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One of the technologies used in electric cars is the in-wheel motor, a motor installed in the car wheels themselves to propel the vehicle. Such motors have the advantage of dispensing with the need for differential gears or drive shifts, but their weak point is the cable between the motor and the battery. This cable supplies the motor with electricity, but is prone to breaking, which can cause a loss of control in vehicles in motion. Looking for a solution to this problem, a team led by the Graduate School of Frontier Sciences, University of Tokyo has developed the wireless in-wheel motor. On August 4th, JQR observed a test drive of a vehicle equipped with this technology.
Associate Professor, Graduate School of Frontier Sciences, University of Tokyo. Chair of the Society of Automotive Engineers of Japan Electric Power Technologies Division.
Research interests: control engineering, motion control, electric vehicle control, high performance control of motors and inverters, wireless power supplies.
The test site was located on the campus of the Graduate School of Frontier Sciences, University of Tokyo in the city of Kashiwa, Chiba prefecture. Upon arriving at the site we saw a parked electric car with wireless in-wheel motors in the rear wheels. When the driver got in and stepped on the accelerator the car, looking rather unfinished with its rear wheels exposed, moved off smoothly. Since we were inside university grounds the speed limit was 30 km/h, but the car drove just like any other, and being electric it hardly made any noise.
When the car stopped and we had a look at the driving motor, we saw a gap the size of a fist between the axle and body of the car. Apart from the joints and suspension supporting the axle, there appeared to be no wires or anything else connecting the wheels to the car. This empty space seemed quite bizarre. How could the motor run if it wasn’t connected to anything? In fact, enough electricity to propel the car is sent wirelessly through that gap. According to project leader Associate Professor Hiroshi Fujimoto, “If you raise the voltage it can reach speeds of up to 75 km/h.”
Merits of the In-Wheel Motor
Applying a driving force directly to the wheels has the merit of being highly energy efficient, and there has been ongoing research on in-wheel motors for the wheels of electric cars. Controlling all four wheels individually according to the state of each makes it possible to reduce electricity consumption and extend the maximum driving range. It also has the potential for better responses to skidding and other sudden movements. It is no exaggeration to say that this will be a vital technology in the future because of its safety and energy efficiency benefits.
However, the tendency of the cables to break and disrupt signals and the supply of electricity from the battery to the motor has been a long-running impediment to practical applications.
・Developed a motor that could transmit power wirelessly
・Succeeded in creating the world’s first drivable electric car with wireless in-wheel motors
・Achieved a stable power supply through control technology
・Placed all driving equipment in the wheels
Safety is naturally the number one requirement for a car, and it is mandatory that all parts must be durable for a period of ten years or more. From this point of view the cables are a major weak point. Wheels in motion are affected by bumps on the road surface and move irregularly, flicking up stones. They travel over asphalt that might be frozen by snow or baked in the sun. It is difficult to develop cables that are guaranteed not to break in such extreme and harsh conditions.
While pondering this issue, it occurred to Fujimoto that the problem could be solved if the cables were removed. If power could be sent from the battery to an in-wheel motor the same way that Wi-Fi operates with the internet, there would be no need for cables.
Fujimoto immediately contacted Assistant Professor Takehiro Imura, who was researching wireless power transmission, and in the autumn of 2012 they began to work on the development of a wireless in-wheel motor with the cooperation of Toyo Denki, a transportation systems manufacturer, and NSK Ltd., a bearings manufacturer.
Magnetic Resonance Coupling and SiC Converter
After many challenges the team completed a prototype. They held a test drive and press conference on May 18, 2015, which was attended by many representatives from the media.The most remarkable thing about the technology the team presented was that it could propel a car using the wireless transmission of electricity. While the technology to transmit power wirelessly to a battery in a stationary electric vehicle was already in existence, as Fujimoto explains, “It is far more difficult to send electricity from the body to the wheels of a moving car, and drive the motor with that power.”
The two technologies that made this possible were a power transmission method known as magnetic resonance coupling and SiC converter control technology.
Because moving wheels always move irregularly, as mentioned above, the alignment of the power transmission and receiving coils is always shifting. To solve this problem the team used magnetic resonance coupling, a technology being researched by Imura. Resonance capacitors installed on both the power supply coil and receiving coil created resonance that made it possible to transfer power at over 90% efficiency over a gap of 10 cm or more.
If a motor receives even slightly too much power, the excess is stored in the condenser and the voltage goes up. Conversely, if not enough power is supplied the condenser voltage drops, the motor stops and the car can’t move. As a result, supplying just the right amount of power, neither too much nor too little, and maintaining a uniform voltage are vital. This issue was solved with a circuit called the SiC converter circuit that controls power on the receiving side.
Electric cars can also have regenerative braking systems, in which the motor produces power during deceleration. Electricity generated by this method must be sent to the car body, so a symmetric circuit for transmission and reception was used to facilitate the transfer of power in both directions.
The Impact of Wireless
Technology on Electric Cars
“Improvements to wireless in-wheel motor technology may make it possible to recharge the car while in motion from supply coils embedded in the road. If this is achieved, you would only need enough power to get from your home to the expressway, and then you could keep driving indefinitely.”
This system of supplying energy wirelessly from power sources in the road will no doubt attract more and more attention in future. The British government has announced that it will conduct its own tests later this year. Basic experiments are already being conducted in Japan, and in South Korea buses with this technology have begun trial operations in specified zones.
With the technology to control power transmission even when the alignment of transmission and receiving coils shifts drastically, the development of electric cars can progress to the next level. The evolution of this technology should be well worth watching.
The main members of the group. “Working with Toyo Denki and NSK as a joint development group allowed us to create a car that could actually be driven,” says Hiroshi Fujimoto.
Graduate School of Frontier Sciences
The University of Tokyo
Kashiwanoha 5-1-5, Kashiwa-shi, Chiba
Tel: 04-7136-3873 (laboratory)