Wireless power transfer is the transmission of energy from one place to another without using wires. Conventional energy transfer is using wires but the WPT is made possible by using various technologies.

As per studies, most electrical energy transfer is through wires so the energy loss is during transmission more than 30%. WPT have some benefits, reduce cost associated with maintaining direct connectors, greater convenience for the charging of everyday electronic devices, safe power transfer to applications that need to remain sterile or hermetically sealed.

Wireless power transfer was first demonstrated by Nikola Tesla in the 1890s, however, it is only really in the last few decades. At the first time he tried to develop a wireless lighting system based on near-field inductive and capacytive coupling.

In recent years, there has been increasing interest in research and development of WPT technology.  Applications that have accelerated the development of WPT are, among others, healthcare, mobile devices such as laptops and cell phones, automobile, charging table and housewares device.

Tesla demonstrating wireless transmission by “electrostatic induction” during an 1891 lecture at Columbia College

Applications of WPT

In recent years, there has been increasing interest in research and development of WPT technology.  Applications that have accelerated the development of WPT are, among others, healthcare, mobile devices such as laptops and cell phones, automobile, charging table and housewares device.

There are the different WPT power technologies:

  • Inductive coupling
  • Resonant inductive coupling
  • Capacitive coupling
  • Magneto dynamic coupling
  • Microwaves

The inductive coupling for short range application and resonant inductive for mid-range applications, they are the most widely used method in most of wireless power applications. In this thesis, the direction of research has a relationship with resonant inductive coupling. The WPT system used this method, it works based on the principle of electromagnetic phenomena between the two coils, in which the first coil is connected to the power source and the second coil is connected to the load, based on the physical concept that two resonators with the same resonant frequency are able to effectively exchange power. The two resonators Tx Coil and Rx Coil, which have the same resonant frequency, were described as Figure:

The short-distance WPT is either based on capacitive or inductive coupling which distance is limited to a few centimeters. Widely method of inductive power transmission is resonant power transmission. Using self-resonant coil in a strongly coupled regime, the efficient non-radiation power transfer over distance up to 8 times the radius of the coils

At the resonance frequency, the distance between the source coil and the Tx resonator, the load coil and the Rx resonator will be adjusted using the variable coupling method for matching impedance.

(a) Schematic of four coil WPT system, (b) lumped circuit model

The currents can be calculated formula:

The voltage can be calculated formula

Rs = Source Resistance

RL = Load Resistance

Vs = Signal Source

k = Coupling coefficients between resonators

The efficiency of the WPT system is defined as the ratio between output power and input power:

Input impedance:

Matching condition is given:

This method is very useful for matching impedance in WPT four coil system. So almost measurement of experiment in thesis had already manipulated it to achieve the most accurate results.

Editor: Thanh Son

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