Toyota Plans to Employ Angular Coil for Wireless Power Transmission

Dec 12, 2012
Naoshige Shimizu, Nikkei Automotive Technology
In September 2012, Toyota showed a prototyped vehicle equipped with a wireless power transmission device. It was based on the Prius PHV. It seems that the company equipped the Prius PHV with the angular coil this time.
In September 2012, Toyota showed a prototyped vehicle equipped with a wireless power transmission device. It was based on the Prius PHV. It seems that the company equipped the Prius PHV with the angular coil this time.
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Toyota Motor Corp is considering employing an angular coil, which is made by winding a copper wire around a quadrangular plate, for its wireless power transmission technology under development.

Toyota prototyped a vehicle equipped with the angular coil and is evaluating its performance. The problem with angular coils is that it is necessary to reduce the leakage of electromagnetic waves to the extent that it meets radio laws. It seems that Toyota is now ready to meet such regulations.

There are two shapes of coils used for wirelessly transmitting electricity to vehicles: circular and angular shapes. A magnetic path is decided depending on the shape. Therefore, to efficiently pass magnetic fluxes, it is necessary to have the same coil shape on the power transmitting and receiving sides.

The two shapes are not compatible with each other, and they both have advantages and disadvantages. The number of automakers that are choosing a circular coil is almost the same as the number of makers that are choosing an angular coil.

Angular coil
An angular coil is made by winding a copper wire around a quadrangular, flat ferrite core. Magnetic fluxes are emitted from the sides of one flat core and received by the sides of the other flat core.

The advantage of this method is that power can be transmitted even when the coils are horizontally displaced to some extent. However, the amount of the leakage of electromagnetic waves tends to be large. Because magnetic fluxes are emitted horizontally from the sides of the lower flat core and received by the sides of the upper flat core, the magnetic path tends to horizontally spread.

It seems that Toyota prevents the magnetic path from spreading by making improvements to, for example, the packages of the coils.

Circular coil
A circular coil is made by placing a spiral copper wire that functions as a coil on a doughnut-shape, flat ferrite core, etc. Compared with an angular coil, the amount of electromagnetic waves leaked in the horizontal direction is small.

The magnetic fluxes emitted from the inside of the coil on the power transmitting side pass through the inside of the coil on the power receiving side. Because the magnetic fluxes pass from top down, it is easy to close the magnetic path by placing a flat electric conductor under the power transmitting coil and above the power receiving coil. On the other hand, this method does not allow the two coils to be horizontally displaced as much as an angular coil.