Effects of intermediate coil on power transfer capability and efficiency in three-coil wireless power transfer system

Intermediate coils are used in wireless power transfer to improve system performance, such as power transfer capability and efficiency. However, they can have adverse effects if used improperly. This paper focuses on the coil coupling stage of three-coil series-compensated wireless power transfer systems and analyses the effects of the intermediate coil under different conditions. When the coupling between the transmitter and the receiver is high, the intermediate coil usually brings negative effects; only when the coupling is weak can it possibly improve power transfer capability or efficiency. Then, the criteria in deciding the necessity of adopting an intermediate coil and the conditions to be met to improve output power and efficiency are proposed. Simulation is carried out for verification. Experiments are done on an EV-based wireless power transfer prototype. When the coupling coefficient is as low as 0.05 between the transmitter and receiver, the coil coupling efficiency is improved from 84.7% to 91.0% with the help of a carefully-designed intermediate coil.