In our earlier articles, we have discussed about Basics of Wireless Power Transfer and Pros and Cons of Qi Charging. Wireless power transfer is the process of transferring electrical energy from one object to another without contact. The most widely used method is inductive energy transfer. At close range of a few centimeters, it has a comparatively high efficiency of about 90%. Examples of applications include the charging of batteries in mobile devices.
Near-field
For inductive energy transfer, an alternating magnetic field is generated in the transmitter by means of an oscillator. The transmission takes place by means of counter-induction between two coils. In the receiving coil, an alternating current is induced by the alternating current in the transmitting coil, which is rectified in applications such as charging accumulators and supplied as a direct voltage to the consumer like a charge controller. The operating principle is the same as that of a transformer with loose coupling of the two coils.
The distance between the two coils represents the wireless transmission distance and should be as small as possible – typically a few centimeters to a few 10 cm distance. If the distance between the two coils is greater, the leakage flux increases sharply, which reduces the inductive coupling and deteriorates the efficiency. Typical distances that can be bridged with this method are approximately the coil diameter to twice the coil diameter, the frequency range used ranges from a few 10 kHz to the MHz range. Typical applications in this area are RFID transponders, contactless chargers or the power supply between moving machine parts or between special rail systems and electrically powered vehicles.
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Resonant inductive coupling is an extension of inductive coupling with the aim of increasing the short range. For this purpose, one or more free oscillating circuits are installed in the free space between the transmitting and receiving coils, as shown in the adjacent principle diagram. Each of these oscillating circuits consists of a capacitor C and a coil L, whose resonant frequency is matched to the transmission frequency. The resonance between the oscillating circuits results in an improved magnetic coupling between the transmitting and receiving coils at the transmission frequency. The oscillating circuits should have the highest possible quality factor. This results in a longer range and better efficiency. Wireless power transmission is thus possible over a distance of 4 to 10 times the coil diameter.
In 2013, a paper was published which, among other things, sheds light on the possibilities of increasing efficiency through the use of resonant inductive couplers. This shows that the efficiency of energy transmission in a coupled transmission system in the near field can only be increased or even maximized by selecting the complex load impedance. If the transmitted power is also to be maximized, it is necessary to adapt the load to the source in addition to adjusting the load. From this point of view, the mode of operation of the adjacent principle illustration can be understood approximately as meaning that the two outer coils cause the adaptation to source and load and that the middle, loosely coupled pair of coils serves to transfer energy. As a result, stable efficiencies of 93% can also be achieved in the commercial sector.
The division into the two inner, loosely coupled energy transfer coils and the outer matching coils makes it clear that the adaptation does not necessarily have to be carried out by additional inductive couplings. Rather, by selecting appropriate adaptation networks, it is also possible to achieve the same or higher efficiency of energy transfer with only two coils.
Capacitive coupling is based on a similar basic structure to inductive transmission, except that it uses the electric field for wireless energy transfer between two metal plates. These metal plates represent an electrical capacitor, the area between the two plates is the distance of wireless power transmission. The two capacitors are powered by alternating voltage, obtained from an oscillator on the transmitter side. On the consumer side, rectification takes place and the DC voltage is supplied to the actual consumer.
Capacitive coupling has little practical significance, since high electrical voltages occur in the space between the metal plates during the transmission of higher powers. The distances between the plates should also be kept as small as possible so as not to reduce the efficiency too much.
Far-field
Electromagnetic waves are used for electromagnetic energy transmission, the principle is the same as for the transmission of radio signals. The energy transfer in the far field, for example, can also be a directed laser beam. The laser beam is directed at a photocell as a receiver, which converts the optical power into electrical power.
While far-field transmission in technical systems is well suited for information and signal transmission, wireless energy transmission is associated with high losses due to free-field attenuation and losses during conversion, as with a laser or photocell, with very low efficiencies overall. A practical example of energy transmission would be the detector receiver, a simple radio receiver for medium wave, which draws its power supply only from the radio signal in the vicinity of powerful transmitters and does not require any additional power supply such as a battery for operation. Due to the very low efficiencies far below 1 % in total, technically feasible wireless power transmissions in the far field, apart from a few special applications such as RFID technology, have virtually no practical significance.
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