# Development of Wireless Power Transmission System for Transfer Cart with Shortened Track
## 1 Introduction  
Wireless power transmission (WPT) is a technology that transmits electrical energy from source to load for a distance wirelessly. It is a continuous technological development, revolutionizing wireless charging and electric vehicle radios [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12]. WPT can also be applied to various other applications, such as radio frequency (RF) identification, wireless buildings, codeless switches, batteryless sensors, and wirelessly powered drug release systems [2]. It converts electrical energy into an electromagnetic wave by converting electrical energy into an RF signal. This radio transmission technology is divided into two types: Short-distance transmission technology using magnetic fields and long-distance transmission technology using an antenna [10; 13]. Most wireless power transmission technologies currently being developed are short-distance using magnetic fields [5; 9]. In connection with this technology, different concepts have been proposed, which can be classified into two methods: The magnetic induction method (MIM) and the magnetic resonance method (MRM) [1; 3; 4; 6; 8; 14; 15; 16]. The MIM transmits electric power using a magnetic field induced in a coil [8; 14]. The concept of the MIM is that most of the magnetic field generated from the current flowing through the primary coil flows through the secondary coil, inducing current flows through the secondary coil to supply energy to the load. Such an energy transfer method is similar to the operationprinciple of a conventional transformer. The characteristic of the MIM is that the inherent resonance frequency of each coil differs from the transmission frequency that carries actual energy. This makes it possible to miniaturize the coil, but it also has the disadvantage that the distance, which can be transmitted, decreases as the size of the coil decreases. Presently, the MIM is applied for the wireless charging of portable devices and is also used for the wireless charging of some electric vehicles [8; 14]. The MRM transfers energy using the resonance phenomenon between coils [1; 3; 4; 6; 14; 15; 16]. The MRM is similar to the MIM in that the magnetic fields generated from the current flowing through the primary coil pass through the secondary coil to induce the induction current, but the resonance frequency of the primary coil and the resonance frequency of the secondary coil are the same. The energy generated in the primary coil is transferred to the secondary coil through resonance mode energy coupling between the coils. The resonance frequency and the energy transmission frequency of each coil should be equal. Compared to the MIM, the MRM is advantageous in terms of the transmission distance as a result of the high-quality factor (Q) of the resonator. However, in order to ensure high Q, the size of each coil must be large compared to the MIM systems. Such a MRM can be applied to various fields, compared to the MIM [14; 15; 16]. All the main power equipment used today is powered through sockets and plugs. As a result, there are many problems to overcome to power all mobile devices. Power supplies such as a trolley bars are now available and devices such as carrier-like vehicles are generally a common way to supply power [17]. The transfer method using the sliding contact can cause serious problems in areas requiring a clean environment such as semiconductor factories due to wear and tear, as well as dirt, which contains metal, copper, and lead components. To solve these problems, a non-contact power (NCP) supply has been developed, which is a method of transmitting electrical energy using magnetic induction phenomena without direct electrical or physical contact. This contactless power supply uses mechanical contactless magnetic coupling to transfer power to a load, eliminating the risk of electric shock in wet and dangerous environments, which enables safe power supplies in a clean environment.