METHOD AND SYSTEM FOR APPLYING AN ISOLATION LAYER TO A BRAZED END OF A GENERATOR ARMATURE WINDING BAR
RELATED APPLICATIONS
This application is related to commonly owned US Patent Application Serial No 10/991,371, entitled "Braze End Isolation Layer For Generator Armature Winding Bar And Method For Applying The Isolation Layer" (attny dkt 839-1597) and US Patent Application Serial No 10/991,501, entitled "Braze Chamber And Method For Applying An Isolation Layer To A Brazed End Of A Generator Armature Wmdmg Bar" (attny dkt 839-1599) which were filed contemporaneously with this application These two applications are incorporated by reference herem
BACKGROUND OF THE INVENTION
The present invention relates to brazing generator armature winding bars to hydrauhc header clips, and to a method for sealing an armature winding bar to its header clips to prevent or reduce corrosion due to coolant water flowing through the end fitting and the armature winding bar
The armature windings on large steam-turbine generators are generally water-cooled The armature windings comprise an arrangement of half coils or armature bars (collectively referred to as "armature bars" or "bars") connected at each end through copper or stainless steel fittings and water-cooled connections to form contmuous hydraulic winding circuits
Water-cooled armature winding bars are compnsed of a plurality of small rectangular solid and hollow copper strands arranged to form a bar The rectangular copper strands are generally arranged in rectangular bundles The hollow strands each have an internal duct for conductmg coolant through the bar The ends of the strands are each brazed to a respective hydrauhc header clip The hydraulic header chp serves as both an electrical and a cooling flow connection for the armature windmg bar
The hydraulic header clip is a hollow connector that includes an enclosed chamber for ingress or egress of a cooling liquid, typically deionized water At one open end, the clip encloses the ends of the copper strands of the armature winding bar A braze alloy bonds the end sections of the strands to each other and to the hydraulic header clip The braze joints between adjacent strand ends and between the strand ends and the clip should retain hydraulic and electrical integrity for the expected lifetime of the winding A typical lifetime of a winding is on the order of tens of years
Internal surfaces of the brazed joints between the clip and the ends of the strands are constantly exposed to the deionized, oxygenated water flowing through the clip and the hollow strands The exposure of the brazed surfaces to the coolant can result in corrosion of the armature winding bar and hydraulic header clip Corrosion tends to occur in the crevices of the joints between the hydraulic header clip and the strand ends of the armature bar, and in the crevices between the strand ends Corrosion of a phosphorous-contaimng braze alloy and adjoining copper strand surfaces can occur if cntical crevice geometry and crevice water chemistry conditions are present Certain conditions promote crevice corrosion in the braze jomts, such as phosphorous, copper, suitable corrosion initiation sites and water If any one of these conditions is eliminated from the clip to bar joints, crevice corrosion should be reduced or eliminated
The corrosion process can initiate if the braze jomt surfaces contain surface crevices, pinholes, or porosity at or near the surface of the jomt and the cntical water chemistry conditions that support corrosion The corrosion process can progress through the braze joints especially when cntical crevice geometry and water chemistry conditions exist Porosity within the braze jomts can accelerate corrosion If allowed to progress through a joint, corrosion will eventually result in a water leak through the entire effective braze joint length and compromise the hydraulic mtegnty of the chp-to-strand joint Accordmgly, there is a long felt need for a corrosion-resistant chp-to-strand braze joint The benefits of crevice corrosion-resistant braze jomt are expected to include improved generator availability and generator reliability
BRIEF DESCRIPTION OF THE INVENTION
A non-crevice-corroding clip-to-strand braze joint has been developed using a silver based braze alloy that is essentially phosphorous-free A method to braze the joint and a brazing chamber assembly has also been developed In preparation for brazing, stops of braze alloy are interleaved between tiered rows of the copper strands such that the stops extend beyond rows of short solid strands but not beyond the free ends of longer hollow strands During induction heating, the braze alloy is briefly heated to above its hquidus temperature such that the alloy pools on the solid strand ends and in crevices between the strands and the internal surfaces of the hydraulic header chp The pooled alloy when cooled forms a layer of braze alloy that isolates the solid strand ends, the joints between strand ends and the joints between strand ends and the clip from the coolant passage m the clip
The brazing chamber includes a split hood that when closed and purged has an essentially oxygen free atmosphere The armature bar is mounted vertically in the chamber such that the free ends of the copper strands are horizontal to allow liquid braze alloy to pool on the solid free ends A cooled heat sink clamps the bar just below the hydraulic chp to chill the bar and sohdify braze alloy flowmg down between the strands A hooked induction coil in the chamber heats the clip, strand ends and stops of braze alloy A mechanical ram compresses the chp, strand ends and braze stops together during the brazing process in the chamber
The invention may be embodied as a method of forming a brazed jomt between an armature bar and a hydraulic header chp including assembling an end of the armature bar, hydraulic header chp and a substantially phosphorous-free braze matenal, positioning the assembly of the armature bar, hydraulic header clip and the braze matenal m a braze chamber, such that the clip is seated m an induction heatmg coil, heating the assembly to a first temperature within 200 degrees Fahrenheit of and below a solidus temperature of the braze matenal by applying electacal current to the induction heat coil, heating the assembly to a second temperature above the hquidus temperature of the braze matenal, poolmg liquid braze matenal on ends of the
armature bar, and cooling the assembly and thereby forming a braze layer on the end of the armature bar
The mvention may be further embodied as a method of forming a brazed joint
between an armature bar and a hydraulic header chp compnsmg assemblmg free ends
of hollow strands and of solid strands of the armature winding bar and positioning the
assembly within the hydraulic header chp, interleaving a silver based braze alloy
between the strands, wherein a portion of the braze alloy extends axially beyond the
free ends of a plurality of the solid strands and the free ends of a plurality of the
hollow strands extend axially beyond the braze matenal, positioning the assembly of
the free ends, hydraulic header clip and the braze alloy m a braze chamber, such that
the clip is seated main induction heating coil of the chamber, heating the
assembly to a first temperature below a solidus temperature of the braze alloy and within a 200 degrees Fahrenheit of the solidus temperature activating the mduction heat coil to heat the clip, heatmg the assembly to a second temperature above the hquidus temperature of the braze alloy, poolmg liquid braze alloy on ends of the armature bar, and cooling the assembly and clip thereby fonning a braze isolation layer on the end of the armature bar
The mvention may also be embodied as a system for fonnmg a brazed jomt between an armature winding bar and a hydraulic header chp compnsmg an assembly of free ends of hollow strands and solid strands of the armature winding bar positioned within the hydraulic header clip, a silver based braze alloy mterleaved between the strands, wherein a portion of the braze alloy extends axially beyond the free ends of a plurality of the solid strands and the free ends of a plurality of the hollow strands extend axially beyond the braze matenal, said assembly of the free ends, hydraulic header clip and the braze alloy mounted a braze chamber, such that the chp is seated in an induction heating coil of the chamber, and a controller receiving a temperature feedback signal from a temperature sensor m the chamber and controlling a temperature in the chamber by controlling power applied to the coil and based on the feedback signal, wherein during a brazing penod said coil heats the assembly to a temperature above the hquidus temperature of the braze alloy
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of a liquid-cooled stator winding arrangement illustrating the armature bars and hydraulic header clips coupled to inlet and outlet coolant headers
FIGURE 2 is a perspective view of the end of an armature winding bar showing the tiered rows of hollow and solid strands, and interleaving sheets of braze material
FIGURE 3 is a perspective exploded view of the end of an armature winding bar inserted into a hydraulic header clip, with braze material and a clip cover shown to the side of the clip
FIGURE 4 is an end view of the strands of an armature winding bar within a hydraulic header end chp with a ram clamping the cover to the clip and a heat sink attached to the bar
FIGURE 5 is a side view of the winding bar, end clip and ram shown in a cross-section taken along lme 5-5 m Figure 4
FIGURE 6 is a perspective side view of a brazmg chamber
FIGURE 7 is an enlarged view of the mtenor of the brazmg chamber that shows an induction heating ceil and armature winding bar heat sink
FIGURE 8 is a flow chart of an exemplary braze process
FIGURES 9 and 10 are end and cross-sectional side views respectively of the hydraulic header chp brazed to an armature bar
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 illustrates a liquid-cooled armature winding arrangement for a stator m a typical liquid-cooled generator A stator core 10 having stator core flanges 12 and core ribs 14 Armature winding bars 16 (also referred to as stator bars) pass through radially extending slots in the stator COIB and are capped at opposite ends by hydraulic header clips 18 fitted to the ends of the bars Inlet hoses 22 connect an inlet clip 18 to an inlet coolant header 24 Outlet hoses 26 connect an outlet clip 18 to an outlet coolant header 28 A copper or stainless steel fittings 20 connect adjacent ends of pairs of armature bars and clips to form complete armature coil elements
FIGURE 2 is a perspective end view of an armature winding 16 bar without a hydraulic header clip The bar is a rectangular array of solid 34 and hollow 36 copper strands FIGURE 3 is a perspective view of the armature winding bar 16 inserted in a clip 18 with braze strips 30 and a braze sheet 50 and a clip cover 32 shown to the side of the clip In figure 2, the braze ships 30 are shown interleaved between tiered rows of sohd the copper strands 34 and rows of hollow strands 36 of the bar 16
Each armature winding bar 16 includes a plurality of solid copper strands 34 and hollow copper strands 36 The strands 34,36 may also be constructed of metals other than copper, such as copper-nickel alloys or stainless steel The ends of the strands 34, 36 form the end of the armature winding bar 16 The free ends of the hollow strands 36 (and optionally some of the ends of the solid strands) extend axially beyond the free ends of short solid strands 34 For example, the free ends of the hollow strands extend approximately 0 31 inch (10 to 500 mils) beyond the free ends of the solid strands
In the armature winding bar 16 shown m figures 2 and 3, the extended hollow strands 36 form tiered rows with respect to the shorter rows of sohd strands 34 A four-tier array is shown in FIGURE 2 It will be appreciated that various numbers of tiers are possible in an armature bar The particular configuration of solid strands 34 and hollow strands 36 within the armature winding bar 16 is a matter of design choice There may be a one to one ratio of solid to hollow strands or a ratio of 6 solid strands
to one hollow strand The ratio may be greater or smaller depending on the capability of the bar design to remove heat dunng generator operation
Braze alloy stnps 30 and sheets 50 of a rolled, essentially phosphorous-free, silver based braze alloy are placed between the tiers of strands and between the strands and the internal surfaces of the hydraulic header clip 18 The silver braze alloy of the strips 30 and sheets 50 may contain other elements, such as tin, zinc or nickel, that can result in sohdus and hquidus modifications to suit specific applications The thickness of the alloy stnps 30 and sheets 50 is a matter of design choice For example, the stnp 30 thickness may be 0 060 inches and the sheet 50 thickness may be 0 020 inches
The braze alloy has minimal phosphorous The phosphorous-containing metallurgical phases of earlier braze alloys are susceptible to crevice conosion Braze alloys with less than 500 ppm (or 0 05 weight percent) phosphorous are considered phosphorous-free The benefits of using a phosphorous free braze alloy include reduced corrosion and hence improved generator availability and reliability
The pre-braze positioned braze alloy stnps extend beyond the ends of the short solid strands After brazing, the braze alloy forms a braze alloy isolation layer 52 over the end of the armature bar (but not the end of the hollow strands) The isolation layer shields the solid strand ends and the joints from the coolant passage m the clip The braze alloy also bonds the clip to the strands and the strand ends to each other
The stnps 30 inserted between the tiers of strands may be rectangular as shown m Figure 3 The braze stnps are shaped to fit between (he strand rows The edges of the braze stnps may be trimmed mto alignment with the outer surfaces of the strands of the bar 16 Substantially square braze sheets 50 may be fitted between the sides of the armature winding bar and the internal sides of the header clip The height of the alloy pre-positioned before brazing is selected so that the braze alloy will entirely melt dunng the braze process and not flow mto the open ends of the extended hollow strands
FIGURE 4 is a cross-sectional end view of the hydraulic header clip 18, the free ends of the solid 34 and hollow 36 strands, a ram 54 pressing the clip cover 34 into the clip and an mduction heating coil 66 to heat the assembly of the clip, strand and braze strips 30 and sheets 50 The hydraulic header clip 18 (also referred to as a stator bar clip) is formed of an electrically conductive material, such as copper The chp 18 is hollow and includes a rectangular collar 38 that slides over the outer side surfaces of the end of the armature winding bar 16 A rectangular slot 39 m the collar receives the end of the armature winding bar and interleaved strips 30 of the braze alloy The clip cover 32 fits into the matching rectangular slot 39 m the side of the collar 38 At the other end of the clip 18 is a cylindrical couplmg end 40 that is configured to connect to the coolant circuit
FIGURE 5 is a cross-sectional side view of a hydraulic header clip 18 receiving an armature winding bar 16 and the ram 54 to press the clip cover 32 mto the clip slot 39 during brazing The solid and hollow copper strands 34, 36 are disposed m a side-by-side and superposed relation one to the other, in a generally rectangular, multi-tier array The array may be compressed within the hydraulic end fitting or header clip 18 by means of the side cover 32 fitted within a similarly shaped slot 39 of the header clip Ram 54 presses the chp cover 32 mto the collar 38 and compress together the ends of the strands 34,36 and mterleaved braze strips
The clip is seated in an mduction heating coil 66 Mica spacers 76 separate the coil from the clip and the ram 54 from the clip cover The mica spacer between the coil and clip may be 0 060 inches and the spacer between the ram and clip cover may be 0 030 inches A cooled heat sink clamp 74 grasps the bar 16 just below the clip during the brazing process
Each hydraulic header clip 18 includes an internal mamfold chamber 42 within the clip collar 38 The manifold chamber 42 receives the strand ends 34, 36 of the armature bar and provides a conduit for coolant flowing through the clip 18 to enter or be discharged from the hollow strands 36 of the armature bar 16 Within the clip, the mamfold chamber 42 is internally open to a necked down internal chamber section 56
and to an expanded sub-chamber 58, which is aligned with the hose couplmg 40 and
configured to receive coolant flowing into or out of a hose The external and internal shapes of a clip may vary to suit different armature bar configurations that are present in large liquid cooled turbine generators
When the bar 16 is brazed to the hydraulic header clip 18, the free ends of the solid copper strands 34 are generally flush with a back wall 48 of the manifold chamber 42 The free ends of the hollow copper strands 36 extend partially into the manifold chamber 42 The ends of the hollow copper strands 36 may extend about 10 to 500 thousands of an inch beyond the ends of solid strands 34 and into the chamber 42
The differential lengths of the solid and hollow strands may be achieved by any suitable means including the use of a cutting tool to shorten the solid strands The alloy ships 30 between the tiers of the solid and hollow strands do not generally extend axially beyond the ends of the hollow strands 36 so that liquid braze when liquefied does not plug the open ends of the hollow strands In addition, filler metal 44 and the braze alloy sheets 50 (Fig 3) are pre-placed along the mtenor walls 46 of the clip to surround the enclosed ends of the hollow and solid strands The filler metal 44 may be a copper-silver alloy mat is positioned between the outer strands and the mtenor of the clip
At the end of the brazing process, a braze alloy isolation layer 52 (Fig 9) extends axially along and between all sides of each of the strands 34, 36 in the array, and also covers the ends (or faying surfaces) of the sohd strands 34 while leavmg the ends of the hollow strands 36 open and unobstructed for free flow of coolant through the hollow strands
The braze joint can be made with the axis of the armature bar in either a horizontal or a vertical onentation The vertical onentation is preferred because it aids alloy retention m the joint and permits pieces of the alloy to be more easily pre-placed on the surface of the assembly inside the hydraulic header clip, thereby providing a source of additional braze alloy and/or filler metal that will melt and flow over the bar 16 end surfaces to create a thicker layer of braze isolation layer 52 (Fig 9)
FIGURE 6 is a side view of a brazing chamber 60 assembly The braze chamber 60 is used to form a brazed connection of a liquid-cooled armature bar strand package to the hydraulic header clip 18 with a corrosion resistant braze alloy that is not susceptible to crevice corrosion initiation and provides for an alloy layer at the hquid-cooled interface surface of the brazement
A split braze chamber has left and right side hood sections 62 that laterally separate to receive the armature winding bar Once the bar 16 is mounted vertically m the left hood section, the right hood section closes against the left hood to form a closed chamber Windows 64 m the hood sections allow the braze process to be viewed The hood can withstand a brazing temperature of 1,000 degrees Celsius (1,832 degrees Fahrenheit) or more
A controlled gas atmosphere is pumped into the chamber to purge oxygen and form an internal substantially oxygen free atmosphere within the chamber The controlled gas atmosphere may comprise mixtures of nitrogen and hydrogen or 100 percent hydrogen After purgmg, the oxygen level is preferably less than 500 parts per million (ppm) oxygen m the chamber A substantially oxygen free atmosphere allows the brazing process to proceed without unwanted oxidation of the braze
FIGURE 7 is a perspective view of the interior of the left hood 62 of the chamber 60, without an armature bar or clip seated m the coil 66 The induction heating coil 66 heats the clip and bar to a predetermined brazing temperature for a prescribed time period The temperature profile of the heatmg coil is a design choice and depends on the brazing process bemg performed
A hook-shaped induction heatmg coil 66 receives the bar end and hydrauhc header clip 18 An upper guide 71 aligns the top of the hydraulic header clip such that the collar is between the legs 78 of the induction coil 66 A heat sink clamp 74 secures the armature bar vertically within the braze chamber and prevents liquid braze from flowing down between the strands of the bar The ram 54 presses the clip cover 32 and strand ends 34, 36 into the clip during the braze process A pneumatic drive cylmder 55 moves the ram and applies a compressive force to the clip cover
The bottom wall 68 of the chamber mcludes a seal to receive the armature bar and prevent leakage of the gas atmosphere m the chamber The inert gases m the chamber may be maintained at an above-atmospheric pressure to ensure that oxygen does not leak into the chamber
Multiple temperature indicators 70 m the chamber and are located at various positions inside die brazing chamber An oxygen sensor 72 within die chamber generates a signal m real time of the oxygen level m parts per million in the chamber atmosphere The oxygen signal may be provided to a programmable logic controller 73 for the brazmg process
The programmable logic controller (PLC) 73 automates the braze process protocol The PLC controls the induction coil and monitors the temperature and oxygen level in the chamber during die brazing process The PLC may also control the force applied by the ram 54, 55 and the linear movement of die ram The control program executed by die PLC may include multiple tune and temperature cycles for heating thee coil and the chp and armature bar assembly
The heat sink 74 is a straight bar clamp diat is spring loaded and grasps the bar 16 just below me chp The heat sink is water cooled to ensure that the armature winding bar 16 below me clip is cooler than me hquidus temperature of die braze alloy The cool armature bar at me clamp point causes liquid braze alloy flowing down between the bar strands to solidify
FIGURE 8 is a flow chart of exemplary steps for brazmg In step 80, the armature bar 16 and clip 18 assembly is seated m the induction heating coil Mica insulation sheets 76 may separate the clip from the mduction coil In step 81, ram 54 is positioned against the cover 32 of die clip to force the cover and bar into the clip The armature bar is mounted vertically such mat the free ends of die solid strands 34 are honzontal during the brazmg process An upper stop guide 71 in the left hood (Fig 7) provides an alignment stop for the free end of die clip In general, the clip and bar are seated such mat me legs 78 of me induction coil 66 are in die same plane as are the extended free ends of the hollow strands 36 In step 82, a heat sink, e g, a cooled bar clamp, is
applied to the armature winding bar 16 at a location below the clip 18 The heat sink cools the armature bar below the clip to prevent liquid braze alloy from flowing down between the bar strands
In step 84, the hood sections 62 of the braze chamber 60 are closed The closed chamber is purged to an oxygen free atmosphere, such as less than 500 parts per million of oxygen The control gas may be a mixture of hydrogen and nitrogen, or alternatively be 100% hydrogen or have some other composition that allows for a good braze joint
In step 86, the clip is heated by the induction coil to braze the strand ends together, braze the clip to the strands, and to form a isolation layer 44 (Fig 9) over the solid ends of the clip To reduce liquation of the braze alloy, the braze assembly is held at a temperature just below sohdus of the braze alloy to allow equalization of temperature within the braze chamber for a period of, for example, 30 to 600 seconds, in step 88 Thereafter, the power applied to the induction coil 66 is mcreased to quickly raise the temperature to above the hquidus temperature of the braze allow but below the maximum allowable braze temperature for the specific alloy, during step 90 This higher temperature is held for a period of, for example, 5 to 100 seconds At the higher temperature the braze alloy bonds to the strands and to the clip In addition, at the higher temperature the braze alloy strips extending beyond the solid strands melts and pools on the ends of the solid strands
At the higher temperature, the assembly of clip, strands and braze alloy softens and partially liquefied The ram 54 pressing against the clip cover 32 causes the cover to slid further into the slot 39 of the clip, in step 91 The hold time above the hquidus temperature may be controlled by the amount of displacement experienced by the clip cover When the desired displacement is met, the braze cycle is terminated Accordmgly, the controller 73 monitors the displacement of the ram against the cover m step 92 The controller terminates the high temperature induced by the coil when the ram displacement exceeds a predetermined level, e g , up to 0 25 mch, in step 94 The volume and placement of the alloy force applied by the ram to the clip cover, and
the temperature profile in the chamber may be controlled by the PLC controller 73
and selected to assist the capillary flow of the liquid braze alloy between the strands and create the desired layer 52 on the ends of the solid strands and between the extended hollow strands
To control the alloy flow at hquidus within the hydraulic header clip, the clip to cover clearance may preferably be between 0 001 to 0 005 mch between mating surfaces The liquid cooled heat sink 74 adjacent to the clip on the strands also controls liquid alloy flow by solidifying the braze alloy below the clip To allow proper alloy flow between the strands, a faying surface allowance of preferably between 0 001 and 0 010 mch is used and a braze assembly force preferably of 100 to 1,800 pounds of force is applied by the ram 54 to the assembly during the braze operation
During heating to above hquidus temperature (step 90), the liquid braze alloy pools on top of the ends of the solid strands The pooled alloy forms a corrosion-resistant isolation layer 52 on the end of the armature winding bar In addition to the braze strips and sheets between adjacent strands and between the strands and the clip, braze material, e g», braze rods or strips, may be prepositioned on the ends of die solid strands or may be added during the braze process to ensure sufficient braze material pools on the end of the solid strands
When heated to its melting temperature, the braze alloy flows and fills in the spaces between the solid and hollow strands 34, 36 and between the strands and the interior surfaces of the header clip, including at the opening of the header clip mto which the strands are inserted At its melting temperature, the alloy remams sufficiently viscous that it does not flow substantially to the free ends of the hollow strands The extended length of the hollow strands 36 provides a safety margin in that the excess alloy material does not flow out as far as the ends of the hollow strands, precluding the possibility of pluggmg the coolmg passages m the hollow strands
The layer 52 has sufficient thickness and quality to fill the area between the extended hollow strands and over the ends of the short solid strands Capillary flow draws the liquid braze alloy into the faymg surfaces between the strands and between the strands and manifold chamber 42 of the clip The layer 52 produces a corrosion resistant
isolation layer on the water inlet end surface of the armature winding bar The isolation layer seals the liquid-cooled stator armature bar strands to the hydraulic header clip
The brazed clip and strands remain in the control atmosphere of the hood untd the temperature decreases to a temperature below which no appreciable oxidation forms on the metal surfaces Thereafter, the hood sections are separated and the armature bar and clip assembly is removed from the braze chamber
FIGURES 9 and 10 are end and cross-sectional side views respectively of the hydraulic header clip brazed to an armature bar The hollow 36 and sohd 34 end strands are brazed to the collar 38 of the clip 18 such that the free ends of the hollow strands are open to the manifold chamber 42 A braze alloy isolation layer 44 has formed over the free ends of the solid strands 34 and in the crevices between the strands and between the strands and internal surfaces of the manifold chamber of the clip 18 The mimmum thickness of the" braze alloy isolation layer 44 may be at least 0 050 of an inch
While the invention has been descnbed m 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.

WHAT IS CLAIMED IS:
1. A method of forming a brazed joint between an armature bar (16) and a hydraulic
header clip (18) comprising:
a) assembling an end of the armature bar, hydraulic header clip and a substantially phosphorous-free braze material (50,52);
b) positioning the assembly of the armature bar, hydraulic header clip and the braze material in a braze chamber (60), such that the clip is seated in aa induction heating coil;
c) heating (86, 88) the assembly to a first temperature withir; 200 degrees Fahrenheit of and below a solidus temperature of the braze material by applying electrical current to the induction heat coil;
d) heating (90) the assembly to a second temperature above the liquidus temperature of the braze material;
e) pooling liquid braze material orr ends of the armature bar, and
f) cooling (94) the assembly and thereby forming a braze layer on the end of the armature bar.
2. The method of claim 1 wherein steps (b) to (f) are performed in cm substantially
oxygen free atmosphere within the braze chamber (60).
3. The method of claim 2 wherein the substantially oxygen free atmosphere is substantially nitrogen and hydrogen.
4. The method of claim 2 wherein the oxygen free atmosphere is substantially all hydrogen.
5. The method of claim 2 wherein the substantially oxygen free autosphere has an oxygen level of less than 400 parts per million.
6. The method of claim 1 further comprising applying a force (81) to the clip during heating to compress the assembly.
7. The method of claim 6 wherein the force (8.1) is applied to a side cover of the clip and said side cover is juxtaposed against the armature bar.
8. The method of claim 7 wherein The force is in a range of 100 to 1,800 pounds.
9. The method of claim 1 wherein said armature bar is positioned vertically in the chamber (60) and the bar comprises solid strands and hollow strands (34, 36) having free hollow ends extending axially beyond solid free ends of said solid strands.
10. The method of claim 1 wherein the armature bar (16) comprises solid and hollow strands (34, 36) formed of at least one of copper, copper-nickel alloys and stainless steel.