Wireless charging
Visualization of the inductive power transfer system during operation: black regions — coil windings of Litz-wire;  long brick shaped bars — ferrite bars with colors that indicate the magnitude of the magnetic flux density;  gray plates — copper plates for passive shielding with induced eddy currents; green curves of varying thickness — magnetic flux density lines where the thickness corresponds to the magnitude of the flux density.
Illustration: Johan Winges/ Thomas Rylander

Prototype for wireless charging of buses

Is it possible to charge electric buses through open air, without physically connecting any electrical equipment to the vehicle? A prototype for wireless charging of city buses is constructed by researchers at Chalmers. The first round of tests is promising.
Yujing LiuIn the laboratory at the division of Electric Power Engineering several prototypes for charging electrical vehicles are under construction and testing.

In this project, the researchers focus primarily on charging of electric buses operated in cities as they traffic pre-determined routes with specified stops that offer good charging possibilities. Frequent charging allows for substantial reductions in battery size, which lowers the weight and cost of the bus. Alternatively, frequent charging can be used to reduce the depth of discharge, which prolongs the lifetime of the batteries.

“The first round of tests on our 50 kW module has been completed in our laboratory and the results are promising so far”, says Yujing Liu, Professor at the department of Electrical Engineering. “The results show a transfer efficiency, from DC to DC, of about 95 percent at the desired power level, across an airgap of 20 centimetres, which is really good.”

Charging the bus from the ground
The wireless charging, or inductive power transfer (IPT) as the researchers call it, allows for contactless transfer of power across an air gap that extends from a charging unit in the ground, located at the bus stop, to a similar unit integrated in the vehicle frame of the electric bus.

The charging unit in the ground contains a coil which creates a magnetic field. In turn, this magnetic field induces a voltage in a similar coil embedded in the unit placed beneath the vehicle and this induced voltage yields a current that charges the batteries in the electric bus.

Thomas Rylander“Essentially, this part of the system is a conventional transformer but, as opposed to a typical transformer, the primary and secondary coils are separated by a relatively large air gap. The gap yields a rather low magnetic coupling and this is compensated for by adding capacitors to the coils such that we get resonance circuits on both the primary and secondary side”, says Thomas Rylander, Professor at the department of Electrical Engineering.

To charge the batteries in a bus would require about 200 kW, which will be made possible by connecting charging modules in parallel.

The possibility to charge city buses at bus stops, so called opportunity charging, may reduce the size of the battery in the bus, perhaps by as much as 70 percent. About 30 seconds charging at every other bus stop will be enough to keep the batteries at a sufficient charging level – just about the time it takes for passengers to get on or off the bus.

Thus, this charging method is different from the one used for the well-known Electricity bus trafficking route 55 in Gothenburg. Bus 55 is charged at the end stops using physical connectors on the roof.

Higher efficiency and reduced battery size
“The two major challenges that may limit the applications of inductive power transfer in electrical vehicles are the transfer efficiency and the size of the equipment”, says Yujing Liu. “However, the progress in fast-switching power electronics and high-frequency electromagnetic materials has led to new opportunities. We want to explore the benefits of using these kinds of new technology and high-quality materials for reducing losses and the size of the equipment.”

Using high electric frequency, it is possible to reduce the magnetic energy and leakage field, which is important for applications in public places like city buses.

The pictures show the first prototype of power electronics (to the left) and the primary side transformer (to the right) used for inductive power transfer. Illustrations: Felix Mannerhagen

“This experimental prototype for inductive power transfer is to be considered state-of-the-art today. It will serve as a platform for several future research projects”, says Thomas Rylander. “The work is interdisciplinary, both experimental and theoretical. The seed project funding from the department of Electrical Engineering has initiated an entirely new and very exciting collaboration for us.”

Facts about the project
Objective: To develop a prototype for wireless charging of electric vehicles, considered state-of-the-art, which can serve as a platform for future research projects in the field
Long-term purpose: To contribute to sustainable, competitive and efficient traffic solutions
Participants: Thomas Rylander, Yujing Liu, Tomas McKelvey, Torbjörn Thiringer, Felix Mannerhagen, Daniel Pehrman, Johan Winges
This seed project is based on the thesis "Multi-Objective Optimization of Inductive Power Transfer Systems for EV Charging” by Roman Bosshard, 2015.​

Text: Yvonne Jonsson
Photo: Oscar Mattsson

For more information, contact
Yujing Liu, Professor, Department of Electrical Engineering, Chalmers

Thomas Rylander, Professor, Department of Electrical Engineering, Chalmers

Published: Wed 23 May 2018.