Note: Descriptions are shown in the official language in which they were submitted.
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MOVABLE HEATING METHOD AND SYSTEM HAVING FIXED HEATING
SOURCE FOR BRAZING STATOR BARS
BACKGROUND OF THE INVENTION
The present invention relates to brazing the ends of large stator bars that
are used in
power and industrial generators, positioning these bars in a brazing station,
and fitting
header clips to the bars during assembly.
Stator bars are typically large, long and heavy, e.g., 35 feet long and
hundreds of
pounds (lbs.). The bars are generally straight and extend the length of a
stator. When
seated in a stator, the straight sections of the stator bars form a
cylindrical array
around a rotor. The ends of the stator bars extend axially from opposite ends
of the
stator. The end portion of the stator bars extend from the stator and are
curved to
form end turns. The ends of stator bars are connected through copper or
stainless steel
fittings and water-cooled connections to form continuous hydraulic winding
circuits.
Each water-cooled stator bar comprises an array of small rectangular solid and
hollow
copper strands. The array of copper strands in each bar are generally arranged
in a
rectangular bundle. The hollow strands each have an internal duct for
conducting
coolant through the bar. The ends of the bars are each connected to a
hydraulic
header clip.
The hydraulic header clip serves as an electrical and a cooling flow
connection for the
armature winding bar. The hydraulic header clip is a hollow connector that
includes
an enclosed chamber for ingress or egress of a cooling liquid, typically
deionized
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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 ends of the solid and hollow
strands are
brazed to a hydraulic header clip fitted to the end of the stator bar.
The stator bar end and clip assembly must be heated to melt the braze alloy
and braze
the assembly together. A system and method is needed for applying heat to the
clip
and stator bar end assembly during the brazing process.
BRIEF DESCRIPTION OF THE INVENTION
A method has been developed to heat a metal assembly in a brazing chamber
comprising: placing the assembly in the brazing chamber, wherein the assembly
is
seated in a heating coil; positioning a conductive mass between a press and
the
assembly; applying the press to the assembly while the assembly is seated in
the coil;
heating the assembly and the mass by applying energy to the coil; brazing the
assembly with the heat from the coil, and removing the press and cooling the
brazed
assembly. The assembly may include a stator bar end, e.g., stator strands, and
a clip.
The method may further comprise heating coil includes a U-shaped section and
the
stator bar end seats in the U-shaped section, wherein the conductive mass
seats in the
U-shaped section and substantially fills an area defined by an entrance to the
U-
shaped section. Further, the conductive mass seats in the coil when abutting
the stator
bar and clip.
The method may also be heating a stator bar and clip assembly in a brazing
chamber
comprising: placing the stator bar and clip assembly in the brazing chamber,
wherein
the assembly is seated in a U-shaped section of the heating coil; seating a
heating
mass in the U-shaped section between a ram and the assembly; extending the ram
to
press the heating mass against the assembly in the coil, wherein the mass fits
into the
U-shaped section; heating the stator bar and clip by applying energy to the
coil;
brazing the stator bar to the clip with the heat from the coil, and removing
the press
and cooling the brazed clip.
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A heating system has been developed for brazing a stator bar to a clip
comprising: a
heating coil having a seat to receive the stator bar and clip; a heating mass
adapted to
fit into the seat of the coil, and an extendible press applying the mass to
the stator bar
or clip. The seat may be a U-shaped section of the coil and the mass
substantially
fills an area defined by an entrance to the seat of the coil.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGURE 1 is a schematic illustration of a liquid-cooled stator winding
arrangement
illustrating the stator, stator 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 clip 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 line 5-5 in Figure 4.
FIGURE 6 is a perspective side view of a brazing chamber.
FIGURE 7 is an enlarged view of the interior of the brazing chamber that shows
an
induction heating coil and armature winding bar heat sink.
DETAILED DESCRIPTION OF THE INVENTION
FIGURE 1 illustrates a liquid-cooled stator bar arrangement for a stator in a
typical
liquid-cooled generator. A stator core 10 has stator core flanges 12 and core
ribs 14.
Stator bars 16 (also referred to as armature winding bars) pass through
radially
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extending slots in the stator core and are capped at opposite ends by
hydraulic header
clips 18 fitted to the ends of the bars. Copper or stainless steel fittings 20
connect
adjacent ends of the stator bar pairs to form the complete armature coil.
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. Each stator bar forms a half an
armature
coil. A pair of stator bars linked at their opposite ends form a complete
armature coil.
FIGURE 2 is a perspective end view of an stator bar 16 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 end of an armature winding bar 16
inserted in
a clip 18 with braze strips 30 and a braze sheet 50. A clip cover 32 is shown
to the
side of the clip 18. In Figure 2, the braze strips 30 are interleaved between
tiered rows
of solid the copper strands 34 and rows of hollow strands 36 of the bar 16.
Just prior
to brazing and at the end of the stator bar, braze strips are inserted between
the strands
34, 36. In addition, the braze sheets 50 and clip 32 are assembled in the clip
18.
As shown in Figure 2, the pre-braze positioned braze alloy strips extend
beyond the
ends of the short solid strands. The height of the alloy pre-positioned before
brazing
is selected so that the braze alloy will entirely melt during the braze
process and not
flow into the open ends of the extended hollow strands.
The hydraulic header clip 18 (also referred to as a stator bar clip) is formed
of an
electrically conductive material, such as copper. The clip 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 in the collar receives the end of the
armature
winding bar and interleaved strips 30 of the braze alloy. A clip cover 32 fits
into the
matching rectangular slot 39 in the side of the collar 38. Sheets 50 of braze
alloy are
arranged around the inside surface of the collar and surrounding the end of
the bar. At
the other end of the clip 18 is a cylindrical coupling end 40 that is
configured to
connect to the coolant circuit.
During brazing, the stator bar is held in a vertical position. When the stator
bar is
vertical the planer end of the bar is horizontal. An end clip 18 is fitted to
the end of
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the stator bar and braze material 30, 50 is placed between the clip and bar.
Melted
braze alloy forms a pool over the solid strand ends of the stator bar. The
braze alloy
material 30, 50 may be a rolled, essentially phosphorous-free, silver based
braze alloy.
After brazing, the braze alloy forms a braze alloy isolation coating 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 in the clip. The
braze alloy
also bonds the clip to the strands and the strand ends to each other.
During brazing, the induction heating coil 66 heats the assembly of the clip,
strand
and braze strips 30 and sheets 50. The heating coil 66 heats the clip and end
of the
stator bar to braze them together. A heating mass 57 may be placed between the
end
of the ram 54 and the clip cover 32. The heating mass 57 may be formed a
thermally
conductive material, such as steel or copper. The heating mass 57 is heated by
the
heating coil 66 and conducts heat to the clip cover. The heating mass may have
an
inverted "C" shape in cross-section to fit the end of the ram and fit into the
"C"
shaped heating coil 66. The heating mass slides between the legs of the
heating coil
without touching the coil. The heating mass may also have a slot on its front
face to
receive a lever arm a clamp used to hold the clip to the stator bar while the
clip and
bar assembly are positioned in the brazing chamber.
The heating mass 57 assists in applying heat to the clip cover 32 during
brazing. The
heating mass is heated by the coil 66. Heat energy is transferred by
conduction from
the mass 57 to the clip cover 32. The heating mass abuts directly against the
clip
cover (or other exposed outer surfaces of the clip and bar assembly) to
promote
conductive heat transfer to the cover or exposed outer surfaces.
Further, the heating mass seats in the U-shaped coil 66. The C-shaped cross-
sectional
profile of the heating mass 57 substantially fills the entrance area 67 of the
U-shaped
seat in the coil. For example, the heating mass may fill at least 75% of the
entrance
area of the seat in the coil. The sides of the heating mass 57 may be
substantially
parallel and adjacent to the sides of the heating coil to ensure that the mass
is heated
by the coil.
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Mica may be used for spacers 76 separating the coil from the clip and the
heating
mass 57 from the ram 54. The mica spacer between the coil and clip may be
0.060
inches. A thermal mass spacer may be used to insulate the shaft of the ram 54
from
the heating mass 57.
A heat sink clamp 69 is attached to the stator bar below the heated end of the
bar. The
heat sink cools the stator bar below the clip. By cooling the bar, liquefied
braze is
prevented from flowing down between the strands when the vertical bar is in
the braze
hood.
The braze joint is preferably made with the stator bar in a vertical
orientation. The
vertical orientation is preferred because it aids alloy retention in 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 over the ends of the solid copper strands of
the bar.
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 induction 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
clip 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 in 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 in the side of the
collar 38. At
the other end of the clip 18 is a cylindrical coupling 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 into the
clip slot 39
during brazing. The solid and hollow copper strands 34, 36 are disposed in a
side-by-
side and superposed relation one to the other, in a generally rectangular,
multi-tier
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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 clip cover 32 into the collar 38 and compress
together the
ends of the strands 34, 36 and interleaved braze strips.
The clip and stator bar assembly is seated in an induction heating coil 66.
Mica
spacers 76 separate the coil from the clip. 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 manifold 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
manifold 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 coupling 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 strips 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 interior
walls 46 of
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the clip to surround the enclosed ends of the hollow and solid strands. The
filler metal
44 may be a copper-silver alloy that is positioned between the outer strands
and the
interior of the clip.
At the end of the brazing process, a braze alloy isolation layer 52 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 solid strands 34 while leaving 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 orientation. The vertical orientation is preferred because it aids
alloy
retention in 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.
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 liquid-
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 in the
left
hood section, the right hood section closes against the left hood to form a
closed
chamber. Windows 64 in 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 purging, the oxygen level is preferably less than 500 parts
per
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million (ppm) oxygen in 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 in 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 heating coil is a design choice and
depends on
the brazing process being performed.
A hook-shaped (or U-shaped) induction heating coil 66 receives the bar end and
hydraulic 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 cylinder 55 moves the ram and applies a compressive force to
the
clip cover.
The bottom wall 68 of the chamber includes a seal to receive the armature bar
and
prevent leakage of the gas atmosphere in the chamber. The inert gases in the
chamber
may be maintained at an above-atmospheric pressure to ensure that oxygen does
not
leak into the chamber.
Multiple temperature indicators 70 in the chamber and are located at various
positions
inside the brazing chamber. An oxygen sensor 72 within the chamber generates a
signal in real time of the oxygen level in parts per million in the chamber
atmosphere.
The oxygen signal may be provided to a programmable logic controller 73 for
the
brazing 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 the brazing process. The PLC may also control the force
applied
by the ram 54, 55 and the linear movement of the ram. The control program
executed
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by the PLC may include multiple time and temperature cycles for heating the
coil and
the clip and armature bar assembly.
The heat sink 74 is a straight bar clamp that is spring loaded and grasps the
bar 16 just
below the clip. The heat sink is water cooled to ensure that the armature
winding bar
16 below the clip is cooler than the liquidus temperature of the braze alloy.
The cool
armature bar at the clamp point causes liquid braze alloy flowing down between
the
bar strands to solidify.
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.
