preface
Magnetically coupled resonant wireless power transfer technology enables longer-range, higher-power energy transfer. Because it has the advantages of convenience and safety, it can be used for wireless charging of a variety of electronic devices, but due to its transmission efficiency and transmission distance limitations, it is currently unable to achieve commercial applications.
These problems propose to replace the conventional relay coil with a superconducting relay coil, which can carry a larger current density than the conventional relay coil, have lower resistance, and a higher figure of merit (Q), so it can reduce the impedance loss of the coil itself and transmit more energy.
The results show that under the same circumstances, the quality factor of the superconducting winding is about 3 times that of the copper winding, and it is demonstrated that the magnetocoupler resonance wireless energy transmission system using the superconducting winding as a relay can improve the transmission efficiency and increase the transmission distance. At present, the technology of low-temperature superconducting magnets is relatively mature, but its development is limited by the critical magnetic field. Compared with low-temperature superconductors, high-temperature superconducting belts have higher critical temperature, higher critical magnetic field, larger magnetic flux current, and better mechanical properties.
The working principle of the combined resonant system
High-temperature superconducting magnets are currently the world's largest DC magnetic field value (45.5 T), which is considered to be the main direction of permanent magnet technology development in the future. However, in extreme environments such as low temperature, large current carrying field, high magnetic field and complex load, due to the joint action of strong magnetic field and large current, the current carrying characteristics of superconducting strip are affected by strain sensitivity, structural deformation and other factors, a series of serious problems such as overrun, structural failure, force and electrical degradation have occurred in the development process.
In recent years, with the popularity of mobile electronic products such as mobile phones, laptops, and electric vehicles, people's research on wireless charging technology has become more in-depth. In 2007, the Massachusetts Institute of Technology (MIT) announced the magnetic phase synthesis resonance wireless energy transfer technology and obtained the test results, which caused great repercussions in the field of wireless energy transfer.
Wireless power transmission mainly includes three modes, namely: magnetic induction wireless power transfer technology, microwave wireless power transfer technology and magnetic coupling resonant wireless power transfer technology. In these three modes, the transmission efficiency and transmission distance of magnetic coupling transmission technology are between magnetic induction and microwave transmission technology, which fills the gap between transmission efficiency and distance between the two.
The work of the magnetic coupling resonance vibration wireless energy transmission system is carried out in the near-field region. The distance r between the field point P and the source point r is less than the wavelength", and the region corresponding to the field point is called the near-field region. Since the phase difference between the electric and magnetic fields in the near-field region is #$2, the average value of the Pointing vector is 0, that is, there is no electromagnetic energy radiated outward by point primitives in the near-field region.
Electromagnetic coupling resonance energy transfer is a method of energy transfer through near-field non-damage, radiation-free resonance coupling. It can be seen that the excitation coil and the load coil use the principle of electromagnetic induction to exchange energy with the transmitting coil and the receiving coil, so that impedance matching can be easily achieved.
The transmit coil, relay coil and receive coil have the same resonant frequency, and they use magnetic coupling resonant technology to transfer energy. In a magnetic coupling, when the signal source frequency of the excitation coil matches the resonance frequency of the magnetic coupling, its energy transmission efficiency in the resonant system is extremely high, and the transmission efficiency between targets far away from the system is extremely low.
At the same time, because the system is in a resonant state, the transmitting coil, relay coil and receiving coil can generate a large current at a low voltage, so the coil can generate a very strong magnetic field. The above is a discussion of the importance of block temperature monitoring in the superconducting state. However, the superconducting state in bulk materials has an external force. Low temperature environment. Therefore, in addition to the heating caused by AC losses, the cooling system of the superconducting block should also be prevented.
The electric field energy contained in the capacitor in the transmitting coil and the magnetic field contained in the coil can continuously exchange energy, and the receiving coil induces current in the alternating magnetic field. The exchange of electromagnetic energy is also carried out, so the load coil can use the principle of electromagnetic induction to receive energy and finally transmit the energy to the load.
Strong magnetic fields can change the internal structure and properties of substances. played a crucial role. Up to now, there have been many important research results and new technologies. For example, the famous quantum Hall effect, magnetically confinement thermonuclear fusion reactors, high-energy particle accelerators, nuclear energy, and so on.
With the development of science and technology, people have higher and higher requirements for steady-state strong magnetic field technology. Although the high-intensity pulse current is used, a pulsed magnetic field of more than 100 T can be generated. And all this is based on a stable magnetic field. A stable strong magnetic field is generated by water-cooled magnets.
Superconducting magnets and hybrid magnets. Compared to water-cooled magnets, hybrid magnets are superconducting materials because their main components are superconducting materials. As a component, superconducting magnets have obvious advantages such as low energy consumption, light weight, compact size and high field uniformity. With the continuous development of superconductivity, superconductivity is expected to make new breakthroughs under higher magnetic fields.
A wireless communication system based on magnetic coupling and resonance
At present, there are two analysis methods for this system, one is the coupling mode theory in the time domain and the other is the concentrated parameter circuit mutual inductance theory in the frequency domain, and these two methods are consistent when deriving the theoretical model. Because electrical staff are more familiar with and understand the theory of circuit mutual inductance, we use the centralized circuit mutual inductance theory for analysis.
The necessary conditions for the effective energy transmission of the magnetically coupled wireless energy transmission system are that the transmitter coil, receiving coil and relay coil have the same resonant frequency and the quality factor number of the coil is high. When winding the coil, when the size of the coil is the same, then the equivalent parameters of the coil can be regarded as the same.
The energy transmitted by the system is stored in a resonant coil, and the coil itself dissipates energy due to its own impedance. The figure of merit (Q) of the coil represents the ratio of stored energy to energy dissipated, and the expression of the figure of merit Q is as follows.
Here, R = Ro+ Rr, Ro is the ohmic resistance, Rr is the radiation resistance, ω is the angular frequency, L is the inductance of the coil, a is the magnetic force of the magnetic dipole, n is the number of turns of the coil, c is the speed of light, μ0 and ε0 are the permeability and permittivity in the vacuum, but Ro is the main influence in the low frequency band, and the main influence in the high frequency band is Rr.
However, the traditional winding resistance is too large to achieve the improvement of the high-quality factor, and when approaching the superconducting temperature, because the winding resistance in the superconducting state is very small, therefore, a very high quality factor can be obtained, so this project proposes a winding design method based on the high-quality factor and applies it to practical engineering.
where, Mxy= Myx, I1, I2, I3 are the transmitting coil, trunk winding, receiving coil, Z1, Z2, Z3 are the transmission coil, the impedance of the trunk winding, and the impedance of the receiving coil. Since the parameters and supply voltage of each coil are known, the current Ix of any coil can be found.
Testing and analysis of the parameters of the relay coil
In previous literature, it has been mentioned that superconducting coils have excellent characteristics such as low resistance and high quality factor. However, this is only a general description from a theoretical point of view, and has not been experimentally verified. In order to better understand the change of resistance and figure of merit of coils with frequency, the impedance and figure of merit characteristics of copper relay coils and superconducting relay coils are tested and analyzed accordingly.
The 7600 PlusLCR bridge tester was used to test the inductance L, resistance RS and figure of merit Q of the relay coil at 300 K and 77 K respectively. Under the condition of 300 K and 77 K, the inductance L of the relay winding is used as a function of frequency, which is between 300 K and 77 K, and the relay coil resistance RS is used as the curve of the frequency function;
The figure below shows the quality factor and frequency relationship of the relay winding under 300 K and 77 K conditions, whether at 300 K or 77 K, the inductance of the superconducting coil and the copper strip coil basically does not change with the change of temperature and frequency, that is, the inductance of the coil is an intrinsic parameter of the coil and will not change with the change of temperature and frequency.
At the same frequency, copper coils resist less at 77 K than at 300 K. This may be due to the fact that at low temperatures, the resistivity of the conductor decreases and, therefore, the electrical resistance decreases. Low temperatures reduce the convergence effect; Relative to this, at 77 K, the resistance of the superconducting coil is minimal, because the DC resistance of the superconducting wire is 0 below the critical temperature.
The resistance at this time includes the flow resistance and the connection resistance; Between 1.5 MHz and 2 MHz, ohmic resistance predominates, while radiation resistance above 2 MHz prevails. The trend of figure of merit Q measured at 300 K and 77 K for superconducting coils and copper strip coils at 300 K and 77 K, respectively, is demonstrated.
At 77 K, the figure of merit of the superconducting coil is three times that of the copper coil at 300 K and about twice that of the copper strip coil at 77 K.
That is, the superconducting coil has the best figure of merit, and at the same time, the copper strip at 77 K has a higher figure of merit Q than at 300 K. This shows that low temperature is beneficial to reduce the skin effect and improve the quality factor Q.
When the distance S12 between the transmitting coil and the relay coil is 4 cm, the transmission efficiency changes with the distance S23 between the relay coil and the receiving coil is described. Among them, when S23 is 17 cm, the transmission efficiency reaches the highest, respectively: & S (77 K)=84.8%, & S (77 K)=49.2%, & S (77 K)=44.7%. After that, as S23 increases, the transmission efficiency first decreases sharply, and then slowly decreases.
When the spacing S12 between the transmitting coil and the relay coil is 6 cm, the transmission efficiency tends to change with the spacing S23 between the relay coil and the collection coil. When the spacing S12 between the transmitting coil and the relay coil is 8 cm, the transmission efficiency tends to change with the spacing S23 between the relay coil and the receiving coil.
All three curves tend to change in the same direction, and all decrease as the spacing increases. The results show that the transmission efficiency of the superconducting relay coil reaches the best at 77 K, indicating that the use of superconducting coil as the relay coil can improve the transmission efficiency of the magnetically coupled resonant wireless energy transmission system.
The emission efficiency of the copper repeater coil at 77 K is significantly higher than that of 300 K. In addition, at lower temperatures, the resistance of the winding can also be reduced, and the influence on the polyester can be reduced, therefore, the quality coefficient of the product is improved. Through the research of this project, it can not only provide a theoretical basis for practical applications, but also provide a theoretical basis for the magnetic coupling resonance energy transfer system.
Under the conditions of 300 and 77 K, the transmission efficiency of superconducting coils above 41 cm is higher than that of copper wires, and the attenuation trend is slower, indicating that superconducting relay coils can achieve longer transmission distances.
epilogue
In this project, it is proposed to replace the traditional copper relay winding with a superconducting winding of the same size, in order to obtain higher quality factor, higher transmission efficiency, larger transmission distance, and increase transmission distance. Therefore, superconducting coils have a higher figure of merit. By measuring the resistance, inductance, and figure of merit between different coils, it is confirmed that superconducting coils have the above excellent characteristics.
Moreover, the experimental results show that the copper coil under 77 K conditions has a higher figure of merit than the copper coil at 300 K, which can improve the transmission characteristics of the system. At 77 K, the AC loss of copper wire windings is still higher than that of superconducting coils at 77 K, which indicates that the use of superconducting coils can significantly improve the energy transfer efficiency of magnetic couplings, thus providing a new idea for the wireless energy transfer system of magnetic couplings.