Analysis of three contactless charging methods for electric vehicles

The charging device of an electric vehicle is equivalent to a fuel filling station for automobiles. It can provide energy for the vehicle to run continuously through repeated charging. When China began to vigorously build wired charging piles and charging stations, three types of contactless electric vehicle charging devices emerged abroad and entered commercial operation to varying degrees. What types of contactless charging devices are there? What is the basic working principle? How is its charging efficiency, safety, and convenience? These are all people's concerns.

Types of contactless charging devices

There are three types of contactless charging devices: electromagnetic induction, magnetic resonance, and microwave.

Advantages of contactless charging devices

Compared with electric vehicles, traditional fuel vehicles not only have many advantages in terms of ease of use, curb weight, driving range, manufacturing and use costs, but also do not require more time to refuel.

Electric vehicles not only take a long time to charge, but also have operational inconveniences in replacing batteries or charging via cables at charging stations. The safety of working on rainy days is even more worrying.

The contactless charging device can directly charge the vehicle quickly without connecting it to the power supply system with a cable. In addition, the contactless fast charging can be arranged in parking lots, residences, roadsides and other places, and can provide charging services for various types of electric (including external plug-in hybrid) vehicles, making it possible to charge electric vehicles anytime and anywhere. For buses, charging facilities can be arranged at terminal stations, hub stations, transfer stations and other places, and fast charging can be completed during short parking time.

Working principle of contactless charging device

1. Electromagnetic induction method

Electromagnetic induction is the most practical charging method that transmits power between the power transmission coil and the receiving coil. When an alternating current flows through the power transmission coil, an alternating magnetic flux is generated between the transmitting (primary) and receiving (secondary) coils, thereby generating an induced electromotive force in the secondary coil that changes with the magnetic flux, and outputs an alternating current through the receiving coil terminals.

a) Basic principles of power transmission

b) Actual wiring method

The basic working principle of electromagnetic induction

The current problems are: the power transmission distance is relatively short (about 100mm), and when there is a large deviation between the power transmission and receiving parts, the power transmission efficiency will drop significantly; the power size is directly related to the coil size. When high-power transmission is required, more investment must be made in infrastructure construction and power equipment.

2. Magnetic Resonance Method

The magnetic resonance transmission method was successfully developed by the Massachusetts Institute of Technology (MIT) in 2007. Since it was made public, it has attracted widespread attention from countries around the world.

It is mainly composed of power supply, power output, power receiving, rectifier and other main parts. The basic principle is basically the same as the electromagnetic induction method. When current passes through the power transmission part, the alternating magnetic flux generated causes the receiving part to generate an electric potential, outputting current when charging the battery.

The difference is that the magnetic resonance method adds a high-frequency driving power supply and adopts an LC resonance circuit that combines a coil and a capacitor, rather than a simple coil that constitutes the power transmission and reception units.

The basic working principle of magnetic resonance method

The value of the resonant frequency changes with the distance between the power transmission and receiving units. When the transmission distance changes, the transmission efficiency will also decrease rapidly, just like electromagnetic induction. For this reason, the resonant frequency can be adjusted by the control circuit so that the circuits of the two units resonate, that is, "resonate". Therefore, this magnetic resonance state is also called "magnetic resonance".

By changing the transmission and reception frequencies under the control of the control loop, the power transmission distance can be increased to about several meters, while the resistance of the two unit circuits is reduced to a minimum to improve the transmission efficiency.

Of course, the transmission efficiency is also related to the diameter of the sending and receiving electrical units. The larger the transmission area, the higher the transmission efficiency. The current transmission distance can reach about 400mm, and the transmission efficiency can reach 95%.

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3. Microwave method

The power is transmitted using a 2.45GHz radio wave generator, and the transmitting device 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 and converted into DC power by rectifier circuits to charge the car battery, and can achieve point-to-multipoint long-distance transmission.

Microwaves can transmit data from one point to multiple points over long distances at the same time

To prevent microwaves from leaking out during charging, the charging part is equipped with a metal shielding device. During use, the effective shielding between the power transmission and reception can prevent microwaves from leaking out.

The main problem at present is that the efficiency of magnetron in generating microwaves is too low, causing a lot of electricity to be converted into heat energy and wasted.

Application of contactless charging devices in Japan

In July 2009, Nissan and Showa Aircraft Company unveiled an electromagnetic induction contactless charging system with a transmission distance of about 100mm and a transmission efficiency of up to 90%.

However, when the parking position is deviated and a large error occurs between the sending and receiving disks, the power transmission efficiency will be seriously affected. Currently, research is being conducted to ensure that the transmission efficiency is above 90% when the parking position has a lateral and longitudinal deviation of 200 to 300 mm.

a) Charging working state, the upper part is the vehicle-mounted part, and the lower part is the transmission part

b) Vehicle-mounted receiving device assembly

Electromagnetic induction non-contact charging device developed by Showa Aircraft Co., Ltd.

In addition, the two companies also studied the adverse effects of animals and metal fragments entering between the transmitter and the receiver. This is because such foreign matter can generate eddy currents between the two, causing heat and affecting transmission efficiency.

Nagano Japan Radio Co., Ltd. announced in August 2009 that it had developed a charging system based on magnetic resonance. Compared with the electromagnetic induction method, the magnetic resonance method has the advantages of long transmission distance and low parking error requirements. It can ensure 90% transmission efficiency within a transmission distance of 600mm. However, the current transmission power is still relatively small (about 1kW), and it is planned to enter the market from the use range of forklifts, etc. With the improvement of technical maturity and transmission power, it is expected to soon enter the field of electric vehicle charging.

The microwave contactless charging system developed by Mitsubishi Heavy Industries uses a group of 48 silicon rectifier diodes as receiving antennas. Each silicon rectifier diode can generate a voltage of 20V and a certain amount of direct current, which can increase the voltage to the required indicator for charging and achieve a power output of 1kW. Its advantage is low cost, and the cost of the whole set is about RMB 20,000. The disadvantage is low transmission efficiency, and the current transmission efficiency is only 38%. In this regard, Mitsubishi Heavy Industries believes: "Although it is not suitable for fast charging, as a night valley charging, the electricity cost is only 10% to 20% of the traditional fuel cost. If the magnetron with excessive heat is used to heat domestic water, the overall efficiency can reach 70%. In addition, in terms of safety, there is also a device to prevent microwave leakage, which will not affect the electronic equipment on the vehicle and the pacemakers of the surrounding personnel during use.

As soon as the contactless charging method came out, it has received widespread attention from countries around the world, and it is also worthy of learning and reference by domestic peers. Compared with the construction investment of charging stations and charging piles, the cost is lower, and the operation and waiting time required for wiring are eliminated. It has absolute advantages such as flexible layout, convenient use, safety and reliability.