METHOD FOR APPLYING AN INSULATING LAYER AND ELECTRONIC COMPONENT

Description

[0001] The invention relates to a method for applying an electrically insulating layer to a metallic surface, and to an electrical component with such a layer.

[0002] Electrical machines (electromechanical converters), such as electric drive motors for motor vehicles, starters, generators, or starter-generators, convert electrical energy into mechanical energy (motors) or mechanical energy into electrical energy (generators). This electromechanical conversion is based on electromagnetic induction. Such electrical machines comprise a stationary stator (also called the stator or primary part), which, in a common design, consists of a stator core (laminated core) with a multitude of stator poles wound with corresponding wire windings. Electrical machines also include a moving component (also called the rotor or secondary part), which in the most common design is a rotor that is rotatably mounted in or around the ring-shaped stator and contains a multitude of permanent magnets.In this design, the moving magnetic field of the rotor generates a current flow in the stator winding (generator), or the magnetic field generated by the stator causes the mechanical movement / rotation of the rotor (motor). Reverse designs, in which the rotor comprises a winding and the stator magnets, are also known.

[0003] The individual windings of the stator or rotor are electrically interconnected with each other and to the outside by a switching ring (also called a connecting ring or contact bridge). The switching ring is usually arranged on the winding head and has several switching ring elements. For example, in three-phase AC machines, the switching ring typically comprises three switching ring elements, each of which electrically connects every third partial winding (coil). EP 1 505 711 A2 (DE 10 2004 036 368 A1) describes a switching ring arrangement for a stator of a three-phase three-phase motor, consisting of three superimposed switching ring elements in the form of copper busbars, each of which connects every third winding of the stator, and a star point ring. An insulating ring made of an electrically insulating material is arranged between each of the individual switching ring elements and the star point ring.DE 10 2008 007 409 A1 also describes a three-part switching ring arrangement for a stator, wherein three switching ring elements and a star point ring are enclosed side by side in a plane within a carrier ring made of a temperature-resistant plastic, such as polyphenylene sulfide. Contact points of the switching ring elements and the star point ring protrude from the carrier ring and are connected to the wire ends of the stator's partial windings. The switching ring arrangement described in DE 102011 115 405 A1 has three substantially coaxially arranged switching ring elements embedded in a plastic and thus insulated from one another. It is manufactured by injection molding. The switching ring arrangements of DE 10 2008 007 409 A1 and DE 10 2011 115 405 A1 are therefore designed as compact components in which the individual switching rings are embedded in solid plastic bodies.

[0004] The electrically insulating material used in electrical components, such as those between switching ring elements, is subject to stringent requirements. It must primarily exhibit high electrical insulation performance and, in particular, high dielectric strength and partial discharge resistance. Furthermore, it must possess high resistance, especially to temperature fluctuations and spikes. These properties should also be achieved at low layer thicknesses of less than 500 µm to minimize the space required for the electrical component.

[0005] It is known to coat individual copper busbars with an electrically insulating plastic layer by powder coating before installation. Epoxy resins are particularly commonly used as materials. DE 30 109 82 A1 describes a typical coating compound consisting of an epoxy resin, rubber components, and inorganic fillers. However, epoxy resins have the disadvantage of being prone to defects, which in turn can lead to electrical breakdowns.

[0006] The powder coating process is generally less complex than extrusion, injection molding and electrodeposition processes, but can lead to problems with regard to surface quality (homogeneous layer thickness), which is particularly disadvantageous for electrical insulation applications.

[0007] Also known is the use of high-temperature polymers such as polyetheretherketone (PEEK) and polyphenylene sulfide (PPS), which are applied to a surface by means of powder coatings according to the documents DE 10 2006 061 940 A1, DE10 2005 009 552 A1 and WO 00 2006 0921 26 A1.

[0008] WO 01 / 48763 A2 discloses a method for applying an electrically insulating layer to an electrical conductor, wherein a powdered epoxy resin is first applied and then the applied epoxy resin is thermally cured.

[0009] From DE 10 2009 047 865 A1 it is known that when applying plastic particles of polyetherketones, a particle size of less than 10 µm should be avoided in order to achieve the thinnest possible layers.

[0010] DE 10 2006 061940 A1 describes a process for producing an electrically insulating layer on a metallic surface, for example, of wires, wherein a powder layer comprising a thermoplastic high-temperature polymer and a metal oxide powder as a filler is applied to the surface and subsequently baked on. Layer thicknesses in the range of 5 to a maximum of 150 µm are produced, wherein the average particle size of the powdered high-temperature polymer is at most twice the layer thickness to be produced.

[0011] US 4,199,651 A describes a method for producing an electrically insulating layer on a metallic surface, for example, of wires, in which a blend of polyetheramide imide and a phenolic polymer is electrostatically deposited on the surface and subsequently sintered. The mean particle diameter of the polymer blend is in the range of 5 to 200 µm.

[0012] EP 3 081 608 A1 describes a method for static powder spraying of an epoxy resin blend onto a metal surface.

[0013] To avoid eddy currents, it is known to construct the stator core not from solid metal, but as a laminated core. For electrical insulation of the individual laminations, these are coated with insulating varnishes, such as C5 varnishes. Furthermore, the copper wire windings required for the stator are wound around the individual teeth of the laminated core, so that the windings are arranged in the slots. However, the copper wire windings must be reliably electrically insulated from the laminated core. To ensure this, insulating papers or sheet insulation materials (for example, consisting of Kapton® and / or Nomex®) are placed between the laminated core and the copper wire windings.

[0014] However, Kapton®, which is made of polyimide, has the disadvantage that it is only available as a film, bandage, or other simple semi-finished products, as it cannot be melted without decomposing. Polyimide is therefore unsuitable for powder coating processes, for example.

[0015] The invention is based on the objective of further improving the insulating effect, the electrical breakdown strength, the partial discharge resistance and the thermal resistance of the insulating layer, and in particular of providing an economical method for producing a very high-quality insulating layer.

[0016] This task is solved by a method and an electrical component having the features of the independent claims.

[0017] Thus, a first aspect of the invention relates to a method for applying an electrically insulating layer to a metallic surface, wherein in a first step a powdered high-temperature polymer with a mean particle diameter [D(v; 0.5)] in the range of 20 to 100 µm is applied to the metallic surface, and in a second step heat is applied to the applied high-temperature polymer, wherein the metallic surface is grounded so that electrical charges dissipate. The powdered high-temperature polymer is a polyetherimide, and the polymer powder has a minimum particle diameter of 5-10 µm [D(v; 0.1)] and a maximum particle diameter of 50-80 µm [D(v; 0.9)].

[0018] It has been shown that the particle diameter of the powder particles used in powder coating has a decisive influence on the homogeneity and adhesion of the resulting layer. Within the scope of the invention, it was found that excessively small particles, due to their low mass, lead to backspray effects, which are particularly noticeable with materials exhibiting very high electrical insulation. It was observed that these backspray effects increase with increasing coating duration and thickness. This is because the polymer particles are electrically charged and are repelled by similarly charged polymer particles already deposited on the metallic surface. Due to the good electrical insulation of the particles, the deposited particles retain their charge and repel newly arriving particles, especially small ones, with the same charge.Depending on the particle size and the material, or rather its capacity to store electrical charges, these backspray effects result in low maximum thicknesses of the insulating layer. Furthermore, the deposition due to these effects is highly irregular and leads to an increased number of areas on the workpiece where little material is deposited. Conversely, excessively large particles, while effectively deposited on the surface due to their mass, exhibit poorer surface adhesion. Additionally, excessively large particles cause inhomogeneities in the applied layer, appearing as so-called orange peel, because the larger particles do not coalesce as well.

[0019] According to the invention, it was discovered that there is an optimal particle size of the polymer powder at which neither relevant backspray effects nor inhomogeneities due to poor interparticle flow and / or poor substrate adhesion are observed.

[0020] It was further observed that after reaching a certain layer thickness, preferably half the final thickness, a tempering step with electrical grounding of the workpiece at a temperature above the glass transition temperature significantly improves the process. This tempering step primarily serves to dissipate electrical charges, but also to homogenize the coating material (more homogeneous layer thickness) and reduce any mechanical stresses. Following the tempering step, another powder coating is applied under the same parameters as the first coating to achieve the final thickness of the polymer layer.

[0021] Due to the improved homogeneity and the choice of materials, the inventive method leads to electrically insulating layers with significantly improved insulating properties. The resulting layers are also preferably transparent and pore-free. Furthermore, the inventive method requires neither pretreatment of the metallic surface nor preheating of the workpiece, as is the case with prior art methods, particularly when copper and steel serve as the metallic surface.

[0022] Metallic surfaces are defined here as surfaces exhibiting metallic electrical properties, namely very good electrical conductivity with the lowest possible electrical resistance, as well as high durability. The method according to the invention is preferably applied to surfaces comprising copper or copper compounds and steel.

[0023] In the first step of the process according to the invention, the high-temperature polymer is applied to the metallic surface using a powder source, for example, a spray gun, spray lance, or similar device. Pretreatment of the metallic surface, such as pickling or preheating, is not required. Advantageously, relative movement occurs between the powder source and the metallic surface to ensure the most uniform possible application of the powder. After reaching a defined layer thickness, preferably half the final layer thickness, the powder-coated metallic surface is subjected to heating in the form of an annealing step with electrical grounding above the glass transition temperature (for amorphous polymers).The heat is preferably applied over a large area rather than at a single point to dissipate electrical charges, homogenize the polymer layer, and reduce mechanical stresses. Heat sources include, for example, a tempering oven, residual or waste heat from other processes, particularly those upstream or downstream, and / or inductive heating. Inductive heating is a method of heating electrically conductive materials by generating eddy current losses within them. The heat is generated directly within the material itself and therefore does not need to be transferred by conduction. The heating output is easily controllable. The electrical power is supplied by special frequency converters (see inverter or resonant converter) or directly from the mains.

[0024] Inductive heating can occur through non-conductive materials; the surroundings are only heated indirectly. The process can be used with any gas or in a vacuum, and no contamination from an external heat source occurs.

[0025] High-temperature polymers are defined as polymers that exhibit high continuous operating temperatures above 180 °C. According to the invention, the high-temperature polymer is polyetherimide. Advantageously, when using polyetherimide (PEI), any inhomogeneities in the layer, particularly on its surface, are very easily detectable, since polyetherimide forms a transparent, amber-colored layer that makes inhomogeneities visible to the naked eye. This offers the advantage of reliable quality assurance. Furthermore, polyetherimides exhibit very good electrical insulation, dielectric strength, and partial discharge resistance, especially even in relatively thin layers.

[0026] The polyetherimide poly-[2,2'-bis(4-(3,4-dicarboxyphenoxy)-phenylpropane)-1,3-phenylene-bisimide] is particularly advantageous for use in the process according to the invention. This is because it is an amorphous material. Therefore, the polyetherimide exhibits very little shrinkage. It has been found that this property is also responsible for its good adhesion to metal surfaces.

[0027] In a further preferred embodiment of the invention, the particles of the powdered high-temperature polymer have a mean diameter [(D(v; 0.5)] in the range of 20 to 50 µm. Additionally, according to the invention, the particle size of the smallest particles [D(v; 0.1)] is 5 to 10 µm and the particle size of the largest particles [D(v; 0.9)] is 50 to 80 µm.

[0028] Studies have shown that particle size distributions in these areas further improve the homogeneity of the layer when the maximum layer thickness is increased, as backspray effects are reduced and at the same time very good adhesion of the layer to the metallic surface is achieved.

[0029] A distribution of particle diameters that is as narrow as possible, especially one like in Figure 5 This has been shown to have a positive effect on the adhesion of the layer to the metallic surface. Furthermore, it was demonstrated that this particle size distribution produces an electrically insulating layer optimized in terms of homogeneity, material yield, and adhesion.

[0030] In a particular embodiment of the invention, it is further provided that heat is applied to the applied high-temperature polymer in a tempering step in the range of 200 to 400 °C, particularly in the range of 250 to 350 °C. The tempering step, with electrical grounding of the workpiece, takes place at a temperature above the glass transition temperature of the high-temperature polymer. The tempering step primarily serves to dissipate electrical charges, but also to homogenize the coating material (more homogeneous layer thickness) and to relieve any mechanical stresses. After the tempering step, a second powder coating is preferably applied under the same parameters as the first powder coating to achieve the final thickness of the polymer layer.

[0031] The heat applied, especially with inductive heating, causes the high-temperature polymer particles to fuse into a continuous layer that exhibits good adhesion properties on metallic surfaces, particularly steel and copper surfaces.

[0032] The preferred temperature ranges are advantageously significantly above the glass transition temperature of the preferably amorphous high-temperature polymer. Within these ranges, the polymer softens, allowing the particles to bond with one another. The softened polymer then fills the surface voids of the coating substrate, resulting in optimal wetting of the metallic surface and a largely pore-free layer. Furthermore, the annealing step causes charges to dissipate from the polymer layer, as it becomes electrically or electrostatically charged due to the deposition of charged and / or polarized particles. When using polyetherimide, an annealing step at temperatures in the range of 300 to 325 °C, particularly 310 °C, for a duration of 10 to 20 minutes, especially approximately 15 minutes, proved particularly advantageous for achieving optimal layer properties.

[0033] Furthermore, it is preferred that the insulating layer has a thickness in the range of 10 to 300 µm, preferably in the range of 80 to 200 µm, and particularly preferred are thicknesses in the range of 90 to 130 µm. In particular, thicknesses in the range of 80 to 200 µm are preferred for the electrically insulating coating of parts of electrical machines, for example, switching rings or laminated cores. For the coating of welded joints, especially on wires, particularly in electrical machines, thicknesses in the range of 10 to 100 µm are preferred. When using polyetherimide according to the inventive method, very good electrical insulation in the form of high dielectric strength and partial discharge resistance can be achieved even at very low layer thicknesses.

[0034] It is particularly advantageous to repeat the first and / or second step of the inventive process. At least one repetition serves primarily to fill any remaining areas of thin material (valleys). Thus, the repetition step(s) significantly increase surface homogeneity and result in uniform layer thicknesses across the entire substrate. Since the thickness of the insulating layer, especially in preferred areas, is proportional to its electrical insulation capacity (dielectrical breakdown strength and partial discharge strength), the quality of the insulating layer is also significantly improved by the repetition process.

[0035] It is preferred that the high-temperature polymer applied in the first and / or second step before the first annealing step with electrical grounding has a layer thickness in the range of 30 to 70 µm, particularly in the range of 45 to 55 µm, especially if a final layer thickness of 100 µm is to be achieved. This advantageously ensures that the total layer thickness does not increase beyond the preferred limits, even when the individual steps are repeated. Furthermore, this embodiment achieves maximum homogeneity of the insulating layer through the multilayer structure. Preferably, after the deposition of the first layer at a layer thickness of 30 to 70 µm, an annealing step with electrical grounding is performed. The annealing step primarily serves to dissipate electrical charges, but also to homogenize the coating material (more homogeneous layer thickness) and to relieve any mechanical stresses.Particularly in areas of the layer where back-spray effects due to charged particles on the surface lead to a stagnation of layer growth, the discharge through the tempering step ensures that a homogeneous total layer thickness is achieved in a second deposition process over the entire substrate, even or especially in areas where no or only inhibited layer growth was possible in the first step.

[0036] The process according to the invention, in particular the tempering step, is advantageously carried out under a protective gas atmosphere. However, a protective gas atmosphere is not absolutely necessary for this process.

[0037] Furthermore, it is preferred that the metallic surface also includes a weld joint and that the process is carried out particularly immediately after welding in order to achieve effective electrical insulation by means of the powder coating according to the invention, especially within a heat-affected zone created by the welding process. Both embodiments serve to electrically insulate the metal in the heat-affected zone of the weld joint and at the same time prevent corrosion of the metal.

[0038] Another aspect of the invention relates to an electrical component comprising a stator lamination stack comprising two metallic surfaces arranged facing each other, wherein an insulating layer is arranged on the metallic surfaces for electrical insulation, which is applied or can be applied using the method according to one of the preceding claims.

[0039] With particular advantage, the electrical component is a component of an electrical machine, in particular a switching ring for an electrical machine, a stator lamination stack, wherein the coating is preferably designed as an inner slot coating or as a coating for the electrical insulation of heat-affected zones of welded joints, for example of welded wire windings.

[0040] In a particularly preferred embodiment of the invention, the metallic surface of two workpieces to be joined by a welding process has two adjacent sections, wherein a first section is coated according to the inventive method and a second section has a comprehensive layer enriched with polyetherimide particles. This layer is, for example, a varnish (wire varnish) or another protective layer, which could potentially reduce the generally good adhesion properties of the coating applied according to the invention or which would result in an interface between two coatings that could lead to defects in the electrical and / or moisture insulation.This configuration is particularly preferred for the coating of components according to the invention that have a welded joint with a heat-affected zone, since the layer is damaged or removed, at least in the area of ​​the heat-affected zone, during the welding process. The presence of polyetherimide in the area of ​​the layer promotes bonding between the layer and the coating according to the invention and thus improves the transition between the paint layer and the powder coating. Furthermore, the occurrence of defects is reduced. The workpieces in question are, for example, wires of a winding of an electric motor that are joined together by welding.

[0041] Further preferred embodiments of the invention result from the other features mentioned in the dependent claims.

[0042] Unless otherwise stated in individual cases, the various embodiments of the invention mentioned in this application can be advantageously combined with one another.

[0043] The invention is explained below using exemplary embodiments with reference to the accompanying drawings. These show: Figure 1 shows a schematic structure of a switching ring and the arrangement of copper rails therein; Figure 2 shows a component with a coating in a first embodiment of the invention; Figure 3 shows another component with a coating in a further embodiment of the invention; Figure 4 shows a schematic representation of the process of the method according to the invention in a preferred embodiment; Figure 5 shows a logarithmic plot of a particle diameter distribution according to a preferred embodiment using the example of a polyetherimide powder for insulating coating; and Figure 6 shows a measured progression of partial discharge inception voltages in an insulating layer according to the invention during a voltage impulse partial discharge.

[0044] The exemplary construction of a switching ring 30 is shown in Figure 1 depicted, with the lower part of the Figure 1 An enlarged section is shown.

[0045] The switching ring 30 shown here, for connection to a stator (not shown), has three switching ring elements 32, which are arranged concentrically within one another and, in the selected illustration, vertically oriented. An insulating layer 10 is arranged between each of the individual switching ring elements 32, so that the switching ring elements 32 are electrically insulated from one another. Each switching ring element 32 has a plurality of contact points 34, which project axially beyond the individual switching ring elements 32 and are angled radially inwards. The contact points 34 serve for the electrical connection of the partial windings of the stator to one another. For this purpose, the contact points 34 are electrically connected to the wire ends of the partial windings, for example, by welding or soldering.Each switching ring element 32 further comprises a connection point 36, which serves for the external connection of the stator, for example to a high-voltage connection. Each switching ring element 32 together with the contact points 34 and the connection point 36 is preferably manufactured in one piece from a metal, for example copper.

[0046] The switching ring 30 can, as in this example, further comprise a star point ring 38, which here is also arranged coaxially with the three switching ring elements 32 as the outer ring layer. An insulating layer 10 is also present between the star point ring 38 and the connecting switching ring element 32. The star point ring 38 forms the electrical star point of the electrical connection for the partial windings. For this purpose, the star point ring 38 has radially inwardly angled contact extensions 40, which, in the assembled state, extend onto the stator poles of the stator core and are wound around by the partial windings. The contact extensions 40 serve to connect one end of the partial windings wound around the poles of the stator.

[0047] The structure of the switching ring 30 described above is known per se. Other designs can also be used within the scope of the present invention.

[0048] The lower detailed illustration shows a further enlarged section of the switching ring 30 and clarifies the arrangement of the insulating layer 10 between the individual switching ring elements 32 of the switching ring 30. These form a layer stack 100, with each switching ring element 32 representing a metallic surface 11 on which the insulating layer 10 is arranged on both sides or around the perimeter, but at least on the side facing an adjacent metallic surface 11. Thus, two insulating layers 10 are arranged between two metallic surfaces 11. Depending on the quality and design of the insulating layer 10, as well as in the case of larger tolerances of the switching ring elements 32, gaps 11a may occur between the insulating layers 11.

[0049] The switching ring 30 is an example of a component 45 according to the invention, which has an insulating coating according to the inventive method. Further components 45 within the meaning of the invention are described in the Figures 2 and 3 shown.

[0050] This shows Figure 2 A stator lamination stack 40. This stator lamination stack 40 consists of a multitude of laminations, which have been assembled into a stator lamination stack 40, for example, by a stamping stacking process. It has a cylindrical outer shell.

[0051] The stator lamination stacks have a yoke 43a. From this yoke 43a, teeth 43 extend radially inwards, leaving a cylindrical area free in the radially inner region of the stator lamination stack. In this free cylindrical area, the rotor, which is mounted to rotate relative to the stator, is positioned in the completed electric motor drive. Grooves are formed between the teeth 43, which must be electrically insulated. End discs 41, which also have an insulating function, are arranged at the openings of the cylindrical base body. The electrical insulation of the grooves and / or the end discs can be achieved by inserting insulating paper or, preferably, by coating using the method according to the invention. A coating 42 according to the invention can be applied partially (A) or completely (B) to the stator lamination stack 40 and the end discs 41.Partial coating offers the advantage of improved heat exchange through the uncoated area. Furthermore, the copper fill factor can be increased because the coating according to the invention requires less space than conventional insulating paper. Moreover, the coating according to the invention is significantly less critical with regard to subsequent process steps, such as full stator potting, since, unlike insulating paper, the coating does not protrude and cannot slip. It also eliminates the formation of additional creepage distances, such as those created by cover slides. Insulating paper also has the disadvantage of often requiring manual application.

[0052] Another example of an electrical component 45, which is preferred for the coating according to the invention, is shown. Figure 3 In Figure 3A so-called hairpin connection of two copper wires in the winding of a stator is shown. Such a connection is formed by joining two metal wires 44, in particular copper wires, by welding. Before welding, the metal wires 44 have a protective layer 48, for example in the form of a wire enamel. As a result of the welding process, this layer is destroyed or at least altered, at least in some areas, namely at the weld point 46 and within the so-called heat-affected zone (HAZ) 47, by the heat supplied. Therefore, the now joined metal wires are recoated with a coating 42. For this purpose, the method according to the invention is preferably used, wherein the metal wires 44 are coated by means of a powder coating according to the invention both in the heat-affected zone 47 and in the area of ​​the weld joint 46.

[0053] The metallic surface on which the coating according to the invention is applied can be divided into two sections. A first section 46, which in the example of the Figure 3 the welded joint 46 corresponds to, and an adjacent second section 47, which in the design of the Figure 3 the heat-affected zone. The coating 42 according to the invention exhibits good adhesion properties even without pretreatment. If the metallic surface in the second section 47 has a further protective layer 48, for example in the form of a lacquer, the adhesion properties can be improved if the protective layer 48 comprises polyetherimide, particularly in particulate form. These are then melted by the heat applied during welding and / or the second step of the process according to the invention and bonded with the coating according to the invention.

[0054] Figure 4Figure 1 shows a schematic representation of the method according to the invention in a preferred embodiment. In the embodiment shown, the method comprises two repetition units I and II, wherein in the first unit I a first layer 14 is applied to a metallic surface 11 in two steps Ia and Ib. In repetition step II, a further layer is applied to the first layer 14 by the same process steps Ia and Ib.

[0055] At the beginning of the process according to the invention, a metallic surface 11, preferably a copper or steel surface, a surface 11 made of copper compounds, or a steel surface, is provided in a first step. The metallic surface 11 has not been pretreated beforehand, for example, by pickling, tempering, or the like. A powdered polyetherimide with a corresponding particle size distribution 12 is applied to this metallic surface 11 using a spray gun 1 or spray lance. In order to achieve the most homogeneous deposition possible, a relative movement takes place between the metallic surface 11 and the spray gun 1. This can be achieved either by moving the metallic surface 11 or, as shown, by pivoting the spray gun 1.It is advantageous that the processing area achievable by swiveling the spray gun 1 corresponds to a width or a length of the metallic surface 11.

[0056] According to the invention, polyetherimide is used as the high-temperature polymer. The particles of the powdered polyetherimide exhibit a size distribution according to Fig. 3 A polyetherimide with a particle size distribution of D(v;0,1)=7 µm, D(v; 0.5)=27 µm and D(v; 0.9)=62 µm is particularly suitable, i.e. a powder with a particle size distribution according to Figure 5 .

[0057] After achieving a specified layer thickness of 80 to 200 µm, in particular approximately 100 µm, preferably half the final layer thickness, the powder application to the metallic surface 11 is interrupted. In the subsequent second step Ib, the applied high-temperature polymer 13 is exposed to heat (annealing with electrical grounding) 3.

[0058] This can be achieved, for example, by placing the metallic surface 11 in a tempering furnace. The heat source 3 has a temperature above the glass transition temperature of the high-temperature polymer, specifically above 180 °C, and more specifically above 250 °C. At the same time, it is ensured that the temperature remains below a critical temperature for the respective high-temperature polymer, at which thermal degradation of the polymer would begin. The tempering process lasts a few minutes, particularly 10 to 20 minutes, and preferably approximately 15 minutes.

[0059] The result is a metallic surface with an insulating layer 10 applied to it. To further improve the quality of this insulating layer 10, process steps 1a and 1b are repeated in a second iteration step II. The aim is to achieve a total thickness of the insulating layer 10 of 80 to 200 µm, in particular 90 to 130 µm, preferably approximately 100 µm. Most known powder coating processes with high-temperature polymers require at least three coating steps for this; however, the choice of particle sizes or particle size distribution in the process according to the invention makes it possible to achieve such a layer thickness after only two coating steps, interrupted by a tempering step with electrical discharge and completed by a final tempering step with electrical grounding. Preferred embodiment:

[0060] In a preferred embodiment, polyetherimide granules (Ultem® < 1000) were milled into polyetherimide powder with varying particle size distributions in a liquid nitrogen-filled pulveriser and dried under vacuum. The powder was drawn in by a rod injector and fluidized into a powder cloud for ejection via the powder lance. Powder coating was carried out with a total volume flow rate of 2.5 m³ / h and a powder output of 100 g / min. The powder was charged via a 30 kV corona system (GEMA system). Powder deposition took place onto a grounded copper plate, with a distance of 125 mm between the powder lance and the copper plate. The copper plate (type Wieland K1, Cu-OF R2401) had dimensions of 100 x 100 x 1 mm. The copper plates serve as simple surrogate models for the geometrically more complex components to be coated.The copper was coated in a cold state (at room temperature). A particle size distribution with a mean particle diameter [D(v; 0.5)] of 27 µm, where the smallest particles had a diameter [D(v; 0.1)] of 7 µm and the largest particles had a diameter [D(v; 0.9)] of 62 µm, yielded the best results. Following the powder coating step, a 15-minute annealing step at 310 °C was performed. This process initially achieved a polyetherimide layer thickness of 50 µm. Subsequently, the powder coating step and annealing were repeated to deposit further polyetherimide in a layer thickness of 50 µm, resulting in a total polyetherimide layer thickness of 100 ± 5 µm. The result was a very homogeneous and well-adhering polyetherimide layer with a very smooth surface.

[0061] In contrast, using an otherwise identical process, the application of polyetherimide powder with a particle size distribution of a mean particle diameter [D(v; 0.5)] of 121 µm, a smallest particle diameter [D(v; 0.1)] of 12 µm, and a largest particle diameter [D(v; 0.9)] of 204 µm showed hardly any backspray effects, but after the tempering step, a clearly recognizable uneven distribution of particles in the layer was observed. The typical "orange peel" pattern of the polyetherimide coating was evident.

[0062] A polyetherimide powder with a particle size distribution of 10 µm mean particle diameter [D(v; 0.5)], 6 µm minimum particle diameter [D(v; 0.1)] and 15 µm maximum particle diameter [D(v; 0.9)] showed significantly increasing backspray effects and the associated inhibited layer growth even at a layer thickness of 50 µm, so that complete coverage of the metallic substrate with polyetherimide could not be achieved.

[0063] Figure 5 Figure 1 represents the particle size distribution of the particles used in the preferred embodiment in a logarithmic plot. The particle diameters plotted on the abscissa are most densely populated in the range of 15, particularly 20, to 40 µm. Based on this distribution, the mean particle diameter [D(v; 0.5)] is 27 µm.

[0064] The polyetherimide coatings produced according to the invention also exhibited very good electrical shock and partial discharge resistance ( Figure 6 For characterizing insulating materials, voltage impulse partial discharge measurements are particularly suitable, as they especially accurately reflect the stress on the insulating material as it occurs in electric machines (rapidly rising voltage edges due to frequency converters). For this purpose, the sample (coated copper plate) is subjected to a rapidly rising voltage pulse ( Figure 6 , black, bold graph), in parallel, the partial discharges occurring in the insulation layer are detected ( Figure 6(Graph with thin lines). The measurement is preferably performed a total of five times, from which the arithmetic mean is then calculated. The later the partial discharges occur in the insulating material, i.e., the higher the inception voltages for the occurrence of the partial discharges, the better the electrical insulation effect. Specifically, the copper plate with polyetherimide coating (coating thickness: 96 µm) described in the preferred embodiment exhibited an inception voltage of 1309 V. Figure 6 shows the stress profile of the described measurement on a layer applied according to the invention with a layer thickness of 96 µm.

[0065] Table 1 shows measured inrush voltages for various powder coatings. A 275 µm thick sample of a commercially available epoxy resin, designated for electrical insulation purposes, serves as a prior art reference sample. The reference sample only achieved an inrush voltage of 1510 V (Table 1). For the polyetherimide coatings according to the invention, however, inrush voltages in the range of 1309–2742 V were found in the layer thickness range of 96–210 µm. The relationship can be mathematically formulated as follows: Sto β − TE − Einsetzspannung Volt = 12,716 Schichtdicke in μm + 28,577 R 2 = 0,9915 Table 1 Powder coating material (layer thickness in µm) Impulse TE induction voltage in V EP resin for powder coating (275) (comparative sample) 1510 Polyetherimide (96), example measurement in Fig. 6 1309 Polyetherimide (123) 1531 Polyetherimide (175) 2213 Polyetherimide (210) 2742 Reference symbol list

[0066] 1. Spray gun 2. Processing area 3. Heat radiation 10 insulating layer 11 metallic surface 11a gap 12 powdered high-temperature polymer 13 applied high-temperature polymer 14 first layer 30 Switching ring 32 Switching ring element 34 Contact point 36 Connection part 38 Star point ring 40 Stator lamination stack 41 End plates 42 Insulating coating 43 Tooth 43a Yoke 44 Metal wire 45 Component 46 First section / Weld 47 Second section / Heat-affected zone 48 Protective layer / Wire enamel I. Arranging a first layer II. Repeat step Ia / IIa first step Ib / llb follow-up step

Claims

1. Method for applying an electrically insulating layer (10, 42) to a metal surface (11, 40), a powdered high-temperature polymer (12) which has an average particle diameter [D(v; 0.5)] in the range of 20 to 100 µm being applied to the metal surface (11, 40) in a first step (Ia / IIa), and heat acting on the applied high-temperature polymer (12) in a second step (Ib / IIb), the metal surface being grounded such that electrical charges flow away, characterized in that the powdered high-temperature polymer (12) is a polyetherimide, and the smallest particles of the powdered high-temperature polymer (12) have a diameter [(D(v; 0.1)] in the range of 5 to 10 µm, and the largest particles of the powdered high-temperature polymer (12) have a diameter [(D(v; 0,9)] in the range of 50 to 80 µm.

2. Method according to any of the preceding claims, characterized in that the powdered high-temperature polymer (12) has an average particle diameter [D(v; 0.5)] in the range of 20 to 50 µm.

3. Method according to either of the preceding claims, characterized in that the heat in the second step (Ib / IIb) acts on the applied high-temperature polymer (12) in the range of 200 to 400°C, in particular in the range of 250 to 350°C.

4. Method according to any of the preceding claims, characterized in that the heat is generated in a tempering furnace, by inductive heating and / or as waste heat of upstream and / or downstream processes.

5. Method according to any of the preceding claims, characterized in that the insulating layer (10, 42) has a layer thickness in the range of 10 to 300 µm, in particular in the range of 80 to 200 µm.

6. Method according to any of the preceding claims, characterized in that the first step (Ia / IIa) and / or the second step (Ib / IIb) are repeated.

7. Method according to claim 5, characterized in that the high-temperature polymer (12) applied in the first step (Ia / IIa) has a layer thickness in the range of 5 to 70 µm, preferably in the range of 30 to 70 µm, in particular in the range of 45 to 55 µm.

8. Method according to any of the preceding claims, characterized in that the metal surface comprises the heat-affected zone of a welded joint, and the method is carried out, in particular immediately, after welding.

9. Component (45) having a layer stack (100) comprising two metal surfaces (11, 40) which are arranged facing one another, wherein an electrically insulating layer (10, 42) is arranged on the metal surfaces (11, 40, 44), which layer is applied using the method according to any of the preceding claims.

10. Component (45) according to claim 9, characterized in that the component (45) is a part of an electrical machine, in particular a switch ring (30), a part of a stator laminated core (40), in particular a groove, or a welded joint.

11. Component (45) according to claim 9 or 10, characterized in that the metal surface (11, 40, 44) has two contiguous portions (46, 47), a first portion (46) being coated with the electrically insulating layer (10, 42) applied according to a method of claims 1 to 8, and a second portion (47) having a layer (48) comprising polyetherimide.