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NCSU team proposes new system for dynamic wireless EV charging

Researchers from North Carolina State University are proposing a new topology appropriate for dynamic wireless charging—i.e., wireless charging on the go. Currently, at peak efficiency, the new system can transmit energy at a rate of 0.5 kilowatts (kW). The team’s goal is to move from 0.5 kW into the 50 kW range.

The system, outlined in a paper published in IEEE Transactions on Power Electronics, features a transmitter made from segmented multiple coils, each of which broadcasts a low-level electromagnetic field and is powered by a single inverter. A receiver coil that is the same size as each of the transmitter coils is placed in a car or other mobile platform. (The size of the coils is important, because coils of the same size transfer energy more efficiently.)

The proposed system uses the reactance reflected by the receiver automatically to increase the field strength in coupled portions of the transmitter-receiver system, thus allowing efficient power transfer and adherence to electromagnetic field emission standards without complex shielding circuits, switches, electronics and communication.

When the receiver comes into range and couples with a transmitter coil, that specific transmitter coil automatically increases its current—boosting its magnetic field strength and the related transfer of energy by 400%. The transmitter coil’s current returns to normal levels when the receiver passes out of the range of the transmitter.

The power transfer is at its peak when the transmitting and receiving coils approach their maximum coupling (as defined by the geometrical constraints of the system), resulting in improved system-level efficiency.

We’ve made changes to both the receiver and the transmitter in order to make wireless energy transfer safer and more efficient.

—Assistant Professor Dr. Srdjan Lukic, senior author

These modifications improve on previous mobile, wireless power transfer techniques.

One previous approach was to use large transmitter coils. But this approach created a powerful and imprecise field that could couple to the frame of a car or other metal objects passing through the field. Because of the magnetic field’s strength, which is required to transfer sufficient power to the receiver, these electromagnetic field “leaks” raised safety concerns and reduced system efficiency.

Another previous approach used smaller transmitter coils, which addressed safety and efficiency concerns. But this approach would require a very large number of transmitters to effectively “cover” a section of the roadway, adding substantial cost and complexity to the system, and requiring very precise vehicle position detection technology.

We tried to take the best from both of those approaches.

—Srdjan Lukic

Lukic and his team have developed a small, functional prototype of their system, and are now working to both scale it up and increase the power of the system.

The research was partially supported by National Science Foundation grant number EEC-0812121.


  • Kibok Lee, Zeljko Pantic, and Srdjan Lukic (2013) “Reflexive Field Containment in Dynamic Inductive Power Transfer Systems” IEEE Transactions on Power Electronics doi: 10.1109/TPEL.2013.2287262



Highway lanes, streets or selected sections equipped with this type of wireless charging could extend EVs range without having to carry 100+ kWh of expensive batteries.

Of course, each user would have to pay for the energy used. Something like $0.25/kWh would not be exaggerated and could produce enough revenues to make such facilities profitable?

Bob Wallace

Modest range EVs would cover well over 90% of all driving days. People can fill their batteries at home, often with cheaper off-peak electricity.

Paying 25c/kWh for the > 100 mile distance part of long drive days would be quite acceptable. That would be about 7.5c per mile for the miles once the battery charge was used up.

The average US car now gets ~24 miles to the gallon so driving with $3.5 gas costs 15c per mile. A 50 MPG hybrid burning fantasy $3/gallon gas would cost 6c.

Henry Gibson

The majority of automobile trips are less than 40 miles, and the Durathon battery can be used for such distances. Automobile fuel use can be cut by up to half without any electric vehicles if hydraulic hybrids are used. Philips proposed a system of very high temperature molten salts that could operate a Stirling engine for many miles. Durathon and ZEBRA batteries are now providing cellphone towers with reliable cheaper electricity and are in volume production. Perhaps China will recognize the need to make them even cheaper and they can be used for 20 years of service. Flywheels tested in race cars can be used for acceleration. Range extender generators need not be large or efficient because they will not be needed often in most electric vehicles. Nature has done billions of years of testing energy storage and use. Hydrocarbons work well enough.

The ultimate automotive fuel known and used right now is Isotope 238 of element 94. It powers the large automobile on Mars and keeps the twin small ones warm. It has killed far few people than gasoline or diesel, probably none, and you never have to refuel. It cannot produce flames to ignite a vehicle. One pound of this material would allow an electric automobile to travel 6 to 12 miles every day. A super high efficiency Stirling engine would work day and night to convert the the 220 watts of heat from the pound of material into as much as 2.5 kWh into the batteries for as much as 12 miles of travel.

People give off natural radioactivity and another person in the automobile will expose you to more radioactivity than this properly contained source of heat which loses half of its energy about every 90 years.

More than one pound can be used, but one pound would fuel a bicycle at full human speed continuously with a gradual loss of speed over 90 years. In raw form the pound would be a cube slightly more than an inch on a side, but would be used in a far different shape for efficiency.

It should be remembered that any frequency energy from a coil can and does heat people and other animals. Arrange three sausages on a plate in a nearly closed triangle and expose them to microwaves and sparks will jump between them. This is hundreds of volts. ..HG..

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