Abstract:
In an axial fan, a case ( 40 ) accommodating the fan motor stator ( 50 ) is anchored into a bearing retainer ( 10 ), in which ball bearings ( 20, 21 ) as a bearing unit are accommodated, along a bulge part ( 13 ) formed on the lower end portion thereof. The case ( 40 ) extends heading axially downward, wherein there is no diametrical overlap between the ball bearings ( 20, 21 ) and the stator ( 50 ). Since only the bulge part ( 13 ) of the bearing retainer ( 10 ) is fixed to the case ( 40 ), heat arising from the stator ( 50 ) is thermally conveyed via the case ( 40 ) only to the bulge part ( 13 ). The fact that the thermal conveyance zone is minimal enables unrestrained curtailment of heat transmitted to the ball bearings ( 20, 21 ). As a result the axial fan can be designed for prolonged bearing lifespan.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   The present invention relates to axial fans (given simply to be “motors” hereinafter), in particular to axial-fan motor structures involving 10,000 rpm and faster high-speed rotation. 
   2. Description of the Related Art 
   Owing to improvements in the processing speed of integrated circuits employed in server computers, the amount of heat emission from servers themselves has been on the rise in recent years. Thus, securing the airflow volume necessary in order stably to cool server-computer integrated circuits has become a critical requirement demanded of server-cooling fan motors. Consequently, there has been an urgently felt need to improve the revs of fan motors. 
   Nevertheless, improving the motor revs entails the problem of heat emission from the motor itself increasing. That is, the goal of raising the revs in a motor gives rise to the necessity of passing greater current to the motor&#39;s stator, and this turns out to cause the stator to emit heat. In conventional motor structures, because the positional relationship between the motor stator and bearing unit is one in which they are radially opposed across a bearing retainer, heat from the stator gets transmitted to the bearing unit via the bearing retainer. On that account, the bearing unit is subjected, more than is necessary, to the influences of the heat produced by the stator, which has led to the problem of the bearing lifespan consequently being shortened. Moreover, the bearing unit is continually subjected to strong rotational impact due to the increase in motor revs. The rotational impact causes the bearing(s) itself to emit heat, shortening the bearing lifespan all the more. 
   In that raising the motor revs also makes the motor&#39;s magnetic-pole switching more frequent, the vibrations produced when the magnetic poles are switching turn out be a serious problem. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention, brought about taking into consideration the issues discussed above, is in the high-speed-rotation motor art to make available a motor lent both heat-dissipating and vibration-resisting properties. 
   In an axial fan of the present invention, the impeller revs at 10,000 rpm or faster, wherein a high-level airflow volume is realized. The bearing(s) is disposed unilaterally set off from the fan&#39;s magnetic drive unit, and is anchored via a bearing retainer. Therefore, direct contact of the bearing(s) on the stator is non-existent. A further feature of the axial fan is that the bearing unit is configured diametrically smaller than the magnetic drive unit. 
   In an axial fan of the present invention, inasmuch as the bearing retainer and the stator do not come into direct contact, stator-originated heat directly heating the bearing(s) is not an issue. As a result, bearing lifespan can be assured even under harsh advanced-rotating conditions in which the speed is 10,000 rpm. In implementations in which two or more bearings are retained, since the bearings together can be retained by a single bearing retainer, compared with implementations in which each bearing is retained with a different component, the coaxial precision along the bearing span is improved by at least the assemblage discrepancies that would exist between the different components. The improved precision makes it possible to reduce tilt in the journal that is inserted through the bearings, whereby vibration due to journal tilt can be kept to a minimum. 
   The fact that the bearing(s) of the present invention are diametrically small by comparison to the magnetic drive unit enables part of the axial flow of air produced by the impeller to strike the magnetic drive unit, which is a heat-emitting component, to effectively cool that area of the motor. A bulge part is formed in the bearing retainer, and axially penetrating through-holes are formed in the bulge part. 
   By means of the configuration in which through-holes are provided in the bulge part, the stator and the external air communicate, whereby heat from the stator is radiated more readily to the external air. What is more, providing the through-holes increases the surface area of contact between the bearing retainer itself and the external air, whereby heat-dissipating effectiveness can be promoted. 
   Through-holes are formed axially penetrating the bottom portion of a motor case of the present invention. Forming through-holes in the bottom makes the fanned air flow all the more readily through the interior of the case in which the stator is accommodated, making it possible to promote heat dissipation from the stator. 
   A circuit board to which coil wires from the stator are connected is fixed to the surface along the underside of the bottom of a case of the present invention. 
   The present invention configuration in which coil wires have in advance been drawn out through the through-holes in the bottom portion of the case facilitates connection of the coil wires to the circuit board. Moreover, the effectiveness with which heat is dissipated from integrated circuits having been loaded onto the circuit board can be enhanced, because the circuit-carrying board is external to the motor. 
   A motor of the present invention is an inner-rotor type in which the stator is disposed so as to radially encompass the magnet, and the outer circumferential surface of the magnet and the inner circumferential surface of the stator are opposed across a gap. 
   Making the rotational mode of a motor of the present invention an inner-rotor type makes it possible to minimize rotational imbalance. Load on the bearing(s) due to rotational imbalance can thereby be reduced, which in particular is optimal in implementations involving high-speed rotation. 
   An impeller furnished with a plurality of vanes is fixedly retained in the present invention on a portion of the journal upward of the bearing. Thus, rotation of the motor produces a stream of air heading from along the upper end to along the lower end of the motor. This airstream cools the bearing(s) and the stator. 
   The present invention configuration in which the impeller is attached to the fore-end portion of the motor journal enables a high airflow-volume airstream to be produced by rotating the motor at 10,000 rpm or faster. The airstream induces a flow of air on the bearing(s) and on the surfaces of the bearing retainer and the stator, making it possible to cool the heat produced in the bearing(s) and stator by the rotation of the motor. A high-volume-airflow fan motor that, while being a 10,000-rpm-and-higher-speed rotating motor, spins with long-term stability can thereby be realized. 
   Therein, the airstream initially passes over the bearing(s) and the surfaces in proximity to the bearing retainer, and next passes over the surfaces in proximity to the stator. Given that the motor is rotated at high speed, it is necessary to supply a large current to the stator coils. The large current is, due to the coil resistance, the causative source of the emission of heat. Furthermore, with brushless motors, flowing to the stator is the high-frequency switching current that goes with running at the 10,000 rpm level, wherein an alternating magnetic field produced in the interior of the stator core by the switching current produces hysteresis loss that is given off as heat. Consequently, in the motor during high-speed rotation, initially the stator heats up. A further consequence, meanwhile, is that during the high-speed rotation the bearing(s) continually undergo impact, which becomes heat produced in the bearing(s). In the present invention, the airstream produced by the fan impeller initially cools the bearing(s) and the bearing retainer, and next cools the stator, which has become the most heated. This enables the bearing(s) to be air-cooled efficaciously, in turn making it possible to prolong the bearing lifespan. Moreover, the airstream enables the stator, having risen to a high temperature, to be air-cooled at the same time, making it possible to prolong the lifespan of the stator coils. These features enable the realization of a high-airflow-volume, long-lifespan fan motor optimal for sever computers and other devices that are operated continuously over the long term without pause. 
   From the following detailed description in conjunction with the accompanying drawings, the foregoing and other objects, features, aspects and advantages of the present invention will become readily apparent to those skilled in the art. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is diagrams illustrating one mode of an embodiment example having to do with the present invention, wherein  FIG. 1A  presents a schematic view in section axially through the motor,  FIG. 1B , an upper-side view, and  FIG. 1C , an underside view; and 
       FIG. 2  is a schematic view in section axially through the motor having been fitted with a fan on the upper portion of the shaft in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A to 1C  illustrate one embodiment of a brushless motor according to the present invention. Throughout the specification, the up/down directions are defined using the up/down orientations along the shaft axis in  FIG. 1A . Accordingly,  FIG. 1A  presents a schematic sectional view of the motor taken along the shaft axis,  FIG. 1B , an upper-side view, and  FIG. 1C , an underside view. 
   A bearing retainer  10  in  FIG. 1A  is made of die-cast aluminum, and is rendered in a hollow cylindroid form. The bearing retainer  10  has a bearing-retaining cylinder  10   a  for retaining later-described bearings. The bearing-retaining cylinder  10   a  has respective recesses  11  and  12  at the upper and lower ends of the bearing-retaining cylinder  10   a . These recesses  11  and  12  are die-cast and thereafter machine-processed to enhance the coaxial precision. These recesses  11  and  12  accommodate respective ball bearings  20  and  21 , which are press-fit into the recesses. The present structure enables the ball bearings  20  and  21  to be retained with the bearing-retaining cylinder  10   a  alone, achieving high coaxial precision between the bearings  20  and  21 . If instead the ball bearings  20  and  21  were retained with two different components, higher machining precision would be demanded of the components and moreover high assembly precision would be required, because dimensional deviations in processing the components and discrepancies in assembling them would become cumulative. 
   A shaft  30 , an axial part that is stainless steel shaped into a columnar form, is mounted on the inner peripheries of the ball bearings  20  and  21 , wherein the shaft  30  is freely rotating. A corrugated spring, constituting a preloaded spring  22 , is attached abutting the bottom side of the lower ball bearing  21 . An annular washer  23  is fixed to the shaft  30  so as to retain the preloaded spring  22 . The washer  23 , made of stainless steel or the like, has an opening on which the shaft  30  abuts. To determine the axial position of the washer  23 , a snap ring  24  is also fixed to the shaft  30  so as to abut on the lower surface of the washer  23 . Another snap ring  25  is fixed to the shaft  30  so as to abut on the upper side of the inner ring of the ball bearing  20 . The preloaded spring  23  and the snap ring  25  impose a load on the inner rings of the ball bearings  20  and  21 , while the bearing retainer  10  imposes a load on the outer rings of the ball bearings  20  and  21 , whereby the positions of the balls inside the ball bearings are fixed radially and axially. 
   When inserting a unit comprised of the shaft  30  to which a yoke  60 , a magnet  70 , and the ball bearing  21  are fixed, through a later-described stator  50  to assemble the motor, the interaction between the stator  50  and the magnet  70  places an axially upward heading force on the assembly. The presence of the preloaded spring  22  on the ball bearing  21  side makes it possible for force that would make the shaft slip axially to be absorbed by the preloaded spring  22 . This action minimizes defects originating in excessive force being applied to the ball bearing  21 , and thus reduces assembly rejects. To guarantee long-term lifespan along with high-speed rotation, it is desirable that the balls accommodated in the ball bearings  20  and  21  be made of ceramic. 
   A bulge part  13  is formed below the bearing retainer  10 . Although the bulge part  13  may be formed independently from the bearing retainer  10 , the bulge part  13  and the bearing retainer  10  should preferably be formed integrally to prevent axial tilt of the bearing-retaining cylinder  10   a . The outer circumferential side of the bulge part  13  is press-fit and fixed to the upper end of a case  40 . The case  40  is made by molding austenite stainless steel, which has good heat-dissipating properties compared to plastic resins, in a press. The case  40 , which is in the form of a hollow round cylinder, and the stator  50  is arranged on the inner circumferential surface of the case  40 . 
   The yoke  60  is fixed to the lower side of, by press-fitting it onto, the shaft  30 . The yoke  60  is formed by shaping stainless steel, being a magnetic body, into a hollow cylindrical form by means of a plastic process such as press-working. The magnet  70  is fixed to the outer circumferential surface of the yoke  60 . The magnet  70  and the stator  50  are arranged so that the axial center of the magnet  70  coincides with the axis of the stator  50 , and are radially opposed across a very slight gap. The magnet  70  and the stator  50  form a magnetic drive unit. As shown in  FIG. 1B , the bulge part  13  has a plurality of axially penetrating through-holes  13   a  arranged at circumferentially equal spacing. The through-holes  13   a  preferably have circularly arcuate apertures because that enables the opening in the through-holes  13   a  to be formed large. In the present invention, because the diametrical size of the ball bearings  20  and  21  is smaller than the diametrical size of the magnetic drive unit, sufficient area for the through-hole  13   a  apertures is assured. This configuration enables more external air to flow onto the stator  50 , facilitating heat dissipation from the stator  50 . 
   The description returns again to  FIG. 1A . An annular protrusion  41  is formed on the inner circumferential surface of the case  40 , for axially positioning the stator  50 . This makes it possible to position the stator  50  axially without using jigs or the like when assembling the motor. The upper surface of the protrusion  41  that is on the far side from the stator  50  abuts on the bearing retainer  10 . Accordingly, the protrusion  41  also serves to axially position the bearing retainer  10 . It should be noted that the protrusion  41  need not have an annular shape. A plurality of axially aligned virgate protrusions may be provided instead, for positioning the stator  50  axially. 
   Moreover, since contact of the case  40  on the bearing retainer  10  is only in its upper portion where the case  40  is in contact with the bulge part  13  of the bearing retainer  10 —that is, since the surface area of contact between the case  40  and the bearing retainer  10  is slight—transmission of heat from the case  40  to the bearing retainer  10  is reduced. Heat generated by the stator  50  is transmitted to the case  40 . Nevertheless, the entire outer circumferential surface of the case  40  is in contact with the external air, ensuring good heat dissipation. Moreover, since only a part of the upper portion of the case  40  is in contact with the bearing retainer  10 , the area of the surface that transmits heat is small, and accordingly heat transmission to the bearing retainer  10  is as a matter of course slight. Thus, it becomes possible to unrestrictedly minimize the heat that is transmitted to the ball bearings  20  and  21  as retained by the bearing retainer  10 , and thereby to prevent shortening of the bearing lifespan due to the impact of heat. 
   The stator  50  is composed of circumferentially evenly spaced, radially inwardly radiating teeth  51  that are formed by stacking steel plates, a stator core that is formed on the outer peripheral portions of the teeth  51  and that has a core backing  52  circumferentially linking the teeth  51 , and coil wires  53  that are wound around the teeth  51 . In a high-speed-rotation motor, the necessity of increasing motor efficiency arises in order to minimize heat generation. To increase motor efficiency, a technique is employed in which the coil wires  53  are wound around the teeth  51  as fully as possible. As one example of this technique, the present embodiment adopts a segmented core design in which a separate core is provided for each of the teeth  51 . The segmented core design serves to increase the slot-fill ratio of the coil wires  53 , thereby improving motor efficiency. 
   A stator mounting member  80  formed of aluminum or the like is arranged on the inner circumferential surface of a lower portion of the case  40 , wherein the stator mounting member  80  abuts on the diametrically outer side of the stator  50 , at the lower end surface of the core backing  52 . The stator mounting member  80  presses the core backing  52  upward evenly around the entire circumference, so that the stator  50  is clamped axially between the stator mounting member  80  and the protrusion  41  in the case  40 . As a result, the stator  50  is put at an even height along its entire circumference, while the center of the stator  50  made uniform along the entire circumference. In other words, the stator mounting member  80  and the protrusion  41  serve to compensate circumferential tilt of the stator  50 . Accordingly, inasmuch as circumferential declination of the axial center of the stator  50  and the magnetic center of the magnet  70 —which is caused primarily by the stator  50 —is not an issue, unevenness in vibration due to the magnetic-pole switching can be prevented. 
   What is more, clamping the stator  50  axially from both sides improves rigidity of the stator  50 , making it possible to prevent the stator  50  from vibrating due to the switching of the magnetic poles. Since the vibration of the stator  50  is transmitted to the ball bearings  20  and  21  via the case  40  and the bearing retainer  10 , reducing the vibration means that vibration imparted to the ball bearings  20  and  21  is lowered accordingly. As a result, the lifespan of the ball bearings is prolonged. This is optimal especially for motors such as that of the present invention in which, due to being revved at 10,000 rpm or faster, the magnetic-pole switching is intense, and in which vibration attendant on the pole switching is large. 
   A plate  90  abuts on the lower end face of the stator mounting member  80 . The plate  90  is formed of aluminum or the like in a circular shape and is attached to the lower end face of the case  40  by plastic process such as crimping to thus form the case bottom. A portion of the stator mounting member  80  that abuts on the plate  80  protrudes radially inwardly, forming a protrusion  81  around the entire circumference. In implementations in which the plate  90  is press-fit to the case  40 , the case  40  is plastic-deformed by applying a load in an axially upward direction. This protrusion  81  serves to bear the load created in the press-fitting. Providing the protrusion  81  prevents the stator mounting member  80  from deforming, so that the plate  90  is kept at a uniform height level over the entire circumference. Moreover, the axially upward heading force in fitting the plate  90  acts also on the stator  80  via the stator mounting member  90 , contributing to putting the stator  50  at an even height along its circumference. 
   The plate  90  has axially penetrating through-holes  91  provided circumferentially at even intervals. The through-holes  91  enable the lower side of the stator  50  to communicate with the external air. The surface area of contact between the stator  50  and the external air thus increases, promoting heat dissipation. The through-holes  91  should desirably be circularly arcuate apertures, for the same reason as with the through-holes  13  of the bearing retainer  10 . The plate  90  also has an opening  92  at its center. 
   To the lower side of the plate  90 , a circuit board  100  is fixed with fastening members  110  such as screws. In a central area of the circuit board  100 , three lead wires  120  are fixed with solder or the like. The portion of each of the lead wires  120  that is attached to the circuit board  100  is provided with a conductor part  121 . The circuit board  100  has corresponding holes to the conductor parts  121 , through which the conductor parts  121  are inserted and soldered to the circuit board  100 . The opening  92  of the plate  90  keeps the conductor parts  121  of the lead wires  120  from being in contact with the plate  90 . 
   Referring to  FIG. 1C , at the portions of the outer circumference of the circuit board  100  that correspond to the through-holes  91  in the plate  90 , the outer circumferential edges of those portions of the circuit board  100  form projections  101  that slightly jut out radially outward of the inner circumferential edges of the through-holes  92 . To provide electrical conduction between the lead wires  120  and the stator  50 , the coil wires  53  of the stator  50  are drawn out and the drawn-out portions are fixed to the circuit board  100  by soldering or the like. The projections  101  have recesses  102  for circumferentially positioning the drawn-out portions of the coil wires  53 . The projections  101  prevent the drawn-out portions of the coil wires  53  from rubbing against the plate  90 . Thus, the drawn-out coil wires  53  are prevented from disconnection when being fixed to the circuit board  100 . Moreover, since the plate  90  and the coil wires  53  do not come in direct contact with each other, good insulation is assured. What is more, providing recesses  102  makes the circumferential positioning of the drawn-out portions of the coil wires  53  easy, improving production workability. 
   By supplying current to the coil wires  53  via the lead wires  120  from an external power supply (not shown), the stator  50  generates a magnetic field, and the interaction between the magnetic field and the magnet  70  generates a rotational force. Because the present embodiment adopts an inner rotor design, in which the stator  50  is arranged surrounding the magnet  60 , the radius of the rotor can be made small. This means that even if rotational imbalance occurs, the small radius of the rotor keeps the rotational imbalance to minimum. Thus, rotational impact on the ball bearings  20  and  21  is reduced. Moreover, the heat produced at the ball bearings  20  and  21  due to the rotational impact is accordingly reduces, making it possible to prolong the lifespan of the ball bearings  20  and  21 . This is optimal for motors such as that of the present invention, which involve high-speed rotation. 
   In press-fitting the bearing retainer  10  into the case  40 , press-fitting the ball bearings  20  and  21  into the bearing retainer  10 , press-fitting the shaft  30  into the yoke  60 , and press-fitting the stator mounting member  80  into the case, the components that are in contact with each other are in an insertion relationship (clearance fitting) to a certain point along the insertion direction, but beyond that point, they turn to be in a press-fitting relationship (interference fitting). Specifically, positioning is carried out in the insertion section, whereas the fixing is effected in the press-fitting section. For example, for press-fitting the bearing retainer  10  into the case  40 , a constriction  13   b  is formed at a lower part of the bulge part  13  of the bearing retainer  10  so as to have a slightly smaller diameter. With the constriction  13   b , the bearing retainer  10  is radially aligned with respect to the case  40 , in other words, the axial center of the bearing retainer  10  is aligned with the axial center of the case  40 , and subsequently, the outer circumferential surface of the bearing retainer  10  is press-fit to the inner circumferential surface of the case  40 . With the constriction  13   b  of the bearing retainer  10  being inserted in the case  40 , the bearing retainer  10  is prevented from tilting, whereby press-fitting can be carried out with the axial center being fixed. It is preferable that the constriction  13   b  have dimensions such as to prevent the center axis of the bearing retainer  10  from tilting. Specifically, it is desirable that the constriction  13   b  account for about one-third the portion where the two parts are fit together. As a result, the bearing retainer  10  does not tilt in the radial orientation, whereby the coaxial precision improves between the ball bearings  20  and  21 , preventing runout of the shaft  30 . Accordingly, adverse effects on the ball bearings  20  and  21  due to the runout of the shaft  30  are reduced, and consequently the lifespan of the bearings is prolonged. What is more, the gap between the magnet  70  and the stator  50  is kept uniform around their circumferences, whereby rotational precision is improved. This serves to reduce vibrations further. 
   Next, referring to  FIG. 2 , the situation with the fan  130  having been fixed to the upper portion of the shaft  30  will be described.  FIG. 2  is a schematic section view along the axis of the configuration in  FIG. 1 , having been fitted with the fan  130 . The dotted-line arrows indicate the paths of the flow of air generated by spinning the fan  130 . 
   Referring to  FIG. 2 , the fan  130  fixed to the upper portion of the shaft  30  is formed by hemispherical cup portions  132  that, from portions  131  of the fan  130  where it is attached to the shaft  30 , encompass part of the upper portion of the bearing retainer  10 , and a plurality of outer-peripheral vanes  133  formed integrally with the outer peripheral faces of the cup portions  132 . The outer-peripheral vanes  133  parallel the axial orientation of the fan  130  and have the same predetermined slant along its circumference, wherein they function to deliver air from the upper side to the lower side of the fan  130  along its rotational axis. 
   By the fan  130  rotating, streams of air as indicated by the dotted-line arrows in the figure are formed. These airstreams pass the through-holes  13   a  in the bearing retainer  10  and collide with the stator  50 . In particular, inasmuch as in the present invention the motor is revved at 10,000 rpm or faster, a large current is passed into the stator  50 , such that the amount of heat it emits is large. Yet by the airstreams being made to collide directly on the stator  50 , the efficiency with which it is cooled is promoted, making it possible to prolong the life of the stator windings. Moreover, the airstreams do not linger in the interior of the case  40 , but are passed out to the external atmosphere by way of the through-holes  91  in the plate  90 . This enables the air that flows inside the case  40  from the exterior to force out to the motor exterior air, present inside the case  40 , which has been heated by the stator  50 . This enables the stator  50  cooling efficiency to be further enhanced. What is more, since the airstreams come into contact not only with the interior of the case  40 , but also with the outer circumferential surface of the bearing retainer  10 , with the abutment between the bearing retainer  10  and the case  40 , and with the outer circumferential surface of the case  40  itself, the efficiency with which these areas are cooled can be enhanced as well. The heat transmitted to the ball bearings  20  and  21  is reduced as a result, which enables the fan to be designed for prolonged bearing lifespan. These features make it possible to provide a high-airflow-volume, long-life fan motor optimal for sever computers and other devices that are operated continuously over the long term without pause. 
   A further feature is that since the outer diameter of the bearing-retaining cylinder  10   a  portion of the bearing retainer  10  is formed smaller than the case  40 , the outer diameter of the cup portions  132  encompassing the bearing-retaining cylinder  10   a  can be made smaller. Accordingly, for the same outer diameter of the outer-peripheral vanes  133 , in this case the outer diameter of the cup portions  132  being made smaller enables a greater airflow volume to be generated. 
   Although an example embodiment having to do with the present invention have been explained in the foregoing, the present invention is not thereby limited, in that various modifications are possible. 
   For example, in the present embodiment the case  40  and plate  90  are separate parts, but may be integrally formed. This makes for a further curtailment of parts, contributing to making the motor low-cost. On the other hand, in implementations in which the coil wires  53  from the stator  50  are to lead out to the motor exterior, having the parts be separate is the superior choice in terms of production workability. 
   A further example of a modification is that in the present embodiment, although the fastening between the case  40  and the plate  90  is by crimping, the fastening method is not thereby limited, and may be welding, gluing, press-fitting, etc. Nevertheless, crimping is the best method in terms of production workability. Further, although the bottom of the case  40  is formed by fastening on the plate  90 , the case bottom is not limited to being formed in that way; the bottom may be formed integrally with the case  40 . 
   A still further example of a modification is that in the present embodiment, although the stator  50  is of segmented core design, it is not thereby limited. The stator  50  may be fabricated by winding the coil wires  53  around an annular core, as well as by winding the coil wires  53  around a straight core, and then transforming the straight core into a loop. 
   Only selected embodiments have been chosen to illustrate the present invention. To those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and not for limiting the invention as defined by the appended claims and their equivalents.