Investigating the frequency for load-independent output voltage in three-coil inductive power transfer system

Using circuit theory, this letter investigates the output voltage characteristics of three-coil inductive power transfer (IPT) system against load variations and determines the operating frequency to achieve a constant output voltage. First, a three-coil IPT system is modeled, and the analytical expression of the root-mean-square of the output voltage is derived. By substituting the coupling coefficient, the quality factor, and the resonant frequency, we propose an intuitive method of calculating the frequency for load-independent output voltage for the first time. When the coupling coefficient is relatively small, there are three frequencies that achieve load-independent output voltage. If the coupling coefficient increases and is greater than 1/root 2, only two frequencies can achieve load-independent output voltage. Experiments are conducted to confirm these conclusions. Copyright (c) 2015 John Wiley & Sons, Ltd.