Generic block diagram of an inductive wireless power system.

The electrodynamics induction wireless transmission technique relies on the use of a magnetic field generated by an electric current to induce a current in a second conductor. This effect occurs in the electromagnetic near field, with the secondary in close proximity to the primary. As the distance from the primary is increased, more and more of the primary's magnetic field misses the secondary. Even over a relatively short range the inductive coupling is grossly inefficient, wasting much of the transmitted energy [2].

In capacitive coupling (electrostatic induction), the dual of inductive coupling, power is transmitted by electric fields between electrodes such as metal plates. The transmitter and receiver electrodes form a capacitor, with the intervening space as the dielectric. An alternating voltage generated by the transmitter is applied to the transmitting plate, and the oscillating electric field induces an alternating potential on the receiver plate by electrostatic induction, which causes an alternating current to flow in the load circuit. The amount of power transferred increases with the frequency and the capacitance between the plates, which is proportional to the area of the smaller plate and (for short distances) inversely proportional to the separation [4].

In magnetodynamic coupling, power is transmitted between two rotating armature, one in the transmitter and one in the receiver, which rotate synchronously, coupled together by a magnetic field generated by permanent magnets on the armatures. The transmitter armature is turned either by or as the rotor of an electric motor, and its magnetic field exerts torque on the receiver armature, turning it. The magnetic field acts like a mechanical coupling between the armatures. The receiver armature produces power to drive the load, either by turning a separate electric generator or by using the receiver armature itself as the rotor in a generator [1].

Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). The main reason for longer ranges with radio wave and optical devices is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated laser beams). The maximum directivity for antennas is physically limited by diffraction. In general, visible light (from lasers) and microwaves (from purpose-designed antennas) are the forms of electromagnetic radiation best suited to energy transfer.

The dimensions of the components may be dictated by the distance from transmitter to receiver, the wavelength and the Rayleigh criterion or diffraction limit, used in standard radio frequency antenna design, which also applies to lasers. Airy's diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture. Electromagnetic radiation experiences less diffraction at shorter wavelengths (higher frequencies); so, for example, a blue laser is diffracted less than a red one [5].

The proliferation of portable wireless communication devices such as cell phones, tablet, and laptop computers in recent decades is currently driving the development of wireless powering and charging technology to eliminate the need for these devices to be tethered to wall plugs during charging. The Wireless Power Consortium was established in 2008 to develop interoperable standards across manufacturers. Its Qi inductive power standard published in August 2009 enables charging and powering of portable devices of up to 5 watts over distances of 4 cm (1.6 inches). The wireless device is placed on a flat charger plate (which could be embedded in table tops at cafes, for example) and power is transferred from a flat coil in the charger to a similar one in the device.

In 2007, a team led by Marin Soljačić at MIT used coupled tuned circuits made of a 25 cm resonant coil at 10 MHz to transfer 60 W of power over a distance of 2 meters (6.6 ft) (8 times the coil diameter) at around 40% efficiency. This technology is being commercialized as WiTricity [3].

Список литературы

1. Angelo, Joseph A. Encyclopedia of Space and Astronomy, New York, 2006

https://books.google.ru/books?id=VUWno1sOwnUC&pg=PA293&redir_esc=y#v=onepage&q&f=false, (accessed April 26, 2015)

2. Baarman, David W.; Schwannecke, Joshua, "White paper: Understanding Wireless Power", December 2009

http://ecoupled.com/system/files/pdf/eCoupled_UnderstandingWirelessPower_WhitePaper.pdf,(accessed April 26, 2015)

3. Global Qi Standard Powers Up Wireless Charging, September 2, 2009

http://www.prnewswire.com/news-releases/global-qi-standard-powers-up-wireless-charging-102043348.html, (accessed April 26, 2015)

4. Gopinath, Ashwin (August 2013). "All About Transferring Power Wirelessly", Electronics For You E-zine, pp. 52–56. http://www.pcmag.com/encyclopedia/term/57396/wireless-energy-transfer (accessed April 26, 2015)

5. Rajakaruna, Sumedha; Shahnia, Farhad; Ghosh, Arindam). Plug In Electric Vehicles in Smart Grids: Integration Technique, New York, 2014

https://books.google.ru/books?id=VYWhBQAAQBAJ&pg=PA35&redir_esc=y#v=onepage&q&f=false (accessed April 26, 2015)

6. Sun, Tianjia; Xie, Xiang; Zhihua, Wang. Wireless Power Transfer for Medical Microsystems. New York, 2013

http://books.google.ru/books?id=kTA_AAAAQBAJ&pg=PA6&dq="wireless+power"&redir_esc=y#v=onepage&q=%22wireless%20power%22&f=false (accessed April 26, 2015)

7. Tesla, Nikola (May 20, 1891) Experiments with Alternate Currents of Very High Frequency and Their Application to Methods of Artificial Illumination, lecture before the American Inst. of Electrical Engineers, Columbia College, New York

http://www.tfcbooks.com/tesla/1891-05-20.htm (accessed April 26, 2015)

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