Developing a dynamic wireless power transfer system that supplies electricity to the car while drivingDeveloping a dynamic wireless power transfer system that supplies electricity to the car while driving

December 24, 2021

Developing a dynamic wireless power transfer system that supplies electricity to the car while driving
―More freedom to drive electrified cars―

(Illustration only. Image does not represent an actual production vehicle.)

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At the Frontier Research Center of Toyota, we are developing car chargers that make it easier to drive battery electric vehicles (BEVs) and plug-in hybrid electric vehicles (PHEVs). When people hear the word ‘charging,’ they usually think of traditional contact-type chargers that use special plug-in cables to charge cars in a parking lot. Well, there are several different types of charging available, including swappable batteries that get replaced with fully charged batteries, and wireless charging that enables batteries to be charged without cables. Here, we are also developing a dynamic wireless power transfer system that charges the cars while driving.

Unlimited range BEVs

Dynamic wireless power transfer uses devices embedded in roads, on highways or at city intersections, to charge cars while driving or parking on those roads. This system eliminates the inconvenience of plugging in charger cables and frees the user from range anxiety if their battery runs out of charge. Also, because batteries are generally expensive and heavy, some people say vehicles like large trucks and buses that need long cruising ranges are not suited to BEV technology. However, if we can realize this dynamic wireless power transfer system, vehicles that require a lot of energy to drive will no longer need to be equipped with large capacity batteries and we will be able to move even more people and things in the future.

  • Dynamic Wireless Power Transfer function layoutDynamic Wireless Power Transfer function layout
    Dynamic Wireless Power Transfer functional layout

Many issues remain

Although dynamic wireless power transfer offers great convenience, it is a system yet to be realized. There are a number of technical issues that remain.

1. Strength: Devices must be strong enough to withstand being run over by heavy vehicles like trucks.
2. Power loss: The amount of energy consumed by the chargers during wireless charging must be reduced.
3. Magnetic flux: The chargers must generate low levels of magnetic flux during wireless charging to avoid causing malfunction of nearby electrical devices.

This report focuses on our current development of a system that generates low levels of magnetic flux (3 above) to eliminate impact on navigation systems, car radios and other devices in the car during wireless charging, and on devices near the roads where chargers are embedded, such as smartphones and pacemakers used by pedestrians.

  • Magnetic flux leakage due to chargingMagnetic flux leakage due to charging
    Magnetic flux leakage due to charging

Good old ‘right handed screw rule’

So, what is the principle on which wireless power transfer is based? One wireless charging device that everyone would be familiar is a smartphone that can be charged wirelessly just by placing the phone on the charger. Actually, smartphone charging and vehicle charging both rely on the same principle of charging using the right handed screw rule.

  • Right handed screw ruleRight handed screw rule
    Right handed screw rule

The right handed screw rule is the familiar ‘thumbs up!’ hand sign, with the thumb representing an electric wire (1), the direction the thumb is pointing representing the direction of current flow, and the other fingers representing the direction of magnetic flux generated when current is flowing.
You can also imagine the wind generated by a passing train, with the magnetic flux (electrical wind) in the case of electricity wrapping the electric wire in a clockwise direction. Conversely, according to this rule, if the magnetic flux generated above hits another nearby electric wire (2), a current will flow in accordance with the size and direction of that magnetic flux.

  • Current and Magnetic FluxCurrent and Magnetic Flux
    Current ⇒ Magnetic FluxMagnetic Flux ⇒ Current

Wireless charging can be achieved by (1) attaching a wire to a charger (ground assembly) embedded under the road surface and (2) using the magnetic flux and current generated to charge the batteries of smartphones or cars equipped with another wire.
However, the electrical wind of this magnetic flux may adversely affect other nearby smartphones or devices, so one of the most important elements of the dynamic wireless power transfer method is to prevent leakage of the magnetic flux as much as possible.

Coils are electric wires formed effectively to generate and receive magnetic flux, with wires wound around in a circle and current passed through to produce a strong magnetic flux (electrical wind) from the center, like an electric fan.
Looking at the relationship between the energy transmitter and receiver, when the coil of each is located close to each other, of the same size, and perfectly centered, it is much less likely that the magnetic flux will leak.

  • CoilCoil
    Coil example
  • Conditions with little magnetic flux leakageConditions with little magnetic flux leakage
    Conditions with little magnetic flux leakage
  • Conditions with a lot of magnetic flux leakageConditions with a lot of magnetic flux leakage
    Conditions with a lot of magnetic flux leakage

To wirelessly charge a smartphone, coils of the same shape are placed about 1 cm apart. Magnets are then used to fix positions and ensure the coils remain centered during charging. In the case of cars, a certain clearance must be maintained between the road surface and the bottom of the car to get over bumps (at least 9 cm according to the Japanese shaken car inspection system) and the size of the coils on the car must be small enough to avoid adding too much weight to the car. Also, because the ground assemblies are not cheap, it is preferable that the full width of the assemblies in the direction of travel (see the following diagram) is used for charging, even to the edges where magnetic flux leaking is more likely. For this reason, parameters applied to these ground assemblies allow much more magnetic flux leakage than those for smartphones.

  • Positional deviation in the traveling direction of the vehicle while drivingPositional deviation in the traveling direction of the vehicle while driving
    Positional deviation in the traveling direction of the vehicle while driving

Adjacent Cancel Coil

The Adjacent Cancel Coil (ACC) concept that we are developing uses coils located in adjacent positions to cancel out magnetic flux. In this way, even if there is some misalignment, both the vehicle coils and road surface coils themselves will cancel any magnetic flux leaked from the opposite coils.
Generally speaking, when electric current is passed separately through two wires to generate magnetic flux, the directions of each magnetic flux align and cooperate when the wires are close together, but they apply an opposing force to cancel each other out when the wires are further apart.

  • Cooperate in close proximity and Cancel at a distant positionCooperate in close proximity and Cancel at a distant position
    Cooperate in close proximity and Cancel at a distant position

ACC employs two adjacently located coils, separated by a distance, to cancel out magnetic flux as explained above. The magnetic flux generated around each of the two middle wires has the effect of canceling the magnetic flux generated around each of the two outside wires. By applying ACC to both the ground and vehicle assemblies, each of the coils cancels any magnetic flux leaked to the outside of the coils to help reduce levels of magnetic flux.

  • Developed coil and current directionDeveloped coil and current direction
    Developed coil and current direction
  • Loop to cancelLoop to cancel
    Loop to cancel
  • Reduces leakage magnetic flux by cancelingReduces leakage magnetic flux by canceling
    Reduces leakage magnetic flux by canceling

World-leading levels achieved with standard for fundamental waves

The International Standard CISPR 11 Class B prohibits magnetic flux above certain levels for devices that employ magnetic flux for wireless charging. The standard specifies maximum strength levels of magnetic flux for various frequencies, which is an electric property. These frequencies include fundamental waves, which are those frequencies in the targeted range when setting up and operating the coils, and harmonics, which are those frequencies in the non-targeted range of magnetic flux. The strength of magnetic flux generated by fundamental waves is determined by the type of coil used, while the strength of magnetic flux generated by harmonics is determined by the device (electric circuit) that operates the coil. We scoured the world for examples of achieving the standard for the purpose of dynamic wireless power transfer, but found none. Therefore, the coil that we developed is the world’s first case of achieving the standard for fundamental waves (250 mm separation of coils and 25 kW of charging power).

  • evaluation of dynamic wireless power transferevaluation of dynamic wireless power transfer
  • evaluation of dynamic wireless power transferevaluation of dynamic wireless power transfer
  • kanesaki and Hashimoto from Toyota Motor Corporation, and a member from Nagaoka University of Technologykanesaki and Hashimoto from Toyota Motor Corporation, and a member from Nagaoka University of Technology
One act of EMI evaluation
Pictured above right: Masaki Kanesaki (author of this report, on the left), a member of Nagaoka University of Technology (joint research partner, in the middle), and Makoto Hashimoto (Electronics Control System Development Division, on the right)
  • Result:Achieved the standard value of the fundamental waveResult:Achieved the standard value of the fundamental wave
    Result:Achieved the standard value of the fundamental wave

Our battle continues

The coil we developed is able to achieve the standard for fundamental waves, but we are still unable to achieve the standard for harmonics. We are currently collaborating with the Nagaoka University of Technology on a study of the electric circuits that produce harmonics. This also is a world’s first trial that we are agonizing over, but we will definitely succeed in achieving the magnetic flux standard for all frequency ranges.
We are still facing a number of challenges, including improving strength, reducing power loss, and reducing costs, but we will continue our research to determine whether this dynamic wireless power transfer system is more rational and lower impact, as social infrastructure, than other charging methods.

Masaki Kanesaki

Author: Masaki Kanesaki

Assistant Manager, Energy Group No.1, R-Frontier Division
Masaki Kanesaki graduated from the Department of Electrical, Electronics and Information Engineering at Nagaoka University of Technology, where he specialized in power electronics, which seeks to reduce energy consumption and improve performance of electrical devices.
He has led the dynamic wireless power transfer project as development leader since December 2019. After one year of development, he was able to achieve the world’s first device that meets the CISPR 11 Class B magnetic flux standard for fundamental waves. He has contributed to 30 patent applications to date. At the same time, he has launched a project that seeks to accelerate the spread of renewable energies through the use of BEVs. Kanesaki dreams of creating the foundations for sustainable society over the next 100 years or longer.

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