Some Major Wireless Power Tranmissions

电子科技大学格拉斯哥学院高浩淳 2017/18级

Abstract:
Wireless power transmission (WPT) technique will become the development trend of energy supply system for electric vehicles (EV), which features the advantages of energy saving, low pollution and low-carbon emission. This paper shows some mainstream models of wireless charging
Keywords: WPT, wireless charging, magnetic field, microwave, electromagnetic
Instruction:
Wireless Power Transmission (WPT), translate electric energy through the electromagnetic field. For civil WPT is setting the primary winding and the secondary winding on or out of the devices translate
the electric energy by coupling of high frequency electromagnetic field. Compared by contact charging, this way is convenient, safe, no danger of electric shock and problem of dirt retention. Besides, it can adapt to severe weather and environment. Because of wireless charging, we charge our devices anytime and everywhere.

1.Some models of wireless charging:

The technology not only can used to charge our electric devices but also has its stage in the field of new energy vehicle and the change of voltage of the power transmission line.Although WPT has so many advantages, the defect of low effective compare to traditional charging limits its development, which challenges the researchers.
There are some models of wireless charging.
1.1microwave WPT
the microwave WPT is to make the microwave be the carrier that can transform the energy wirelessly and freely in the space. Figure 1 is the model of microwave WPT, using the source of microwave to change the electric energy to the microwave and then send by antenna. Received by antenna far from the sender after a long time spreading, finally it is changed to the electric energy again by the rectifier.
Because of the characters of” certain direction”, “strong penetrating power” and so on, the way of translating has concerned by our humans since 1960s. the WPT always used on microwave plane, spaceport or other long-way occasions.At present, the main reason which limit the technology on this stage mainly is lick of the effective rectifier element, electromagnetic field and high powerful antenna. In other hands, the influences of the environment and the safety of biology is another dilemma. Besides, the technology is not suitable to the short distance transforming.
1.2Electromagnetic WPT
The WPT is based on the principle of electromagnetic, using the primary and secondary segregate transformer transforming in the short distance. Nowadays, the majority of mature ways of wireless charging are based on this technology. Some applicants are shown in the figure 2 below.It is shown in figure 2 that whether low power electronics or EV wireless charging system, the electromagnetic WPT realize wireless charging. Many scientific and technical corporations apply this technology in their productions or experimental devices.However, the WPT also exists some questions, like: can not transform too long, the effect decreases rapidly, sensitive to the position relations between the primary and secondary segregate transformer and so on.

1.3Magnetic resonance WPT
Magnetic resonance WPT, which was a breakthrough technique raised by MIT Soljač ič in 2007.They used two equal-inherent frequency copper winding (equal to the “potential transformer”). The effect reached 40%. Compared to electromagnetic WPT, magnetic resonance WPT can increase the coupling and the effect of transforming, concerned by researchers in the industry. Many companies like Intel and Haier give the priority to this technology and invested a lot of money to explore it. Besides, in the process of producing, we also find that the WPT also has the advantage of the low standard of the
position. A Japanese company has achieved the experience of the vertical winding
magnetic resonance WPT. Furthermore, using the serious standard of operating frequency, we can charge the specified devices through the setting the excitation frequency to increase the safety.However, the WPT either just a theory, lack of the experience’s support, or the power is small, can not used to charge our devices or EV. But it is do an important invent and odder a new direction of WPT.

2.Basic structure of WPT
The figure below indicates the basic structure of a WPT，including Power Factor Correction, convertor, inverter, loosely coupled transformer, non-contact feedback and receiving circuit and so on. The majority of elements of WPT is similar to the translated charger. The difference is the design of noncontact transformer, converter topology and its control. We can see that the research of the converter topology and its control and the noncontact feedback are the points of WPT. The noncontact transformer is the code element of the non-contact charger.
3.Some deep researches of increasing the effect of wireless charging
3.1Fully separated charging device
It is a perfect model at present, the interspace of the special charger usually between 10mm and 50mm, and the mobile charger’s interspace can reach 150mm or even more. According to the data of analysis,??/?= ?, because the large value of g, k is so small that the effect of the transformer is small(usually below 70 present or even less than 50 percent. The best result data is 82%.
3.2Increase the coupling coefficient of the non-contact transformer
Change the magnetic core and the shape of winding are basic methods of increasing the coupling coefficient. As we know, increasing the distance of magnetic core, the resistance of magnetic will also increase, which is good for coupling coefficient’s increasing. Compared with the traditional core, the U-form of core can get larger distance.In other hands, using plane winding with U-core can increase the distance of magnetic path and the resistance, thus increase the resistance furtherly. There are two forms of plane winding: centrallayout and distributed. The magnetic flux produced by circuit can not only let L close, but also decrease the coupling coefficient. Therefore, we should use distributed plane winding.
3.3 increasing the rate of unity coupling magnetic flux
Beside the planarity, Zhang W, Chen Q H, Wong S C, et also raised a new theory of increasing the rate of unity coupling magnetic flux. This theory orders a new effective way of optimizing the chargers limited by certain volume and weight. The essay divides the magnetic field into three parts: unity coupling, part coupling and leakage magnetic flux. And then set the model of transformer based on the theory, finally summarized the new method: decrease ??1 and ??2, increase ??1, ??2 ??? ??3.hen join up the 60W non-contact transformer, the result shows k increases to 0.68, and in the case of the same k, the weight of magnetic core also decreases about 60g. the theory also suitable to the high power EV charger.
3.4other new ways
In order to increase the effect, we can set series or parallel model to replace the single structure non-contact transformer. That can simplify the process of production and has advantages of heat dissipation. Another dilemma is the problem of nonuniform distribution of magnetic flux. To avoid the problem, we can use hollow transformer to replace the traditional transformer which has magnetic core. Of course, doing this must increase the volume and influence the coupling coefficient.
Conclusion
Wireless charging is the trend of transforming energy between devices. This essay introduces threemainstream WPTs: microwave WPT, Electromagnetic WPT and Magnetic resonance WPT. The essay also introduces some way which can increase the effect of charging.
References
[1] 陈清泉, 孙逢春, 祝嘉光.现代电动汽车技术 [M] .北京：北京理工大学出版社 , 2002.
[2] Papastergiou K D, Macpherson D E. An airborne radar power supply with contactless transfer of energy-part I
rotating transformer[J]. IEEE Transactions on Industrial Electronics, 2007, 54(5): 2874-2884.

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