**Analysis of Wireless Power System Efficiency in Dependency on Configuration of Resonant Tank**

Michal Frivaldsky, Pavol Spanik, Peter Drgona, Viliam Jaros and Marek Piri

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/62998

#### **Abstract**

This chapter compares various compensation methods for resonant coupling of the wireless energy transfer system. A proposed analysis is particularly relevant to any application where contactless battery charging is used. Main parameters that are investigated include efficiency and electrical variables (current and voltage) of the circuit. In order to analyze the most suitable solution of coupling compensation, the relevant equations are graphically interpreted for each discussed circuit topology. Finally, this chapter provides the recommendations how to design the wireless powertransfer system with the highest possible efficiency for the given system parameters (switching frequency and transmitting distance).

**Keywords:** Efficiency, wireless power transfer, configuration, resonant compensation, distance

#### **1. Introduction**

Nowadays, electrically powered automobiles show higher interest in the automotive indus‐ try. Electric vehicles gain this interest thanks to high efficiency. In the case of the asynchro‐ nous motor, the electrical energy can be converted into the mechanical with an efficiency of 90%, which is much more than the efficiency of the combustion engine that has an efficiency of only 25‒34%. Due to the limitation of natural resources such as petroleum and natural gas, the power electronics and energy conversion along with the development of new battery types contrib‐ ute to the development and application in the automotive industry. The main disadvantage of the electric car is the energy accumulation. Batteries are large, heavy, with short lifetime, and

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longchargingtimes.These reasons showhownecessaryitis tosolve anissueofbatterycharging [1]. In general, charging systems can be classified as wired and these could be replaced with wireless (inductive) systems.

Wireless power transfer and issues relevant to wireless charging of various types of electronic devices are still important and emerging trends in electrical engineering [2‒4]. Regarding of wireless power transfer, each application requires several self-specific operational properties or transfer characteristics. The most important of them are transmitting efficiency, value of the output power, mutual position between the transmitter and the receiver, and their geometric dimensions. One possible way how to influence these parameters lies in the main circuit configuration of a compensation network of the wireless energy transfer (WET) system. Inductively coupled power-transfer systems have been suited for a wide spectrum of consumer applications including electromobility (battery charging of vehicles). Even high system efficiency has been achieved (approximately 70%); however, the restriction to close range, typically shorter than 30% of the coil diameter, is the main limitation for perspective usage in high-efficiency or high-performance systems [5, 6].

It the beginning of 2000s, a team of researchers under charge of Prof. Soljacic introduced a new concept of strongly coupled magnetic resonance for wireless energy transfer systems [7]. In comparison with an inductively coupled system, the coils with resonant inductive coupling have several significant advantages. The most important of them are as follows: the coupling may be very efficient even at large transmission distances, it has low environmental impact, and can be variously tuned/configured based on the requirements of target application. The solution of the WET system based on the magnetic-resonance coupling represents constantly investigated phenomena. Most of the analyses are based on the pure theory of physical interactions, which are unable to provide clear and consistent overview of knowledge for researchers in the field of electrical engineering [8, 9]. Based on this, it can be said that some important relationships between system efficiency, amount of transmitted power, and transmitting distance for magnetic-resonance coupled WET systems need to be introduced.

This chapter describes a simple equivalent circuit model for magnetic-coupled and resonant magnetic-coupled WET systems. The resonant coupling is further described in details, whereby all key system equations for various configurations of a resonant compensation network with pure resistive load are derived. From the component design point of view, it is also important to have knowledge about values of voltage and current in the main circuit. In the last part of the chapter, the general recommendations for practical use of each variant are provided.
