Derivation of Coupling Factors for Different Wireless Power Transfer Systems: Inter- and Intralaboratory Comparison

In this study, the inter- and intralaboratory comparison of coupling factors is discussed for human exposure to nonuniform magnetic field from different wireless power transfer (WPT) systems. In order to derive the coupling factors, different laboratories computed the internal electric field and specific absorption rate (SAR) for different WPT configurations. The concept of coupling factors was originally introduced in the International Electrotechnical Commission (IEC) 62311 and 62233 standards, for product safety assessment. For WPT systems, extension of the coupling factors is discussed in the IEC working group. This factor enables the estimation of the internal electric field and the SAR without detailed computation using human anatomical models. The coupling factors were computed for different WPT systems, such as an electric vehicle charging at approximately 100 kHz, induction coupling systems (140 kHz band), and home appliances (6.78 MHz). Four in-house codes were used in this inter- and intralaboratory comparison study to compute the coupling factors for the systems. Similar tendencies were observed in the coupling factors obtained by the groups. The difference between the coupling factors of different research groups is 30% when considering the same exposure scenarios. The coupling factors for different WPT systems are different because of the frequency, coil size, and existence of the magnetic sheet (shield). The variability of the internal electric field for WPT systems is also investigated to set the coupling factor required for practical use.