Wireless Charging of Electric Vehicles seminar report
1. WIRELESS CHARGING OF ELECTRIC VEHICLES
Dept. of Mechanical Engineering, AIT CHIKMAGALUR 1
Chapter - 1
WPT technique requires no physical contact between vehicle and charging device, therefore
overcomes the inconvenience and hazards caused by traditional conductive method.
The initial objective is replacing conductive charging method by the novel WPT technology,
while maintaining a comparable power level and efficiency. The long-term goal is to
dynamically power the moving vehicles on road. This will lead to a much reduced battery pack
but extended driving range. Then, the main concerns of EV, namely the high battery price and
the range anxiety, will be addressed.
The growing EV market stimulates the demand for more convenient and reliable means
to recharge the Great effort has been put on WPT technology. Feasibility of its application on
wireless EV charging has been proved by institutes through various demonstrations. Leading
manufacturers and major global automobile suppliers are seeking opportunities for
commercialization. Nissan and Chevrolet have developed wireless charging system in
corporation with Evatran for their EV models, the Nissan LEAF and Chevrolet Volt.
Meanwhile, Audi, Toyota, and Mitsubishi are integrating magnetic resonance WPT technology
into their EV models in collaboration with Delphi, and WiTricity, using the technologies from
MIT. In 2011, Qualcomm acquired the former HaloIPT company owned by the University of
Auckland and announced the biggest pre-commercial trial of wireless EV charging in Europe.
In this paper, current WPT technologies will be reviewed on the perspective of electric vehicle
charging. For each technology, basic principle will be explained with summary of its potential
and constraints on EV charging. For the two promising techniques, namely coupled magnetic
resonance and magnetic gear; key issues, research challenges the latest developments will be
noted. Finally, the technology trends will be introduced.
Fig. 1.1- An example of wireless power transmission.
2. WIRELESS CHARGING OF ELECTRIC VEHICLES
Dept. of Mechanical Engineering, AIT CHIKMAGALUR 2
Chapter – 2
WIRELESS CHARGING TECHNOLOGY
The idea of transmitting power through the air has been around for over a century, with Nikola
Tesla’s pioneering ideas and experiments perhaps being the most well-known early attempts
to do so. He had a vision of wirelessly distributing power over large distances using the earth’s
ionosphere. Most approaches to wireless power transfer use an electromagnetic (EM) field of
some frequency as the means by which the energy is transferred. At the high frequency end of
the spectrum are optical techniques that use lasers to send power via a collimated beam of light
to a remote detector where the received photons are converted to electrical energy. Efficient
transmission over large distances is possible with this approach; however, complicated pointing
and tracking mechanisms are needed to maintain proper alignment between moving
transmitters and/or receivers. In addition, objects that get between the transmitter and receiver
can block the beam, interrupting the power transmission and, depending on the power level,
possibly causing harm. At microwave frequencies, a similar approach can be used to efficiently
transmit power over large distances using the radiated EM field from appropriate antennas.
However, similar caveats about safety and system complexity apply for these radiative
approaches. It is also possible to transmit power using non-radiative fields. As an example, the
operation of a transformer can be considered a form of wireless power transfer since it uses the
principle of magnetic induction to transfer energy from a primary coil to a secondary coil
without a direct electrical connection. Inductive chargers, such as those found commonly in
electric toothbrushes, operate on this same principle. However, for these systems to operate
efficiently, the primary coil (source) and secondary coil (device) must be in close proximity
and carefully positioned with respect to one another. From a technical point of view, this means
the magnetic coupling between the source and device coils must be large for proper operation.
But what about going over somewhat larger distances or having more freedom in positioning
the source and device relative to each other? That’s the question that a group at the
Massachusetts Institute of Technology (MIT) asked themselves. They explored many
techniques for transmitting power over “mid-range” distances and arrived at a non-radiative
approach that uses resonance to enhance the efficiency of the energy transfer. High quality
factor resonators enable efficient energy transfer at lower coupling rates, i.e., at greater
distances and/or with more positional freedom than is otherwise possible (and therefore, this
approach is sometimes referred to as “highly resonant” wireless energy transfer or “highly
3. WIRELESS CHARGING OF ELECTRIC VEHICLES
Dept. of Mechanical Engineering, AIT CHIKMAGALUR 3
resonant” wireless power transfer (HR-WPT)). The MIT team demonstrated the highly
resonant technique using a magnetic field to transfer energy over a mid-range distance of 2
meters, and an industry was born. In some instances, this technology is also referred to as
“magnetic resonance”, and it is often contrasted to “induction” for its ability to efficiently
transfer power over a range of distances and with positional and orientational offsets. Since
that initial demonstration, the use of HR-WPT, or magnetic resonance, has enabled efficient
wireless energy transfer in a wide range of applications that was not possible before.
2.2. THE BASICS
WiTricity technology is transferring electric energy or power over distance without wires. with
the basics of electricity and magnetism, and work our way up to the WiTricity technology.
The flow of electrons (current) through a conductor (like a wire), or charges through the
atmosphere (like lightning). A convenient way for energy to get from one place to another!
A fundamental force of nature, which causes certain types of materials to attract or repel each
other. Permanent magnets, like the ones on your refrigerator and the earth's magnetic field, are
examples of objects having constant magnetic fields. Oscillating magnetic fields vary with
time, and can be generated by alternating current (AC) flowing on a wire. The strength,
direction, and extent of magnetic fields are often represented and visualized by drawings of the
magnetic field lines.
Fig.2.1-An illustration representing the earth's magnetic field
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When electric current passes through a conductor, a circular electromagnetic field is created
around it. The direction of the current decides the direction of rotation of the created magnetic
field. The current strength as well as length of the conductor decides the force of
electromagnetism developed. Change of the magnetic field can produce electricity.
Fig.2.2 As electric current, I flow in the circuit it gives rise to a magnetic field, which
wrap around wire and when current is reversed magnetic field also get reversed.
2.2.4 MAGNETIC INDUCTION
the process by which an object or material is magnetized by an external magnetic field.
If a conductive loop is connected to an AC power source, it will generate an oscillating
magnetic field in the vicinity of the loop. A second conducting loop, brought close enough to
the first, may capture" some portion of that oscillating magnetic field, which in turn, generates
or induces an electric current in the second coil. The current generated in the second coil may
be used to power devices. This type of electricalpower transfer from one loop or coil to another
is well known and referred to as magnetic induction. Some common examples of devices based
on magnetic induction are electric transformers and electric generators.
Fig.2.3- the blue lines represent the magnetic field when current flows through a coil
and current is reversed, magnetic field also gets reversed.
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2.2.5 ENERGY/POWER COUPLING
In electronics and telecommunication, coupling is the desirable or undesirable transfer of
energy from one medium, such as a metallic wire or an optical fiber, to another medium.
Energy coupling occurs when an energy source has a means of transferring energy to
another object. One simple example is a locomotive pulling a train car the mechanical coupling
between the two enables the locomotive to pull the train, and overcome the forces of friction
and inertia that keep the train still and, the train moves. Magnetic coupling occurs when the
magnetic field of one object interacts with a second object and induces an electric current in or
on that object. In this way, electric energy can be transferred from a power source to a powered
device. In contrast to the example of mechanical coupling given for the train, magnetic
coupling does not require any physical contact between the object generating the energy and
the object receiving or capturing that energy.
Fig. 2.4 An electric transformer is a device that uses magnetic induction to transfer energy
from its primary winding to its secondary winding, without the winding being connected
to each other. It is used to transform AC current at one voltage to AC current at a
Resonance is a property that exists in many different physical systems. It can be thought of as
the natural frequency at which energy can most efficiently be added to an oscillating system.
A playground swing is an example of an oscillating system involving potential energy and
kinetic energy. The child swings back and forth at a rate that is determined by the length of the
swing. The child can make the swing go higher if she properly coordinates her arm and leg
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action with the motion of the swing. The swing is oscillating at its resonant frequency and the
simple movements of the child efficiently transfer energy to the system.
In this example, the wine glass is the resonant oscillating system. Sound waves traveling
through the air are captured by the glass, and the sound energy is converted to mechanical
vibrations of the glass itself. When the singer hits the note that matches the resonant frequency
of the glass, the glass absorbs energy, begins vibrating, and can eventually even shatter. The
resonant frequency of the glass depends on the size, shape, thickness of the glass, and how
much wine is in it.
2.2.7 Resonant Magnetic Coupling
Magnetic coupling occurs when two objects exchange energy through their varying oscillating
magnetic fields. Resonant coupling occurs when the natural frequencies of the two objects are
approximately the same.
Fig. 2.5 Two idealized resonant magnetic coils, shown in yellow. The blue and red colour
bands illustrate their magnetic fields. The coupling of their respective magnetic fields is
indicated by the connection of the colour bands.
2.2.8 WiTricity (WIRELESS ELECTRICITY) TECHNOLOGY
WiTricity power sources and capture devices are specially designed magnetic resonators that
efficiently transfer power over large distances via the magnetic near-field. These proprietary
sources and device designs and the electronic systems that control them support efficient
energy transfer over distances that are many times the Size of the sources/devices themselves.
7. WIRELESS CHARGING OF ELECTRIC VEHICLES
Dept. of Mechanical Engineering, AIT CHIKMAGALUR 7
Fig. 2.6 The WiTricity power source left is connected to AC power. The blue lines
represent the magnetic near field induced by the power source. The yellow lines present
the flow of energy from the source to the WiTricity capture coil, which is shown powering
a light bulb.
2.3 METHODS OF WIRELESS TRANSMISSION OF ELECTRICAL
The principle of mutual induction between two coils can be used for the transfer of electrical
power without any physical contact in between. The simplest example of how mutual induction
works is the transformer, where there is no physical contact between the primary and the
secondary coils. The transfer of energy takes place due to electromagnetic coupling between
the two coils.
2.3.2 Electromagnetic Transmission
Electromagnetic waves can also be used to transfer power without wires. By converting
electricity into light, such as a laser beam, then firing this beam at a receiving target, such as a
solar cell on a small aircraft, power can be beamed to a single target. This is generally known
as “power beaming”.
2.3.3 Evanescent Wave Coupling
Researchers at MIT believe they have discovered a new way to wirelessly transfer power using
non-radiative electromagnetic energy resonant tunnelling. Since the electromagnetic waves
8. WIRELESS CHARGING OF ELECTRIC VEHICLES
Dept. of Mechanical Engineering, AIT CHIKMAGALUR 8
would tunnel, they would not propagate through the air to be absorbed or wasted, and would
not disrupt electronic devices or cause physical injury like microwave or radio transmission.
Researchers anticipate up to 5 meters of range.
2.3.4 Electrodynamic Induction
Also known as "resonant inductive coupling" resolves the main problem associated with non-
coupling for wireless energy transfer; specifically, the dependence of efficiency on
When resonant coupling is used the transmitter and receiver inductors are tuned to a
mutual frequency and the drive current is modified from a sinusoidal to a non-sinusoidal
transient waveform. Pulse power transfer occurs over multiple cycles. In this way significant
power may be transmitted over a distance of up to a few times the size of the transmitter.
2.3.5 Radio and Microwave
Power transmission via radio waves can be made more directional, allowing longer distance
power beaming, with shorter wavelengths of electromagnetic radiation, typically in the
microwave range. A rectenna may be used to convert the microwave energy back into
electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power
beaming using microwaves has been proposed for the transmission of energy from orbiting
solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been
2.3.6 Electrostatic Induction
Also known as "capacitive coupling" is an electric field gradient or differential capacitance
between two elevated electrodes over a conducting ground plane for wireless energy
transmission involving high frequency alternating current potential differences transmitted
between two plates or nodes.
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Chapter – 3
Wireless Charging System Overview
3.1 System Description
Across an application space that spans power levels from less than a watt to multiple kilowatts,
a wireless energy transfer system based on HR-WPT often has a common set of functional
blocks. A general diagram of such a system is shown in figure3.1.
Fig.3.1- Block diagram of a wireless energy transfer system.
Progressing from left to right on the top line of the diagram, the input power to the system is
usually either wall power (AC mains) which is converted to DC in an AC/DC rectifier block,
or alternatively, a DC voltage directly from a battery or other DC supply. In high power
applications, a power factor correction stage may also be included in this block. A high
efficiency switching amplifier converts the DC voltage into an RF voltage waveform used to
drive the source resonator. Often an impedance matching network (IMN) is used to effectively
couple the amplifier output to the source resonator while enabling efficient switching-amplifier
operation. Class D or E switching amplifiers are suitable in many applications and generally
require an inductive load impedance for highest efficiency. The IMN serves to transform the
source resonator impedance, loaded by the coupling to the device resonator and output load,
into such an impedance for the source amplifier. The magnetic field generated by the source
resonator couples to the device resonator, exciting the resonator and causing energy build-up.
This energy is coupled out of the device resonator to do useful work, for example, directly
powering a load or charging a battery. A second IMN may be used here to efficiently couple
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energy from the resonator to the load. It may transform the actual load impedance into an
effective load impedance seen by the device resonator which more closely matches the loading
for optimum efficiency. For loads requiring a DC voltage, a rectifier converts the received AC
power back into DC.
In the earliest work at MIT, the impedance matching was accomplished by inductively
coupling into the source resonator and out of the device resonator. This approach provides a
way to tune the input coupling, and therefore the input impedance, by adjusting the alignment
between the source input coupling coil and the source resonator, and similarly, a way to tune
the output coupling, and therefore the effective loading on the device resonator, by adjusting
the alignment between the device output coupling coil and the device resonator. With proper
adjustment of the coupling values, it was possible to achieve power transfer efficiencies
approaching the optimum possible efficiency. Figure 3.2 shows a schematic representation of
an inductive coupling approach to impedance matching. In this circuit, M! is adjusted to
provide the desired input impedance for the given loading of the source resonator. The device
resonator is similarly loaded by adjusting M! the mutual coupling to the load. Series capacitors
may be needed in the impound output coupling coils to improve efficiency unless the reactance
of the coupling inductors are much less than the generator and load resistances.
Fig.3.2. Schematic representation of inductively coupling into and out of the resonators.
It is also possible to directly connect the generator and load to the respective resonators with a
variety of IMNs. These generally comprise components (capacitors and inductors) that are
arranged in “T” and/or “pi” configurations. The values of these components may be chosen for
optimum efficiency at a specific source-to-device coupling and load condition (“fixed tuned”
impedance matching) or they may be adjustable to provide higher performance over a range of
source-to device positions and load conditions (“tuneable” impedance matching).
Requirements of the particular application will determine which approach is most appropriate
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from a performance and cost perspective. A common question about wireless charging is: How
efficient is it? The end-to-end efficiency of a wireless energy transfer system is the product of
the wireless efficiency (see Physics of Highly Resonant Power Transfer for an explanation)
and the efficiency of the electronics (RF amplifier, rectifier and any other power conversion
stages, if needed). In high power applications, such as the charging of electric vehicles at multi
kilowatt levels, end-to-end efficiencies (AC input to DC output) greater than 94% have been
demonstrated. Such efficiencies require that each stage in the system have an efficiency at 98-
99% or greater. Careful design in each stage is required to minimize losses to achieve such
In mobile electronic devices, space is usually of utmost importance, so incorporating
resonators generally involves some trade-offs in resonator size and system efficiency to
accommodate the space restrictions. Also, the application use-case may involve a wider range
of magnetic coupling between source and device which can also present a challenge for the
design of the impedance matching networks. However, coil-to-coil efficiencies of 90% or more
and end-to-end efficiencies over 80% are achievable in these lower power applications.
3.2 POWER ELECTRONICS CONVERTER AND POWER CONTROL
Fig.3.3 Typical wireless EV charging system.
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In a WPT system, the function of the primary side power electronics converter is to generate a
high-frequency current in the sending coil. To increase the switching frequency and efficiency,
usually a resonant topology is adopted. At the secondary side, a rectifier is adopted to convert
the high-frequency ac current to dc current. Depending on whether a secondary side control is
needed, an additional converter may be employed. The primary side converter may be a voltage
or a current source converter. As a bulky inductor is needed for the current source converter,
the most common choice at the primary side is a full bridge voltage source resonant converter.
In the primary side, the full bridge converter outputs a high-frequency square voltage. By
adopting the LC compensation network, a constant high frequency current can be maintained
in L1. An additional capacitor C1s is introduced here to compensated part of the reactive power
on L1. Thus, the power rating on L f 1 could be reduced. The system design flexibility could
also be improved. At the secondary side, the parallel compensation is adopted. With a constant
primary coil, current and parallel secondary side compensation, the output is like a current
source. At a certain coupling, the current in L3 is almost constant. By changing the duty ratio
of switch S5, the output power can be controlled.
Many different control methods were proposed to control the transferred power.
Depending on where the control action is applied, the control method could be classified as
primary side control, secondary side control and dual-side control. In most cases, the primary
side and dual-side control is only suitable for power transfer from one primary pad to one
pickup pad. The secondary side control could be used in the scenario where multiple pickup
pads are powered from one primary pad or track.
The control at the primary side can be realized by changing the frequency, duty cycle
and the phase between the two legs. Since the characteristic of a resonant converter is related
to the operating frequency, a frequency control at the primary side is adopted in some designs.
When adjusting the frequency, it should be noted that the bifurcation phenomenon in the
loosely coupled systems. The power versus frequency is not always a monotonic function.
Also, the frequency control method takes up a wider radio frequency bandwidth, which may
increase the risk of electromagnetic interference. When the switching frequency is fixed, the
control can be carried out by duty cycle or phase shift. The problem of duty cycle or phase shift
control is that there is a high circulating current in the converter. Also, the ZVS or ZCS
switching condition may be lost. To ensure ZVS, an alternative way to control the system
output power is to adjust the input dc voltage VS . An asymmetrical voltage cancellation
method, which uses an alternative way to change the duty cycle, was proposed to increase the
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ZVS region. A discrete energy injection method, which could achieve ZCS and lower the
switching frequency at light load condition, was proposed in and.
At the secondary side, with parallel compensation, a boost converter is inserted after
the rectifier for the control. Correspondingly, with series compensation, a buck converter can
be used. When the control is after the rectifier, an additional dc inductor, as well as a diode on
the current flow path, should be introduced. The University of Auckland proposed a control
method at the ac side before the rectifier. By doing so, the dc inductor and additional diodes
could be saved. Because of the resonating in the ac side, ZVS and ZCS could be achieved. The
detailed designs for series compensation as well as a LC compensation network are presented
in and. The dual-side control is a combination of both primary and secondary side control. The
system complexity and cost may increase, but the efficiency can be optimized by a dual-side
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Chapter – 4
Human Safety Considerations
A common question about wireless power transfer using magnetic resonance systems is: Are
they safe? Perhaps because these systems can efficiently exchange energy over mid-range
distances, people may assume that they are being exposed to large and potentially dangerous
electromagnetic fields when using these systems. Early popular press descriptions of the
technology as “electricity-in-the-air” have done little to calm people’s potential fears. Of
course, WiTricity’s technology is NOT “electricity-in-the-air”, but rather a technology that
uses oscillating magnetic fields to mediate the wireless energy exchange. With proper design
the stray electric and magnetic fields can be kept below the well-established and long-standing
human safety limits that regulate all electro-magnetic consumer devices including cell phones,
wireless routers, Bluetooth headphones, radio transmitters, etc. At WiTricity, we perform a
detailed electromagnetic analysis, using measurements and sophisticated numerical modelling
tools, of each system we design and application area we explore, including systems transferring
more than 10kW of power, to ensure that the systems will meet all applicable human safety
The fig 4.1 shows the working of power transmitting unit, which transmits wireless
power to that particular section of the road above which the electric vehicle is passing.
Fig.4.1 An example of real time wireless power transfer to the vehicles on road
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ADVANTAGE, DISADVANTAGES AND THE FUTURE
➢ 80 % reduced operating cost than equivalent gas powered vehicles.
➢ Lower maintenance costs than gas powered vehicles.
➢ Pollution free.
➢ Zero recharging time and unlimited range (when operating on an electric road)
➢ Light weight vehicles.
➢ A number of devices can be charged at a time
➢ Electrically safe
➢ Low maintenance cost
➢ Charging is convenient
➢ Initial installation cost is very high.
➢ Working area is limited.
➢ Currently the speed is limited to 40 miles per hour.
➢ power outage might cause the EVs to run out of power.
➢ Heat generation is more than traditional charging.
➢ Complex design.
➢ Our current electrical grid could not support mass market adaptation of the online
5.3 The Future
With a maturing technology base and a broad application space, wireless power transfer will
become prevalent in many areas of life in the coming years. Since the original demonstrations
at MIT early this century, the technology of magnetic resonance has moved from a scientific
experiment to the production line where it will be incorporated into mass-produced consumer
electronics such as laptops and mobile phones. Electric vehicles, both plug-in hybrids and full
battery electric vehicles, will soon offer wireless charging so that plugging in to charge will no
longer be a requirement. Development of world-wide standards for wireless power in both of
these application areas is underway to ensure interoperability across products and brands,
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facilitate the deployment of wireless charging infrastructure and help to accelerate adoption of
the technology. Some advanced automotive technology, such as vehicles with autonomous
navigation and ultimately driverless operation, along with the expansion of car sharing services
to provide better utilization of such vehicles, will benefit greatly from the ability to charge
without human intervention. In fact, wireless charging is almost essential for the deployment
of autonomous vehicles where there may not be anyone around to connect a wired charger (or
otherwise add fuel). Imagine a fleet of autonomous vehicles, offering ride services for example,
that automatically find the nearest charging spot when charging is needed and go back into
service once recharged. Research into dynamic charging of vehicles, using the same basic
technology of magnetic resonance, is underway and may someday lead to real charging on the
move. Another promising application area for wireless power transfer is in the medical arena.
The use of medical implants for innovative therapies for a variety of chronic conditions is
growing, and the ability to safely get power to such devices opens the door to new treatment
options. For example, wireless power offers the ability to extend the useful lifetime of an
implant because its battery can be recharged, or even eliminate the need for a battery in some
cases. Of course, there will likely be applications for wireless power that we cannot envision
today. With the pace of technology innovation, expect to see wireless power technology
deployed not only in the areas mentioned here, but in many more applications.
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This paper presented a review of wireless charging of electric vehicles. It is clear that vehicle
electrification is unavoidable because of environment and energy related issues. Wireless
charging will provide many benefits as compared with wired charging. In particular, when the
roads are electrified with wireless charging capability, it will provide the foundation for mass
market penetration for EV regardless of battery technology. With technology development,
wireless charging of EV can be brought to fruition. Further studies in topology, control, inverter
design, and human safety are still needed in the near term.
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