What are the wireless charging methods for electric cars?
1. Electromagnetic induction coupling method
Electromagnetic inductive coupling is all too familiar to electrical engineers, and transformers use this principle to transfer energy. If the two windings of the transformer are separated, it is a wireless power supply in a sense. However, the transmission of electrical energy by electromagnetic coupling has a great disadvantage. Without a magnetic core with high magnetic permeability as a medium, the magnetic field lines will seriously diverge into the air, resulting in a decrease in the transmission efficiency, especially when the two coils are far away. Powerful, so it is not suitable for high-power, long-distance power transmission.
The electromagnetic induction coupling charging method does not have direct electrical contact, but uses a separate high-frequency transformer to transfer energy through inductive coupling without contact, as shown in Figure 1. The inductive coupling charging method can solve many defects of the contact charging method.
Transmission of electric power through electromagnetic induction between the power transmitting coil and the receiving coil is the most practical wireless charging method. When there is an alternating current in the transmission coil, alternating magnetic fluxes are generated between the sending (primary) and receiving (secondary) coils, thereby generating an induced electromotive force that changes with the magnetic flux in the secondary coil, and passes through the terminals of the receiving coil. An alternating current is output externally, thereby transferring energy from the transmitting end to the receiving end. At present, the most common wireless charging solution uses electromagnetic induction technology.
Inductive wireless power transmission technology is a relatively mature technology at present. Many mobile phones are wirelessly charged, and even common induction cookers use this principle. The digital device has a small space and a small receiving coil. In addition, the power of the charging device is small. Usually, the charging distance is short (even it needs to be in contact with the charging base), and the relative electromagnetic radiation is also small.
The simplified power flow of the inductively coupled charging system for electric cars is shown in Figure 2. The AC input from the grid is rectified, passed through the high-frequency inverter link, transmitted through the cable and passed through the inductive coupler, and then transmitted to the input end of the electric car, and then passes through the rectification and filtering link to charge the battery of the electric car.
When using an electric car inductive coupling charging system, it is required that the distance between the two devices must be very close, the power supply distance should be controlled between 0mm and 10cm, and charging can only be performed after a pair of coils (power supply coil and power receiving coil) are aligned. The energy conversion rate of electromagnetic induction wireless charging is high, and the transmission power range is large, which can be from several watts to several kilowatts.
The inductive coupling charging method can also be further designed as a fully automatic charging method that does not require personal personnel, that is, the primary and secondary sides of the magnetic coupling device of the inductive coupler are separated by a greater distance, and the charging power supply is installed at a fixed location. In a fixed area, the energy of the charging source can be received in a non-contact manner to realize inductive charging, so as to realize fully automatic charging without the intervention of electric car users or charging station staff.
In inductive charging, a charging pad is buried under the road surface in a suitable location, such as the driveway of a home garage. The charging pad consists of coils that generate a magnetic field. The car is parked on the road above the charging board without physical contact with the car. Electric energy can be transmitted from the charging board to the car’s inductive sensor (Inductive Pick-up) through the magnetic field to form alternating current. The rectifier circuit of the on-board voltage converter (Voltage Converter) converts alternating current into direct current and stores it in the car power battery pack.
In 2011, Volvo Car Group announced that it had officially launched a research project on Inductive Charging. The project, called “Continuous Electric Drive CED (Continuous Electric Drive)”, was jointly developed by Volvo, Belgian technology development agency Flanders’ Drive, Belgian bus manufacturer Van Hoo1 and tram (Tram) manufacturer Bombardier. . Volvo is mainly responsible for the development of charging methods and system products without sockets and cables. Using inductive charging technology, energy can be wirelessly transmitted to the car battery through a charging pad buried under the road surface, which has many conveniences. The schematic diagram of Volvo induction charging structure is shown in Figure 3. Electric car on-board electrical equipment includes AC/DC Rectifier (AC/DC Rectifier), energy management system and battery pack. Inductive sensor is installed on the bottom of the car. When the inductive sensor is facing the charging conductor (Charging Conductor) plate connected to the power grid under the road , the user can activate the charging function, and an electromagnetic field (Electromagnetic Field) is formed near the charging board to transmit energy to the inductive sensor.
At present, the practical wireless charging system mainly adopts the electromagnetic induction method. However, the electromagnetic induction wireless charging system has the following problems:
(1) The power transmission distance is relatively short (about 100mm). If the lateral deviation between the power transmission and reception coils is large, the transmission efficiency will drop significantly, and the transmission distance can only be achieved about 10cm. Therefore, further research and development is required. At the same time, it is also necessary to consider the heat dissipation problem, such as the heating of the coil.
(2) The size of the power is directly related to the size of the coil. When high-power transmission of power is required, investment in infrastructure construction and power equipment must be increased.
(3) Coupling radiation, whether there will be a large magnetic field leakage in the coupling of electromagnetic waves. Electromagnetic induction transmits power between coils. Like a magnet, there will be a certain leakage. How to avoid being affected by the leakage magnetic field also requires further research and development and solutions.
(4) There may be debris entering between the coils, and some animals (cats and dogs) entering. Once an eddy current is generated, just like an induction cooker, the safety problem is very obvious, that is, when foreign objects enter, there will be local fever.
Due to the above problems of the electromagnetic induction wireless charging system, the development of the magnetic resonance wireless charging system is more active. For example, the magnetic resonance wireless charging system being developed by the Takenaka Public Works Store in Japan can solve the problems of local heating and electromagnetic wave and high-frequency protection in the electromagnetic induction system.
2. Magnetic resonance methods
The term electromagnetic resonance is a bit unfamiliar. Its principle is similar to the principle of acoustic resonance. Two media have the same resonance frequency and can be used to transfer energy, which is called non-radiative electromagnetic resonance. This is not to say that the technology does not emit radiation, but it is very different from the common concept of electromagnetic radiation.
The magnetic resonance method uses the phenomenon of electromagnetic induction and the principle of resonance, which can improve the efficiency of wireless charging. The transmission distance of the magnetic resonance method is longer than that of the ordinary magnetic induction type. The magnetic resonance method consists of an energy transmitting device and an energy receiving device. When the two devices are tuned to the same frequency, or resonate at a specific frequency, they can exchange energy with each other. The research team, led by Marin So1jacic, professor of physics at the Massachusetts Institute of Technology (MIT), used magnetic resonance technology to light up a 60W light bulb two meters away and named it WiTricity. The diameter of the coil used in the experiment is 50 cm, which cannot be commercialized yet. If the size of the coil is to be reduced, the received power will naturally decrease.
The advantage of the magnetic resonance method is that the transmission power is large, which can reach several kilowatts, and multiple devices can be charged at the same time, and the coils between the two devices are not required to correspond; the disadvantage is that the loss is high, and the longer the distance, the greater the transmission power and the loss. The larger it is, the frequency band used must be protected.
The basic principle of the magnetic resonance method is the same as that of electromagnetic induction. When a current flows through the coil, a magnetic flux is generated, and current flows through the induction coil. The special point is that a coil and a capacitor are used to form an LC resonance circuit, and a control circuit is used to form the same The resonant frequency is shown in Figure 4. At resonance, the resistance between the two coils can be minimized, reducing losses and enabling power transmission over distances of several meters. From the current point of view, the magnetic resonance method can ensure 90% efficiency within the transmission distance of 60cm, which is in line with the height of the electric car chassis.
The difference between the magnetic resonance charging method and the inductive coupling charging method is that the magnetic resonance charging method is equipped with a high-frequency drive power supply, and uses an LC resonance circuit with both a coil and a capacitor, instead of a simple coil that constitutes two units of sending and receiving. .
The resonance frequency value of the magnetic resonance charging method will change with the distance between the power transmission and the receiving unit. When the transmission distance changes, the transmission efficiency will also decrease rapidly like electromagnetic induction. To this end, the resonant frequency can be adjusted by the control circuit so that the circuits of the two units resonate.
By changing the frequency of transmission and reception, the power transmission distance can be increased to several meters, and the resistance of the two-unit circuit can be minimized to improve the transmission efficiency. Of course, the transmission efficiency is also related to the diameter of the transmitting and receiving electrical units. The larger the transmission area, the higher the transmission efficiency.
In development, the magnetic resonance charging method utilizes electric field coupled resonance technology as shown in Figure 5. Although this technology needs to make the power transmitting end and the power receiving end close to each other, it can also supply power in the state of horizontal dislocation. Problems such as overheating and leakage of electromagnetic waves and high-frequency waves occur. A power supply system utilizing the principle of electric field coupling is being developed, and a power supply system using series resonance has successfully supplied 100W of power to an incandescent light bulb with 90% efficiency. Unlike the electromagnetic induction method, the resonance method does not use ferrite and litz coils, so the weight and cost of the equipment can be reduced. In addition, it is also an advantage that high-power appliances can be powered only by expanding the contact area.
Compared with the traditional magnetic induction charging technology, the magnetic resonance charging technology transmits electric energy over a long distance, and does not need to accurately locate the position of the device to be charged during charging. It can also charge multiple charging devices at the same time, so it is better than the magnetic induction charging. Technology. In addition, the magnetic resonance charging technology can make the mobile charging device do not need to touch the power source, and can be wirelessly charged only within a certain range near the power source. In the traditional magnetic induction charging technology, one charging coil can only charge one charging device, and the mobile charging device must be placed on top of the power supply during charging to receive the charge.
3. Microwave method
Microwave transmission is another method of contactless power transmission, but it is not practical on a large scale due to the limitation of transmission power. The biggest advantage of microwave transmission is that the transmission distance is long, and it can even achieve energy transmission between the spacecraft and the ground. At the same time, it can also achieve directional transmission (the transmitting antenna is directional). The future prospects are worth looking forward to.
The working principle of the wireless power supply system based on microwave mode: firstly, the microwave energy is emitted by the Wave guide slot antenna (Wave guide Slot Antenna) set on the ground, and then the power is received and rectified through the rectenna installed at the bottom of the car, and finally the power is stored in the charging in the battery. The rectenna consists of a patch antenna (Patch Antenna) and a rectifier circuit.
At present, the microwave method is in the research and development stage, mainly by Mitsubishi. The 2.45GHz radio wave generator “Magnetron” is used, and the transmitter is basically the same as the “magnetron” used in microwave ovens. The transmitted microwaves are also AC waves, which can be received by antennas in different directions, converted into DC power by a rectifier circuit to charge the car battery, and can realize point-to-multipoint long-distance transmission.
The microwave method has radiation problems. Since this power source is larger than that of a microwave oven, it is necessary to fully consider the shielding design to prevent the microwave from leaking out between the transmitting and receiving parts during charging. In the design, the microwave leakage must be limited below the legal value, so that the electronic equipment equipped in the car and the nearby pedestrians are not affected. At present, the main problem of the microwave method is that the efficiency of the magnetron to generate microwaves is too low, causing a lot of electric energy to be consumed as heat energy.