Electromagnetic characteristics of basic structure of wireless power transmission coil for electric vehicles

Abstract

Research on wireless power transmission is a hot area at present [1]–[3]. Coil design is one of the key technologies in wireless power transmission [3]–[6]. A core goal of coil design is to study how to improve coupling coefficient. Higher coupling coefficient means greater transmission power and higher efficiency under the same conditions. Many types of coil have been designed, such as square coils, circular pad, four-square coils, DD coupler, DDQ coupler [4], [5]. A comparison is made between circular and elliptical coils by studying the shape of the magnetic field and the path of magnetic flux with a concept of flux pipe [5], [6]. Experimental results show that, in terms of the type of coil, coupling coefficient of the circular pad and DD coupling coils is higher and the cost is lower. Most of the existing researches focus on the practical application and comparison of various types of coils, and rarely compare the characteristics of the two basic circular and square coils. In this paper, the basic characteristics are studied about the two types of the most basic coil, which provides the basis for how to use two kinds of basic coils. For the two most common types of circular and rectangular coils, the structure is axisymmetric. For any one of the above two types of coils as a transmitting coil, the case of a magnetic field coupling is considered using two types of coils in the single turn as receiving coil, respectively. There are three cases: (1) the area of the two receiving coils is equal; (2) the circumference of the two receiving coils is equal; (3) the outer diameters of the two receiving coils are equal in space. It is easy to know that the maximum area of the rectangle coil is square under the same circumference of coil, and so we use square and circular coils for comparative analysis. The circumference, area and outer diameter of the circular coil are equal to the square coil, respectively, when the area and circumference of the square coil are kept constant, as shown Fig. 1. Magnetic field characteristic analysis: For the circular and square transmitting coil with given structural parameters, the curves of the similar Magnetic flux density B can be obtained by numerical calculation and electromagnetic simulation, as shown in Fig. 2a. For cases where the area of the two receiving coils is equal in Fig. 1a, the magnetic flux is equal in the area where two receiving coils overlap. For the non-coincident part, the magnetic flux of the circular coil will be greater than the magnetic flux of the square coil, according to the results of Fig.2a. From the above analysis, it is easy to know that the magnetic flux of the circular coil will be greater than the magnetic flux of the square coil for the same circumference of the two coils as shown in Fig.1a. Because the area of the circular coil in Fig.1b is larger than the area of the circle coil in Fig.1a, the Fig.1b has a result similar to that of Fig.1a. For the case of the same outer diameter shown in Fig. 1c, the magnetic flux of the square coil is larger than the circular coil because the square coil completely contains the circular coil. through the above analysis, the circular receiver coil flux greater than the square coil flux in the case of equal area or equal perimeter, namely in these two cases, the circular receive coil mutual inductance is greater than the square receive coil. the simulation results of the normalized coupling coefficient of three types of coils at different offset distances according to the design parameters of the electric vehicle wireless power supply coil are shown in Fig.2b. The cost and loss of the circular and square coil are equal when the circumference is equal. The mutual inductance of the circular coil is obviously better than that of the square coil, so the efficiency is higher. When the enclosed area is equal, the cost and loss of the circular coil is lower than that of the square coil, and the mutual inductance is slightly larger than that of the square coil, so the efficiency is also higher than that of the square coil. Therefore, it is the best choice to choose a circular coil with no space constraints. Under the condition of limited space, the square coil can achieve greater mutual inductance, but the cost and loss of the coil will be higher than that of the circular coil.