Takehiro Imura

Maximizing Air Gap and Efficiency of Magnetic Resonant Coupling for Wireless Power Transfer Using Equivalent Circuit and Neumann Formula

The progress in the field of wireless power transfer in the last few years is remarkable. With recent research, transferring power across large air gaps has been achieved. Both small and large electric equipment have been proposed, e.g., wireless power transfer for small equipment (mobile phones and laptops) and for large equipment (electric vehicles). Furthermore, replacing every cord with wireless power transfer is proposed. The coupled mode theory was proposed in 2006 and proven in 2007. Magnetic and electric resonant couplings allow power to traverse large air gaps with high efficiency. This technology is closely related to electromagnetic induction and has been applied to antennas and resonators used for filters in communication technology. We have studied these phenomena and technologies using equivalent circuits, which is a more familiar format for electrical engineers than the coupled mode theory. In this paper, we analyzed the relationship between maximum efficiency air gap using equivalent circuits and the Neumann formula and proposed equations for the conditions required to achieve maximum efficiency for a given air gap. The results of these equations match well with the results of electromagnetic field analysis and experiments.

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.

Automated Impedance Matching System for Robust Wireless Power Transfer via Magnetic Resonance Coupling

Recently, a highly efficient midrange wireless transfer technology using electromagnetic resonance coupling has been proposed and has received much attention due to its practical range and efficiency. The resonance frequency of the resonators changes as the gap between the resonators changes. However, when this technology is applied in the megahertz range, the usable frequency is bounded by the industrial, scientific, and medical (ISM) band. Therefore, to achieve maximum power transmission efficiency, the resonance frequency has to be fixed within the ISM band. In this paper, an automated impedance matching (IM) system is proposed to increase the efficiency by matching the resonance frequency of the resonator pair to that of the power source. The simulations and experiments verify that the IM circuits can change the resonance frequency to 13.56 MHz (in the ISM band) for different air gaps, improving the power transfer efficiency. Experiments also verified that automated IM can be easily achieved just by observing and minimizing the reflected wave at the transmitting side of the system.

Basic study of improving efficiency of wireless power transfer via magnetic resonance coupling based on impedance matching

Wireless power transfer is essential for the spread of Electric Vehicle(EV) usage as it provides a safe and convenient way to charge the EVs. Recently, a highly efficient mid-range wireless power transfer technology using electromagnetic resonance coupling, WiTricity, was proposed. Studies show that the resonant frequencies of the two antennas change according to the air gap in between the antennas. To achieve maximum efficiency using this system, the resonance frequencies of the antennas and the frequency of the system has to be matched. However, when this technology is applied in the MHz range (which allows small sized antennas), the usable frequency is bounded by the Industrial, Scientific, and Medical(ISM) band. Hence a method to fix the resonance frequency within the ISM band is required. In this paper, the possibility of using impedance matching (IM) networks to adjust the resonance frequency of a pair of antennas at a certain distance to 13.56MHz is studied. We studied the electrical characteristics of the antenna with equivalent circuits, electromagnetic analysis and experiments. The equivalent circuits are used as reference to calculate the parameters of the IM circuits. The simulations and experiments shows that the IM circuits can change the resonance frequency to 13.56MHz for different air gaps, thus improving the power transfer efficiency.

Study on open and short end helical antennas with capacitor in series of wireless power transfer using magnetic resonant couplings

In the technology of wireless power transfer, electromagnetic coupling using resonance can transfer across a large air gap without losing its high efficiency and power. In this technology, the role of antennas is very important. In this paper, magnetic resonant coupling using helical antennas is proposed. The helical antennas are two types- one is open end and the other is short end helical antennas and specifications of these antennas were studied which is related to the efficiency of power transfer wirelessly. The capacitor is installed to antennas in series and parallel is studied. High efficiency power transfer is possible when resonance is in series. The short end helical antennas with a capacitor in series make series resonance which makes resonance frequencies lower than open end helical antennas.

Impedance Matching and Power Division Using Impedance Inverter for Wireless Power Transfer via Magnetic Resonant Coupling

Future applications of wireless power transfer will include powering various devices in a room, charging electric vehicles in a parking area, charging moving robots, and so on. Therefore, practical wireless power transfer must be able to support complicated configurations, for example, combination of multireceiver and repeaters. Many past works have discussed methods for improving efficiency and, more recently, extended the methods to multireceiver systems. However, controllable power division among receivers is also an important feature as receivers nearer to the transmitter tend to absorb more power compared to further ones. In this paper, a new impedance matching and power division method utilizing impedance inverters only at the receiver sides is proposed. The mathematical equations in the proposed method are then generalized for arbitrary number of receivers and arbitrary number of repeaters.

Basic study on reduction of reflected power using DC/DC converters in wireless power transfer system via magnetic resonant coupling

Nowadays, wireless power transfer via magnetic resonance coupling is researched and developed actively. This method enables power transfer with high efficiency across large air gap and is robust to displacement compared to traditional method. One of the problems for realization of such system is the increase of reflected power due to load variation and displacement. Reflected power will become loss in power source and result in declining efficiency of the power transfer. Therefore, impedance matching circuit becomes crucial for realization of high efficiency wireless power systems. In this paper, a novel method for reduction of reflected power using DC/DC converter is proposed. DC/DC converter operates like variable impedance by changing duty cycle of switching devices and therefore functions as impedance matching circuit. As a basic phase, reduction of reflected power and efficiency improvement by proposed method are investigated by experiment.

Coupling Coefficients Estimation of Wireless Power Transfer System via Magnetic Resonance Coupling Using Information From Either Side of the System

Wireless power transfer via magnetic resonance coupling method has opened a new possibility to the electric vehicle system. It allows the wireless charging system of moving vehicles, using charging lanes. However, although the efficiency of power transmission is relatively high, the efficiency still depends on displacement of coils. There have been several researches on methods to maintain power transmission at the highest efficiency. However, in such systems, the information on system parameter especially coupling coefficients is needed, and in the charging lane system, such information is unlikely to be obtainable without a communication system. Therefore, it has come to attention that parameter estimation is a crucial factor to implement a charging lane system. This paper presents derivations of equations for estimating coupling coefficients in several configurations of wireless power transfer system, using information from only one side, either the transmitting side or the receiving side, of the system. The presented equations are both applicable to the case of single receiving coil and are also generalized for the case of multiple receiving coils. Each equation is verified by both simulations and experiments. An experimental system of the coupling coefficient estimation system is constructed for estimation from the receiving side using a dc/dc converter.

Study on maximum air-gap and efficiency of Magnetic Resonant Coupling for Wireless Power Transfer using Equivalent Circuit

The progress of technology in the field of Wireless Power Transfer in the last few years is remarkable. The difference between traditional technology and todays technology is that a transforming power across large air pas can be achieved through recent research. Much electric equipment is proposed- both small one and large. For example, Wireless Power Transfer for small equipment (mobile phone and laptop) and for large equipment (electric vehicle). Furthermore, replacing every cord Wireless Power Transfer is proposed. The Coupled Mode Theory was theorized in 2006 and proven in 2007. Thus, magnetic and Electric Resonant Coupling allows power to traverse large air gaps with high efficiency. This technology is closely related to Electromagnetic Induction, antenna technology and resonator for the filters of communication technology. We have studied these phenomena and technologies using Equivalent Circuits- which is a more familiar and comprehensible format for electrical engineers than the Coupled Mode Theory. In this paper, we study maximum efficiency vs. air gap by Equivalent Circuit and propose simple equations about maximum efficiency of air gaps. The results of these equations are well matched with results from Electromagnetic Field Analysis.

Study on maximize efficiency by secondary side control using DC-DC converter in wireless power transfer via magnetic resonant coupling

Wireless power transfer via magnetic couple resonance has been widely studied as an attractive research target. In order to improve the transmitting efficiency, not only increasing performance of antenna is important but also controlling secondary (receiver) side input impedance value. As a method of controlling secondary input impedance, the method of using a DC-DC converter is proposed in previous research. Mutual inductance what related to transmitting distance and load resistance value is needed to know for control the DC-DC converter. Moreover, the transmitting distance and load resistance has changed in actual WPT situation. Therefore, control method for DC-DC convertor what keep best efficiency in actual WPT situation is needed. In this paper, the novel method of controlling a DC-DC converter for maximize efficiency which used for wireless power transfer via magnetic couple resonance is proposed. A mutual inductance can be known the voltage sensor in secondary side by setting power supply voltage constant. Furthermore, efficiency could be maximized regardless of the state of load by controlling a DC-DC converter so that the secondary side voltage constant value. This principle was explained by equation. Thereby, even when transmission distance and a load resistance value change, wireless power transfer system keep best efficiency automatically without communication between primary to secondary side.