Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVEMENTS IN ELECTRICAL MACHINES
This invention relates to electrical machines and in particular to insulated
electrical
conductors suitable for use as excitation windings on the stators of high
voltage
electrical machines, for example, machines operating at voltages greater than
3KV.
Known types of excitation winding for the stators of such machines generally
comprise
solid rectangular copper conductors which are electrically insulated from each
other
and from an earthed laminated steel core on which they are wound. Materials
used to
insulate the conductors are chosen so as to have properties - such as
thickness, thermal
conductivity, dielectric strength, and permittivity - which are appropriate to
the size of
the machine, applied voltage and temperature rise.
The power output of a rotating electrical machine (whether a motor or a
generator) is
a function of the properties of the laminated magnetic steel core, the
excitation
windings and their operating temperature. An "output coe~cient" or "specific
torque
coe~cient" of such machines is often quoted as a useful means of comparing the
power outputs of machines of differing design. Its units are torque per unit
volume and
it may be derived by dividing the machine's power output by the volume of the
stator
within the air-gap periphery.
Having been the subject of industrial manufacture for over 100 years, rotating
electrical machines are considered a mature product using mature technologies.
Over
the century of production the materials and processes used in the construction
have
evolved slowly resulting in a steady increase in output coefficient (about
3.0% per
annum) for the most popular machines. Recently, their evolutionary progress
has
slowed down and has now reached a plateau, suggesting that further development
is
either unlikely or will be very slow.
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It is necessary to externally insulate the stator windings of such machines
from each
other and the stator core. It is also necessary to internally insulate the
windings by
insulating the conductors from each other within the winding. The insulation
materials
presently used to perform the insulation function have only limited ability to
withstand
high temperatures, with modest electric strength properties and generally poor
thermal
conductivity. Furthermore, during operation of the machines, heat is generated
due to
electrical losses in the winding conductors, but the poor thermal conductivity
of the
insulation materials results in poor transfer of the heat, and this in turn
inhibits the
output coefficients of the machines on which the insulation materials are
used.
Since there are only small differences between the stator winding insulation
systems
used currently by leading manufacturers of machines, the thermal and
dielectric
performance of such systems is similar. Typically, all use combinations of
mica,
polyester film and woven glass materials impregnated with synthetic resins.
The mica is
used in the form of a so-called "paper" which is supported by either polyester
film or
woven glass and is wrapped around the conductors to insulate them from
external
contact. To complete the insulation process, they are vacuum pressure
impregnated
with the synthetic resin (for example, an epoxide resin) as the last process
after
positioning the windings into the stator.
It is an object of the present invention to improve the output coefficients of
electrical
machines and hence reduce their capital cost per KW output of electrical
energy by
increasing the heat transfer capability of their stator conductor windings.
This is
achieved in the invention by means of a novel type of composite conductor.
According to the present invention, a composite conductor for use as a winding
of a
high voltage electrical machine comprises:
a plurality of strands of conductor material forming a conductor bundle which
in cross-section is of generally rectangular shape, the strands being
insulated from each
other within the bundle.
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an insulating sleeve of substantially homogeneous polymeric material
surrounding the conductor bundle; the insulating sleeve also having a
generally
rectangular shape in cross-section and the polymeric material being filled
with at least
one electrically insulating filler material which conducts heat more
efficiently than the
polymer alone, and
conductive material forming a corona shield coating at the inner and outer
surfaces of the insulating sleeve.
The conductor bundle is impregnated with a curable high-temperature resistant
insulating material, such as a synthetic resin or polymer material, whereby
the cured
composite conductor is rendered sufficiently strong and rigid to enable its
use in the
windings of electrical machines.
It should be noted that the term "rectangular" as used herein includes shapes
which are
square (having four sides all of substantially equal dimensions) and
rectangles and
squares having rounded corners.
A composite conductor in accordance with the invention, by virtue of its
construction
and the materials it uses, provides a high efficiency winding for the stator
of an
electrical machine. Due to its filler material(s), the insulating sleeve not
only has
superior dielectric strength properties which permits operating at reduced
sleeve wall
thickness and/or increased electric stress, but also has much higher thermal
conductivity and thermal capability (temperature resistance) than known
windings. The
higher thermal conductivity permits a considerable increase in the heat
transfer
capability from the composite conductor into the stator core, which may be
cooled by
heat exchange with ducted airstreams. Furthermore, insulation of the conductor
strands from each other substantially reduces high frequency eddy current
losses in the
stator winding, and the conductive surface coating on the insulation sleeve
contributes
to enabling safe operation at higher electric stresses at the surface of the
sleeve. The
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net result of the above is that the output coefficient is substantially
increased and hence
the capital cost of the delivered power is substantially decreased.
Preferably, the corners of the conductor bundle's rectangular shape are
radiused to
minimise electrical stress concentrations. The radius dimension may be up to 5
mm.,
preferably between 2-3 mm., but it is preferred that the corners of the
insulating sleeve
are substantially rectilinear, having a radius of not more than about lmm.
Preferably, the strands of conductor material are collectively twisted around
the
longitudinal centreline of the conductor bundle in a similar manner to which
the strands
in a rope are twisted about the longitudinal centreline of the rope. This
angular/spatial
transposition of the strands of conductor material longitudinally of the
bundle due to
its twist inherently cancels eddy currents as they arise, substantially
reducing or
eliminating the associated losses.
The at least one insulating filler material in the polymer insulating sleeve
is preferably a
metallic oxide and/or a metallic nitride.
The polymeric sleeve material preferably comprises a high temperature polymer,
e.g. a
polymer selected from the groups comprising fluoropolymers or aromatic
polymers,
and the conductive coating material for reducing electric stress variations at
the
surfaces of the composite conductor may comprise a graphitic or silicon-based
material, preferably a high-temperature resistant polymer or paint material
which has
sufficient of the conductive material incorporated therein to render it
conductive.
Preferably the strands of conductor material comprise copper, but other
materials may
be useable, such as aluminium or silver. Whatever the material from which the
conductor strands are made, it is preferred they are insulated from each other
by means
of a coating of high-temperature resistant synthetic resin or polymer material
on each
strand. This may be achieved either by impregnation of the conductor bundle
with a
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resin or precursor polymer material in its uncured state, or perhaps more
conveniently
and reliably by using conductor strands that have been previously manufactured
with
an insulating coating before being incorporated into the conductor bundle.
Both these
conductor coating techniques are well known in the art.
The invention further envisages a process for making the above composite
conductor,
comprising the steps of
gathering together a plurality of strands of conductor material to form a
conductor bundle,
impregnating the conductor bundle with a curable high-temperature resistant
insulating material, the impregnation occurring one of simultaneously with the
gathering process and subsequent thereto,
applying a coating of conductive material to the exterior of the twisted
conductor bundle to form a first, inner, corona shield,
extruding an insulating sleeve of homogeneous polymeric material onto the
coating of conductive material on the conductor bundle, the polymeric material
having
been previously filled with at least one insulating filler material which
conducts heat
more efficiently than the polymer alone, and
applying a coating of conductive material to the outer surface of the
insulating
sleeve to form a second, outer, corona shield;
wherein each strand of conductor material is provided with an insulating
coating by at
least one of coating the strands before the formation of the conductor bundle,
and
coating the strands during the impregnation step.
We prefer that the conductor bundle is formed into a generally rectangular
shape
during or subsequent to the gathering process. We further prefer that
subsequent to the
formation of the conductor bundle, the bundled strands are twisted bodily
about a
longitudinal centreline of the bundle to form a twisted conductor bundle.
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Whereas it is known to impregnate the insulating wrappings of bar conductor
windings
with synthetic resin after they have been assembled onto the stator, then
partially or
wholly to cure the resin, it is envisaged that in the present invention,
impregnation
occurs before the composite conductor is wound onto the stator, the composite
conductor then being wound onto the stator while the resin or precursor
polymer
material is uncured or only partly cured. This facilitates the impregnation
process while
allowing easy manipulation of the composite conductor during the stator
winding
process before the resin is fully cured.
The invention also includes a stator for a rotary electrical machine
comprising a
laminated steel core provided with a plurality of radially oriented slots
extending
longitudinally of the stator, each slot housing a winding comprising either a
plurality of
turns of a single length of the above composite conductor, or a plurality of
turns
comprising a plurality of lengths of the above composite conductor, successive
turns of
the composite conductor being in contact and in radial registration with each
other.
The winding is retained in its slot, preferably by a high thermal
conductivity,
electrically insulating retaining means fixed in the radially outer end of the
slot.
Preferably, the retaining means is a filled polymer composition having
relatively high
thermal conductivity compared to the polymer in its unfilled state.
The retaining means may comprise an extrusion which is forced into the end of
the
slot.
It should be understood that the materials mentioned herein in connection with
composite conductors according to the invention are best estimates of which
materials
will probably be suitable.
Exemplary embodiments of the invention will now be described with reference to
the
accompanying drawings, in which:
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_7_
Figure 1 is a perspective view of part of the inside of a stator of a rotary
electrical machine provided with a conventional winding;
Figure 2 is a cross-section of a composite conductor according to the present
invention;
Figure 3 is a cross-section of an alternative composite conductor according to
the present invention;
Figure 4 is a radial section through part of a stator according to the
invention,
showing a stator slot containing a winding comprising six turns of the
composite
conductor shown in Figure 2;
Figure 5 is a diagrammatic representation of a process for manufacturing a
composite conductor according to the invention.
Referring to Figure 1, there is shown the inner circumference of one end of a
stator 10
of an A.C. electrical machine. The stator is intended to encircle the rotor
(not shown)
of the electrical machine. As is conventional, the stator body is composed of
a large
number of steel laminations and is formed with a large number of radially
oriented slots
12 which extend longitudinally of the stator body.
Each slot 12 houses an excitation winding 14, successive "turns" 14A, 14B of
which
contact each other and are in radial registration within their slot. The
windings are
retained in their slots by wedges 16, made of a suitable polymer insulation
material,
such as filled epoxide resin. During manufacture of the stator, wedges 16 are
forced
into position within the mouths of the slots 12.
The loops or "turns" of the windings each comprise solid rectangular copper
conductor bars 14A, 14B which are pre-formed to the correct shape to enable
their
installation into the stator slots. The conductor bars are electrically
insulated from each
other and from the earthed steel core by wrappings of mica paper on polyester
support
film, the wrappings being pressure-impregnated with epoxide resin and cured
later.
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_g_
To enable the construction of the stator winding 14 from pre-formed lengths of
conductor bar, each turn of the winding consists of a plurality of lengths of
conductor
bar whose ends 15 projecting from the stator core are bent at compound angles
as
shown, these ends being brazed or otherwise securely joined together in
electrical
contact (not shown) to complete the turn. The ends of radially adjacent
conductor bars
14A, 14B are bent in opposing directions.
As Figure 1 shows, where the ends of the conductor bars 14A, 14B, etc.,
project from
the ends of the slots 12, packing blocks 18, 20 are inserted between adjacent
conductor bars, packing blocks 18 being adjacent the end of the stator core
and
packing blocks 20 being spaced away from the stator core. Various of the
packing
blocks 18, 20 and the bent portions of some of the conductor bars 14A and 14B
have
been removed at the lower left of Figure 1 to show the construction more
clearly. The
packing blocks are moulded to shape, being made up from glass cloth bags
containing
glass mat laminate filler pieces impregnated with epoxide resin. Packing
blocks 20 are
bound to the adjacent conductor bars with glass fibre tape 22.
After assembly of the stator winding and insertion of the packing blocks,
assembly of
the stator is completed by tying the bent portions 15 of the conductor bars
14A, 14B,
to support rings 24, 26 using glass fibre cord 28. The packing blocks and the
support
rings provide support to the ends of the conductor bars and prevent vibration
during
service. Finally, the stator assembly is heat-treated at an appropriate curing
temperature for its resin-impregnated portions, this temperature being below
that
which would cause deterioration of the polyester film component of the
conductor
wrappings.
Turning now to Figure 2, a composite conductor 30 is shown in cross-section
and is
intended to be used as a stator winding of a high voltage electrical machine,
thereby
substituting for the wrapped solid conductor bars 14A, 14B of Figure 1. Unlike
the
solid conductor bars, however, the core of composite conductor 30 comprises a
large
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number of strands 31 of conductor material forming a conductor bundle 32.
Preferably,
the strands 31 of conductor material comprise copper, but other materials may
be
useable, such as aluminium (cheaper, but not such a good electrical conductor
as
copper) or silver (expensive, but a better electrical conductor). A typical
dimension for
an individual strand of conductor material is likely to be of the order of
O.lmm, so it
will be appreciated that the number of strands 31 needed to form a conductor
bundle
32 is likely to be much larger than that diagrammatically indicated in Figure
2, and
their cross-sections will consequently appear smaller relative to the total
cross-section
of the composite conductor 30.
Conductor bundle 32, although of generally rectangular shape, has rounded
corners 34
to minimise electrical stress concentrations during operation of the
electrical machine.
The dimensions of the conductor bundle's corner radii R are in the range 0.5
to 5 mm.;
2-3 mm. may be optimum.
To reduce high frequency eddy current losses in the stator winding, the bundle
of
copper strands 32 is bodily twisted around the longitudinal centreline C of
the
composite conductor 30 in a similar manner to which the fibres in a rope are
twisted
about the longitudinal centreline of the rope. It should be noted that the
spatial
transposition of the strands of conductor material longitudinally within the
bundle due
to its twist inherently cancels eddy currents as they arise.
An insulating sleeve 36 of high temperature polymer material (e.g., a
fluoropolymer or
an aromatic polymer) surrounds the conductor bundle. In accordance with the
invention, the polymer is homogeneously filled with an insulating material
which
conducts heat more efficiently than the polymer alone. This is explained
further below.
The insulating sleeve has a rectangular shape in cross-section, but unlike the
shape
adopted for the conductor bundle, it is preferred that the corners of the
insulating
sleeve are not substantially rounded, i.e. the long and short sides of the
insulating
sleeve meet substantially at right angles. However, there may be a small
radius present,
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say up to about 1 mm., due to the manufacturing process and the need to apply
a
corona shield coating evenly to the surface of the insulation sleeve. The
total width W
and height H of the insulating sleeve 36 is substantially uniform in the
lateral and
lengthwise directions of the composite conductor. The sleeve's wall thickness
Tl is
similarly uniform, except of course in the regions adjacent the rounded
corners 34 of
the conductor bundle 32.
Thickness Tl should ideally be as small as possible consistent with adequate
electrical
insulation properties, because a thin wall for the sleeve 36 enables more
rapid
conduction of heat away from the bundle of conductor strands 32 and also
allows a
larger conductor bundle to be included in the composite conductor for the same
overall
size of composite conductor. The latter point is illustrated by reference to
Figure 3,
which shows an alternative embodiment of the invention in which the thickness
T2 of
the sleeve is about twice that of thickness Tl in Figure 2. It will be noticed
that the
number of conductor strands 31 that can be included in the composite conductor
is
much less in Figure 3 than in Figure 2.
Ordinary polymer sleeves for electric cables are extruded onto the conductor
or
conductor bundle by means of an extrusion head through the centre of which
runs the
conductor to which the sleeve is applied. This is a well known and understood
process.
A polymer sleeve having a wall thickness of about 0.1 mm can be produced by
known
extrusion processes and it is preferred that the thickness of the polymer
sleeve in the
invention (excluding the corner regions) should be in the range 0.4 to 2.OOmm.
For
example, a typical value for T1 could be about O.Smm, preferably 0.65mm, and a
typical value for T2 could be about lmm.
In order to permit operation of the composite conductor 30 with as small a
value of T1
as possible, the invention provides that the polymeric material from which the
sleeve
36 is made is homogeneously compounded with one or more powdered materials
which conduct heat more efficiently than the polymer alone. These materials
may be
CA 02368557 2001-10-O1
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metallic oxide or metallic nitride fillers, such as aluminium oxide or
aluminium nitride.
As a result, the insulating sleeve 36 not only has superior dielectric
strength properties
which permits operating at reduced thickness, but also has much higher thermal
conductivity and temperature resistance than known windings. The approximate
volume ratio of polymer to filler material may be 10% to 75%.
To obtain low eddy current losses and hence good electrical efficiency of the
composite conductor 30 in its role as a current carrying stator winding in an
electrical
machine, the individual conductor strands 31 in the conductor bundle 32 are
insulated
from each other according to the invention. As discussed in more detail below,
this can
be achieved by impregnating the conductor bundle with a synthetic resin
material, such
as an epoxide resin, and/or using conductor strands that have been previously
provided
with an insulating coating. Such pre-coated wires are routinely used in the
production
of windings for small electrical machines, being wound straight onto a rotor
or stator.
Before or during the application of the insulating sleeve to the conductor
bundle a thin
coating of conductive material e.g., a graphitic or silicon-based material,
such as a
carbon-filled high-temperature resistant polymer, is applied so as to form a
corona
protective shield on the inside surface of the of the extruded polymer, i.e.,
at the
interface between the insulating sleeve and the conductor bundle. During or
after the
application of the insulating sleeve to the conductor bundle, a thin coating
of the same
or a similar conductive material is applied to the outside surface of the
insulating sleeve
so as to form a corona protective shield thereon. The external coating is
indicated by
the dotted line 38 surrounding the sleeve 36 in Figure 2 (the inner corona
shield is not
shown). The purpose of the conductive coating material is to equalise the
electric
stresses on the surface of the composite conductor during operation of the
electrical
machine and thereby avoid localised breakdown of the insulation afforded by
the
insulating sleeve 36. We have found that both inside and outside surfaces of
the
insulating sleeve should be provided with the conductive corona shield coating
to
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provide good equalisation of the electric stresses at the surface of the
composite
conductor.
A suitable thickness for the inner and outer corona shields is in the range
about 0.1 to
0.3mm, preferably at the lower end of this range. As already stated, a polymer
film of
this thickness is known to be extrudable and therefore the inner and outer
corona
shields may be applied by means of extrusion, as hereafter described.
Alternatively, the
inner corona shield may be applied by winding a thin tape of the conductive
material
onto the conductor bundle before the insulating sleeve is applied thereto and
the outer
corona shield may be applied by winding a thin tape onto the outside of the
insulating
sleeve after the insulating sleeve has been applied to the conductor bundle.
As yet a
further alternative, the corona shields may be applied in the manner of a coat
of paint;
for instance, before application of the insulating sleeve to the conductor
bundle, the
latter may pass through a bath of the corona shield material held as a
suspension or
solution in a suitable liquid and after the insulating sleeve has been applied
it may
similarly be passed through such a bath. However, in such a process it will be
necessary to ensure that the inner corona shield has adequately dried or cured
to form
a flexible high temperature resistant coating before application of the
insulating sleeve
to the conductor bundle occurs. Similarly, the outer corona shield coating
must have
dried or cured to form a flexible high temperature resistant coating before
fizrther
handling of the composite conductor occurs, such as winding it into the slots
of the
electrical machine.
To facilitate the impregnation process while allowing easy manipulation of the
composite conductor during the process of producing the winding on the stator
core,
the composite conductor is impregnated before the composite conductor is wound
onto the stator and fully cured only after it has been wound onto the stator.
As
explained later, impregnation may occur at the time when the individual
strands of
conductor material are gathered together and consolidated into the conductor
bundle,
before application of the insulating sleeve. We prefer then to partially cure
the resin, so
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that the composite conductor is still flexible enough to be wound directly
onto the
stator, with optional intervening storage on a drum for later use. The
completed stator
is heat-treated at a temperature below that at which deterioration of the
filled polymer
sleeve and the corona shield occurs, thereby fully curing the resin to make
the finished
stator winding rigid.
Turning now to Figure 4, it is shown how the composite conductor 30 of Figure
2 may
be used to form a complete stator winding. The stator 40 of a rotary
electrical machine
comprises a laminated steel core 42 provided with radially oriented slots
extending
longitudinally of the stator. In the broken-away sectional view, only one of
the slots 44
is shown. It houses a winding 46 comprising a plurality of turns or loops of
the
composite conductor 30. Each turn conveniently comprises a single length of
the
composite conductor, or alternatively, two or more shorter lengths of
composite
conductor may be spliced together to complete one turn. As can be seen,
successive
turns 30A, 30B, etc, of the composite conductor are in contact and in radial
registration with each other. It should be noted that the rectangular shape of
the
composite conductor minimises air voids in the finished winding, enabling
geometrically exact production of the winding without pre-forming of rigid bar
conductors, as was necessary in the prior art. The winding is retained in its
slot 44 by a
high thermal conductivity, electrically insulating retaining wedge 48 fixed in
the radially
outer end of the slot. The wedge may be an extrusion comprising a filled
polymer
material, such as an epoxide resin, and may be driven into the slot from the
end face of
the stator.
In connection with the rectangular shape of the conductor bundle and the
insulating
sleeve, it should be particularly noted that besides the better packing
characteristics of
the composite conductors in the machine slots, the extra thickness T3-Tl
(Figure 2) of
the insulation at the corner of the conductor bundle effectively reduces the
electric
stress at the corner to approximately the same value as that on the flat sides
of the
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conductor bundle. Such reduction in peak electric stresses in the windings
contributes
to an ability of the machine to operate at higher loadings.
During production of the stator, the inherent flexibility provided by the
polymer
insulation sleeve, the thin corona shield coatings, the thin conductor strands
and the
uncured or partially cured resin on the conductor strands, permits ease of
positioning
of the conductor in the slots in the stator core to form a winding. However,
after heat-
treatment to cure the resin (curing being at a temperature lower than the
temperature
at which the filled polymer insulation sleeve and the corona shield coatings
begin to
deteriorate), the rigidity provided by the fully cured resin on the conductor
strands 31
enables the finished stator winding to withstand the operational impressed
forces. Note
that where the composite conductor winding 30 is not within the slots 44, it
may be
supported by support rings and packing blocks as shown for the known
arrangement in
Figure 1.
A possible process for making the composite conductor will now be described
with
reference to Figure 5. At the left of Figure 5, many pre-insulated wire
strands 31 of
copper or other conductor material are drawn together from storage reels (not
shown)
and passed through a bundle forming head I to produce a conductor bundle 32.
Within
the head I are means, well known in the art of cable-making, whereby the
strands 31
are brought together, formed into a bundle and the bundle is bodily twisted
around the
centreline of the bundle. At the same time as the strands 31 are being formed
into a
bundle, the bundle is impregnated with an epoxy resin or similar binding and
strengthening agent resistant to high temperatures; alternatively, this
impregnation
process may be a pressure impregnation process, as known, the pressure
impregnation
being accomplished immediately after the conductor bundle has been formed and
either
within the head I or following it. The final process within head I (which may
in practice
be carried out in a separate head, not shown) is to finally form and
consolidate the
conductor bundle into the required rectangular shape of the present invention
by
passing it through a suitably shaped die.
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The head I is heated so as to partially cure the resin as the conductor bundle
passes
therethrough, thereby producing a conductor bundle which has good cohesion,
yet is
still sufficiently flexible to be wound onto a large diameter storage drum S 1
for later
use. Alternatively, the partly cured bundle passes straight on as indicated
through the
centre of an annular die in extruding head II, whereby a conductive filled
polymer film
is extruded onto the outside of the conductor bundle 32 to form a first,
inner, corona
shield. Thereafter, the coated conductor bundle passes through the centre of a
fizrther
annular die in extruding head III, whereby a filled polymer insulating sleeve
is extruded
onto the outside of the first corona shield. Effectively, this first corona
shield thereby
forms a conductive coating on the inner surface of the insulating sleeve.
Finally, by a
similar process to that described for head II, a second, outer, corona shield
is applied
to the insulating sleeve in extruding head IV, so completing the formation of
a
composite conductor 30. The composite conductor 30 can then be wound onto a
large
diameter storage drum S2 for later use. Alternatively, as indicated by the
dashed arrow
line, it can pass straight into a fi~rther manufacturing stage for producing
windings for
electrical machines. As mentioned previously, final curing of the conductor
bundles is
accomplished by heating, after the composite conductor has been fixed into the
winding slots in an electrical machine, as shown in Figure 4.
It has been said above that the wire strands 31 are pre-insulated, meaning
that during
their manufacture they have been provided with a thin coating of suitable
epoxy resin
or high temperature resistant polymer, as known. However, as an alternative to
the use
of pre-insulated wire strands, it may be possible to rely on the resin
impregnation
process which occurs in head I to insulate the strands 31 from each other
within the
finished conductor bundle. This will be a matter for determination by routine
experimentation.
Although in Figure 5 the strands are shown being gathered together into a
bundle in a
one-stage process, in practice, due to the large number of strands required to
make a
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conductor bundle, this first part of the process may require a number of
parallel stages
in which a number of sub-bundles are produced in corresponding bundle-
producing
heads, each bundle having been impregnated with resin therein as described
previously,
the sub-bundles then being brought together to form the final twisted
rectangular
conductor bundle 32.
It will be evident that the above process may be used to produce composite
conductors
having cross-sectional shapes other than rectangular, e.g. circular.
