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.
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