A COST-EFFECTIVE SYSTEM FOR WIRELESS POWER

TRANSMISSION

SEMINAR REPORT

Submitted in partial fulfillment of the requirements for the award of B.tech degree in Electrical and Electronics Engineering by the University of Kerala

Submitted By

NABEEL.PM

S7 EEE

ROLL NO: 33

University NO: 08401017

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

GOVERNMENT ENGINEERING COLLEGE BARTON HILL

TRIVANDRUM

2011

DEPARTMENT OF ELECTRICAL & ELECTRONICS ENGINEERING

GOVERNMENT ENGINEERING COLLEGE

BARTON HILL

THIRUVANANTHAPURAM

CERTIFICATE

This is to certify that this report titled “A COST-EFFECTIVE SYSTEM FOR WIRELESS

POWER TRANSMISSION” is a bonafide record of the seminar presented by NABEEL.PM

towards the fulfillment of the requirements for the award of B-Tech Degree in Electrical &

Electronics Engineering of the University of Kerala during the year 2011. Guided by

Prof.Jayaprakash Prof. Sheela.S

Professor and Head Asst. Professor

Dept. of EEE Dept. of EEE

Prof.B.Mayadevi Prof.K.L.Sreekumar

Asst. Professor Asst. Professor

Dept. of EEE Dept. of EEE

INDEX OF CONTENTS

1. INTRODUCTION 06

2. WITRICITY TECHNOLOGY 10

2.1 ELECTRICITY 10

2.2 MAGNETISM 10

2.3 ELECTROMAGNETISM 10

2.4 MAGNETIC INDUCTION 10

2.5 RESONANCE 10

2.6 RESONANT MAGNETIC COUPLING 11

3. WITRICITY TECHNOLOGY 12

4. HOW WIRELESS POWER COULD WORK 13

4.1 SHORT RANGE WIRELESS POWER TRANSMISSION 13

4.2 LONG DISTANCE WIRELESS POWER TRANSMISSION 14

5. WIRELESS POWER TRANSMISSION SYSTEM 16

6. COMPONENTS OF WPT SYSTEM 17

6.1 MICROWAVE GENERATOR 17

6.2 TRANSMITTING ANTENNAT 17

6.3 RECTENNA 17

7. A COST EFFECTIVE SYSTEM DESIGN 19

7.1 SYSTEM DESIGN 20

7.2 DESIGN OF CONTROL SIGNAL WAVEFORMS 20

7.3 PHYSICAL ISOLATION 22

7.4 INVERTER CIRCUIT 24

7.5 LOAD DESIGN 26

i) Induction Coil Design: 26

(ii) Snubber circuit 27

8. DIFFERENT WAVEFORMS IN THE CIRCUIT 29

9. ADVANTAGES OF WITRICITY 33

10. DISADVANTAGES OF WITRICITY 34

11. BIOLOGICAL IMPACT OF WITRICITY 35

12. WITIRICITY APPLICATIONS 36

13. FREQUENTLY ASKING QUESTIONS 37

14. WHAT IS THE FUTURE OF WITRICITY 38

15. CONCLUSION 39

16. REFERENCES 40

ABSTRACT

In this paper, we present the concept of transmitting power without using wires i.e., transmitting

power as microwaves from one place to another is in order to reduce the transmission and

distribution losses. This concept is known as Microwave Power transmission (MPT). We also

discussed the technological developments in Wireless Power Transmission (WPT). The

advantages, disadvantages, biological impacts and applications of WPT are also presented.

In recent years, the notion of transfer of power using wireless techniques has attracted many

researchers. Transfer of power by wireless means has been recently demonstrated in the

Massachusetts Institute of Technology (MIT). This system operates at 9.9 MHz. In this paper, we

have discussed the design of a simple and cost-effective system which can enable transmission of

power over short distances. We have employed the H – Bridge Inverter configuration to convert

DC power to high frequency (100 kHz) which is then radiated with the help of a suitable loop

antenna. It is observed that this system can also be used as a induction heating unit. In this form

it can be used to replace conventional convection heating based electric stoves.

ACKNOWLEDGEMENT

I express my sincere gratitude to Prof.Jayaprakash, Professor and Head, Prof.Sheela.S, Staff

Advisor, Department of Electrical & Electronics Engineering, Government Engineering College,

Barton Hill, Thiruvananthapuram, for their expert advice and timely guidance in preparation of

the seminar.

I express my heartfelt thanks to Prof.K.L.Sreekumar, Assistant Professor, Prof.B.Mayadevi,

Professor, Smt.Arlene Davidson, Lecturer, Smt.Anu, Lecturer, Mr. Vinod, Lecturer, Guest

Lecturer Mr.Vinith, Department of Electrical & Electronics Engineering, Government

Engineering College, Barton Hill, Thiruvananthapuram, for their kind co-operation,

encouragement and help.

I thank God Almighty for showering his blessings on me without which this report would have

been impossible.

Last but not the least, I wish to place on record my gratefulness to my parents and friends for

their suggestions, criticisms and assistance towards the improvement and successful completion

of the report.

NABEEL.PM

NOMENCLATURE

PRU: Power Reciving Unit

PTU: Power Transmission Unit.

Fig (1) Nikola Tesla

Fig (2): Magnetic lines from a coil

Fig (3): Nikola Tesla‘s Wardenclyffe tower built on Long Island

Fig (4): A Schematic arrangement of Short range power transmission and reception

Fig (5): A Schematic arrangement of Long distance Wireless power transmission and

reception

Fig (6): Functional Block Diagram of Wireless Power Transmission System

Fig (7): Functional Diagram.

Fig (8): Control Signal Pulse Generator

Fig (9): Pulse Transformer Arrangement

Fig (10): Driver Circuit

Fig (11): H-Bridge Inverter

Fig (12): MOSFET IRFPG50

Fig (13): PCB Design of H-Bridge inverter.

Fig (15): Pancake Induction coil

Fig (16): Turn- OFF Snubber Circuit.

Fig (17): Waveform across the load without snubber circuits

Fig (18): Waveform across the load with the snubber circuits

Fig (19): Waveform at A

Fig (20): Waveform at B

Table 1. Performance of Printed Rectenna

Table2. Rectenna Efficiency for Various Diodes at Different Frequency

1. INTRODUCTION

In this era of modernization, electricity has become the cup of life. A moment without electricity

makes your thinking go dry. The major source of conventional form of electricity is through

wires. The continuous research and development has brought forward a major breakthrough,

which provides electricity without the medium of wires. This wonder baby is called WiTricity.

There are certain small but very useful discoveries made in history, which changed the world

forever, Newton‘s gravitational law, Watt‘s steam engine, Thomson‘s bulb and many more. But

a renaissance occurred with the invention of Electromagnetic Waves by Maxwell. Sir Jagdish

Chandra Bose successfully generated electromagnetic waves having wavelength in the range of

5mm to 25 mm. Thereafter an Italian scientist named Marconi succeeded in transmitting

electromagnetic waves up to a distance of several miles and with this there started a new era

called WIRELESS TECHNOLOGY. Today, as we can see the word ‗wireless‘ is common in day

– to – day life. Wireless communication has made the world smaller. Almost each and

everything is wireless or cordless. Cordless mouse, cordless keyboard, satellite communication,

mobiles, cordless microphones and headphones, wireless internet service i.e. WI-FI, etc. And

these have definitely increased the standard of living. In fact it dates back to the 19th century,

when Nikola Tesla used conduction-based systems instead of resonance magnetic fields to

transfer wireless power. As it is in Radiative mode, most of the Power was wasted and has less

efficiency. Further, in 2005, Dave Gerding coined the term WiTricity which is being used by the

MIT researchers today.

Fig (1) Nikola Tesla was the first to experiment with wireless electricity, but ultimately failed after losing his key

financial backing in the late 1800's

Moreover, we all are aware of the use of electromagnetic radiation (radio waves) which

is quite well known for wireless transfer of information. In addition, lasers have also been used

to transmit energy without wires. However, radio waves are not feasible for power transmissions

because the nature of the radiation is such that it spreads across the place, resulting into a

largeamount of radiations being wasted. And in the case of lasers, apart fromrequirement

of uninterrupted line of sight (obstacles hinders the transmission process). It is also very

dangerous. WiTricity is nothing but wireless electricity. Transmission of electrical energy from

one object to another without the use of wires is called as WiTricity.

WiTricity will ensure that the cellphones, laptops, iPods and other power hungry devices get

charged on their own, eliminating the need of plugging them in. Even better, because of

WiTricity some of the devices won't require batteries to operate. Nikola Tesla was the first to

experiment with wireless electricity, but ultimately failed after losing his key financial backing in the

late 1800's.

The transmission of power without wires is not a theory or a mere possibility, it is now a reality.

The electrical energy can be economically transmitted without wires to any terrestrial distance.

Many researchers have established in numerous observations, experiments and measurements,

qualitative and quantitative. Dr.N.Tesla is the pioneer of this invention.

Wireless transmission of electricity have tremendous merits like high transmission integrity and

Low Loss (90 – 97 % efficient) and can be transmitted to any where in the globe and eliminate

the need for an inefficient, costly, and capital intensive grid of cables, towers, and substations.

The system would reduce the cost of electrical energy used by the consumer and get rid of the

landscape of wires, cables, and transmission towers. It has negligible demerits like reactive

power which was found insignificant and biologically compatible.

2. WITRICITY TECHNOLOGY

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.

2.1 ELECTRICITY

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!

2.2 MAGNETISM

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.

2.3 ELECTROMAGNETISM

A term for the interdependence of time-varying electric and magnetic fields. For example, it

turns out that an oscillating magnetic field produces an electric field and an oscillating electric

field produces a magnetic field.

2.4 MAGNETIC INDUCTION

A loop or coil of conductive material like copper, carrying an alternating current (AC), is a very

efficient structure for generating or capturing a 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

asmagnetic induction. Some common examples of devices based on magnetic inductionare

electric transformers and electric generators.

Fig (2): Magnetic lines from a coil.

2.5 RESONANCE

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 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. Another example of resonance

is the way in which a singer can shatter a wine glass by singing a single loud, clear note. 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. Whenthe 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.

2.6 RESONANT MAGNETIC COUPLING

Magnetic coupling occurs when two objects exchange energy through their varying or oscillating

magnetic fields. Resonant coupling occurs when the natural frequencies of the two objects are

approximately the same.

WiTricity power sources and capture devices are specially designed magnetic resonators that

efficiently transfer power over large distances via the magnetic near-field. The proprietary source

and device designs and the electronic systems the control them support efficient

energy transfer over distances that are many times the size of the sources/devices themselves.

At first glance, WiTricity technology for power transfer appears to be traditional magnetic

induction, such as is used in power transformers, where conductive coils transmit power to each

other wirelessly, over very short distances. In a transformer, an electric current running in a

sending coil (or ―primary winding‖) induces another current in a receiving coil (or ―secondary

winding‖). The two coils must be very close together, and may even overlap, but the coils do not

make direct electrical contact with each other. However, the efficiency of the power exchange in

traditional magnetic induction systems drops by orders of magnitude when the distance between

the coils becomes larger than their sizes. In addition to electric transformers, other devices based

on traditional magnetic induction include rechargeable electric toothbrushes, and inductive

―charging pads‖ which require that the object being charged be placed directly on top of, or very

close to, the base or pad supplying the power.

3. WITRICITY TECHNOLOGY

In the late 1800‘s and early 1900‘s, at the dawn of the electrification of the modern world, some

scientists and engineers believed that using wires to transfer electricity from every place it was

generated to every place that it could be used would be too expensive to be practical. Nikola

Tesla, one of the most well-known of these scientists, had a vision for a wireless world in which

wireless electric power and communications would reach around the world, delivering

information and power toships at sea, factories, and every home on the planet. Tesla contributed

significantly toour understanding of electricity and electrical systems and is credited with

inventingthree-phase AC power systems, induction motors, fluorescent lamps, radio

transmission, and various modes of wireless electric power transfer.

WiTricity mode of wireless power transfer is highly efficient over distances ranging from

centimeters to several meters. Efficiency may be defined as the amount of usable electrical

energy that is available to the device being powered, divided by the amount of energy that is

drawn by the WiTricity source. In many applications, efficiency can exceed 90%. And

WiTricity sources only transfer energy when it is needed. When WiTricity powered device no

longer needs to capture additional energy, the WiTricity power source will automatically reduce

its power consumption to a power saving ―idle‖ state.

Fig (3): Nikola Tesla’s Wardenclyffe tower built on Long Island

4. HOW WIRELESS POWER COULD WORK

Researchers have developed several techniques for moving electricity over long distances

without wires. Some exist only as theories or prototypes, but others are already in use. Magnetic

resonance was found a promising means of electricity transfer because magnetic fields travel

freely through air yet have little effect on the environment or, at the appropriate frequencies, on

living beings and hence is a leading technology for developing WiTricity.

4.1 SHORT RANGE WIRELESS POWER TRANSMISSION

Power supply for portable electronic devices is considered, which receives ambient radio

frequency radiation (typically in an urban environment) and converts it to DC electricity that is

stored in a battery for use by the portable device. A Power transmission unit (PTU) is connected

to the electrical utility, typically in a domestic and office environment, and uses the electricity to

generate a beam of electromagnetic radiation. This beam can take the form of visible light,

microwave radiation, near infrared radiation or any appropriate frequency or frequencies,

depending on the technology chosen. The beam can be focused and shaped using a focusing

mechanism: for example, a parabola shape may be chosen to focus light waves at a certain

distance from the PTU. A Power reception unit (PRU) receives power from one or several

PTU's, and converts the total power received to electricity, which is used to trickle charge a

storage unit such as a battery or transferred directly to the appliance for use, or both. If

transferred to the storage unit, the output of the storage unit can power the appliance. Similarly to

the focusing of the transmitted power, it is possible to concentrate the received power for

conversion, using receiving arrays, antennas, reflectors or similar means.

It is possible to construct power "relay units", consisting of PRU's powering PTU's, whose

function is to make the transmitted power available at further distances than would normally be

possible.

Fig (4): A Schematic arrangement of Short range power transmission and reception

4.2 LONG DISTANCE WIRELESS POWER TRANSMISSION

Some plans for wireless power involve moving electricity over a span of miles. A few proposals

even involve sending power to the Earth from space. The Stationary High Altitude Relay

Platform (SHARP) unmanned plane could run off power beamed from the Earth. The secret to

the SHARP's long flight time was a large, ground-based microwave transmitter. A large, disc-

shaped rectifying antenna, or rectenna, near the system changed the microwave energy from the

transmitter into direct-current (DC) electricity. Because of the microwaves' interaction with the

rectenna, the system had a constant power supply as long as it was in range of a functioning

microwave array. Rectifying antennae are central to many wireless power transmission theories.

They are usually made of an array of dipole antennae, which have positive and negative poles.

These antennae connect to semiconductor diodes. Here's what happens:

1. Microwaves, which are part of the electromagnetic spectrum, reach the dipole antennae.

2. The antennae collect the microwave energy and transmit it to the diodes.

3. The diodes act like switches that are open or closed as well as turnstiles that let electrons flow

in only one direction. They direct the electrons to the rectenna circuitry.

4. The circuitry routes the electrons to the parts and systems that need them.

Fig (5): A Schematic arrangement of Long distance Wireless power transmission and reception

5. WIRELESS POWER TRANSMISSION SYSTEM

William C. Brown, the pioneer in wireless power transmission technology, has designed,

developed a unit and demonstrated to show how power can be transferred through free space by

microwaves. The concept of Wireless Power Transmission System is explained with functional

block diagram shown in Fig (6). In the transmission side, the microwave power source generates

microwave power and the output power is controlled by electronic control circuits. The wave

guide ferrite circulator which protects the microwave source from reflected power is connected

with the microwave power source through the Coax – Waveguide Adaptor. The tuner matches

the impedance between the transmitting antenna and the microwave source. The attenuated

signals will be then separated based on the direction of signal propagation by Directional

Coupler. The transmitting antenna radiates the power uniformly through free space to the

rectenna. In the receiving side, a rectenna receives the transmitted power and converts the

microwave power into DC power. The impedance matching circuit and filter is provided to

setting the output impedance of a signal source equal to the rectifying circuit. The rectifying

circuit consists of Schottky barrier diodes converts the received microwave power into DC

power.

Fig (6): Functional Block Diagram of Wireless Power Transmission System

6. COMPONENTS OF WPT SYSTEM

The Primary components of Wireless Power Transmission are Microwave Generator,

Transmitting antenna and Receiving antenna (Rectenna). Refer the figure Fig (6).

6.1 MICROWAVE GENERATOR

The microwave transmitting devices are classified as Microwave Vacuum Tubes (magnetron,

klystron, Travelling Wave Tube (TWT), and Microwave Power Module (MPM)) and

Semiconductor Microwave transmitters (GaAs MESFET, GaN pHEMT, SiC MESFET,

AlGaN/GaN HFET, and InGaAS). Magnetron is widely used for experimentation of WPT. The

microwave transmission often uses 2.45GHz or 5.8GHz of ISM band. The other choices of

frequencies are 8.5 GHz , 10 GHz and 35 GHz . The highest efficiency over 90% is achieved at

2.45 GHz among all the frequencies.

6.2 TRANSMITTING ANTENNAT

The slotted wave guide antenna, micro strip patch antenna, and parabolic dish antenna are the

most popular type of transmitting antenna. The slotted waveguide antenna is ideal for power

transmission because of its high aperture efficiency (> 95%) and high power handling capability.

6.3 RECTENNA

The concept, the name ‗rectenna‘ and the rectenna was conceived by W.C. Brown of Raytheon

Company in the early of 1960s . The rectenna is a passive element consists of antenna, rectifying

circuit with a low pass filter between the antenna and rectifying diode. The antenna used in

rectenna may be dipole, Yagi – Uda, microstrip or parabolic dish antenna. The patch dipole

antenna achieved the highest efficiency among the all. The performance of various printed

rectenna is shown in Table I. Schottky barrier diodes (GaAs-W, Si, and GaAs) are usually used

in the rectifying circuit due to the faster reverse recovery time and much lower forward voltage

drop and good RF characteristics. The rectenna efficiency for various diodes at different

frequency is shown inTable II.

Table 1. Performance of Printed Rectenna

Table 2. Rectenna Efficiency for Various Diodes at Different Frequency

7. A COST EFFECTIVE SYSTEM DESIGN

Wireless power transmission is still in its infancy as a topic. Although devices and applications

have been designed that transmit power over short distances, their price and design complexity

keep them out of the reach of ordinary users. There is a need for a simple, efficient and cost-

effective system which can create the changing electromagnetic field required to initiate wireless

power transfer. In addition, this can be used to heat vessels as an induction coil based stove. By

suitably replacing the induction coil element with an efficient antenna, the system can be applied

to deliver limited amounts of power wirelessly, for small applications like the charging of mobile

handsets, or laptops.

The aim is to design a system that can be constructed from easily available and low cost

electronic components, thus facilitating the transfer of this technology for the benefit of

humanity. The fundamental principle guiding this system is the use of a suitable inverter circuit

to convert D.C voltage into an alternating supply. Such an alternating voltage would create a

rapidly changing magnetic flux, as per the equation: φ = B * A * Cosθ

Where ‗φ‘ is the magnetic flux,

‗B‘ is the magnetic field density,

‗A‘ is the cross sectional area of the loop and

‗θ‘ is the angle between the magnetic field density and the surface of the loop.

This flux change induces an e.m.f (electromotive force) in any wire loop or metal surface that

cuts the magnetic flux lines. Such an e.m.f, if suitably tapped, can be used either wireless power

transfer

7.1 SYSTEM DESIGN

The goal is to develop a system capable of operating at a frequency of 100 KHz, with an inverter

supply voltage of 300V DC. This frequency is chosen because efficient low cost power

MOSFET switches can be operated efficiently at these frequencies. If the frequency of operation

is further increased, we will have to use expensive power devices which would not be readily

available and would increase the cost of the system. The system as designed by us can be

partitioned into 4 functional blocks, as described in Figure 6.

Fig (7): Functional Diagram.

7.2 DESIGN OF CONTROL SIGNAL WAVEFORMS

It is required to produce two sets of non-overlapping pulses to drive the inverter circuit. The

SG3524 circuit manufactured by Philips semiconductor is ideal for this purpose. It allows for

duty ratio variation up to a maximum of 40%. In addition, a blanking pulse to both outputs rules

out the possibility of pulse overlap. The SG3524 is connected as shown in figure 2 with the

output pulses seen at pins 11 and 14.

Fig (8): Control Signal Pulse Generator

The SG2524 and SG3524 incorporate on a single monolithic chip all the function required for

the construction of regulating power supplies inverters or switching regulators. They can also be

used as the control element for high power-output applications. The SG3524 family was

designed for switching regulators of either polarity, transformer- coupled dc-to-dc converters,

transformer less voltage doubles and polarity converter applications employing fixed-frequency,

pulse-width modulation techniques. The dual alternating outputs allows either single-ended or

push-pull applications. Each device includes an on-ship reference, error amplifier, programmable

oscillator, pulse-steering flip flop, two uncommitted output transistors, a high-gain comparator,

and current limiting and shut-down circuitry.

7.3 PHYSICAL ISOLATION

The H-bridge inverter employs high side and low side switches (four switches in all). The pulses

used to drive the high side switches are derived from the pulses used to drive the low side

switches. However, physical isolation using a suitable isolating technique needs to be

implemented before these pulses are used to drive the high side MOSFETs. This precaution is

essential, for if the same signal is used to drive both the high side and low side MOSFETs in the

circuit without physical isolation, a short circuit will result due to creation of a parasitic path

between the ground and the source of the high side MOSFET. This can result in serious damage

to the MOSFET switches. Physical isolation of the high side drive waveforms can be

implemented using several methods. One such method is to use a high side driver which is

available from several manufacturers. However, we have used pulse transformers to realize this

function in the interests of simplicity, ease of availability and low cost. A 1:1:1 pulse transformer

is used. The output of the SG3524 is used to drive the primary winding of the transformer after

suitable current amplification. Current amplification becomes necessary because the outputs of

the SG3524 are not designed to drive inductive loads directly. The drive waveform is replicated

at the two secondary windings which are physically isolated from the primary windings. A

freewheeling diode (BA 159) is connected across the primary winding of the transformer. A

combination of a resistor and a zener diode is used across the secondary windings of the

transformer to obtain pulses suitable for driving the MOSFET switches. These details are

described in Fig (9).

Fig (9): Pulse Transformer Arrangement

Fig (10): Driver Circuit

7.4 INVERTER CIRCUIT

The outputs from the secondary windings of the pulse transformers are sent to the H-bridge

inverter circuit detailed in Fig (11). One of the advantages of using an H-bridge inverter is that

the load experiences a peak-to-peak voltage of 2Vcc. The inverter works in the required manner

i.e. when Q is high, M1 and M2 are turned ON and current flows from Vcc to Gnd via the path

M1- - A -- Load -- B -- M2. At this stage, the Other two MOSFETs will not be conducting

because their input Q' will be low. When Q becomes low turning Off M1 and M2, Q' becomes

high after sometime, which turns on M3 and M4. Now, the current flows from Vcc to Gnd via

the path M3 -- B - - Load -- A -- M4. The power MOSFETs used to build the inverter are of type

IRFPG50. The VDS=1000 V, ID (max) = 6.1 Amp and on resistance Ron =2 ohms.

Fig (11): H-Bridge Inverter

MOSFET, N, 1000V, 6.1A, TO-247AC; Transistor Type:MOSFET; Transistor Polarity; Voltage,

Vds Typ:1000V; Current, Id Cont:6A; Resistance, Rds On:2ohm; Voltage, Vgs Rds on

Measurement:10V; Voltage, Vgs th Typ:4V; Case Style:TO-247AC; Termination Type:Through

Hole; Current, Idm Pulse:24A; Lead Spacing:5.45mm; No. of Pins:3; Power Dissipation:180W;

Power, Pd:180W; Temperature, Current:25°C; Temperature, Full Power Rating:25°C; Thermal

Resistance, Junction to Case A:0.65°C/W; Transistors, No. of:1; Voltage, Vds:1000V;

Voltage, Vds Max:1000V

Fig (12): MOSFET IRFPG50

Fig (13): PCB Design of H-Bridge inverter.

7.5 LOAD DESIGN

(i) Induction Coil Design:

A wide variety of coil designs are available for several applications. The choice of shape depends

on the nature of radiation pattern to be established (if the application is wireless power transfer)

and the shape of the vessel to be heated (if the system is also to be employed as an induction

cooking unit). We have found the pancake design to be most suitable for a wide range of

applications. Hence, we have designed and fabricated a pancake coil shown in Fig (13) for use in

this system. As a result of the pancake coil design, the energy is focused in the region of space

immediately above the surface of the coil. It is well known that the coupling efficiency increases

with frequency. The coupling efficiency of pancake coil with magnetic steel is 0.35 at a

frequency of 10 Hz and this figure increases to 0.5 at a frequency of 450 kHz. It is for this reason

that we have chosen an operating frequency of 100 kHz. The pancake coil is constructed out of

enamel-coated copper of size SWG 22. Giving a clearance of approx. 1.5 cm on each side, and

the gap between subsequent turns at 3mm, the length of wire required is estimated to be 20m

before braiding. Nails are placed on the board in perpendicular directions at the specified interval

to keep the coil in place while winding. After winding is completed, further nails are driven in to

ensure no two consecutive turns are touching each other. In order to make the coil permanent, the

gaps are filled with Araldite adhesive and allowed to set overnight, after which the nails are

removed. On analyzing the coil characteristics, its parameters were found to be: L = 60.2 μH.R =

0 .790 Ω.

.

Fig (15): Pancake Induction coil

(ii) Snubber circuit

Due to the high speed switching coupled with the presence of an inductive load, the switch

experiences a huge amount of back e.m.f during the turn off stage described by the equation:

E = -L*(di/dt).

Where ‗E‘ is the induced e.m.f, ‗L‘ is the inductance of the coil, and ‗di/dt‘ is the change in

current with respect to time. This leads to the spikes in voltage across the switches, which can

damage the device in the long term. In order to combat these spikes, turn-off snubber circuits

have been designed and put in place across every switch to absorb the back e.m.f and protect the

device. The value of Capacitance and Resistance are given by the following equations:

C = iL * tf / (2Vcc)

Vcc/ (Icm-iL) < R < Ton-min/5C

Where iL is the load current at the collector, if is the fall time of the signal, Icm is the maximum

collector current rating of the MOSFET, and Ton-min is the minimum time during which the

MOSFET should remain ON so that the capacitor can fully discharge. Assuming iL to be 3A and

tf to be 0.12 μSec and since we are operating at a Vcc of 200V, C is found to be 1nF with 1 kV

rating. As per device characteristics, Icm and Ton-min were found to be 8A and 4 μSec

respectively since the duty ratio is 40%. From these, we choose the resistance value to be 470Ω.

The addition of snubber circuits lead to signification suppression of spikes across the switching

devices as shown in fig (17) and Fig (18). On connecting the circuit to the Induction Coil in

series with a 60W bulb at an operating voltage of 60V, the waveforms listed in Fig 19, 20 and 21

were observed. We increased the operating voltage to 120 V. When a coil of wire (closed wire

loop) connected to a 6V bulb was brought close to the Induction coil, it was observed to light up.

This is shown in Fig (20).

Fig (16): Turn- OFF Snubber Circuit.

8. DIFFERENT WAVEFORMS IN THE CIRCUIT

Fig (17): Waveform across the load without snubber circuits

Fig (18): Waveform across the load with the snubber circuits

Fig (19): Waveform at A in Fig(11)

Fig (20): Waveform at B in Fig (11)

Fig (21): Waveform across the LOAD in Fig (11).

A few observations were noted

On increasing the distance between the loop and the coil, the glow of the bulb gradually

diminishes.

For a given distance, the intensity of the bulb‘s glow is maximum when the surface of the

wire loop is parallel to the surface of the coil. The intensity reduces when the loop is

placed at an angle to the coil.

For a given distance, the intensity of the bulb‘s glow is maximum when the loop is placed

near the center of the coil, and reduces as the loop is moved away from the coil.

Thus, it is demonstrated that wireless power transmission is practically possible on such a

system. The system provides conclusive evidence that despite the absence of an antenna of

suitable directivity, a sizeable amount of power can be wirelessly transmitted over short

distances. The next step in the development of this system is to design and build a helical

transmitter and receiver antenna system of suitable dimensions, which can increase the distance

of transmission by improving directivity and gain. By increasing the operating voltage through

repeated testing, the Coil can be applied for use as a heating stove.

9. ADVANTAGES OF WITRICITY

1. Completely eliminates the existing high-power transmission line cables, towers etc…

2. The cost of transmission and distribution become less

3. WiTricity uses resonant magnetic fields to reduce wastage of power.

4. So efficiency of this method is very much higher than wired transmission.

5. The power failure due to short circuit and fault o cables would never exist.

6. The power could be transmitted to the places where the wired transmission is not

possible.

7. Do not interfere with radio waves.

8. No need of power cables and batteries - WiTricity replaces the use of power cables and

batteries

10. DISADVANTAGES OF WITRICITY

1. The resonance condition should be satisfied and if any error exists, there is no

possibility of power transfer.

2. If there is any possibility of very strong ferromagnetic material presence causes low

power transfer due to radiation.

3. The Capital Cost for practical implementation of WPT seems to be very high and the

other disadvantage of the concept is interference of microwave with present

communication systems

11. BIOLOGICAL IMPACT OF WITRICITY

Common beliefs fear the effect of microwave radiation. But the studies in this domain repeatedly

proves that the microwave radiation level would be never higher than the dose received while

opening the microwave oven door, meaning it is slightly higher than the emissions created by

cellular telephones. Cellular telephones operate with power densities at or below the ANSI/IEEE

exposure standards [18]. Thus public exposure to WPT fields would also be below existing

safety guidelines.

12. WITIRICITY APPLICATIONS

WiTricity wireless power transfer technology can be applied in a wide variety of applications

and environments. The ability of our technology to transfer power safely, efficiently, and over

distance can improve products by making them more convenient, reliable, and environmentally

friendly. WiTricity technology can be used to provide:

Direct Wireless Power— when all the power a device needs is provided wirelessly and no batteries are

required. This mode is for a device that is always used within range of its WiTricity power

source.

Automatic Wireless Charging —when a device with rechargeable batteries charges itself while

still in use or at rest, without requiring a power cord or battery replacement. This mode is for a

mobile device that may be used both in and out of range of its WiTricity power source.

WiTricity technology is designed for Original Equipment Manufacturers (OEM‘s) to embed

directly in their products and systems.

WiTricity technology will make your products:

More Convenient

No manual recharging or changing batteries.

Eliminate unsightly, unwieldy and costly power cords.

More Reliable

Never run out of battery power.

Reduce product failure rates by fixing the ‗weakest link‘: flexing wiring.

More Environmentally Friendly

Reduce use of disposable batteries.

Use efficient electric ‗grid power‘ directly instead of inefficient battery charging.

13. FREQUENTLY ASKING QUESTIONS

The concept being so new and innovative brings in so many questions. Hereafter, some questions

are being answered on the basis of study done on the topic and relevant topics.

Is WiTricity technology safe?

Human beings or other objects placed between the transmitter and receiver do not hinder thetrans

mission of power. WiTricity technology is a non-radiative mode of energy transfer, relying

instead on the magnetic near field. Magnetic fields interact very weakly with biological

organisms—people and animals—and are scientifically regarded to be safe.

WiTricity products are being designed to comply with applicable safety standards and regulations.

How much power can be transferred?

Till now, Scientists has been able to transfer more than 60W power. The technology by itself is

capable of scaling from applications requiring mill watts to those requiring several kilowatts of

power.

Over what distance can WiTricity technology transfer power?

WiTricity technology is designed for ―mid-range‖ distances, which we consider to be anywhere

from a centimeter to several meters. The actual operating range for a given application is

determined by many factors, including power source and capture device sizes, desired efficiency,

and the amount of power to be transferred.

How efficient is WiTricity technology?

The power transfer efficiency of a WiTricity solution depends on the relative sizes of the power

source andcapture devices, and on the distance between the devices. Maximum efficiency is

achieved when the devicesare relatively close to one another, and can exceed 95%.

14. WHAT IS THE FUTURE OF WITRICITY

MIT's WiTricit is only 40 to 45% efficient and according to

Soljacic, they have to be twice as efficient tocompete with the traditional chemical batteries. The

team's next aim is to get a robotic vacuum or a

laptopworking, charging devices placed anywhere in the room and even robots on factory floors.

Theresearchers are also currently working on the health issues related to this concept

and have said that in another three to five years‘ time, they will come up with a WiTricity system

for commercial use.

“WiTricity, if successful will definitely change the way we live.

Imagine cellphones, laptops, digital camera’s getting self-charged! Engineers have got job on

hand to research and commercialize the technology. Till then, it is a wait in anticipation”

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15. CONCLUSION

The concept of Wireless Power Transmission system is presented. The technological

developments in Wireless Power Transmission (WPT), the advantages, disadvantages, biological

impacts and applications of WPT are also discussed. The system provides conclusive evidence

that despite the absence of an antenna of suitable directivity, a sizeable amount of power can be

wirelessly transmitted over short distances. The next step in the development of this system is to

design and build a helical transmitter and receiver antenna system of suitable dimensions, which

can increase the distance of transmission by improving directivity and gain.

This concept offers greater possibilities for transmitting power with negligible losses and ease of

transmission than any invention or discovery heretofore made. Dr. Neville of NASA states ―You

don‘t need cables, pipes, or copper wires to receive power. We can send it to you like a cell

phone call – where you want it, when you want it, in real time‖. We can expect with certitude

that in next few years‘ wonders will be wrought by its applications if all the conditions are

favorable.

16. REFERENCES

[1] Nikola Tesla, ―The Transmission of Electrical Energy Without Wires as a Means for

Furthering Peace,‖ Electrical World and Engineer. Jan. 7, p. 21, 1905.

[2] Nikola Tesla, My Inventions, Ben Johnston, Ed., Austin, Hart Brothers, p. 91,1982.

[3] Thomas F. Valone, ― Tesla‘s Wireless Energy... For the 21st Century!!! One Step Beyond

Direct TV!!!‖ Extra Ordinary Technology, 1, no. 4, Oct / Nov / Dec 2003.

[4] James O. McSpadden, ― Wireless Power Transmission Demonstration‖, Texas A&M

University, June, 1997.

[5] Thomas W. Benson , ― Wireless transmission of power now possible‖, News Letter, pp1118 –

9, March , 1920.

[6] Charych Arthur (Setauket, NY), ― System and method for wireless electrical power

transmission‖, Patent No. 6,798,716, September 28, 2004.

[7] Joe T. Howell, et. al , ―Advanced receiver / converter experiments for laser wireless power

transmission‖5th. Wireless transmission conference, pp 1-8, Garanda, Spain,2004.

[8] Nikola Tesla, ― The true wireless‖, Electrical Experiments ,May, 1919.

[9] Toby Grotz,‖ Wireless transmission of power‖, Courtesy of the Tesla BBS at 719 486-2775,

August 28, 1990.

[10] L.Umanand and S.R.Bhat, ―Design of Magnetic Components for Switched Mode Power

Converters‖, Wiley Eastern Limited, 1992.

[11] Zinn and Semiatin, "Coil Design and fabrication", Heat Treating, p. 32- 36, June 1988.

[12] IRF840 datasheet, Fairchild Semiconductor, p. 1-2, January 2002.

[13] IRFPG50 datasheet, International Rectifier, p. 1-2, October 1997.

[14] SG3524 datasheet, Philips Semiconductors, p.1-2, August 1994.