Performance comparison of two-, three- and four-coil e-textile wireless power transfer systems

Magnetically coupled resonant wireless power transfer (MR-WPT) can efficiently power e-textile systems. Different coil configurations (2, 3 and 4-coil structures) were investigated using planar textile coils of size 14.5 x 15 cm fabricated on cotton substrates using an embroidery process whereby silk-coated copper Litz wire forms the conductive track. For each configuration, the efficiency of the MR-WPT has been explored at different separation distances, misalignments and when subject to bending. The results show that the 2-coil structure was the most efficient at short distances (below 10 cm). However, the 4-coil system has the best performance at distances over 11 cm and when subject to axial misalignment, variations in coil orientation and when the coils are deformed by bending. The planar 4-coil configuration is therefore recommended for use in most practical applications. The coils were wash tested, and whilst they continue to function up to 15 wash cycles, performance does deteriorate due to a change in the coil properties. The coils can withstand 1000 bending cycles but there is a drop in performance of 43% in power transfer efficiency due to increasing coil resistance. Performance over longer separation distances is limited due to the properties of textile coils.