# Textile-Based Coils for Inductive Wireless Power Transmission
## 3 Results  
Efficiency is the most important parameter for power transfer, as it is the ratio of output to input. The evaluation criteria are the transmission quality, as well as the maximum achievable efficiency of the test objects (Table 6). All values were measured three times and averaged. The coils are subdivided according to their manufacturing method and the coil material. The results are listed in the following tables. Table 7 contains the calculated results for the transmission quality \(Q_{k}\) and the maximum achievable efficiency \(\eta\) of the stitched coils from test series S, with and without the use of the ferrite foil. In the following, we present the results from coils, which show peculiarities for example by a very good or very bad result. The 50E15 coil without ferrite has a transmission quality of 5.15 and a max. efficiency of 0.675. Considering the results of the measurements with ferrite, a significant increase of both parameters can be observed. The transmission quality can be increased to 8.02 and the max. efficiency up to 0.780, which means an increase of 10.5%. For the 40E9 coil, we observe a 14% improvement of the max. efficiency from 0.620 to 0.760. While the highest transmission quality of the ELITEX coils with ferrite is 8.02, with a max. efficiency of 0.780 (78%). The coil 50L15 already achieves a transmission quality of 35.5 without ferrite, which \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Transmitter Coil** & **Receiver Coil** & \(Q_{k}\) & \(\eta_{max}\) \\ \hline
760308100111 & 760308100111 & 93.84 & 0.9789 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Measurement results of the copper coils. Figure 13: Picture of the test setup. even increases to 49.45 with ferrite, which corresponds to a max. efficiency of 0.960 (96%). The other two coils made out of stranded copper wires, (60L12, 40L9) also have a high transmission quality of >40 and a max. efficiency greater than 0.90 with and without ferrite. Coil 60U12 achieves a max. efficiency of 0.76 (76%) and a transmission quality of 7.35. Coil 50U15 was not possible to connect to the measurement system. Table 8 contains the results of the wound coils of ELITEX and wrapping yarn. Coil WPRE4 achieved a max. efficiency of 0.743 with a transmission quality of 6.81 without the use of the ferrite. In the measurements with ferrite, an increase in max. efficiency to 0.862 (86.2%) was achieved. Hence, this coil achieves the highest possible efficiency with a textile conductor material. This is probably due to the fact that the two inputs and outputs of the coil are connected in parallel, which halves the electrical resistance. WPRU4 achieves the second-highest results (7.01/0.75) with a fully textile structure. The coil test conditions for the measurements from test series Z (embroidered coils with Shielddex yarms) were slightly different from the others. The test frequency was reduced to 100 kHz and we used a different ferrite foil. The transmission quality was still significantly below those of the other textile coils. Nevertheless, coil Z580 with ferrite foil could achieve the best test result with a transmission quality of 2.6499 and a max. efficiency of 0.452 (Table 9). The printed coils were also tested, but it was not possible to measure inductance. The capacitive component seemed to outweigh the inductive component. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Name** & \(Q_{k}\) & \(\eta_{max}\) & \(Q_{k}\) & \(\eta_{max}\) \\ \hline \multicolumn{5}{c}{Without ferrite foil} & With ferrite foil \\ \hline
**60E12** & 3.99 & 0.564 & 5.52 & 0.693 \\
**50E15** & 5.15 & 0.675 & 8.03 & 0.778 \\
**40E9** & 4.28 & 0.620 & 7.23 & 0.760 \\ \hline
**60U12** & 3.11 & 0.514 & 7.35 & 0.760 \\
**50U15** & & No measurement possible & & \\
**40U9** & 2.93 & 0.490 & 5.41 & 0.688 \\ \hline
**60L12** & 25.02 & 0.923 & 41.44 & 0.950 \\
**50L15** & 35.5 & 0.945 & 49.45 & 0.960 \\
**40L9** & 29.7 & 0.935 & 42.15 & 0.954 \\ \hline \hline \end{tabular}
\end{table}