When electric vehicles (EVs) are so smart that they can do self-driving and self-parking and make route-planning decisions to save time and fuel, will you have to hire someone to plug them in and unplug them?
To that end, Omer Onar, the Vehicle Power Electronics group leader at Oak Ridge National Laboratory (ORNL) and his colleagues, in collaboration with Volkswagen Group of America, have created a 270-kilowatt wireless power transfer system that has successfully charged a Porsche Taycan.
The breakthrough was a refinement of—and a jump in power from—a similar 120-kilowatt demonstration system they developed for the DOE in 2018. The receiver coil on that version weighed a bit more than 114 pounds, was two inches thick, and more than two-and-a-half feet wide.
Because the size and heft of that prototype would be a deal killer for car manufacturers doing everything to keep their EVs light, the DOE asked Onar and his team to come up with something smaller, sleeker, and more powerful.
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The coils of existing wireless charging systems pulse power, from zero to peak, across an air gap between the transmitter and the receiver. That means they have a “very low space-time utilization of the magnetic field,” Onar said. In other words, their power rating is limited, making charging slow.
To overcome this, the ORNL researchers turned to a coil geometry that energizes three phases to generate rotating magnetic fields. The coils are designed to have three sections, shifting every 120 degrees. Each phase carries about a third of the power. “So, when you sum up those powers at any time from the three phases, you always have constant power transfers delivered from primary to secondary,” Onar explained.
The polyphase geometry also reduced magnetic field emissions. The magnetic field lives at the outer edge with a single circular coil. With the 120-degree design, it sits at the centre, so the system requires no shielding to remain compliant with international guidelines for electromagnetic field emission limits.