Method for reworking defective welds of hairpin windings

Abstract

The invention relates to a method for reworking defective welds (13, 14) of a hairpin winding for a rotor or stator (1) of an electric motor, the method comprising the following steps: providing a hairpin winding applied to a winding support (2), wherein the hairpin winding has at least one conductor (3, 4) formed by a plurality of conductor segments (5), wherein the conductor segments (5) of the respective conductors (3, 4) are welded together in pairs at respective welds (13, 14) by a first welding method, wherein at least one of the welds (13, 14) is a defective weld (13), separating the defective weld (13), and connecting the conductor segments (5) to be welded, which were previously connected by the defective weld (13), by a connecting method different from the first welding method, in particular by a second welding method.

Description

Technical Field

[0001] The invention relates to a method for reworking a defective weld of a hairpin winding for a rotor or stator of an electric machine. Background Art

[0002] Electrical machines, such as motors or generators, typically use a stator with coils mounted thereon. This stator is used to drive the rotor by supplying the coils with a time-varying current, or to convert the time-varying magnetic field generated by the rotation into an induced current. For example, a cylindrical winding carrier with multiple slots extending in the axial direction can be used, through which the conductors forming the coils extend. In principle, it is possible to wind a separate winding around each coil tooth delimiting a slot. However, it may be advantageous to guide a continuous conductor in a meandering pattern through multiple slots.

[0003] In electric motors with high power density, such as those used in motor vehicles, a so-called hairpin design is now often used to create windings that meander through slots. This involves first bending a profiled wire, typically a rectangular wire, into a U-shape—a hairpin or "hairpin" shape. The legs of the hairpin are then arranged in a circular shape and inserted into the slots of the stator lamination stack. In the next step, the free ends of the respective hairpins are twisted concentrically by a defined angle relative to the stator axis or axis of rotation of the motor. All ends located on the corresponding diameter, i.e., at specific positions in the slots, are twisted alternately clockwise and counterclockwise. This is also known as "twisting." Adjacent ends are then electrically conductively connected to one another. Depending on the winding plan, interconnecting bridges are placed at the winding ends and electrically connected to the respective ends of the hairpins to interconnect the hairpins to form the overall winding. If necessary, the hairpin ends are then insulated, and the entire stator is impregnated. This technology enables high automation and a high copper filling factor (slot fill factor) in the stator slots, that is, a high ratio between copper area and slot area. For example, DE 10 2018 200 035 A1 uses this hairpin design. To achieve high production volumes, the conductive connections between the individual hairpins are typically established by welding, for example, using laser welding.

[0004] When welding hairpin windings for rotors or stators, a very large number of welds are required, typically over 200. Consequently, even a low defect rate in the welding process can result in a high scrap rate. For example, even a defect rate of approximately 0.5% will result in an average of one defective weld per stator. For example, if 200 stators are produced per day, each with 216 welds, even with a defect rate of 0.1% per welding process, this still results in a daily scrap rate of up to 432 stators that are rejected. For economic and ecological reasons, such high scrap rates should be avoided. Therefore, reworking strategies are advantageously employed, in which defective welds are reworked as much as possible to produce a fully functional stator. This also applies to the production of rotors with hairpin windings.

[0005] In the simplest case, reworking is accomplished by laser welding, which is typically also used to create the original weld. However, the problem here is that the initial, defective weld modifies the existing welding conditions for reworking in an undefined manner, since the surface conditions resulting from the defective weld are unknown. This typically results in the surface no longer being perpendicular to the incident laser beam. Furthermore, the copper typically used for such windings has a relatively low absorption rate for infrared laser light, resulting in a low energy input. Consequently, different energy inputs are required for different welding processes during reworking, which in turn can lead to defective welds or uncertain electrical conductivity of the winding. Summary of the Invention

[0006] It is therefore the object of the present invention to provide a correspondingly improved solution for reworking defective welds of hair pin windings.

[0007] According to the invention, this object is achieved by a method for reworking a defective weld of a hairpin winding of a rotor or stator of an electric machine, the method comprising the following steps:

[0008] - providing a hairpin winding applied to a winding support, wherein the hairpin winding has at least one conductor formed from a plurality of conductor segments, wherein the conductor segments of the respective conductor are welded together in pairs at respective welds by a first welding method, wherein at least one of the welds is a defective weld,

[0009] - Separate defective welds, and

[0010] The conductor sections to be welded, which were previously connected by the defective weld, are connected by a connecting method different from the first welding method, in particular by a second welding method.

[0011] According to the invention, the defective solder joint is first separated, in particular by breaking the region containing the solder joint. This allows a defined geometry of the ends of the conductor segments to be connected to be achieved, thus avoiding the problems explained at the outset regarding an undefined initial geometry for subsequent processing.

[0012] If no further measures are taken, this leads to the problem of relatively high energy input in conventional welding methods for producing hairpin windings, such as laser welding. However, separating a defective weld, for example by cutting it to length, results in a length of exposed / removed insulation that is shorter than the length during the original welding process. Therefore, repeating the welding process using the same welding method could damage the insulation of the conductor segment due to the shorter distance to the insulation, which could again result in a defective stator or rotor.

[0013] Furthermore, the reworked welded portion may have a significantly different shape than the unworked welded portion, especially a significantly different length in the axial direction of the rotor or stator, which may be disadvantageous for subsequent steps such as insulation of the welded portion.

[0014] Therefore, according to the present invention, a different connection method, in particular a second welding method, is used to reconnect the previously separated conductor segments. This can avoid the above-mentioned problems or at least reduce their effects. Suitable connection methods will be discussed later.

[0015] The conductor segments can be particularly clip-shaped, i.e., formed, for example, from two slot segments that are guided through corresponding slots of the winding support and a connecting segment that connects the slot segments. Such clip-shaped conductor segments, also known as hairpins, are common and known in hairpin windings. However, conductor segments of different shapes can also be used, such as straight conductor segments, also known as I-pins.

[0016] Weld defects can occur for various reasons. For example, shrinkage cavities or pores—that is, air inclusions—can form. These defects reduce the copper cross-section at the weld point, thereby shortening the service life of the rotor or stator due to locally higher thermal loads. Furthermore, the resulting smaller connection cross-section can lead to mechanical stresses that can cause the connection to break during operation of the rotor or stator. If too much energy is applied during welding, the insulation of the corresponding conductor segments can be damaged, which in turn reduces the service life of the rotor or stator. However, applying too little welding energy can also be problematic because it leads to inadequate welding of the conductor segments, potentially resulting in locally higher resistance and, in turn, higher thermal loads, as discussed above. Excessive thermal loads can reduce the service life of the rotor or stator. Such inadequate welds are also more likely to break during operation under mechanical stress. Another possible defect is a geometrically poorly formed weld, such as a weld bead with lateral protrusions, sharp edges, or depressions. This can cause problems during the subsequent impregnation process, for example.

[0017] Concepts for identifying corresponding defective welds are generally known and are already used to screen out defectively welded rotors or stators or to forward them to further processing. For example, optical inspections can be used, which can be performed manually and / or automatically. For example, algorithms for image recognition can be used. Additionally or alternatively, conductivity measurements can be performed to identify locally increased resistance, which can be performed using X-ray imaging methods, in particular CT methods or similar methods.

[0018] A joining method that applies additional material to the conductor segments to be welded can be used as a joining method. This ensures, in particular, that even when a defective weld is separated by shortening the conductor segment, the resulting joint has essentially the same shape and length as the remaining, unprocessed welds. Furthermore, as will be explained later, joining methods that apply additional material can be performed at lower temperatures than beam welding methods or energy beam welding methods, or require less energy to be introduced into the conductor ends of the conductor segments. Furthermore, as the amount of material applied increases, the energy input is directed further away from the insulation of the respective conductor segment. These two factors can help avoid the already discussed damage to the insulation that can occur during reprocessing, such as with laser welding.

[0019] Disadvantage in the joining methods that apply additional material is that these joining methods typically require longer process times. However, within the scope of reprocessing solutions, this is less important, so that these joining methods are still very suitable for reprocessing.

[0020] The additional material can be applied in the welding method by melting the welding rod and partially applying it to the welding point. As an alternative to the second welding method, for example, a 3D printing method can be used as a connecting method, as disclosed in the printed document DE 10 2018 200 035 A1 cited at the beginning.

[0021] The first welding method can be a beam welding method. For example, laser welding or electron beam welding can be used. In the first welding process, only the material of the conductor segments is used and melted to connect them. The beam welding method advantageously allows for short process times. This is particularly important because multiple welds must be created when manufacturing hairpin windings.

[0022] Gas metal arc welding can be used as a joining method. In addition to the heat input by the welding arc, the shielding gas used can transfer heat to the conductor segments. Therefore, it can be advantageous to prevent gas from flowing along the conductor into the insulation region of the respective conductor segment using a tool, which can, for example, be sleeve-shaped around the ends of the conductor segments to be welded.

[0023] The joining method used can be CMT welding, inert gas metal arc welding, or active gas metal arc welding. Inert gas metal arc welding and active gas metal arc welding are well-known welding processes in which the electrode is partially melted and forms part of the weld. These welding methods are also collectively referred to as MIG / MAG welding.

[0024] CMT welding is also a gas-shielded metal arc welding method. The term CMT welding comes from the English expression "cold metal transfer." This welding method is a further development of MIG / MAG welding, in which the welding wire is moved back and forth at a high frequency. This improves the detachment of droplets from the electrode and allows for a lower temperature input into the conductor section to be welded. As a result, an exposed length of, for example, 5 to 6 mm at the conductor end is sufficient for welding the conductor end without damaging the insulation. Conventional laser welding processes require an exposed length of approximately 10 mm.

[0025] Defective welds can be separated by disconnecting the area containing the defective weld from the conductor segments to be connected. This is also known as cutting to length and can be achieved, for example, by severing the conductor ends of the conductor segments. This provides a defined surface for the connection method.

[0026] The volume of additional material that can be applied using the second joining method is between 50% and 200% or between 80% and 120% of the volume of the disconnected region. By applying a material volume that is approximately equal to the volume of the disconnected region, a shape of the reworked weld can be achieved that is similar to the shape of the unworked weld, which is advantageous for subsequent processing steps, such as insulation processes.

[0027] After the conductor segments to be connected are connected, at least the welded portion of the winding can be immersed in a bath of insulating material in the axial direction of the rotor or stator. For example, this can be done in resin or powder coating. To achieve uniform insulation, it is advantageous if the conductor ends or welded portions of the conductor segments extend approximately the same distance beyond the winding support. This can be achieved by applying additional material approximately equal in volume to the disconnected area.

[0028] Providing a hairpin winding applied to a winding support may comprise the following steps:

[0029] - providing a winding support and a plurality of clip-shaped conductor segments,

[0030] - axially moving the conductor segments into the winding support so that the two slot sections of each conductor segment extend into corresponding slots of the winding support,

[0031] - bending the free ends of the slot segments in the corresponding direction along the circumference of the winding support,

[0032] The respective free end is connected to the free end of the respective further conductor section by a first welding method to form at least one conductor, wherein at least one of the welds is a defective weld.

[0033] In other words, known steps for producing a hair pin winding for a rotor or stator can also be part of the method according to the invention, so that the method according to the invention can include defective production and subsequent reworking of a hair pin winding. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Further advantages and details of the present invention can be derived from the following examples and the accompanying drawings, which schematically illustrate:

[0035] Figure 1 A detailed cross-sectional view showing a hairpin winding for a rotor or stator of an electric machine;

[0036] Figure 2 The states of a weld to be reworked are shown in different steps of an exemplary embodiment of the method according to the invention. DETAILED DESCRIPTION

[0037] Figure 1A detailed view of a stator 1 of an electric machine is shown. The stator 1 comprises a winding support 2 with a plurality of slots 15, which carries a plurality of continuous conductors 3, 4 of a hairpin winding. For example, the conductors 3, 4 can form windings for different phases or parallel windings of one phase. For reasons of clarity, Figure 1 Only two such conductors 3 , 4 are shown in FIG, wherein in a real electric machine one or more conductors 3 , 4 are used for typically three phases.

[0038] The winding formed by the conductors 3, 4 is produced by a method in which a winding support 2 and a plurality of clip-shaped conductor segments 5 are provided, after which the conductor segments are arranged in a basket shape and axially inserted into the winding support 2 so that the two slot segments 6, 7 of each conductor segment 5 extend in corresponding slots 15 of the winding support, wherein the slot segments 6, 7 are connected by a connecting segment 8. The free ends 9 to 12 of the slot segments 6, 7 or of the conductor segments 5 are Figure 1 The free ends 10, 11 and 9, 12 are then bent in the manner shown in FIG. 1 so that each two free ends 10, 11 or 9, 12 are directly adjacent to each other. This is also referred to as a twist. The free ends 10, 11 and 9, 12 are then welded together using a first welding method, such as laser welding or another beam welding method, to form the corresponding welds 13, 14.

[0039] In this procedure, a plurality of welds 13 , 14 , for example more than 200 welds, are produced between different conductor segments 5 , so that even with a relatively low defect rate of the welding process, for example a defect rate of 0.1%, a relatively high proportion of defective stators is produced, in which at least one of the welds 13 , 14 is defective.

[0040] Defective welds 13, 14 can be identified, for example, by manual or automatic inspection. In the following, it is assumed that the weld 13 is defective. For the weld 13, a method for reworking the weld 13 is then performed, which method will be referred to below. Figure 2 For a more detailed explanation.

[0041] In this case, in step S1, a hairpin winding is first provided as described above and applied to a winding support 2. After the defective weld 13 is detected, in step S2, the region 15 including the defective weld 13 is severed from the conductor segments 5 to be connected or from the free ends 10, 11, for example, by a cutting process.

[0042] from Figure 2As can be seen in the figure, the exposed length 16 of the conductor end is thereby reduced to a shorter exposed length 17. If the beam welding method is now used again to reconnect the conductor sections 5 or free ends 10, 11, this may, on the one hand, result in a high heat input in the region of the insulation, which may damage this region. On the other hand, the weld 13 after such reworking will be much shorter than the unreworked weld 14, which is disadvantageous, for example, if welds 13, 14 are immersed in an insulating bath after connection, because in this case the reworked weld 13 may not be reliably insulated.

[0043] Therefore, in step S3, the conductor segments 5 or their free ends 10, 11 are reconnected using a connection method different from the first welding method, that is, in particular, not using a beam welding method. To avoid the aforementioned disadvantages, a connection method is used in which additional material 18 is applied to the conductor segments 5 or their free ends 10, 11 to be welded. Inert gas metal arc welding is used as the connection method, in which the welding rod is partially melted and forms part of the weld. CMT welding can be particularly advantageous because the heat input in this method is particularly low, and therefore insulation damage to the conductor segments 5 can be particularly reliably avoided.

[0044] Preferably, the volume of the additional material 18 applied during the joining process is substantially equivalent to the volume of the region 15 to be severed, so that Figure 2 Comparing the state of step S1 with the state after step S3 shows that the overall shape of the weld 13 is at least similar to that before the reprocessing. In particular, the exposed length 16 or the length of the weld 13 extending beyond the winding support 2 can be substantially equivalent to the corresponding length of other welds 14 that have not been reprocessed.

[0045] After all defective welds 13 have been reworked, interconnection bridges can be placed and / or the welds 13 , 14 can be insulated, for example by immersing them in an insulating bath in the axial direction of the rotor or stator, to complete the manufacture of the rotor or stator.

Claims

1. A method for reworking a defective weld (13) of a hairpin winding for a rotor or stator (1) of an electric machine, the method comprising the following steps: - providing a hairpin winding applied to a winding support (2), wherein the hairpin winding has at least one conductor (3, 4) formed from a plurality of conductor segments (5), wherein the conductor segments (5) of the respective conductors (3, 4) are welded together in pairs at respective welds (13, 14) by a first welding method, wherein at least one of the welds (13, 14) is a defective weld (13), - causing the defective weld (13) to be separated, and - connecting the conductor sections (5) to be connected, which were previously connected by the defective weld (13), by a connecting method different from the first welding method.

2. The method according to claim 1, characterized in that The connection method is a second welding method.

3. The method according to claim 1, characterized in that A connecting method in which an additional material (18) is applied to the conductor sections (5) to be connected is used as the connecting method, and / or the first welding method is a beam welding method.

4. The method according to claim 3, characterized in that Shielded gas metal arc welding is employed as the joining method.

5. The method according to claim 4, characterized in that CMT welding, inert gas shielded metal arc welding, or active gas shielded metal arc welding is used as the connection method.

6. The method according to any one of claims 1 to 5, characterized in that The defective weld (13) is separated by disconnecting the region (15) including the defective weld (13) from the conductor section (5) to be connected.

7. The method according to any one of claims 3 to 5, characterized in that The defective weld (13) is separated by disconnecting a region (15) including the defective weld (13) from the conductor section (5) to be connected, the volume of the additional material (18) applied by the connecting method being between 50% and 200% of the volume of the disconnected region (15).

8. The method according to claim 7, characterized in that The volume of the additional material (18) applied by the joining method is between 80% and 120% of the volume of the disconnected region (15).

9. The method according to any one of claims 1 to 5, characterized in that After the conductor segments (5) to be connected are connected, at least the welding parts (13, 14) of the winding are immersed in an insulating agent bath in the axial direction of the rotor or stator (1).

10. The method according to any one of claims 1 to 5, wherein Providing a hairpin winding applied to a winding support (2) comprises the following steps: - providing a winding support (2) and a plurality of clip-shaped conductor segments (5), - axially moving the conductor segments (5) into the winding support (2) so that the two slot segments (6, 7) of each conductor segment (5) extend in corresponding slots (15) of the winding support (2), - bending the free ends (9-12) of the slot sections (6, 7) in corresponding directions along the circumference of the winding support (2), - connecting each free end (9-12) to the free end (9-12) of a corresponding further conductor segment (5) by a first welding method to form at least one conductor (3, 4), wherein at least one of the welds (13, 14) is a defective weld (13).

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