Flexibility of Contactless Power Transfer using Magnetic Resonance Coupling to Air Gap and Misalignment for EV

Nowadays, the establishment of the technologies of contactless power transfer (CPT) for electric vehicles (EVs) are needed. The system can provide people with a convenient charging system which can powers their EVs wirelessly and automatically when they park. Traditional CPT solutions have the problem of distance between transmitting antennas and receiving antennas (air gap), as well as from the misalignment of the antennas. Those systems cannot power at a great distance and the efficiency is very low. In this paper, CPT by magnetic resonance couplings and small antennas are proposed. These techniques more easily span air gaps and are more forgiving to antennas misalignments than traditional solutions. The Efficiencies are still high when the air gaps are longer than the diameter of the antennas. As well, the efficiencies are over 90% when the antennas are half a diameter out of alignment. For example, when the air gap and the displacement is 150mm, the efficiency is about 96%. The magnetic resonant couplings can power wirelessly even if there is a weak connection. Thus, a large air gaps with high efficiency are possible. The relation between two resonant frequencies and the coupling coefficient are described using the method of equivalent circuits. As well, the relation between resonant frequencies and the efficiencies, especially the moment of decreasing, are described.


Introduction
Nowadays, EVs are being widely researched and developed.Due to the high power density enhanced in rechargeable batteries, a wide use of Electric vehicles (EVs) could soon become available.But, this energy is not enough for the EVs to be driven for a week without being charged.
The EVs need to be charged once a day.EV batteries are commonly recharged by connecting them to power sources.However, in this way, a code is needed and it needs to be plugged in.In that way, recharging is not convenient.A novel solution that does not require the user to connect EVS24 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 1 World Electric Vehicle Journal Vol. 3

Experimental setup
In this paper, parameters and designs of suitable antennas for EVs are proposed.The entire system of contactless power supply is shown in Fig. 1.The system consists of the contactless power supply of a high frequency power source, a transmitting antenna, a receiving antenna, a rectifying circuit and electric power storage (batteries or electric double layer capacitors [EDLCs]).In this paper, antennas will be the main focus.In this paper, r = 150mm and p = 5mm.
The experimental set up is shown in Fig. 3 where the vector network analyzer (VNA) is used.The VNA put out power from port 1 to the transmitting antenna through a coaxial cable and swept the frequencies.The power was transferred wirelessly to the receiving antenna and put in port 2. For the electromagnetic field analysis, the method of moments (MoM) is used.axis alignment are discussed in paragraph 3.2.( ) The coupling coefficient (k) is shown in Fig. 13.When the gap and the displacement are short, the value of the coupling coefficient is large.When the gap and displacement is large, the coupling coefficient is close to zero, however, the efficiencies are still high.This means that even when the coupling is weak, the power can be transferred wirelessly.Thus, power can still be efficiently transferred through large air gaps.

Efficiency vs. air gaps
k is defined by equation ( 6); however, the strictly accurate k is defined by f m and f e under Z 0 = 0 ohm, but this electromagnetic field analysis and experiment was done by Z 0 = 50 ohms is why there is small error.These results imply that the magnetic resonance couplings with the use of these antennas are better able to more efficiently and reliably cross air gaps and more flexibly alignment than more traditional solutions.
The minimum ground clearance of passenger vehicles is between about 90 mm to 200 mm.Therefore, 10 turn antennas are cover wider than 5 turn antennas, so 10 turn antennas are more suitable for EVs.

Conclusion
In this paper, the CPT system by electromagnetic resonant couplings is described and the feasibility of it is studied-especially about antenna characteristics.
The magnetic resonant couplings can power wirelessly even if there is a small connection, therefore, large air gaps with high efficiency can be achieved.
The efficiencies are higher than 90% between the length of transmitting antenna and the receiving antenna are less than 200 mm at resonant frequencies.
The minimum ground clearance of passenger vehicles is between about 90 mm to 200 mm.Therefore, 10 turn antennas are cover wider than 5 turn antennas, so 10 turn antennas are more suitable for EVs.
These results imply that the CPT system by electromagnetic resonant couplings is better suited to traverse air gap and easily align than more traditional solutions and are suitable to wireless power transfer to electric vehicles.
-ISSN 2032-6653 -© 2009 AVERE the batteries to a power source could be Contactless Power Transfer (CPT).In this way, CPT can add more value to EVs to surpass the gasoline vehicle in terms of convenience.In fact, by using this technology, the user does not have to think about connecting the EV to a power source, because the charging is done automatically by the wireless technology.However, the traditional CPT has the problem of distance between the transmitting and receiving antennas (air gap) as well as from the antennas' misalignment.This system needs the flexibility to be effective when the antennas are not aligned; otherwise we need to perfectly align the vehicle directly over the transmitting antenna so that the receiving antenna under the chassis can properly receive the charge.CPT technology can be classified into: electromagnetic induction, microwave power transmission and electromagnetic resonant coupling.In electromagnetic induction, the air gap is short (10 cm) and the alignment must be accurate (within 1-2cm) [1].In microwave power transmission, air gap is very long but efficiency is very low [2][3].The electromagnetic resonant couplings method has a long air gap and is forgiving when it comes to alignment [4][5].Antennas are very important for contactless power transfer technology.In this paper, parameters and design of suitable antenna EVs are proposed.The proposed antennas are forgiving for air gaps and axis variations.

Fig. 1
Fig. 1 System of contactless power transfer

Fig. 6 Fig. 6
Fig.6shows the result of the efficiencies of CPT using the magnetic resonant couplings in respect to

Fig. 10 Electric fields at resonant frequencies 3 . 2
Fig. 8 shows the diagrams of magnetic field and currents from the side.Fig. 8 (a) is a depiction of

Fig. 14
Fig. 14 Contour of efficiency with axis alignment at 5 turns