Technology for the Production of Stator and Rotor Packets for Electric Motors by Adhesive Bonding Holger Thiede, Andreas Winkel, Sascha Mechtold, Stefan Böhm Institute of Cutting and Joining Manufacturing Processes (tff), University of Kassel Kurt-Wolters-Straße 3, 34125 Kassel, Germany, [email protected] Introduction The battery is not only of vital importance for the efficiency of future electric vehicles, but also the electric motor. Essential components of an electric motor are the stator and the rotor, which are composed of stacked magnetic steel sheets and fixed to so-called packets. In this paper a new adhesive-based production technology is presented which enables a greater degree of freedom when designing the geometry of the plates. Simultaneously, the efficiency of electric motors is optimized. Moreover, a more efficient manufacturing process lowers the manufacturing costs. This innovative approach involves the integration of the gluing process in the cutting process, so that full surface bonding is enabled in the punching cycle. Especially the short time available for curing the precoated adhesive is taken into account. Therefore, suitable adhesive coatings are developed and tested. In this paper, first results of the investigations are presented. State of the Art In literature, numerous methods are known for the manufacture of stator and rotor packets for electric motors. The processing step which involves joining singular lamellae together to form a packet of metal sheets is generally referred to as packetizing; the according methods are called packetizing methods. The aim of the packetizing is to support the ensuing assembly processes. Depending on the area of application, quantity, or requirements regarding the efficiency, various packetizing methods are employed. At present, interlocking [1], the baked paint system method [2], the Glulock method [3, 4] and packetizing by welding [2] have the highest significance in industry. Conventionally, the separation of the sheets is carried out in progressive cutting tools, in which the geometry of the metal sheet lamellae is cut in a series of steps. In order to save material, the rotor and stator are punched out of the same band of magnetic steel sheets; the rotor and stator are simply pressed out in different places. During interlocking, so-called packetizing studs are embossed in the material by means of additional punching sequences. They are pressed into one another so that a mechanical grouting of the metal sheet lamellae is achieved [1]. Disadvantages include, in particular, eddy current losses, which occur due to damage to the insulating layer located on the metal sheet and as a result of electric contact of the metal sheet lamellae in the vicinity of the packetizing studs. The positions of the packetizing studs has to be specifically selected to ensure that the lowest possible losses arise. The positioning also restricts the constructive freedom of design. Furthermore, the joints by interlocking are characterized by a low strength, which in turn leads to high scraps. In addition, high investments are necessary for the required tools. In the baked paint system method, the metal sheet is entirely coated with an adhesive that functions as an insulator, but also bonds the metal sheet lamellae together. In contrast to interlocking, no mechanical studs are embossed into the material during the baked paint system method. Instead, the metal sheet lamellae are packetized without a joint after separation, and positioned in a clamping device. In a subsequent processing step, the adhesive is hardened in an oven at temperatures ranging from about 150 to 200 °C for 30 to 240 minutes [2, 5]. The benefits of the baked paint system method are a high strength and temperature stability of the joint. Additionally, this process makes it possible to obtain a high degree of freedom regarding construction, as well as good insulation of the singular lamellae against one another and thus, good electric properties. The high costs and expenditure of energy that arise when hardening the adhesive in the oven are a definite disadvantage. Consequently, the utilization of this method is economically only useful for small series. An adhesive bonding of metal sheet lamellae is also produced when using the Glulock method. A low-viscosity adhesive is selectively applied onto the metal sheet; either before it is or whilst it is in the punching tool. Chemically cross-linking adhesives with a short hardening time, i.e. cyanoacrylate, are employed and require constant and clean production and environmental conditions to guarantee a safe process. The complex dosing units are also disadvantageous because they demand high precision concerning the location and amount of the dose. Moreover, this method only enables selective adhesion, not complete adhesion. An additional varnish is required to ensure electric insulation. Here, compatibility of the varnish with the employed adhesives is only partially on hand. The strength and temperature stability are limited. The speed of the method and the simultaneously low expenditure of the packetizing tool are advantageous [3, 4]. During packetizing by welding, the punched metal sheet lamellae are positioned in a unit, and connected on the outer radius using several welding seams that are orthogo- nal to the metal sheet lamellae level. Via these welded seams, an electric contact is created between the metal sheet lamellae. Moreover, the insulating layer between the singular lamellae is damaged, leading to an increased amount of eddy current losses. The advantages include a high strength and high temperature stability in regards of the joint. [2] 1465 was carried out. The tensile shear samples are produced using a hot press. The pressing time equaled < 0.5 s. Moreover, the reaction of the adhesives was characterized using Differential Scanning Calorimetry (DSC). In particular, isothermic DSC measurements are completed. By means of the assessment of the time span till the end of the reaction, a measure for the reaction speed is generated. Solution Statement Results and Discussion In order to eradicate the disadvantages of thus far known packetizing methods, so-called thermic packetizing by gluing of magnetic metal sheets is being examined at the tff. As is the case for interlocking, the metal sheet lamellae should be combined together in the punching step. However, the joining process is not accomplished via a mechanical joint, but by means of adhesive bonding. The adhesive bond is characterized by the fact that it covers the complete area of the material and an initial strength can be achieved during punching. An adhesive is applied onto the magnetic metal sheet before punching and represents a tack-free surface before the material is inserted into the punching tool. The adhesive is applied to the underside of the material, in order to enable contact with the source of heat on the upper side. The metal band is punched throughout a series of steps in a modified packetizing tool using a classic method. In doing so, the coated adhesive is activated and a joint is generated. Essential adhesive technical challenges which result from observations of the new technology include the extremely short time (<1 s). Simultaneously, the requirements for the temperature stability are just as high, because temperatures higher than 180 °C can occur both in the rotor and stator when utilizing these components in electric motors. In addition, the provision of an adhesive as a dry, tack-free and punchable coating represents a special challenge. Adhesives The identification of an adhesive suitable for this technology is of decisive significance. The adhesive is either commercially available, or is a commercial adhesive that is modified. Besides the classic magnetic metal sheet varnishes, which are employed for reference purposes, epoxy dispersions are employed which are made into adhesive coatings; they fulfil the requirements by means of the addition of different hardeners and catalysts. The adhesives are then converted into adhesive varnishes, which are applied in thin coats in order to create sufficiently thin layers. Experimental The adhesives are characterized according to their achievable temperature and ageing resistances in correlation with the hardening time. The results are then compared to existing packetizing methods. In order to determine the strengths, a tensile shear experiment according to DIN EN Numerous examinations were carried out which aimed to obtain a suitable formulation. Here, we chose to leave out an illustration of the previous investigations. Instead, only the best result is listed, and compared to a classic magnetic metal sheet. The goal of the isothermic DSC examinations is, above all, to gain an impression of the possibilities concerning the reaction rate of the adhesive. dispersion with accelerator dispersion without accelerator baked coating Figure 1. Isothermic DSC measurements at 200 °C Figure 1 depicts the result of the isothermic DSC measurements for a baked coating and a newly formulated adhesive system (dispersions). One dispersion is formulated with an accelerator and one without an accelerator. While the reaction of the baked coating completes after approximately 7.5 minutes, the newly formulated system achieved the hardening maximum after only 1.5 minutes (with accelerator) respectively 4.0 minutes (without accelerator). When regarding the fact that heat for hardening is not only available during punching when using packetizing by gluing, but also when the metal stamp consisting of several hundred sheets heats with every punching action, the time for hardening should be sufficient for a temperature and ageing resistant adhesive bond. In order to examine this, tensile shear examinations in accordance with DIN EN 1465 were carried out. The sample sheets were coated with both adhesives. Coating thicknesses of 3 to 6 µm were used. The samples were then bonded in a hot press for 0.5 s using a heating plate tempered to 200 °C. Testing was performed at room tempera- ture, 150 °c and 190 °c, as well as after completing the ageing test VDA 621.415. The results of the examinations are shown in Figure 2. References 1. 2. 3. 4. Figure 2. Results of tensile shear tests In Figure 2, the results for the baked coating are shown in red. Here, it is already evident that only very low strengths of 0.5 MPa are achieved when testing at room temperature. After testing at higher temperatures and after ageing, strength is no longer discernible. This result was to be expected, for the DSC tests already proved that a significantly longer hardening time period is necessary to achieve complete cross-linking. The newly formulated adhesive coating displays strengths between 2 and 8 MPa at room temperature. At 150 °C, the material reaches a strength of 1 MPa, and, at 190 °C, the strength equals approximately 0.2 MPa. After ageing, strengths of up to 2 MPa (testing at room temperature) or 0.7 MPa (testing at 150 °C) can be achieved. The enhanced cross-linking of the adhesive identified in the DSC tests is confirmed by the achieved strengths. Conclusion A new technology for packetizing magnetic metal sheets was introduced, which requires new adhesive coatings which cure in extremely short amounts of time. A possible adhesive formulation was introduced with which the requirements could be fulfilled. However, further examinations are necessary, especially those which concentrate on enhancing hardening, examining precoating, and characterizing the storage suitability for the formulated adhesives. Acknowledgements The studies presented here are part of the joint project „ProStaR- Produktionstechnologie für die serienflexible Herstellung von Stator- und Rotorpaketen von EAntrieben“. The project is funded by the “Bundesministerium für Bildung und Forschung” (BMBF) in Germany in the context of „Forschung für die Produktion von morgen“, and is supervised by the “Projektträger Karlruhe” (PTKA). 5. 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