Resonance and Efficiency in Wireless Power Transfer System

Many papers about wireless power transfer use resonant phenomena. In general if input frequency is adjusted to the resonant frequency, the amplitude of current or voltage will be maximal. Since the power of resistive load is determined by its current or voltage, the power will be maximal at the resonance. As for wireless power transfer, it is one of significant specifications to transfer power as much as possible. Then it is natural to try to maximize transmitted power with resonant phenomena. On the other hand, efficiency is another significant specification on wireless power transfer. However efficiency is defined as a ratio with two different powers, and it cannot be concluded that efficiency is always maximal when either power is maximal. Therefore efficiency has no straightforward relation with resonance in contrast with the relation between resonance and power. In this paper, we show an example which cannot realize the maximal efficiency, although resonant phenomena are caused and discuss why the situation is caused. An analytical method for wireless power transfer is clarified and how to choose an optimal frequency is scrutinized. The power of transmitting and receiving sides and efficiency are calculated by using mathematical models, e.g. a state equation, a transfer function. Key–Words: wireless power transfer, highly efficient power transmission, electromagnetic resonance