IEEE Electrification Magazine - September 2013 - 59

an Incomplete History of eV
Wireless Charging

The history of the wireless power
transfer began in 1891 when Nikola
Tesla invented his famous Tesla coil or
magnifying transmitter. The system
contains two loosely coupled and
tuned resonant circuits: a primary and
a secondary. The coils were built using
large, single-layer solenoids, which significantly reduces the coil resistance
and increases the quality factor. The
primary and secondary coils were
tuned using an external capacitor and
the parasitic self-capacitance, respectively. Periodic spark gap discharges
were used to short out the primary resonant circuit and
initiate the power transfer. Even with the significant spark
losses, the Tesla coil was able to transfer power with 85%
efficiency. Tesla's experiments demonstrate the majority of
modern IPT design concepts: 1) Tesla applied the strongly
coupled resonant circuit to enhance the power transfer
capability of the system, 2) he used the self-capacitance to
tune the secondary and to obtain a high quality factor, and
3) he used the spark discharge over the air gap to control
the power in the resonant circuit, similar to how modern
resonant converters use electronic switches.
The next milestone in vehicle dynamic charging took
place in 1894, when Hutin and LeBlanc filed a patent that
describes a transformer for powering streetcars without
contact. The proposed system included a single-wire
elongated primary coil carrying 2-kHz ac and coupled by
multiple secondary windings. They used ferromagnetic
materials and suspension systems that lower the receivers to increase the coupling. Although the proposed
topology has some similarities to modern solutions,
because of component limitations at the time, the system was not a commercial success.
In the 1990s, researchers at the University of California,
Berkeley, built a proof-of-concept roadway-powered
35-passanger electric bus. The complete infrastructure
was built for a 213-m-long test track with two 120-m
powered sections. The bipolar primary track was supplied
with 1,200-A, 400-Hz ac current and coupled to a receiver

Compensation
Tank

Components of the Ipt system
A typical IPT system consists of two physically detached
subsystems with power transfer through induction.
Typically, the system supplying the power is stationary
and is named the primary, transmitter, or source. The system receiving the power is attached to a movable frame
and is named the secondary, pickup, or receiver. The power
is transferred via induction between two magnetically
coupled coils, much like in a transformer. The coupling
medium between the coils is air, which has a much higher magnetic reluctance than do the ferromagnetic materials used in transformers. As a result, the coupling coefficient is in the range of 0.1-0.2 for stationary charging
applications and less than 0.1 for midrange resonant
applications. Therefore, these systems are usually
referred to as loosely coupled systems to distinguish
them from the tightly coupled transformer coils.

Compensation
Tank

Power
Conditioner

Load

Ferromagnetic

High-Frequency
Power
Supply

with an area of 4.3 m2, at a distance
of 7.6 cm. The system efficiency was
around 60%. These results proved the
potential of the technology but were
limited by the size of the system due
to the very low operating frequency.
Researchers at Auckland University
laid the theoretical groundwork in the
1990s for much of the research that is
presently ongoing in the design of
wireless chargers. It is worth noting
their recent achievement in designing
the optimal pad for the stationary
charging of EVs. One of the designs is
a 766 mm × 578 mm pad that delivers
7 kW of power with more than 90%
efficiency at a distance of 20 cm. They also proposed using
multicoil track designs for dynamic charging applications.
Starting in 2008, researchers at the Korea Advanced
Institute of Science and Technology (KAIST) have built
several prototypes of roadway powered EVs, which they
named online EVs. Three generations of IPT systems have
been developed, and three different vehicles have been
tested, with system efficiency peaking at about 70%. In
each generation, a different structure of the ferromagnetic
material and a different track layout has been designed.

The longer-term
vision for wireless
charging is to enable
the power transfer
between the grid
infrastructure and
the vehicle while the
vehicle is moving.

Ferromagnetic
Figure 2. The typical topology of a high-power IPT system.

IEEE Elec trific ation Magazine / s ep t em be r 2 0 1 3

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2013