Abstract:
A method for assembling an inside out motor includes the step of stacking a plurality of laminations together to form a pole member. A stator is molded around the pole member so that an end of the pole member is located adjacent an exterior surface of the molded stator. An electrically conductive magnet wire is wound around the molded stator adjacent the pole member to form a winding of the motor.

Description:
This application is a divisional of U.S. patent application Ser. No. 08/955,515, now U.S. Pat. No. 5,962,938, filed Oct. 21, 1997, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to electric motors and more specifically to a motor with an external rotor and a method for assembling the motor. 
     Motors with external rotors or “inside out motors” of the type to which the present invention generally relates have magnetic elements mounted on a rotor. These magnetic elements may include permanent magnets and/or electromagnets. A stator located inside the magnet elements includes a bearing for rotatably mounting a rotor shaft on the stator so that the rotor may rotate relative to the stator as a result of the magnetic interaction of the magnetic elements and magnetic fields generated by energizing windings of the stator. Typically, only one or two windings are wound on a bobbin of an inside out motor. Pole members provided around the bobbin, between the magnetic elements and the windings, direct the magnetic flux generated by the energized windings to improve the performance of the motor. 
     SUMMARY OF THE INVENTION 
     Generally, a method for assembling an inside out motor generally includes the steps of stacking a plurality of laminations together to form a pole member. A stator is molded around the pole member so that an end of the pole member is located adjacent an exterior surface of the molded stator. An electrically conductive magnet wire is wound around the molded stator adjacent the pole member to form a winding of the motor. 
     In another aspect of the invention, a method for molding a stator for use in an inside out motor includes stacking laminations together to form a pole member. The stator is molded around the pole member so that an end of the pole member is located adjacent an exterior surface of the molded stator. An electrically conductive magnet wire is wound around the molded stator adjacent the pole member. 
     Other objects and features of the invention will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 vertical cross section of an inside out motor of the present invention; 
     FIG. 2 is a cross section of the motor taken in the plane of line  2 — 2  of FIG. 1; 
     FIG. 3 is a perspective of a pole member of the motor of the present invention; and 
     FIG. 4 is a bottom plan of a stator of the motor. 
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, and in particular to FIG. 1, an inside out motor constructed according to the principles of the present invention is designated generally by the reference numeral  10 . The motor  10  generally comprises a stationary assembly or stator (generally designated  12 ), a rotating assembly or rotor (generally designated  14 ) rotatably mounted on the stator, a control housing (generally designated  16 ) attached to the stator, and a mount (generally designated  18 ) connected to the control housing for mounting the motor on equipment (not shown). 
     The stator  12  of the motor  10  includes a generally cylindrical bobbin  20  having a channel  22  extending circumferentially around it. A base  24  is formed at one end of the bobbin  20  for connecting the bobbin to the control  10  housing  16 . As illustrated in FIG. 4, the bobbin base  24  includes four tabs  26  having holes  28  through which connectors (not shown) such as screw fasteners or rivets may be installed to connect the base to the control housing  16 . Alternatively, the bobbin base  24  may be connected to the control housing  16  by heat staking or by ultrasonic welding. A generally annular rim  30  extending around the base  24  centers the housing  16  on the base. Two opposing latches  32  extending from the base  24  connect a circuit board  34  to the base  24  for controlling the operation of the motor  10 . Although the bobbin  20  may be made of other materials without departing from the scope of the present invention, the bobbin of the preferred embodiment is made of an injection molded, electrically insulating polymeric material. 
     As illustrated in FIG. 2, four laminated pole members  40  are molded into the bobbin  20  for directing magnetic flux through the motor  10  as will be understood by those skilled in the art. Although these members  40  may be made of any magnetically soft material (i.e., material having a high magnetic permeability within a range of about 5000 gauss/oersted to about 7000 gauss/oersted) without departing from the scope of the present invention, the pole members of the preferred embodiment are made of motor steel. 
     As illustrated in FIG. 3, the pole members  40  are formed by stacking thin sheets of material or laminations together in a conventional manner. The sheets may be secured together in the stack using any suitable method such as by welding. Other methods for securing the sheets together include forming interlocks (not shown) on the sheets. Each sheet is provided with an oxide layer on its surface which resists the passage of electrical current between the sheets. Although other constructions are envisioned, the pole members  40  of the preferred embodiment are constructed so that they have two spaced-apart legs  46  extending from a cross-piece  48 . The bobbin  20  is injection molded around the pole members  40  so that the legs  46  extend radially outward from the cross-piece  48 . Moreover, one of the legs  46  is positioned on each side of the channel  22 . The pole members  40  are constructed so their inner surfaces  42  and outer surfaces  44  are generally arcuate as shown in FIG. 2 to conform to the shape of the rotor and stator  14 ,  12 , respectively. There may be more or fewer pole members  40  depending on the number of magnetic poles desired for the motor  10 . A four pole motor is illustrated in a preferred embodiment. 
     As illustrated in FIG. 1, each pole member  40  is molded inside the bobbin  20  so that a portion  50  of the bobbin extends between the legs  46  of the pole member to electrically insulate the pole member from two windings (collectively designated  52 ) which are circumferentially wound around the bobbin between the legs of the pole member. The windings  52  of the preferred embodiment are made from copper wires wound in the channel  22  of the bobbin  20 . Although two windings  52  are used in the preferred embodiment, a single winding or more than two windings may be used without departing from the scope of the present invention. The windings  52  are electrically connected to the control board  34  by conventional means. 
     A bearing  54  is molded into the bobbin  20  so it extends along a central axis A of the bobbin for rotatably receiving the rotor  14 . Preferably, the bearing  54  is made from a powdered metal and is impregnated with a lubricant so it is self-lubricating over the entire life of the motor  10 . Although the bearing  54  of the preferred embodiment is molded into the bobbin  20 , it is envisioned that a bearing could be attached to the bobbin by other means, such as by press fitting or adhesive bonding, without departing from the scope of the present invention. In addition, it is envisioned that any bearing  54  may be located between the stator  12  and rotor  14  to permit the rotor to freely rotate with respect to the stator. 
     The rotor  14  comprises a bell  60  having an interior surface  62  defining an interior space  64 , a shaft  66  having a longitudinal axis extending into the interior space, and two permanent magnetic strip elements  68  mounted on the rotor for directing a magnetic flux within the interior space of the rotor. The bearing  54  has a central longitudinal hole  70  which has a smaller diameter at each end than in the middle so the rotor shaft  66  only contacts the bearing near the ends of the hole. This arrangement provides solid support for the rotor  14  at two spaced locations and minimizes the overall contact area between the shaft  66  and the bearing  54 . The shaft  66  is received through the bearing  54  and retained in position so that each of the pole member legs  46  is spaced radially inward from and at least partially aligned with one of the magnetic elements  68 . 
     Although magnetic elements  68  having fewer or more pole pairs are envisioned as being within the scope of the present invention, each of the magnetic elements of the preferred embodiment has four pole pairs extending circumferentially around the interior space  64  of the rotor  14 . In contrast, each sheet in the pole members  40  lies in a plane which is parallel to the flux paths of the magnetic fields generated by the windings  52 . Therefore, the pole members  40  facilitate magnetic flux while inhibiting eddy currents. 
     The rotor shaft  66  is mounted on the rotor bell  60  by a cast insert  80  made of a mounting material (e.g., a zinc alloy). The insert  80  around the shaft  66  forms a spacer for operatively engaging the bearing  54  to axially space the rotor bell  60  from the bearing and stator  12 . Other mounting means may be used without departing from the scope of the present invention. A disk  82  made of low friction material such as nylon or phenolic is disposed between the insert  80  and the bearing  54  to reduce friction between the insert and bearing. 
     In the preferred embodiments, the motor  10  is electronically commutated and is controlled by the printed circuit board  34  mounted on bobbin base  24 . The control devices mounted on the circuit board  34  have not been illustrated in the drawings for clarity. As an example and not by way of limitation, the control circuitry may be a capacitively powered motor and constant speed control as described in co-assigned, co-pending U.S. patent application Ser. No. 08/761,748, filed Dec. 5, 1996, the entire disclosure of which is incorporated herein by reference. 
     The pole members  40  are asymmetrical as shown in FIG. 2 because the sheets at one end are offset inward from the other sheets. As will be understood by those skilled in the art, the asymmetry of the pole member  40  causes the rotor  14  to come to rest at a position where the poles of the permanent magnet strip elements  68  on the rotor  14  are not positioned halfway between adjacent poles of the stator  12 . Further, the air gap  72  between the pole members  40  and the permanent magnet strip elements  68  is asymmetrical. Accordingly, a reluctance torque is produced during startup which urges the rotor  14  to rotate in a desired direction. 
     A preferred application for the inside out motor of the present invention is to drive a fan (not shown). As shown in FIG. 1, a cup-shaped hub  90  of the fan fits over the rotor bell  60  and is integrally formed with fan blades. A shroud  92  attached to the control housing  16  by four struts  94  (only two struts are visible in FIG. 1) is provided to attach the motor  10  to the equipment being cooled. Thus, it may be seen that the shroud  92  provides the sole means of support for the motor  10 . The struts  94  are formed to hold the motor and fan rigidly against pitch and yaw motion, but to permit some small, dampened roll motion. 
     The inside out motor  10  of the present invention may be rapidly and accurately assembled from its component parts. The pole members  40  are formed by stamping the C-shaped sheets from magnetically soft sheet material, and stacking the sheets together as described above. Four of the pole members  40  are positioned in a mold along with a bearing  54  and plastic is injected into the mold around the members and bearing to form the bobbin  20 . The bobbin  20  is preferably molded from a suitable polymeric material and wound with one or more windings  52 . When more than one winding  52  is used, the windings may be bifilar or wound one over the other in a layered arrangement. In the illustrated embodiment, there are two windings  52 , the terminal ends of which are received in respective connector portions (not shown) of the bobbin  20 . 
     The rotor shaft  66  is attached in the interior space  64  of the rotor bell  60  with the insert  80  leaving the spacer formed from the mounting material. The permanent magnet strip elements  68  formed as annular strips of magnetized material are mounted on the interior surface  62  of the rotor bell  60 . However, there may be separate magnets (not shown) spaced around the interior surface  62  of the rotor bell  60  without departing from the scope of the present invention. The magnetic elements  68  are magnetized to have eight distinct poles spaced around the element. In this preferred embodiment, the poles of the magnet elements  68  are circumferentially offset with respect to each other. The angle of offset is preferably about 10°-15° for the eight pole motor of the illustrated embodiment. It is to be understood that other angles of offset are also envisioned as being within the scope of the present invention. 
     The fan is formed in a suitable manner, such as by molding the hub  90  and fan blades as one piece from polymeric material, and fitted over the rotor bell  60 . The hub  90  is secured to the rotor bell  60  in a suitable manner such as by heat staking, snap fitting or press fitting. The low friction disk  82  is placed on the rotor shaft  66 , which is then inserted through the hole  70  in the bearing  54  of the stator. A C-clip  96  is snapped onto a grooved distal end of the rotor shaft  66  to secure the shaft in the bearing  54 . 
     The printed circuit board  34  for the motor  10  is attached to the bobbin base  24  without the use of fasteners. More specifically, the circuit board  34  has a pair of diametrically opposed notches (not shown) in its periphery which are aligned with snap latches  32  formed in the base  24 . The elasticity of the latches  32  permits them to flex outwardly as the circuit board  34  is pushed toward the base  24 , and the resiliency of the latches causes them to snap radially inwardly so that they overlie the circuit board to hold it on the base. As the printed circuit board  34  is mounted on the bobbin base  24 , electrical connections for the windings  52  are made. The control housing  16  and mount  18  are them attached to the base  24  by fasteners are described above. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.