Magnet-based wireless EV charger could be smaller, safer and cheaper

March 11, 2014
The new magnet-based wireless EV charger system has operated for about a year, avoids the usage of power cords.

Electric car owners' least-favorite feature is probably the battery-charging cord. After all, no one is fond of having to remember to plug in their car every evening or fumbling around with the power cord each time.

It's no surprise then that several companies such as Delphi, Infiniti/Nissan, Qualcomm, Plugless Power, Rolls-Royce, and WiTricity have either developed or tested wireless technology that requires motorists merely to park their EVs over a device that's embedded in a garage floor or parking space, enabling them power up without even leaving their seats. Many auto industry marketing experts believe, in fact, that wireless battery charging is perhaps needed for widespread adoption of EVs in the U.S.

Existing EV charging systems are based on a well-known technique called electromagnetic induction to accomplish the task. A varying electric current in a conducting wire coil (transmitter) that is placed in the road bed produces via resonance a similarly varying current in a noncontacting coil (receiver) sitting just above in the vehicle. The transferred electrical power then feeds into the battery, recharging it.

Electromagnetic induction is not considered by all to be absolutely ideal, however, because it can emit stray radio waves or heat up nearby metal objects (via ohmic heating) unless it is engineered correctly—two issues that evoke perceived public safety concerns. The industry strongly affirms, it must be said, that their power transfer technology has been fully tested and shown to be completely safe.

Researchers at the University of British Columbia (UBC) in Vancouver believe that they have developed a better, simpler way to wirelessly charge EVs. They have produced a safe, high-efficiency method that employs what inventor Lorne Whitehead, an applied physicist there, calls remote magnetic gears.

The new UBC noncontact power-transfer approach, Whitehead says, relies on magneto-dynamic coupling, or MDC—a magnetic field interaction between two rotating permanent magnets that are separated by an air gap of 4 to 6 in (102 to 152 mm). The system consists of a magnet on the transmitter side of the air gap between the road and the car, and another magnet on the receiver side. When a small electric motor turns the lower magnet, the upper one is caused to turn, "much as a compass follows a changing magnetic field," he explained. The top magnet then drives a small generator that charges the car battery.

"This magnetic field between them," he continued, "essentially acts as a mechanical coupling—an invisible magnetic pulley/belt system, but it requires no direct contact and is almost perfectly energy-efficient."

In production, Whitehead added, the two magnets can be integrated, respectively, into the motor and the generator, making for a reliable and compact (better for packaging) power-transfer system. In addition, relatively cheap ferrite magnets can be employed rather than more costly and potentially supply-challenged rare-earth permanent magnets.

Tests show the system is more than 90 percent efficient compared to a cable charge, and perfect alignment of the car with the device is not needed. The new-fangled EV charging technology would be incorporated into low street curbs over which the car-borne magnet (which is installed under hood) would hang.

For around a year or so, four demonstration MDC systems have operated successfully at UBC, wirelessly charging electrically powered campus-service vehicles that have been retrofitted, Whitehead noted. He expects that the patent-pending technology will be licensed to other manufacturers, probably through a spin-off company.

"One of the major challenges of electric vehicles is the need to connect cords and sockets in often cramped conditions and in bad weather," said David Woodson, Managing Director of UBC Building Operations. "Since we began testing the system, the feedback from drivers has been overwhelmingly positive—all they have to do is park the car and the charging begins automatically."

Other experts on wireless EV charging technology doubted the ultimate practicality of the UBC development. "It's really nothing new," said John M. Miller, Center Director in the Power Electronics and Electric Machinery Research Group at Oak Ridge National Laboratory in TN. "Too many moving parts; it all comes down to the number of energy conversions that are involved. Here, it goes from electrical to mechanical to electric field and back to mechanical and then to electrical," the electrical engineer explained. Each step entails a loss in energy efficiency that adds up.

A top maker of automotive recharging equipment also was similarly skeptical. "First, let me emphasize that our technology is fully safe," says David Schatz, Vice President of Sales & Business Development at WiTricity in Watertown, MA. "We crossed that threshold two years ago in certifying that to the satisfaction of the carmakers," who must concern themselves with potential liability issues beforehand. "The codes and standards are being written now."

"Leading edge automakers are designing wireless EV technology that will be produced and introduced around 2015 to 2017," he said. The biggest interest is to place them in so-called plug-less hybrids, Schatz continued. In addition, the industry expects the forthcoming devices to "be approaching the size and shape of a sheet of paper and lightweight to boot." WiTricity's power-transfer technology, which has no moving parts, exploits a special-design, high-efficiency coupling between transmitter and receiver—a one-step energy transfer.

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