Method for the electromagnetic isolation of electromagnetic conductors

Abstract

Method for electromagnetic isolation of an electromagnetic conductor comprises the following steps: - coating of said first electromagnetic conductor and with at least one insulating compound configured, in use, to electromagnetically isolate said conductor wherein said at least one insulating compound comprises at least one preceramic polymer and at least one photosensitive element, - irradiation with ultraviolet rays of the conductor coated with the insulating compound for a predetermined time, so as to allow the cross- linking of the preceramic polymer and to obtain the coated and electrically insulated electromagnetic conductor.

Description

[0001] METHOD FOR THE ELECTROMAGNETIC ISOLATION OF ELECTROMAGNETIC CONDUCTORS

[0002] The present invention relates to a method for producing an insulating coating made of ceramic material for high temperatures, obtained by means of preceramic polymers.

[0003] In particular, the invention relates to the electrical insulation of copper conductors with which electrical devices such as electric motors and / or generators are constructed.

[0004] In such electric motors and / or generators, both the stator and the rotor may consist of a core of magnetically conductive material (e.g. a ferromagnetic material), on which current-carrying conducting filaments (typically made of copper), adapted to generate a (fixed or variable) magnetic field, are wound.

[0005] The operation of such motors or generators is linked to the magnetic induction of the current-carrying windings; conducting filaments are wound, around ferromagnetic material elements, in multiple superimposed coils. The magnetic induction is affected by the mutual insulation of the coils; "short-circuit", i.e. the electrical contact between coils, leads to a degradation of the device functionality, since superimposed coils would behave as a single conductive loop.

[0006] In such motors, the need for electrical insulation involves both the core elements (typically made of metal) on which the copper conductors are wound, and the conducting filaments that generate the windings.

[0007] Typically used conductors show limitations in the tightness of the insulation, consisting of a thin polymer layer, in relation to thermal degradation phenomena. These limitations in turn affect the field of use of electric motors and generators, making the coupling with thermal machines operating at high temperatures difficult (thermal shielding, for example, limits the compaction of any thermal-electric hybrid system). Ceramic-based insulators with very high thermal stability are difficult to produce, unless special non-carbon chain polymers, which can be transformed into ceramic material by thermal transformation, are used. Patent Application WO2023012662 proposes a solution using silicone polymers transformed, by oxidation, into a silica-based deposit; the strong dimensional changes and loss of integrity of the polymeric deposit resulting from its ceramic transformation are compensated for by the introduction of dielectric oxide fillers. Conductors coated with a silicone / filler mixture, after being wound on a ferromagnetic core, are further insulated by the deposition, on the same winding, of a second silicone-based mixture.

[0008] Specifically, the method proposed by this Patent Application provides to coat the conductors (both the ferromagnetic core and the copper wires that are wound on it) with an insulating compound comprising a preceramic polymer. The method further comprises the subsequent steps of drying the conductors coated with said insulating compound, further coating the core already coated with the wound copper wire, in turn already coated with a sealing compound comprising a preceramic polymer and configured to promote the adhesion of the conductors to each other when wound in coils, and finally heat treating the assembly of core and copper wire in coils, which comprises heating to a temperature of at least 350°C and subsequent cooling.

[0009] It can be observed that the method proposed by this Patent Application is satisfactory when using highly flexible conductors comprising a weave of thin filaments, while it has criticalities as the diameter and stiffness of the conductors increases. When working with highly flexible conductors, it is possible to perform the coating by simply immersing the conductors themselves, followed by extraction and casting of the excess liquid material; with rigid conductors, this operation is impossible, except for very limited lengths and therefore unfeasible for manufacturing windings consisting of several mutually insulated coils. Flexible filaments (i.e. in the form of a 'braid') are also little suitable for homogenisation operations, by drawing, and for automation in general, given their low stiffness.

[0010] The present invention is instead characterised by being also applicable to monolithic conductors, without diameter and stiffness constraints, in a sequence of operations that can be easily automated. This condition is eased by the use of a liquid mixture based on silicone polymer and oxide fillers, stabilised by UV irradiation, thanks to the introduction of photosensitive components into the mixture.

[0011] An aspect of the present invention relates to a having the characteristics of the appended claim 1.

[0012] Further features of the present invention are contained in the dependent claims.

[0013] The characteristics and advantages of the present invention will become more apparent from the following description of an embodiment of the invention, provided by way of non-limiting example, with reference to the schematic attached drawings, wherein:

[0014] • Figure 1 shows a perspective view of a magnetic core of an electric motor according to an aspect of the present invention;

[0015] • Figure 2 shows a perspective view of a magnetic core of an electric motor coated with an insulating compound according to an aspect of the present invention;

[0016] • Figure 3 shows a perspective view of a conducting filament coated with an insulating compound and assembled on the core of Figure 2 coated with an insulating compound according to an aspect of the present invention;

[0017] • Figures 4a and 4b show the steps of coating a conducting filament with an insulating compound according to a first embodiment of the present invention;

[0018] • Figure 5 shows the steps of coating a conducting filament with an insulating compound according to a second embodiment of the present invention;

[0019] • Figure 6 shows the steps of coating and assembling a conducting filament provided with insulation on a conducting core of the present invention.

[0020] With reference to the above-mentioned figures, the method for electromagnetic isolation of electromagnetic conductors refers, by way of example, to the coating of a first type of conductor and a second type of conductor forming part of a stator and / or rotor of an electric motor.

[0021] Preferably the first type of conductor and the second type of conductor are conductors of a stator of an electric motor.

[0022] Preferably, the first type of conductor is a ferromagnetic core N of a stator, and the second type of conductor is a metal filament F (single wire or braid) wound on said core.

[0023] The method according to the present invention provides a step of coating the electromagnetic conductor with an insulating compound I comprising at least one preceramic polymer and at least one photosensitive element.

[0024] For the purposes of the present invention, "preceramic polymer" means a polymeric compound that can be converted into a ceramic compound, advantageously provided with high thermal and chemical stability.

[0025] Following the deposition step on the conductor, the method comprises a step of UV irradiation (with ultraviolet rays) for a predetermined time of the insulating compound, so as to allow the cross-linking of the preceramic polymer and obtain the electromagnetic conductor coated and electrically insulated.

[0026] Preferably, such an insulating compound is a substantially liquid mixture comprising a silicone-based preceramic polymer, a glassy material, preferably in powder form such as boron-alumino-silicate (BAS), and aluminium oxide or alumina, also preferably in powder form.

[0027] Preferably, such a photosensitive element is an acrylic photosensitive resin.

[0028] The irradiation step with ultraviolet rays to allow the cross-linking of the silicone material of the insulating compound has a minimum duration of 5 minutes, preferably about 10 minutes (λ=365 nm).

[0029] The insulating compound is made from this liquid mixture, which is cross-linked by UV irradiation. In detail, according to a preferred aspect of the present invention, the insulating compound has the following composition (wt%):

[0030] • 38.5 % silicone, preferably H44®;

[0031] • 34.6% isopropanol;

[0032] • 3.8 % acrylic photosensitive resin, preferably 3DM Tough resin, Prusa Research a.s.

[0033] • 15.4 % AI2O3powder (<10 μm)

[0034] • 7.7 % BAS boron-alumino-silicate glass powder (<10 μm) The preparation of the mixture provides the complete dissolution of the silicone (powder) in isopropanol, under mechanical mixing (2000 RPM) for 5 minutes. The solution is then first added with photosensitive resin and then with ceramic and glass powders. The input of each additive into the initial solution is always carried out while maintaining mechanical mixing. At each addition, the compound is kept stirred for 1 minute before the next addition. Further mixing for 2 minutes at 2000 RPM removes excess air from the mixture. When the insulating compound I is used to coat the first type of electromagnetic conductor, i.e. a monolithic conductor such as the ferromagnetic core N of a stator, as the one shown in Figure 1, it is spread on the monolith until a thin, uniform layer S (as shown in Figure 2), free of discontinuities and comprising the edges, is obtained; UV cross-linking is then applied to stabilise the deposit.

[0035] When the insulating compound is used to coat the second type of electromagnetic conductor, i.e. a filament, the liquid mixture can be deposited according to different modes.

[0036] For example, a first mode provides that the long filament F (e.g. over 2 metres) is folded several times to form a helix (as schematised in Figure 3a). This helix is coated with the mixture M using a pipette and then subjected to cross-linking by exposure to UV rays from a suitable source.

[0037] A second mode provides to impregnate the stretched filament manually or pass it through an initial tank V containing the liquid mixture M and then passing it through a chamber C provided with a UV source for stabilisation.

[0038] This process involves a deposition machine, capable of taking uncoated monolithic filament from a reel, and placing it, continuously, in this tank for impregnation in a photosensitive mixture and then in a UV cross-linking chamber, before collection in a second reel. The uniformity and continuity of deposition must be optimised by means of excess material removal systems (e.g. by passing the coated filament through a die), or by replacing the impregnation tank with a spray deposition station. As a result of this optimisation, the deposition layer can be minimised to incorporate fillers that increase the strength and breakdown voltage of the silicone-based coating, before or after ceramisation. The impregnation and stabilisation times must be set, in case of a continuous process, by modulating the feed rate depending on the extent of the UV deposition and irradiation chambers (e.g. irradiation time = length of UV chamber / feed rate).

[0039] The first tank and chamber are advantageously placed in series with each other and the filament can be fed, as shown in Figure 5.

[0040] The succession of multiple deposition and stabilisation actions can be repeated while keeping the filament straight, producing a seamless coating. A filament that has already been coated can be treated anew in the same way as a new filament.

[0041] When the method according to the present invention is used to assemble the stator of an electric motor, the following process is followed.

[0042] First, the magnetic core N is electrically insulated with the layer S of insulating compound as indicated above, then the liquid mixture is spread on the core and UV cross-linking is applied to stabilise the deposit. The filament is then electrically insulated by depositing the liquid mixture on the filament and then subjecting it to UV irradiation. Then, the insulated filament F is wound on the core for a first layer of coils SP (as shown in Figures 3 and 6).

[0043] A sealing mixture SIG, comprising a preceramic polymer, is spread on the core and on the first layer of coils wound on the core.

[0044] Additional sealing mixture is spread at each overlapping layer of coils. The filaments are tensed, so that the excess mixture is removed. The mixture is deposited to seal the entire winding system.

[0045] This sealing mixture preferably comprises the following composition:

[0046] - silicone powder H62C® / 11.9 %

[0047] - isopropanol / 4.7 %

[0048] - liquid silicone H44® / 7.5%

[0049] - Alumina (powder) / 56.8 %

[0050] - Glass (silica-sodium-calcium glass powder) 8.5 %

[0051] - Triethylamine / 2.1 %

[0052] - Distilled water / 8.5 %

[0053] This mixture is prepared as follows. Silicone H44 (Wacker Chemie AG), in powder form, is dissolved in isopropanol until a gel is obtained. Silicone H62C (Wacker Chemie AG, in liquid form) is added to this gel and mixed until a uniform compound is obtained. Alumina and glass powders are then added and homogenisation is carried out in a mixer at 2000 RPM for 2 minutes. Triethylamine (which acts as a cross-linker) is added and the mixture is mixed until it has a uniform viscosity. Water is added until a thick paste is obtained. Finally, further mixing for 2 minutes at 2000 RPM removes excess air from the mixture.

[0054] The sealing mixture thus applied on the stator, after removal of the alcohol solvent (approx. 2 hours at room temperature), is ceramised by heating to 600 °C preferably for 5 °C / min up to 350 °C; 20 °C / min from 350 to 600 °C and subsequently maintained for 3 hours, followed by natural cooling by convection.

[0055] As mentioned above, the present invention is characterised, firstly, by its applicability to monolithic conductors, with no diameter and stiffness constraints. Secondly, a liquid silicone-based mixture achieves stabilisation according to a mechanism different from the one found in the prior art. The cross-linking of the silicone component, which is thermally activated with the support of a catalyst, is replaced by cross-linking by UV irradiation through the introduction of photosensitive acrylic monomers as additional components. These monomers perform a solvent function with respect to (solid) silicone, forming liquid wherein the fillers remain suspended. Cross-linking occurs much faster, without the involvement of catalysts (potentially toxic such as amines).

Claims

CLAIMS1. Method for electromagnetic isolation of an electromagnetic conductor includes the following steps:• coating of said first electromagnetic conductor and with at least one insulating compound (I) configured, in use, to electromagnetically isolate said conductor wherein said at least one insulating compound comprises at least one preceramic polymer and at least one photosensitive element,• irradiation with ultraviolet rays of the conductor coated with the insulating compound for a predetermined time, so as to allow crosslinking of the preceramic polymer and obtain the coated and electrically insulated electromagnetic conductor.

2. Method according to claim 1, wherein said insulating compound is a substantially liquid mixture (M) comprising a silicone-based preceramic polymer, a glassy material, preferably in powder form, and aluminum oxide or alumina, also preferably in dust.

3. Method according to claim 1, wherein said photosensitive element is an acrylic photosensitive resin.

4. Method according to claim 1, wherein the irradiation phase with ultraviolet rays to allow the cross-linking of the silicone material of the insulating compound has a minimum duration of 5 min, preferably about 10 minutes at a wavelength λ of 365 nm .

5. Method according to claim 1, wherein the insulating compound (I) has the following composition (wt%):• 38.5 % silicone, preferably H44®;• 34.6% isopropanol;• 3.8 % acrylic photosensitive resin, preferably 3DM Tough resin, Prusa Research a.s.;• 15.4 % AI2O3powder (<10 μm);• 7.7 % BAS boron-alumino-silicate glass powder (<10 μm).

6. Method according to claim 1, wherein the coating is carried out for a first type of conductor and for a second type of conductor forming part of a statorand / or a rotor of an electric motor.

7. The method according to claim 6, wherein the first type of conductor and the second type of conductor are conductors of a stator of an electric motor and the first type of conductor is a ferromagnetic core (N) of a stator and the second type The conductor is a metal filament (F) that wraps around said core.

8. Method according to claim 7, wherein the coating step of the first type of conductor comprises spreading the insulating compound (I) on the monolith, until obtaining a thin and uniform layer (S), free of discontinuities and also including the edges and subsequently UV cross-linking is applied to stabilize the deposit.

9. Method according to claim 7, wherein the step of coating the second type of conductor includes the step of folding the filament (F) several times to form a helix, coating this helix with the mixture (M) and then subjecting this helix coated to cross-linking through exposure to UV rays.

10. Method according to claim 7, in which the phase of coating the second type of conductor includes the phase of impregnating the stretched filament (F) with the insulating compound (I) by making it pass through a first tank (V) containing said compound in form of a substantially liquid mixture and then subsequently pass the impregnated filament into a chamber (C) equipped with a UV source for stabilization.

11. Method for assembling a stator of an electric motor including the following steps: a) electrically insulate the magnetic core (N) of the stator with the insulating compound (I) according to the method of claim 8, b) electrically isolate the filament (F) capable of being wound on said core according to the method of claim 9 or 10, c) wind the isolated filament on the core (N) for a first layer of turns (SP), d) spread a sealing mixture (SIG), including a preceramic polymer, on the core and on the first layer of coils (SP) wrapped on the core itself, e) wind a further layer of coils on said first layer, f) spread the sealing mixture, including a preceramic polymer, on the newly wrapped layer of coils,g) repeat steps e) and f) until the core has been wrapped with the filament.

12. Method according to claim 11, wherein the sealing compound (SIG) has the following composition:- silicone powder H62C® / 11.9%; - isopropanol / 4.7 %;- liquid silicone H44® / 7.5%;- Alumina (powder) / 56.8 %;- Glass (silica-sodium-calcium glass powder) 8.5%;- Triethylamine / 2.1 %; - Distilled water / 8.5%.

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