# Textile-Based Coils for Inductive Wireless Power Transmission
## 4 Discussion  
This paper deals with the development of textile induction coils for contactless energy transfer. For this purpose, three different methods for the possible fabrication of textile coils were investigated. Five varying coil materials were used for this project. The result of the manufacturing process resulted in total of 26 different coils, which differ in their shape and size, number of turns, and starting material. The different coils are critically examined on the base of their transmission properties. After an initial examination of all the results, it is already clear that the 60L12, 50L15, and 40L9 coils deliver significantly higher results. This is mainly due to the fact that the coils 60L12, 50L15, and 40L9 consist of stranded copper wire. These coils deliver the best transmission characteristics, even without ferrite. Coil 50L15 achieves the best test results of the entire test (\(\eta_{max}=0.960\)). It is clear that textile conductors cannot compete with these electrical properties. Nevertheless, presentable results could be obtained even with the coils made entirely of textile fiber material. The wound WPRE4 coil from test series W achieved the best result of all textile coils with a maximum efficiency of 0.862 (86.2%). This unexpectedly positive result can be attributed to the high number of turns and the small distance between the turns. However, this result can also be significantly improved by using a ferromagnetic material. The TWP coils became slightly larger compared to the reference designs of the Qi coils This does not mean that the TWP coils should be rated worse than smaller sized coils. Conversely, the size of the transmitting coil should be adjusted to achieve the best possible coupling between the two components. A closer look at the individual results reveals that, among other things, there are differences between the various geometries. It turns out that the coils with a high number of turns and very small distance between the turns show the highest test results for the max. efficiency within their category (see 50E15, 50L15, WPRE4, WPRU4). The results of coils made out of wrapping yarn bad regarding \(\eta_{max}\) and \(Q_{k}\). Because of its much lower resistance, the fine copper wires around the yarn should result in better transmitting properties than a coated yarn. The wrapped yarns seem to occur eddy current which might negatively influence the magnetic field. Consequently, wrapped yarns are not the best solution for a final application. Furthermore, the contacting of the material is difficult. The printed coils (test series D) did not provide values for inductances. Rather, capacitances were measured instead of inductances. This could be due to the fact that the inductance of the coils is very low and the capacitive component outweighs the inductive component. For this reason, no results are given for the printed coils. Under these conditions, a suitability of the printed coils cannot be verified. Therefore, we conclude printed coils we produced are not suitable for wireless energy transmission. The printed coils from test series D, as well as the embroidered coils, proved poor or non-measurable transmission properties. An initial determination of the coil resistance already indicated a considerable low energy transmission. Another problem in the processing of Shieldex yarns is the absence of insulation. Since the windings are close together, it is indispensable to prevent short circuits. For this reason, we do not recommend coils with Shieldex yarns. However, it would be possible to improve the suitability for wireless transmission by increasing the degree of metallization as well as additional insulation. Due to the relatively high resistance, the yarn length should be reduced.