Wireless Power Transfer Based on Magnetic Coupling

Abstract

Wireless transmission of electric energy has always been a dream for people. The wireless power transmission technology based on induction coupling is the easiest way. We studied the factors that affect the magnetic coupling wireless power transmission system by combining the theory, the software simulation and the experiment. The main work of this paper is as follows. We analyzed the circuit model of the magnetic coupling wireless power transmission system, studied the reactive power compensation problem under power frequency and high frequency, produced a kind of loosely coupled transformer and calculated its parameters, used ANSYS software to simulate the loosely coupled transformer. Significance of wireless power transmission The advancement of human civilization has been accompanied by the development of energy. Events of the first industrial revolution represented by steam power and the second industrial revolution represented by electric power indicate that the advancement of energy application technologies has always been the precondition of cultivation’s progress[1]. The traditional power transmission mode has its limitations, such as transmission loss, aging lines, point discharge, which could significantly reduce the reliability and safety of the system. The traditional power supply with cable in some special areas including mine fields and seabed has serious safety flaws. It’s inconvenient for us to live in the surroundings with crisscross cables. Correspondingly, wireless power transmission has its own advantages. First, it could avoid the crisscross cables. In addition, for the above-mentioned problems occurring in special areas, the special wireless transmission could prevent safety accidents. Some mobile electric devices, such as electric vehicles, could be charged at any time, which reduces the demands for the greater capacity of battery. Especially, with wireless charging, there is no need to carry the various complicated power lines for mobile device such as mobile phone and digital camera[2,3,4,5]. Compared with the traditional power transmission, the wireless transmission has the advantage of not only flexibility and convenience but also higher level of safety. Without complicated power lines and power sources, the potential safety risks could be significantly reduced. Thus, the technology has important application values in some fields, such as military, aerospace, aviation, urban electrified transportation, industrial robots and transportation equipments. It also has bright future when it’s applied in domestic electronic products, biomedicine, and disaster-prevention and rescue[6,7]. Evaluation and analysis of coreless coils It’s the theory of mutual inductance that the inductive coupling technology applies. The primary circuit and secondary circuit have no directly electric connection, but are connected through magnetic coupling. Meanwhile, the change of the number of coil’s turns could achieve the purpose of voltage transformation. At present, in studies and applications of transmitting electric energy through inductive coupling technology, the theory of separation transformer has been put into usage. This technology’s greatest advantages are its easy theory, easiness to be realized, and high efficiency of transmitting energy at short distance, which is as high as 99%. Meanwhile, it has obvious disadvantages, such as that only if the primary circuit and the secondary circuit are aligned 4th International Conference on Computer, Mechatronics, Control and Electronic Engineering (ICCMCEE 2015) © 2015. The authors Published by Atlantis Press 441 with each other as much as possible could the requirements of transmission could be met, otherwise, the efficiency would be reduced significantly. Therefore, this technology has strict requirements for the positions of the first and second loop as well as the iron core[8,9,10]. Circuit of the system The wireless transmission system based on the inductive coupling technology designed by us has four parts, which are respectively the power supply, emitter, receiver and load. Fig.1 Circuit of the system Fig 2-1 is the system circuit based on the mutual inductance. In the system designed by us, the signals produced by the power supply are transmitted into inductance coil L1, then, the emitting coils transmit the electric energy into receiving coils L2 by way of magnetic inductance coupling. The energy is transmitted by the emitter into the receiver with certain distance away, then, transmitted into the load, which completes the task of wireless transmission of energy. At first, we will analyze the coreless coil which is easily achieved and explore the working situation under power frequency. During the exploration, based on coreless coils with 275 turns, and given domestic electric energy usage, the number of turns of the mutual inductance coils is gradually adjusted under the condition of load voltage being 220V to make sure that system’s requirement for voltage is about 220V. The inside and outside radius of coil with 275 turns is respectively 0.25m and 0.36m, and the self-inductance is computed to be 0.071919H; the diameter of the enameled wire is 2.04mm; the distance between the centers of two coils is 2cm; the filling coefficient of coil is computed to be 0.7442; the load is 13.75Ω; the voltage on the load is 220V; the capacitance compensation employs completely compensation. With the self-designed volume integral software to compute the self-inductance and mutual inductance of mutual inductance coils, the exploration is gradually conducted and the gotten main data is shown in table 1. It’s shown that under condition of power frequency, since the mutual inductance among coils is relatively small and the coupling of mutual inductance coils is relatively bad, when transmission of 220V voltage is required by the load, the input voltage and input current are relatively high, and the efficiency of the system isn’t good. In addition, for the coils with 275 turns, its self-inductance is 0.071919H and the compensation capacitance required by complete compensation is 221μF. Problems may occur when the compensation capacitor is chosen, such as that film capacitor is strong enough to withstand the voltage but its capacitance is too small; the capacitance of the supercapacitor is too big and it can’t withstand voltage; the electrolytic capacitor can’t be applied in AC circuit and the electrolytic capacitance without polarity is used in the circuit and couldn’t run long. Therefore, there are two resolutions for the above-mentioned problems, i.e. improving the working frequency of the system or employing the structure of iron core to improve the self-inductance and mutual-inductance of the coils. The method of improving the working frequency is used by many electric devices with low power to get charged without wire. Given that the electric devices confronted by us are the ones with relative big power and the requirement of simplifying system needs to be met, we make our mind to use the iron core to improve the self-inductance and mutual inductance of coils.