Abstract:
A system and method for reducing the cost of producing a brushless DC motor ( 58 ) is presented. The brushless DC motor ( 58 ) provides higher power density and efficiency with an increased tool run time. The brushless DC motor ( 58 ) includes a rotor assembly ( 72 ) that has an unmagnetized permanent magnet ( 74 ) affixed to a shaft. The permanent magnet ( 74 ) remains unmagnetized until the motor is partially assembled. A plurality of coils ( 94 ) for producing a magnetic field are wound about the rotor assembly ( 72 ). The coils ( 94 ) include end turns that enclose the rotor assembly ( 72 ) such that the rotor assembly ( 72 ) is not removable. Since the windings ( 94 ) are wound with the rotor assembly ( 72 ) already enclosed, the windings ( 94 ) do not require large end coils to allow subsequent insertion of the rotor ( 72 ). Minimizing the end coils reduces the length of wire required per turn, thereby reducing the resistance of the winding ( 94 ). Also, since the permanent magnet ( 74 ) is unmagnetized when the coils ( 94 ) are wound around the rotor assembly ( 72 ) the winding process is simplified by not coupling energy into the wire which would influence the winder operation. The wound assembly is inserted into a stator stack ( 96 ) comprised of magnetic material that provides a magnetic flux return path for the magnetic flux generated by the permanent magnet ( 74 ). Using an unmagnetized permanent magnet ( 74 ) facilitates easy insertion of the wound assembly into the stator stack ( 96 ), reduces the accumulation of magnetic debris during the manufacturing process, and permits the motor assembly to be sealed prior to magnetizing the permanent magnet ( 74 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of the filing date of U.S. provisional application Ser. No. 60/175,133 filed on Jan. 7, 2000. 
     
    
     
       BACKGROUND AND SUMMARY OF THE INVENTION  
         [0002]    The present invention relates generally to electrically operated power tools and in particular, to power tools that are powered by a brushless DC motor.  
           [0003]    Over the past couple of decades the use of cordless power tools has increased dramatically. Cordless power tools provide the ease of a power assisted tool with the convenience of cordless operation. Generally, cordless tools are driven by a Permanent Magnet (PM) brushed motor that receives DC power from a battery assembly or converted AC power. The motor associated with a cordless tool has a direct impact on many of the operating characteristics of the tool, such as output torque, time duration of operation between charges and durability of the tool. The torque output relates to the capability of the power tool to operate under greater loads without stalling. The time duration of the power tool operation is strongly affected by the energy efficiency of the motor. Since, during some operating modes cordless tools are powered by battery modules that contain a limited amount of energy, the greater the energy efficiency of the motor, the longer the time duration that the tool can be operated. The durability of a power tool is affected by many factors, including the type of motor that is used to convert electrical power into mechanical power. Brushed motors such as the PM brushed motors that are generally employed in power tools are susceptible to damaged brushes during rough handling.  
           [0004]    Conventional permanent magnet brushless DC motors provide an ineconomical alternative to brushed DC motors. Although brushless DC motors generally are more durable and provide higher speed and torque performance than similar size brushed motors, conventional brushless motors have daunting cost disadvantages. Before expanding on the cost disadvantages of brushless DC motors, first an overview of the operating characteristics of the two types of motors will be presented.  
           [0005]    The main mechanical characteristic that separates Permanent Magnet brushless motors from Permanent Magnet brushed motors, is the method of commutation. In a PM brushed motor, commutation is achieved mechanically by means of a commutator and brush system. Whereas, in a brushless DC motor commutation is achieved electronically by controlling the flow of current to the stator windings. A brushless DC motor is comprised of a rotor for providing rotational energy and a stator for supplying a magnetic field that drives the rotor. Comprising the rotor is a shaft supported by a bearing set on each end and encircled by a permanent magnet (PM) that generates a magnetic field. The stator core mounts around the rotor maintaining an air-gap at all points except for the bearing set interface. Included in the air-gap are sets of stator windings that are typically connected in either a three-phase wye or delta configuration. Each of the windings is oriented such that it lies parallel to the rotor shaft. Power devices such as MOSFETs are connected in series with each winding to enable power to be selectively applied. When power is applied to a winding, the resulting current in the winding generates a magnetic field that couples to the rotor. The magnetic field associated with the PM in the rotor assembly attempts to align itself with the stator generated magnetic field resulting in rotational movement of the rotor. A control circuit sequentially activates the individual stator coils so that the PM attached to the rotor continuously chases the advancing magnetic field generated by the stator windings. Proper timing of the commutation sequence is maintained by monitoring sensors mounted on the rotor shaft or detecting magnetic field peaks or nulls associated with the PM.  
           [0006]    Generally, existing brushless DC motors that provide a specified power output within a volume that is appropriate for portable power tools are too costly for the consumer market (by a factor of 10). The most significant factors driving the cost of a brushless DC motor are the power density, the cost of the permanent magnets and elecontronic components, and complex production procedures. Therefore, to reduce the cost of producing brushless DC motors either the cost of the permanent magnets must be reduced, the method of assembling the devices must be improved, or the power density must be increased. The cost of the permanent magnets can be reduced by using either smaller or less powerful permanent magnets. The power density of a brushless DC motor can be increased by using higher power PMs or reducing the resistance of the stator windings.  
           [0007]    The present invention provides a system and method for reducing the cost of producing brushless DC motors. The brushless DC motor includes a rotor assembly that has an unmagnetized permanent magnet affixed to a shaft. The permanent magnet remains unmagnetized until the motor is partially assembled. A plurality of coils for producing a magnetic field are wound about the rotor assembly. The coils include end turns that enclose the rotor assembly such that the rotor assembly is not removable. Since the windings are wound with the rotor assembly already enclosed, the windings do not require large end coils to allow subsequent insertion of the rotor. Minimizing the end coils reduces the length of wire required per turn, thereby reducing the resistance of the winding. Also, since the PMs are unmagnetized when the coils are wound around the rotor assembly the winding process is simplified by not coupling energy into the wire which would interference with the winder operation. In addition, enclosing the rotor assembly with the coils improves the coupling between the permanent magnet and the coils, thereby permitting the use of a smaller permanent magnet. The wound assembly is inserted into a stator stack comprised of ferrous material that provides a magnetic flux return path for the magnetic flux generated by the PM and stator windings. Using an unmagnetized PM facilitates easy insertion of the wound assembly into the stator stack, reduces the accumulation of metallic debris during the manufacturing process, and permits the motor assembly to be sealed prior to magnetizing the PM.  
           [0008]    For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification and to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a three dimensional view showing a present embodiment of a portable power tool including a brushless DC motor in accordance with the principles of the invention;  
         [0010]    [0010]FIG. 2 is a cross-sectional view illustrating a presently preferred embodiment of a brushless DC motor in accordance with the principles of the invention;  
         [0011]    [0011]FIG. 3 is an exploded view showing a presently preferred embodiment of a brushless DC motor in accordance with the principles of the invention;  
         [0012]    [0012]FIG. 4A, is a perspective view of the winding form enclosing the rotor assembly;  
         [0013]    [0013]FIG. 4B is an end view of a wound assembly illustrating the arrangement of the coils;  
         [0014]    [0014]FIG. 5 is a schematic diagram showing a controller for generating drive signals for the coils;  
         [0015]    [0015]FIG. 6A is a two-dimensional view of the relationship between the sensor magnet and the sensor card;  
         [0016]    [0016]FIG. 6B is an end view of the rotor assembly and sensor card; and  
         [0017]    [0017]FIG. 7 is a flow diagram illustrating a process for producing a power tool in accordance with the principles of the present invention;  
         [0018]    [0018]FIG. 8 is a two-dimensional view illustrating a bearing system in accordance with the teachings of the present invention; and  
         [0019]    [0019]FIG. 9 is a two-dimensional view illustrating a sealing system in accordance with the teachings of the present invention.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0020]    Referring to FIG. 1, a portable power tool  50  according to the present invention is shown. While the present invention is shown and described with a power drill  50 , it will be appreciated that the particular tool is merely exemplary and could be a circular saw, a reciprocating saw, or any similar portable power tool constructed in accordance with the teachings of the present invention. The power tool  50  includes a tool interface  53  which is driven through a gear train  56  by a DC brushless motor  58 . The tool interface in the preferred embodiment includes a chuck  52  secured to a rotatable spindle  54 . The motor  58  is mounted within a housing  62  that includes a handle  64  extending therefrom. A trigger switch  66  is mounted in the handle  64  below the motor  58 . A controller  65  coupled to the trigger switch supplies drive signals to the motor  58 . The controller  65  is mounted below the motor  58  within the housing  62 . Alternative locations for mounting the controller  65  include within the handle  64 , adjacent to the motor  58  and around the motor  58 . A recess  63  is provided in the handle  64  to accept a power module (not shown). The power module is installed within the handle recess  63  to supply electrical power to the motor  58  through the controller  65  in response to actuation of the trigger switch  66 . The handle  64  is configured to accept either a cordless battery power module  68  or a corded line power module  70 . The battery power module  66  includes a battery assembly (not shown) that provides  18  VDC power to the motor  58 . The AC converter power module  70  converts 120 VAC, 60 Hz power to regulated 18 VDC. Although in the preferred embodiment, the power modules  68  and  70  provide  18  VDC power to the motor  58 , it is within the scope of the invention to provide any DC voltage that is required by the power tool  50 , such as regulated 12 volts or unregulated 100 volts.  
         [0021]    Referring to FIGS. 2 and 3, cut-away and exploded views of the DC brushless motor  58  are illustrated. The motor  58  includes a rotor assembly  72  having a magnetic field for supplying rotational energy to the chuck  52  through the gear train  56 . A wound assembly  78  encloses the rotor assembly  72  providing a rotating magnetic field that the rotor assembly magnetic field is drawn towards. A stator assembly  96  provides a magnetic flux return path for the magnetic field generated by the rotor assembly  72 . A pair of bearings  82  and  84  located between the rotor assembly  72  and the stator assembly provide a mechanical interface to permit rotation of the rotor assembly  72 . A fan  80  attached to the rotor assembly  72  supplies cooling air to the motor  58  and the controller  65 .  
         [0022]    The rotor assembly  72  comprises a permanent magnet  74  that is bonded to a shaft  76 . The shaft  76  in the preferred embodiment is made from magnetic steel although other materials such as stainless steel are within the scope of the invention. The permanent magnet  74  is a one-piece sintered Neodymium Iron Boron (NIB) magnet that is left unmagnetized until the motor  58  is partially constructed. The permanent magnet  74  is then transversely magnetized to provide a two-pole magnet. Although a two-pole NIB magnet is used in the preferred embodiment, it is within the scope of the invention to employ other permanent magnets such as axially magnetized Samarium-Cobalt magnets and Ferrite magnets having four or more poles. To form the NIB magnet, a quantity of Neodymium alloy is milled down to approximately  5  micron. A transverse field is then applied and the milled Neodymium is formed by a press made of ferrous material. Finally, the pressed material is sintered resulting in a near net shaped magnet. The final shape for the magnet is attained by machining the material. The resulting permanent magnet  74  is attached to the shaft  76 . The methods of attaching the magnet  74  to the shaft include injection molding and bonding. In the injection molding version, the rotor assembly  72  is inserted into an injection mold. Plastic or epoxy that serves as a bonding agent is injected between the shaft  76  and the permanent magnet  74 . The shape of the magnet  74  inside diameter is preferably elliptical while the shaft is round. There is a minimum gap of 0.5 mm per side to allow for the plastic to flow through. In the bonding version, the clearance between the shaft  76  and the magnet is smaller than that required for the injection molding version. This is to compensate for the decreasing strength of cylindrical metal part bonders with increasing gap between bonding surfaces. The rotor assembly  72  is then placed into a winding form  89  prior to winding the coils.  
         [0023]    Referring to FIG. 4A, a perspective view of the winding form  89  enclosing the rotor assembly  72  is shown. The winding form  89  includes insulating tube  88  and two end plugs  90  that are formed from plastic. In addition, six plastic teeth  92  are integrated to the end plugs  90  to provide winding posts for a set of coils. Although a plastic winding form with plastic teeth is used in the preferred embodiment, the scope of the invention includes using other materials such as magnetic steel and insulated powder metal. Three sets of coils (not shown) are wound onto the winding form  89  so that the coils (not shown) lie substantially parallel to the shaft  76 . The coils are constructed with multi-strand magnet wire to obtain a better fill, for ease of winding, and to reduce resistance. A quasi-tumble winding method is used for winding the coils. The winding method is a variation of layer winding that is similar to tumble winding, except for using multiple wire feeds instead of a single wire feed.  
         [0024]    Referring to FIG. 4B, an end view of the wound assembly  78  after winding is shown. The wound assembly  78  includes the winding form  89  enclosing the rotor assembly  72  with the coils  94  wound about the form  89 . The coils  94  include end turns  93  that enclose the ends of the rotor assembly such that the rotor assembly  72  is not removable from the wound assembly. The end turns  93  are wound so that the length of wire required for each of the coils  94  is minimized. Minimizing the wire length leads to coils  94  having a lower resistance and therefore lower resistire losses. The resulting increased efficiency of the motor  58  increases the power density and reliability of the motor  58  and reduces the complexity of thermal management circuitry. As well as minimizing the wire length, the end turns  93  are arranged to minimize any gap between the end of the rotor assembly  72  and the end turns  93 . Minimizing the gap provides increased coupling between the coils  94  and the permanent magnet  74 . Due to the improved coupling, a smaller, less costly permanent magnet  74  can be employed for the motor  58 . The preferred embodiment uses a set of three coils connected in a three-phase wye configuration. However, the scope of the invention includes other coil configurations such as two-phase bifilar wound, three-phase delta, and other multiphase configurations.  
         [0025]    Again referring to FIGS. 2 and 3, the stator assembly  86  includes a stator stack  96 , an end ring  98 , and front end bell  100 . In the preferred embodiment, the stator stack is constructed from laminated silicon steel. However, the scope of the invention encompasses using other magnetic materials such as insulated powder metal. The inside of the end ring  98  contains features that mate with one of the end plugs  90  and position the winding form  89  in one of six positions. Although it is preferable that the winding form  89  be keyed in one of the six positions, it is not critical that a certain orientation be achieved. The two end bells  100  and  102  serve as a means of supporting the rotor assembly  72  and retaining it concentric to the winding form  89  while allowing the rotor assembly  72  to spin freely. In the preferred embodiment, the end bells  100  and  102  are made from aluminum, however it is within the scope of the invention to use other materials such as plastic. The forward side of the front end bell  100  is modeled to interface with the gear train  56 . The other side of the front end bell  100  includes two posts  104  that mate with the stator stack  96 . The front end bell posts  104  are used for location and retention of concentricity between the end bell bearing bore  106  and the stator stack  96 . The back end bell  102  includes a sleeve bearing (not shown) and two posts  108  that mate with the end ring  98 , which in turn has two posts  110  that interface with the stator stack  96 . Two steel pins  112  are inserted through the back end bell  102 , the end ring  98 , and pressed into the stator stack  96 . The steel pins  112  and the posts  110  ensure concentricity between the bearing bore  106  and the stator stack  96 .  
         [0026]    Referring to FIG. 8, a detailed two-dimensional view of a bearing system  160  of the presently preferred embodiment of the invention is illustrated. The bearing system  160  compensates for radial and axial thrust forces existing at the interface of the stator  96  and rotor assembly  72  (see FIG. 3). The bearing system  160  includes a set of caged balls  162  bearing on the front face  165  of a sleeve bearing  164 . The sleeve bearing  164  serves two functions, being used as a race for the caged balls  162  in addition to compensating for radial forces. The caged balls  162  are held against the sleeve bearing  164  by a ball race  166  formed in the back side of the fan  80 . Although in the presently preferred embodiment, the ball race  166  is integrated into another assembly (the fan  80 ), it is within the scope of the invention to use a ball race that is not integrated into another assembly.  
         [0027]    Referring to FIG. 9, a two-dimensional view highlighting a sealing system of the presently preferred embodiment of the invention is illustrated. The sealing system prevents contamination of the region between the winding form  88  and, shaft  76 , and end bell  100  during operations such as impregnating the windings with resin. A compliant seal  170  is applied from the winding form end cap  92  to the end bell  100 . The seal  170  adheres to a bearing boss on the end bell  100  as well as the end cap  92 . The seal  170  blocks off the interface area from outside airborne contaminants as well as contamination that might occur due to the process of impregnating the windings.  
         [0028]    Referring to FIG. 5, the controller  65  includes a control circuit  114 , a coil driver module  116 , and a heat sink. Signals from the trigger switch  66  are coupled to the control circuit  114 , which generates drive signals for controlling the coil driver module  116 . The output of the coil driver module  116  couples to the coils  94  providing drive power for the motor  58 . The coil driver module  116  attaches to the heat sink  118 , which provides a thermal path for power losses. In the preferred embodiment the controller  65  is attached to the side of the motor  58  above the handle  64 , however it is within the scope of the invention to locate the controller  65  elsewhere within the power tool  50  such as behind the motor  58  in the back of the power tool  50  and within the handle  64 . The controller  65  sequentially switches a DC voltage across each of the phase coils in a manner that generates a rotating magnetic field. In response, the rotor assembly  72  rotates in an attempt to align the magnetic field generated from the permanent magnet  74  with the rotating magnetic field. Controllers for brushless DC motors are well known to those skilled in the art. The scope of the invention encompasses using a controller to provide drive power for a brushless DC motor constructed in accordance with the principles of the invention.  
         [0029]    Referring to FIGS. 6A and 6B, the position sensor circuit is illustrated. The position sensor circuit senses the orientation of the permanent magnet  74  with respect to the coils  94 . A sensor magnet  124  is mounted on the shaft  76  external to the front end bell  100  to provide a marker of the relative position of the coils  94 . The sensor magnet  124  is a 2-pole ring magnet that is unmagnetized until the motor  58  is assembled, at which time an external field is applied to magnetize the sensor magnet  124  in addition to the permanent magnet  74 . A sensor card  120  having three Hall sensors  122  spaced  120  degrees apart is mounted so that the Hall sensors  122  detect the sensor magnet  124  as the shaft  76  revolves. The sensor card  120  has an inner clearance hole and is mounted such that the shaft  76  passes through the card  120  and the Hall sensors  122  are maintained in close proximity to the sensor magnet  124 . The outputs of the Hall sensors  122  are coupled to the controller  65  which uses the position sense in conjunction with an input from the trigger switch to determine the timing of the drive signals to the power module  116 . Although the presently preferred embodiment of the invention employs three Hall sensors spaced  120  degrees apart for sensing the position of the shaft  76 , it is within the scope of the invention to use other means of position sensing such as three Hall sensors spaced  60  degrees apart with an inverted center signal, sensing leakage flux, sensorless operation, and other position sensing circuits that are known to those skilled in the art.  
         [0030]    Referring to FIG. 7, the process of assembling the motor  58  is illustrated. At step  130 , an unmagnetized permanent magnet  74  is attached to the shaft  76 . The permanent magnet  74  has an elliptical center through which the shaft  76 , which is round, is inserted. A Liquid Crystal Polymer (LCP) is injected between the shaft  76  and the permanent magnet  74 . A minimum gap of 0.5 mm is maintained to allow for the LCP to flow through. At step  132 , the rotor assembly  72  is placed into the winding form  89 . A light press fit is used for retaining the end plugs  90  on the insulating tube  88  since the coils  94  will enclose the end plugs. Once assembled, the combination rotor assembly  72  and winding form  89  is placed into a winder for winding. The coils  94  are wound onto the winding form  89  using a quasi-tumble winding method that is similar to tumble winding, except for using multiple wire feeds instead of a single wire feed, step  134 . Winding the coils  94  with an unmagnetized permanent magnet  74  provides for a simpler more repeatable process by eliminating the errors associated with a magnetic field from the magnet coupling into the wire as the coils  94  are wound. Without the impinging magnetic field causing eddy currents that heat the wire and Lorentz forces that deflect the wire, it is possible to wind flatter, tightly layered coils  94  that are consistent from one coil to the next. Generally, the coils for motors are wound without the permanent magnet installed to avoid the aforementioned problems with magnetic fields. To provide clearance for the later insertion of the permanent magnet inside the coils, end turns that provide clearance are generally used. In the preferred embodiment, since the coils  94  are wound with the permanent magnet  74  and rotor assembly  72  enclosed by the winding form  89 , it is not necessary to provide end turns that have clearance for inserting the rotor assembly  72 . Instead, the coils  94  are wound so that the wire encloses the rotor assembly  72 , thereby providing shorter wire turns resulting in reduced coil resistance and lower power losses for the motor. Both conduction and eddy current losses are decreased by shortening the length of the wire turns for the coils  94 . The reduced power losses result in an increased motor power density. In addition, by enclosing the rotor assembly  72  with the coils  94 , the magnetic coupling from the permanent magnet  74  to the coils  94  during normal operation of the motor  58  is enhanced. After winding, the coils  94  are then interconnected in a delta configuration. At step  136 , the wound assembly  78  is inserted into the stator stack  96  which is pre-fitted with the end ring  98 . The wound assembly  78  is subsequently impregnated, step  138 . A compliant seal is applied to the bearing boss on the end bells  100  and  102  and to the neck of the end plugs  90 . The seal blocks off the interface area from any type of contamination. The coils are then heated and the resin is applied to the winding end coils. When the impregnation resin is fully cured, it forms a solid mass that holds the coils  94  in place, not allowing the wires to abrade due to vibration. The solid mass provides an improved thermal path from the coils  94  to the stator stack  96 . The seal also provides a barrier that protects the winding area from outside airborne contaminants. The inside of the end ring  98  contains female features that accept the teeth  92  of the end plugs  90  and position the wound assembly  68  in one of six positions. A particular orientation is not required, however the wound assembly  78  is preferably keyed in one of the six positions. The easy insertion of the wound assembly  78  into the stator stack  96  is facilitated by the unmagnetized permanent magnet  74  which is not magnetically attracted to the stator stack  96  like conventional brushless DC motors are typically. At step  140 , the bearings  82  and  84  are attached to the shaft  76 . Debris is removed from within the partially assembled motor prior to attaching the end bells  100  and  102 . Removal of debris is also facilitated by the unmagnetized permanent magnet  74 , which does not provide a magnetic force attracting metallic and magnetic debris, which would thereby prevent removal of the debris. The front and back end bells  100  and  102  are then attached, step  142 . The end bells  100  and  102  serve as a means of supporting and retaining the rotor assembly  72  concentric to the winding form  89  while allowing the rotor assembly  72  to spin freely. The end bells  100  and  102  also provide a sealed air-gap construction. By sealing the motor  58  prior to magnetizing the permanent magnet  74 , debris is prevented from contaminating the cleaned assembly. At step  144 , the sensor magnet  124  is attached to the shaft  76 . A low-level magnetic field is then applied to the assembled motor to align the permanent magnet  74 , step  146 . The rotor assembly  72  rotates to orient the assembly so that the permanent magnet  74  is aligned with the external field. At step  148 , a high-level magnetic field is then applied to the assembled motor  58  to transversely magnetize the permanent magnet  74  and the sensor magnet  124 . At step  149 , a potting boat (not shown) that contains the sensor card  120  is slid over the shaft  76  and secured to the back end bell  100 . Tabs on the potting boat assembly that interface with the back end bell  100  set the hall cells in the proper position relative to the sense magnet. Next, the controller  65  is bolted to the side of the motor  58 . The phase leads from the motor are attached to the controller  65  using either ultrasonic welding or Fast-on terminals. A ribbon cable for coupling the hall sensor signals to the controller  65  is then attached to the sensor card. At step  150 , the final assembly steps such as insertion into the power tool  50  are completed and the finished power tool  50  is ready for operation.  
         [0031]    From the foregoing it will be understood that the invention provides a novel brushless DC motor. The brushless DC motor includes a permanent magnet that is unmagnetized during assembly of the motor resulting in simplified assembly procedures that reduce the cost of the motor. In addition, the DC motor has improved power efficiency resulting from lower resistance coils that enclose the rotor assembly. Also, the motor is less susceptible to contamination by metallic and magnetic debris leading to improved reliability. Additionally, improved coupling from the coils to the permanent magnet permit the use of smaller, less costly permanent magnets. In addition, the reduced power losses permit the use of simpler cooling techniques for the motor. Additionally, the brushless DC motor can be employed in a portable power tool to provide faster operating speed and higher torque due to the higher power density of the brushless DC motor. The power tool reliability is improved by eliminating the brushes associated with brushed DC motors and employing the higher reliability brushless DC motor.  
         [0032]    The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.