Process for annealing of helical wound cores used for automotive alternator applications

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred exemplary embodiments/best mode illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and such alterations and further modifications in the illustrated embodiments and such further applications of the principles of the invention as illustrated as would normally occur to one skilled in the art to which the invention relates are included.

It is known that both elastic stress and plastic deformation both increase core loss and adversely affect other magnetic properties of electrical steels. The effect can be very significant, especially in the case of plastic deformation.

The prior art manufacturing process by stamping and then bending the lamination strip to form a helical wound alternator core produces significant plastic deformation and strain in the back-iron 12A illustrated in Figures 2 and 3 by pressure applied at both the stamped edges 12E and/or 12F and possibly also to the teeth

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12B by pressure at teeth edge 12G (first prior art bending method). Also the previously described bending methods introduce both elastic and plastic stress from compression of the core. The effects of stamping and bending in causing plastic deformation at stamped edges 12E, 12F and 12G can be clearly shown by a crosssection prior art photo-micrograph.

Figure 6 shows an illustration of a typical stamped stator tooth 12B extending from a back-iron 12A. If this tooth 12B is cross-sectioned through line AA using standard metallographic techniques, it is possible to make microhardness measurements in progressive steps from the stamped (or possibly compressed) edge 12G of the tooth into the middle of the tooth. Figure 6 is thus a schematic of a typical stamped stator tooth illustrating the location of cross-section AA for Figure?.

Figure 7 (the photo-micrograph) shows the cross-section through the tooth 12B at a magnification of 50 x. For those skilled in the art, Figure 7 clearly shows the rolled edge 50 (resulting from the punch entry), the shear section 51 from the punch, the tensile break section 52, and the small burr 53 at the bottom as the punch exits the material. Figure 7 also shows lines of black dots 54, each of which represents a micro-hardness reading shown in Fig. 8. Thus Figure 7 is a crosssection photomicrograph at 50 x magnification, of a typical stamped tooth 12B with the stamped edge 12G on the left. The black dots 54 (or squares) are the locations for each micro-hardness reading.

The table of Fig. '8 shows the results for one line of micro-hardness readings. The data shows that, at a distance of 0.0015” from the stamped edge 12G, the hardness reading is 232 HV (Vickers Hardness scale), and this reduces to 202 HV at 0.0030” from the stamped edge, and continues to decrease until a distance of 0.025” from the stamped edge is reached, where the hardness is 98 HV. The clear

CA 02811540 2013-04-02 conclusion is that stamping has caused deformation adjacent to the stamped edge

12G and has resulted in an increased in hardness (and deformation) into the body of the tooth 12B. It is also clear that the same phenomenon occurs at each stamped edge 12E and 12F ofthe back-iron 12A.

In contrast to normal, non-helically wound motor cores, the width WT of the stator teeth (Fig. 2) and width WB of the back-iron 12A are relatively small for a helical wound alternator core. As a result, the ratio of plastically deformed steel compared to the total volume of steel is very high in a prior art helical wound alternator core. It is also known that flux is concentrated at the edges and surfaces of electrical cores (called the “skin effect), and that the depth of the skin effect reduces as frequency is increased. This means that any plastic deformation on the edges and surface, especially in a helical wound core where the ratio of plastically deformed steel compared to the total volume of steel is very high, will result in a significant degradation of electrical properties for the steel.

To solve the problem previously known in the prior art described above, according to a preferred exemplary embodiment a properly controlled annealing is provided to the lamination core having a helical lamination strip 18 after welding, as shown in Fig. 9, to relieve the stress and strain in the electrical steel material of the back-iron 19 and the projecting teeth 20 subjected to elastic strain and recrystallization at areas of plastic deformation caused by bending forces applied to the strip during helical winding. Indentations 21 are provided with a respective weld 22 before the annealing.

As shown in Fig. 10, instead of T-shaped teeth, the teeth may be straight as shown at 60 and have a slight concavity at the stamped edge facing the rotor.

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Thus an important feature of this preferred exemplary embodiment is the application of controlled annealing to helical wound alternator cores. The result is a significant improvement in electrical properties of the steel, including core loss, which results in an increase in current output and increase in efficiency. This may be shown by the following plots of core loss vs. induction (Fig. 11) and induced field (B) vs. applied field (H) (Fig. 12) for both a core made using a regular prior art production process and a core that has been annealed under controlled atmosphere and temperature conditions according to a preferred exemplary embodiment. The plot of core loss vs. induction (Fig. 11) shows reduced losses for the annealed core at the same flux level (Induction). The plot of induced field (B) vs. applied field (H) (Fig. 12) shows that the annealed core carries a much higher flux or induced field (B) for a fixed level of applied field. The direct result or interpretation is that, for a fixed applied voltage in a stator or core winding the annealed core will provide a higher excitation current and consequently a higher efficiency. Thus Figure 11 shows the comparison of Core loss vs. Induction for helical wound alternator cores using a regular or normal (prior art) production method and cores that have been annealed according to a preferred exemplary embodiment.

Figure 12 shows the comparison or induced field (B) vs. applied field (H) for helical wound alternator cores using the regular or normal (prior art) production method and cores that have been annealed. Thus the situation and results are different for welded stacks of helically wound cores using regular or normal (prior art) production method and cores that have been annealed.

In prior art manufacturing of loose laminations, welding of a stack is performed after annealing. Welding is not performed before annealing. If welding of a stack is performed before annealing the core loss is usually worse. If the weld is

CA 02811540 2013-04-02 made after annealing, the weld acts as a partial short circuit, but the resistance is high because of the fine-grained microstructure of the weld. If the weld is made before annealing, the grain size microstructure of the weld increases (as a result of annealing) and the resistance decreases, resulting in a greater short circuit and higher electrical losses. Note that this applies for the prior art manufacturing method of welded stacks of loose laminations.

It is believed no one has used a process for annealing of welded helically wound cores in the manner described herein.

Based on the above observations, one skilled in the art would previously have expected that the performance of a helically wound core that has been annealed after welding would be worse than normal production prior art cores (without annealing) in view of the short circuit effect described above. Surprisingly, however, it has been discovered this is not the case, as shown in Figures 11 and 12. Degradation of performance has been measured after annealing of older style prior art alternators using thick commercial quality steel which has high core losses. This fits the “short circuit model” but does not explain the improved performance using thinner fully processed electrical steels. One further anomaly is that, based on the “short circuit model”, a helically wound core that is formed, annealed and then welded and coined, should give excellent results. In fact, the performance of cores produced using this sequence is not much better than cores made with the normal production method and without annealing.

It has been discovered that the improved performance of annealed, welded, helically wound cores (as opposed to the decreased performance of annealed, welded progressively stamped cores formed of a plurality of separate stacked laminations) is that the positive effects of the removal of plastic stress and strain

CA 02811540 2013-04-02 from the body or back-iron of the core far exceed the negative effects of the reduced resistance or short circuit effects of the welds. The positive effect of removal of plastic stress and strain .from the stamped edges as a result of annealing is the same in both cases.

The conditions for annealing of helically wound alternator cores are carefully defined and are similar to conditions for annealing of stacked, separate loose stator and rotor laminations and interlocked stacks, all using progressively stamped separate and stacked laminations.

The method steps of the preferred exemplary embodiment are shown in Fig.

13. In step 100 a lamination strip having a thickness in a range from 0.35 mm to 1.00 mm, and preferably having a thickness of approximately 0.05 mm, and having a back-iron and projecting teeth for an alternator core to be helically wound is created by stamping. Thereafter in step 200, the stamped strip is helically wound by bending to form a helically wound alternator core. In step 300 the core is welded at peripherally spaced locations as in the prior art and also the space between adjacent teeth is coined. Then in step 400, the helically wound core is annealed in a neutral or decarburizing atmosphere at temperatures above 750°F. More particularly, the basic conditions for the annealing of the helically wound alternator cores include:

(a) A neutral or decarburizing atmosphere, which is preferably based on nitrogen, hydrogen/nitrogen combinations or atmospheres generated by controlled combustion of natural gas, propane or other similar hydrocarbon fuels; and (b) Temperatures above 750^eF (minimum temperature for stress relief in steel) and preferably in the range of 1300°F to 1600°F to allow for both stress relief and recrystallization to occur.

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Although preferred exemplary embodiments are shown and described in detail in the drawings and in the preceding specification, they should be viewed as purely exemplary and not as limiting the invention. It is noted that only preferred exemplary embodiments are shown and described, and all variations and modifications that presently or in the future lie within the protective scope of the invention should be protected.