DEVELOPMENT OF A HIGHLY RESONANT WIRELESS

POWER TRANSFER SYSTEM

PROJECT REFERENCE NO.: 41S_BE_0793

COLLEGE : ALLIANCE COLLEGE OF ENGINEERING AND DESIGN, BENGALURU

BRANCH : DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

GUIDE : Dr. ULLAS GURUDAS

STUDENTS : Mr. KOWSHIK KUMAR R.

Ms. RAJESHWARI MUTHAPPA

Mr. YASH T. TATED

Key Words:

Wireless power transfer, Magnetic resonant coupling Power transmission efficiency,

Impedance matching.

Introduction:

In the early days of Electromagnetism, serious interest and effort were devoted towards the

development of schemes to transport energy over long distances wirelessly by many scientists. Most

notably, Nikola Tesla had a vision of wirelessly distributing power over large distances using the

earth’s ionosphere.[1] But his efforts appear to have met with little success. Radiative modes using

Omni-directional antennas which are presently well used for information transfer are not possible

for wireless energy transfer because a large amount of energy is wasted into free space. Another

mode of Radiative energy transfer is the Directed radiation mode, using Lasers or highly directional

antennas which can efficiently transfer energy over a long distances, but require an uninterruptible

line-of-sight and complicated tracking systems in the case of mobile devices. [2]

It is also possible to transmit power using non-radiative fields. For 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. Similarly, most of the currently used schemes rely on non-radiative modes

(magnetic induction) that are restrict to very close range or very low power energy transfer [3-7].

Long-lived oscillatory resonant electromagnetic modes, with localized slowly-evanescent field

patterns on the other hand found suitable for efficient wireless non-radiative mid-range energy

transfer. It is based on the well-known principle of resonant coupling while interacting weakly with

other off-resonant objects. Resonance is a phenomenon wherein two objects of same resonant

frequency tend to exchange energy efficiently, while dissipating relatively little energy in extraneous

off resonant objects. This leads to the result that energy can be efficiently coupled between objects

in the extremely near field.

The above mentioned considerations apply for resonance, irrespective of the physical nature

of the resonances. In these physical realizations, magnetic resonances are of particularly suitable for

everyday applications because most of the common materials do not interact with magnetic fields,

so interactions with environmental objects are suppressed even further. The strongly coupled regime

in magnetic resonance is more efficient than mid-range energy transfer which can be achieved at

megahertz frequencies in a system of two coupled magnetic resonances.[8]

The behavior of the resonator can be described by two fundamental parameters that is its

frequency at resonance ωoand the intrinsic loss rate Ί . The quality factor can be defined from these

two factors and the amount of energy stored in the system can be known(Q=ωo/2 Ί). The

electromagnetic resonator is shown in the Figure 1 which consists of an inductor, capacitor and a

resistor.

FIGURE 1: Example of a Resonator

In the above resonator circuit the energy oscillates at resonant frequency between the

inductor (energy stored in the form of magnetic field) and the capacitor (energy stored in the form of

electric field) which gets dissipated through the resistor. The resonant frequency of the circuit is

given by

𝜔0 =1

𝐿𝐶 (1)

The Quality factor of the circuit is given by

𝑄 =𝜔0

2𝛤=

𝐿

𝐶

1

𝑅=

𝜔0𝐿

𝑅 (2)

The Quality factor should be significantly high for the efficient wireless power transfer to

take place. The high Q factor electromagnetic resonators are typically made up of components and

conductors with low radiative losses and low absorptive losses and have narrow resonant frequency

widths. And also the resonators can be designed to reduce their interactions with the extraneous

objects.

Self Resonator Coil

The theoretical model of the self-resonant coils relays on the interplay between the

distributed inductance and distributed capacitances to achieve the resonance. Considering an

electrically conducting coil of total length 'l' and area of cross section 'a' wound into a helix of 'n'

turns having radius 'r' and height 'h'. At time 0, the current at the ends of the coil will be 0 and will

vary sinusoidally along the length of the coil. When such a coil comes in resonance, the current and

charge density profiles will be out of phase with each other with a phase difference of π/2 which

means that at one point energy is completely due to current and on the other part it is due to the

charge density. This energy due to current and charge density forms an effective inductance and

effective capacitance in the coil respectively.

The effective inductance is given by

𝐿 =𝜇0

4𝜋 𝐼0 2 𝑑𝑟𝑑𝑟 ′ 𝐽 𝑟 .𝐽(𝑟

′)

𝑟−𝑟 ′ (3)

The effective capacitance is given by

1

𝐶=

1

4𝜋𝜀0 𝑞0 2 𝑑𝑟𝑑𝑟 ′ 𝜌(𝑟)𝜌(𝑟

′)

𝑟−𝑟 ′ (4)

Total energy contained in the coil is given by

𝑈 =1

2𝐿 𝐼0

2 =1

2𝐶 𝑞0

2 (5)

Coupled Resonators:

Two resonators are placed in such a way that they can exchange energy through coupling.

Such a combination of resonator coils is called Coupled Resonators.[9] The energy coupling rate k

and the characteristic parameters of each resonators are responsible for the efficiency of energy

exchange. The equivalent circuit for coupled resonators is the series resonant circuit as shown in the

Figure 2.

FIGURE 2: Equivalent circuit for the coupled resonator system.

A sinusoidal voltage source with amplitude Vg at frequency ω having generator resistance

Rgis fed to the source resonator coil represented by the inductor Ls and capacitor Cs. Ld and Cd

represents the device resonator. The source and the device resonator coils are coupled through

mutual inductance M which is given by

𝑀 = 𝑘 𝐿𝑠𝐿𝑑

The resistances Rs and Rd are the parasitic resistances which include both Ohmic and

radiative losses of the coil. The load is represented by an equivalent AC resistance RL .

Analysis of the Figure 2 gave rise to an important parameter called figure-of-merit(U) for the

system which is defined as the ratio of power delivered to the load resistor to the maximum power

available from the source when both the source and the device are in resonance with each other that

is given by

𝑃𝐿

𝑃𝑔 ,𝑚𝑎𝑥=

4𝑈2𝑅𝑔

𝑅𝑠

𝑅𝐿𝑅𝑑

1+𝑅𝑔

𝑅𝑠 1+

𝑅𝐿𝑅𝑑

+𝑈2 (6)

where

𝑈 =𝜔𝑀

𝑅𝑠𝑅𝑑=

𝑘

𝛤𝑠𝛤𝑑= 𝑘 𝑄𝑠𝑄𝑑 (7)

The efficiency of the power transmission is maximized by choosing the generator and load

resistances and is given by the equation

𝜂𝑜𝑝𝑡 =𝑈2

1+ 1+𝑈2 2 (8)

Once the quality factor and the range of magnetic coupling is known efficiency of the system

can be easily determined. A wide range of applications can thus be supported by this wireless

transfer system using Highly Resonant Wireless Power Transfer.

The high quality factor resonators used in wireless systems have very low loss rates, and so

can efficiently store energy and transfer power efficiently over distance, even when the magnitude

of the magnetic fields is very low.

The safety limits for human exposure to electromagnetic fields are determined by on-going

reviews of scientific evidence of the impact of electromagnetic fields on human health. The World

Health Organization (WHO) is expected to release a harmonized set of human exposure guidelines

in the near future. In the meantime, most national regulations reference, and the WHO recommends

[10], the human exposure guidelines determined by the Institute of Electrical and Electronic

Engineers (IEEE) and by the International Commission on Non-Ionizing Radiation Protection

(ICNIRP).

References:

1. Nicola Tesla, “The transmission of electrical energy without wires”, Electrical World

andEngineer, March 1905. http://www.tfcbooks.com/tesla/1904-03-05.htm, (acc. Dec. 08).

2. William C. Brown, “The history of power transmission by radio waves”, Microwave Theory

and Techniques, IEEE Transactions, 32(9):1230-1242, September 1984.

3. J.M. Fernandez, and J.A.Borras, contactless battery charger with wireless control link, US

patent number 6,184,651, issued in February 2001.

4. L. Ka-Lai, J.W. Hay, and P. G. W. Beart, Contact -less power transfer, US patent number

7,042,196, issued in May 2006 (SplashPower Ltd., splashpower.com) Esser, H.-C. Skudenly,

IEEE Trans. Industry Appl. 27(1991) 872.

5. J. Hirai, T.-W. Kim, A. Kawamura, IEEE Trans. Power Electron.15 (2000)21.

6. G. Scheible, B. Smailus, M. Klaus, K. Garrela, and L. Heinemann, "System for wirelessly

supplying a large number of actuators of a machine with electrical power". US patent

number 6,597,076, issued in July 2003 (ABB, <www.abb.com>)

7. K. O’Brien, G. Scheible, H. Gueldner, in IECON ’03, 29thAnnual Conference of the IEEE

(http://ieeexplore.ieee.org/xpl/RecentCon.jsp?punumber=9011) (2003).

8. A.B. Kurs, A. Karalis, R. Moffatt, J.D. Joannopoulos, P.H. Fisher, and M. Soljacic,

“WirelessPower Transfer via Strongly Coupled Magnetic Resonances”, Science, 317, pp.

83-86,(2007).

9. World Health Organization, “Electromagnetic fields and public health”, Fact Sheet No.304,

May 2006.

10. "IEEE Standard for Safety Levels with Respect to Human Exposure to Radio

FrequencyElectromagnetic Fields, 3 kHz to 300 GHz”, IEEE Std. C95.1-2005.

11. “Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic andElectromagnetic

Fields (Up to 300 GHz)”, ICNIRP Guidelines, International Commissionon Non-Ionizing

Radiation Protection, Health Physics, 74, no. 4, pp. 494-522, (1998).

12. Highly Resonant Wireless Power Transfer System, Dr. Morris Kesler, WiTricityCorporation,

2013.

13. Wireless Power Transfer via Strongly Coupled Magnetic Resonances, Andre Kurs, et al.

(Science, 83, 317, 2007)

Objectives:

The ultimate goal of this project is to build a highly resonant wireless power transfer device.

As an essential step towards the ultimate goal the main objective of this project is to develop

instrument that can transfer power safely and efficiently over distances. Some of the main

motivations are as follows.

Make the electronic devices more convenient by eliminating the need for power cord,

connectors or battery replacement

Make the devices more environmental friendly and cost effective. Avoids the need for

disposable batteries. Using grid power is more environmental friendly and economical.

Make the devices safer by eliminating sparking hazards associated with conductive

interconnections

Reduce system cost by providing the ability to power multiple devices from a single

source.

Methodology:

Block Diagram and description

The system description of the proposed device is depicted in the picture shown below. It

consists of two self-resonant copper coils. One coil (the source coil) coupled inductively to an

oscillating circuit while the other (the device coil) is coupled inductively to a resistive load. Self-

resonant coils rely on the interplay between distributed inductance and distributed capacitance to

achieve resonance. The coils are made of an electrically conducting wire of radius a total length l

and cross-sectional radius a wound into a helix of n turns, radius r, and height h. We will use the

quasi-static[14] model proposed by Andre Kursetfor the design of the helix coil that forms the heart

of the system. The input power to the system is from the AC main which is converted to DC that is

shown in the AC/DC rectifier block. A high efficiency switching amplifier converts the DC voltage

into an RF voltage waveform used to drive the source resonator.

FIGURE 3: Block diagram of Wireless Energy Transfer System

The impedance matching network (IMN) is used to efficiently couple the amplifier output to

the source resonator for the efficient switching-amplifier operation. The magnetic field generated by

the source resonator couples to the device resonator, exciting the resonator and causing energy to

build up in it. 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 is used to efficiently couple energy

from the resonator to the load. For loads requiring a DC voltage, a rectifier converts the received AC

power back into DC.

Transmitter Block

The input to the block is given from the AC main which is then converted in to the required

DC supply using a Bridge Rectifier. A sine wave is generated using an oscillator which generates

the radio frequency signal. The generated radio signals are considered to be weak signal for wireless

transfer due to the losses occurring over the transmission medium. Hence the signal is further

amplified using an RF Tuned Amplifier. The amplified signal is then fed to the Source Resonator by

introducing Impedance Matching Network for the effective transfer of signal between them.

The design and optimization of each of the transmitter blocks are explained as follows:

Driver Power Supply

This unit is used to drive the RF Generator and Amplifier Circuits of the transmitter block.

The AC mains power had to be converted to the desired DC supply using a bridge rectifier. The

circuit is as shown in the figure 4.

FIGURE 4: DC/RF Converter

The rectifier circuit consist of a 12-0-12 (2 Ampere) transformer, BY127 diodes, regulator

IC, a transistor, high power rated resistors and capacitors. The transformer’s primary is fed by the

AC input (220 V-50Hz) signal. The high power diodes used are responsible for the rectification of

the AC input into DC. The capacitor resistor combination in the circuit is used for the filtering

action. 7812 IC is the regulator IC used to obtain the required regulated driving power.

Oscillator

A Colpitts oscillator was used to generate the RF waves that is desired for wireless transmission of

power using highly resonant wireless power transfer method. The oscillator was designed in such a

way that it can generate a RF signal of frequencies ranging between 1-10MHz. The circuit is as

shown in the figure 5.

FIGURE 5: Colpitt Oscillator Circuit

The inductance and capacitance of the tank circuit for a desired frequency in a Colpitts

Oscillator can be calculated by using the following equation

𝑓 =1

2𝜋 𝐿𝐶 (9)

The Initially designed circuit was generating a signal of frequency 6.423MHz (10V). This

signal was tuned in such a way that it could generate a frequency as less as possible close to 1MHz

to maintain the IEEE standards as mentioned earlier. Hence tuning was done by varying the

capacitance and inductance in the circuit.

Possible combinations of capacitors were used to tune the signal keeping the inductance in

the circuit constant and the least achieved frequency by doing this was 4.623MHz (8V). Therefore

inductance tuning was also required in the circuit. Since the commercially available standard

inductors were not sufficient for tuning, coils of different turns and radius were made manually and

used in the circuit. A coil of radius 3.5cm having 11 turns offered the least possible frequency of

3.42MHz (11.48V)as shown in the Figure 6. Since the output voltage level of the generated signal

was considerably less for wireless transfer, there was a need for amplification.

FIGURE 6: RF Signal waveform generated by the Oscillator

Tuned Amplifier

A tuned amplifier circuit is used to amplify the obtained generated RF signal of frequency

3.42MHz. The designed single tuned amplifier circuit is as shown in the figure 7.

FIGURE 7: RF Tuned Amplifier Circuit

This amplifier circuit had a disadvantage that if the signal frequency goes 1 MHz, the

amplification was found reduceddrastically. Therefore the effects of each and every parameter in the

circuit were studied and we observed that the tank circuit along with the output capacitor are

responsible for the change in the amplification factor beyond the above mentioned limit. Hence the

circuit tuning was done by using the combination of capacitors along with varying the inductance of

the tank circuit. The suitable amplification was achieved by using a coil of 2 turns having a radius of

7cm along with some capacitors. This amplified output was fed to the source resonator coil.

FIGURE 8: Amplified output waveform from the tuned amplifier

Source resonator

This device was used to radiate the power wirelessly from the transmitter to the receiver end.

It is made up of a coil of 17 turns having radius of 13 cm and cross sectional area of 12.56 m2.

FIGURE 9:Source resonator used in the Setup

Receiver Block

The device resonator is the first part of the receiver block. When the device resonator comes

in resonance with the source resonator transfer of power occurs from the source resonator to the

device resonator of the receiver block separated by few metres of distance. The device resonator coil

output is rectified to DC voltage by using a high frequency rectifier circuit. The obtained DC power

is used for the load circuits.

Device resonator

This device was used to receive the power radiated by the source resonator coil. It is

identical in dimensions with the source resonator so that the natural frequency of the coils match

with each other.

RF to DC Converter

A high frequency rectifier circuit was used to convert the received RF power to the corresponding

DC voltage. The circuit is as shown in the figure 10

FIGURE 10: RF/DC Converter

The circuit consists of switching diodes -1N448, ceramic and electrolytic capacitors. The design was

based on Villard voltage multiplier circuit. A four stage schottky diode voltage multiplier was

designed to convert the RF energy signal into DC voltage which can be used to energise the low

power electronic device.

Impedance matching network

Impedance matching is very important to deliver maximum power to the load with properly matched

transmission line and to avoid reflections back to the source.

The method used to match the impedance of any two circuits in the project was matching using L

networks. Matching was done at all the required stages of the system to match the load with the

transmission line.

FIGURE 11: The experimental setup

The final experimental setup is as shown in figure 11. The experiment was performed on the setup

by matching the impedance of all the stages. Initially, the two coils were aligned in such a way that

the axis of the two coils were coinciding with each other and transferred power signal was observed

and recorded using the Digital Signal Oscilloscope. The changes in the output waveform was noted

by varying the distance between the two coils. The experiment continued by varying the orientation

of the coil also at different distances. The transmitter and receiver blocks separated by certain

distance, transferring the energy from source resonator to device resonator(shown by DSO) are

shown in the figure 11.

1. Results and Conclusion

The experimental setup was developed (as shown in the figure 11) for Highly Resonant Wireless

Power Transfer to occur for efficient transfer of power wirelessly. Certain experiments were

performed on the system to study its behaviours.

Study on Distance between the Coils

The developed system had a very good response when it was observed for the smaller

distances (1-30cm). The expectation from the system was to give the similar response at few metres

of distance also. Hence an experiment was carried out by varying the distance of the separation

between the resonator coils. The following table shows the corresponding AC and DC voltages at

the receiver for various distance of separation.

DISTANCE (in cm) Vpp (in

Volts)

Vdc (in Volts)

25 130 104.2

30 140 39

35 80 21.6

40 50 12

45 35.2 7.9

50 24 4.9

55 15 3.3

60 12 2.28

65 8.4 1.66

70 7.6 1.28

75 6 0.92

80 4.24 0.64

85 3.52 0.43

90 2.88 0.3

95 2.5 0.17

100 2.5 0.12

TABLE 1: Output AC and DC voltages at different distances

The signal was recorded at different distances to identify the changes and for the further studies on

the system developed. The figure 12 shows various output waveforms at different distances.

FIGURE 12: Output signal at different distances

From the Figure 12, it can be noted that the signal strength reduces as distance between the source

resonator and the device resonator increases. Hence it infers that there is a limit for the wireless

transfer of power using Highly Resonant Wireless Power Transfer Method.

FIGURE 13: Graph showing the Output AC and DC voltages at different distances

INSET: The voltage variation between 20 and 30cm

From the Figure 13, it is observed that there is drastic change in the voltage levels between 25cm to

30cm. In this range the AC signal increases from 130 volts to 140 volts while the DC voltage

decreases from 104.2 volts to 39 volts. We are not in a position to give a proper explanation to this

effect. We offer the following tentative explanation to this effect. If we look carefully the waveform

we can see that another mode start oscillating with lesser amplitude and probably that mode is

contributing the DC voltage signal. Detailed study in this direction is underway and will report in

due course of time.

Study on the Orientations of Receiver

Another experiment was carried out on the system by varying the orientation of the receiver coil

with respect to the transmitter by keeping the distance between them constant (say 35cm) but

adjusting the receiver in certain angles like 450, 90

0, 135

0 and 180

0. The corresponding AC and the

DC voltages were noted as in Table 2 and a graph was plotted as shown in the figure 14.

Distance (in

cm)

Angle (in

degree)

Vpp (in Volts) Vdc (in Volts)

35 0 35.2 8.13

35 45 66.0 14.39

35 90 32.0 10.25

35 35 21.6 4.25

35 180 27.2 6.99

TABLE 2:Output AC and DC voltages at a distance with different orientations

FIGURE 14: Graph showing Output AC and DC voltages with different orientations

We have not noticed any marked difference in the output signal so the orientation of the receiver is

has no significant role. This is a welcome sign as the normal users need not worry about orienting

the receiver for better reception.

Conclusion:

We have developed a Highly Resonant Wireless Power Transfer System that can be used to

power up the devices wirelessly, effectively and efficiently within considerable distances. Few

parametric studies such as variation of voltage with distances and different orientation were made.

We have found some interesting effect in the distance range of 25 to 30 cm that the AC signal

increases but the DC signal decreases drastically. A tentative explanation to this effect was given.

Many studies still remains and that will be the future direction of this project. These studies are

underway and will report in due course of time.

Scope for Future Work:

Miniaturization is one important aspect that is essential for the receiving part as it needs to

be integrated with hand held instruments such as mobile phones. The device developed by us is

capable of transferring power to a distance of 1 m. It needs to be improved and transfer power

efficiently to a minimum distance of two meters. Many parametric studies need to be undertaken to

understand the complete mechanism. Different types of receivers also to be tested and optimized for

various practical applications. These objectives will be our future direction on this project.