SPECIFICATION

TITLE Laminate for electrical machines

TECHNICAL FIELD

The present invention concerns a laminate for electrical machines, preferably for the insulation of conductors from the rotor or from the stator of an electrical machine.

BACKGROUND OF THE INVENTION Laminates are used in electrical machines in order to insulate conductors from the rotor or from the stator. Laminates are multi-layer structures and consist of several thin films laminated on top of each other. Nomex® paper (aramide fiber based sheet structure), polyethylene terephtalate film or other materials can compose the laminate individual layers.

Rotors and stators are mainly made of an assembly of coils and a stack of metallic sheets. The sheets are shaped so that slots are formed when the sheets are stacked. Laminates are laid into the slots and preformed conductor bundles consisting of insulated wires are put on top of the laminate. The slots are then closed by another layer of laminates. The stacks of sheets are usually not perfectly aligned and not perfectly cut, creating a micro -roughness on the surface of the rotor or the stator that can reach up to 500 microns or above. Thereby, a gap which prevents good thermal conductivity is formed between the conductors and the stack of sheets when laminates are applied against the rotor or the stator.

When the coils of the rotor and of the stator are assembled, they are dried and impregnated with a curable varnish using e.g. the vacuum pressure impregnation (VPI)

process in order to provide electrical insulation and mechanical strength. The impregnation is performed by immersing the coils into a varnish (e.g. epoxy, polyurethane, phenolic, silicon, polyester etc.), applying vacuum to remove trapped air, and then applying pressure to ensure that every part of the coils is impregnated. However, the gap caused by the micro -roughness cannot be filled because it is closed by the laminate (slot liner). The coils are then cured, generally in a conventional oven, and the electrical machine is assembled.

The misalignment of the sheets composing the stack creates said micro-roughness. It leaves an air gap between the stack of sheets and the slot liner laminates. Air is a poor thermal conductor, impairing the heat dissipation capabilities of the machine. It could lead to overheating, loss of efficiency and shortened life-time of the complete system.

The air gap can be filled by applying manually a heat sink compound on the slot before installing the slot liner laminate. Nevertheless, this process is very labor intensive and costly. Another alternative is to use an active cooling system such as water cooling. Again, such a system is expensive, consumes energy and creates reliability issues.

US 5,341,561 discloses a process for producing an electric insulation of electric machine windings. The objective of this process is to provide a better mechanical fixation of the windings in the grooves in the laminate housing of the rotor or stator in which the windings are placed. In this document, the grooves, into which the windings are to be laid, are filled with a compressed or pre-stressed laminate. As also in GB 1 504 106, the laminate itself is compressible, by containing an elastically compressible, highly elastic fiber material. The laminate is compressed when applied to the grooves. One disadvantage of such a pre-tensioned layer is that the pre-tensioned material usually has the tendency to elastically decontract upon storage prior to installation e.g. by thermal influences.

SUMMARY OF THE INVENTION

One objective of the present invention is to provide a functional laminate structure, which allows a better coupling of the coils to the metallic stacks without introducing costly extra steps in the manufacturing.

The present invention solves the above problems by applying a heat expandable coating and/or impregnation layer to at least one side of the laminate, wherein said laminate preferably is a slot liner laminate between a stack of metallic sheets and conductor bundles of a rotor or stator.

Heat expandable coating compositions as such are well known in the state of the art in different fields. E.g. US 2,862,834 mentions a coating containing heat-expandable plastic material can be applied in a mixture with paint or varnish for providing a coating film which can be heated or baked. Named compositions are substantially made of binder, a solvent vehicle and expandable resin beads, or a resin, methanol/ethanol and polystyrene (beads), or a sodium silicate solution and polystyrene beads. However, this composition is used for coating an adherent, decorative film in order to achieve a pleasing and decorative finish of the surface structure.

The heat-expandable coating material described in JP 6240179 for the purpose of obtaining a fire-resistant and heat-resistant coating material is a mixture of a carbide containing a graphite-type substance with a substance containing one or more kinds of different metal elements composed mainly of metal salts, plus solvent adhesives and a thermally decomposable expandable foaming agent composed mainly of azodicarbonamide etc.

One key feature of the invention is therefore the finding that a laminate for the insulation of conductors of a rotor or stator of an electrical machine, as for example a

generator or motor, is provided, which comprises a substrate and a heat-expandable coating. Said coating can be adhesive. The coating is to be understood as either a coating, which is applied to the surface of the substrate, or as an impregnation layer, which can at least partially enter the substrate and/or be absorbed by it. For the purpose of this invention, in the following the term 'coating' is used for both options.

In a first preferred embodiment of the invention, said laminate comprises a substrate that is not pre-tensioned, preferably not mechanically pre-tensioned, and preferably not elastically compressed either. That means, that the substrate is non compressed or pre- stressed in any other way before its application onto the surface of the slot. This has the advantage, compared to the expandable laminate of the state of the art, that no additional compression step is necessary before the application of the laminate to the slot. This simplifies the production of the laminate and/or shortens the process of assembly of the electrical machine.

In another preferred embodiment, the heat-expandable coating is not pre-tensioned/pre- stressed and preferably not elastically compressed either.

Preferably, the coating is applied on one or both sides of the substrate. The coating can be applied preferably to at least the side of the substrate facing the metal sheets and/or on both sides of the substrate. The coating can be applied before or after application of the substrate to the slot. In the latter case, the coating might be injected into the slot and onto the substrate.

According to another preferred embodiment of the invention, said coating comprises, as a matrix, a B-stage resin and at least one constituent having heat expandable properties. The B-stage resin can be a phenolic-, epoxy-, silicone-, polyimide-, polysilazane-, cyanate-, cyanate ester-, polyester-, or polyurethane resin or any other suitable resin, preferably a thermosetting resin or a mixture thereof. With respect to the type of resin

that can be used, WO 2005/096320 A2, the contents of which shall be incorporated herein, discloses a wide array of resin types, including e.g. Novolak.

The B-stage is known as a secondary/intermediate stage in the reaction of a number of thermosetting resins, characterized by softening of the resin when heated and swelling when in the presence of certain liquids, but without complete fusing or dissolving. The B-Stage, also known as "resitol" or "resolite", is also characterized by a progressive increase in viscosity; the resin portion of an uncured thermosetting adhesive/molding compound is usually in this stage. The expansion capabilities can be achieved by adding one or several ingredients providing expandable properties to the coating. Besides being a carrier for an expandandable constituent, the B-stage resin can also provide mechanical and/or dielectric strength to the laminate and/or the coating and/or give the laminate and/or the coating a specific shape.

The constituent having expandable properties is an expandable filler and/or a foaming agent (e.g. p-p-oxybisbenzenesulphonylhydrazine or any similar agent, etc.) and/or an expanding polymer or a mixture thereof. The expanding polymers can be used either as additional additive or as the matrix of the coating. The expanding properties of the constituent usually result from gas-formation and/or fluid vaporization and/or gas expansion and/or change in molecular structure and mostly are the result of carbon dioxide- and/or nitrogenous compounds and/or sulphurous compounds-release. Examples of said expandable filler agents are expandable micro fillers or nano fillers, preferably expandable nanospheres or expandable microspheres like Expancel® microspheres, Advancell® or an expandable graphite or a mixture thereof. The gas- driven fillers, like Expancel or Advancell, can be spherical or lens-shaped (uniaxially compressed) in their unexpanded state.

Besides expandable microspheres, the expandable filler may also be some other filler

with an expansion that can be triggered on demand. It can also be possible to trigger the expansion of such an expandable filler agent in some other way than by an increased temperature, e.g. chemically (water, solvent, acid, gas), electromagnetically (MW, IR, UV, VIS), mechanically, etc. Preferably, the expandable filler is unexpanded when added to the matrix of the coating and expanded at a later stage.

The Expancel fibre composite as described in US 6,864,297 is a foam composition comprising a fibrous material and microspheres, for the use in a wide range of engineering applications. Microspheres in general can be selected from the group comprising a glass, a silica-alumina ceramic, an epoxy resin, an unsaturated polyester resin, a silicone resin, a phenolic, a polyvinyl alcohol, a polyvinyl chloride, a polypropylene, a polystyrene, a polyacrylonitrile, a polyamide, a polyacrylate and any combination thereof. Expancel microspheres are small spherical plastic particles consisting of a polymer shell encapsulating a gas. When the gas inside the shell is heated, it increases its pressure and the thermoplastic shell softens, resulting in a dramatic increase in the volume of the microspheres. When fully expanded, the volume of the Expancel microspheres increases more than 40 times. Alternatively, the shell can also encapsulate a fluid or a fluid-gas-combination.

The thermal expandable microsphere of Advancell contains a liquid hydrocarbon of low boiling point in a thermoplastic polymer cell. The Advancell microsphere becomes a balloon with hollow inside by the heated expansion of the contained liquid hydrocarbon of low boiling point and simultaneously by the softening of the thermoplastic shell. The balloons with hollow make a closed-cell structure. The temperature to start expansion and the cell size, etc. can be optimized by selecting appropriate grades. Another expandable filler is disclosed in WO 01/72399 as a component of an expanding glue for filters.

According to a further preferred embodiment, the coating further comprises at least one

thermally conductive constituent. This constituent preferably is of insulating, conducting or semi-conducting nature. It can contain particles of different morphologies and/or of different sizes. The thermally conductive constituents may be added to the formulation in order to further increase the thermal conductivity of the laminate. These fillers can be of insulating nature, like boron nitride, aluminium nitride, MgO, TiO ₂ , (synthetic or natural) diamond, mica, Ba-sulphate, oxinitride or any other oxide/nitride/carbide. It can also be of conducting or semiconducting nature, like carbon black, expandable graphite (which may be modified to low expansion temperatures) or metal flakes or particles, coated mica-platelets (see e.g. US 5,341,561), doped or undoped semi-conductors like SiC, ZnO, etc.. Semiconducting or conducting fillers add the additional function to electrically screen the voids produced in the expansion process and hence reduce the risk of increased partial discharge activity and electrical failure. The filler can be of different morphology (equiaxed particles, fibres, flakes, etc.) and/or of different size (nanoparticles to several 10-100 microns, as well as mixtures thereof).

The laminate, preferably the coating, can also contain an additive, preferably selected from the group comprising a catalyst, a latent hardener, an adhesion promotor, a coupling agent, a solid filler, a liquid metal filler, a flame retardant, a processing aid, a dispersion aid, or any other suitable additive, or combinations thereof. For instance, adhesion promotors or coupling agents can be used e.g. as a processing- and/or dispersion aid and/or for the improved adhesion of fillers and/or substrates and/or metal plates.

In another preferred embodiment, the laminate comprises more than one layer of substrate. If so, inside the laminates the adhesives used to bond the individual layers of substrate can be provided with fine (nano-type) fillers of good thermal (and/or conducting/semi-conducting) properties, in order to further reduce the thermal resistance of the base laminate.

The substrate comprises at least one foil or film, preferably metallic foil, more preferably Al or Cu foil, and/or mats- and/or felts made of glass, mineral or metal fibers, polyester, polypropylene, polyethylene, polyethylene terephtalate or aramide (e.g. Nomex® (available e.g. at DuPont Advanced Fibers Systems, Richmond, VA 23234, USA), etc.)). Such an insulating film provides the mechanical and/or dielectric strength. If thin metal foils or metal mats, e.g. made of Al, are used, they can act as lateral heat spreaders.

According to another preferred embodiment of the invention, the expanded coating has a thickness of 1-500 μm , preferably 10-250 μm, more preferably 80-150 μm. In an unexpanded state, the coating has a thickness of 1-300 μm, preferably 1-150 μm, more preferably 5-100 μm.

The purpose of the invention is also achieved by the use of said laminate for the filling, preferably void-free filling, of gaps and/or the impregnation between metal sheets and conductors in a rotor or stator of an electrical machine.

Furthermore, to achieve the purpose of the present invention, a method of producing said laminate is provided, wherein a heat-expandable coating is applied to at least one side of a substrate by applying a heat expandable coating and/or impregnation layer to at least one side of the laminate, wherein said laminate preferably is a slot liner laminate between a stack of metallic sheets and conductor bundles of a rotor or stator.

Another preferred embodiment of the invention is an electrical machine, e.g. a generator or motor, comprising a rotor and a stator, at least one of which has radially directed slots between metal sheet stacks and conductor bundles, which are axially extending in the slots. Said rotor and/or stator have gaps between the metal sheet stacks and the

conductor bundles, which are filled with one or more layers of the laminate according to one of the embodiments described above for the insulation of said conductor bundles. In the case where multiple layers of laminate are present, it is possible to combine conventional layers of laminate, without the coating, with layers of laminate according to the present invention. Therefore, in the case of multiple layers of laminate in an electrical machine, preferably at least one layer of laminate according to one of the embodiments of the present invention (laminate comprising a substrate and a heat- expandable coating) is provided among the layers. Preferably the at least one layer of laminate according to the invention is provided on the side of the stack of laminates facing the metal sheets of the machine.

The purpose of the present invention is also achieved by providing a method for the void-free filling of gaps and/or the impregnation between metal sheets and conductors in a rotor or stator of an electrical machine. According to this method, at least one layer of the laminate according to any one of the embodiments described above is applied to the surface of the slot. In other words, the slot liner laminate is applied onto the surface of the slot, into which the conductor bundles are to be laid, thereby covering the stack of metallic sheets of the rotor or stator before the conductor bundles are positioned. In principle several laminates could be stacked for higher thickness as well. Thereafter, the conductors are positioned on said at least one layer of slot lining laminate and then covered by at least one layer of slot closure laminate. After the application of the laminate and the positioning of the conductors, heat is applied to the rotor or stator. Thereby, the coating is cured from A-stage to B-stage and then expanded and cured from B-stage to C-stage. Before reaching B-Stage (as is defined above), coatings are in a so-called A-stage: an initial stage in the reaction of some thermosetting resins wherein the resin continues to be soluble and fusible. "A-stage" is characterized by an initial lowering of viscosity and is also called "Resol". The C-stage is known as the third and final stage in the reaction of some thermosetting resins, characterized by the relatively insoluble and infusible state of the resin. Some thermosetting resins in this stage are fully cured; also called "resite". Therefore, when heat is applied to the rotor or the

stator for drying, the coating of the slot liner laminate expands, closes the air gap with the slot and can form an adhesive bond with the sheets, the substrate, and/or the conductor. The coating fully cures to C-stage and remains irreversibly in position. The expansion can be independent of the drying.

In a preferred embodiment of said method for filling gaps between metal sheets and conductors according to the invention, the coating is cured to B-stage for about 0.5-120 minutes, preferably about 5-60 minutes, more preferably about 30 minutes at about 30- 100 ⁰ C, preferably about 50-90 ⁰ C, more preferably about 80 ⁰ C. Curing to C-stage is carried out at about 60-240 ⁰ C, preferably about 100-150 ⁰ C, more preferably about 140 ⁰ C for 5-240 minutes, preferably 10-180 minutes, more preferably about 60 minutes.

The advantages of the present invention are among others an improved heat dissipation of the electrical machine, providing a better efficiency, longer life-time and lower service temperature. The preparation time and labor are also reduced because there is no need of applying manually any heatsink compound in order to overcome the micro- roughness issue, nor to use active cooling. Existing automation techniques can be used.

Further embodiments of the present invention are outlined in the dependent claims.

SHORT DESCRIPTION OF THE FIGURES

In the accompanying drawings preferred embodiments of the invention are shown in which:

Figure 1 shows an embodiment of the state of the art, representing a cut of a rotor slot perpendicular to the main axis of the machine,

Figure 2 shows an embodiment of the state of the art, representing an axial cut of a

rotor slot;

Figure 3 represents a graph showing the coating thickness in microns, wherein the expanded thickness 11 is plotted against the non-expanded thickness 10.

Figure 4 schematic view of a layer of laminate 2 according to the present invention, representing an embodiment in which only one side of the substrate 7 has a heat-expandable coating applied to it;

Figure 5 represents an embodiment, in which both sides of the substrate 7 have a heat-expandable coating 6 applied to them.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same, figures 1 and 2 show a cut of a rotor slot perpendicular to the main axis of the machine (Fig. 1) and an axial cut (Fig. 2) of said rotor slot. Rotors and stators are mainly made of an assembly of coils and a stack of metallic sheets 1. The sheets are shaped so that slots are formed when the sheets are stacked. Laminates/slot liners 2 are laid into the slots and preformed conductor bundles 3 consisting of insulated wires are put on top of the laminate. The slots are then closed by another layer of laminates, the slot closure laminate layer 4.

The stacks of sheets are usually not perfectly aligned and not perfectly cut, as shown in the axial cut through a rotor slot in Fig. 2, creating a micro-roughness that can reach up to 500 microns (The surface of the rotor or the stator exhibits a micro -roughness that can reach 500 micrometers or above.). Thereby, a gap 5 which prevents good thermal conductivity is formed between the conductors 3 and the stack of sheets 1 when laminates are applied against the rotor or the stator.

In the following, the properties and the making of a laminate according to two

embodiments of the present invention is described.

Examples:

Several coating thicknesses were applied to a substrate 7 using a hand coater (K Hand coater 620, supplied by Erichsen Testing Equipment). The coatings 6 on the substrates 7 were cured 30 minutes at 80 ⁰ C until they reached B-stage. The thicknesses of the coatings 6 were measured and are called "non-expanded thickness" 10. The B-stage coatings 6 on the substrates 7 were then cured further for Ih at 140 ⁰ C. The thicknesses were measured and are called "expanded thicknesses" 11.

The formulation of the coatings was as follows:

Components phr (parts per hundred resin)

Bakelite PF 9968 LG 100

Expancel 421 DE 20 10 Bakelite PF 9968 LG was supplied by Hexion Speciality Chemicals GmbH, Germany. Expancel 421 DE 20 is available e.g. at Expancel Inc., Duluth, GA 30096, USA.

The slot liner laminate 2 is laid normally into position, followed by the conductors 3 and by the slot closure laminate 4, which is another layer of laminate, added to the conductors 3 after they have been positioned in the slots for insulation purposes. Curing of the coatings 6 can take place in an oven or by any other way of heat application. The temperature range for curing is 60-160 ⁰ C. Preferably, the curing is carried in two steps. If curing is carried out in two steps, at the first step, A-stage coatings are cured until they reach B-stage, followed by a second step in which B-stage coatings are cured until they reach C-stage. Usually, if the curing is carried out in two steps, the curing in the first step is exercised at 30-100 ⁰ C, preferably 50-90 ⁰ C, more preferably about 80 ⁰ C. The curing in the second step is usually exercised at 60-240 ⁰ C, preferably 100-150 ⁰ C,

more preferably about 140 ⁰ C. Preferably, the first step of the curing is about 0.5-120 minutes, preferably 5-60 minutes, more preferably about 30 minutes in length, while the second step of the curing takes about 5-240 minutes, preferably 10-180 minutes, more preferably about 60 minutes in length.. The ranges depend on the type of matrix and filler chosen and therefore vary according to the composition chosen for the laminate and/or the coating.

The graph of figure 3 shows the expanded thickness 11 against the non-expanded thickness 10. It shows that the thicker the coating, the greater the expansion. Also, the increase is exponential in the chosen thickness range and in this particular example. For instance, a non expanded coating thickness of about 50 micron results in an expanded coating thickness of about 160 micron, according to the graph depicted. Coating thicknesses of about 10 micron to about 80 micron were tested. It is however possible, that smaller or larger amounts of coating 6 are applied to the substrate 7 and are useful as well.

Figures 4 and 5 show a schematic detailed view of a laminate 2. Figure 4 exhibits a substrate 7 coated only on one side, e.g. on a first side 8 of the substrate 7. The coating 6 can either be applied to the side 9 of the substrate 7 facing the metal sheets 1, or to the side 8 of the substrate 7 facing the conductor bundles 3. In Figure 5, the substrate 7 has a coating 6 applied both of its sides 8, 9. Therefore, the side 9 of the substrate 7 facing the metal sheets 1 is coated as well as the side 8 of the substrate facing upwards, toward the conductor bundles 3.

LIST OF REFERENCE NUMERALS

Metallic sheets

Slot lining laminate layer

Conductor bundle

Slot closure laminate layer

Gap

Coating

Substrate

First side of 7

Second side of 7

Non-expanded thickness

Expanded thickness