Abstract:
A direct-drive generator and assembly procedure for that generator include a permanent magnet assembly that is partitioned and assembled piece-by-piece after the rotor and stator have been attached. 
     The magnets are attached to a plate in columns, and adjacent columns have a N-S orientation. The air gap between the rotor and stator is variable, and application of an input torque produces a first cogging torque in a first direction due to the variable air gap that offsets a second cogging torque in a second opposite direction.

Description:
RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Patent Application Ser. No. 60/831,510 filed Jul. 18, 2006, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Brushless permanent-magnet (PM) motors and generators are commonly used small devices and/or devices that operate at high-speed. They are less commonly utilized in slow speed applications due to the assembly difficulties associated with maneuvering a large magnet and because of the difficulty of eliminating enough cogging torque for slow-speed applications. 
     An example of a slow-speed application that has rarely used a brushless PM generator is wind power. Slow-speed operation often utilizes a gearbox to turn the generator at a higher speed. In addition to efficiency losses, the gearbox necessitates a larger structure to support the additional weight of the gearbox/generator assembly, Thus, there is a need in the art for a generator that includes a permanent magnet, and reduces cogging torque for use in slow-speed applications. 
     SUMMARY 
     This present disclosure relates to a slow-speed, large-scale generator and assembly procedure for that generator. The magnet is partitioned and assembled piece-by-piece after the rotor and stator have been attached. Guides are used to arrange the magnets so as to give skew to the overall magnet assembly. The shoes of the teeth of the stator have subteeth. 
     The direct drive generator makes manufacturing easier and has reduced cogging torque, to facilitate large-scale, slow-speed operation. The efficiency and thus reduced weight of the direct-drive improves the packagability of the generator, since the generator can be smaller in size, easier to construct and have fewer components. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The above, as well as other advantages of the present disclosure will become readily apparent to those skilled in the art from the following detailed description when considered in the light of the accompanying drawings in which: 
         FIG. 1  illustrates a cutaway view of a generator; 
         FIG. 2  illustrates an individual tooth for the generator of FIG.  1 .; 
         FIG. 3  illustrates a stator lamination stack for the generator of FIG.  1 .; 
         FIG. 4  illustrates a magnet assembly for the generator of FIG.  1 .; 
         FIG. 5  illustrates a sectional view of a rotor mount for the generator of FIG.  1 .; 
         FIG. 6  illustrates a generator disposed in a windmill; and 
         FIG. 7  illustrates a magnet assembly installation alignment for the generator of  FIG. 1 . 
     
    
    
     DESCRIPTION 
       FIG. 1  is a cross-sectional view of the generator. An outside rotor  10  includes a rotor mount  12 , permanent-magnet assemblies  14  lining the inside of the rotor mount  12 , and a top flange  16  rigidly connected to the top  18  of the rotor mount  12 . An inside stator  20  includes a stator mount  22  and a stator lamination stack  24 . The rotor  10  and stator  20  are rotatably connected by two bearings  26 , one near the top  18  and the other near the bottom  28 . The stator  20  is situated inside the rotor  10  so as to leave a small air gap  30  between the permanent magnets  14  and the stator lamination stack  24 . 
       FIG. 2  shows a stator lamination stack  24  with teeth  32 . The Detail A part of  FIG. 2  shows an individual tooth  32  of the stator lamination stack  24 . There are approximately 144 teeth around the circumference of the stator lamination stack  24 . The space between the teeth is known as a slot opening  31 . Each tooth  32  contains a shoe  34  with three or so subteeth or protrusions  35 ,  37  facing the air gap  30 , which improve flux flow and reduce cogging torque. Detail B of  FIG. 2  shows an individual tooth  32 , a center protrusion or subtooth  35 , and two protrusions to the side of the center protrusion,  37 . Between the protrusions there is are notches  33 . 
     When a permanent magnet rotor turns in an airgap a cogging torque is generated due to variations in the air gap  30 . This generates a torque due to variation of magnetic reluctance which causes a cogging of the shaft torque. The major variation in airgap  30  is due to the slot opening  31  between the teeth  32  so that the winding may be put around the teeth. The cogging torque can be reduced by skewing the poles of the magnet. However, to reduce the cogging torque to acceptable levels by skewing alone the amount of skew may be so great the BEMF of the motor may be reduced or the wave shape of the BEMF of the motor may be compromised thus reducing the performance of the motor. 
     The effect of cogging torque can also be reduced by introducing deliberate variations in the airgap  30  of the motor which generate a reluctance torque which is opposite of the reluctance torque generated by the slot opening  31 . 
     Introducing a particular ratio into the lamination can assist in providing deliberate variations in the airgap  30 . For example, where the number of stator teeth is 1.5 times the number of rotor poles, such as 144 stator teeth and 96 magnetic poles on the rotor, this will provide variation in the airgap. The cogging due to the slot opening  31  can also be reduced by making the center protrusion  35  the same width as the slot opening  31 . The notches  33  on either side of the center protrusion  35  would have a width of half the distance from the edge of the center protrusion  35  to the edge of the slot opening  31 . The protrusions next to the slot opening  37  would have a width the same as the width of the notches  33 . 
     As the rotor  10  is turned, while one pole of the magnet is seeing the slot opening  31  another magnetic pole of the rotor  10  is seeing the protrusion  35  in the center of another tooth  32 . When the first magnetic pole on the rotor is transitioning from the slot opening  31  to the tooth  32 , another magnetic pole on the rotor  10  is transitioning from a protrusion on the center of the tooth  35  to the notch  33  on the tooth  32 . Every time a magnetic pole of the rotor is transitioning from a larger airgap to a smaller airgap another pole of the motor is transitioning from a smaller airgap to a larger airgap. While each magnetic pole of the rotor is producing cogging due to the variation of the reluctance of the airgap, half the magnetic poles are producing a torque in one direction while the other half of the magnetic rotor poles are producing a cogging torque in the opposite direction. The net result is a cancellation of the cogging torque. 
       FIG. 3  shows the stator lamination stack  24 . It has a coil of electrically-conducting wire wound around the teeth  32 . The stator lamination stack  24  shown from a top view in  FIG. 3  is a schematic end view of the generator. The outer ring of segmented parts shown in  FIG. 3  are the magnets  36  of the generator  48 .  FIG. 3  also shows that the rotor  10  is on the outside of the assembly and the stator  20  is on the inside. The center portion depicts the stator  20 . The actual number of magnets  36  and the number of stator teeth  32  may vary. 
       FIG. 4  shows a magnet assembly  14 . The magnets  36  are affixed to a curved plate  38 , approximately 96 of which line the inside surface of the rotor mount. The magnets can be arranged in a linear arrangement so that the north and south magnets are perpendicular to one another in columns, as shown in  FIG. 4D . 
     Alternatively, the rotor magnetic pole can also be skewed for additional reduction of cogging. There are at least two ways to skew the magnetic pole. One way, as shown in  FIG. 4B  is to stagger or displace the magnets  36  so that the north and south magnets do not line up exactly, resulting in a skewed magnetic pole. The degree to which the magnets  36  are staggered may vary and result in variations of skew to the magnetic pole. Another way to skew the magnetic pole is shown in  FIG. 4C , where the magnets  36  are placed or created using magnetizing equipment in strips that are affixed at an angle relative to the interior of the plate  38  such that the magnets  36  are continuously skewed. As with the staggered magnetic skew, the continuous skew in  FIG. 4C  may be accomplished at a variety of angles to result in variations to the skew of the magnetic pole. 
       FIG. 5  shows a small section of the rotor mount  12 . The permanent magnets  36  are affixed to plates  38  which are then bolted to the inside  40  of the rotor mount  12 . As one travels up the side of the rotor mount  12 , the bolt holes  42  are skewed slightly. 
     The skew of the magnets  36  from the holes  42  reduces the cogging torque and improves the wave shape of the voltage by reducing harmonic content. 
       FIG. 5A  is an assembly of twelve magnets  36  referred to as a magnet-hub assembly  14 . A total of ninety-six magnet-hub assemblies  14  are used in the complete rotor  10 . 
     The magnets  36  are arranged in a N-S-N-S pattern along the circumferential direction of the hub and three magnets of like polarity are arranged lengthwise on the hub. Lengthwise is shown in  FIG. 5A  in the vertical direction while the circumferential direction is shown in the horizontal direction. Twelve magnets  36  are attached to a magnetic iron hub. 
     Ninety-six magnet-hub assemblies  14  are attached to rotor  10  with bolts using the holes  42  shown in the sheet in  FIG. 5B . The lengthwise direction in  FIG. 5B  is shown horizontally while the circumferential direction is shown vertically. 
     Twenty-four magnet-hub assemblies  14  are first bolted to the rotor  10  in a circumferential direction using the first two rows of holes  42 . Then twenty-four magnet-hub assemblies  14  are bolted to the rotor  10  in the circumferential direction using the next two rows of holes  42 . The second row of holes  42  are staggered from the first set of holes  42  by 0.15625 radial degrees, or approximately 2.05 mm. Third and fourth sets of holes  42  are likewise staggered by the distance. 
     When the rotor  10  is finished along the lengthwise direction there are three magnets of the same polarity directly in line, and three more magnets directly in line with each other but staggered by approximately 2.05 mm from the first set of the magnets, with three more magnets in line with each other but staggered 4.1 mm from the second set of magnets and then three more magnets in line with each other but staggered 4.1 mm from the third set of magnets. This produces a staggered skew of the rotor magnets  36 . 
       FIG. 6  shows an application of a device that utilizes the generator, which in this example is a windmill  44  of the “egg beater” design with an airfoil  56  connected to a direct-drive generator  48 . The windmill  44  rests atop a tower  50  and it is understood that the generator assembly  48  can be located at any point along the tower  50 , or multiple generators, possibly of different horsepower, can be distributed along the tower  50  in segments. As described above, the rotor  10  and stator  20  of the generator  48  can be inside out and still located at any point along the tower  50 . The invention also may be used with wind turbines of other designs, including those with horizontal and vertical turbines. 
       FIG. 7  shows a rotor for a slow-speed generator. In building the generator, the rotor  10  without the magnet-hub assemblies  14  attached is placed over the stator  20 . A Teflon spacer or plate  58  is placed next to the stator  20 . The thickness of the plate is slightly less than the airgap  30  between the magnets  36  and the stator  20 . The magnet-hub assembly  14  is pushed lengthwise in the space between the rotor  10  and the Teflon plate  58  until it is aligned with a pre-drilled hole  60  in the rotor  10 . Guide poles  54  are used to position the magnet-hub assembly  14  circumferentially for proper alignment with the predrilled holes  60  in the rotor  10 . 
     The magnet assemblies  14  are assembled by gluing or otherwise fixing the magnets  36  to the curved plate  38  ( FIGS. 4 and 7 ); the magnets  36  are aligned such that they will be alternating polarity as one travels around the rotor  10 . Alternatively, the magnets  36  are magnetized with magnetizing equipment to achieve the desired pattern of magnets with respect to the distribution of north and south poles. 
     The stator mount  22  has the lamination stacks  24  built around and affixed to it. The bearings  26  are also attached to the stator mount  22  by the inner rings  52 , one bearing near the top  18  and the other near the bottom  28 . The rotor  10 , without the magnet assemblies, is also attached to the bearings  26 . 
     Each magnet assembly  14  is attached to the rotor mount  12  individually. First, a curved Teflon or similar nonmagnetic material plate  38  is slid into the air gap  30 , and two guide poles  54  are placed on either side of the location where the magnet assembly  14  will be placed. Next, the magnet assembly  14  is lowered to the bottom of the gap  30  created between the rotor mount  12 , Teflon plate  38 , and guide poles  54 , all of which collectively hold the magnet assembly&#39;s radial and circumferential positions (see  FIG. 6 ). Finally, the magnet assembly  14  is secured to the rotor, such as by bolting, gluing or applying another type of fastener. The process is repeated for each magnet assembly  14 . 
     Finally, the top flange  16  is fixed to the rotor mount  12 . 
     In operation, a slow-speed torque input (such as that provided by a windmill  44 ) is applied to the rotor  10  via the top flange  16 . There does not need to be a change in gear ratio between a slow-speed input and the generator. The rotor rotates about the stator  20  on the bearings  26 , and the motion of the magnets  36  passing the coiled lamination stack  24  produces electrical current in the coiled wires. The skew of the magnet alignment relative to the stator teeth reduces the cogging torque, as do the subteeth on the stator teeth. 
     The present generator has been described in an illustrative manner. It is understood that the terminology which has been used is intended to be in the nature of words of description, rather than of limitation. Many modifications and variations are possible in light of the above teachings. Therefore it should be noted that the generator can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.