Editing Wireless power transfer (section)

=== Resonant inductive coupling ===
[[File:Wireless power system - inductive coupling.svg|thumb|upright=1.4|Generic block diagram of an inductive wireless power system]]
{{main|Resonant inductive coupling}}
{{Further|Tesla coil#Resonant transformer}}

[[Resonant inductive coupling]] (''electrodynamic coupling'',<ref name="Valtchev" /> ''strongly coupled magnetic resonance''<ref name="Karalis" />) is a form of inductive coupling in which power is transferred by magnetic fields ''(B, green)'' between two [[resonant circuit]]s (tuned circuits), one in the transmitter and one in the receiver.<ref name="Sazonov" /><ref name="Valtchev" /><ref name="Agbinya" /><ref name="Wilson" /><ref name="Shinohara" /> Each resonant circuit consists of a coil of wire connected to a [[capacitor]], or a [[self-resonant frequency|self-resonant]] coil or other [[resonator]] with internal capacitance. The two are tuned to resonate at the same [[resonant frequency]]. The resonance between the coils can greatly increase coupling and power transfer, analogously to the way a vibrating [[tuning fork]] can induce [[sympathetic vibration]] in a distant fork tuned to the same pitch.

[[Nikola Tesla]] first discovered resonant coupling during his pioneering experiments in wireless power transfer around the turn of the 20th century,<ref name="Wheeler">{{cite journal |last1=Wheeler |first1=L. P. |title=II — Tesla's contribution to high frequency |journal=Electrical Engineering |date=August 1943 |volume=62 |issue=8 |pages=355–357 |doi=10.1109/EE.1943.6435874 |s2cid=51671246}}</ref><ref name="LeeZhongHui">{{cite conference |first1=C.K. |last1=Lee |first2=W.X. |last2=Zhong |first3=S.Y.R. |last3=Hui |title=Recent Progress in Mid-Range Wireless Power Transfer |conference=The 4th Annual IEEE Energy Conversion Congress and Exposition (ECCE 2012) |pages=3819–3821 |publisher=Inst. of Electrical and Electronic Engineers |date=5 September 2012 |location=Raleigh, North Carolina |url=http://hub.hku.hk/bitstream/10722/189863/1/Content.pdf |access-date=4 November 2014}}</ref><ref name="Sun1">{{cite book |url=https://books.google.com/books?id=kTA_AAAAQBAJ&q=%22resonate+inductive+coupling%22+tesla&pg=PA3 |last1=Sun |last2=Xie |last3=Wang |year=2013 |title=Wireless Power Transfer for Medical Microsystems |page=3 |publisher=Springer |isbn=9781461477020}}</ref> but the possibilities of using resonant coupling to increase transmission range has only recently been explored.<ref name="Beams">{{Cite book |doi=10.1109/MWSCAS.2013.6674697 |isbn=978-1-4799-0066-4 |chapter=Design and simulation of networks for midrange wireless power transfer |title=2013 IEEE 56th International Midwest Symposium on Circuits and Systems (MWSCAS) |pages=509–512 |year=2013 |last1=Beams |first1=David M. |last2=Nagoorkar |first2=Varun |s2cid=42092151}}</ref> In 2007 a team led by [[Marin Soljačić]] at MIT used two coupled tuned circuits each made of a 25&nbsp;cm self-resonant coil of wire at 10&nbsp;MHz to achieve the transmission of 60 W of power over a distance of {{convert|2|meters|feet}} (8 times the coil diameter) at around 40% efficiency.<ref name="Valtchev" /><ref name="Karalis" /><ref name="Wilson" /><ref name="LeeZhongHui" /><ref name="Kurs">{{cite journal |last1=Kurs |first1=A. |last2=Karalis |first2=A. |last3=Moffatt |first3=R. |last4=Joannopoulos |first4=J. D. |last5=Fisher |first5=P. |last6=Soljacic |first6=M. |title=Wireless Power Transfer via Strongly Coupled Magnetic Resonances |journal=Science |date=6 July 2007 |volume=317 |issue=5834 |pages=83–86 |doi=10.1126/science.1143254 |pmid=17556549 |bibcode=2007Sci...317...83K |citeseerx=10.1.1.418.9645 |s2cid=17105396}}</ref>

The concept behind resonant inductive coupling systems is that high [[Q factor]] [[resonator]]s exchange energy at a much higher rate than they lose energy due to internal [[Damping ratio|damping]].<ref name="Karalis" /> Therefore, by using resonance, the same amount of power can be transferred at greater distances, using the much weaker magnetic fields out in the peripheral regions ("tails") of the near fields.<ref name="Karalis" /> Resonant inductive coupling can achieve high efficiency at ranges of 4 to 10 times the coil diameter (''D''<sub>ant</sub>).<ref name="Wong" /><ref name="Baarman" /><ref name="Agbinya3" /> This is called "mid-range" transfer,<ref name="Baarman" /> in contrast to the "short range" of nonresonant inductive transfer, which can achieve similar efficiencies only when the coils are adjacent. Another advantage is that resonant circuits interact with each other so much more strongly than they do with nonresonant objects that power losses due to absorption in stray nearby objects are negligible.<ref name="Agbinya" /><ref name="Karalis" />

A drawback of resonant coupling theory is that at close ranges when the two resonant circuits are tightly coupled, the resonant frequency of the system is no longer constant but "splits" into two resonant peaks,<ref>{{Cite journal |doi=10.3390/s16081229 |title=Frequency Splitting Analysis and Compensation Method for Inductive Wireless Powering of Implantable Biosensors |journal=Sensors |volume=16 |issue=8 |pages=1229 |year=2016 |last1=Schormans |first1=Matthew |last2=Valente |first2=Virgilio |last3=Demosthenous |first3=Andreas |pmid=27527174 |pmc=5017394 |bibcode=2016Senso..16.1229S |doi-access=free}}</ref><ref>{{Cite journal |doi=10.3390/en10040498 |title=Combined Conformal Strongly-Coupled Magnetic Resonance for Efficient Wireless Power Transfer |journal=Energies |volume=10 |issue=4 |pages=498 |year=2017 |last1=Rozman |first1=Matjaz |last2=Fernando |first2=Michael |last3=Adebisi |first3=Bamidele |last4=Rabie |first4=Khaled |last5=Kharel |first5=Rupak |last6=Ikpehai |first6=Augustine |last7=Gacanin |first7=Haris |doi-access=free}}</ref><ref>{{cite web |last=smith |first=K.J. |url=http://www.lessmiths.com/~kjsmith/crystal/resonance.shtml |title=A graphical look at Resonance}}</ref> so the maximum power transfer no longer occurs at the original resonant frequency and the oscillator frequency must be tuned to the new resonance peak.<ref name="Wong" /><ref name="Neo">{{Cite web |url=http://blog.livedoor.jp/neotesla/archives/51508967.html |title=Reconsideration of Wireless Power Transfer principle which presented by MIT |website=ニコラテスラって素晴らしい |date=30 March 2017}}</ref>

Resonant technology is currently being widely incorporated in modern inductive wireless power systems.<ref name="Davis" /> One of the possibilities envisioned for this technology is area wireless power coverage. A coil in the wall or ceiling of a room might be able to wirelessly power lights and mobile devices anywhere in the room, with reasonable efficiency.<ref name="Wilson" /> An environmental and economic benefit of wirelessly powering small devices such as clocks, radios, music players and [[remote control]]s is that it could drastically reduce the 6 billion [[Electric battery|batteries]] disposed of each year, a large source of [[toxic waste]] and groundwater contamination.<ref name="Tan" />

A study for the Swedish military found that 85&nbsp;kHz systems for [[dynamic wireless power transfer]] for vehicles can cause electromagnetic interference at a radius of up to 300 kilometers.<ref>{{citation |url=https://www.electronic.se/en/2021/05/25/interference-risks-from-wireless-power-transfer-for-electric-vehicles/ |title=Interference Risks from Wireless Power Transfer for Electric Vehicles |author=Sara Linder |publisher=Swedish Defence Research Agency (FOI) |date=2 May 2021}}</ref>