Study on the Transmission Efficiency of Wireless Charging System using Asymmetrical Flat Spiral Coil

Abstract

This paper represents the research finding of wireless electric power transmission using same or different outer diameter flat spiral coil. The coupling factor, transmission power and transmission efficiency of asymmetrical coil which has same inner diameter but different turn numbers was compared with symmetrical coil which has same inner diameter and same turn numbers. In the application of wireless electric power transmission for electric assisted bicycles, small size diameter coil (less than ø10 cm) is more suitable than large size diameter coil (larger than ø50 cm), therefore, a small size coil is used. In the results, an electric power transmission by using asymmetrical coil was found the same as that of symmetrical coil. However, the distance for power transmission by asymmetrical coil was found smaller than symmetrical coil. When a secondary battery was connected with wireless power transmission as a load, the apparent resistance of secondary battery had influence on charging rate. This resistance change was used for battery charging apparatus instead of feedback control.