http://dx.doi.org/10.11142/jicems.2015.4.1.90
90
Journal of International Conference on Electrical Machines and Systems Vol.4, No.1, pp.90~94, 2015
The Influence on Characteristics of Movable Loosely Coupled
Transformer from Metal Units in Urban Railway System
Yajie Zhao*, Yumei Du*, Hua Cai*, Ruihua Zhang*, and Liming Shi*
Abstract – Contactless power transfer (CPT) system is widely used in high power recently.
The characteristics of contactless transformer in CPT system have powerful influences on
the performance of whole system. This paper studies the performance on movable loosely
coupled transformer (MLCT) for CPT system, which can be used in rail transit systems.
Making use of 3D finite element methods, this paper investigates how much influence on
MLCT produced by metal units in urban railway system, including the fixing units on the
vehicle and the metal units on the ground. The results of such studies have great practical
significance when the CPT system applied to urban railway system.
Keywords: Loosely coupled transformer, Multi-secondary, Contactless power transfer
1. Introduction
The traditional method of power supply is contact power
supply, which used cable or other contact equipment to
realize the power transmission. However, this method has
the defect off friction, sparks, risk of electric shock, and
high maintenance cost [1]. Thus, contact power supply will
reduce the performance of security and stability, and
shorten the lifetime of the power supply system. These
shortages are very serious especially in applications of
industrial production, biomedicine, and transportation.
Nowadays, rail transit system has been widely used for
transportation. As for disadvantages of contact power
supply mentioned above, CPT system is an alternative
proposal for rail transit system since it works without a
third rail or overhead line and the system is spark and
residue free, water and dust proof, reliable, flexible and safe.
Fig. 1 shows the topology of movable contactless power
transfer (MCPT) system. It contains the ground part and
Fig. 1. Topology of MCPT system
* Key Laboratory of Power Electronics and Electric Drive, Institute of
Electrical Engineering, Chinese Academy of Sciences, Beijing
China.(zhaoyajie@mail.iee.ac.cn)
Received 14 March 2014; Accepted 16 October 2014
vehicle part; the primary winding is powered by converter
located on the ground and the secondary winding picks up
the power and then transfers it to vehicle load [2].
There are about four kinds of MLCT for MCPT system
at home and abroad. First one is with magnetic core on both
the primary and secondary coils of transformer, which is
suitable for movable load under the low power application
area [3]; the second one only has magnetic core on the
secondary with different shapes. The primary coils always
be designed as lead rail and the secondary coils are around
the magnetic core. It reduces the number of magnetic core
and manufacturing costs, and is suitable for linear
motion [4]. The third one reduces all magnetic core on the
basis of the second kind. Although it reduces more
manufacturing costs, it also reduces the coupling ability of
MLCT and make the performance of MCPT system
poorer [5]. The last one designs secondary coils around the
circular primary coils. It is suitable for curvilinear
motion [6].
Fig. 2. Typical MLCT system
Considering the advantages and disadvantages of these
types above, this paper colligates the second and third type.
It is shown in Fig. 2.To satisfy the power supply while the
train is moving, primary coils of MLCT are lengthened and
Yajie Zhao, Yumei Du, Hua Cai, Ruihua Zhang, and Liming Shi
buried under the ground, and the secondary coils of MLCT
is fixed on the vehicle to pick up power and drive the
vehicle. By designing reasonable primary coils structure
and taking a primary segment control method, the loss of
cost by converters and inverters can be made up very well.
The CPT system with MLCT has been studied and
applied by many research institutions around the world.
Maglev train used MCPT system with the third kind of
MLCP when it’s running at a speed below 100km/h in
Switzerland and Germany [7]. A sightseeing bus is powered
in 600W used the structure that this paper had been
research out. The bus applied in park by electronics
research center, university of Auckland, New Zealand [8].
In addition, Institute of Electrical Engineering, Chinese
Academy of Sciences has already set up an experimental
platform of MCPT with transfer power of 20kVA [9].
MLCT, whose characteristics almost directly decide the
performance of the whole system, is the most important
part of the MCPT system. However, under the real
operating situation, there are always a lot of metal units
both at ground and at the vehicle like railway track and
fixing devices. They change the magnetic circuit of
transformer, and further influence other parameters, such as
inductance, frequency, and output voltage of transformer.
This results the change of the MCPT system’s performance.
2. Natural Frequency Analysis
A. Model
In order to keep the output voltage more stable when the
primary of MLCT supplies the power segmented, a
structure of multi-secondary is adopted [10]. The structure
of MLCT is shown in Fig.3. It is the ideal base model of
this paper. The red parts are the primary coils, called P1 and
P2; the yellow parts are the secondary coils, called S1 and
S2; the gray parts are the ferrite cubes. With this kind of
structure, the output voltage can be smoother and steadier
compared with the structure of only one secondary coils.
Fig. 3. The structure of movable loosely coupled
transformer
91
Some parameters of MCPT system in this paper are
shown in Table 1. The system gets power from three-phase
voltage. So, set 380V AC voltage as input voltage of
system Uin. The typical voltages of railway system are
750V, 1000V DC. In this paper, the target output voltage of
system Uout. Is 750V DC. Considering the performance of
switching devices in high frequency situation, the transfer
frequency f is 23kHz. According to these parameters, a
specific MLCT model can be built. Table 1 shows some
parameters of it.
Table 1. Parameters of MPCT system and MLCT
MPCT system
MLCT
Uin
Uout
f
Np
Yp
Wp
Ns
Ys
Ws
d
380V
750V
23kHz
2turns
5884mm
694mm
3turns
1134mm
694mm
90mm
Np and Ns are the primary and secondary coils turns
respectively; Yp and Ys are the primary length and secondary
length respectively; Wp and Ws are the primary width and
secondary width respectively; d stands for the air gap
between primary and secondary.
The other models with metal units are built based on the
structure above. Fig. 4 (a) and (b) show the models of
different metal units (green part) added to base model.
Fig. 4 (a) takes the railway track into consideration, and
Fig. 4 (b) takes the fixing devices on vehicle.
Fig. 4. Models of different metal units added
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The Influence on Characteristics of Movable Loosely Coupled Transformer from Metal Units in Urban Railway System
The model of metal units in this paper strictly accords
with the real situation. So the railway track uses the model
of GB 50kg/m. The distance between railway track and the
primary coils is 388mm in the direction of X axis. The
fixing devices connect secondary coils and vehicle through
a kind of epoxy resin board, and the distance between them
is 20mm in the direction of Z axis.
B. Theoretical Analysis
The traditional transformer is strongly coupled between
primary coils and secondary coils because the magnetic line
of force is closed through iron core of transformer. It has
small leakage flux and high coupling coefficient (generally
between 0.95 and 0.98). Being different with traditional
transformer, loosely coupled transformer has poor coupling
ability since a large part of magnetomotive force is
consumed in the air through air gap. As one kind of loosely
coupled transformer, MLCP has high leakage flux, high
reluctance and low coupling coefficient. Due to this, MLCT
has low transfer efficiency; its equivalent circuit should be
built by mutual inductance model. Besides, high frequency
current introduces high impedance of the winding, which
decreases the system efficiency. Compensation capacitors
are used in the primary resonant circuit to improve such
condition through decreasing the apparent capacity of the
system, and increasing the received power in the secondary
resonant circuit. Typical compensation topologies are series
capacitor, parallel capacitor and other complex resonant
circuit. This paper use series capacitor in both primary
circuit and secondary circuit. The equivalent circuit is
shown in Fig. 5, the dotted rectangular is the MLCT.
Fig. 5. The equivalent circuit of MLCT
Up is the high frequency source. Lp and Ls stand for total
inductance of the primary windings and secondary
windings respectively. M is the mutual inductance between
primary and secondary. Rp and Rs stand for total resistance
of the primary winding and secondary winding respectively.
Re is the equivalent resistance of load. f is the resonant
frequency, and ω is resonant regular frequency. Cp and Cs
are chosen as (1).
1
1
f =
=
(1)
2π Lp Cp 2π Ls Cs
From Fig. 5, the voltage of equivalent load is given as (2).
jω I p Re
(2)
U RL =
1
+ Rs + Re
jω Ls +
jωCs
It shows that the frequency and the mutual inductance
influence the voltage of equivalent load. As mentioned
before, taking switching devices in high frequency situation
into consideration, the transfer frequency is controlled in
very small fluctuations. Load voltage URL is basically
proportional to mutual inductance M. As a result, M
between primary and secondary coils is an important
parameter and deserves further study.
Besides, the efficiency of MCPT system can be
calculated from Eq.(3). In conclusion, on one hand, the
addition of metal units influences the magnetic field and
temperature field distribution. On the other hand, metal
units will change M, then have influence on URL, output
power Pout and system efficiency.
P
η = out
(3)
Pin
3. Simulation and Analysis
Ansoft Maxwell 3D is helpful software using 3D finite
element methods to solve electromagnetic field problems.
In this part, it is used to set up the model of MLCT
described above, then to calculate magnetic field distribution,
self-inductances of primary coils and secondary coils, and
mutual inductances between primary coils and secondary
coils. At last, the fluctuations from the metal units in
railways are obtained through the simulation.
Besides, Power Simulation (PSIM) is a kind of
simulation software in the field of power electronics and
motor control. In this paper, it is used to study some
electrical characteristics of the whole MCPT system such as
output voltage, output power and efficiency.
A. Influence on Magnetic Field Distribution
The simulation model is built as Fig. 3 and Fig. 4 (a) and
(b). In order to simulating real working environment, the
material of metal units is steel 1008 in the simulation. To
make sure of that the three models work in the same
environment, the simulation have the same condition of the
excitation source, the boundaries, the mesh operation, the
analysis setup, and the parameters.
Fig. 6 shows the different magnetic field distribution of
three models. (a) is the magnetic field distribution of ideal
base model; (b) is the magnetic field distribution of model
Yajie Zhao, Yumei Du, Hua Cai, Ruihua Zhang, and Liming Shi
with railway track; (c) is the magnetic field distribution of
model with fixing devices.
From Fig.6, it can be seen that the existence of metal
devices changes the original distribution of the magnetic
field to a certain extent compared (b) and (c) with (a).
However, it is obviously that the railway track has a very
little influence on magnetic field; and the fixing devices
influence magnetic field strongly. Seen from the values of
magnetic field intensity in Fig.6, it can be known that the
magnetic field intensity of ideal base model ranges from
about 0.11T to 0.37T and the maximum value is less than
0.55T. The magnetic field intensity of model with railway
track ranges from about 0.12T to 0.35T and the maximum
value is also less than 0.55T. Magnetic field intensity on
railway track is less than 0.002T and can be ignored. The
magnetic field intensity of model with fixing devices ranges
from about 0.15T to 0.46T. What should be noticed is that
the magnetic field intensity on railway track is range from
0.85T to 1.32T. It will change the self inductance and
mutual inductance of the transformer.
93
also studied in this paper. The parameters of MLCT in
PSIM are based on the results of Ansoft Maxwell 3D,
RL=3.75Ω. Table II shows the result of calculation.
From Table 2, it can be seen that when the system
working steady under the same condition, the ideal base
model has a higher output voltage, output power and
efficiency. When the system uses the model with railway
track, URL decreases about 0.167%, Pout decreases about
0.176%, and Pout decreases about 1.18%. When the system
uses the fixing devices, URL decreases about 3.73%, Pout
decreases about 7.10%, and Pout decreases about 0.30%.
Table 2. Output parameters of MCPT system with different
model
Ideal base
Model with
Model with
model
railway track fixing devices
URL(V)
700.25
699.08
674.16
Pout(kW)
13.056
13.033
12.133
84.55
83.55
84.3
η (%)
Compared the two model with metal units, it can be seen
that railway track on the ground has small influence on
output voltage and output power, but has a little bit big
influence on the efficiency. Fixing devices on the vehicle
decreases output voltage and output power seriously, but
have some influence on the efficiency.
Fig. 7 shows the current of S1 and output voltage of
MCPT system. IRL_ideal, Is1_FT and Is1_FD are currents with
three model of MLCT, and URL_ideal, URL_FT and URL_FD are
Fig. 6. The magnetic field distribution of different model
B. Influence on Output Power of MCPT system
On the foundation of simulation above, the output
voltage, output power and efficiency of MCPT system are
Fig. 7. The Is1 and URL of three model MCPT system with
different load
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The Influence on Characteristics of Movable Loosely Coupled Transformer from Metal Units in Urban Railway System
voltages with three model of MLCT. It can be seen that
when RL=6Ω, the fixing devices have a relatively large
influence on output voltage compared with the situation of
RL=3.75Ω. Besides, the currents have good shapes when
RL=3.75Ω, that result few harmonic of output. Because the
metal units change the inductance of MLCT, the resonant
frequency of the system also be changed. That makes the
output frequency of current varied with different models.
4. Conclusion
This paper focuses on the influence on characteristics of
movable loosely coupled transformer from metal units in
urban railway system. By setting up the model and
theoretical analysis, it is known that the output voltage pf
system is almost proportional to mutual inductance. The
influence from metal units on magnetic field distribution,
the value magnetic field intensity and the output parameters
of system are researched. As a result, fixing devices on
vehicle has a bigger influence on magnetic field distribution
and the value magnetic field intensity than railway track on
the ground. And railway track has bigger influence on the
efficiency and small influence on output voltage and power.
Fixing devices decreases output voltage and output power
seriously, but have samller influence on the efficiency. The
results show that the influence from these metal units can
not be ignored and play an important role on the output
characteristic of the MCPT system. These results also make
a foundation on the further experiment.
References
[1] Wu Ying, “Research on new contactless power supply
system.” Ph.D. dissertation, Institute of Electrical Engineering,
Chinese Academy of Sciences, China, 2004.
[2] Cai Hua, Shi Liming, and Li Yaohua, “Harmonic-based
phase-shifted control of inductively coupled power transfer.”
IEEE Transactions on Power Electronics, Vol.29, No.2, pp.
594-602, 2014.
[3] A. W. Kelley, and W. R. Owens, “Connectorless power
supply for an aircraft-passenger entertainment system.” IEEE
Trans on Power Electronics, Vol.4, No.3, pp. 348-354,1999.
[4] J. M. Barnard, J. A. Ferreira, and J. D. van Wyk, “Sliding
transformers for linear contactless power delivey.” IEEE
Trans on Industrial Electronics, Vol.44, No.6, pp. 774-779,
1997.
[5] J. M. Barnard, J. A. Ferreira, and J. D. van Wyk, “Optimising
transformers for contactless power transmission systems.”
IEEE PESC 1995, Vol. 1, pp. 245-251, 1995.
[6] A. Esser, and Hans-Christoph Skudelny, “A new approach to
power supplies for robots.” IEEE Transactions on Industry
Applications, Vol.27, No.5, pp. 872-875, 1991.
[7] Wang C., Stielau O. H., Covic G. A., et al., “Design
Considerations for a Contactless Electric Vehicle Battery
Charger.” IEEE Transactions on Industrial Electronics, Vol.52,
No.5, pp. 1308-1314, October 2005.
[8] E. Abel, and S. M. Third, “Contactless Power Transfer an
Exercise in Topology.” IEEE Transactions on Magnetics,
Vol.20, No. 5, pp. 1813-1815, September 1984.
[9] Li M, Chen Q, Hou J, et al., “8-Type contactless transformer
applied in railway inductive power transfer system.” Energy
Conversion Congress and Exposition (ECCE)2013, pp. 22332238, 2013.
[10] Ruihua Z, Ying Z, and Hua C., “Study and experiment of
large power linear contactless power supply system for
moving apparatus.” Electrical Machines and Systems
(ICEMS), pp.1-4, 2011.
Yajie Zhao was born in 1989. She is
currently working towards the Master's
degree in electric engineering from Institute
of Electrical Engineering, Chinese Academy
of Sciences, Beijing China. Her current
research interests are power electronic and
contactless transformer.
Yumei Du was born in 1964. She received
M.S and Ph.D. degree in electrical
engineering from Institute of Electrical
Engineering, Chinese Academy of
Sciences, Beijing China. Her research
interests are electrical machine , magnetic
levitation and linear drive.
Hua Cai was born in 1987. He received
the Ph.D. degree in electrical engineering
from Institute of Electrical Engineering,
Chinese Academy of Sciences, Beijing
China in 2015. His research interests
include contactless power supply,
converter and control.
Ruihua Zhang received Ph.D. degree in
electrical engineering from Institute of
Electrical Engineering, Chinese Academy
of Sciences, Beijing China in 2004. Her
research interests are power electronic,
contactless power supply and linear drive.
Liming Shi was born in Henan, China, in
1964. He received the Ph.D degree in
1998 from Kyushu University, Fukuoka,
Japan. His current research interests are
analysis and control electrical machines,
contactless power supply.