Research Trends and Results

Wireless power supply system for running electricvehicles
OBARA Hiroshi, Senior researcher
YOKOJI Katsunori, Guest Research Engineer
Information Technology Division, Research Center for Advanced Information Technology(key word)electric vehicle,

noncontact electricity supply,

1. Electric vehicle
Electric vehicles available in the market today are
considered one of the most remarkable ways to approach
environmental issues we have, due to their highefficiency in energy utilization.
However, those electric vehicles have their own
restrictive conditions, such as “extreme increase inelectricity consumption caused by high speed run andtheir heavy weight” and “short running distance in orderto lower the price”. In order to improve these issues,much research has been carried out on improving
efficiency in electricity consumption, as well as
advancing storage battery technologies. These studies
have been effective in improving the usability and uptakeof electric vehicles currently available, but at the sametime, are limited to the technology in the vehicles
themselves. Instead, our focus here is to develop
technology to charge the battery by supplying electricity
to moving vehicles. For that reason, technologicaldevelopment on road facilities also plays an essential part.
The National Institute for Land and InfrastructureManagement has recently commenced basic researchusing several models, in partnership with Tokyo
University, in order to achieve this goal.
2. Wireless power supply technology
The most common way to charge battery for an electricvehicle today is using a cable, however, many studies
have been carried out on developing wireless charging
technology. While many of these use microwaves orelectromagnetic induction techniques, we have been
looking at the “magnetic field resonance system”,
recently introduced by MIT, as a way of establishing awireless power supply system on roads.
The benefits of the magnetic field resonance system are
its longer transmission distance and higher resilience topositional gaps, compared to the electromagnetic
induction technique. In addition, as opposed to themicrowave system which uses the far-field (the region
where the electromagnetic field disperses), it offers
magnetic coupling between transmitter and receiver in
the near-field (the region where electromagnetic wave
doesn't disperse but stays around the transmission device),thus we believe it to be an easier approach to provide
safety measures for optimizing controls and preventing
leaks of electromagnetic radiation.
3. Wireless power supply using a Magnetic
Resonance System
When installing a transmission system in a road, it iselectricity supply in running state

best not to change the current road format in order tomaintain the level of convenience for driving. Also, it ismost desirable to place the transmitter under the road, sothat stable power supply conditions can be achieved
while accommodating different car sizes, without
demanding special driving skills. For road structure andmaintenance reasons, it should be installed within theroadbed, more than 60cm deep, rather than in the asphalt
surface.
The magnetic field resonance system is regarded to be
excellent as a power supply technology for runningvehicles for the following reasons: (i) The magnetic field
being hardly affected by the gravel or asphalt used forbuilding roads, (ii) the distance between transmitter andreceiver can be as much as 1~2 meters, (iii) a higher
tolerance for positional gaps.
4. Supplying electricity for running vehicles
The majority of recent studies on wireless power supply
technologies target off-road electricity supply while thevehicle is parked; thus many of existing magnetic fieldresonance systems use transmitters and receivers of same
size and shape, which make them inappropriate forsupplying power to running vehicles. At the National
Institute for Land and Infrastructure Management, wehave verified transmission capability in an experimentusing a model, enabling a stable electricity supply on themove by using a transmission coil and a receiver coil thatare very different in size. (See the image below)

Image: The experimental model
In the future, aiming for a life-sized experiment, we will
work on improving transmission distance and efficiency,
as well as attempting to eliminate magnetic radiation
leaks to the surroundings. We will continue to develop
technology to enable electricity supply for runningelectric vehicles on the road.