IEEE Power Electronics Magazine - March 2014 - 15

N

Coefficient Coupling (#)

(a)
Coefficient of Coupling, k, 380-mm Coil,
Nc = 7 Turn, f = 48 kHz
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
50

Chassis
Ground

75

100
125
150
Coil Separation, d(mm)
(b)

175

Resistance, Rac (mX)

Gate Driver 2

HF
Transformer
Primary Coil
VP

T4

VS

C2

(a)

200

Litz Coil ac Resistance

160

Rac (mX)
Measured Data

140
120
100
80
60
40
20
0

10 20 30 40 50 60 70 80 90 100
Frequency (kHz)

fig 4 The measured and modeled R ac for a WPT coil. (Figure
courtesy of ORNL.)

All couplers are characterized in the laboratory for selfinductance, ac resistance, and coupling coefficient as a
function of gap. Figure 3 shows the circular coil and characterization results. The coil is planar spiral wound with
seven turns using cable guides interspersed with wedgeshaped ferrite flux guides. The ferrite plates are covered
with a Kapton sheet for voltage isolation.
The circular coil shown in Figure 3 was designed for
operation at 48 kHz and a working gap of 100 mm, which is
the ground clearance of the GEM EV. For this ground clearance, the coil should have a diameter of approximately four
	

T2

T3

OC
Comp
L-Aid

fig 3 (a) The circular coil and (b) characterization results.
(Photo courtesy of ORNL.)

0

Lc2

C1

Gate Driver 1

ac 240 V

T1
Power
Factor Corrector

Lc1

(b)
fig 5 (a) A schematic view of an HF power inverter and
tuned primary coil. (b) The experimental HF SiC power
inverter. (Photo courtesy of ORNL.)

times the gap, d, or 400 mm. The results of coupling coefficient, k, testing are shown in Figure 3, where three independent test methods were used. Note that at the working gap
of 100 mm, both the open circuit and compensated methods
are in excellent agreement, while the inductance-aiding
method obtained using an Agilent instrument is somewhat
lower [2], [3]. A comprehensive list of WPT references that
cover the main aspects of wireless and IPT is included in the
"References" section. The coupling coefficient is defined as
the ratio of secondary coil captured flux to the primary coil
total flux generated by a specified current, taken as 10 A rms
in these laboratory characterization tests.
Coil resistance is a dominant contributor to coupler inefficiency, plus the core loss contribution of the soft ferrite
materials used to guide flux and minimize fringe fields. Figure 4 shows the results of ac resistance testing on the planar
coil of Figure 3 at both 22 and 48 kHz. The experimental fit
to characterization data is given as (1) and shows that R ac
increases at the square of the frequency for this design. In
later designs, the exponent was reduced to 1.4 and lower.
The importance of R ac will become evident in later discussions on efficiency. The key components of frequencydependent R ac shown in Figure 4 include R dc + R skin + R prox,
where the proximity effect resistance R prox depends strongly
on the cable type (i.e., Litz cable bandwidth) and spacing.
	

f kHz 2
R ac = R dc ;1 + 0.147 a 20 k E + R prox .(1)
March 2014	

z	IEEE Power Electronics Magazine	

15



Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2014

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