Resonant energy transfer or resonant inductive coupling is the short-distance wireless transmission of energy between two coils that are highly resonant at the same frequency. The equipment to do this is sometimes called a resonant transformer. While many transformers employ resonance, this type has a high Q and is nearly always air-cored to avoid 'iron' losses. The coils may be present in a single piece of equipment or in separate pieces of equipment.
Resonant transfer works by making a coil ring with an oscillating current. This generates an oscillating magnetic field. Because the coil is highly resonant any energy placed in the coil dies away relatively slowly over very many cycles; but if a second coil is brought near to it, the coil can pick up most of the energy before it is lost, even if it is some distance away.
One of the applications of the resonant transformer is for the CCFL inverter. Another application of the resonant transformer is to couple between stages of a superheterodyne receiver, where the selectivity of the receiver is provided by tuned transformers in the intermediate-frequency amplifiers. Resonant transformers such as the Tesla coil can generate very high voltages without arcing, and are able to provide much higher current than electrostatic high-voltage generation machines such as the Van de Graaff generator.
Resonant energy transfer is the operating principle behind proposed short range wireless electricity systems such as WiTricity and similar systems. These types of systems generate magnetic fields that are unlikely to cause health issues in humans.
Non resonant coupled inductors, such as transformers, work on the principle of a primary coil generating a magnetic field and a secondary coil subtending as much as possible of that field so that the power passing though the secondary is as similar as possible to that of the primary. This requirement that the field be covered by the secondary results in very short range and usually requires a magnetic core. Over greater distances the non-resonant induction method is highly inefficient and wastes the vast majority of the energy in resistive losses of the primary coil.
Using resonance can help efficiency dramatically. If resonant coupling is used, each coil is capacitively loaded so as to form a tuned LC circuit. If the primary and secondary coils are resonant at a common frequency, it turns out that significant power may be transmitted between the coils over a range of a few times the coil diameters at reasonable efficiency.
The general principle is that if a given amount of energy is placed into a primary coil which is capacitively loaded, the coil will 'ring', and form an oscillating magnetic field. The energy will transfer back and forth between the magnetic field in the inductor and the electric field across the capacitor at the resonant frequency. This oscillation will die away at a rate determined by the Q factor, mainly due to resistive and radiative losses. However, provided the secondary coil cuts enough of the field that it absorbs more energy than is lost in each cycle of the primary, then most of the energy can still be transferred.
The primary coil forms a series RLC circuit, and the Q factor for such a coil is:
Because the Q factor can be very high, (experimentally around a thousand has been demonstrated with air cored coils) only a small percentage of the field has to be coupled from one coil to the other to achieve high efficiency, even though the field dies quickly with distance from a coil, they can be several diameters apart.
The coupling coefficient is the fraction of flux of the primary that cuts the secondary coil, and is a function of geometry of the system The coupling coefficient is between 0 and 1.
Systems are said to be tightly coupled, loosely coupled, critically coupled or overcoupled. Tight coupling is when the coupling coefficient is around 1 as with conventional transformers. Overcoupling is when the secondary coil is close enough that it tends to collapse the primary's field, and critical coupling is when the transfer in the passband is optimal. Loose coupling is when the coils are distant from each other, so that most of the flux misses the secondary, in Tesla coils around 0.2 is used, and at greater distances, for example for wireless power transmission, it may be lower than 0.01.
Because the Q can be very high, even when low power is fed into the transmitter coil, a relatively intense field builds up over multiple cycles, which increases the power that can be received—at resonance far more power is in the oscillating field than is being fed into the coil, and the receiver coil receives a percentage of that.
The voltage gain of resonantly coupled coils is proportional to the square root of the ratio of secondary and primary inductances.
Unlike the multiple-layer secondary of a non-resonant transformer, coils for this purpose are often single layer solenoids (to minimise skin effect and give improved Q) in parallel with a suitable capacitor, or they may be other shapes such as wave-wound litz wire. Insulation is either absent with spacers or low permittivity, low loss materials such as silk to minimise dielectric losses.
In Tesla coils an intermittent breaker system is used to inject an impulsive signal into the coil; the coil then rings and decays.
The secondary receiver coils are similar designs to the primary sending coils. Running the secondary at the same resonant frequency as the primary ensures that the secondary has a low impedance at the transmitter's frequency and that the energy is optimally absorbed.
To remove energy from the secondary coil, different methods can be used, the AC can be used directly or rectified and a regulator circuit can be used to generate DC voltage.
In 1902 Nikola Tesla patented a device, he called the device a "high-voltage, air-core, self-regenerative resonant transformer that generates very high voltages at high frequency"; it was a Tesla coil that transferred its energy using resonant transfer from the bottom coil a few feet through air to the top coil. This avoided arcing and permitted very high voltages to be created, and is one of the more common types built today.
In the early 1960s resonant inductive wireless energy transfer was used successfully in implantable medical devices  including such devices as pacemakers and artificial hearts. While the early systems used a resonant receiver coil, later systems  implemented resonant transmitter coils as well. These medical devices are designed for high efficiency using low power electronics while efficiently accommodating some misalignment and dynamic twisting of the coils. The separation between the coils in implantable applications is commonly less than 20 cm. Today resonant inductive energy transfer is regularly used for providing electric power in many commercially available medical implantable devices.
Wireless electric energy transfer for experimentally powering electric automobiles and buses is a higher power application (>10 kW) of resonant inductive energy transfer. High power levels are required for rapid recharging and high energy transfer efficiency is required both for operational economy and to avoid negative environmental impact of the system. An experimental electrified roadway test track built circa 1990 achieved 80% energy efficiency while recharging the battery of a prototype bus at a specially equipped bus stop. The bus could be outfitted with a retractable receiving coil for greater coil clearance when moving. The gap between the transmit and receive coils was designed to be less than 10 cm when powered. In addition to buses the use of wireless transfer has been investigated for recharging electric automobiles in parking spots and garages as well.
Some of these wireless resonant inductive devices operate at low milliwatt power levels and are battery powered. Others operate at higher kilowatt power levels. Current implantable medical and road electrification device designs achieve more than 75% transfer efficiency at an operating distance between the transmit and receive coils of less than 10 cm.
In 1995, Professor John Boys and Prof Grant Covic, of The University of Auckland in New Zealand, developed systems to transfer large amounts of energy across small air gaps.
In November 2006, Marin Soljačić and other researchers at the Massachusetts Institute of Technology applied this near field behavior, well known in electromagnetic theory, the wireless power transmission concept based on strongly-coupled resonators. In a theoretical analysis, they demonstrate that, by designing electromagnetic resonators that suffer minimal loss due to radiation and absorption and have a near field with mid-range extent (namely a few times the resonator size), mid-range efficient wireless energy-transfer is possible. The reason is that, if two such resonant circuits tuned to the same frequency are within a fraction of a wavelength, their near fields (consisting of 'evanescent waves') couple by means of evanescent wave coupling (which is related to quantum tunneling). Oscillating waves develop between the inductors, which can allow the energy to transfer from one object to the other within times much shorter than all loss times, which were designed to be long, and thus with the maximum possible energy-transfer efficiency. Since the resonant wavelength is much larger than the resonators, the field can circumvent extraneous objects in the vicinity and thus this mid-range energy-transfer scheme does not require line-of-sight. By utilizing in particular the magnetic field to achieve the coupling, this method can be safe, since magnetic fields interact weakly with living organisms.
Compared to inductive transfer in transformers, except when the coils are well within a diameter of each other, the efficiency is somewhat lower (around 80% at short range) whereas conventional transformers may achieve greater efficiency (around 90-95%), and for this reason, it's unlikely it will be used very much at larger distances where high energy is transferred.
However, compared to the costs associated with batteries, particularly non rechargeable batteries, the costs of the batteries are hundreds of times higher. In situations where a source of power is available nearby, it can be a cheaper solution. In addition, whereas batteries need periodic maintenance and replacement, resonant energy transfer could be used instead, which would not need this. Batteries additionally generate pollution during their construction and their disposal which largely would be avoided.
Unlike mains-wired equipment, no direct electrical connection is needed and hence equipment can be sealed to avoid the risks of electrocution.
Because the coupling is achieved using magnetic fields; the technology is believed to be relatively safe. Safety standards and guidelines do exist in most countries for electromagnetic field exposures(e.g. ), and the system can usually be designed to meet them.
There have, however, been incidents with implanted medical devices (especially pacemakers) where somebody fitted with one approaches within a foot or so of an inductive transmitter coil. As some pacemakers are recharged using this technology, so it is not an inherent risk of the technology, but may be considered to be an EMI issue that might be handled with standardisation and testing.