Note: Descriptions are shown in the official language in which they were submitted.
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RO~ARY ~LECTRIC MACt~INF WITH AXIAL COOLI~G
TECHNICAL FIELD:
The present invention relates to high-voltage rotating .~?lertrjc machines,
5 e.g., synchronous machines, but also double-fed machines, applications in
asynchronous static current .:ol.v-elL~r ~rA~l~s, outer pole machines and
synchronous flux machines, as well as alternating current machines
intended prim~rily as generators in a power station for generating electric
power. The i~,ve,.lion relates particularly to the cooling of such machines.
BACKGROUND ART:
Rotating electric machines for high-voltages, i.e. voltages exceeding about
10 kV with the maximum of 30-35 kV, are ~ormAlly arranged with a
cooling system for forced cooling of the machine.
Rotating high-voltage electric machines are usual~y built with a stator body
of sheet steel with a welded construction. The laminated core is norm~lly
made from varnished 0.35 or 0.5 mm electrical steel. The stator winding is
located in slots in the sheet iron core, the slots normally having a
20 rectangular or trapP7.0i~l cross section. Each winding phase comprises a
number of coil groups connected in series and each coil group comprises a
number of coils connected in series. The different parts of the coil are
designated coil side for the part which is placed in the stator and end
winding for that part which is located outside the stator. A coil comprises
25 one or more conductors brought together in height and/or width.
Between each conductor there is a thin insulation, for example epoxy/glass
fibre.
30 The coil is insulated from the slot with a coil insulation, that is, an
insulation intended to withstand the rated voltage of the machine to earth.
As insulating material, various plastic, varnish and glass fibre materials
may ~e used. Usually, so-called mica tape is used, which is a mixture of
mica and hard plastic, especially produced to provide resistance to partial
35 discharges, which can rapidly break down the insulation. The insulation is
applied to the coil by winding the mica tape around the coil in several
layers. The insulation is impregnated, and then the coil side is painted
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with a graphite-based paint to improve the contact with the surrounding
stator which is connected to earth potential
In the case of generators, this usually must be connected to the power
S network via a transformer which steps up the voltage to the level of the
power network - in ~e range of approximately 130~00 kV. The present
invention is intended for use with high voltages, in the working range of
36-800 kV.
10 By using high-voltage insulated ~lecfric conductors, in the following
termed cables, in the stator winding, with solid insulation similar to that
used in cables for transmitting ~lectric power, e.g., crosslinked
polyethylene (XLPE) cables, the voltage of the machine can be increased to
such levels that it can be connected directly to the power network without
15 an intermediate transformer. The conventional transformer can thus be
eliminated. The concept generally requires the slots in which the cables are
placed in the stator to be deeper than with conventional technology
(thicker insulation due to higher voltage and more turns in the winding).
This means that the loss distribution will be different from that in a
20 conventional machine, which in turn entails new problems with regard to
cooling, e.g., the stator teeth.
For cooling the windings of for example a synchronous machine there are
three systems which can be used. With air-cooling, the winding of the
25 stator as well as the winding of the rotor is cooled by air flowing through
the windings~ Air-cooling ducts are arranged in the laminated sheets of
the stator as well as in the rotor. With radial veIttil~ti-)n and cooling by airthe laminated core is, at least for medium sized and large sized machines
divided in packets comprising radial and axial ventilation ducts disposed
30 in the core. The cooling air can be ambierLt air but at powers above 1 MW
mainly a closed cooling system with a heat exchanger is used. Hydrogen
cooling is normally used in larger turbo-generators and in large
synchronous compensators. The cooling method works in the same way as
air-cooling with a heat exchanger, but instead of air as cooling medium
35 hydrogen is used. Hydrogen has better cooling capabilities than air, but
~iffic~llties arise at sea~ings and to detect leakage. I~Le cooling ducts are
made by punching the laminated stator sheets which will constitute the
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ducts when the sheet are laid for building a segment. The consequence is
an increase of the pressure drop over the machine due to many small
irreg~ riti~ in the gas cooling ducts.
In turbo-generators of power range 500 - 1000 MW it is also known to use
water cooling of the winding of the stator as well as of the winding of the
rotor. The cooling ducts are made as tubes placed inside conductors in the
winding of the stator. A problem in large machines is that the cooling tend
to become non-uruform and, therefore, temperature variations arise in the
machine.
It is considered that coils for rotating generators can be manufactured with
good results within a voltage range of 3 - 25 kV.
Attempts to develop generators for higher voltages however, however,
been in progress for a long time. This is obvious, for instance from
"E~lectrical World", October 15,1932, pages 524-525. This describes how a
generator designed by Parson 1929 was arranged for 33 kV. It also
describes a generator in Langerbrugge, Belgium, which produced a
voltage of 36 kV. Although the article also speculates on the possibility of
increasing voltage levels still further, the development was curtailed by
the concepts upon which these generators were based. This was primarily
because of the shortcomings of the insulation system where varnish-
impregnated layers of mica oil and paper were used in several separate
layers.
In a report from the Electric Power Research Institute, EPRI, EL-~391 from
April 1984, an account is given of generator concepts for achieving higher
voltage in an electric generator with the object of being able to connect
such a generator to a power network without intermediate transformers.
Such a solution is assessed in the report to offer good gains in efficiency
and considerable financial advantages. The main reason that it was
~le~me~ possible in 1984 to start developing generators for direct
connection to power networks was tt~at a superconducting rotor had been
developed at that time. The considera~le excitation capacity of the
superconducting ~ield winding enables the use of airgap-winding with
sufficient thickness to withstand the electrical stresses.
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By combining the concept ~lP~me~1 most promising according to the
project, that of designing a magnetic circuit with winding, known as
"monolithe cylinder armature", a concept in which two cylinders of
conductors are enclosed in three cylinders of insulation and the whole
structure is attached to an iron core without teeth, it was assessed that a
rotating ~lectri~ machine for high voltage could be directly connected to a
power network. The solution entailed the main insulation having to be
made sufficiently thick to withstand network-to-network and network-to-
earth potentials. Obvious drawbacks with the proposed solution, besides
10 its demanding a superconducting rotor, are that it also requires extremely
thick insulation, which increases the machine size. The end windings must
be insulated and cooled with oil or freones in order to control the large
~lec~ric fields at the ends. The whole machine must be hermetically sealed
in order to prevent the liquid r~ ctri~ medium from absorbing moisture
15 from the atmosphere.
Certain attempts to a new approach as regards the design of synchronous
machines are described, inter alia, in an article entitled "Water-and-oil-
cooled Turbogenerator TVM-300" in J. Elektrotechnika, No. 1, 1970, pp. 6-
20 8, in US 4,429,244 "Stator of Generator" and in Russian patent document
CCCP Patent 955369.
The water- and oil-cooled syr~hronous machine described in J.
Elektrotechnika is intended for voltages up to 20 kV. The article describes
25 a new insulation system consisting of oil/paper insulation, which makes it
possible to immerse the stator completely in oil. The oil can then be used
as a coolant while at the same time using it as insulation. To prevent oil in
the stator from leaking out towards the rotor, a dielectric oil-separating
ring is provided at the internal surface of the core. The stator winding is
made from conductors with an oval hollow shape provided with oil and
paper insulation. The coil sides with their insulation are secured to the
slots made with rectangular cross section by means of wedges. As coolant,
oil is used both in the hollow conductors and in holes in the stator walls.
Such cooling systems, however, entail a large number of connections of
both oil and electricity at the end windings. The thick insulation also
entails an increased radius of curvature of the conductors, which in turn
results in an increased size of the winding overhang.
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s
The above-mentioned US patent relates to the stator part of a synchronous
machine which comprises a magnetic core of laminated sheet with
trapezoidal slots for the stator winding. The slots are tapered since the
5 need of inslllation of the stator winding is less towards the interior of the
rotor where that part of the winding which is located nearest the neutral
point is located. ~n addition, the stator part comprises a ~i~lectric oil-
separating cylinder nearest the inner surface of the core which may
increase the magnetization requirement relative to a machine without this
10 ring. The stator winding is made of oil-immersed cables with the same
diameter for each coil layer. The layers are separated from each other by
means of spacers in the slots and secured by wedges. Special for the
winding is that it comprises two so-called half-windings connected in
series. One of the two half-windings is located, centred, inside an
15 insulation sleeve. The conductors of the stator winding are cooled by
surrounding oil. The disadvantages with such a large quantity of oil in the
system are the risk of leakage and the considerable amount of deaning
work which may result from a fault condition. Those parts of the
insulation sleeve which are located outside the slots have a cylindrical part
20 and a conical termination reinforced with current-carrying layers, the duty
of which is to control the electric field strength in the region where the
cable enters the end winding.
From CCCP 955369 it is clear, in another attempt to raise the rated voltage
25 of the synchronous machine, that the oil-cooled stator winding comprises
a conventional medium voltage insulated conductor with the same
dimension for all the layers. The conductor is placed in stator slots formed
as circular, radially placed openings corresponding to the cross-section
area of the conductor and the necessary space for fixing and for coolant.
30 The dif~erent radially located layers of the winding are surrounded by and
fixed in insulated tubes. Insulating spacers fix the tubes in the stator slot.
Because o~ the oil cooling, an internal dielectric ring is also needed here for
sealing the coolant against the internal air gap. The design shown has no
tapering of the insulation or of the stator slots. The design exhibits a very
35 narrow radial waist between the different stator slots, which means a large
slot leakage flux which significantly influences the magnetization
requirement of the machine.
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EP 0342554 discloses a rotating electric machine comprising a stator cooled
by liquid cooling me~7il1m The stator of the machine is arranged with
trap~7ni~1~1 stator slots with corresponding cross section of winding
5 mounted therein.
EP 0493704 discloses an electric motor comprising cooling channels in the
stator teeth. The windings are in this motor arranged irregularly i n the
stator in slots formed between the teeth.
EP 0684682 discloses a rotating electric machine. The stator of the machine
is arranged with rectangular stator slots and a winding packed with a
conductor having a rectangular cross section. Axial gas cooling ducts run
through the stator teeth in order to achieve heat transport from the
15 co~ 7c~rs. The cool~ng ducts are inserted in order to cool a larger part of
the radial depth of a stator tooth. This implies problems in the design of
higher power machines which will contain more turns of winding and
therefore deeper stator slots which result in worse mechanical stability.
EP 0155405 discloses a gas cooled arrangement for rotating dynamoelectric
machine in order to improve the capacity o~ machines which earlier have
been cooled by air to reach capacities which only water-cooled machines
has.
US 2,975,309 discloses an oil-cooled stator for rotating machines, especially
turbo generators. The stator of the machine is arranged with rectangular
stator slots and a winding with corresponding cross-section.
US 3,675,056 discloses a hermetically sealed dynamoelectric machine
connected to a fluid refrigerant circuit. The inside of the machine is filled
with refrigerant flowing through ducts with triangular cross section in the
stator for cooling the windings with rectang~ar cross-section.
US 3,801,843 discloses a rotating electrical machine having rotor and stator
cooled by means o~ heat pipes with the aid of a two-phase ~luid coolant.
The stator heat pipes are located in the stator slots and extend axially to a
remote location beyond the stator and the rotor. The rotor heat pipes also
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serve as electrical conductors as well as heat exchangers for cooling the
rotor.
OBJECT OF THE INVENIION:
5 The object of the present invention is to provide a cooling system for a
high-voltage rotating electric machine in the range from 10 kV and up to
the voltage level of the power network. Such a rotating electric machine
can be directly connected to the power network without a transformer
therebetween.
SUMMARY OF THE ~VENIION:
The present invention relates to a means for cooling the stator teeth, and
indirectly the stator winding, in a high-voltage electric machine such as a
high-voltage alternating current generator.
The arrangement comprises axially-running tubes, electrically insulated,
which are drawn through axial apertures through the stator teeth. The
tubes are permanently glued in the apertures to ensure good cooling
capacity when coolant is circulated in the tubes. The tubes run along the
20 entire axial length of the stator teeth and are spliced in the stator ends.
According to a particularly preferred embodiment of the invention, at
least one of the semiconducting layers, preferably both, have the same
coe~ficient of thermal expansion as the solid insulation. The decisive
25 benefit is thus achieved that defects, cracks or the like are avoided at
thermal movement in the winding.
The means comprises axially runn~ng cooling tubes made of a dielectric
material such as a polymer, and drawn through axial apertures in the
30 stator yoke and in the stator teeth. The tubes are embedded in the
apertures to ensure good heat transfer when coolant is circulated in the
tubes. The tubes run in the stator yoke and the stator teeth, along the entire
length of the stator and, if necessary, they can be spliced in ~e stator ends.
35 Polymer cooling tubes are non-conducting and the risk of short-circuiting
is thus eliminated. Nor can eddy-currents occur in them. Polymer cooling
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tubes can also be cold bent and drawn through several apertures without
splicing, which is a great advantage.
Polymer cooling tubes can be produced from many materials such as
5 polyethylene, polypropene, polybutene, polyvinylidene fluoride,
polytetrafluoroethylene, as well as filled and reinforced elastomers. Of
these polyethylene with high density, HDPE, is preferred since the
thermal conductivity increases with increased density. If the polyethylene
is cross-linlced, which can be achieved by peroxide-splitting, silane cross-
10 linking or radiation cross-linlcing, its ability to withstand pressure at
increased temperature is increased at the same time as the risk of stress
corrosion disappears. Cross-linked polyethylene is used for water pipes,
e.g. ~CLPE pipes from Wirsbo Bruks AB, Sweden.
15 One single tube is in one embodiment thread through more than one
aperture without being joined to another tube part.
BRIEF DESCRIPTION OF THE DRAWINGS:
The invention will be described in more detail with reference to the
20 accompanying drawings.
Figure 1 shows schematically a perspective view of a section taken
diametrically through ~e stator of a rotating electrical
machine.~5 Pigure 2 shows a cross-sectional view of a high-voltage cable according
to the present invention,
Figure 3 shows schematically a sector of a rotating electric machine,
Pigure 4 shows a sector corresponding to a slot division in a stator tooth
with its yoke as indicated by the broken lines in Pigure 3,~0 Figure 5 shows an alternative sector cc~ s~onding to a slot division of a
stator tooth with its yoke provided with axial cooling tubes in
accordance with the present invention.
Figure 6 shows a cooling circuit in accordance with the present
invention.~5 Figure 7 shows a combination of holes for an arrangement in accordance
with the present invention.
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Figure 8 shows an alternative arrangement of the holes in accordance
with the present invention.
DESCRIPTION OF TH~ INVENTION:
5 Figure 1 shows part of an electric machine in which the rotor has been
removed to show more clearly the arrangement of a stator 1. The main
parts of the stator 1 constitute a stator frame 2, a stator core 3 comprising
stator teeth 4 and a stator yoke 5. The stator also comprises a stator
winding 6 composed of high-voltage cable situated in a space 7 shaped
10 like a bicycle chain, see Figure 3, formed between each individual stator
tooth 4. In Figure 3 the stator winding 6 is only indicated by its electric
conductors. As can be seen in Figure 1, the stator winding 6 forms a end-
winding package 8 on both sides of the stator 1. It is also clear from Figure
3 that the insulation of the high-voltage cables has several dimensions,
15 arranged in groups depending on the radial position of the cables in the
stator 1. For the sake of simplicity only one end-winding package is shown
at each end of the stator in Figure 1.
In larger conventional machines the stator frame 2 often consists of a
20 welded sheet steel construction. ~ large machines the stator core 3, also
known as the laminated core, is generally formed of 0.35 mm core sheet
divided into stacks with an axial length of approximately 50 mm,
separated from each other by 5 mm ventilation ducts forming partitions. In
a machine according to the present invention, however, the ventilation
25 ducts are eliminated. In large machines each stack of laminations is
formed by fitting punched segments 9 of suitable size together to form a
first layer, after which each subsequent layer is placed at right angles to
produce a complete plate-shaped part of a stator core 3. The parts and the
partitions are held together by pressure legs 10 pressing against pressure
30 rings, fingers or se~ments, not shown. Only two pressure legs are shown
in Figure 1.
Figure 2 shows a cross-sectional view of a high-voltage cable 11 according
to the invention. The high-voltage cable 11 comprises a number of strands
35 12 of copper (Cu), for instance, having circular cross section. These strands 12 are arranged in the mi~ of the high-voltage cable 11. Around the
strands 12 is a first semiconducting layer 13, and around the first
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semiconducting layer 13 is an insulating layer 14, e.g. XLPE insulation.
Around the insulating layer 14 is a second semiconducting layer 15. Thus
the concept "high-voltage cable" in the present application does not
include the outer protective sheath that normally surrounds such cables
5 for power distribution. The high-voltage cable has a diameter in the range
of 20-200 mm and a conducting area in the range of 80-3000 mm2.
Figure 3 shows schematically a radial sector of a machine with a segment 9
of the stator 1 and with a rotor pole 16 on the rotor 17 of the machine. It
lO can also be seen that the high-voltage cable 11 is arranged in the space 7 in the shape of a bicycle chain, formed between each stator tooth 4.
Figure 4 shows a tooth sector 18 corresponding to one slot pitch in~ t.o-l
by the broken lines extending radially in Figure 3, defining the tooth
15 height as the radial distance from the tip 19 of a tooth to the outer end 20
of the space 7 resembling a bicycle d~ain. The length of a stator tooth is
thus equivalent to the tooth height. Furthermore, the yoke height is
defined as the radiaI distance from the outer end 20 of the space 7
resembling a bicycle chain, to the outer end 21 of the stator core 3. This
20 latter distance denotes the width of an outer yoke portion 22. A tooth waist
23, furthermore, is defined as being one of several narrow parts formed
along each stator tooth by the space 7 resembling a bicyde chain between
the stator teeth. Thus a number o~ tooth maxima 24 are formed radially
between each tooth waist 23, their dimensions increasing from a sm~ st
25 maximum nearest the tooth tip 19 and a largest maximum nearest the
outer end 20 of the space 7 resembling a bicyde chain. As is clear from the
figure, the width of the outer yoke portion in the sector shown increases
towards the outer edge 21 of the stator core 3.
30 In a high-voltage rotating electric machine of the type described above at
least one stator tooth 4 is provided according to the present invention, see
Figure 5, with at least one cooling duct running substantially axially,
preferably in the form of a cooling tube 25, connected to a cooling circuit
in which coolant is arranged to circulate. In a possible embodiment the
35 cooling ducts may use oil as coolant. To achieve efficient cooling, cooling
ducts/tubes are ~refeL~bly arranged in every stator tooth. According to
the embodiment of the invention shown in Figure 5 four cooling tubes are
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11
arranged to run axially through the actual tooth, whereas another two
cooling tubes are arranged to run axially through the outer yoke portion
22 of the sector shown. ~s can be seen in the figure, two narrow cooling
tubes may be arranged beside each other instead of a single thicker one, in
S at least one tooth maximum. Each of these two tubes belongs to its own
parallel coolant circuit. The advantage is that narrower cooling tubes are
more easily bent to a sm~ r radius. Another advantage of narrow tubes is
that these do not obstruct the magnetic flux to the same extent as a thick
tube. This advantage is also gained with elliptical and oval tubes with
10 their large axis in the radial direction of the tooth. According to one
embodiment, all tooth maxima are provided with double coolin~ tubes
and all the cooling tubes in a slot pitch sector are radially aligned.
According to another embodiment cooling tubes in a slot pitch sector are
also radially aligned. The cooling also occurs on the earth potential in all
lS embodiments.
Other embodiments with cooling tubes arranged in conjunction with the
stator winding 6 also lie within the scope of the appended daims, e.g.
cooling tubes placed between the windings in a triangular space 26 in the
20 form of attachment elements to the windings, for instance, or in specially
provided grooves in a tooth side 27.
Each cooling tube 25 is provided with an insulating layer 28 in order to
avoid contact with the metal in the stator tooth 4 or in the outer yoke
25 portion 22. A th~rm~lly conducting glue may alternatively be used for
attachment. In another ~reLelled embodiment each cooling tube 25 is
made of a dielectric material such as a polymer, ~ref~Ldbly XLPE, in order
to avoid electrical contact with the metal in the stator tooth 4 or in the
outer yoke portion 22. All cooling tubes are embedded in the apertures 28
30 running in the stator 1 with a cold-vulcanized, two-component silicon
rubber provided with filler to increase the thermal conductivity. The filler
material is pressed into the apertures 28 after that the tube 25 has been
mounted. The filler material is also arranged to be pressed into the
aperture 28 from one side of the stator with overpressure before
35 hardening.
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12
All cooling tubes 25 are connected to a closed cooling circuit 29, see Pigure
6, which in the embodiment shown comprises a tank 3Q containing coolant
31 which may be water, hydrogen gas or other coolant for the circuit. The
tank 30 is provided with a level indicator for control and monitoring of the
5 cooLant level. The tank 30 is also connected to two annular conduits
consisting of an inlet loop 32 and an outlet loop 33. Between the inlet loop
32 and outlet loop 33 a number of parallel circuits are connected, the
number generally cclle~onding to the number of stator teeth or tooth
sides provided with cooling tubes, of which one parallel circuit 34 is
10 shown in Figure 6. The coolant 31 is arranged to circulate from the inlet
loop 32 simultaneously through every parallel circuit 34 to the outlet loop
33 and on to a circulation pump 35 and a circulation filter 36 through a
heat exchanger 37, i.e. a plate heat exchanger, and then back to the inlet
loop 32. Water from a water reservoir is supplied through one end of the
~5 heat exchanger 37 via the exchanger filter, not shown, of an exchanger
pump 38. The water is pumped ~rough the exchanger and back to the
water reservoir.
Figure 7 is showing an alternative design of the cooling tubes 25 placed in
20 a eight-formed double hole 39. This arrangement makes it possible to
combine two cooling tubes 25 in this hole in a tooth maximum 24 in the
stator tooth as is showed in figure 8. Furthermore, the double hole
arrangement is radially aligned as is also showed in figure 8.
2:5 When manufacturing a stator cooled in accordance with the present
inVentiQn, the first cooling circuit 29 is dimensioned taking into
consideration possible distances between the cooling tubes 25. The
distance between tubes must be chosen so that they can be placed in the
middle of the broadest parts of the stator tooth 4 at a tooth maximLun 24.
30 This is important from the magnetic point of view in order to avoid
magnetic saturation in the stator teeth. A thermal ( ~lc~ tion is performed
in order to ensure the correct number of tubes, with radial and axial
spacing so that a uniform temperature distribution is obtained in the high-
voltage cables. The apertures are inserted in the punching template for the
35 stator laminations and no additional operation is thus required. The
cooling tubes 25, ~ref~ldbly of stainless steel, are inserted after the
laminations have been stacked but before the stator is wound. The tubes
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13
are first insulated and glued in the apertures by pressurizing the glue and
inserting the tubes from below. The tubes may be spliced by means of
welding. However, an electric~Ally insulating tube part must be provided
in each parallel circuit 34. This can be achieved by choosing tubes of
5 polymer material for the connections to the ring circuits 32, 33 above the
generator.
~e tubes must be embedded since the thermal resistance between the
tube and the stator core will otherwise be too high. In order to increase
10 heat transfer between tube and stator core, the space is filled with a cross- linkable casting compound. This may consist of a polymer which has low
viscosity and can therefore withstand being filled with a high content of
heat-conducting filler material before being injected into the space where
it is converted through chemical reaction to a non-fluid compound.
1~ Examples of suitable compounds are acryl, epoxy, unsaturated polyester,
polyurethane and silicon, the latter being preferred since it is non-toxic.
Heat-conducting filler may also comprise oxides of aluminium,
magnesium, iron or zinc, nitrides of boron or aluminium, silicon carbide.
20 The embedding compound may be a silicon rubber, i.e. a mixture of for
instance aluminium oxide and silicon, i.e. polydimethyl siloxane with
vinyl groups which react with hydrogen polydimethyl siloxane in the
presence of a platinum catalyst. This is forced into the aperture 28 between
~e XLPE tube and the stator core at over-pressure, after which curing
25 occurs by the hydrogen atoms being added to the vinyl groups.
A magnetic flux flows between the cooling tube and earth which would
induce a circulating current if the tube were uninsulated from the metal
laminations. The tube insulation should be thin but at the same time so
30 resistant to wear that the tube can be inserted into the aperture without
damage to the insulation. The tube could be coated with a layer of varnish
or wound with insulating fabric.
One single tube 25 is arranged to be tread through more than two aperture
35 28 without being jointed to another tube part. U-shaped tubes may be used
in order to reduce the number of joints in the cooling tubes. Welding is the
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14
~rer~led splicing method but other solutions are possible such as O-rings,
coupIings, glueing, soldering, etc.
The invention is not limitPrl to the embodiments shown by way of
5 example. Several modifications are feasible with~n the scope of the
invention. Thus the tubes in each slot division need not be connected in
series but may sometimes be connected in parallel. !Sim;l~rly, several slot
divisions may be arranged in series. The joints may be performed in
several different ways, e.g. soldering welding, screwed joint elements,
10 tube damps, etc. ~he cooling circuit need not be connected as shown in
Figure 6. Instead it may be open, in which case the heat exchanger is
eli~Linated. The glue can be introduced by other means that under
pressure depending, for instance, on its viscosity. Finally, the tubes may be
made of dif~erent m~t~ri~l, even polymer m~t~ri~l, depending on the need
15 for tube insulation.
