15th International Power Electronics and Motion Control Conference, EPE-PEMC 2012 ECCE Europe, Novi Sad,
Serbia
Layout and Operation of a Non-Contact Charging
System for Electric Vehicles
Benedikt Schmuelling1, Sarp G. Cimen1, Thomas Vosshagen2, Faical Turki2
1
University of Wuppertal, E-Mobility Research Group, Wuppertal, Germany,
schmuelling@uni-wuppertal.de/cimen@uni-wuppertal.de
2
Vahle Inc., Kamen, Germany, thomas.vosshagen@vahle.de/faical.turki@vahle.de
Abstract — Actually, there is a controversial discussion in
automotive industry if the deployment of inductive power
supply is able to avoid the disadvantages of conductive
vehicle charging systems.
Available charging systems for electric vehicles use wires
to connect the vehicle to the grid. This leads to several
disadvantages such as vandalism and additional effort for
the driver. Furthermore, loose or faulty cables are a safety
risk. As an alternative, non-contact charging of electric
vehicles allows for a simple, reliable and safe charging
process. It improves the user acceptance and contributes to
the integration of electric vehicles into the market. Thus, the
availability to the grid is increased, which offers a higher
degree of flexibility for the storage management.
Keywords — Inductive power supply, electric vehicles,
adaptive frequency inverter.
I. INTRODUCTION
A major aim of the German government’s energy
concept is the reduction of CO2 emissions by increasing
the percentage of renewable energies in the federal
energy mix as well as promoting electro mobility (emobility) [1, 2]. If charged by renewable energies,
electric vehicles can significantly contribute to a
reduction of CO2 emissions. Additionally, they can be
employed to store and balance the fluctuating energy
production of renewable sources. Therefore, a high
availability of electric vehicles in the grid needs to be
guaranteed.
Available charging systems for electric vehicles use
wires to connect the vehicle to the grid. These systems
are called conductive supply systems. Compared to
conventional vehicles, driven by combustion engines,
conductively charged electric vehicles feature several
disadvantages. The perseverative handling with plug and
cable means an additional effort for the driver. Unclean
cables and plug-in activities during rainy weather at
public charging stations are a permanent annoyance.
Furthermore, free accessible cables are an invitation to
vandalism and unplugged or broken cables are a safety
risk, which is not acceptable. If the vehicle is not charged
at public areas, the grid connectivity time is shorter than
desired. The results are uncharged batteries, which cause
a minimization of mobility and a further reduction of the
available range of electric vehicles.
An alternative is non-contact charging on the basis of
inductive energy transfer. This technique allows for a
simple and reliable charging process. It is assumed, that
automatic inductive charging will improve the user
acceptance of electric vehicles in general. Furthermore, it
accounts for the integration of electric vehicles into the
market and supports the full exploitation of all e-mobility
based benefits. Moreover, the deployment of a noncontact solution causes a low deterioration compared to
plug afflicted systems.
This paper presents a solution for inductive vehicle
charging, which is called e-mobile contactless power
supply (eCPS). In a first section, the setup of the entire
system is introduced. Afterwards, the system’s inductive
parts and their compensation are described as well as the
operation principles and the configuration of the adaptive
frequency inverter. Measurement results of several
laboratory experiments and an analysis of the system’s
performance conclude the work presented.
II. SYSTEM DESCRIPTION
The eCPS system is a reasonable alternative to the
conventional vehicle charging via cable. It consists of an
on-board unit, which is directly mounted on the vehicle’s
underbody structure. To start the process of charging the
vehicle has to be parked above the stationary unit, which
can be placed at the central position of a parking lot, i.e.
directly beneath the parked vehicle.
eCPS consists of a number of components. The power
coupling between ground and vehicle is established by
the stationary transmitter unit and the on-board receiver
unit (called pick-up) mentioned above. The stationary
transmitter as a charging unit is linked via magnetic flux
with the pick-up. According to the principle of
electromagnetic induction, the pick-up can receive
electric power from the stationary transmitter, without an
electrical connection.
The transmitter is galvanically connected to the
primary inverter. This unit converts the grid voltage to a
higher frequency alternating voltage with a nominal
frequency of fN = 140 kHz. The advantages of an energy
transfer performed at high frequencies (f>100 kHz) are
already proven by a multitude of scientific examinations
[3-10]. The stationary power transmitter, called primary
part, generates a higher frequency (HF) electromagnetic
field. This electromagnetic field is coupled with the
windings of the pick-up.
Fig. 1: Simplified block diagram of the complete eCPS system.
To produce an appreciable part of active power in the
on-board system, a compensation of the system’s
reactances is required. Therefore, the primary inverter
unit is equipped with a capacitor assembly in series
connection to the primary coil. A similar compensation is
established at the on-board unit. The high frequent
alternating current of the pick-up is converted into a
direct current by a HF-rectifier, to be connected to the onboard charging device. Figure 1 depicts a schematic
diagram of the entire system.
III. THE INDUCTIVE SYSTEM AND ITS COMPENSATION
To transfer a higher performance (P>3000W), a PFC
(Power Factor Correction) device is integrated into the
inverter. This PFC provides the compensation of reactive
power to the grid. Reactive power on the primary side of
the primary inverter is usually caused by switching
distortion. The PFC produces a direct voltage of UDC =
400 V as input voltage for the primary inverter.
Fig. 4: Stationary transmitter and pick-up of the inductive charging
system.
Fig. 2: Structure of inductive power transfer system.
The inductive primary part consists of a flat coil with
several windings, which is mounted to a cover plate made
of glass fiber reinforced plastic, which has no influence
on the magnetic field. This assembly group is directly
placed above an aluminum plate populated with soft
magnetic ferrites. Basically, the construction principle of
the pick-up is the same. A difference between transmitter
unit and receiver unit only exists in size, in number of
windings, and in housing shape. The outer dimensions of
the pick-up unit are presented in Table 1.
TABLE I
DIMENSIONS OF THE STATIONARY TRANSMITTER AND THE PICK-UP UNIT
Stationary Transmitter
Pick-Up Unit
length
1089mm
825mm
width
1089mm
825mm
height
24mm
16mm
weight
approx. 49kG
approx. 14kG
The large dimensions of the inductive unit are caused
by the safety limit value of the electromagnetic field
defined in the German application guideline [11].
A schematic diagram of the inductive system’s crosssection is shown in Figure 3 and a photographic
illustration of that part is presented in Figure 4.
Fig. 5: The inductive’s part (i.e. stationary transmitter and pick-up)
equivalent circuit drawing depicting inductances and additional
capacitors.
The electrical transmission characteristic is described
by a transformer equivalent circuit drawing, which is
presented in Figure 5. In this drawing, L1σ is the primary
leakage inductance, L’2σ is the secondary leakage
inductance, and L1h is the mutual inductance. The
capacitors C1 and C’2 are the respective compensation
elements of primary and secondary winding. All values
marked with an inverted comma are not the measurable
values but related to the primary side by the ratio of the
primary and secondary winding factors w1/w2. The
secondary output voltage U2 is calculated as follows, for
example:
′
Fig. 3: Schematic diagram of the inductive system’s cross section.
(1)
Compensation of inductive power supply systems is
required due to their high demand of reactive power [12,
13, 14]. The correspondent reactances are presented in
Figure 6.
Fig. 6: The equivalent circuit drawing of the system’s inductive part
depicting all reactances.
The leakage flux is very high, when compared to a
conventional transformer. This is caused by the high air
gap reluctance and leads to a low coupling factor and
with it to a small value for mutual reactance X1h in
comparison to leakage reactances X1σ and X’2σ. Therefore,
the transformer equivalent circuit drawing cannot be
simplified.
To analyze the operational behavior of the
transmission path, open circuit and short circuit
measurements of the windings’ inductances (without
capacitors) are conducted. The results vary for different
distances between pick-up and stationary transmitter (i.e.
different heights of the cars’ undersurfaces). By utilizing
these measurement results, the inductive elements of the
equivalent circuit drawing are calculated.
The values of the capacitors C1 and C’2 are chosen in a
way, that the resonant frequency of the system is fN =
140kHz during operation with a nominal charger unit
distance:
·
·
·
·
·
· ′
′
(2)
(3)
The total impedance of the transmission system is
calculated from the equivalent circuit drawing in Fig. 6
(inclusive ohmic load on the secondary side of the
equivalent circuit drawing):
′
′
′
′
′
′
(4)
Here, X represents the respective capacitive or
inductive reactance and RL describes the resistive load
of rectifier and battery lying behind the transmission link.
The following reactance conditions are mandatory for an
exactly compensated transmission link:
′
0
0
′
(5)
Hereby, equation (4) can be simplified to
′
It can be seen, that the total impendance Z only possess
a real part. Transmission link and load are compensated
and do not require further reactive power supply from
outside.
The variability of the inductance values represents a
great challenge: both mutual and leakage inductances
depend on diverse factors, such as plate distance, lateral
positioning and temperature. Fig. 7 shows the changing
of inductances at lateral displacement of stationary
transmitter and pick-up, which for example regularly
appears in case of parking.
Fig. 7: Changing of mutual inductance and leakage inductance at
lateral displacement of stationary transmitter and pick-up (measured
without vehicle).
Here, a clear difference arises between the
conventional contactless energy transmission systems (up
to now situated in the market for industrial applications,
such as electric monorail systems or forklifts [13, 14])
and charging systems for electric motor vehicles.
Different inductance values after every park process
cause detuning of the entire system and thereby lead to a
change of the resonant frequency. However, for operation
with a constant frequency this would lead to a necessity
of reactive power, which causes inductive and capacitive
switching behavior of the semiconductor switches and
higher losses within them. These higher losses could
cause a thermal destruction of the power electronics.
The problem could be solved by variable compensation
capacitors, which would mean higher costs and higher
system complexity. A smarter and also effective solution
is an adaptive frequency adjustment, which actually is
realized at the system presented. This frequency
adjustment can clearly be declared by an example taking
the primary compensation condition (shown in equation
(2)) into account. If the reactances in this equation are
replaced by the characteristic variables, the following
formula results:
2·
· · ·
·
·
0
(7)
Here, f is the system’s operating frequency. After a
rearrangement of the equation follows
(6)
· ·
·
(8)
Therefore, it is possible to compensate a change of
inductance by a change of operating frequency. With this
frequency adaption, the resonant circuit is still in resonant
operation and does still not require any reactive power,
i.e. the value of the total impedance Zges is still an ohmic
value.
The voltage induced in the pick-up fluctuates
depending on the varying of the plate distance, which
means a further difficulty. The part of the magnetic flux
generated by the stationary transmitter, which is coupled
with the secondary coil, decreases if the distance between
both inductive parts increases. The dependence between
coupling and plate distance is shown in Fig. 8.
200 KHz. However, the nominal frequency of eCPS is fN
= 140kHz in accordance with application guide VDEAR-E 2122-4-2[11].
The inverter consists of a 400V DC-Link, which feeds
an H-bridge of four MOSFETs. This semiconductor
switches transform the DC current to a high frequent AC
current. In Figure 10, a picture of one printed circuit
board of a 2011th inverter generation is displayed.
Fig. 10: One printed circuit board of the primary inverter unit.
Fig. 8: Changing of coupling with different plate distances
(measured without vehicle).
Here, the pick-up’s lateral deviation is zero, i.e. it is
centrally located (x=y=0) above the stationary
transmitter. It can be seen that the coupling is nearly
reduced by half on the distance of z=70mm to z=170mm.
The same statement is valid for U2 since the induced
voltage is proportional to coupling.
Here, the operation frequency fN is not firm adjusted.
In contrast to conventional inverters of non-contact power
supply systems [13, 14](e.g. electrification systems for
industrial applications), the controller of the eCPSinverter observes the resonant circuit and adapts its
resonant frequency in real time. The procedure of
observing occurs by measuring and analyzing the
sinusoidal current waveform, which is presented in
Figure 9. The inductances of the resonant circuit (Fig. 5),
which depend on the actual pick-up position, directly
adjust the current’s frequency. The voltage is forced to
follow this frequency. This behavior is defined within the
primary inverters microcontroller. This means, the
primary inverter is an adaptive unit, which exclusively
permits resonant switching.
The output voltage of the pick-up, i.e. the input voltage
of the battery charging system, can also be controlled by
the primary inverter. The inductive power transmission
system’s output voltage U2 is proportional dependent on
the input current i1:
(9)
Fig. 9: Voltage and current waveform at the primary inverter unit.
IV. THE PRIMARY INVERTER
The greatest challenge in developing the contactless
charging system is the design of a reliable primary
inverter. Today, the 140kHz-technology is not state of the
art in common power electronic systems within this
power range. Therefore, the selection of reasonable
components, the interaction between them and the
ensuring of the electromagnetic compatibility are major
requirements in the development work. The primary
inverter of the eCPS-system is able to excite an
alternating voltage within a frequency range of 1Hz < f <
Here, c describes the correlation between U2 and i1 as a
constant value. This interdependence exits since the
series compensation of primary side and secondary side
causes that the inductive transmission system behaves
like a gyrator. This gyrator characteristic is exemplarily
described in [14]. Equation (9) shows, that the output
voltage of the inductive transmission system can easily be
controlled by adjusting the primary current. Therefore, a
voltage feedback is required. Figure 11 shows the
uncontrolled rectifier operation between pick-up and onboard charging device. It can be seen, that the value of
the secondary voltage is U2 ≈ 380V. As aforementioned,
this value can be adjusted by changing primary current i1.
Future steps in power electronic development can
obtain an easier and more inexpensive assembly and
further efficiency improvements.
The influence of the self-heeating of the inverter on the
system stability was analyzed and compensated by
cooling. Thereby a safe and sttable operation is protected
independent on the power oveer the complete operational
temperature range.
To investigate the implemenntation of pick-up and onboard rectifier the eCPS-Systeem is mounted beneath the
undersurface of a testing vehiicle. This construction can
be seen in Figure 13.
Fig. 11: Voltage and current waveform at the on-board rectifier.
V. OPERATION OF THE ENTIREE SYSTEM
To fulfill the electrical specificatiions by law, the
primary inverter has to be equipped with
w a power factor
correction unit (PFC). This unit compennsates the reactive
power and the deformed power produced
p
by the
inverter’s switching operations and proovides the primary
inverter’s DC intermediate circuit with a voltage of UDC =
400V. A measurement of the invverter born grid
disturbances shows a sinusoidal currentt consumption and
a power factor of cosφ > 0.98.
Further laboratory measurements utiilizing the existing
system show a satisfying performance with an efficiency
of η = 91% ascertained between grrid and on-board
charging device at an optimal placemeent. The efficiency
measurements are performed duuring a power
consumption of the on-board charginng device of P =
3kW.
f
safety,
To meet the requirements in functional
application guide AR-E 2122-4-2[11] specifies two
pieces for checking the maximum
m ratings of the
electromagnetic field. One testing piece made of electric
conductible material is placed inn an alternating
electromagnetic field and heats duee to magnetically
excited eddy currents within. The risse of the heating
temperature is directly dependent onn the field’s flux
density. Therefore, the rule speciffies a maximum
temperature rise of ΔT = 40K. Figure 12 presents the
m
of structural
temperature profile of a testing piece made
steel (ST37). It can be seen, that the tem
mperature rise at an
ambient temperature of Troom = 20.5°C
C is less than 30K.
This is the worst measured case. Hencee, eCPS meets the
flux density safety requirements defined by the
application guideline.
Fig. 13: Pick-up beneath thee vehicles undersurface.
Fig. 14: Changing of output pow
wer at horizontal moving in xdirectioon.
With the aid of this vehicle integration, the effect of a
p
in x-direction is
horizontal moving of the pick-up
measured. The results are show
wn in Fig. 14. It can be seen
that moving in x-direction doess not have a large influence
on the output power. Adittionallly, the diagram shows that
the value of the power transm
mitted is proportional to the
plates’ distance in z-direction.
50
45
Temperature T [°C]
40
35
30
25
20
15
10
5
0
4
8
12
16
20
24
28
32
36
40
44
48
52
56
60
64
68
72
76
80
84
88
92
96
100
104
108
112
116
120
0
Time t [min]
Fig. 12: Temperature profile of testing piece “ST
T37” during operation
with a maximum flux density.
Fig. 15: Changing of output power at horizontal moving in x- and ydirectioon.
Fig. 15 shows the influence of the lateral displacement
synchronistic in x- and y-direcction. Compared to Fig. 14
the changing of the power exceeds. These results are
measured on a system with a constannt primary current.
Therefore, the transmitted power changges in dependence
on the flux linkage, e.g. by replaccing the pick-up.
However, the full power can be trannsmitted (up to a
lateral deviation of minimum 100mm
m) by utilizing the
voltage control described in section IV. This voltage
control is actually tested under laboratoory conditions and
promises a stable system behavior and
a
a decoupling
between plate distance and transmittedd power. However,
this control procedure is not tested on a vehicle until now.
Further measurements show a well ruunning system and
propose the possibility of a further improvement in
efficiency.
able to transmit the nominal power within the parking
deviation range excepted.
VI. AN ALTERNATIVE TO THE UNDERSU
URFACE SYSTEM:
CHARGING VIA LICENSE TAG
An alternative construction of the prresented system is
deployed in the contactless chargingg system via the
vehicle’s license tag (license tag charginng system, LTCS).
On 18 May 2012 Vahle Inc. presented a charging system,
which consists of a charging sttation (stationary
transmitter) and an on-board receiver unit
u (pick-up). This
pick-up is located under the surfacee of the vehicles
license tag.
Fig. 16: Charging process via inductive power traansmission through the
license tag.
To start the charging process, the veehicle has to drive
up to the station and carefully push the stationary
transmitter. At this moment, stationarry transmitter and
pick-up are coupled. The electrical components and the
functional principles of this system are the same as
described for the undersurface system. The prototype of
this contactless charging station can be seen in Figure 16
and Figure 17.
VII. CONCLUSION
Feasibility and advantages of a nonn-contact charging
system for electric vehicles are introdduced. Setting up
operation of the entire system is shownn under laboratory
conditions: An active power of up to 3 kW
k is transferred.
In a further step, a vehicle was equippped with one pickup and several transmission tests aree performed. It is
shown that a lateral deviation of the picck-up compared to
the stationary transmitter only leads to a small decrease in
transmitted power. However, the transsmission system is
Fig. 17: Front view on the liceense tag charging station.
Moreover, a system is preesented, which is able to
fulfill the requirements of thhe corresponding German
application guideline. By em
mploying this solution, a
simple and reliable charging syystem can be established.
As renewable energies and electro mobility
complement each other there iss a need for a user-friendly
charging infrastructure. A nonn-contact charging system
on the basis of inductive energyy transfer can contribute to
an improved user acceptaance and supports the
exploitation of the renewable ennergies’ potential.
Further research activities are
a required to guarantee a
successful market launch off inductive power supply
systems for vehicle charginng. On the one hand,
interoperability
between
systems
of
different
manufacturers must be possiblle without any restrictions.
For this, the existing standardds have to be enhanced by
detailed interface definitions for communication and
h
a further downsizing
magnetic field. On the other hand,
of the system is of high inteerest for both automotive
manufacturers and componennt supplier. Therefore, an
optimization of coupling, i.ee. an optimization of the
inductive part’s design, is a reeasonable next step in the
development of the non-contact charging system.
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