Over the next decade, the most vibrant Wireless Power Transmission (WPT) markets will involve the contactless charging of portable and mobile equipment, in particular consumer electronics and electric vehicles. Today, development teams worldwide are stretching the boundaries of physics using available components to create systems that are able to compete with the efficiencies of wired solutions while offering the conveniences of wire free connections. In the 1890's famous scientist and engineer Nikola Tesla dreamt of wireless transfer of power. He envisioned a world, in which all electricity is transferred wirelessly, but the idea was more or less abandoned and highly efficient copper cables became the basis for modern electricity infrastructure. Then, a few years ago, Marin Soljačić, an assistant professor of physics at MIT, was dragged out of bed by the insistent beeping of a cell phone. In his exhausted state, he wished the phone would just begin charging itself as soon as it was brought into the house. So, Soljačić started searching for ways to transmit power wirelessly. Soljačić found magnetic resonance a promising means of electricity transfer because magnetic fields travel freely through air yet have little effect on the environment or, at the appropriate frequencies, on living beings. Working with MIT physics professors John Joannopoulos and Peter Fisher and three students, he devised a simple setup that wirelessly powered a 60-watt light bulb. The MIT work has attracted the attention of consumer-electronics companies and the auto industry. The U.S. Department of Defense, which is funding the research, hopes it will also give soldiers a way to automatically recharge batteries.
1) Inductive coupling or magnetic induction Two conductors are referred to as mutual-inductively coupled or magnetically coupled when they are configured such that change in current flow through one wire induces a voltage across the ends of the other wire through electromagnetic induction. In the above figure there are two coils. The red color coil is the primary coil, which is carrying an AC current, and is generating an oscillating magnetic field. Blue coil is the secondary coil and when brought close enough to the first, it captures a portion of the oscillating magnetic field and receives energy from the magnetic field due to mutual induction. In induction coupling both coils are kept very close to each other so that secondary coil can capture the magnetic field of primary coil without any leakage of power.
For example, wireless charging pad (WCP) and electric toothbrushes. On a WCP, devices like mobiles are to be kept, battery will be automatically charged. Electric toothbrushes also charges using inductive coupling. 2) Resonant inductive coupling Resonant induction is an improvement on electromagnetic induction that increases the allowable distance between magnetic coils. Radial expansion of a standard magnetic field is possible, but it is also extremely inefficient.
Researchers at MIT found that resonance; "a phenomenon that causes an object to vibrate when energy of a certain frequency is applied" can be used to extend the magnetic field and target only devices, which resonate at a certain frequency. Resonant induction
could work within a range of a few meters. Furthermore, resonant induction only affects devices that resonate at the same frequency as the transmitting coil, so other electronic or wireless devices would not be affected. In this case additional capacitors are connected in parallel to both coils, which will make the primary coil to produce strong magnetic field with bigger diameter. And thus secondary coil can capture the magnetic field from a large distance. Resonance inductive coupling is highly efficient and this cordless power technology uses wireless antenna to transmit power in house and requires no charging pads...
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