be attached directly to coil ends  305 . Lengths of coil ends  305  can be insulated by various methods, such as shrink tubing, various wall thickness TFE or PTFE tubing, and the insulated lengths provide the lead wires  302 . The use of tubing, such as TFE or PTFE tubing, in addition to insulating the coil ends  305 , further provides the advantages of strain relief and added rigidity to lead wires  302 . Sliding tubing such as TFE or PTFE tubing over the coil ends  305  shields them and the tubing can be retained by any type of end termination. 
   At step  216 , the formed coil  614  is insulated to form field coil  304 . The formed coil  614  can be insulated by encapsulating it with an encapsulation material  309  that forms an encapsulation  313 . The encapsulation material  309  is illustratively an elastomeric thermoplastic or thermoset plastic, such as thermoset liquid silicon rubber. Encapsulation material  309  is illustratively injection molded around field coils  304 . It should be understood that other processes and materials can be used to encapsulate the formed and bonded coils with encapsulation material  309 , such as transfer molding or spraying the encapsulation material  309 . The encapsulation material could also be a more rigid thermoset. The encapsulation material may illustratively be thermally conductive and could also be a more rigid type of thermally conductive plastic, such as a Konduit® thermoplastic commercially available from LNP Engineering Plastics of Exton, Pa. The encapsulation material may illustratively be applied using the known vacuum impregnation process. The formed field coil  614  would be placed in a vacuum chamber and the encapsulation material wicks onto the field coil  614 . 
   Encapsulating the field coils  304  with the appropriate encapsulating material enhances abrasion protection and improves tracking resistance. Some types of power tools, such as grinders that are used to grind metal and remove mortar between bricks (called tuck pointing), generate a lot of abrasive particles that are drawn into the motor during operation and thus pass over the stator and rotor coil windings. These particles abrade the insulation of the wire, and also tends to abrade the extra trickle varnishes or slurries that may be used to coat the coil windings. Eventually, the wires electrically short and the motor burns up, resulting in an inoperable power tool. Tracking is a condition where an alternate conductive path is created outside the motor, thus carrying electrical current where it normally doesn&#39;t go, such as outside of the motor windings. This path is normally created by metal debris drawing into the motor during operation of the power tool that collects in the tool housing and contacts exposed elements of the electrical system of the power tool, such as brush boxes, exposed motor field windings, and lead wires. 
   Silicon rubber, such as liquid silicon rubber, is one such encapsulating material that can be used to enhance abrasion protection and improve tracking resistance. Silicon rubber is an elastomeric material and cushions the particles drawn into the motor when the particles impact it. Using a grade of silicon rubber with an appropriate durometer gives a desirable balance of functionality in terms of mechanical strength, abrasion resistance, tear resistance, and manufacturability. Illustratively, the liquid silicon rubber has a durometer in the range of 40 to 70 Shore A, and illustratively greater than about 50, and a high tear strength, that is, a tear strength of 200 pounds per inch or greater. It should be understood that other elastomers having comparable properties can also be used as the encapsulating material. The silicon rubber, or similar elastomers, can be applied by various means in addition to injection molding, such as spray-on, brush-on and compression molding and can be cured by any appropriate method, such as heat cure, room temperature cure, moisture cure and UV light cure. 
   Alternatively or in addition to encapsulating the field coils, insulating slot liners, such as slot liner  322  ( FIG. 3A ), can be placed in the slots of the stator core between pole pieces  308  and inner surfaces of return path pieces  310 . Such a slot  503  is shown more specifically in the embodiment of  FIG. 5A  between pole pieces  404  and an inner surface  505  of return path pieces  402 . The insulating slot liners may illustratively be known types of insulating slot liners, such as those made of fiber or rag-polyester. 
   Insulated field coils  304  are assembled with stator core pieces  306  to form stator  300 . Stator core pieces  306  include pole pieces  308  and back iron or return path pieces  310 . 
   Stator core pieces  306  are formed at step  220  out of steel laminations, as discussed above. In this regard, the laminations can be stacked and bonded together, such as by welding, or the laminations  706  ( FIGS. 7A and 7B ) stamped with interlocks, such as interlocks  704  ( FIGS. 7A and 7B ), which interlock the laminations together as the laminations are stamped. Each core piece  306  may illustratively be seam welded separately across its laminations to strengthen it during handling, assembly of stator  300  and during operation of the motor in which stator  300  is used. Stator core pieces  306  can also be made by molding or pressing them out of an iron powder, illustratively, insulated iron powder, such as a sulfate coated iron powder. One such sulfate coated iron powder is SOMALOY™ 500 available from Höganäs AB of Sweden through its U.S. subsidiary, North American Höganäs, Inc., 111 Hoganas Way, Hollsopple, Pa. 15935-6416. It should be understood that stator core pieces  306  could also be formed from other iron powders that can be pressed or molded, such as sintered iron powder. 
   It should be understood that forming the stator core pieces  306  is illustratively carried out independently of forming field coils  304  and vice versa. Consequently, stator core pieces  306  and field coils  304  can be made on separate lines and stockpiled until needed. It also allows the geometry of field coils  304  and stator core pieces  306  to be optimized. Moreover, pole pieces  308  are illustratively made separately from return path pieces  310 . This allows the geometry of the pole pieces  308  and the return path pieces  310  to be separately optimized. Preferably, the pole pieces  308  are identical as are the return path pieces  310  and the field coils  304 . 
   Each pole piece  308  illustratively has a neck  311  with a rectangular outer base  312  and an intermediate portion  315 , illustratively recessed at  317 , that connects the outer base  312  with an inwardly opening arcuate cylindrical pole tip section  314  thereon having pole tips  318 . Each return path piece  310  is illustratively semi-cylindrical with opposed ends  316  shaped to attach to one or both of the opposed ends  316  of the other return path piece  310  and the rectangular outer bases  312  of pole pieces  308 . In assembling encapsulated field coils  304  and stator core pieces  306 , encapsulated field coils  304  are placed over the necks  311  of respective pole pieces  308 . Return path pieces  310  are then secured to pole pieces  308 , such as by snapping together, welding, riveting, with screws, forming operations, or the like. 
   An armature, such as armature  352  ( FIG. 3B ) is then placed in stator  300  in making an electric motor, such as electric motor  350  ( FIG. 3B ). 
   The process just described provides a number of advantages. A relatively simple, inexpensive machine can be used to wind the field coils  304 . Moreover, multiple magnet wires can be wound at the same time to form the field coils  304 . It also provides for a higher slot fill factor (total area of wire in the winding slot, including wire insulation, divided by available or total area of the winding slot), particularly when the wires of the coils are compressed during bonding. Looked at a different way, it provides for denser field coil that has a higher packing factor (total area of the wire, including wire insulation, divided by the area of the envelope of the field coil defined by the inner and outer perimeters of the field coil). 
   Compressing the wires during bonding improves bonding by assuring that adjacent wires of the coil are firmly together resulting in increased bond strength. Also, by pressing the wires of the coil together, many of the voids from the winding process are eliminated. This reduces or eliminates air pockets in the coil resulting in improved heat transfer because the inner wires of the coil are in direct contact with the outer wires, which are exposed to airflow when the motor is in operation. The resistive heat generated during operation of the motor can thus be dissipated through the coil quicker by being conducted through adjacent wires rather than convection through an air pocket. Finally, by compressing the wires of the coil together, a higher slot fill factor and packing factor can be achieved compared to conventional winding techniques. This allows for more turns of wire or equal turns of larger gauge (thicker) wire than provided by conventional winding techniques. Field coils having packing factors of greater than sixty, seventy, eighty and up to about eighty five percent can be achieved with this process. 
   In an aspect of the invention, multi-stranded wire is used to wind the field coils  304  which also provides for more slot fill. A commercially available wire of this type is commonly known as litz wire. 
   In an aspect of the invention, multiple magnet wires having different functions and, illustratively, different sizes, can be wound to form the field coils  304 . For example, eighteen gauge magnet wire can be wound in each field coil  304  to form one or more coils that are energized to provide the magnetic field that interacts with the armature of the electric motor to rotate the armature. Twenty-one gauge wire can be wound in each of field coils  304  to form coils that are energized to brake the armature. In this regard, the magnet wires of different sizes are wound sequentially, that is, first one size of magnet wire is wound and then the second size of magnet wire is wound, or they are wound at the same time. The twenty-one gauge wire is illustratively wound with more turns than the eighteen gauge wire to produce the needed amount of flux to brake the armature quickly. 
   Forming the field coils  304  into predetermined shape(s), such as by winding them to pre-determined shape(s), and then bonding the magnet wires  303  allows the field coils  304  to be wound so that they extend beyond edges  320  of pole tips  318  of pole pieces  308  when field coils  304  are assembled in stator  300 . That is, the field coils  304  can extend beyond the edges  320  of pole tips  318  of pole pieces  308 . Also, the coil forming step allows the field coils  304  to be formed more compactly, as discussed, and thinner. By being able to form the field coils  304  so that they extend beyond edges  320  of pole tips  318  of pole pieces  308  and be more compact, applicants have determined that at least ten percent more output power can be achieved as well as providing better thermal characteristics for a given size field. For example, applicants found that an electric motor having a 59 mm diameter stator made in accordance with the invention has about thirty-six more percent output power than an electric motor having a 59 mm diameter conventionally made stator. This also permits a smaller diameter stator to be used for a given amount of output power. For example, applicants found that an electric motor having a 55 mm diameter stator formed according to the invention has about the same output power as an electric motor having a 59 mm diameter conventionally formed stator. 
   Forming the field coils  304 , illustratively into net shapes, and then assembling the field coils to the pole pieces also allows the overall diameter of stator  300  for a given diameter motor to be kept the same but allows a larger diameter armature to be used. As is known, the maximum motor performance measured by cold or hot max watts out increases as the size of the armature increases. More specifically, as the diameter of a motor armature increases, the power of a motor goes up by the square of the armature diameter. But with conventional motors, every incremental increase in the diameter of the armature results in a corresponding increase in the diameter of the stator and thus of the motor. A motor using a stator made in accordance with the invention discussed above and as further discussed below allows the windings of the field coils, such as field coils  304 , to be packed more tightly. It also allows them to be packed more thinly which in turn allows the thickness of the stator core pieces to be reduced. Packing the windings of the field coils  304  thinner allows, as discussed above, the diameter of the motor to be reduced or a larger diameter armature used for a given diameter motor. The above motor having a 55 mm diameter stator constructed in accordance with this invention (which is also the diameter of the motor) for use in a small angle grinder provides a power output of about 1000 W. To achieve a power output of 1000 W using a conventional stator requires a 59 mm stator. 
   Using the above referenced motor with the conventional 59 mm diameter stator as an example, which has field coils wound about the pole tips of the poles by a needle-winder as is conventional, this motor has a total slot area for the field coils (slot area being the area in which the field coils can be disposed which in the case of the conventional needle-wound field is limited by the width or arc of the pole tips of the poles) of about 90 mm 2  and radial dimensions as follows: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               Armature radius: 
               17.5 mm 
             
             
                 
               Airgap 
                0.5 mm 
             
             
                 
               Field coil thickness: 
                6.5 mm (includes thickness of pole tip) 
             
             
                 
               Back iron thickness: 
                 5 mm 
             
             
                 
                 
             
             
                 
               (The air gap is the gap between the field coils or faces of the pole tips, whichever is closer to the armature, and the armature.) 
             
          
         
       
     
   
   The above referenced motor with the 55 mm diameter stator made in accordance with this invention where the field coils  304  can extend beyond the edges  320  of the pole tips  318  has a total slot area for the field coils of about 100 mm 2  with the following radial dimensions: 
   
     
       
         
             
             
             
           
             
                 
                 
             
           
          
             
                 
               Armature radius 
               17.5 mm 
             
             
                 
               Airgap 
                0.5 mm 
             
             
                 
               Coil thickness 
                4.5 mm (includes thickness of pole tip) 
             
             
                 
               Back iron thickness 
                 4 mm 
             
             
                 
                 
             
          
         
       
     
   
   The armature winding in both cases is eight turns of 0.52 mm wire and winding of each field coil in both cases is sixty-two turns of 0.75 mm wire. 
   Alternatively, a 59 mm diameter stator constructed according to this invention could be used allowing for the diameter of the armature to be increased 4 mm, with a commensurate increase in power. 
   Table 1 below shows the armature OD, Field OD, Armature OD/Field OD ratio, and power output at 38,000 RPM for conventional AC motors having a Field OD of 57 mm and 59 mm and Table 2 below shows the same information for AC motors with fields made in accordance with the foregoing aspect of the invention having a field O.D. of 55 mm and 59 mm. 
   
     
       
         
             
             
             
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Field O.D. 
               Armature O.D. 
               Ratio 
               RPM 
               Watts 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
             
          
             
                 
               56.96 mm 
               35.19 mm 
               0.618 
               38000 
               800 
             
             
                 
               59.00 
               35.19 mm 
               0.596 
               38000 
               1000 
             
             
                 
                 
             
          
         
       
     
   
   
     
       
         
             
             
             
             
             
           
             
               TABLE 2 
             
             
                 
             
             
               Field O.D. (D f ) 
               Armature O.D. (D a ) 
               Ratio 
               RPM 
               Watts 
             
             
                 
             
           
          
             
               55.00 mm 
               35.19 mm 
               0.640 
               38000 
               1050 
             
             
               59.00 
               37.00 mm 
               0.627 
               38000 
               1600 
             
             
                 
             
          
         
       
     
   
   Referring to the AC motor having a 59 mm field O.D. as an example, as can be seen from Tables 1 and 2, the motor made in accordance with the foregoing aspect of the invention allows use of a 37 mm O.D. armature with a commensurate increase in power to 1600 Watts at 38,000 RPM compared to a conventional AC motor which utilizes a 35.19 mm O.D. armature and has a power output of 1000 Watts at 38,000 RPM. Also as can be seen from Tables 1 and 2, a motor having a 55 mm O.D. field made in accordance with this aspect of the invention allows use of a 35.19 mm O.D. armature resulting in a power output of 1050 Watts at 38,000 RPM, which is more than 1.25 times the power of an existing AC motor having a 56.96 mm O.D. field which also uses a 35.19 mm O.D. armature. In accordance with the foregoing aspect of the invention, for a given motor volume (motor outside diameter x motor length) an AC electric motor  350  ( FIG. 3B ) made in accordance with the foregoing aspect of the invention has an armature  352  and a field or stator  300  with an armature O.D. (D a ) to field O.D. (D f ) ratio of at least 0.625 which results in motor  350  having at least 1.3 times the power of an existing AC electric motor with a field having the same O.D. but with the smaller O.D. armature. The motor is also thermally balanced with the operating temperature of the field being about the same as the operating temperature of the armature at the current or power rating of the motor, such as the Underwriter Laboratories&#39; rating for the motor. 
   Forming the stator core pieces  306  separately from each other and particularly from the field coils  304  decouples an important aspect of the design and configuration of the field coils from the design and configuration of the stator core pieces  306 , the pole pieces  308  in particular. In conventional stators with needle-wound field coils, the field coils can&#39;t extend beyond the edges of the pole tips since the pole tips are used to hold the wires of the field coils during winding and before bonding or application of the trickle resin. The usable field winding area is thus defined by the width or arc (included angle) of the pole tips. While the arc of the pole tips can be increased to increase the area in which the field coils can be wound, this causes performance problems, particularly, commutation performance. Extending the arc of the pole tips too much degrades commutation. Thus, commutation performance limits the degree to which the area in which the coils are wound can be increased by increasing the arc of the pole tips. 
   In contrast, in a stator made in accordance with the invention as described above and below, such as stator  300 , the arc of the pole tips does not limit the area in which the field coils can be disposed, and thus does not limit the size of the field coils  304 . As discussed, the field coils  304  can be formed so that they extend beyond the edges  320  of the pole tips  318 . That is, the arc or included angle of the field coil is greater than the arc or included angle of the pole tips. Thus, in a two pole stator such as stator  300 , the two field coils  304  can be formed so that their respective edges are almost adjacent each other, that is, each field coil  304  has an arc (included angle) of almost one-hundred and eighty degrees, as shown representatively by field coils  614  in  FIG. 15 . Comparing the above discussed 55 mm motor having a stator made in accordance with this invention to the above discussed 59 mm motor having a conventional needle-wound stator, the pole tips of the 55 mm motor have an arc or included angle  710  ( FIG. 7A ) of 110 degrees and the field coils have an arc or included angle  712  of 158 degrees, whereas the field coils of the conventional 59 mm motor have an arc or included angle of 125 degrees which is the arc or included angle of the pole tips. Stators made in accordance with this invention can have field coils that have arcs or included angles of that are more than 100% of the arcs or included angles of the pole tips and up to about 163% of the arcs or included angles of the pole tips, such as, by way of example and not of limitation, at least 110%, 125%, 140%, 155% of the arcs or included angles of the pole tips. 
   Forming the field coils  304  before assembling them in stator  300  also provides the advantage of simplifying “leading” them. “Leading” the field coils  304  is the process of bringing out or attaching lead wires, such as lead wires  302 . In conventional stators where the field coils are needle-wound around the poles, a length of the magnet wire must be brought out from the wound coil and either attached to a terminal placed in the end ring or if used as the lead wire, terminals attached. If the magnet wire is used as the lead wire, it must be strain relieved. This process typically results in a length of wire (magnet wire, lead wire, or both) that is longer than needed for the actual lead wire which must then be routed through the stator to secure it and keep it from touching the armature when the motor in which the stator is assembled in use. In contrast, by forming field coils  304  separately from the stator core pieces  306  and before they are assembled in stator  300 , the “leading” process is simplified as it is much easier to get access to the coil since it is not in the stator. The lead wire can be attached directly adjacent the coil with little magnet wire needed to be brought out from the coil. If the magnet wire is used as the lead wire, only the length needed for the lead wire need be brought out. A further advantage is that if an unrepairable mistake is made in “leading” the field coil  304 , only that field coil  304  need be scrapped and it can be scrapped without any disassembly. In contrast, if a mistake is made in leading a field coil in a conventional stator, either the entire stator has to be scrapped or the field coils disassembled from the stator and new field coils wound, which is usually impractical if not impossible. 
     FIGS. 3C and 3D  show a variation of the above described aspect of the invention illustrated in  FIGS. 3–3B . In an aspect of the invention, the return path pieces  310  and the pole pieces  308  are formed such that the return path pieces  310  are axially longer than the pole pieces  308 . That is, an axial length X of the return path pieces  310  is longer than an axial length Y of the pole pieces  308 . Accordingly, the geometry of the field coils  304  can have a shorter axial length. Therefore, the preformed field coils  304  can be wound so that when the stator  300  is assembled the amount that the field coils  304  extend beyond ends  330  of the pole pieces  308  can be minimized. Similarly, this reduces or may even eliminate the amount that the field coils  304  extend beyond ends  332  of the return path pieces  310 . That is the field coils  304  can have an axial length Z that is less than the axial length X of the return path pieces  310 . Minimizing the amount that field coils  304  extend beyond ends  330  of field coils  304  decreases the end coil height and total volume of space consumed by the field. Alternatively, the pole tip section  314  could have the axial length Y, while the base  312  of the neck  311  of the pole pieces  308  could have an axial length greater than Y and less than or equal to X. 
   The stator core pieces  306  of this embodiment can be constructed of stacked laminations or pressed iron powder, as described above. For example, if the core pieces  306  are fabricated using stacked laminations, the pole pieces  308  would be formed to have fewer or thinner steel laminations than the return path pieces  310  so that the axial length Y of the pole pieces  308  is shorter than the axial length X of the return path pieces  310 . If the core pieces  306  are fabricated from pressed iron powder, the pole pieces  308  and return path pieces  310  would be molded or pressed so that the axial length Y of the pole pieces  308  is shorter than the axial length X of the return path pieces  310 . 
   Referring to  FIG. 3E , in accordance with an aspect of the invention, a variation of the stator  300  illustrated in  FIGS. 3C and 3D  includes a field assembly  360  having an illustratively substantially annular back iron portion  362  and at least two pole portions  364 . Field coils (not shown) are illustratively needle-wound around the pole portions  364  in a conventional manner. However, in accordance with this embodiment of the invention, the pole portions  364  have axial lengths YY that are shorter than an axial length XX of the back iron portion  362 . Therefore, the magnet wire can be wound so that the amount that the field coils extend beyond ends  366  of the pole portions  364  is minimized. This also minimizes or even eliminates the amount that the field coils extend beyond ends  368  of the back iron portion  362 . This decreases the end coil height and the total volume of space consumed by the field assembly  360 . Back iron portion  362  and pole portions  364  may illustratively be formed by stacking laminations together, with a set of laminations stamped to have the shape of the back iron portion  362  and pole portions  364  stacked with sets of laminations stamped to have the shape of the back iron portion  362  but without pole portions  364  on either side of the set of laminations stamped to have the shape of the back iron portion  362  and pole portions  364 . Alternatively, back iron portion  362  and pole portions  364  can be formed of pressed iron powder. 
     FIGS. 3F and 3G  show a variation of the above described aspect of the invention illustrated in  FIGS. 3–3B . In an aspect of the invention, axial opposed ends  330  of pole pieces  308  gradually decrease in width, as shown at  340 . In this regard, opposed ends  330  may be incrementally stepped. Illustratively, those portions of ends  330  around which the field coils, such as field coils  304 , are disposed, are the portions that gradually decrease in width. As most clearly shown in  FIG. 3G , each end  330  decreases in width from a width P at a point axially inwardly spaced from an outermost axial end of end  330  to a width Q at the outermost axial end of end  330 , wherein the width Q is less than the width P. Alternatively, instead of being stepped, ends  330  can gradually taper at  340  from width P to width Q to thereby have a substantially rounded shape at each end  330  of the pole pieces  308 . Therefore, each pole piece  308  will at its ends  330  more closely match the shape of the preformed field coils  304  which will decrease the amount that the field coils  304  extend beyond ends  330  of the pole pieces  308 . This decreases the end coil height and the total volume of space consumed by the field. As used herein in the context of the axial opposed ends  330  of pole pieces  308  gradually decreasing in width, it is meant that the opposed end  330  of each pole piece has a portion that decreases in width from width P to width Q where widths of the opposed end  330  in this portion are less than P but greater than Q. 
   The width of the pole pieces  308  at ends  330  can be stepped or tapered from width P to width Q by incrementally sizing the laminations of the pole pieces  308  or by forming the described shape from pressed iron powder. 
   Referring to  FIGS. 3C ,  3 D  3 F and  3 G, in an aspect of the invention, the pole pieces  308  having axial lengths Y shorter than the axial length X of the return path pieces  310 , as shown in  FIGS. 3C and 3D , can include ends  330  that gradually decrease in width, as shown at  340  in  FIGS. 3F and 3G . 
   Referring to  FIG. 3H , in accordance with an aspect of the invention, a variation of the stator  300  illustrated in  FIGS. 3F and 3G  includes a field assembly  375  having an illustratively substantially annular back iron portion  376  and at least two pole portions  378 . Field coils (not shown) are illustratively needle-wound around the pole portions  378  in a conventional manner. However, in accordance with this embodiment of the invention, pole portions  378  have opposed ends  382  that gradually decrease in width, as shown at  384 . Therefore, magnet wire can be wound around the pole portion  378  to form a field coil (not shown) having a reduced end coil height. Additionally, the total volume of space consumed by the field assembly  375  will be reduced. Alternatively, ends  382  can gradually taper so that each pole portion  378  has a substantially rounded shape at each end  382 . 
   Referring to  FIGS. 3E and 3H , in an aspect of the invention, pole portions  364  having axial lengths YY shorter than the axial length XX of the back iron portion  362 , as shown in  FIG. 3E , can include ends  382  that gradually decrease in width, as shown at  384  in  FIG. 3H . 
   Referring to  FIG. 3I , a stator  390  that is a variation of the stators shown in  FIGS. 3–3H  is shown. Stator  390  has a continuous annular back iron portion  391  and separately formed pole pieces  392 , only one of which is shown in  FIG. 3I . A neck  393  of each pole piece  392  includes at a radial outer end  394  a dovetail feature  395 . In this regard, radial outer end  394  of neck  393  of pole piece  392  may illustratively be formed as the dovetail feature  395 . Back iron portion  391  includes a corresponding recess  396  shaped to received dovetail feature  395 . Illustratively, each recess  396  extends the axial length of back iron portion  391  and the dovetail feature  395  of each pole piece  392  extends the axial length of that pole piece  302 . 
   Pole pieces  392  and back iron portion  391  are formed separately, such as from stack of laminations or from iron powder, as discussed above and below. Field coils, such as field coils  304 , are also separately formed, as discussed above, and placed over necks  393  of pole pieces  392 . The dovetail feature  395  of each pole piece  392  is then inserted in the corresponding recess  396  in back iron portion  391  and the pole piece  392  slid into place. Each pole piece  392  is then secured to back iron portion  391 , such as by staking, welding, riveting or the like. This variation provides the advantage of reducing part count in the stator  390  as the back iron portion  391  is one piece compared to those aspects discussed above that have multiple return path pieces, such as return path pieces  310 . It should be understood that pole pieces  392  can have axial lengths the same or less than the axial lengths of back iron portion  391 . Pole pieces  392  can also have axial ends that decrease in width. 
   Referring now to  FIGS. 3A–3I , pressing the stator core pieces  306  out of iron powder provides additional advantages to those described above. The stator core pieces  306 , the pole pieces  308  in particular, can be formed in one operation as a three-dimensional part. In contrast, in the conventional process described above, the pole pieces of the stator are made by stacking an appropriate number of laminations, in effect, stacking the appropriate number of two-dimensional pieces to arrive at the resulting three-dimensional pole piece. 
   Using insulated iron powder as the iron powder provides additional advantages in that insulated iron powder has low eddy current losses. 
     FIGS. 4A–4C  show a variation of the above described aspect of the invention. A field assembly, stator  400  in this instance, has first and second return path pieces  402 , first and second pole pieces  404 , and first and second field coils  406 . Field coils  406  are illustratively pre-formed coils encapsulated with an elastomeric encapsulation  408 . Field coils  406  are illustratively wound to the predetermined shape as described above with reference to the embodiment shown in  FIG. 3 . Illustratively, elastomeric encapsulation  408  is liquid silicon rubber, as described above. It should be understood that field coils  406  can be insulated in other manners as described above. 
   To assemble stator  400 , field coils  406  are placed over necks  414  of pole pieces  404 . Necks  414  have opposed receiving pockets  504  ( FIGS. 5A–5C ) therein between pole tip section  522  of pole pieces  404  and base portion  524  of necks  414  of pole pieces  404 . Circumferentially and radially outer edges  526  of pole tip section  522  project circumferentially outwardly to provide lips  528  (in other words, pole tip portions  522  have undercuts  527 ). Edges  526  may illustratively be recessed and have a radius as shown in  FIG. 5D  to ease the assembly of field coils  406  to pole pieces  404 . If edges  526  are sharp edges, the insulation on field coils  406  could catch and possibly be displaced from its correct position on the coil. With edges  526  having a smooth radius, the insulation on field coils  406  more freely slides onto pole pieces  404  and facilitates keeping the insulation correctly positioned on field coils  406 . 
   Field coils  406 , when encapsulated with an elastomeric encapsulation material such as liquid silicon rubber, snap over lips  528  and into undercuts  527 , which retains them in place during further assembly of stator  400 . Bumps or other interference features may illustratively be formed of the encapsulation material where the field coils abut the pole tip portions  522  to further retain the field coils  406  to the pole pieces. In a variation, lips  528  may also be staked over field coils  406  in one or more places, shown illustratively at  529 , to provide further retention of field coils  406  as shown in  FIG. 5F . 
   Ends  418  of field coils  406  may extend beyond pole tips  420  of pole pieces  404 . Return path pieces  402  are then brought in radially (laterally) and mated to the pole pieces  404 . Opposed edges  423  of radial outer ends  422  of pole pieces  404  have mating features  424  that mate with corresponding mating features  426  in edges  428  of return path pieces  402 , as described in more detail below. 
   In an aspect of the invention, field coils  406  may have mating features  410  formed in encapsulation  408 . Pole pieces  404  have corresponding mating features  412  formed therein, and in this regard, pole pieces  404  may be encapsulated with an encapsulation material with the mating features  412  formed in this encapsulation, or the mating features  412  formed directly in the soft magnetic material of which pole pieces  404  are made. Mating features  410  may illustratively be a projection or detent and mating feature  412  would then be a corresponding hole or recess. The converse could also be used—that is, mating feature  412  is the projection or detent and mating feature  410  is the corresponding hole or recess. Mating features  410  of field coils  406  and mating features  412  of pole pieces  404  mate together when field coils  406  are placed over the necks  414  of pole pieces  404 , holding each field coil  406  to a respective pole piece  404 , making coil/pole subassemblies  416 . Pole pieces  404  may illustratively be made of laminations or of iron powder, such as insulated iron powder, such as described above with reference to  FIGS. 2 and 3 . Similarly, return path pieces  402  can be made of laminations or insulated iron powder. 
   Turning to  FIGS. 5A and 5B , an embodiment of mating features  424 ,  426  is shown. Mating feature  426  of each edge  428  of each return path piece  402  is a projection  500  that extends from the respective edge  428  of the return path piece  402 , with a recess  502  at a junction of projection  500  and edge  428  of return path piece  402 . Mating feature  424  in each opposed edge  423  of each radial outer end  422  of each pole piece  404  ( FIG. 4B ) comprises receiving pocket  504  defined between outer finger  506  of base portion  524  of pole piece  404  and pole tip portion  522  of pole piece  404 . Mating feature  424  further includes outer finger  506  having a projection  510  extending radially inwardly from an outer end  512  of finger  506 . 
   Each receiving pocket  504  is illustratively larger than the projection  500  of the respective return path piece  402  so that projection  500  is easily received in the receiving pocket  504 . This is accomplished by forming finger  506  so that it is at an angle  514  with respect to projection  500 , as shown in  FIG. 5B , when projection  500  is first inserted into receiving pocket  504 . Additionally, mating radii of receiving projection  500  and receiving pocket  504  are sized so that there is always an appropriate clearance  516  between them taking tolerances into account. 
   Once the projections  500  of return path pieces  402  are inserted into receiving pockets  504  of respective pole pieces  404 , the fingers  506  of pole pieces  404  are deformed radially inwardly so that projections  510  extending radially inwardly from outer ends  512  of fingers  506  are received in recesses  502  of respective return path pieces  402 . The mating of projections  510  in recesses  502  forms mating detents  518  ( FIG. 5A ) that mechanically lock pole pieces  404  and return path pieces  402  together. Return path pieces  402  and pole pieces  404  are thus mechanically interlocked by mating detents  518  and held together by friction. Pole pieces  404  can also be welded to return path pieces  402  to further strengthen the attachment of pole pieces  404  to return path pieces  402 . Alternatively, pole pieces  404  and return path pieces  402  could just be welded together. 
     FIG. 5C  shows a variation of the mating features  424 ,  426  of  FIGS. 5A and 5B  which is almost identical to the embodiment shown in  FIGS. 5A and 5B , and only the differences will be discussed. Elements of  FIG. 5C  common with the elements of  FIGS. 5A and 5B  are identified with the same reference numbers. The difference is that the mating detent  518  is moved distally outwardly along projection  500 . This increases the “critical length” designated by reference numeral  520  compared with the length of the same segment in the embodiment shown in  FIGS. 5A and 5B . This critical length is the length of the segment of return path piece  402  and pole piece  404  through which the majority of the magnetic flux is carried. Maximizing this critical length benefits motor performance. 
   Illustratively, when return path pieces  402  are mated with pole pieces  404 , they are brought together radially shown by arrow  440  in  FIG. 4B , as opposed to axially. The return path piece  402  radially compresses respective sides of the field coils  406  mounted on pole pieces  404 . This eliminates the return path piece  402  sliding axially across the field coils  406  and the possible damage to the insulation surrounding the field coils  406  due to the return path piece  402  sliding across them. Also, the tolerances, particularly of the field coils  406 , can be somewhat looser when the return path pieces  402  and pole pieces  404  are mated by bringing them together radially as opposed to axially. 
   Making the return path pieces  402  separately from the pole pieces  404  also provides the advantage that not only can different materials, such as different magnetic grades of steel, be used to make them, but different construction techniques can be used. For example, the pole pieces  404  could be made of stacks of laminations as described above and the return path pieces made of solid steel. The pole pieces  404  would then include deformable portions that would be deformed against corresponding portions of return path pieces  402  to fasten the return path pieces  402  and pole pieces  404  together. 
   While stators  300  and  400  ( FIGS. 3 and 4 ) have been described in the context of having two poles with two return path pieces and two pole pieces, it should be understood that other configurations can be used that are within the scope of the invention. For example, only one return path piece could be used, which would illustratively be a cylindrical piece, with the two pole pieces being affixed to an inner side of the return path piece on opposite sides thereof. Each return path piece could be made of multiple pieces that are joined together, such as by welding or by forming mating features therein that snap together. The stator core pieces could also be held together by being inserted in a stator housing. The stators could also have more than two poles, such as four, six, eight or other multiples of two. In this regard, at least one pole piece would be provided for each pole and they would be spaced equidistantly around the stator. Each pole piece could be made of multiple pieces that are joined together. 
     FIG. 17  shows such a stator  1700  having more than two poles, illustratively, four poles. Stator  1700  illustratively includes four return path pieces  1702 , four pole pieces  1704  and four field coils  1706 . Return path pieces  1702 , pole pieces  1704  and field coils  1706  are all separately formed in the manner described above. Field coils  1706  are then placed over necks  1708  of pole pieces  1704  so that they abut pole tips  1710  of pole pieces  1704  and pole pieces  1704  and return path pieces  1702  mated together. 
   In an aspect of the invention, the core pieces of the stator include at least three pieces—two pole pieces and one return path piece. In an aspect of the invention, the pole pieces, return path piece or pieces and the field coils are all separately formed and then assembled together. By separately formed, it is meant that the pole pieces are formed separately from the return path piece or pieces which are in turn formed separately from the field coils. 
     FIG. 6  shows an illustrative embodiment of a mold  600  that can be used to mold the encapsulation material, such as encapsulation material  309  ( FIG. 3 ) that forms the encapsulation, particularly when an elastomeric encapsulation material such as liquid silicon rubber is used. Mold  600  has a core plate  602  having a plateau  604  from which locating posts  606  extend. On either side of plateau  604 , core plate  602  has raised pads  608  and holes  610 . Raised pads  608  are illustratively oval shaped and extend the majority of the way between plateau  604  and edges  612  of core plate  602 . Mold  600  also has a cavity plate, not shown, that mates with core plate  602  when mold  600  is closed. The cavity plate may also have raised pads  608  and holes  610 . 
   Raised pads  608  maintain coil  614  in centered spaced relation to a surface  620  of core plate  602  facilitating the flow of the encapsulating material  309  around the radial inner side  622  of coil  614 . Holes  610  result in compression tabs or projections  624  being formed in encapsulation  313  on the radial inner side  622  of field coil  304  and, if provided in the cavity plate of mold  600 , on the radial outer side  628  of field coil  304 . (For continuity, reference number  622  is used to identify the radial inner side of coil  614  and of field coil  304 ). Raised pads  608  form recesses  626  in the encapsulation  313  on radial inner side  622  of field coil  304  and, if provided in the cavity plate of mold  600 , on the radial outer side  628  of field coil  304 . In addition to providing spacing between coil  614  and core plate  602 , and the cavity plate of the mold  600  if provided on the cavity plate, raised pads  608  can also be used to thin out the walls of the encapsulation  313  that encapsulates coil  614  of field coil  304 . Compression tabs  624  provided added areas of compression between field coil  304  and the pole pieces  308  (compression tabs  624  on the radial inner side  622  of field coil  304 ) and between the field coil  304  and the return path pieces  310  (compression tabs  624  on the radial outer side  628  of field coil  304 ) when field coil  304  is assembled into stator  300  ( FIG. 3 ). Compression tabs  624  are dimensioned so that they are small compared to the overall area of field coil  304  so that they provided added retention without significantly increasing the assembly interference forces when field coil  304  is assembled with stator core pieces  306  ( FIG. 3 ) to form stator  300  ( FIG. 3 ). 
   With reference to  FIGS. 6A–C , the molding of a field coil, such as field coil  304  ( FIGS. 3 and 6C ), is described. The magnet wires  303  are wound in a coil  614  ( FIG. 6B ) having a predetermined shape, which is illustratively a section of a cylinder with a central open rectangular section  616  ( FIG. 6B ), which is also the final shape of the field coil  304  as can be seen from  FIG. 6C . Coil  614  is placed in mold  600  so that plateau  604  extends through central open rectangular section  616 . Central open rectangular  616  of coil  614  is placed around locating posts  606  when coil  614  is first placed in mold  600  which assist in properly locating coil  614  on core plate  602  as coil  614  is being placed in mold  600 . Lead wires  302  are placed in slots  618  in core plate  602 , only one of which is shown in  FIG. 6A . The cavity plate of mold  600  is closed over core plate  602  and the encapsulation material  309  ( FIG. 3 ) injected into mold  600 , encapsulating coil  614  to form field coil  304  with magnet wires  303  encapsulated in encapsulation  313  made of encapsulation material  309 . 
   Coil  614  of field coil  304  can be insulated by processes other than encapsulation, such as applying a resin coating to them by using the trickle resin process, applying an epoxy coat to them by dipping the formed coil  614  in a tank of epoxy, a powder coat process where heated coil windings cure powdered epoxy on the coil wires, applying an electrically insulating foam to them, or winding insulating tape, such as electrical insulating tape or epoxy tape, around them. In one type of powder coat process, heated coils are placed in a fluidized bed of epoxy. When the coils are insulated by coating, the coating can be applied to the coils before they are assembled in the stator or after. It should also be understood that the coils may be encapsulated or coated to improve abrasion protection and tracking resistance and the coils further insulated to provide insulation between the coils and the stator core pieces, such as with insulated slot liners or winding insulating tape around the encapsulated or coated coils. 
     FIG. 7  shows a cross section of stator  400  ( FIG. 4C ) in which the field coils  700  are insulated with a layer of insulating material  702  such as insulating paper, electrical insulating tape, epoxy tape, or electrical insulating foam. Insulating material  702  is wrapped around the coils of field coils  700  in the area abutting the field laminations, such as return path pieces  402  and pole pieces  404 . 
   Such electrical insulating material, other than electrical insulating foam, is not compliant, so clearances must be left between the insulating material  702  and the field laminations, such as return and pole pieces  402 ,  404 . These clearances result in a degree of looseness of field coils  700  in stator  400 . To enhance product life and durability, these clearances need to be eliminated, or at least minimized. To do so, a compliant material  708  ( FIG. 7B ) is placed between the return path pieces  402  and the field coils  700 . Compliant material  708  may illustratively be a foam having a suitable temperature rating. Compliant material  708  may also have adhesive on one or both sides to facilitate retaining it in place during assembly of stator  400  and improve retention of field coils  700  relative to return path pieces  402 . 
   If foam is used as electrically insulating material  702  or compliant material  708 , it may illustratively be thermally conductive to enhance heat transfer. In this regard, it may contain fillers such as ceramics to increase thermal conductively. Other types of fillers can be used, such as carbon which is cheaper than ceramic, if suitable for the electrical design of the product. 
   Referring now to  FIG. 8 , a power tool  800  is shown. Power tool  800  is illustratively a hand-held power tool and is illustrated as a drill, however, any type of power tool may be used in accordance with the present invention. The power tool  800  includes a housing  802  which surrounds a motor  803 . An activation member  804  is coupled with the motor and a power source  806 , illustratively AC. The motor  803  is coupled with an output  808  via a drivetrain  810 . Output  808  includes a chuck  812  having jaws  814  to retain a tool such as a drill bit (not shown). The motor  803  includes an armature  816  and a stator  818  made in accordance with this invention, such as stator  300  or  400  ( FIGS. 3 and 4 ). 
     FIGS. 9–12  show an insulating sleeve  900  that can be used as the insulating slot liner  322  ( FIG. 3A ) and in lieu of encapsulating the field coils, such as field coils  1104  ( FIG. 11 ). For convenience, insulating sleeve  900  will be described with reference to the stator  400  of  FIG. 4 . Insulating sleeve  900  may illustratively be made of compliant material, such as liquid silicon rubber, and may illustratively be molded. Insulating sleeve  900  includes an outer section  902 , inner section  904  and a bight section  906  bridging inner and outer sections  904 ,  902  at one edge thereof. Locating or centering tabs  908  extend from opposed ends  910  of bight section  906 . An outer surface  912  of outer section  902  has laterally extending outwardly projecting compression ribs  914  formed thereon. A pocket  1000  ( FIG. 10 ) may be formed in an outer surface  1002  of inner section  904  for receiving one of the pole tips  420  of pole piece  404  ( FIG. 4 ). Outer and inner sections  902 ,  904  and bight section  906  of insulating sleeve  900  define a slot  916  in which one of sides  1102  of field coil  1104  ( FIG. 11 ) is received. 
   The use of insulating sleeve  900  is now described. In assembling the stator  400 , two insulating sleeves  900  are placed on field coil  1104  with opposite sides  1102  ( FIG. 11 ) of the field coil  1104  received in the slots  916  of the respective insulating sleeves  900  to form field coil/sleeve assembly  1100 . The width of the outer section  902  of the insulating sleeve  900  may illustratively be the same or preferably slightly greater than the width of the side  1102  of the field coil  1104  that is received in the slot  916  of the insulating sleeve  900  to insulate the field coil  1104  from an inner surface of the return path piece  402  that is adjacent the side  1102  of the field coil  1104  when the field coil  1104  is assembled in stator  400 . The width of the inner section  904  of the insulating sleeve  900  may illustratively be the same or preferably slightly greater than the width of the section of the pole tip  420  of pole piece  404  that is adjacent the side of the field coil  1104  when the field coil  1104  is assembled in stator  400  to insulate the field coil from the surface of the pole tip  420  adjacent the side of the field coil  1104 . 
   A field coil/sleeve assembly  1100  is then placed over the neck  414  of each of the pole pieces  404  and the pole pieces  404  mated with the return path pieces  402 . The pole tips  420  of each pole piece  404  are received in the pockets  1000  ( FIG. 10 ) of the respective insulating sleeves  900  disposed over the opposite sides  1102  of that field coil  1104  to aid in retaining the field coil/sleeve assembly  1100  in place. Centering tabs  908  of the insulating sleeves  900  center the pole piece  404  and the field coil/sleeve assembly  1100  with respect to each other. Compression ribs  914  compress against respective inner surfaces  434  ( FIG. 4B ) of respective return path pieces  402  and aid in securing the field coil/sleeve assembly in place in stator  400  so that the field coil/sleeve assembly  1000  will not vibrate loose during operation of the motor in which it is used, such as in power tool  800 . 
   Turning to  FIG. 18 , an insulating sleeve  1800  that is a variation of insulating sleeve  900  is shown. Insulating sleeve  1800  is also made of complaint material, such as silicon rubber, but is extruded instead of molded. Insulating sleeve  1800  includes an outer section  1802 , an inner section  1804  and a bight section  1806  bridging inner and outer sections  1804 ,  1802  at one edge thereof. An outer surface  1808  of outer section  1802  has outwardly projecting compression ribs  1810  formed thereon that extend across outer section  1802 . Outer and inner sections  1802 ,  1804  and bight section  1806  define a slot  1812  in which one side of a field coil, such as field coil  1104  ( FIG. 11 ) is received. Compression ribs  1810  allow tuning adjustments in the tool used to extrude insulating sleeve  1800  so that the retention force on the field coil, such as field coil  1104 , when it is assembled as part of a stator such as stator  400  can be optimized. 
   With reference to  FIG. 5E , edges  526  of radially outer section  521  of pole tip section  522  are recessed and have a radius at  530 . However, edges  526  are not formed to include lips  528  ( FIG. 5D ) so that a radially extending outer surface  532  of radially outer section  521  of pole tip portion  522  presents a smooth wall free of detents, lips or the like. This improves assembly when the field coils are insulated with compliant insulating sleeve  900  and insulating slots liners made of paper such as embodiments of insulating slot liners  322  ( FIG. 3A ),  1300  and  1600  (described below.) The radius  530  and the smooth wall presented by surface  532  helps prevent displacing the insulating sleeve  900  and insulating slot liners  1300 ,  1600  from their proper position around the field coils. 
   As mentioned, insulating sleeve  900  may illustratively be made of compliant material, such as liquid silicon rubber, and may illustratively be used in lieu of encapsulating the field coils. This provides the benefit of not having to insert mold the field coils with an encapsulant. Insulating sleeves  900  can be molded separately at a rate that applicants expect will be much faster than the rate at which the field coils can be wound and the mold(s) used to mold the insulating sleeves will likely be able to have more cavities than the mold(s) used to insert mold the field coils. 
     FIGS. 13–15  show an insulating slot liner  1300  in accordance with an embodiment of the invention that can be used as insulating slot liner  322  ( FIG. 3A ) and in lieu of encapsulating the field coils. Insulating slot liner  1300  includes a substrate  1302  made of insulative material, such as insulating paper, insulating plastic film, or the like having an outer section  1301  and an inner section  1303 . Illustrative materials of which substrate  1302  can be made include various grades of Nomex® paper or tape, polyester/glass fiber, polyester/rag, Nomex®/polyester, or polyester/Dacron® laminates. An inner adhesive strip  1304  is disposed on an inner surface  1306  of outer section  1301  of substrate  1302  and an outer adhesive strip  1308  is disposed on an outer surface  1310  of inner section  1303  of substrate  1302 . An outer surface  1404  ( FIG. 14 ) of outer section  1301  may also have an adhesive strip (not shown) disposed thereon as may an inner surface  1406  of inner section  1303 . Inner and outer adhesive strips  1304 ,  1308  may illustratively include non-stick overhanging cover strips  1400  ( FIG. 14 ) that can be easily removed from inner and outer adhesive strips  1304 ,  1308  during assembly. One or both of opposed upper and lower edges  1312  of substrate  1302  may illustratively be folded over cuffed edges. 
   Insulating slot liner  1300  may illustratively be “C” or “U” shaped and may illustratively be preformed so that it fits the contours of the field coils, such as field coils  614 , and radially outer surfaces  1500  ( FIG. 15 ) of pole tips  420  of pole pieces  404  of stator  400  that abut field coils  614  and inner surfaces  1502  of return path pieces  402 . This aids in adhesive retention such as between inner adhesive strip  1304  and field coil  614  and/or between outer adhesive strips  1308  and the surfaces  1500  of pole tips  420  of pole pieces  404 . This also aids in assembly. Insulating slot liner  1300  may illustratively be sized so that a distal edge  1505  ( FIG. 15 ) of outer section  1301  extends beyond a distal edge  1506  of field coil  614  and a distal edge  1508  of inner section  1303  extends beyond an outer edge  1510  of pole tip  420 . In a 59 mm. O.D. stator, this distance is illustratively a minimum of 2 mm. Cuffed edge(s)  1312  of substrate  1302  extend over axial edge(s)  436  ( FIG. 4B ) of return path piece  402  and axial edge(s)  438  ( FIG. 4A ) of pole piece  404  to locate insulating slot liner  1300  on return path piece  402  and pole piece  404  and, when both opposed edges  1312  of substrate  1302  are cuffed, to capture insulating slot liner  1300  on return path piece  402  and pole piece  404 . 
   The use of insulating slot liners  1300  is now described. In assembling the stator  400 , cover strips  1400  are removed from the inner adhesive strips  1304  of two insulating slot liners  1300  which are then placed on field coil  614  with the opposites sides of the field coil  614  received in respective ones of the insulating slot liners  1300 . If an adhesive strip is provided on inner surface  1406  of inner section  1303 , its cover strip is removed before placing the insulating slot liner  1300  over the side of field coil  614 . Inner adhesive strip  1304  secures the insulating slot liner  1300  to the side of the field coil  614  over which the insulating slot liner  1300  was placed. The cover strips  1400  are then removed from outer adhesive strips  1308  of the insulating slot liners  1300  and field coil/insulating slot liner assembly  1514  ( FIG. 15 ) placed over the neck  414  of a pole piece  404 , bringing the outer adhesive strips  1308  of the insulating slot liner  1300  into contact with the surfaces  1500  of the pole tips  420  of the pole piece  404  so that the adhesive on the outer adhesive strips  1308  contacts the surfaces  1504  of the pole tips  420 . The return path pieces  402  are then mated with the pole pieces  404 . If an adhesive strip is provided on the outer surface  1404  of outer section  1301  of insulating slot liner  1300 , its cover strip is removed before the return path piece  402  that will abut that insulating slot liner  1300  is mated to the pole piece  404 . It should be understood that while only one insulating slot liner  1300  is shown in  FIG. 15 , all of field coils  614  would be insulated with insulating slot liners  1300 , illustratively, two insulating slot liners  1300  for each field coil  614 . 
   Inner adhesive strip  1304  may illustratively be a pliable adhesive strip, such as a foam or gel strip ranging from 0.001″ to 0.250″ in thickness, to take up clearances and fill into component contours of field coil  614  to provide a robust retention force. Outer adhesive strip  1308  may also be a pliable adhesive strip. 
   Outer adhesive strip  1308  may illustratively be sized so that there is a gap between its edges and the edges of substrate  1302 , shown representatively at  1316 . That is, outer adhesive strip  1308  is smaller than the outer surface  1310  on which it is disposed. By having a gap between the edges of substrate  1302  and outer adhesive strip  1308 , that is, sizing outer adhesive strip  1308  so that it is smaller than the outer surface  1310  on which it is disposed, the adhesive on outer adhesive strip is completely covered by inner surface  1502  of return path piece  402  when insulating slot liner  1300  is assembled in stator  400 . This minimizes or eliminates any dust or chips contacting the adhesive on outer adhesive strip  1308  and being retained thereon. Similarly, inner adhesive strip  1304  may illustratively be sized so that it is smaller than the inner surface  1306  of substrate  1302  on which it is disposed. It should be understood that the insulating slot liner  1300  could have multiple inner and outer adhesive strips  1304 ,  1308 . 
   The inner and outer adhesive strips  1304 ,  1308  of the insulating slot liners  1300  serve three purposes. They retain the field coils  614  to the return path pieces  402  and pole pieces  404  and prevent slippage between field coils  614  and the return path pieces  402  and pole pieces  404 . They act as a secondary support to hold together the windings of field coil  614 . They also act as a secondary support to hold together the return path piece  402  and the pole piece  404 . 
   The thickness of the substrate  1302  of insulating slot liner  1300  may illustratively be optimized to take up clearances thus keeping the assembly of the field coils  614  and the return path and pole pieces  402 ,  404  tight and keeping pressure on inner and outer adhesive strips  1304 ,  1308  as they contact field coils  614  and the inner surfaces  1502  of return path pieces  402 , respectively. In a 59 mm O.D. stator  400 , the optimum thickness of substrate  1302  is in the range of 0.002″ to 0.030″. The distal edge  1505  of outer section  1301  may also be folded over as shown at  1402  in  FIG. 14 . Doing so helps take up clearances, increases the interference in a localized area for a tight fit in that localized area. It may also allow a thinner, better conforming, lower cost paper to be used for substrate  1302 . 
   Certain materials, such as some types of insulated paper, that can be used for substrate  1302 , have a smooth surface on one side and a rough surface on the other side. For these materials, insulating slot liner  1300  may illustratively be formed so that the smooth surface is the outer surface of substrate  1302  that contacts the surfaces  1500  of pole tips  420  and inner surfaces  1502  of return path pieces  402  to facilitate assembly. 
   As shown in  FIG. 15 , field coil  614  could in an alternative embodiment be insulated with a full wrap of insulated material, such as insulated paper, as shown in phantom at  1512 . This reduces the likelihood of the insulated paper curling up into the armature of the motor in which stator  400  is used and prevents slippage of the insulated paper during assembly. 
     FIGS. 16A and 16B  show an insulating slot liner  1600  which is a variation of insulating slot liner  1300 . Like elements will be identified with the same reference numbers and only the differences will be described. Insulating slot liner  1600  includes compliant material  1602  disposed on inner and outer surfaces  1406 ,  1310  of inner section  1303  and inner and outer surfaces  1306 ,  1404  of outer section  1301  of substrate  1302 . The compliant material  1602  provide an interference between the substrate  1302  of the insulating slot liner  1600 , the field coil, such as field coil  614  ( FIG. 15 ), and the return path pieces  402  and pole pieces  404 . It should be understood that compliant material  1602  can be disposed on one as opposed to both of the inner and outer surfaces  1306 ,  1404  of outer section  1301  of substrate  1302  and on one as opposed to both of the inner and outer surfaces  1406 ,  1310  of inner section  1303  of substrate  1302 . It should also be understood that compliant material  1602  can be strips of complaint material, beads or other shapes. It should further be understood that complaint material  1602  can be any suitable complaint material, such as compliant polymers such as silicon, resins, foams or epoxies. 
   Alternatively or in addition to encapsulating the field coils and or using insulating slot liners, the stator core pieces  306  or appropriate portions of the stator core pieces can be encapsulated or covered with an encapsulating or coating material, such as thermoplastics and thermosets, which may illustratively be thermally conductive or not. By way of example and not of limitation, the stator core pieces  306  (or appropriate portions of them) can be covered with an epoxy coating that is either sprayed on or applied using an electrostatic coating process. With reference to  FIG. 19 , a layer  1900  of insulation is applied to surfaces  1902  of pole tip portion  522  of pole piece  404  and to radially inner facing surfaces  1904  of return path pieces  402  (only one of which is shown in  FIG. 19 ). 
   Referring to  FIGS. 20 and 21 , an insulating slot liner  2000  that is a variation of insulating sleeve  1300  ( FIG. 13 ) is shown. Insulating slot liner  2000  is made of a layer of insulation material, such as one of the above referenced insulation papers, having both sides or surfaces coated with a thin layer of a B-stage thermosetting adhesive, such as VonRollIsola 6001 (phenolic) or 6351 (epoxy). A B-stage thermosetting adhesive is one that is dry to the touch and not tacky and is in a state to be cured by the application of heat. An insulating slot liner  2000  is wrapped around each portion of a field coil that is disposed between a pole piece and a return path piece mated to that pole piece. Insulating slot liner  2000  is illustratively formed with creases to contour around the field coil. Additionally, for lower temperature applications, a thermoplastic adhesive could be used, such as VonRollIsola HS2400. Moreover, pre-laminated films with adhesives could also be used, such as 3M bonding film 583 or 588 (heat or solvent cure), or 3M ENPE-365 (UV light cure). The film containing the resin is itself adhered to the insulation paper used for the slot liner. 
   In assembly, insulating slot liner  2000  is wrapped around the appropriate portion of the field coil, such as field coil  2100  ( FIG. 21 ), and secured with a thin tape, such as 0.025 mm thick acrylic adhesive tape, to form insulated field coil  2102 . The insulated field coil  2102  is then placed over the neck  414  of a pole piece  404  ( FIG. 4 ). Preferably, there will be enough pressure between the insulating slot liner  2000  and pole piece  404  to hold the two together during assembly of stator  400 . If not, a temporary adhesive may be used, such as thin double-sided taped, one or two part adhesives, and UV light cure adhesives. 
   The thickness of the material, such as insulating paper, used for insulating slot liner  2000  is chosen so that there is a slight pressure between field coil  2100 , the insulating slot liner  2000 , and the return path pieces  402  and the pole pieces  404  after final assembly. This will hold the field coil  2100  in the proper position until the B-stage adhesive is activated and cured. If there is not sufficient pressure, a temporary adhesive can be used until the B-stage adhesive is cured. The B-stage adhesive on both sides of the material used for insulating slot liner  2000  adheres to both the field coil  2100  and the return path pieces  402  and the pole pieces  404 , and secures them to each other. This facilitates the motor in which the stator  400  is used withstanding heavy vibrations that are seen in some motor/power tool applications. The B-stage adhesive also acts to bond the individual laminations of the return path pieces  402  and pole pieces  404  together. 
     FIG. 22  shows a field (stator) made in accordance with this invention utilizing the insulated field coils  2102 . Elements in common with those described above with reference to previously discussed figures are identified with the same reference numerals used for those elements in those figures. In the illustrative embodiment of  FIG. 22 , after field coils  2102  are placed on the necks of pole pieces  404  and pole pieces  404  mated with return path pieces  402 , field coils  2102  are coated with epoxy using one of the processes described above. illustratively, field coils  2102  are coated with epoxy by placing the field  2102  in a fluidized bed of epoxy and heating field coils  2102 , such as by running electrical current through them. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.