IEEE Electrification Magazine - March 2020 - 42

Electromagnetic Design of a Dynamic
Wireless-Charging System
A wide range of system configurations for dynamic wireless charging have been proposed in the literature, and
several different concepts have been deployed in existing
demonstrators. However, a dynamic-charging system can
be implemented by using rather simple coil shapes while
still obtaining operational characteristics and a performance that are largely comparable to more complex
arrangements. For the small-scale demonstration, rectangular-shaped planar coils were employed for their simple
structure. A plate of magnetic material (ferrite) was placed
behind each coil to improve the quality factor as well as
the mutual coupling, ensuring a reasonably high powertransfer efficiency. As usual in resonant inductive powertransfer systems, Litz wires were used for the coils to limit
losses at the target resonant frequency.
The size of the onboard coil is largely determined by
the available space on the vehicle. On the other hand, the
design of the coils for the roadside infrastructure has a
fundamental degree of freedom in the length of each section that can be independently energized. Long sections
result in a simplified power supply (a lower number of
power-electronics converters and/or a simpler switching

network) but normally result in a lower efficiency. Moreover, long energized sections can create challenges for
controlling the electromagnetic emissions and can be difficult to manage when vehicles that are not equipped for
wireless charging are mixed with charging vehicles in the
normal traffic flow.
Small, discrete roadside coils-ideally, with the same
size as the onboard coil-yield the highest theoretical efficiency when they and cars are perfectly aligned. However,
frequent end-coil effects that result from vehicle motion
cause a pulsating power flow and effective reduction of
the overall efficiency and total energy transfer. End-coil
effects can be avoided by overlapping adjacent coils (multicoil systems) at the expense of additional materials and
a more complex power supply.
The tradeoff between the coil length and efficiency can
be qualitatively and quantitatively evaluated in the case of
simple rectangular coils. As an example, consider a roadcoil section of length l 0 that is at least twice the length of
the onboard coil in the direction of travel, having selfinductance L P,0 and resistance R P,0 . Extending the coil
length to l 2 l 0 results in new inductance and resistance
values, given approximately by

Maximum Transfer Efficiency

0.9

0.7
0.6

0.5
0.4
0.3

l/l0

Figure 2. The impact of the roadside coil length on the maximum
power-transfer efficiency of dynamic wireless-charging systems.

TABLE 1. The assumed specifications of a

full-scale dynamic-charging system for an
electric truck.

l
;
l0

Parameter

Specification

Nominal power, P0

200 kW

Onboard coil maximum planar size

1.4 × 1.4 m

Road coil maximum width

1.4 m

Road coil length

5-15 m

Coil-to-coil minimum clearance

0.3 m

I E E E E l e c t r i f i cati o n M agaz ine / MARCH 2020

L P ( l ) . L P, 0 $

l
& Q P ( l ) . Q P,0 .(3)
l0

The quality factor of the road coil is therefore approximately independent of its length. The mutual flux is also
roughly unbound by the road coil's length, since the latter
is much longer than the onboard coil. The coupling factor
can then be expressed as

k0 = 0.15
k0 = 0.1
k0 = 0.05

0.8

R P ( l ) . R P, 0 $

k( l ) =

M( l )
. k0 $
L P( l ) $ L S

l0
.(4)
l

The change in the maximum efficiency due to the
extended length can be evaluated by the fundamental
equation (1). Some numerical examples are shown in
Figure 2 by assuming reasonable values for the quality
factors and three different values for the initial coupling coefficient.
For designing the small-scale demonstration platform,
reference target specifications for a full-scale dynamiccharging system were assumed according to Table 1. The
specifications are based on the general expected requirements in terms of the power consumption for a future
electrified heavy vehicle, its actual size, and the necessary
clearance between the receiving coil mounted on the
vehicle and the road surface. A wide range of values was
allowed for the road-coil length, as different design concepts should be tested. The operating frequency of the
system has not been specified, as no standard has been
established for such systems. Although most conventional
static wireless electric-vehicle charging systems operate in
the common frequency window of 80-90 kHz, it is possible
that systems requiring considerably higher power levels

IEEE Electrification Magazine - March 2020

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