1_230 Lab Manual Winter2016.pdf

Radiative methods using antennas can achieve far

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radiated. Radiative methods using antennas can achieve far greater power transfer distances, but they cause heating and suffer from spatial interference and poor efficiency. Recently, interest has been revived in wireless power transfer using resonant magnetic induction: the same phenomenon used by Hutin, LeBlanc, and Tesla in the late 1890's. In 2007, an MIT research team led by Professor Marin Soljačić wirelessly powered a 60W light bulb using resonant magnetic coupling over distances 8 times the radius of the selfNresonant coils ( A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljačić, “Wireless power transfer via strongly coupled magnetic resonances,” Science, vol. 317, no. 5834, pp. 83–86, 2007). Their experiments demonstrated efficient wireless energy transfer over larger coupling distances than experiments performed in the past. Since its revival in 2007 at MIT, nonNradiative wireless power transfer (resonant magnetic coupling) has gained strong scientific and commercial interest. In fact, the global wireless charging market is projected to be worth over 7 billion by 2017 ( “Wireless charging market (2012 N 2017) global forecast & analysis,” April 2012. Available Online: chargingNmarketN640.html). Projections are that it will be used to wirelessly charge mobile devices such as laptop computers, digital cameras, cell phones and even electric vehicles. Devices would gradually charge throughout the course of the day and have no need for a power cord connection. Magnetic induction is attractive for several reasons. It is: safe: magnetic fields interact weakly with dielectric objects and living organisms, compared to electrostatic induction methods, which interact strongly, affordable, and simple to implement. During the past few years, shieldedNloop resonators have been developed at the University of Michigan. These loops are lowNloss, selfNresonant structures tailored for efficient wireless nonN radiative power transfer systems using magnetic induction. They offer several advantages: low loss, high power handling capability, confined electric fields, which could otherwise couple to nearby dielectrics, and simple to feed at their input. Magnetic Coupling and Resonance From Faraday's Law of Induction, we know that a timeNvarying magnetic field can induce a current in a conducting loop of wire. Therefore, a timeNvarying current in one loop can generate a magnetic field in the surrounding area which in turn induces a current in another loop. In this manner, electromagnetic energy can be transferred wirelessly between two or more conductive loops. Such loops are said to be inductively coupled . Magnetic coupling is a nonNradiative mechanism, which is less susceptible to spatial interference and does not wastefully broadcast power. The circuit representation for magnetic coupling between two loops/coils is shown in Fig. 4.1 . The symbol ! denotes the mutual inductance between the two loops, whereas !
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