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WITRICITY

WIRELESS ELECTRICITY USING MAGNETIC RESONANCE

CH Roja

Electrical & Electronics Engineering GMRIT

Rajam, AP [email protected]

Abstract — This paper deals with a new mode of

generation of power i.e. wireless electricity. Witricity or

wireless energy is the transfer of electrical energy without

using wires. At present electrical generation system more than

half of the energy gets wasted till it reaches for utilization.

And moreover there is a rapid development in the electronics

such as mobiles, laptops, robots and various equipment which

rely on battery power. As these technologies are becoming

present generation needs therefore wireless energy would be

useful for powering up these devices since they need mid

range energy. We propose the use of a new technology, based

on strongly coupled magnetic resonance. It consists of a

transmitter, a current carrying copper coil, which acts as an

electromagnetic resonator and a receiver, another copper coil

of similar dimensions to which the device to be powered is

attached. The transmitter emits a non-radiative magnetic

field resonating at MHz frequencies, and the receiving unit

resonates in that field. Using wireless electricity this would be

helpful as there wouldn’t be any need to replace batteries as

energy could be transferred from one sensor to other. .

Present day wireless transmission and reception are, Dr

Nichola Tesla based this shows indeed he is the “Father of

Wireless”.

Keywords – wireless energy transfer; near field;

evanescent wave, magnetic resonance; self-resonance

I. ORIGINATION

The wireless power inventor Nichola Tesla performed the

first experiments at the turn of the 20th

century. Nichola

Tesla one of these scientists, had a vision for wirelessworld in which wireless electric power would reach around

the world delivering information and power to ships at sea,

factories and even home on the planet. Nichola Tesla had

devised a type of resonant transformer called Tesla coil inthe year 1899.A major popularity came through his work

when he transmitted 100 million volts of electric powerwireless over a distance of 26 miles to light up a bank of

200 light bulbs and run one electric motor. Tesla claimed to

achieve around 95% efficiency. Tesla claimed to be able to

transmit power on a worldwide scale, using a method that

involved conduction efficiency. Tesla claimed to be able to

transmit power on a worldwide scale, using a method thatinvolved conduction through the Earth and atmosphere.The

proposal suggested that receiving stations would consist of

terminals suspended in the air at above 30,000 feet

(9,100 m) in altitude, where the pressure is lower than at sea

level. At this altitude, Tesla claimed, electricity could besent at high voltages (millions of volts) over long distances.

Fig. 1

But this idea was adjourned as the effects of transmitting

such high voltages in electric arcs would have been

disastrous to human beings and nearby electrical

equipment. Similarly William C Brown demonstrated a

micro wave powered model helicopter in 1964. This

receives all the power needed for flight from a micro wave beam. In 1975 Bill Brown transmitted 30kW power over a

distance of 1 mile at 84% efficiency without using cables.

Researchers developed several technique for moving

electricity over long distance without wires. Some exist

only as theories or prototypes, but others are already in use.Consider an example, in this electric devices rechargingwithout any plug-in. The device which can be recharged is

placed on a charger. Supply is given to the charger and

there is no electrical contact between charger and device.

The recharging takes place in following steps.

1. Current from the wall outlet flows through a coil

Inside the charger, creating a magnetic field. In atransformer, this coil is called the primary winding.

2. When the device placed on the charger, the

magnetic field induces a current in another coil, or

secondary winding, which connects to the battery.

3. This current recharges the battery.

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Fig. 2

II. PRINCIPLE OF WIRICITY

Resonant inductive coupling is the near field wireless

transmission of electrical energy between two magneticallycoupled coils that are part of resonant circuits tuned

to resonate at the same frequency. This process occurs in aresonant transformer, an electrical component which

consists of two high Q coils wound on the same core

with capacitors connected across the winding to make two

coupled LC circuits. Resonant transformers are widely

used in radio circuits as band pass filters, and in switching power supplies. Resonant inductive coupling is also being

used in wireless power systems. Here the two LC

circuits are in different devices; a transmitter coil in one

device transmits electric power across an intervening space

to a resonant receiver coil in another device. This

technology is being developed for powering and charging portable devices such as cell phones and tablet computers

at a distance, without being tethered to an outlet. Thus, the

main idea behind the working is electromagnetic resonance

between two circuits. The Electrical resonance occurs in an

electrical circuit at a particular resonance frequency when

the impedance between input and output of the circuit isminimum. Often this happens when the impedance

between the input and output of the circuit is almost zero.

As an LC circuit is formed there is always tendency of

exchanging their reactive powers. Resonance of a circuit

involving capacitors and inductors occurs because thecollapsing magnetic field of the inductor generates an

electric current in its winding that charges the capacitor,and then the discharging capacitor provides an electric

current that builds the magnetic field in the inductor, and

the process is repeated continually. Formula of resonance

frequency:

Where, ω = 2πf, in which f is the resonance frequency in

hertz, L is the inductance in henries, and C is thecapacitance in farads when standard SI units are used.

Fig. 3

III. COULPLING

NEAR FIELD INDUCTIVE COUPLING

Unlike the previously used far field electromagnetic waves,

we propose the use of near field inductive coupling

through magnetic resonance. A near field is the region

around the source of electromagnetic radiation within a

radius r << λ, where λ is the wavelength of the transmitted

wave. The total energy per unit area at a distance r from the

transmitter is proportional to 1/r2. Since the receiver is

within a very small radius of the transmitter, most of the

transmitted energy appears at the receiving end. Thus,

losses are less. Another advantage of using near fields is

that energy is available to the receiver only if the energy is

tapped, and this is sensed by the transmitter by means of

answering electromagnetic near fields emanated by the

receiver. This is different from the far field, which draws

energy constantly from the transmitter, whether it is

immediately received, or not. This non-radiative energy

transfer is mediated through evanescent waves. The

theoretical realization of the scheme, Fig. 4, consists of two

self-resonant coils. One coil (the source coil 2S) is coupled

inductively to an oscillating circuit; the other (the device

coil 2D) is coupled inductively to a resistive load.

Fig. 4. Theoretical model for self-resonant coils 2A is a

single copper loop of radius 25 cm that is part of the

driving circuit, which outputs a sine wave with frequency 1

MHz. 2D is a loop of wire attached to the load (light bulb).The various K’s represent direct coupling constants

between the objects indicated by the arrows. The direct

coupling constants between 2B and 2A and between 2B

and 2S are negligible.

Fig. 4

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EVANESCENT WAVE COUPLINGAn evanescent wave is a damped wave, whose power

exponentially decays with an increase in distance.

Evanescence basically means ‘tending to vanish’. The

wave is most intense within a distance of one-third

wavelength from an electromagnetic source.Evanescent

wave coupling is accomplished by placing two or moreelectromagnetic elements close together so that the

evanescent field generated by one element does notdecay much before it reaches the other element.

Referring to Fig. 5, in evanescent wave coupling, both the

transmitter and receiver emit evanescent waves of the same

frequency. However, they are traveling in opposite

directions. This forms a standing wave, which facilitates

efficient transfer of energy between the transmitter and

receiver. Initially, the energy is being transmitted from the

transmitter to the receiver. As the receiver utilizes

this energy, the standing wave collapses. This process

repeats itself and the energy is transferred in bursts.

Fig. 5. Propagation of energy using a standing wave.

IV. METHODS FOR WIRELESS POWER

TRANSMISSION

These techniques are briefly classified into three depending

on the distance between the transmitter and receiver. These

are: Short range, Moderate range and Long range.

SHORT DISTANCE RESONANCE : These methods can

reach at most a few centimeters. The action of an electricaltransformer is the simplest instance of wireless energy

transfer. The primary and secondary circuits of a

transformer are electrically isolated from each other. The

transfer of energy takes place by electromagnetic coupling

through a process known as mutual induction. The electric

toothbrush charger is an example of how this principle can be used. Toothbrush's daily exposure to water makes a

traditional plug-in charger potentially dangerous. Ordinary

electrical connections could also allow water to seep into

the toothbrush, damaging its components. Because of this,most toothbrushes recharge through inductive coupling.

MODERATE DISTANCE RESONANCE : Householddevices produce relatively small magnetic fields. For this

reason, chargers hold devices at the distance necessary to

induce a current, which can only happen if the coils are

close together. A larger, stronger field could induce current

from farther away, but the process would be extremelyinefficient. Since a magnetic field spreads in all directions,

making a larger one would waste a lot of energy. An

efficient way to transfer power between coils separated by

a few meters is that we could extend the distance between

the coils by adding resonance to the equation. Induction

can take place a little differently if the electromagneticfield around the coils resonate at the same frequency.

Fig. 4

A capacitance plate, which can hold a charge, attaches to

each end of the coil. As electricity travels through this coil,

the coil begins to resonate. Its resonant frequency is a product of the inductance of the coil and the capacitance of

the plates. The wireless power project uses a curved coil

and capacitive plates. Electricity, travelling along an

electromagnetic wave, can tunnel from one coil to the other

as long as they both have the same resonant frequency. In a

short theoretical analysis these can be demonstrated bysending electromagnetic waves around in a highly angular

waveguide, evanescent waves are produced which carry no

energy. An evanescent wave is near field standing wave

exhibiting exponential decay with distance. If a proper

resonant waveguide is brought near the transmitter, the

evanescent waves can allow the energy to tunnelspecifically evanescent wave coupling, the electromagnetic

equivalent of tunneling to the power drawing waveguide,

where they can be rectified into DC power. Since the

electromagnetic waves would tunnel, they would not

propagate through the air to be absorbed or dissipated, andwould not disrupt electronic devices. The multiple-layer

secondary of a non-resonant transformer, such receiving

coils are single layer solenoids with closely spaced

capacitor plates on each end, which in combination allow

the coil to be tuned to the transmitter frequency thereby

eliminating the wide energy wasting "wave problem" andallowing the energy used to focus in on a specific

frequency increasing the range.

LONG DISTANCE WIRELESS POWER : Whether or not

it incorporates resonance, induction generally sends power

over relatively short distances. But some plans for wireless

power involve moving electricity over a span of miles. Afew proposals even involve sending power to Earth from

space. In the 1980s, Canada's Communications Research

Centre created a small airplane that could run off power

beamed from the Earth.

IV. BENEFITS

1) Highly Resonant Strong Coupling Provides High

Efficiency Over Distance.

2) Energy Transfer via Magnetic Near Field Can

Penetrate and Wrap Around Obstacles.

3)

Non-Radiative Energy Transfer is Safe for People and

Animals.

4) Scalable Design Enables Solutions from Milli watts

to Kilowatts.

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5) Lower Harmonics.6) Higher Efficiency Adaptive Resonant System.

V. DEMERITS

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 ofmicrowave with present communication systems.

VI. WITRICITY 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 :

1) Automatic wireless charging of mobile electronics in

home, car, office, Wifi-hot spots while devices are inuse and mobile.

2) Direct wireless powering of stationary devices (flat

screen TV’s, digital picture frames, screens etc).

3) Direct wireless powering of desktop PC peripherals:

mouse, keyboard, printer etc eliminating disposable

batteries and cables.4) Direct wireless power and communication

interconnections within rotating and moving joints

(robots, packaging machinery, machine tools).

5) Automatic wireless charging for existing electrical

vehicles.

6) Automatic charging for future hybrid and allcommercial vehicles at home, at parking garages and

at remote areas.

Need for wireless power transmission Wireless

transmission is employed in cases where instantaneous or

continuous energy transfer is needed, but interconnectingwires are inconvenient, hazardous, or impossible.

Number of household points receives electricity at the

same frequency using single transmitting coil as long as

they all are at resonance. So this setup could recharge all

the devices in a room at once. The unmanned planes or

robots (where wires cannot be involved viz oceansvolcanic mountains etc.) which are run by the wireless

power over an area, as they could fly for months at a time,could be used for research as well as a mini satellite.

A few proposals even involve sending power to the Earth

from space

VII. CONCLUSION

The concept of Witricity is discussed. The transmission of

power without wires is no more a theory or a mere

possibility, it is reality now. The electrical energy can be

transmitted to any terrestrial distance. The system would

reduce the cost of electrical energy used by consumer

and get rid of landscape of wires,cables, and transmissiontowers.The technological advancements its merits,

demerits and applications are also mentioned.Wireless

power transmission is more advantageous having

efficiency of 90%. It eliminates the capital cost for the

requirement of cables, towers and substations. It has a

great economic impact on human economy.

ACKNOWLEDGMENT

We hereby wish to acknowledge our professors andfriends who have shared their knowledge and guided usin preparing this work.

REFERENCES

[1] Nikola Tesla, My Inventions, Ben Johnston,

Ed., Austin, Hart Brothers, p. 91, 1982.

[2] http://www.WiTricity.com

[3] http://www.WiTricitypower.com

[4] Benson, Thomas W."Wireless Transmission of

Power now Possible”

[5] U.S. Patent 787, 412, "Art of Transmitting

Electrical Energy through the Natural

Mediums".

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