Patent Application
20140033523
The invention described herein relates generally to brazing. More specifically, the invention relates to a method of brazing.
Armature stator bars in large generators are usually formed of many individual strands interleaved in a predetermined pattern. The bars exit the stator and are retained by the end-winding support system. To form a coil, upper and lower stator bars are joined together in the end-winding region. Previous approaches have used multiple connector plates (or series straps) that connect both the upper and lower stator bars.
However, the end-winding region is a very crowded area and space is at a premium. In addition, multiple connector plates may impede flow of cooling gases or make routing of other elements more problematic. Connector plates may also suffer from vibration and their connection to the stator bars may become compromised over extended operating periods.
Thus, there is a need for an improved brazing method that improves joint quality of stator bars in the end-winding region while simplifying construction and increasing reliability.
In an aspect of the present invention, a brazing method is provided including the steps of, preplacing a braze alloy on a first plurality of conductive strands, the first plurality of conductive strands comprising a first stator bar, preplacing a braze alloy on a second plurality of conductive strands, the second plurality of conductive strands comprising a second stator bar, and heating at least a portion of the first stator bar to join the first plurality of conductive strands and the second stator bar to join the second plurality of conductive strands. Another step is used for electrically connecting the first stator bar to the second stator bar.
In another aspect of the present invention, a brazing method is provided including the steps of, preplacing a braze alloy on a first plurality of conductive strands, the first plurality of conductive strands comprising a first stator bar, preplacing a braze alloy on a second plurality of conductive strands, the second plurality of conductive strands comprising a second stator bar, heating at least a portion of the first stator bar to join the first plurality of conductive strands, and the second stator bar to join the second plurality of conductive strands, and electrically connecting the first stator bar to the second stator bar.
One or more specific aspects of the present invention will be described below. In an effort to provide a concise description of these aspects, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with machine-related, system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.
When introducing elements of various aspects of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. Any examples of operating parameters and/or environmental conditions are not exclusive of other parameters/conditions of the disclosed embodiments. Additionally, it should be understood that references to “one embodiment”, “one aspect” or “an embodiment” or “an aspect” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments or aspects that also incorporate the recited features.
A dynamoelectric machine is defined as a machine that converts mechanical energy to electrical energy or vice-versa, including but not limited to generators and motors. However, it is to be understood that the present invention could also be applied to turbomachines as well, or any application where an improved brazing method is desired.
It is important to have good quality connections in the region of the connector plate 234, and in some applications multiple connecting plates are used for each pair of stator bars. Unfortunately, this approach takes up a lot of valuable space in the end winding region. It would be beneficial if a higher quality connection could be used in the end winding region to simplify machine construction, reduce components, improve airflow and improve overall machine reliability.
In order to obtain high-quality brazed joints, the parts must be closely fitted, and the base metals must be exceptionally clean and free of oxides. In most cases, joint clearances of about 0.002 inches to about 0.008 inches are recommended for the best capillary action and joint strength. However, in some brazing operations it may be desirable to have joint clearances above or below this range. Cleanliness of the brazing surfaces and any preplaced braze alloy is also important, as any contamination can cause poor wetting (i.e., flow of the braze alloy), lack of adhesion to the parent metals, or unacceptable porosity in the resultant joint. The parts to be joined by brazing should be clean. Two methods for cleaning parts prior to brazing, are chemical cleaning, and abrasive or mechanical cleaning In the case of mechanical cleaning, it may be desirable to maintain a predetermined surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry. The conductive strands 120, 122, in the region of the brazed joint, should be cleaned (or pre-cleaned) before brazing is initiated.
According to an aspect of the present invention, a method is provided for brazing the conductive strands in the end winding region of the stator bars. Insulation is removed on the stator bar in the region of the connecting plate attachment location. This will expose the individual conductive strands 120, 122. A braze alloy is preplaced on the first conductive strands 120. The braze alloy, in powder or particulate form, may be preplaced using cold spray deposition. However, preplacing may include deposition, mechanical placement, chemical placement or any other suitable method for preplacing the braze alloy. For mechanical placement methods, the braze alloy may be sheets, bands or strips of metal. However, a preferred method for preplacing the braze alloy is by cold spray deposition.
In cold spray deposition, the braze alloy plastically deforms the surface material on the conductive strands 122, 124. The copper material of the conductive strands is typically softer than the braze alloy. The plastic deformation of the conductive strands yields a superior surface for subsequent brazing by increasing the wettability of the brazed surfaces. The braze alloy may be comprised of an alloy from the BCuP family of braze alloys, in which the phosphorus present in the alloy functions as a flux, removing copper oxides during brazing and allowing a well-adhered joint to form without the need for a separately applied flux and/or a reducing atmosphere.
The BCuP braze alloy may be BCuP-5, which contains about 15% silver, 5% phosphorus, a balance of copper, and has a liquidus temperature of around 1,475° F. A brazing method using a combination of these types of braze alloys may also be referred to as a fluxless brazing method.
After the braze alloy is preplaced on the conductive strands, heating can be performed to braze the strands together. Brazing is generally defined as a joining process wherein coalescence is produced by heating to a suitable temperature above about 800° F. and by using a non-ferrous braze alloy, having a melting point below that of the materials to be joined. The heating step may be performed by induction heating, and the conductive strands 120, 122 may be heated to about 1,400° F. to about 1,550° F., or any other suitable temperature range as required by the specific material compositions. Other heating methods (e.g., torch heating, furnace, carbon arc, resistance, etc.) and other temperature ranges above or below those listed may also be used as desired in the specific application.
The previously described preplacing and heating steps may be repeated for the conductive strands of the second stator bar. This “first shot brazing” step brazes the individual conductive strands in each stator bar. A “second shot brazing” step connects (i.e., brazes) the connector plate 234 to both stator bars, thereby forming an electrical connection therebetween.
The preplacing steps described above may be accomplished by using cold spray deposition. During the cold spray deposition preplacing steps, the braze alloy plastically deforms the surface material on the conductive strands. The conductive strands may be formed or comprised of copper. The heating steps may be performed by induction heating or torch heating. The braze alloy may be a BCuP alloy or any other suitable braze alloy material. The electrical connection step connects the first stator bar to the second stator bar by brazing a connector plate to both stator bars. All the methods described herein may also employ an inert gas purge atmosphere around the joint to be brazed, which may include the brazed region of the first and second plurality of conductive strands and the connector plate.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
