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Wireless Power Transfer for Moving Vehicles Proposed by NC State Researchers

November 13, 2013 by Jeff Shepard

Researchers from North Carolina State University have developed new technology and techniques for transmitting power wirelessly from a stationary source to a mobile receiver – moving engineers closer to their goal of creating highway "stations" that can recharge electric vehicles wirelessly as the vehicles drive by. The researchers developed a series of segmented transmitter coils, each of which broadcasts a low-level electromagnetic field. The researchers also created a receiver coil that is the same size as each of the transmitter coils, and which can be 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. These modifications improve on previous mobile, wireless power transfer techniques.

The researchers modified the receiver so that when it 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 percent. The transmitter coil’s current returns to normal levels when the receiver passes out of the range of the transmitter.

“We’ve made changes to both the receiver and the transmitter in order to make wireless energy transfer safer and more efficient,” says Dr. Srdjan Lukic, an assistant professor of electrical engineering at NC State and senior author of a paper on the research.

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,” Lukic says. 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. Currently, at peak efficiency, the new system can transmit energy at a rate of 0.5 kilowatts (kW). “Our goal is to move from 0.5 kW into the 50 kW range,” Lukic says. “That would make it more practical.”

The complete paper describing this research, “Reflexive Field Containment in Dynamic Inductive Power Transfer Systems,” is published online in IEEE Transactions on Power Electronics. Lead author of the paper is NC State Ph.D. student Kibok Lee. The paper was co-authored by Dr. Zeljko Pantic, a former Ph.D. student at NC State. As discussed in the paper: Single-coil designs simplify system control, and provide a relatively constant coupling coefficient as the vehicle moves in the design space, but they suffer from three key drawbacks. First, for high power applications, the elongated coil requires compensation capacitors to be distributed along the coil to maintain the voltage below prescribed standards. Second, the field emitted in uncoupled sections of the coil needs to be contained to ensure that emission standards are not violated. Third, the resulting coupling coefficient is fairly low, due to the large uncoupled flux of the transmitter coil, which results in low total efficiency.

With a segmented transmitter coil design, the issues of field containment, large transmitter coil self-inductance and difficulties with coil impedance compensation can be addressed. Reduction in coil segment size exacerbates the issues and advantages associated with coil segmentation: small coils can further contain the leakage flux, thus improving the coupling and efficiency, but results in a complicated design with many bypass switches and sensors.

Still, developing a segmented transmitter coil design that contains the field by only powering the coils that are coupled with a receiver is quite challenging: First, there is a need for precise receiver position feedback to identify which coils should be powered. Second, a method to energize or bypass coils is needed. One option is to power each coil with a dedicated inverter, which would allow complete control over the power transfer; however, this approach is typically cost prohibitive especially in applications where a large area needs to be powered, as for example, in the EV dynamic charging application.

The segmented transmitter coil design with co-measurable transmitter and receiver coils has found application in low-power designs, where the bypass switches are not as complex or as expensive. For example, in a mesh of coils has been proposed for wireless charging pads of portable electronics. The transmitter is composed of multiple unloaded coils that cover the entirety of the area where the receiver is expected to be placed, with each unloaded coil commeasurable with the receiving coil. This system requires a position identification algorithm and uses a set of relays to energize only the coils that are best coupled with the receiver, allowing free-positioning of the receiving coil.

The authors observed that the main benefit of the proposed system over other methods of powering a sectionalized transmitter coil is that it obviates the need for receiver position sensors and for switches required to energize selected portions of the sectionalized coil. As a result, the field strength in the uncoupled potions is reduced, and there is no need for complex methods of field containment proposed in other work. It should be noted that this current research simply introduces a new compensation circuit, and that therefore this approach can be used in conjunction with other segmentation methods that have been proposed, as well as methods to contain the leakage flux in dynamic IPT systems.

“We envision this dynamic IPT system to operate by delivering power in pulses to a constant dc load as the receiver moves along the sectionalized transmitter coil. Power flow regulation can then be controlled by modulating the number of pulses rather than by changing the power and energy transferred in each pulse,” the authors concluded. The research was partially supported by National Science Foundation grant number EEC-0812121.