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Wireless electricity by resonant magnetic coupling

Work in progress -- Last modification: Sep 22 2009

This page aims to gather all relevant information that can be found on the Web in order to explain how wireless power transfer using magnetic resonance can be created easily.

History

Nikola Tesla's works, more than a century ago, proved that it was possible to have a wireless transmission of electricity. He came out with a huge emitting tower, but it was destroyed before he could realize his plans of providing free electricity anywhere in the world (which was not good for money profit).

In 2006, MIT researchers and Intel worked again on this topic and realized a few mid-range prototypes. Intel's is called WREL, for Wireless resonant energy link (video). MIT research is led Marin Soljacic and was called WiTricity. His webpage contains research papers, for theory and experiments. MIT project has now created the WiTricity Corp., and wireless power market products are announced for 2011.

It's very hard to find (I did not find any actually) information on the Web about how to build a wireless power transmission system and calculate values of electronics components and properties for the system you want to build, in a non-complicated-equations form. In this article, we will study the theory paper in order to design easily wireless transmission systems based on resonant magnetic coupling.

Three kinds of wireless power transmission can be identified: short-range, based on induction coupling, mid-range, based on resonant magnetic coupling, which is an enhanced induction coupling, and long-range transmission, based on electromagnetic waves, such as microwaves. To understand the difference between those three kinds, you can read that introduction to wireless power and that one for details on induction wireless power.

Resonant magnetic coupling

By using magnetic resonance between coils, the transmission efficiency is highly raised over simple induction coupling. Higher frequencies are used, and all objects that are resonant to the same frequency and in the field are able to be powered wirelessly. It does not require a direct line-of-sight between two coupled devices, and they don't have to be aligned in parallel.

In august 2009, 95% efficiency transfer over 40cm and 90% over 60cm have been demonstrated.

Designing a RMC system

1. The oscillator

MIT project used a Colpitts oscillator.

2. The resonant coils

How to build identical coils and determine their resonant frequency, or how build coils for a specific resonant frequency when you already have the oscillator circuit? Resonant frequency is given by 1/2.pi.sqrt(LC) (see also other formulas).

Open questions

Is it safe regarding magnetic storage devices, like hard disk drives?

External resources

Commented article on WiTricity, with good information on the design of Soljacic's device.

wapedia articles on resonant energy transfer and quantum tunnelling.

MIT teachings videos. Very interesting and easy to understand. Here is the lecture on LRC circuits.


1 comment

HARSHAL DEOPARE wrote, on Fri, 07 Feb 2014 20:13:13 +0100

plz tell me complete calculations for design of coil and resonance circuit.

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