IEEE Power Electronics Magazine - June 2022 - 25
Two main families of cores are available for MV transformer
design: the powder type and the tape type. Although
the powder types are generally referred to as ferrites, a
variety of materials can be used in terms of loss and saturation
levels. One challenge is that ferrite cores are not easily
manufactured in larger sizes. Therefore, nowadays such
materials are mainly applied in low-power applications.
Additionally, ferrites usually have relatively low flux density
saturation levels (e.g., ~0.3-0.5 T). Tape type cores, in theory,
have unlimited size. Therefore, they can be produced in
much larger sizes than ferrites. The main material types for
these cores are amorphous, nanocrystalline, nickel iron, and
cobalt iron. The main core parameters are shown in Table 1. As
a summary, the XFC design challenges nowadays are more
on the materials instead of power electronics control.
Wireless Charging
In 2007, a group of scientists in MIT successfully delivered
60 W at 40% efficiency over a 2 m distance between coupled
coils of 30 cm radius [19]. They co-founded WiTricity
later which has been working closely with major automakers
like BMW and Hyundai by licensing its technology
and has demonstrated a series of wireless charging prototypes
for next generation vehicles. In 2018, BMW introduced
the 530e iPerformance, the world's first electric
sedan that is factory equipped with a wireless charger
using WiTricity's techniques.
Meanwhile, Qualcomm's Halo collaborated closely with
the University of Auckland and has developed a number of
coil pad geometries suitable for wireless EVs, including the
patent for the double D coil pad [20]. In 2019, it was acquired
by WiTricity with its over 1500 patents or patent applications.
Plugless Power provides wireless chargers with
3.6 - 7.2 kW power rating and allows customers to directly
install the charger on their vehicles. The supported models
include Telsa Model S, BMW i3, and Nissan Leaf. In 2021,
HEVO licensed a series of wireless charging technologies
from Oak Ridge National Laboratory (ORNL), including a
unique polyphase coil structure that enables a very high
coil surface power density at 1.5 MW/m2. HEVO is working
with ORNL to build a 300 kW system to meet the 15-min
charging goal for EVs with 100 kWh battery packs [21].
Images of wireless chargers
from different companies
are given in Figure 14, with their specifications
compared in Table 2. All IPT systems use inductive coupling
of a magnetic field between two coils. Controlled
Table 1. Comparison of Main Core Parameters.
Core type
Saturation induction at 20 °C (T)
Curie temperature (°C)
Core losses at 10 kHz (W/kg)
Saturation magnetostriction (ppm)
Ferrite MnZn
Powder
0.43
140
70.0
-0.6
Amorphous
(iron-based)
Tape
1.56
395
250.0
27.0
Amorphous
(cobalt-based)
Tape
0.57
225
4.0
1.0
Nano
crystalline
Tape
1.23
600
28.7
0.5
Nickel
iron (79%)
Powder/tape
0.88
450
50.0
12.0
Cobalt
iron (50%)
Tape
2.1
940
400.0
70.0
(a)
(b)
(c)
(d)
FIG 14 (a) Commercially available prototype from WiTricity. (b) Manhole-like charger from HEVO. (c) Electric bus from Momentum.
(d) Customized charger from Plugless Power.
June 2022 z IEEE POWER ELECTRONICS MAGAZINE 25
IEEE Power Electronics Magazine - June 2022
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