# Development of Wireless Power Transmission System for Transfer Cart with Shortened Track
## 2 Power Transfer System Design  
In general, coil design is considered the most important step in the physical implementation process of the WPT system to achieve high efficiency and maximum power transfer [16; 24; 25]. In the design of the coil, there are key design parameters of the coil including the resonance of the coil, ferromagnetic materials, and form factor [5; 9; 16; 20]. We carefully designed a magnetic field according to the change in the coil position to take into account the ceiling type, sidewall type, and window type. This allows us to optimize the coil design for various supplied powers. The design of the magnetic field according to the change in the shape, size, and the first and second coil geometries of the used coil was performed. Coils are generally made using wound Litz wire in order to optimize energy efficiency [14]. As the energy transmission efficiency strongly depends on the coil materials, we studied the effects of various coil materials such as FPC (flexible printed circuit), PCB, metal, stranded wire, and Litz wire together with maximizing the field control and efficiency using the ferromagnetic substance. Details in the design of the coils and cores using the commercial simulation software ANSYS and SIMULINK are introduced in [21; 22].  
It is well known that the coil resistance generally increases as the frequency increases due to the skin effect [16]. Therefore, a coil design with a low amount of loss, which also helps to increase Q, is important [16]. Thus, the coil design for improved WPT efficiency should be built upon to minimize the skin effects. The estimation of skin depth due to the effects will be discussed later. The power transfer efficiency is a function of the passive circuit parameters and operating frequency,  
Figure 1: Schematics for modeling of Litz wire: (**a**) Distribution of actual wire and (**b**) effective area (\(A_{\text{eff}}\)) of Litz wire due to skin effect. Here, \(A\) is the overall area of the cable, \(r_{\text{e}}\) is the effective radius, and \(\delta\) is the skin depth in the cable.  
and the power transfer efficiency is generally maximized at the resonance frequency [16]. To select an appropriate frequency, a correlation analysis was performed with respect to the variations in the frequency, coil size, coil shape, and the optimum frequency study with minimum EMF [9]. We studied the use of other devices in the selected frequency band and occupation relationship of the frequency spectrum. Then, we determined the efficiency of the active power conversion circuit according to the selected frequency. Through this process, we can select the optimum frequency according to the amount of power generation in our devices.  
An active shielding technique was also employed by using the RLCS canceller by following the three steps below [16]. First, a study of the generation type of the anti B-field and form control technology according to the loop shapes was performed, and a beamforming study was then conducted using the loop array. This allowed us to draw an optimal layout. Second, using a controllable capacitor array, the loop impedance was controlled and the optimum impedance values were determined. Finally, we studied adaptive RLCS to dampen the EMF with feedback in real-time by measuring the EMF. This method is described at some length in [21,22].  
The EMI minimization technology was also employed through the power circuit application and analysis of the spread spectrum technique [24]. By using the spread spectrum technique, the EMI can be reduced significantly [9].  
In addition, to optimize the range of frequency control, we controlled the frequency of the transmission side. In addition, we used the spread spectrum implementation technique using the delay cell array and the programmable digital interface study. As a detailed discussion of the design processes described in this section is covered in [21,22], we just introduce the results of the implemented design in this study.