Method for the substitution of copper windings in generator rotor fields originally having aluminum windings
TECHNICAL FIELD 
     The invention relates to dynamoelectric machines and more particularly to the replacement of aluminum rotor windings with copper windings. 
     BACKGROUND 
     Due to the unavailability of an abundant supply of copper during periods of world conflict, electric generators which were built, for example, in the 1940&#39;s and early 1950&#39;s included aluminum or aluminum alloy field windings. The aging of such machines makes them prime candidates for extensive maintenance or complete rewinding as well as developing a need for effective methods for rewinding where necessary or desirable. That is to say, replacement of an original aluminum winding which is determined to be in good condition may not be necessary in that such windings may be reused with new field insulation installed. However, customer preference for copper may also require the replacement of the original aluminum windings. Moreover, where an original aluminum winding has been damaged, obtaining a suitable replacement alloy is not always possible or may involve substantial expense. Accordingly, replacement of original aluminum windings with appropriate copper substitutes is often necessary. 
     As will be appreciated by the artisan, such substitution to be effective and economically feasible is not as simple or as straightforward as it initially appears. For example, the use of aluminum field windings required the use of deep rotor slots in order to accommodate the approximately 80% increase in needed cross section for the windings. Additionally, since aluminum is approximately 35% as dense as copper, relatively thin or shallow rotor wedges were conventionally used. Furthermore, the use of a number of copper turns per coil, which is different than the number used in the original aluminum windings, would clearly have adverse effects on the field winding excitation requirements and thus require further modifications. Still further, the thermal performance of the rewound rotor should exhibit a performance at least equivalent to that of the original rotor with aluminum windings so as to eliminate extensive reevaluation and modifications pertaining to winding clearance requirements and cooling circuit capabilities. 
     Moreover, the mass of the substitute conductors should be approximately the same as the original windings in order to avoid over stressing of the wedges which serve to contain the windings within the rotor slots. In this regard, a substitute winding of copper, for example, may occupy only 50-60% of the original slot volume, while nevertheless weighing approximately the same as the original aluminum windings that have been replaced. However, prior attempts to simply replace the aluminum windings with copper located at the top of the existing rotor slots with the balance of the excess slot space occupied by fillers of insulating material, resulted in the rotor wedges having to restrain not only the copper winding, but also required restraint of the weight added by a stack of fillers. Thus, the addition of insulating fillers in the required quantities added significant weight and shear forces on the relatively thin rotor wedges. Such conditions resulted in over stressing both the rotor slot wedges, as well as the rotor teeth included in the rotor shaft drive mechanism. Still further, the fillers clearly present a significant barrier to the transfer of heat from the windings to the surrounding forged steel rotor, as well as serving as a barrier to cooling gases, such as air. Moreover, attempts to add conductor material for the purpose of reducing current density and hence a decrease in heating also serves to overload the rotor slot wedges and the rotor teeth. 
     DISCLOSURE OF THE INVENTION 
     We have discovered an effective technique using existing field insulation systems to provide a copper substitution for the original aluminum or alloy windings in a practical manner whereby the cross section of the replacement copper winding is minimized, while additionally providing superior thermal performance through the utilization of channel insulation elements formed to support the copper, as well as providing substantially a full width subslot below the windings so as to provide direct cooling of the conductors. That is to say, the integral subslot is formed so as to minimize the amount of fillers required, as well as providing direct cooling of the windings by way of the subslot and openings through the windings, as well as through vented wedges. Thus, a directly cooled field conversion is possible for producing improved thermal performance as well as avoiding the over stressing of the rotor slot wedges. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These, as well as other objects and advantages will be better appreciated by a careful study of the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a cross sectional view of a prior art rotor slot with aluminum windings; 
     FIG. 2 is a cross sectional view of the rotor slot of FIG. 1 after substitution of copper windings for the aluminum; 
     FIG. 3 illustrates a portion of a copper field winding including a ventilation slot for use in rotor slots as illustrated in FIG. 2; 
     FIGS. 4(a) and 4(b) are top and side views,respectively, of a rotor slot wedge including cooling air exhaust passages; and 
     FIGS. 5-8 illustrate a portion of a copper field winding including a ventilation slot for use in rotor slots as illustrated in FIG. 2, and various configurations for the slot armor and subslot channel. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     As may be seen from a consideration of a typical cross section of a rotor slot of a prior art generator with an aluminum or aluminum alloy winding, such windings 1 occupy substantially all of the available space in the rotor slot. Moreover, since the aluminum alloy used in the field windings had approximately one-half the conductivity of copper, the conductors require about 80% more cross sectional area in the slot than a copper equivalent. Accordingly, the rotor slots of such rotors were deeper than that which would be required for copper windings. Additionally, since the aluminum had about 35% of the mass density of copper for an equivalent cross section, relatively thin steel wedges 2 were used. Such slot structures additionally included slot insulation or armor 3, as well as turn insulation 4 to insulate the winding conductors from each other, as well as the steel rotor forging. Additionally included in such conventional rotor slot structures were insulating filling mediums 5, as well as a chafing strip 6, provided between the wedge and the remainder of the slot structure. 
     Rewinding such rotor slots using substitute copper windings results in a slot structure as illustrated in FIG. 2, for example, where the substitute copper winding 11 although including the same number of turns per coil as the original winding also occupies about 50-60% of the original slot volume when substantially equal weights of copper, vis-a-vis, aluminum alloy are used. It is contemplated that such rotor structure may employ conventional molded slot insulation or armor 13 having a generally U-shaped cross section and contemporary turn insulation materials 14 for insulating the winding conductors from each other, as well as the rotor. Of additional benefit is the inclusion of a creepage block 15, formed from an insulating material such as grade G11 fiberglass composites, between the conductors and the wedge 12. The creepage block provides a longer path to ground for voltage buildup in the winding 11, and accordingly enhances insulation integrity. 
     The substitute copper windings must also be located at the top of the rotor slot just below the wedge and creepage block, and accordingly, an appropriate medium must be incorporated to fill the portion of the slot no longer needed for the new windings. As may be seen from FIG. 2, the use of a U-shaped subslot channel element 16 in addition to serving to support the windings and adding negligible weight, effectively converts the previously wasted space into an integral subslot through which cooling gases may be forced to flow by way of either fan or self-pumping design arrangements known to the artisan. The subslot channel element can be formed from conventional insulating material, preferably meeting ANSI Class F temperature requirement and ASTM D709 strength requirements, such as fiberglass by conventional methods such as molding, extrusion, or composite lay-up. The subslot 16 not only minimizes the use of filler, but also provides the opportunity to utilize the thus formed subslot to directly cool the copper by way of inclusion of radially directed ventilation slots 17 in the copper conductors. Thus, the present method in addition to effectively substituting copper windings for the prior art aluminum alloy field winding conductors also contemplates conversion to directly cooled field windings having improved and more uniform thermal performance characteristics. 
     The method of this invention provides the subslot channel 16 to support the conductor windings 11, and suitable slot armor 13 to minimize voltage buildup creepage from the conductor windings 11 to the rotor slot. Therefore, other configurations of the subslot channel and slot armor to provide such support and minimzed voltage buildup creepage are within the contemplation of this invention. For example, FIG. 5 shows two L-shaped sections of slot armor 13 forming the U-shaped slot armor. FIG. 6 shows slot armor 13 extending along the sides of the slot, and the subslot channel 16 extending from the bottom of the slot to the windings 11. FIG. 7 shows the slot armor 13 mortised into the subslot channel 16 a sufficient to minimize the voltage buildup creepage. FIG. 8 shows slot armor 13 having an L-shape extending between the conductor 11 and the subslot channel 16. 
     FIG. 3 illustrates an exemplary ventilation opening located uniformly along the length of the rotor winding copper conductors. Preferably, the opening is elongated to provide increased surface area for better cooling of the conductor. Similar openings would be included in the turn insulation 14 which is bonded between the conductors, as well as similar openings in the creepage block 15. By properly aligning all of the ventilation openings through the turn insulation, copper conductors and creepage block as the winding and other elements are stacked in the rotor slot, cooling passages are provided so that cooling gases supplied by way of the subslot may flow through the rotor windings in a radial direction toward the airgap between the rotor and stator. 
     To complete the radial cooling paths between the airgap and the subslot for the direct cooling of the rotor winding conductors, holes in the slot wedges of the exemplary nature illustrated in FIGS. 4(a) and 4(b) may be used. Preferably, the holes are circular to minimize stress concentration and crack initiation sites in the wedges. Preferably, the creepage block has openings formed to transition from the elongated openings in the conductors to the circular openings in the wedges. The length of the vent wedges is coordinated with the number of winding vents and their spacings. A uniform axial spacing of the cooling vents along the axial length of the rotor provides good thermal performance, as well as providing the advantage of ease of manufacture. 
     The present method provides an effective technique for replacing an aluminum winding with one made of copper wherein the replacement winding will have the same number of turns per coil as the original winding while maintaining substantially equal conductor mass and enhanced thermal performance. As will be appreciated by those skilled in the art, maintaining a constant number of turns per coil will prevent adverse effects on the field winding excitation requirements. Moreover, the maintenance of at least equivalent thermal performance avoids the necessity of completely reevaluating winding clearance requirements and cooling circuit capabilities. Additionally, the use of copper windings of substantially the same weight as the aluminum or aluminum alloy windings being replaced avoids over stressing of the slot wedges which serve to maintain the windings within the rotor slots. Furthermore, the inclusion of a fiber glass U-shaped channel element, as well as radial ventilation openings through the conductors, insulation and wedges provide uniform and improved thermal performance by establishing an integral subslot and cooling gas passages for directly cooling the new field copper. Still further, the presently disclosed method provides an economical technique for rewinding such existing rotors through the use of commonly available materials, such as existing insulation systems and manufacturing processes without requiring expensive modifications to the generator excitation system. For example, the U-shaped channel element 16 may be of conventional insulating material but dimensioned so as to support the replacement copper windings at the top of the rotor slot while simultaneously forming the integral subslot. The subslot channel element may be about 0.100 inches thick and be bonded to the slot armor 13. However, extremely deep subslots may require the use of thicker material to form the subslot channel element. Thus, it is believed that the herein disclosed technique may be used to efficiently replace the field windings for existing machines having aluminum or aluminum alloy windings wherein the original aluminum windings have been damaged or exhibit anomalies such as high average winding temperature or hot spots near the center of the field windings. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.