Finite Element Assisted Numerical Comparison of Single and Two Phase Inductively Coupled Power Transfer Systems

doi:10.4236/jemaa.2013.57048

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Journal of Electromagnetic Analysis and Applications, 2013, 5, 312-315

http://dx.doi.org/10.4236/jemaa.2013.57048 Published Online July 2013 (http://www.scirp.org/journal/jemaa)

Finite Element Assisted Numerical Comparison of Single

and Two Phase Inductively Coupled Power Transfer

Systems*

Pratik Raval#, Dariusz Kacprzak, Aiguo Patrick Hu

Department of Electrical and Computer Engineering, The University of Auckland, Auckland, New Zealand.

Email: #[email protected]

Received May 13th, 2013; revised June 13th, 2013; accepted June 21st, 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

Inductively coupled power transfer systems (ICPT) are becoming ubiquitous in industry. Many such systems are excited

with single or multi-phase input current. This leads to increased complexity in comparing such systems when solely

using the magnetic frequency analysis. This paper utilizes modern finite element method analysis software to propose a

novel software methodology for the numerical comparison of single and two phase ICPT systems as demonstrated on a

three dimensional (3D) battery charging system. The sinusoidal magnetic frequency response of a single phase system

is compared to the magnetic transient response of a multi-phase cu rrent system by u se of a novel software methodology

proposed in this paper. This consists of a transient response analysis to determine compute the resulting magnetic re-

sponse over the duration of an input current period on the two phase system. The resulting non-sinusoidal response is

then integrated over a whole period to extract the root-mean-square value for comparison with that of a single phase

system across a 3D cubic power zone.

Keywords: Finite Element Method; Electromagnetics; Magnetics; Induction; Wireless-Power-Transfer

1. Introduction

The technique termed inductively coupled power transfer

(ICPT) is a wireless-power-transfer technique that trans-

fers power across an air-gap by means of magnetic in-

duction [1]. This technique removes the inconveniences

caused from physical wires by providing power in hard

to reach places where conventional direct electrical con-

nections are inconven ient, hazardou s, or impossible, low-

ers maintenance requirements as there is less wear and

tear from wet, dirty, moisturized and hazardous envi-

ronments, and provides enhanced safety as it is free of

sparking and can be used in potentially explosive atmos-

pheres and supports freedom of mechanical movement of

any load(s) as opposed to a localized load(s). Naturally,

this has led to many ICPT applications including mono-

rail systems [2,3], people mover or transportation appli-

cations such electric cars, trains and buses [4], biomedi-

cal implantation and more recently low power consumer

battery charging applications [5]. However, a majority of

current such applications largely only support unidirec-

tional and/or bidirectional load movement. That is, there

is no support for three-dimensional (3D) load movement

along orthogonal axes. This is due to a lack of generation

of an omnidirectional primary link that provides a 3D

power transfer window. This paper uses the developed

3D cage—like primary magnetic structures that compose

a unique 3D ICPT system. Furthermore, depending on the

application requirements, the primary AC excitation cur-

rents in these systems may be applied as single or multi-

phase. For analysis, such excitation of primary currents

must be numerically computed using advanced modern

finite element method (FEM) analysis software as in [6].

The FEM software being used is JMAG designer.

This paper proposes a methodology for the numerical

comparison of a single and two phase 3D battery charg-

ing ICPT system in terms of the resulting magnetic field.

2. Proposed ICPT System

The battery charging system operating at 155 kHz is il-

lustrated in Figure 1. The power converter inverts a DC

input to distribute an A C power transfer window through

the primary track coils. Typically, the power converter

*The author declares no conflict of interest.

#Corresponding author.

Finite Element Assisted Numerical Comparison of Single and Two Phase Inductively

Coupled Power Transfer Systems 313

Figure 1. Battery charging ICPT system.

may consist of a p rimary compensation network with the

purpose of providing a resonant AC current. This current

is fed into the primary magnetic winding structure. The

objective of the primary magnetic structure is to distrib-

ute magneto-motive force (MMF) uniformly throughout

the entire cubic power transfer volume. The secondary

magnetic structure, often termed a pick-up, induces an

AC voltage. This voltage is often weak and noisy. So,

typically pick-up compensator networks are used to make

the induced signal stronger. The result is rectified before

operating a load. The presented single and two phase

systems are excited with an equivalent ampere-turns ratio

[7]. This ensures that an equal MMF is input in ampere

current-turns to produce the same magnetic flux density

within a pre-defined cubic power transfer volume of

1920 cm3. The only differences between the two systems

presented next are the orientatio n of the primary winding

structures and the input phase of the primary excitation

currents.

3. Single Phase System

The single phase system is shown in Figure 2. This sys-

tem consists of several over-layed planar rectangular

winding structures that are vertically displaced. This gen-

erates a predominant normal or vertical component in

magnetic flux density. The winding structures are sur-

rounded by a ferromagnetic material casing. This ferrite

material acts as a magnetic circuit with three main func-

tions. Firstly, to provide a low magnetic reluctance path-

way to the magnetic flux vectors to guide the flux

through the ferrite and then disperse the field back into

the intended power zone. Secondly, to confine the mag-

netic flux thereby reducing the flux vectors leaked out-

wards in an effort to reduce flux leakage so as to reduce

Figure 2. Single phase system (Left) and MFD vectors

(Right).

potentially harmful electromagnetic interference. Thirdly,

to enhance the field with a high relative permeability of

1200. In this single phase system, the AC sinusoidal ex-

citation primary currents are applied simultaneously and

in phase to the primary track coils. This produces a net

vertically directed with the simulated magnetic flux den-

sity (MFD) distribution vectors also shown in Figur e 2.

4. Two Phase System

The two phase system is shown in Figure 3. This system

consists of two Helmholtz coil pairs along orthogonal

axes. The first Helmholtz coil pair is input with an AC

sinusoidal primary current that is in phase but of opposite

polarity. The second Helmholtz coil pair is similarly in-

put with AC sinusoidal primary current that has a 90 de-

grees phase difference to the current applied in the first

Helmholtz coil pair. That is, the primary currents are

applied sequentially and in phase quadrature to parallel

but adjacent coil windings so as to produce an elliptical

rotating magnetic field resulting from fields





0B and





90B. The resulting winding structures are su rrounded

by ferromagnetic material casing. In this case, the ferrite

acts as an electromagnetic attractor that essentially at-

tracts the flux from unlike magnetic poles. This aids the

propagation of the magnetic field from one side of the

box to the other that in essence distributes the flux

throughout the 3D cubic power transfer volume. The

resulting instantan eous MFD distribution vectors are also

shown in Figure 3.

5. System Comparison

The numerically computed parameter of comparison

MS is shown below. This is defined as the root-

mean-square (RMS) value of the averaged magnitude

magnetic flux density





Bavg throughout the cubic

intended power zone and N is the number of steps in the

transient response. The intended power region to com-

puting the cubic average is illustrated in Figure 4.



RMS i

BBa

N

vg (1)

For a single phase system,

B is readily derived

Finite Element Assisted Numerical Comparison of Single and Two Phase Inductively

Coupled Power Transfer Systems

314

Figure 3. Two phase system (Left) and MFD vectors (Right).

Figure 4. Intended powe r zone : Cubic volume.

via the simulated magnetic frequency responses instant-

taneous peak due to the sinusoidal nature. However, for a

two phase system this parameter is no longer sinusoidal

so a transient response system model as a simulation

methodology is developed as in Figure 5. Firstly, a tran-

sient response analysis is performed on the two phase

model consisting of 400 steps corresponding to time in-

tervals of 32.25805 ns. Next, the cubic averaged MFD is

computed in the cubic volume. Finally, an RMS integra-

tor is performed on the resulting signal, shown in Figure

6, to attain the parameter of comparison

The resulting comparison of B.

MS is shown in Table

1. This result shows that the single phase system is 5.8%

more efficient at distributing the MMF throughout the

intended cubic power transfer volume. It is expected that

this novel numerical FEM computable comparison tech-

nique may be applied to other ICPT systems and multi-

phase systems. More specifically, this methodology of

comparison overcomes a limitation in the FEM software

by manual determination of the net peak electromagnetic

wave in a multi-phase system that exhibits a non-sinu-

soidal response over a 3D power transfer volume.

6. Conclusion

This paper has proposed a novel FEM software method-

ology for the numerical comparison of single and two

Figure 5. Two phase system software methodology.

Figure 6. Transient Response of Two Phase System.

Table 1. Comparison of single and two phase ICPT sys-

tems.

ICPT systems input current phase:



RMS

B:

Single phase 33.590

Two phase 31.745

phase ICPT systems via a transient response analysis

where the resulting signal is not of sinusoidal nature over

a 3D power transfer volume in terms of a performance

governing metric.

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