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Coupling Efficiency Improvement and Power Transfer Enhancement in Wireless Power Transfer System for Electric Vehicle Charging Applications
A Thesis submitted for the award of the degree of Doctor of Philosophy by Gautam Rituraj
Department of Electronics and Electrical Engineering Indian Institute of Technology Guwahati
Abstract
In recent years, wireless power transfer (WPT) technology is gaining popularity for
electric vehicle (EV) charging. This technology has advantages such as safety, reliability,
ease of charging, and robustness over a conventional plug-in charging system. In the WPT
system, transmitter and receiver coils (loosely coupled) play an essential role in power
transfer. Since the power transfer happens through air-medium, the high leakage inductance
results in reduced coupling coefficient (k). This reduced k decreases power transfer
capability and transmission (coil-to-coil/coupling) efficiency. The transmission efficiency
is also affected due to the varying nature of the load during battery charging (i.e., change
in the equivalent load resistance due to change in the battery’s state of charge). In this
context, to improve the transmission efficiency, an experimental study is performed (using
the developed experimental setup) while charging the battery bank (sealed lead-acid)
using a series-parallel (SP) compensated WPT system. Charging of the battery bank is
done using constant current (CC) and constant voltage (CV) modes. For these charging
modes, the equivalent load resistance curve is experimentally determined. Moreover, an
expression of optimum operating frequency is derived, which incorporated the load. At
this frequency, the system is operated in both charging modes where it maintains the maximum
possible transmission efficiency throughout the load variation compared with the
operation at the resonant frequency. Furthermore, this optimum frequency gives a stable
voltage waveform of the inverter in the CC charging mode compared with the resonant
frequency. Besides that, it (optimum frequency) provides zero voltage switching throughout
the charging process (i.e., in CC and CV charging modes). Moreover, the variation
of transmission efficiency and the load phase angle for different operating frequencies in
CC and CV charging modes are verified with the measurement to identify the possible
control parameters.
Furthermore, the power transfer capability and transmission efficiency (performance parameters)
are enhanced by improving factor k. k of air-core coils can be improved by
either doing modifications in the design of the coils or using the ferrite core (or by doing
both). Since the ferrite core increases the weight and cost, it is not the first resort toward
improving k; instead, an attempt to enhance k should involve modification in the design
of the coils. For doing this, different conventional unipolar coils are investigated to find
that approach of improving k, which enhances both performance parameters. Generally,
the 3-D finite element analysis (FEA) software (ANSYS Maxwell or JMAG) is used to
analyse the coils (i.e., magnetic field, self- and mutual inductance, and k). However,
the simulation of various 3-D models with FEA software is a time-consuming process
due to high mesh-density. Therefore, a 3-D analytical model is developed to analyse the
air-core rectangular (or square) coils, used in the WPT systems. The developed 3-D analytical
model calculates the magnetic field and k faster than 3-D FEA and also gives good
accuracy (verified using the simulation and experimental results). Based on the observations
obtained from the investigation, a unipolar coil arrangement method (UCAM) for
improving k compared with conventional coils of the same self-inductance and outer dimensions
is proposed. This method does not require ferrite materials and is applicable for
different popular unipolar coils’ geometry (i.e., rectangular, square, and circular) used in
the static and dynamic WPT systems. Besides that, the developed 3-D analytical model
(for rectangular and square coils) is extended for the coils designed using the proposed
UCAM. For the unipolar rectangular coil system with 400 mm × 300 mm outer dimensions,
6.78%–27.04% improvement in k is achieved at the 150 mm air gap for the case
3 coil system compared with the different conventional unipolar coil systems. Moreover,
the interoperability between the proposed and conventional coils, the impact of various
misalignments of the receiver coil on improvement in k, and the impact of improved k on
the performance parameters are examined. Prototypes of proposed and conventional coil
both vertical and horizontal misalignments and to confirm the improvement in k. Moreover,
for the square and circular coil systems, up to 26.02% and 26.41% improvements
in k at the 150 mm air gap have been found with the proposed UCAM for the outer dimensions
of 350 mm × 350 mm and 400 mm × 400 mm, respectively, compared with
conventional coil systems. Besides that, the second resort (using ferrite) of improving k
is used to enhance the factor k of air-core coils (proposed and conventional). Traditionally,
the ferrite core size is kept approximately equal to the outer dimensions of the coil,
which increases the overall weight and volume of the system. With the traditional ferrite
arrangement, the impact of improved k of the proposed coil system on performance
parameters is examined in comparison to the proposed air-core coil system. To maintain
the obtained enhancements in performance parameters and minimise the weight and volume
of the system, a novel (and simple) ferrite arrangement of unipolar rectangular (and
square) coils is proposed. The proposed arrangement maintains the maximum achievable
k and minimises the volume of ferrite used compared with the traditional arrangement.
