MASTER'S THESIS - diva- 1031874/ Physics and Electrical Engineering at Lulea University

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  • MASTER'S THESIS

    Optimization of Wireless Power

    Oskar Rönnbäck 2013

    Master of Science in Engineering Technology Engineering Physics and Electrical Engineering

    Luleå University of Technology Deptartment of Computer Science, Electrical and Space Engineering

  • Optimization of Wireless Power

    Oskar Rönnbäck

    Lule̊a University of Technology Dept. of Computer Science, Electrical and Space Engineering

    December 5, 2013

  • ABSTRACT

    Today, the limit of wireless devices lays in the way they are powered. Imagine a device

    that doesn’t need a charger or even a battery, which instead gets the power wirelessly

    over the air. To make such a device possible the transfer distance of currently known

    systems have to be increased. That will be the aim of this thesis, to investigate how

    to increase the transfer distance of a wireless power system, WPS, purposed to charge

    low power electronic devices. In order for the system to be usable certain design limits

    are set to restricts the size of the coils, flat spiral coils with diameter < 90mm and wire

    diameter < 2mm, and thereby also narrowing the scope of the thesis.

    This thesis starts with a presentation of the theoretical framework behind wireless

    power, including techniques for modeling a complete system. The framework is then

    broken down to its basic components which generates expressions with geometrical and

    material properties as variables. These expressions are implemented in Matlab creat-

    ing a simulator, which finds optimal values of geometrical and material properties that

    maximizes the transfer distance.

    The simulator is set up and ran for each system, 2, 3 and 4 coils, this because each

    system behaves differently and all have some desirable properties. The findings are

    implemented in Comsol which provides verification and illustrates the electromagnetic

    fields that are generated. The results from Comsol and Matlab are similar and shows

    that a 2-coil system can transfer power with 40% efficiency over a distance of ≈ 150mm. While 3- and 4-coil systems significantly improve the transfer distance and can transfer

    power with the same efficiency over a distance of ≈ 350mm. As a last step were WPS’s built using the findings from the simulations. The coils were

    made according to the optimal parameters and capacitors were added to tune them to

    the same resonance frequency. An E-class amplifier was designed and built to excite the

    transmitting coil in the real system. The measurements made are the power delivered

    to the amplifier and the power delivered to the load. From that the efficiency of the

    complete system can be calculated. The measurements made in this thesis don’t hold

    up to the simulations in the sense of transfer distance. The main reasons for that is that

    the amplifier is included in the measured PTE and not in the simulations and that the

    coils are not perfectly built or tuned.

    iii

  • PREFACE

    This thesis work were conducted as the last part of the Master Programme in Engineering

    Physics and Electrical Engineering at Lulea University of Technology, LTU.

    During my project work course I first came into contact with wireless power and I

    thought is was a fascinating technology. Seeing the possibilities for wireless power it is

    clear that it will play a huge part in the future of electronics. I was not aware of any

    research in this area in Sweden, therefore I made the thesis work as a project on my own

    initiative which I carried out at LTU.

    I would like to thank Kalevi Hyyppa for his understanding and guidance and my family

    for always supporting me.

    Oskar Ronnback

    v

  • CONTENTS

    Chapter 1 – Introduction 1

    1.1 Wireless power today . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

    1.2 Benefits of wireless power systems . . . . . . . . . . . . . . . . . . . . . . 2

    1.2.1 Environmental . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    1.2.2 Social . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

    1.3 Baseline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.4 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

    1.5 Goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.6 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

    1.7 Frequently used variables and abbreviations . . . . . . . . . . . . . . . . 4

    Chapter 2 – Theory 7

    2.1 Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    2.1.1 Resistance in a wire . . . . . . . . . . . . . . . . . . . . . . . . . . 7

    2.1.2 Litz wire resistance . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    2.2 Inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    2.2.1 Self inductance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

    2.2.2 Inductance of pancake coil . . . . . . . . . . . . . . . . . . . . . . 9

    2.2.3 Inductor quality factor . . . . . . . . . . . . . . . . . . . . . . . . 9

    2.2.4 Mutual inductance . . . . . . . . . . . . . . . . . . . . . . . . . . 9

    2.2.5 Coupling coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    2.3 Induction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

    2.4 Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

    2.4.1 Electrical Resonance . . . . . . . . . . . . . . . . . . . . . . . . . 11

    2.5 Wireless power using magnetic resonance . . . . . . . . . . . . . . . . . . 12

    2.6 Coupled Mode Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

    2.6.1 Lossy model with source excitation . . . . . . . . . . . . . . . . . 13

    2.6.2 Model of lossy 2-coil coupled system . . . . . . . . . . . . . . . . 13

    2.6.3 Wireless Power Transfer Efficiency . . . . . . . . . . . . . . . . . 14

    2.6.4 WPT expanded to 3- and 4-coil systems . . . . . . . . . . . . . . 14

    2.7 Reflected Load Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

    2.7.1 Expanded for m-coil systems . . . . . . . . . . . . . . . . . . . . . 15

    2.8 Unified theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

  • Chapter 3 – Simulations 17

    3.1 Matlab simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

    3.1.1 Finding the optimal PTE . . . . . . . . . . . . . . . . . . . . . . 18

    3.1.2 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

    2-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

    3-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

    4-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

    3.2 Comsol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    3.2.1 Simulation setup . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

    3.2.2 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    2-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

    3-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

    4-coil system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

    Chapter 4 – Electronic design 31

    4.1 Tuning of the coils . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

    4.2 Source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

    4.3 E-class amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

    4.4 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

    4.4.1 Component selection . . . . . . . . . . . . . . . . . . . . . . . . . 33

    4.4.2 PSpice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

    Chapter 5 – Real testing 37

    5.1 Measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    5.2 Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

    5.3 Measurements on 2-coil system . . . . . . . . . . . . . . . . . . . . . . . 38

    5.4 Measurement on 3-coil system . . . . . . . . . . . . . . . . . . . . . . . . 39

    5.5 Measurements on 4-coil system . . . . . . . . . . . . . . . . . . . . . . . 40

    Chapter 6 – Discussion 41

    6.1 Simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

    6.2 Real tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    6.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

    6.4 Health issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    6.5 Future work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

    viii

  • CHAPTER 1

    Introduction

    Wireless power is an old concept, Nikola Tesla experimented with it in the late 1800’s.

    He was considering it as an alternative to building the electric grid. History tells us

    that wireless power were never realized at a consumer level and the concept was almost

    forgotten. Induction stoves and transformers transfers powe