Basic experimental study on helical antennas of wireless power transfer for Electric Vehicles by using magnetic resonant couplings

Wireless power transfer is required for the diffusion of Electric Vehicles (EVs) because it makes possible the process of automatically charging EVs. The technology of wireless power transfer requires three main elements: large air gaps, high efficiency and a large amount of power. Though, there has been no such technology, recently, the technology of electromagnetic resonant couplings was proposed and named WiTricity. With this technology there are large air gaps, high efficiency and large amounts of power. In this paper, the feasibility of wireless power transfer for EVs by electromagnetic resonance coupling is studied. We studied small sized antennas that can be equipped on the bottom of a vehicle and we studied the electrical characteristics of the antenna with equivalent circuits, electromagnetic analysis and experimentation. The length of the air gaps between a transmitting antenna and a receiving antenna affect resonance frequencies. The resonance frequency changes from two to one depending on the length of the air gap. Until a certain distance, maximum efficiencies are not changed. Large air gaps are weak couplings. In a weak coupling at resonance, magnetic resonance couplings can transfer energy with high efficiency. The specification results at high power are proposed. In this paper, the feasibility of wireless power transfer with large air gaps and high efficiency by small sized antennas that can be equipped on the bottom of EVs is proposed.