Magnetics Business & Technology - November/December 2023 - 18

RESEARCH & DEVELOPMENT
New Technology Makes Wireless Charging of Electric Vehicles and Ferries Attractive
Professor Yujing Liu shows the 500 kW inductive charging system which can be suitable for charging of electric ferries, trucks, and buses. DC (direct
current) to DC efficiency can reach 98 percent. It transfers 500 kW per 2 square meters with 15 cm air gap between the ground pad and the onboard
pad.
Researchers at Chalmers have developed an induction technology
that enables battery charging without the help of a human or
robotic arm. The technology is also so complete that it can soon be
presented to industry.
A new type of semiconductor based on silicon carbide. And a newly
developed copper wire, as thin as a human hair. These are a couple
of factors that suddenly made it more realistic to transmit high
power through air.
Charging induction is the new thing
Electric toothbrushes have been doing it for decades. In recent
years, cell phones and other portable electronics have picked up
the technology. But for the high power required to charge the batteries
in an electric vehicle, the wireless option has so far appeared to
be too complicated and ineffective.
However, charging with the help of induction now seems to be facing
a breakthrough even in the case of battery vehicles - above all
when charging needs to happen often and where the environment
is demanding. For example, for an electric city ferry.
Charge without a human or robotic arm
This would mean that electric ferries in regular traffic across waterways
in cities, which can be found in Gothenburg and Stockholm,
do not need the help of a person or robotic arm to charge batteries.
The same applies to city buses or driverless electric vehicles used
in industry, mining, and agriculture.
Yujing Liu, Professor of Electric Power at the Department of Electrical
Engineering at Chalmers, is particularly focused on renewable
energy conversion and electrification of the transportation system.
" You can have a system built into the wharf that charges the ferry
at some stops, at the same time as passengers get on and off.
Automatic and completely independent of weather and wind, charging
can take place 30 - 40 times per day. This is probably the most
Four times higher frequencies
" Previous systems for vehicle wireless charging have used frequencies
of around 20 kHz, much like a normal stove top. They became
bulky and the energy transfer was not very efficient. Now we work
with frequencies that are four times higher. Then induction suddenly
becomes attractive " , explains Yujing Liu.
He adds that his research group is in close contact with the leading
manufacturers of SiC modules in the world, two companies based
in the US and Germany respectively.
" With them, rapid product development takes place towards even
higher currents, voltages, and effects. Every two or three years,
new versions are launched, which can withstand more. These types
18 Magnetics Business & Technology * November/December 2023
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obvious application, " says Yujing Liu.
" Even for electric powered trucks of the future there is perhaps a
potential application. Then the motive is rather that these will have
to be charged with such high power that a charging cable becomes
very thick and heavy and thus difficult to handle " .
New possibilities thanks to the development of materials
According to Yujing Liu, it is the rapid development of a handful of
components and materials in recent years that has opened for new
possibilities.
" A key factor is that we now have access to high power semiconductors
based on silicon carbide, so-called SiC components. As
power electronic products, these have only been on the market for a
few years. They allow us to use higher voltage, higher temperature,
and much higher switching frequency, compared to classic, siliconbased
components " , he says.
This is important, because it is the frequency of the magnetic field
that sets the limit to how much power can be transferred between
two coils of a given size.

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