Wireless power transfer is expected in the use of an electric vehicle and a chip card. However, it requires a high efficiency and takes a long distance. In this paper, we propose the use of a magnetoplated wire (MPW), which is a copper wire (COW) whose circumference is plated with a magnetic thin film, to improve transmission efficiency. The MPW can reduce resistances due to the proximity effect comparison with the COW. The inner diameter of COW and MPW coils is d i = 37 mm and their number of turns is n = 10. As a result, the resistances of the COW and MPW at the frequency f = 12 MHz are 6.8  and 4.1 , respectively, which show a reduction of 40%. The quality factors of the COW and MPW at the frequency f = 12 MHz are 83 and 138, respectively, which show an increase of 66%. The efficiencies of the COW and MPW at a transmission distance of 10 mm are 69.8% and 77.7%, respectively, which show an increase of 7.9%.Index Terms-wireless power transfer, magnetic resonant coupling, efficiency, magnetoplated wire, litz wire, quality factor.

Wireless power transfer is expected to be applied to portable devices and electric automobiles in the future. To achieve this, it is necessary to improve the transmission efficiency of the coils used in wireless power transfer, that is, to improve the quality factor and coupling coefficient of the coils. To improve the quality factor of the coils, the authors propose the use of a litz wire with a magnetoplated wire (MPW), which is a copper wire plated with a thin iron film. The MPW increases the quality factor of the coils by reducing the AC resistance owing to the proximity effect. In this study, the effect of the number of strands of a litz wire on the quality factor of the coils and efficiency characteristics is considered. Moreover, the quality factor of the coils and efficiency characteristics using three types of coil-a solid copper wire (COW), a litz wire with a copper wire (LCW), and a litz wire with an MPW (LMW)-are considered. At the transmission frequency f = 13.56 MHz, it is experimentally demonstrated that many strands, whose number becomes the highest in terms of the quality factor of the coils, exist. The transmission efficiencies of the COW, LCW and LMW coils at an output power of 5 W and a transmission distance of 9 mm are 89%, 84%, and 91%, respectively, and the efficiency of the LMW coil is the highest. Also, in this case, the temperature increases of the COW, LCW, and LMW coils are 12 • , 16 • , and 10 • , respectively, and the LMW coil reduces the temperature increase.

Wireless power transfer is expected in the use of an electric vehicle. It requires a high efficiency and takes a long distance. In this paper, we propose the use of a litz magnetoplated wire (LMW), which is a copper wire (COW) whose circumference is plated with a magnetic thin film, to improve transmission efficiency. The LMW can reduce resistances due to the proximity effect comparison with the COW and Litz wire (LCW) using COW. The inner diameter of coils is d i = 36 mm and their number of turns is n = 8. As a result, the resistances of the COW, LCW and LMW at the frequency f = 6.78 MHz are 1.36 ���1.91 � and 0.77 �, respectively, which show a reduction of resistance of LMW. The efficiencies of the COW, LCW and LMW at a transmission distance of 18 mm are 85.2%, 79.8% and 89.9%, respectively, which show an increase of efficiency of LMW. The temperature rise values of COW, LCW and LMW at received power 5 W are 14 deg., 17 deg.�and 10 deg.�respectively, which show a decrease of temperature rise of LMW.