Efficiency-Oriented Design of Litz Wire for Several kW Power Experimented on 20 kW Prototype

Inhalt:
Wireless Power Transfer (WPT) from a few kW to hundreds of kW is the future in order to simplify the recharging of electric vehicles (EVs). For ensuring high current ratings in the magnetic couplers used for this transfer, Litz wire conductors are used to deal with the skin effect due to the 85 kHz operating frequency imposed by SAE standards. These conductors are made by paralleling a more or less large number of strands of Litz wires. Depending on the Litz wire design, inhomogeneous current distributions between these strands can be observed. This article highlights this phenomenon by real measurements carried out on two 20 kW WPT prototypes and study the relevant parameters to propose solutions. Tests on different designs, different connectors at the ends, as well as an easy way to perform Litz wire test method are provided. Finally, the proposals developed and validated experimentally made it possible to obtain a new prototype with a 20% reduction in winding losses while dividing the necessary quantity of copper volume by 2.5.