Method for manufacturing a squirrel-cage rotor for an electric machine

Abstract

The invention relates to a method for manufacturing a squirrel-cage rotor for an electric machine, this rotor including: - a squirrel cage (10) including two short-circuit rings (12, 13) which extend about a single axis (A) and are at a distance from each other, and conducting bars (14) which extend between the rings (12, 13) and which have longitudinal ends (14a) connected to these rings; and - magnetic sheets (16) inserted between the rings (12, 13) and stacked along the axis (A) thereof, characterised in that it comprises the production of at least one portion of the squirrel cage (10) by additive manufacturing, or even the production of the magnetic sheets (16) by additive manufacturing.

Description

[0001] DESCRIPTION

[0002] TITLE: METHOD FOR MANUFACTURING A SQUIRREL CAGE ROTOR FOR AN ELECTRIC MACHINE

[0003] Technical field of the invention

[0004] The present invention relates to a method of manufacturing a squirrel cage rotor for an electric machine.

[0005] Technical background

[0006] The technical background includes in particular documents US-A1- 2021 / 028675, JP-B1 -5745703, US-A1-2017 / 0141661 , US-A1 -2013 / 0106233, US-B2-9,071 ,112 and JP-A-2007 / 202235.

[0007] An electrical machine, such as an asynchronous motor, may have a squirrel-cage rotor.

[0008] A rotor of this type is illustrated in Figure 1a and comprises:

[0009] - a squirrel cage 10 comprising two short-circuit rings 12, 13 which extend around the same axis A and are at a distance from each other, and conductive bars 14 which are arranged around the axis A and extend between the rings 12, 13, these bars 14 having longitudinal ends 14a, 14b connected to these rings 12, 13, and

[0010] - magnetic sheets 16 interposed between the rings 12, 13 and stacked along the axis A.

[0011] In the example of Figure 1a, the rotor further comprises a shaft 18 which extends along the axis A and inside the squirrel cage 10, as well as guide bearings 20 mounted on the shaft 18, on either side of the cage 10. The cage 10 is fixed to the shaft 18 and is therefore rotationally integral with the shaft 18. Figure 1b shows the squirrel cage 10 of the rotor of Figure 1a. The rings 12, 13 and the bars 14, the longitudinal ends 14a of which are connected to the rings 12, 13, can be clearly seen. The bars 14 may be parallel to the axis A or inclined relative to this axis A, as in the example shown. The manufacture of a rotor of this type includes a step of manufacturing the cage 10. In the current art, there are two technologies for manufacturing a squirrel cage 10.

[0012] The first technology consists of assembling the rings 12, 13 and the bars 14 and then joining them together by brazing. This technology is illustrated in Figures 2a to 2d.

[0013] The sheets 16 are stacked and crossed by the bars 14 whose longitudinal ends 14a, 14b are engaged in orifices 12a of the rings 12, 13. On the side opposite the sheets 16, each of the rings 12, 13 comprises a surface 12b on which recesses 12c are formed which communicate with the orifices 12a. The longitudinal ends 14a, 14b of the bars 14 project at the bottom of these recesses 12c (figure 2a).

[0014] A brazing material 22 is deposited on the longitudinal ends 14a of the bars 14 and at the bottom of the recesses 12c. The parts are held by specific tooling and the brazing material 22 is heated by induction to melt it. The molten material fills the spaces between the bars 14 and, upon cooling, forms a brazing joint 24 which makes it possible to secure the bars 14 to the ring 12, 13 (Figures 2b and 2c). The surface 12b of the ring 12, 13 is then machined to plan it and bring the ring 12, 13 to the desired dimensions. The brazing material 22 is deposited in the liquid state and has a melting temperature lower than that of the material of the cage 10 which therefore does not participate in the formation of the brazing joint 24.

[0015] This manufacturing technology of the cage 10 requires particular vigilance and is likely to generate certain defects which can affect the performance of the rotor.

[0016] Among the sensitive defects, there are solder drips which can penetrate into the stack of sheets 16 generating short circuits between bars 14 and directly affecting the performance of the machine (starting time and current, speed under load, etc.).

[0017] Other defects are the lack of material in the brazing area which affects the mechanical strength and resistivity of the cage 10. This can lead to changes in the operating point of the machine (speed under load) and can also affect its lifespan (mechanical weakness and hot spots in the connections).

[0018] The resulting 24 solder joint is heterogeneous and systematically contains porosities or air bubbles. These inclusions are very variable and very difficult to control by non-destructive methods and lead to uncertainty in the resulting quality of the parts produced.

[0019] Another technology is to make the cage 10 from a single casting.

[0020] However, this solution also leads to quality problems similar to those encountered by brazed cages, namely material shortages, poor repeatability, material leaks between the sheets 16, etc. This technology is also not very suitable for small series given the investment required for the tools.

[0021] Therefore, a more reliable and repeatable process would be an asset for improving the quality of rotors (better repeatability of the process), and could make it possible to extend the operating range of the machines (increase in temperatures and admissible currents in the cage), but also the reliability of the products (lifespan of the bearings) and noise reduction.

[0022] At least some of the prior art documents cited above propose the use of additive manufacturing for the production of all or part of the cage. However, these documents focus on the use of the cold spray manufacturing technique which consists of projecting solid material. Cold spray projection on non-smooth surfaces or cavities is very complex and it becomes almost impossible to fill these spaces or interstices. The deposition efficiency is extremely low because a lot of powder is expelled by the air flow.

[0023] Other techniques involve using powder that is melted without being projected.

[0024] The present invention aims to provide a solution to this need, which is simple, effective and economical.

[0025] Summary of the invention The invention relates to a method of manufacturing a squirrel cage rotor for an electric machine, this rotor comprising:

[0026] - a squirrel cage comprising two short-circuit rings which extend around the same axis and are spaced apart from each other, and conductive bars which extend between the rings and which have longitudinal ends connected to these rings, and

[0027] - magnetic sheets interposed between the rings and stacked along their axis, characterized in that it comprises the production of at least part of the squirrel cage by additive manufacturing by projection of molten or molten material.

[0028] The present invention thus proposes using additive manufacturing to produce all or part of the squirrel cage. Several variant embodiments are possible, additive manufacturing being able to be used to produce a part, to produce part of a part, or to join parts together, the latter not necessarily being produced by additive manufacturing.

[0029] The advantage of additive manufacturing is that the filler material used by this technology can be the same as that of the part to be produced or joined.

[0030] Spraying molten or molten material, i.e. in a liquid state, is advantageous over spraying or using solid material for several reasons. It allows for limiting defects, increasing yield, achieving significant deposit thicknesses, etc.

[0031] The method according to the invention may comprise one or more of the following features or steps, taken alone or in combination with each other:

[0032] + the process includes:

[0033] - a first step during which a first of the rings and the conductive bars are produced by additive manufacturing using a first material, these conductive bars having first longitudinal ends connected to the first ring and extending from this first ring at the end of this first step, - a second step during which the magnetic sheets are positioned on the first ring, second free longitudinal ends of the bars, located on the side opposite the first ring, projecting from a surface of the magnetic sheets, and

[0034] - a third step during which the second ring is produced by additive manufacturing directly on the magnetic sheets using the first material, by depositing, and in particular by projecting, this first material onto said surface to secure the second ring to the second longitudinal ends of the bars;

[0035] + the process includes:

[0036] - a first stage of assembly of the magnetic sheets with the conductive bars, and

[0037] - a second step during which at least one of the rings is produced at least in part by additive manufacturing, or is fixed to the conductive bars by additive manufacturing;

[0038] + during the first step, the magnetic sheets and the conductive bars are assembled together and with a first of the rings which comprises orifices for the passage of first longitudinal ends of the bars, these orifices each comprising a flare on the side opposite the magnetic sheets, the longitudinal ends of the bars being projecting at the bottom of these flares at the end of the first step, and in which, during the second step, the flare of each of these orifices is filled by additive manufacturing to secure the longitudinal ends of the bars to the ring;

[0039] + in the first step, the magnetic sheets and the conductive bars are assembled together and with a preform of a first of the rings which comprises orifices for the passage of first longitudinal ends of the bars, the longitudinal ends of the bars passing through orifices of the preform at the end of the first step, and in which, during the second step, the ring is completed by additive manufacturing and the longitudinal ends of the bars are secured to the ring by depositing, and in particular by projecting, a material by additive manufacturing onto an annular surface of the preform opposite the magnetic sheets; + the longitudinal ends of the bars protrude from said annular surface of the preform at the end of the first step;

[0040] + the longitudinal ends of the bars are flush with said annular surface of the preform at the end of the first step;

[0041] + the longitudinal ends of the bars are set back on said annular surface of the preform at the end of the first step;

[0042] + during the first step, the magnetic sheets and the conductive bars are assembled together, first longitudinal ends of the bars protruding from the magnetic sheets and projecting onto a first surface of these sheets at the end of the first step, and in which, during the second step, a first ring is produced by additive manufacturing and the longitudinal ends of the bars are secured to this ring by depositing, and in particular by projecting, a material by additive manufacturing onto said surface of the sheets;

[0043] + the two rings are made at least partly by additive manufacturing, or are secured to the longitudinal ends of the bars in the same way;

[0044] + additive manufacturing is not carried out by a technique called cold spray or cold projection (also called CSAM, acronym for Cold Spray Additive Manufacturing);

[0045] + additive manufacturing is thus chosen from a technique which excludes the cold spray technique;

[0046] + additive manufacturing is chosen from the wire arc or WAAM technique (and for example hot-melt sprayed copper) and the HVOF or HVAF (High-Velocity Oxygen Fuel / High-Velocity Air Fuel) technique; alternatively, other types of additive manufacturing would be possible;+ the rings and bars are made from the same material;

[0047] + the rings and bars are made of a material chosen from pure aluminum, an aluminum-based alloy, pure copper, and a copper-based alloy;

[0048] + the magnetic sheets are clamped between the rings without play or spacer;

[0049] -- the flares each have a chamfer; -- the free longitudinal ends of the bars are chamfered;

[0050] -- the preform of a ring is in the form of a disc;

[0051] -- the orifices have any shape, and / or the bars each have any section, this shape or section being for example round, elliptical, polygonal, etc.;

[0052] -- the rings or ring preforms are supported or applied (directly) to the magnetic sheets.

[0053] Brief description of the figures

[0054] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which:

[0055] [Fig.1 a-1 b] Figures 1 a and 1 b are schematic perspective views respectively of a rotor and a squirrel cage of this rotor;

[0056] [Fig.2a-2d] Figures 2a to 2d are very schematic partial sectional views of a squirrel cage rotor, and show steps in a method of manufacturing the rotor by brazing the cage;

[0057] [Fig. 3a-3b] Figures 3a and 3b are very schematic partial sectional views of a squirrel cage rotor, and show steps of a first embodiment of a method of manufacturing a rotor according to the invention;

[0058] [Fig.4a-4c] Figures 4a to 4c are very schematic partial sectional views of a squirrel cage rotor, and show steps of a second embodiment of a method of manufacturing a rotor according to the invention;

[0059] [Fig.5a-5b] Figures 5a and 5b are very schematic partial sectional views of a squirrel cage rotor, and show steps of a third embodiment of a method of manufacturing a rotor according to the invention;

[0060] [Fig.6a-6c] Figures 6a to 6c are very schematic partial sectional views of a squirrel cage rotor, and show steps of a fourth embodiment of a method of manufacturing a rotor according to the invention;

[0061] [Fig.7a-7c] Figures 7a to 7c are very schematic partial sectional views of a squirrel cage rotor, and show steps of a fifth embodiment of a method of manufacturing a rotor according to the invention. [Fig.8a-8c] Figures 8a to 8c are very schematic partial sectional views of a squirrel cage rotor, and show steps of a sixth embodiment of a method of manufacturing a rotor according to the invention.

[0062] Detailed description of the invention

[0063] Figures 1a-1b and 2a-2d have been described in the above.

[0064] The present invention relates to a method of manufacturing a squirrel cage rotor for an electrical machine, this rotor being as illustrated in Figure 1a and comprising:

[0065] - a squirrel cage 10 comprising two short-circuit rings 12 which extend around the same axis A and are at a distance from each other, and conductive bars 14 which extend between the rings 12, 13 and which have longitudinal ends 14a, 14b connected to these rings 12, 13, and

[0066] - magnetic sheets 16 interposed between the rings 12, 13 and stacked along the axis A.

[0067] The rotor may further comprise a shaft 18 which extends along the axis A and inside the squirrel cage 10, as well as guide bearings 20 mounted on the shaft 18, on either side of the cage 10. The cage 10 is fixed to the shaft 18 and is therefore rotationally integral with the shaft 18.

[0068] The bars 14 may be parallel to the axis A or inclined relative to this axis A, as in the example shown. They are arranged around the axis and are identical. They have any cross-sectional shape, for example square, rectangular, round, polygonal, elliptical, etc.

[0069] The particularity of the method according to the invention is that at least part of the squirrel cage 10 is produced or assembled by additive manufacturing.

[0070] Several embodiments are possible and described below with reference to figures 3a and following.

[0071] A first embodiment is illustrated in Figures 3a and 3b.

[0072] The rotor manufacturing process includes:

[0073] - a first step of assembling the magnetic sheets 16 with the conductive bars 14, and with at least one of the rings 12, 13, and

[0074] - a second step during which at least one of the rings 12, 13 is fixed to the conductive bars 14 by additive manufacturing. It is therefore understood that the sheets 16, the bars 14 and the ring(s) 12 are manufactured beforehand, not necessarily by additive manufacturing.

[0075] A first of the rings 12 comprises orifices 12a for the passage of first longitudinal ends 14a of the bars 14. These orifices 12a have any shape and each comprise a flare 12d on the side opposite the magnetic sheets 16. The longitudinal ends 14a of the bars 14 protrude at the bottom of these flares 12d at the end of the first step illustrated in figure 3a.

[0076] The flares 12d may have any shape and may, for example, have a chamfer to facilitate the deposition of the filler material. The ends 14a of the bars 14 may also have chamfers to facilitate this deposition.

[0077] During the second step illustrated in figure 3b, the flare 12d of each of these orifices 12a is filled by additive manufacturing to secure the longitudinal ends 14a of the bars 14 to the first ring 12.

[0078] A necessary and sufficient quantity of filler material 26 can be deposited in the flare 12d by additive manufacturing so as to obtain an external surface 26a of the filler material 26, which is aligned with the surface 12b of the ring 12. It is thus not necessarily useful to machine this surface 12b to flatten it. The filler material 26 can solidify immediately after deposition, thus avoiding any type of dripping or lack of material and ensuring a homogeneous joint between the parts. The filler material 26 by additive manufacturing is advantageously identical to that of the ring 12 and the bars 14.

[0079] The second ring 13 can be secured in the same way to the opposite longitudinal ends 14b of the bars 14. It is then understood that the second ring 13 comprises orifices 12a for the passage of the second longitudinal ends 14b of the bars 14, these orifices 12a each comprising a flare 12d on the side opposite the magnetic sheets 16. The longitudinal ends 14b of the bars 14 project at the bottom of these flares 12d and these flares 12d are filled by additive manufacturing to secure the longitudinal ends 14b of the bars 14 to the second ring 13. Although figures 3a and 3b show only one of the rings of the cage, it can be considered that the rings are similar and therefore that figures 3a and 3b serve to illustrate each of the rings of the cage.

[0080] A second embodiment is illustrated in Figures 4a to 4c.

[0081] The rotor manufacturing process includes:

[0082] - a first step of assembling the magnetic sheets 16 with the conductive bars 14, and with at least one preform 12' of at least a first of the rings 12, 13, and

[0083] - a second step during which the preform 12' of the ring or of each ring 12, 13 is completed and fixed to the conductive bars 12 by additive manufacturing.

[0084] We therefore understand that the sheets 16, the bars 14 and the preform(s) 12' are manufactured beforehand, not necessarily by additive manufacturing.

[0085] In the example shown, the or each preform 12' is in the form of a disc and therefore has an annular shape around the axis A.

[0086] The preform 12' of a first of the rings 12 comprises orifices 12'a for the passage of first longitudinal ends 14a of the bars 14. The orifices 12'a have any shape. The longitudinal ends 14a of the bars 14 project from an annular surface 12'b of the preform 12' opposite the magnetic sheets 16 at the end of the first step (figure 4a). During the second step, the ring 12 is completed by additive manufacturing and the longitudinal ends 14a of the bars 14 are secured to the ring 12 by depositing, and in particular by projecting, a filler material 26 by additive manufacturing onto the surface 12'b of the preform 12' (figures 4b and 4c).

[0087] A necessary and sufficient quantity of material 26 can be deposited on the surface 12'b by additive manufacturing depending on the final dimensions desired for the ring 12. The filler material 26 can solidify immediately after deposition, thus avoiding any type of dripping or lack of material and ensuring a homogeneous joint between the parts. The filler material 26 by additive manufacturing is advantageously identical to that of the ring 12 and the bars 14.

[0088] The second ring 13 can be produced in the same way at the opposite longitudinal ends 14b of the bars 14, from another preform 12'. It is then understood that this other preform 12' comprises orifices 12a' for the passage of second longitudinal ends 14b of the bars 14, which project from an annular surface 12'b of the preform 12' opposite the magnetic sheets 16. The ring 13 is completed by additive manufacturing and the longitudinal ends 14b of the bars 14 are secured to the ring 13 by depositing, and in particular by projecting, a material by additive manufacturing onto the surface 12'b of the preform 12'.

[0089] Although Figures 4a to 4c show only one of the cage rings, the rings can be considered similar and therefore Figures 4a to 4c serve to illustrate each of the cage rings.

[0090] A third embodiment is illustrated in Figures 5a and 5b.

[0091] The rotor manufacturing process includes:

[0092] - a first step of assembling the magnetic sheets 16 with the conductive bars 14, and

[0093] - a second step during which at least one of the rings 12, 13 is produced by additive manufacturing and secured to the conductive bars 16 by additive manufacturing.

[0094] We therefore understand that the sheets 16 and the bars 14 are manufactured beforehand, not necessarily by additive manufacturing.

[0095] First longitudinal ends 14a of the bars 14 protrude from the magnetic sheets 16 and project onto a first surface 16a of these sheets 16 at the end of the first step (figure 5a).

[0096] During the second step, a first ring 12 is produced by additive manufacturing and the longitudinal ends 14a of the bars 14 are secured to this ring 12 by depositing, and in particular by projecting, a filler material 26 by additive manufacturing onto said surface 16a of the sheets (figure 5b).

[0097] A necessary and sufficient quantity of material 26 can be deposited on the surface 16a by additive manufacturing depending on the final dimensions desired for the ring 12. The filler material 26 can solidify immediately after deposition, thus avoiding any type of dripping or lack of material and ensuring a homogeneous joint between the parts. The filler material 26 by additive manufacturing is advantageously identical to that of the ring 12 and the bars 14. The second ring 13 can be produced in the same way at the opposite longitudinal ends 14b of the bars 14. It is then understood that second longitudinal ends 14b of the bars 14 protrude from the magnetic sheets 16 and project onto a second surface 16a of these sheets 16.The second ring 13 is produced by additive manufacturing and the longitudinal ends 14b of the bars 14 are secured to this ring 13 by depositing, and in particular by projecting, a material 26 by additive manufacturing onto the second surface 16a of the sheets 16.

[0098] Although Figures 5a and 5b show only one of the cage rings, the rings can be considered similar and therefore Figures 5a and 5b serve to illustrate each of the cage rings.

[0099] A fourth embodiment is illustrated in Figures 6a to 6c.

[0100] In this variant, the method comprises:

[0101] - a first step during which a first of the rings 12, 13 and the conductive bars 14 are produced by additive manufacturing using a first filler material 26, these conductive bars 14 having first longitudinal ends 16a connected to the first ring 12 and extending from this first ring 12 at the end of this first step (figure 6a),

[0102] - a second step during which the magnetic sheets 16 are positioned on the first ring 12, second free longitudinal ends 16b of the bars 14, located on the side opposite the first ring 12, projecting from a surface 16a of the magnetic sheets 16 (figure 6b), and

[0103] - a third step during which the second ring 13 is produced by additive manufacturing directly on the magnetic sheets 16 using the first material 26, by depositing, and in particular by projecting, this first material onto said surface 16a to secure the second ring 13 to the second longitudinal ends 14b of the bars 14 (figure 6c).

[0104] The filler materials 26, 28 by additive manufacturing are different, but the same filler material 26 can be used for the manufacture of the rings 12, 13 and the bars 14. These filler materials 26, 28 can solidify immediately after deposition, thus avoiding any type of dripping or lack of material and ensuring homogeneous joints between the parts. The embodiment of Figures 7a-7c differs from the embodiment of Figures 4a-4c in that the longitudinal ends 14a of the bars 14 are flush with the annular surface 12'b of the preform 12' opposite the magnetic sheets 16 at the end of the first step (Figure 7a).

[0105] The embodiment of figures 8a-8c differs from the embodiment of figures 4a-4c in that the longitudinal ends 14a of the bars 14 are set back relative to the annular surface 12'b of the preform 12' opposite the magnetic sheets 16 at the end of the first step (figure 8a).

[0106] Additive manufacturing or each additive manufacturing step is preferably not carried out by a technique called cold spray. Cold spray projection on non-smooth surfaces or cavities is very complex and it becomes almost impossible to fill spaces or interstices. The deposition yield is extremely low (a lot of powder driven out by the air flow). On the contrary, a technique is preferred that produces molten or molten material and therefore in a liquid state, which can penetrate into spaces and interstices. The particles projected by this technique are adhered directly to the point of contact of the part or parts. With this technique, it therefore becomes possible to avoid the defects present in the classic brazing process: drips and porosities.

[0107] Additive manufacturing is preferably carried out using a technique called HVOF / HVAF (High-Velocity Oxygen Fuel / High-Velocity Air Fuel). A wire of material is then melted by combustion and the generated liquid is projected by a gas flow using a nozzle. This technique produces a flow of molten material which is projected at high speed towards the deposition surface. The powder partially melts inside the flow and then deposits on the substrate. The resulting coating has exceptional adhesion strength and very low porosity. With this technique, it is therefore possible to avoid the defects present in the classic brazing process: drips and porosities. In addition, this technique allows for very high yields and the achievement of significant deposit thicknesses, very suitable for the application.

[0108] An alternative additive manufacturing technique also suitable for this common application is wire arc (or WAAM). A wire of material is then melted by an electric arc and the generated liquid is projected by a gas flow using a nozzle. In this case, the projection speed is lower, but the molten particles that are projected also allow for even higher deposition yields and lower costs.

[0109] Other types of additive manufacturing are possible.

[0110] The rings and bars of the squirrel cage are advantageously made of the same material. This material is, for example, chosen from pure aluminum, an aluminum-based alloy, pure copper, and a copper-based alloy.

[0111] The invention is applicable to any asynchronous machine with a squirrel cage rotor.

[0112] The invention brings several advantages including:

[0113] - improve the performance of machines and engines,

[0114] - reduction of performance dispersion,

[0115] - homogeneity of rotor resistance (reduction of torque ripple, reduction of vibrations, reduction of noise, etc.),

[0116] - better electrical conductivity of the cages,

[0117] - possibility of making bars of any shape (not necessarily round or rectangular),

[0118] - better mechanical and temperature resistance of the cage and rotor,

[0119] - possible automation of the manufacturing process,

[0120] - no handling of high temperature parts unlike brazing,

[0121] - the magnetic sheets are preferably clamped between the rings without play or spacer, which ensures the mechanical support and compactness of the cage,

[0122] - etc.

Claims

CLAIMS 1. Method of manufacturing a squirrel cage rotor for an electric machine, this rotor comprising: - a squirrel cage (10) comprising two short-circuit rings (12, 13) which extend around the same axis (A) and are at a distance from each other, and conductive bars (14) which extend between the rings (12, 13) and which have longitudinal ends (14a) connected to these rings, and - magnetic sheets (16) interposed between the rings (12, 13) and stacked along their axis (A), characterized in that it comprises the production of at least part of the squirrel cage (10) by additive manufacturing by projection of molten or molten material.

2. Method according to claim 1, comprising: - a first step during which a first of the rings (12) and the conductive bars (14) are produced by additive manufacturing using a first material, these conductive bars (14) having first longitudinal ends (14a) connected to the first ring (12) and extending from this first ring (12) at the end of this first step, - a second step during which the magnetic sheets (16) are positioned on the first ring (12), second free longitudinal ends (14b) of the bars (14), located on the side opposite the first ring (12), projecting on a surface (16a) of the magnetic sheets (16), and - a third step during which the second ring (13) is produced by additive manufacturing directly on the magnetic sheets (16) using the first material, by depositing this first material on said surface (16a) to secure the second ring (13) to the second longitudinal ends (14b) of the bars (14).

3. Method according to claim 1, comprising: - a first step of assembling the magnetic sheets (16) with the conductive bars (14), and - a second step during which at least one of the rings (12, 13) is produced at least in part by additive manufacturing, or is fixed to the conductive bars (14) by additive manufacturing.

4. Method according to claim 3, in which, during the first step, the magnetic sheets (16) and the conductive bars (14) are assembled together and with a first of the rings (12) which comprises orifices (12a) for the passage of first longitudinal ends (14a) of the bars (14), these orifices (12a) each comprising a flare (12c) on the side opposite the magnetic sheets (16), the longitudinal ends (14a) of the bars (14) projecting at the bottom of these flares (12c) at the end of the first step, and in which, during the second step, the flare (12c) of each of these orifices (12a) is filled by additive manufacturing to secure the longitudinal ends (14a) of the bars (14) to the ring (12).

5. Method according to claim 3, wherein, during the first step, the magnetic sheets (16) and the conductive bars (14) are assembled together and with a preform (12') of a first of the rings (12) which comprises orifices (12a) for the passage of first longitudinal ends (14a) of the bars (14), the longitudinal ends (14a) of the bars (14) passing through orifices (12'a) of the preform (12') at the end of the first step, and wherein, during the second step, the ring (12) is completed by additive manufacturing and the longitudinal ends (14a) of the bars (14) are secured to the ring (12) by depositing a material by additive manufacturing on an annular surface (12b) of the preform (12') opposite the magnetic sheets (16).

6. Method according to claim 5, in which the longitudinal ends (14a) of the bars (14) project from said annular surface (12b) of the preform (12') at the end of the first step.

7. Method according to claim 5, in which the longitudinal ends (14a) of the bars (14) are flush with said annular surface (12b) of the preform (12') at the end of the first step.

8. Method according to claim 5, in which the longitudinal ends (14a) of the bars (14) are set back from said annular surface (12b) of the preform (12') at the end of the first step.

9. Method according to claim 3, wherein, during the first step, the magnetic sheets (16) and the conductive bars (14) are assembled together, first longitudinal ends (14a) of the bars (14) protruding from the magnetic sheets (16) and projecting on a first surface (16a) of these sheets (16) at the end of the first step, and wherein, during the second step, a first ring (12) is produced by additive manufacturing and the longitudinal ends (14a) of the bars (14) are secured to this ring (12) by depositing a material by additive manufacturing on said surface (16a) of the sheets (16).

10. Method according to one of claims 4 to 9, in which the two rings (12, 13) are produced at least in part by additive manufacturing, or are secured to the longitudinal ends (14a, 14b) of the bars (14) in the same way.

11. Method according to one of the preceding claims, in which the additive manufacturing is chosen from a technique which excludes the technique called cold spray.

12. Method according to one of the preceding claims, in which the additive manufacturing is chosen from the wire arc or WAAM technique and the HVOF or HVAF technique.

13. Method according to one of the preceding claims, in which the rings (12, 13) and the bars (14) are made of the same material.

14. Method according to the preceding claim, in which the rings (12, 13) and the bars (14) are made of a material chosen from pure aluminum, an aluminum-based alloy, pure copper, and a copper-based alloy.

15. Method according to the preceding claim, in which the magnetic sheets (16) are clamped between the rings (12, 13) without play or intermediate piece.

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