Abstract:
A method for fabricating a gearless grinding mill motor includes fabricating a plurality of linear stator portions and assembling a grinding mill stator from the linear stator portions.

Description:
BACKGROUND OF INVENTION 
     This invention relates generally to mining operations and, more particularly, to grinding mills utilized in mining operations. 
     Currently, there are two main types of mills employed in mining operations, geared mills and gearless mills. Geared mills typically are power limited to approximately 9000 horsepower per pinion or 18,000 horsepower for a dual-pinion driven mill. Gearless mills, also called Ring Motor mills are employed when a mine operator desires a mill of greater than 18,000 horsepower, or in such cases, where the economics benefits justify the use of a Gearless mill with less than 18,000 horsepower. A typical gearless mill&#39;s ring motor works similar to a synchronous machine with a direct current field exciter. Accordingly, a gearless grinding mill motor includes a stator including a bore and one or more field windings. A rotor assembly extends at least partially through the stator bore and includes a rotor core and a rotor shaft/structure extending through the rotor core. The rotor core includes one or more armature windings. The stator of a gearless grinding mill is large and cannot fit in a Vacuum Pressure Impregnation (VPI) tank, which is typically utilized during the manufacture of stators for synchronous machines and other rotating and linear electrical machines. Available VPI tanks typically have a diameter of twelve feet or less and a depth of ten feet or less. Additionally, a grinding mill&#39;S stator is sufficiently large that the stator can not be transported in one piece. 
     Accordingly, the stator is split into several segments that are individually transported from a motor manufacturer&#39;s plant to a customer&#39;s site. The number of segments depends on a size of the stator and shipping conditions or restrictions but typically the stator is segmented into three or four or more segments. After the segments arrive at the final assembly site, the segments are reassembled. Because segmenting the stator involves segmenting the core including the windings or coils, reassembling the stator involves reconnecting or closing the windings at the customer&#39;s site. However, closing the windings at a customer&#39;s site involves significant costs associated with employing skilled laborers (winders) to close the windings and a higher risk of contamination because the customer&#39;s site (a mine) is typically dirty and constitutes a contaminated environment. Additionally, the closed winding can not be factory tested as a winding assembled in a factory can be. 
     Accordingly, a need exists for providing a large gearless grinding mill including stator windings that are closed at a factory and not segmented for transfer to a customer&#39;s site. 
     SUMMARY OF INVENTION 
     In one aspect, a method for fabricating a gearless grinding mill motor is provided. The method includes fabricating a plurality of linear stator portions and assembling a grinding mill stator from the linear stator portions. 
     In another aspect, the method includes fabricating a plurality of substantially identical linear stator portions each including a substantially identical linear drive wherein one drive is programmed to be a master drive. Each stator portion includes a plurality of dimensions each less than three meters, and each linear stator portion further includes one three phase winding electrically connected to the linear drive. Each three phase winding is substantially galvanically isolated from all other three phase windings. The method further includes assembling the linear stator portions to form a stator including a bore therethrough. 
     In another aspect, a grinding mill is provided. The grinding mill includes a stator including a bore therethrough and a plurality of linear stator portions. The grinding mill further includes a shell rotatably mounted at least partially within the bore and at least one winding mounted on the shell and separated from the stator by an air gap. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a partially cut away perspective view of one embodiment of a linear grinding mill motor. 
     FIG. 2 is a schematic view of the linear drives shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a partially cut away perspective view of one embodiment of a linear grinding mill  10  including a shell  12  including a mill head  14  rotatably supported by a feed end trunion bearing  16  and a discharge end trunion bearing  18 . A plurality of rotor field windings or rotor poles  20  are mounted on shell  12  at a periphery  22  of mill head flange  14  and extend away from first trunion bearing  16  toward a back end  24  of mill  10 . A stator  26  including a bore  28  is positioned such that shell  12  extends at least partially through bore  28 . Stator  26  includes a plurality of linear stator portions  30  circumferentially encircling rotor poles  20 . Linear stator portions  30  are separated from rotor poles  20  by an air gap  32 . Each linear stator portion  30  includes at least one three phase winding (not shown in FIG. 1) and at least one linear drive  34  which powers and controls each linear stator portion  30 . In an exemplary embodiment, each linear stator portion  30  includes a core section and a single three phase winding and is powered and controlled by a single linear drive  34 , wherein all linear drives  34  are substantially identical in power and control components and all linear drives  34  are in electrical communication with each other and one particular linear drive  34  is programmed to be a master drive. The linear motor driving grinding mill  10  further includes a solidified load circuit (not shown) electrically connected to the master drive. Trunion bearings  16  and  18  are each mounted to a respective concrete support  36 . 
     In an alternative embodiment, all linear drives  34 , as shown in FIG. 2, are substantially identical in power and control components except for the master drive which is different from all other linear drives  34 . The master drive generates an overall torque reference control signal which controls all the other linear drives  34  (slave drives) to maintain whatever speed the master drive is programmed to operate the mill at. Additionally, the master drive can quickly stop all drives upon detection of a single drive failure reducing the risks associated with an air gap collapse. Furthermore, because all linear drives  34  are substantially identical, the customer need only stock one replacement unit and inventory costs are, hence, reduced. In addition, since all linear stator portions  30  are substantially identical, the customer also need only stock one replacement linear stator portion for repairs if a core section or a winding needs to be repaired, hence, lowering inventory costs further. 
     In an exemplary embodiment, each linear stator portion  30  includes one three phase winding that is separate from the windings in other linear stator portions  30  and each set of windings is galvanically isolated from other three phase windings. Each winding is wound substantially identically, containing an identical even number of poles, and is controlled by one respective linear drive  34 . In one embodiment, each linear drive  34  utilizes cycloconverters (CCV) drive technology. In an alternative embodiment, each linear drive  34  utilizes pulse width modulated (PWM) drive technology. 
     In a linear grinding mill utilizing CCV technology, linear stator portions  30  are arranged and controlled to provide for either a three phase twelve pulse phase control or a three phase twenty-four pulse phase control to lower harmonic impact on each linear drive  34 . Alternatively, in a linear grinding mill utilizing a PWM linear drive  34 , harmonic impact is controlled through the use of an isolated gated dipolar transistor (IGBT) PWM drive, an integrated gate commutated thyristor (IGCT) PWM drive, and/or an injection enhanced gate transistor (IEGT) PWM drive. 
     Each linear stator portion  30  is sized to fit within conventional Vacuum Pressure Impregnation (VPI) tanks. In an exemplary embodiment, each linear stator portion has dimensions less than or equal to three meters. Accordingly, each linear stator portion  30  is fully manufactured in a manufacturing plant and is factory tested. The portions are then assembled at a customer&#39;s site. In one embodiment, the portions are assembled at the customer&#39;s site to form an integral stator. Utilizing a plurality of linear stator portions  30  with individual linear drives  34  allows for reduced costs due to smaller inventory costs and reduced assembly time as explained above, and by testing all stator windings at the factory. In addition, since occurrences of open windings at the customer&#39;s site is reduced, stator coil failures are reduced which increases motor and system reliability. Additionally, a motor manufacturer can easily provide grinding mills of different sizes by altering the number of linear stator portions the manufacturer incorporates into a particular mill. Therefore, inventory costs for the manufacturer are reduced because grinding mills of different sizes can share the same replacement parts. 
     In an exemplary embodiment, each linear drive  34 , including the particular drive programmed to be a master drive is substantially identical and only one drive need be inventoried for repair reasons, thus reducing inventory costs. Installing a linear grinding mill is less expensive than traditional gearless motors because the time required for installation and assembly is shorter and fewer skilled people are needed for the assembly and installation. 
     During operation of linear grinding mill  10 , the master drive controls all other linear drives  34  causing shell  12  to rotate. Large pieces of material (charge) to be reduced in sized (comminution) are fed into shell  12  through an opening (not shown) proximate to feed end trunion bearing  16 . Since shell  12  is rotating, the charge tumbles and breaks into small pieces. When the charge is as crumbled as desired the charge is removed from shell  12  through an opening (not shown) proximate to discharge end trunion bearing  18 . If a drive  34  should fail, the master drive receives feedback of the drive failure and the master drive quickly stops motor  10  by directing all linear drives  34  to stop rotation of shell  12 . The master drive also receives signals from the solidified load protection circuit and upon receiving an indication of a solidified load within shell  12 , the master drive stops rotation of shell  12 . 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.