DESIGN AND OPTIMISATION OF THREE-PHASE

SALIENT ROTOR WOUND FIELD FLUX

SWITCHING MOTOR

FAISAL KHAN

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

DESIGN AND OPTIMISATION OF THREE-PHASE SALIENT ROTOR WOUND

FIELD FLUX SWITCHING MOTOR

FAISAL KHAN

A thesis submitted in

fulfilment of the requirement for the award of the

Doctor of Philosophy in Electrical Engineering

Faculty of Electrical and Electronic Engineering

Universiti Tun Hussein Onn Malaysia

November 2016

iii

This dissertation is dedicated to my parents, my wife, my brother, and sisters, who

have always encouraged me with their love and prayers.

DEDICATION

iv

ACKNOWLEDGEMENT

I am very thankful to Almighty ALLAH for without His graces and blessings,

this study would not have been possible.

Since I started my PhD research work, several people and organisations have

been involved directly and indirectly with my project. They have loyally provided

me encouragement, motivation, moral, and financial support. Hereby, I express my

gratitude to them.

I would like to thank my supervisor, Assoc. Prof. Ir. Dr. Erwan Sulaiman, for

providing the opportunity to be one of his students. His guidance and support during

the entire study made it possible for me to write this thesis. I am continually amazed

by his discipline, research, and scientific skills.

This PhD project has been made possible with the financial support from the

ORICC, Universiti Tun Hussein Onn Malaysia, and Ministry of Higher Education

(MOHE), Malaysia. I would like to record my gratitude to the staffs from ORICC

who have been involved in my project. I would like to thank COMSATS Institute of

Information Technology, Abbottabad Campus for granting me study leave for PhD

studies.

My sincere thanks go to Mr. Md Zarafi Ahmad for his convincing

explanations and important contributions. I also want to thank Mr. Mohd Fairoz

Omar and Mr. Gadafi M. Romalan for sharing practical and theoretical knowledge.

Furthermore, I wish to express my sincere gratitude to my family for their

moral support and prayers. I would like to thank my grandfather, Mr. Haji Aslam

Khan Malik and father, Mr. Nifaz Ali for their prayers and support, and also to my

mother, the one who had made me successful with her caring and gentle love. I

would also like to thank to my wife for her support, endless encouragement, prayers,

and love. She had spent lots of time taking care of our lovely daughter Burooj during

my study period. To my brother Mr. Shah Saud, I would like to say thanks for

supporting my family throughout my PhD studies.

v

ABSTRACT

Permanent magnet-free wound field flux switching machine (WFFSM) with a

segmented rotor and non-overlapping windings is an attractive alternative for driving

high torque density applications due to their low cost, high efficiency, high average

torque and power. However, a rotor with segments makes the motor less robust and

difficult to be assembled, while WFFSM with salient rotor and overlapping windings

inherit high copper losses and less efficiency due to their long end-windings. This

thesis deals with a novel structure of WFFSM employing a salient rotor with non-

overlapping field and armature windings on the stator and a presentation of an

unexcited rotor. The non-overlapping winding arrangement on the stator consumes

less copper material, thus improves the efficiency. Moreover, the salient rotor

structure with high mechanical strength is suitable for high-speed operation. The

design restrictions and specifications of the proposed motor are keep similar as

WFFSM with a segmented rotor. The JMAG-Designer ver.14.1 was used to verify

the motor’s operating principle and performance characteristics. Three-phase

configurations of WFFSM with non-overlap windings and salient rotor were studied,

from the design features to performance analysis. For the three-phase operation, 11

topologies were feasible when employing a 12-tooth and 24-tooth stator. The

subsequent optimisation work carried out using deterministic optimisation approach

and Genetic Algorithm (GA) method to achieve the target average torque of 25.8 Nm

and power of 6.49 kW. Designed and analysed by 2D and 3D finite element analysis

(FEA), the optimised 12S-10P configuration had achieved high torque and power of

4.6% and 4.8% respectively, as compared to 12S-8P WFFSM with segmental rotor

and non-overlapping windings. Moreover, the torque and power of the optimised

design were also greater than 12S-8P WFFSM with salient rotor and overlapping

windings. The 12S-14P topology had produced high average torque and power at low

armature and field currents compared with all designs.

vi

ABSTRAK

Mesin fluks beralih medan-belitan tanpa magnet kekal (WFFSM) dengan pemutar

bersegmen dan belitan tidak bertindih adalah alternatif yang menarik terhadap

aplikasi yang memerlukan ketumpatan tork yang tinggi. Ini disebabkan oleh kos

yang rendah, kecekapan, tork dan kuasa purata yang tinggi. Walau bagaimanapun,

pemutar bersegmen menjadikan motor kurang teguh serta sukar untuk dipasang,

manakala, WFFSM dengan pemutar menonjol dan belitan bertindih menyebabkan

kerugian pada penggunaan tembaga yang tinggi dan kecekapan yang kurang akibat

daripada penghujung belitan yang panjang. Tesis ini berkaitan struktur baru WFFSM

menggunakan pemutar menonjol yang mempunyai medan dan angker tanpa belitan

bertindih pada pemegun dan pemutar yang tidak teruja. Susunan penggulungan tidak

bertindih pada pemegun yang menggunakan kurang bahan tembaga, sekaligus

mampu meningkatkan kecekapan. Selain itu, struktur pemutar menonjol dengan

kekuatan mekanikal yang tinggi adalah sesuai untuk operasi mesin pada kelajuan

tinggi. Had rekabentuk dan spesifikasi motor yang dicadangkan adalah masih sama

seperti WFFSM dengan pemutar bersegmen. Perisisan JMAG-Designer versi 14.1

telah digunakan untuk mengesahkan prinsip operasi dan prestasi motor. Konfigurasi

tiga fasa WFFSM dengan belitan tidak bertindih dan pemutar menonjol telah dikaji,

bermula dengan ciri-ciri rekabentuk sehingga analisis prestasi. Bagi operasi tiga fasa,

11 topologi telah dilaksanakan apabila menggunakan 12-gigi dan 24-gigi pemegun.

Kerja-kerja pengoptimuman berikutnya dijalankan menggunakan pendekatan

pengoptimuman berketentuan dan kaedah algoritma genetik (GA) untuk mencapai

sasaran tork purata 25.8 Nm dan kuasa 6.49 kW. Setelah mesin direka dan dianalisis

menggunakan kaedah 2D dan 3D analisis unsur terhingga (FEA), konfigurasi 12S-

10P yang telah dioptimumkan telah mencapai tork yang tinggi dan kuasa sebanyak

4.6% dan 4.8% masing-masing, berbanding 12S-8P WFFSM dengan pemutar

bersegmen dan belitan tidak bertindih. Selain itu, tork dan kuasa rekabentuk yang

vii

dioptimumkan juga lebih tinggi daripada 12S-8P WFFSM dengan rotor menonjol

dan belitan bertindih. Topologi 12S-14P telah menghasilkan tork purata dan kuasa

yang tinggi pada arus angker dan arus medan yang rendah berbanding dengan

keseluruhan rekabentuk motor.

viii

TABLE OF CONTENTS

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS viii

LIST OF TABLES xiii

LIST OF FIGURES xiv

LIST OF SYMBOLS AND ABBREVIATIONS xviii

LIST OF APPENDICES xx

LIST OF PUBLICATIONS xxi

LIST OF AWARDS xxiv

CHAPTER 1 INTRODUCTION 1

1.1 Research Background 1

1.2 Problem Statement 3

1.3 Objectives of the Study 4

1.4 Scope 4

1.5 Motivations for Research 5

1.6 Contribution to Knowledge 6

1.7 Thesis Organisation 6

ix

CHAPTER 2 OVERVIEW OF FLUX SWITCHING MOTORS 9

2.1 Introduction 9

2.2 Operation and classification of the Flux Switching

Machine 10

2.3 Permanent Magnet Flux Switching Motor (PMFSM) 12

2.3.1 Various Configurations of PMFSMs 12

2.3.2 Modelling Techniques of PMFSMs 13

2.4 Hybrid Excitation Flux Switching Motor (HEFSM) 15

2.5 Field Excitation Flux Switching Motor (FEFSM) 17

2.5.1 Structure review of FEFSM 17

2.5.2 Design restrictions and specifications for FEFSMs 20

2.5.3 Flux linkages of various FEFSMs 21

2.5.4 Cogging torque of FEFSMs 22

2.5.5 Average torque and efficiency 23

2.5.6 Analytical comparison of FEFSMs 26

2.6 Summary 29

CHAPTER 3 DESIGN METHODOLOGY 30

3.1 Introduction to JMAG-Designer software 30

3.2 Project methodology of proposed SalRoN WFFSM 30

3.2.1 Design process and coil arrangement test 31

3.2.2 Performance analysis of SalRoN WFFSM 36

3.2.2.1 Rotor mechanical properties 38

3.2.2.2 Torque density/ Power density 38

3.2.3 Optimisation of SalRoN WFFSM 39

3.3 Summary 41

CHAPTER 4 CHARACTERISTICS INVESTIGATION OF

THE PROPOSED SALRON WFFSM 42

4.1 Various rotor pole topologies comparison of three-phase

12 slots SalRoN WFFSM on the basis of FEA 42

4.1.1 Coil test analysis and operating principle of

SalRoN WFFSM 43

4.1.2 Development of mathematical model and

equivalent circuit of SalRoN WFFSM 45

x

4.1.3 Cogging torque 49

4.1.4 Induced EMF at open circuit condition 50

4.1.5 Effect of rotor pole width variation on

12S-8P SalRoN WFFSM 51

4.1.6 Torque versus Ja characteristics at various Je 52

4.1.7 Torque and power vs. speed characteristics of

12S-8P and 12S-10P SalRoN WFFSM 55

4.2 Various rotor pole topologies comparison of three-phase

24 slots WFFSM on the basis of FEA 56

4.2.1 Coil test analysis 56

4.2.2 Cogging torque 57

4.2.3 Induced EMF at open circuit condition 58

4.2.4 Torque versus Ja characteristics at various Je

for 24 slots topologies 59

4.2.5 Torque and power versus speed characteristics

of 24S-10P and 24S-14P SalRoN WFFSM 59

4.3 Other performances of SalRoN WFFSM 60

4.4 Performance comparison of SalRoN WFFSMs with

existing WFFSMs 64

4.4.1 Comparison of flux linkage 64

4.4.2 Comparison of cogging torque and average

torque 65

4.4.3 Torque and power versus speed characteristics

of 12S-8P SegRoN and SalRO WFFSMs 67

4.4.4 Efficiency analysis of 12S-8P SegRoN and

SalRO WFFSMs 68

4.5 Summary 69

CHAPTER 5 DESIGN OPTIMISATION 70

5.1 Design Optimisation of 24S-10P, 12S-8P, 12S-10P,

and 12S-14P SalRoN WFFSMs 70

5.1.1 Comparison between GA and deterministic

optimisation methods for cycle 1 73

xi

5.1.2 Influence of design parameters on flux

distribution and flux linkages 76

5.1.3 Cogging torque and induced EMF at

open circuit condition 77

5.1.4 Torque and power comparison at maximum

Je and Ja 81

5.1.5 Transformation of trapezoidal slots of 12S-10P

SalRoN WFFSM to rectangular slots 82

5.1.6 Design optimisation of 12S-10P and 12S-14P

SalRoN WFFSMs with rectangular slots 82

5.1.6.1 Performance analysis comparison of optimised

design of 12S-10P and 12S-14P SalRoN

WFFSMs 88

5.1.6.2 Design techniques for cogging torque

reduction of 12S-10P and 12S-14P 92

5.1.7 Torque and power versus speed characteristics

of optimised design of SalRoN WFFSMs 95

5.1.8 Torque and power densities of optimised

SalRoN WFFSMs 96

5.1.9 Losses and efficiencies of optimised SalRoN

WFFSMs 97

5.1.10 Performances comparison of optimised

SalRoN WFFSMs 98

5.2 Comparison of optimised SalRoN WFFSMs with

same and different FEC polarities 99

5.2.1 2D and 3D FEA performance comparison

of SalRoN WFFSMs 101

5.3 Cost analysis comparison of SalRoN WFFSM with

brushless machines 103

5.4 Practical applications of proposed SalRoN WFFSMs 104

5.5 Summary 105

CHAPTER 6 CONCLUSION AND FUTURE WORKS 106

6.1 Conclusion 106

xii

6.2 Future works 107

REFERENCES 109

APPENDIX 121

VITA 131

xiii

LIST OF TABLES

2.1 FEFSMs Comparison 21

3.1 Design parameters of SalRoN WFFSM 36

4.1 Torque and power densities 63

4.2 Losses and efficiencies of the 12S-8P SegRoN

and SalRO WFFSMs 68

5.1 Design parameters for improved designs 73

5.2 Design restrictions and specifications of 12S-10P

SalRoN WFFSM 83

5.3 Initial and optimised design parameters 87

5.4 Comparison of torque and power densities of

optimised designs 96

5.5 Losses and efficiencies at various operating points 97

5.6 Performances comparison of optimised SalRoN

WFFSMs 98

5.7 Main parameters of SalRoN WFFSMs 99

xiv

LIST OF FIGURES

1.1 Three-phase FEFSM 3

2.1 A flux switching inductor alternator 10

2.2 Basic operation principle with constant field winding

excitation and single-phase armature current 11

2.3 Classifications of FSM 12

2.4 Topologies of PMFSMs 14

2.5 HEFSM examples 16

2.6 Single-phase FEFSMs 18

2.7 Three-phase FEFSM 20

2.8 Flux distribution 22

2.9 Cogging torque of three-phase FEFSM 23

2.10 Average torque-current characteristics of FEFSMs 25

2.11 Total efficiency of FEFSM with segmental rotor 25

3.1 General work flow of project implementation 31

3.2 Work flow to design the SalRoN WFFSM and

investigate the coil arrangement test 32

3.3 Three-phase SalRoN WFFSM topologies have 12slots

and different rotor pole numbers 34

3.4 Three-phase SalRoN WFFSM topologies have

24slots and different rotor pole numbers 35

3.5 Performance analysis based on FEA 37

3.6 Estimated coil end volume of the SalRoN WFFSM 39

3.7 Deterministic optimisation procedure 40

3.8 Optimisation procedure using GA 40

4.1 Flux linkage of 12S-10P WFFSM in terms of U, V,

and W 44

4.2 U-phase flux linkage of 12 Slot WFFSMs SalRoN 44

xv

4.3 Principle of operation of WFFSM 44

4.4 Equivalent circuit of SalRoN WFFSM 487

4.5 Equivalent circuit of a SalRoN WFFSM 48

4.6 Cogging torque of 12 slot WFFSMs 49

4.7 Induced Voltage at 500 rpm 50

4.8 Flux linkage and cogging torque effect with variation

of rotor pole width 51

4.9 Harmonic magnitude with various rotor pole widths 52

4.10 Torque vs. Ja 53

4.11 Flux distribution at Je of 5 A/mm2 54

4.12 Torque and power versus speed characteristics of

12S-8P and 12S-10P SalRoN WFFSMs 55

4.13 Flux linkage of 24S-10P WFFSM topology in terms

of U, V, and W 57

4.14 Flux linkage in U-phase of 24 Slot WFFSMs 57

4.15 Cogging torque of 24 slots WFFSMs 58

4.16 Induced Voltage at 500 rpm 58

4.17 Torque vs. Ja at various Je of 24slot-10Pole 60

4.18 Torque and power vs. speed characteristics 60

4.19 Rotor stress analaysis at 500 rev/min 61

4.20 Mechanical stress versus speed 61

4.21 Motor losses and efficiency for operating shown

in Figure 4.12 63

4.22 Iron losses and copper losses at various points 63

4.23 12S-8P WFFSM 64

4.24 Flux distribution 65

4.25 Flux linkage of WFFSM at maximum Je 65

4.26 Cogging torque of WFFSM 66

4.27 Average torque of WFFSM at maximum Je 66

4.28 Torque and Power versus speed characteristics 67

5.1 Design parameters of SalRoN WFFSM 71

5.2 Effect of various motor parameters on average torque 72

5.3 Optimisation cycles of 12S-8P SalRoN WFFSM 73

5.4 Improved design of SalRoN WFFSMs 74

xvi

5.5 Average torque versus rotor pole width based on

deterministic optimisation method 76

5.6 Average torque versus design variables based on

GA method 76

5.7 Effect of rotor pole width, R2 78

5.8 Flux distributions of 12S-8P 78

5.9 Flux distributions of 12S-10P 78

5.10 U-phase flux linkages of 24S-10P, 12S-8P and

12S-10P SalRoN WFFSMs 79

5.11 Comparison of cogging torque 79

5.12 Comparison of induced emf of SalRoN WFFSM 80

5.13 Comparison of torque and power of SalRoN

WFFSMs at maximum Jeof 30 A/mm2 and various Ja 81

5.14 12S-10P SalRoN WFFSM with rectangular slots 83

5.15 Design parameters of 12S-10P SalRoN WFFSM

with rectangular slots 84

5.16 Optimisation cycles to achieve better torque and

power characteristics 85

5.17 Torque and power versus number of turns 85

5.18 Optimised design of 12S-10P SalRoN WFFSM 87

5.19 Optimised design of 12S-14P SalRoN WFFSM 88

5.20 Average torque versus variable rotor pole width 88

5.21 Flux linkages comparison of optimized design of

12S-10P and 12-14P SalRoN WFFSMs 89

5.22 Cogging torque comparison 89

5.23 Torque versus Ja at various Je for optimized designs

of SalRoN WFFSM 90

5.24 Flux distribution of optimised design of 12S-14P

at Je of 5 A/mm2 and Ja of 15 Arms/mm2 91

5.25 Cogging torque reduction techniques 93

5.26 Comparison of various cogging torque reduction

schemes 94

5.27 Maximum and minimum value of cogging torque

for different schemes 94

xvii

5.28 Torque versus speed characteristics of optimised

designs 95

5.29 Power versus speed characteristics of optimised

designs 95

5.30 12S-10P SalRoN WFFSM with identical polarity

of FECs 99

5.31 Torque versus various armature currents at field

current of 20 A 100

5.32 3D model of 12S-10P SalRoN WFFSM 101

5.33 Flux distribution of 12S-10P SalRoN WFFSM 102

5.34 Torque-armature current curves, Ie=20 A 103

xviii

LIST OF SYMBOLS AND ABBREVIATIONS

- Efficiency

- Flux

exc - Flux linkage due to field excitation

- Electrical angular position of rotor

r - Rotational speed

- Copper resistivity

cog - Electrical angle of rotation for each period of cogging

torque

f - Filling factor

B - Magnetic flux density

F - Magnetomotive force

ef - Electrical frequency

mf - Mechanical rotation frequency

ai - Armature current

di - d-axis current

qi - q-axis current

fi - Field current

aJ - Armature current density

eJ - Field current density

k - Natural number

- Stack length

xix

N - Number of turns

pN - Number of periods

rN - Number of rotor poles

sN - Number of stator slots

cP - Copper loss

iP - Iron loss

mechP - Mechanical power

q - Number of phases

UR - Armature winding resistance per phase

- Reluctance

aS - Slot area

TL - Load torque

CAD - Computer Aided Design

CNC - Computer Numerical Control

HE - Hybrid Excitation

EV - Electric Vehicle

FSM - Flux Switching Motor

FE - Field Excitation

FEC - Field Excitation Coil

FEA - Finite Element Analysis

GA - Genetic Algorithm

HEV - Hybrid Electric Vehicle

IPMSM - Interior Permanent Magnet Synchronous Motor

MEC - Magnetic Equivalent Circuit

SalRoN - Salient Rotor and Non-overlapping windings

SalRO - Salient Rotor and Overlapping windings

SegRoN - Segmental Rotor and Non-overlapping windings

WFFSM - Wound Field Flux Switching Machine

WFSM - Wound Field Synchronous Machine

xx

LIST OF APPENDICES

APPENDIX TITLE PAGE

A. Conversion of U, V and W three-phase system

into d- and q-axis using Park’s transformation 121

B. Optimal values of design variables based on

Genetic Algorithm Method 123

C. Structure development of 12S-14P SalRoN WFFSM 125

D. Technical Drawing of 12S-14P SalRoN WFFSM 126

E. Gantt chart research activities 130

xxi

LIST OF PUBLICATIONS

Journals:

(i) Faisal Khan, Erwan Sulaiman, Md. Zarafi Ahmad, “Review of Switched Flux

Wound-Field Machines Technology,” IETE Technical review, accepted,

DOI:10.1080/02564602.2016.1190304, 2016. (ISI, Scopus, Q2, IF: 1.30)

(ii) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, “A novel wound field flux

switching motor with non-overlapping windings and salient rotor,” Turkish

Journal of Electrical Engineering and Computer Science, accepted, DOI:

10.3906/elk-1507-80, 2016. (ISI, Scopus, Q2, IF: 0.507)

(iii) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, “Design and Analysis of

Wound Field Three-Phase Flux Switching Machine with Non-overlap

Winding and Salient Rotor,” International Journal of Electrical Engineering

and Informatics, vol.7, no. 2, pp. 323-333, 2015. (Scopus, Q3)

(iv) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, Zhafir Aizat, “Design

Refinement and Performance Analysis of 12Slot-10Pole Wound Field Salient

Rotor Switched-Flux Machine for Hybrid Electric Vehicles,” Journal of

Applied Science and Agriculture, vol. 9, no. 18, pp. 148-155, 2014. (ISI)

(v) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, Zhafir Aizat Husin, “Low

Cost and Robust Rotor Three-Phase Wound-Field Switched-Flux Machines

for HEV Applications,” APRN Journal of Engineering and Applied Sciences,

vol.10, no. 19, pp. 8858-8865, 2015. (Scopus, Q3)

(vi) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, “Performance comparison

of 24slot-10pole and 12slot-8pole wound field three-phase switched-flux

machine,” International Journal of Energy and Power Engineering Research,

vol. 2, pp. 17-21, 2014. (Non-indexed)

xxii

(vii) Faisal Khan, Erwan Sulaiman, Mohd Fairoz Omar, “Flux Linkage Analysis

and Cogging Torque Reduction Techniques for Field-Excited Flux Switching

Machines,” Journal Technology, In press, 2016. (Scopus)

(viii)

(ix)

Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad and Zhafir Aizat,

“Parameter Sensitivity Study for Optimization of 12Slot-8Pole Three-Phase

Wound Field Switched-Flux Machine,” Journal- Applied Mechanics and

Materials, vol. 695, pp. 765-769, 2014.(Scopus)

Faisal Khan, Erwan Sulaiman, Zhafir Aizat Husin, Mubin Aizat Mazlan,

“Deterministic Optimization of Single Phase 8S-4P Field Excitation Flux

Switching Motor for Hybrid Electric Vehicle,”APRN Journal of Engineering

and Applied Sciences, vol. 11, no. 8, pp. 5084-5088, April 2016. (Scopus)

Proceedings:

(i) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, “Coil Test Analysis of

Wound-Field Three-Phase Flux Switching Machine with Non-overlap

Winding and Salient Rotor,” IEEE 8th International Power Engineering and

Optimization Conference, pp. 243-247, Langkawi, Malaysia, 24-25 March,

2014.

(ii) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, Zhafir Aizat Husin,

Mohamed Mubin Aizat Mazlan “Topologies for Three-Phase Wound-Field

Salient Rotor Switched-Flux Machines for HEV Applications,” International

Conference on Mathematics, Engineering & Industrial Applications, AIP

Publishing, vol. 1660, pp. 070097, Penang, Malaysia, 2015.

(iii) Faisal Khan, Erwan Sulaiman, Mohd Fairoz Omar, “Design and

Characteristics Investigations of 12Slot-8Pole and 12Slot-10Pole Wound

Field Three-Phase Switched-Flux Machines,” IET International Conference

on Clean Energy and Technology, pp. 1-5, Kuching, Malaysia, 24-26

November, 2014.

(iv) Faisal Khan, Erwan Sulaiman, Md Zarafi Ahmad, Hassan Ali, “Design

Refinement and Performance Analysis of 12Slot-8Pole Wound Field Salient

Rotor Switched-Flux Machine for Hybrid Electric Vehicles,” IEEE

xxiii

International conference on Frontier of Information Technology, pp. 197-201,

Islamabad, Pakistan, 17-19 December, 2014.

(v) Faisal Khan, Erwan Sulaiman, Md. Zarafi Ahmad, Zhafir Aizat Husin,

“Comparative study of 24Slot-10Pole and 24Slot-14Pole Wound filed

switched flux machine,” 8th Malaysian Technical Universities Conference on

Engineering & Technology, Melaka, Malaysia, 10-11 November, 2014.

(vi) Erwan Sulaiman, Faisal Khan, Md Zarafi Ahmad, Mahyuzie Jenal,

“Investigation of field excitation switched flux motor with segmental rotor,”

IEEE Conference on Clean Energy and Technology, pp.317-322, Langkawi,

Malaysia, 18-20 November, 2013.

(vii) Faisal khan, Erwan Sulaiman, “Design Optimization and Efficiency Analysis

of 12Slot-10Pole Wound Field Flux Switching Machine,” IEEE Magnetics

Conference, pp.1, Beijing, China, 11-15 May, 2015.

(viii) Faisal khan, Erwan Sulaiman, “Performance comparison of wound field Flux

switching machine,” IEEE Conference on Energy Conversion, pp. 310-314,

Johor Bahru, Malaysia, 19-20 October, 2015.

(ix) Faisal Khan, Erwan Sulaiman, Mohd Fairoz Omar, Mahyuzie Jenal, “3D FEA

based Performance of Wound Field Flux Switching Machine using JMAG,”

IEEE Student conference on Research and Development, pp. 362-366, 13-14

December, Kuala Lumpur, Malaysia, 2015.

(x) Erwan Sulaiman, Fasial Khan, Md Fairoz Omar, Gadafi M. Romalan,

Mahyuzie Jenal, “Optimal Design of Wound-Field Flux Switching Machines

for an All-Electric boat,” IEEE International Conference on Electrical

Machines, pp. 2464-2470, 4-7 September, Lausanne, Switzerland, 2016.

xxiv

LIST OF AWARDS

(i) Gold Medal in Seoul International Invention Fair, South Korea, [SIIF 2014]:

Erwan Sulaiman, Faisal Khan, Zhafir Aizat, Mubin Aizat, Sharifah Binti

Saon, “Field Excitation Flux Switching Motor”.

(ii) Silver Medal at Malaysia Technology Expo, Kuala Lumpur, [MTE 2014]:

Erwan Sulaiman, Faisal Khan, Zarafi Ahmad, Zhafir Aizat “12Slot-10Pole

Field Excitation Flux Switching Motor for Hybrid Electric Vehicles”.

(iii) Bronze Medal at Research & Innovation Festival, UTHM, [R & I 2013]:

Erwan Sulaiman, Faisal Khan, Zarafi Ahmad, Zhafir Aizat “Field Excitation

Flux Switching Motor for Hybrid Electric Vehicles”.

(iv) Silver Medal at Malaysia Technology Expo, Kuala Lumpur, [MTE 2015]:

Erwan Sulaiman, Mohd. Fairoz Omar, Faisal Khan, Hassan Ali Soomro,

Kamaluddin Hassan, “LoCo FEFS motor”.

(v) Copyright No. LY2015000027, Erwan Sulaiman, Faisal Khan “Wound Field

three-phase Flux Switching Motor with Non-overlap Winding and Salient

Rotor for Hybrid Electric Vehicles”.

(vi) Industrial Design No. 14-01422-0101, Erwan Sulaiman, Faisal Khan

“12Slot-10pole Wound Field three-phase Flux Switching Motor with Non-

overlap Winding and Salient Rotor for Hybrid Electric Vehicle”.

(vii) FRGS MOE Grant Vot. 1508, “Characteristic investigations of a New Field

Excitation Flux Switching Machine using Finite Element Analysis for Electric

Vehicles”.

(viii) Participated in 3 minutes PhD thesis competition, 2015 at national level held

at Kuala Lumpur, organised by UTM.

(ix) Patent File No. PI2016000833, Erwan Sulaiman, Faisal Khan, “Salient Rotor

Non-Overlapping Wound Field Flux Switching Motor”

(x) 3rd Prize in postgraduate poster competition, 2015 at UTHM.

CHAPTER 1

INTRODUCTION

1.1 Research Background

Nowadays, a majority of industrial and domestic applications require more compact,

more efficient, robust, light weight, and low-cost electric motors. As electric motors

are the core of both industrial and domestic appliances, there is a pressing need for

researchers to develop advanced electric motors. Latest research has made them

literally invisible, installed within thousands of everyday products. They can be

found in hybrid electric vehicles, fan, washing machine, air conditioners, vacuum

cleaner, aerospace application, and rigid disc drives [1]. There are several classes of

electric motors with standardised dimensions and characteristics. The development

of the flux switching motor (FSM) [2], a new class of brushless motor is as recent as

just over a decade ago. All the excitation sources of FSMs are located on the stator,

providing simpler cooling options, while the rotor consists of only a single piece

iron, neither magnet nor windings. FSMs are considered to be the most promising

electric motor with high efficiency, easy thermal management, robust rotor structure,

light weight, and cost-competitive. In addition, FSMs have been effectively applied

to domestic and industrial applications [3-6].

FSM is a combination of a switched reluctance motor and an inductor

alternator [7-8]. FSM can be categorised into three groups based on the method of

excitation to stator: (1) Permanent Magnet (PMFSM), (2) Field Excitation (FEFSM),

and (3) Hybrid Excitation (HEFSM). PM and field excitation coil (FEC) are the main

sources of flux in PMFSM and FEFSM respectively while for HEFSM, both PM and

FEC generate the flux [9-11]. Recently, the research work on PMFSM is dominated

2

by employing PM for primary excitation because of its high torque density, fault

tolerance, and high efficiency [12]. However, with the growing demand of PM for

electric motors, the annual consumption of rare-earth magnet has increased

accordingly. This causes the prices of Neodymium (Nd) and Dysprosium (Dy) – an

essential additive to provide the rare-earth magnet with high coercivity to rise

markedly. Together, they raised serious concerns about soaring cost, security issues,

and supply shortages. Hence, continuous research effort to develop electric motors

with high power density and robust rotor structure without relying on rare-earth

magnet is of the utmost importance [13]. It is desirable to completely replace the PM

with FEC or diminish the usage of PM. In order to compete with existing PMFSM,

some FEFSMs have been proposed for low cost and high-speed applications. The

FEFSM has advantages of low cost, simple construction, magnet-less machine, and

variable flux control capabilities suitable for various performances. The power

density of FEFSM is higher, as the working temperature of FEFSM will not be

limited by the working temperature of PMFSM [14-16]. The type of FSM that would

be discussed within the context of this thesis is the FEFSM.

The three-phase FEFSM was simply designed by replacing the PM of 24Slot-

10Pole PMFSM with FEC. However, the total flux generation was limited due to

isolation of DC FEC and thus reducing the performances [14]. Moreover, the

configuration of non-overlapping winding, instead of overlapping winding is

preferred for machine design because of its advantages of short end windings and

simple structure. If the slot pitch was close to the pole pitch, as in the case of similar

numbers of slots and poles, then copper losses were reduced and high torque was

produced in a non-overlapping winding, as discussed in [17-19]. To improve the

drawbacks of FEFSM, 12S-8P FEFSM with segmental rotor was designed and

experimentally tested [20]. Concentrated winding arrangement was used in this

design, which gave shorter end windings of DC FEC and armature coil when

compared with tooth rotor structure with distributed windings. This design had

significant gains over other designs, as it utilised less conductor materials, thus

improving the overall efficiency because of the reduction of copper loss. Examples

of FEFSMs with single and dual polarity FEC were discussed in [21] and [5]. The

advantages of FEFSM with single polarity FEC are low copper loss due to less

volume of FEC, less leakage flux when compared with dual FEC windings. The

field-weakening capability of FEFSM was improved using toroidal DC FEC [22].

3

1.2 Problem Statement

FEFSM was developed to overcome the drawbacks of rare earth materials used in

PMFSM [13], demagnetisation problem [23], uncontrollable flux and less robust

rotor owing to interior PM [24]. Various FEFSM structures are shown in Figure 1.1.

There are certain problems in FEFSM such as segmental rotor that made the rotor

less robust and cannot be used for high-speed application, as illustrated in

Figure 1.1(a) [15], and overlapping armature and field windings that raised the

copper losses in these machines [5], as shown in Figure 1.1(b). The segmented rotor

FEFSM design has noteworthy gain over other designs as it consume less conductor

(a) (b)

(c) (d)

Figure 1.1: Three-phase FEFSM, (a) 12S-8P FEFSM with segmental rotor, (b) 24S-10P FEFSM with dual FEC, (c) 24S-10P FEFSM with single FEC,

(d) 12S-14P FEFSM with toroidal DC winding

A2

C2 B2

A1

C1 B1

FEC-2

FEC-1

FEC-2

FEC-1

FEC-2

FEC-1

FEC-1

FEC-2 A1

B1

C1

A2

B2 C2 A3

B3

C3

A4 B4 C4

A1

B1 C1

A2

B2

C2

A3 B3

C3

A1

B1

A2

B2 C2

A3

B3

C3

A4 B4 C4

FEC FEC

FEC

FEC FEC

FEC

FEC

FEC

FEC

FEC FEC

FEC

FEC

C1

4

materials, accordingly enhanced the overall efficiency due to reduction of copper

loss. To deal with the problems, continuous research and development on electric

machines with no PM, stable current, high synchronous speed and high efficiency

would be very important.

FEFSM with single polarity FEC was discussed in [21], as shown in

Figure 1.1(c). FEFSM with single polarity FEC has advantages of less copper loss

when compared with dual FEC windings but the torque generation is limited. The

field-weakening capability of FEFSM is improved using toroidal dc field winding, as

shown in Figure 1.1(d). The proposed machine had limited torque generation,

because the field flux produced by DC field conductors close to the outer surface of

the stator went through the outside of the stator instead of the rotor, as well as high

copper losses owing to overlapped armature and field windings.

1.3 Objectives of the Study

The main objective of this research is to develop a novel structure of salient rotor and

non-overlapping windings (SalRoN) wound field flux switching motor (WFFSM)

for high-speed applications. In achieving the main objective, there are some specific

objectives that have to be fulfilled, which are:

(i) To design a novel SalRoN WFFSM and investigate its operating principle.

(ii) To investigate and analyse the significance of the new machine; the torque

speed characteristics, mechanical stress of the rotor, torque density, power

density, iron losses, copper losses of windings, and efficiency should be

examined.

(iii) To optimise the SalRoN WFFSM for better average torque, output power,

efficiency and torque-speed characteristics.

1.4 Scope

Commercial FEA package, JMAG-Designer ver.14.1, released by Japan Research

Institute (JRI), was used as 2D-FEA solver for this design. 12 stator-slot topologies

with the number of rotor poles limited to 5, 7, 8, 10, 14 and the 24 stator-slot

topologies with number of rotor poles limited to 10, 14, 16, 20, 22 were investigated.

5

A coil test analysis was performed for feasible topologies of SalRoN WFFSM to

confirm the operating principle. Assuming that only a water cooling system was

employed, the limit of the current density was set to the maximum 30 Arms/mm2 for

armature winding and 30 A/mm2 for FEC, respectively. The electromagnetic

performance, including back EMF, cogging torque, and average torque had been

analysed and compared using 2D-FEA. The torque-speed characteristics were

evaluated by varying the armature phase angle, θ. The iron and copper losses were

calculated based on 2D-FEA and formula, which assisted in calculating the efficiency

of the proposed SalRoN WFFSM. The design restrictions, target specifications, and

parameters of the proposed SalRoN WFFSM were based on the WFFSM employing

a segmented rotor and non-overlapping windings. The outer diameter, the motor

stack length, the shaft radius, and the air gap, having dimensions 150 mm, 70 mm, 24

mm and 0.3 mm, respectively, were kept constant. The electrical restrictions related

with the inverter such as maximum 415V DC bus voltage was set similar as in the

WFFSM with a segmented rotor and non-overlapping windings. The field current

applied to SalRoN WFFSM was limited to 50 A.

1.5 Motivations for Research

For the past 10 years, the automotive, aerospace and shipbuilding industries have

been working to design electric motors to improve the efficiency, torque and power

as well as minimise the cost. In order to reduce the cost, rare earth material has been

removed from the existing PM electric machines and their various characteristics are

examined. Moreover, the efficiency is improved by employing non-overlapping

windings that reduce the copper loss. The performance of WFFSM has been

investigated, in response to economical and environmental pressures. As the

capabilities of semiconductor devices and power converter grow, improvements in

the performance of the WFFSM and considerations of new configurations are

required.

The desktop workstation of modern technology has high processing power

and speed and handles repetitive design problems with an ease not experienced

before. JMAG tool with numeral analysis function for electric machine design using

2D and 3D-FEA are now routinely applied in design, analysis and optimisation of

6

machine dimensions. In the design and analysis of SalRoN WFFSM of this study,

extensive use of 2D and 3D-FEA has been applied.

1.6 Contribution to Knowledge

The contributions of this research are as follows:

(i) Various structures of WFFSM have been reviewed and published.

(ii) It was explained that SalRoN WFFSM is particularly suitable for low cost

and high-speed applications. A novel structure of light weight, more

efficient, and robust rotor WFFSM was validated by 2D-FEA. This thesis

elaborates the feasible topologies for three-phase SalRoN WFFSM.

(iii) Deterministic optimisation and GA methods were adopted to enhance the

motor characteristics and compared with the existing designs of

WFFSMs. The optimized design has achieved better characteristics than

the existing designs and published.

(iv) Different methods were applied to reduce the cogging torque effect and

the flux linkage were made sinusoidal by the rotor pole width variation.

(v) 3D-FEA modelling was examined for the precise analysis of SalRoN

WFFSMs.

1.7 Thesis Organisation

This thesis deals with the design and optimisation of SalRoN WFFSM for high-speed

applications. The thesis is divided into six chapters and the summary of each chapter

is listed as follows:

(i) Chapter 1: Introduction

The first chapter explains the importance of magnetless high-speed

electric machines and provides the broad context in which the motivation

and research background with this work are embedded. The problems of

current WFFSMs are highlighted and research objectives to solve the

problems together with the major contributions are outlined in this

chapter.

7

(ii) Chapter 2: Overview of Flux Switching Motors (FSMs)

The second chapter is a literature review that summarises the basic theory

of flux switching and classification of FSM including the examples of

PMFSM, FEFSM, and HEFSM. The structures, modelling techniques,

and various characteristics of these FSMs are discussed in detail and

compared. In the last section, a brief explanation is provided for structure

review, flux linkage, cogging torque, average torque, efficiency, and

analytical modelling of FEFSM.

(iii) Chapter 3: Design Methodology

The third chapter describes the design methodology of the proposed

SalRoN WFFSM using commercial 2D-FEA, JMAG-Designer ver. 14.1,

released by JSOL Corporation. Design process, coil arrangement test, and

performance analysis are highlighted to investigate the performance of

various topologies of SalRoN WFFSM. In a later section, the procedure

of deterministic optimisation and GA methods are explained to treat

various parameters of the machine and enhance its characteristics.

(iv) Chapter 4: Characteristics Investigation of the Proposed SalRoN WFFSM

In this chapter, the principle of flux switching by means of a salient rotor

is portrayed with the help of elementary structure, and a calculating

method of average torque is introduced for SalRoN WFFSM. Further

design evaluations based on 2D-FEA and the changing pattern of 12S-8P

SalRoN WFFSM characteristics with the variation of rotor pole width are

presented in this chapter. Torque density, power density, and efficiency

are calculated for the initial designs of SalRoN WFFSMs.

(v) Chapter 5: Design Optimisation

Among the various slot combinations of SalRoN WFFSM discussed in

chapter 4, the 12S-8P, 12S-10P, 12S-14P, and 24S-14P are optimised

using deterministic optimisation and GA methods until the target

performances are achieved. The results of the initial and optimised design

that met the target performances are analysed and discussed. Various

cogging torque reduction techniques are adopted to minimise the effect of

cogging torque. Moreover, this chapter also explains 3D-FEA modelling

of the design for precise analysis of axial flux.

8

(vi) Chapter 6: Conclusion and Future Works

The final chapter describes the major results and concludes the summary

of the research as well as recommendations for future work.

CHAPTER 2

OVERVIEW OF FLUX SWITCHING MOTORS (FSMs)

This chapter reviews the types of rotating electric machine based on the principle of

flux switching. It explains the flux switching mechanisms of different FSMs types,

from the early concepts to the modern designs. The merits and demerits of various

FSMs as well as numerous approaches to evaluate their performances are also

highlighting. As the thesis is on WFFSMs that employ salient rotors, the last section

provides more detail and significance of these machines.

2.1 Introduction

The variation of permeance seen by armature with respect to the rotor position

changes the armature flux between the maximum (high state) and minimum (low

state) values, and introduces a new term called ‘flux switching’. Electric machines

that operate on this principle are called flux switching machines. The theoretical

design of the inductor alternator based on flux switching has been known since 1940s

[7], but the analytical approach appears to have come into use a decade later [25]. In

[26], a PMFSM, i.e. PM single-phase limited angle actuator, or more well-known as

Laws’ relay, has four stator slots and four rotor poles developed, while in [25] it was

extended to a single-phase generator with four stator slots, and four or six rotor

poles. Research work has been conducted on these machines for several years

without using the term flux switching [27-30]. During the development of power

electronics technology in the late 1990s, this term was used and investigated [31],

[32]. Computer-aided design (CAD) tools with numeral analysis functions, normally

Finite Element Analysis (FEA) using 2D or 3D, contributed a lot in design, analysis,

10

and optimisation of FSM. Over the past ten years or so, many novel FSM topologies

have been developed for various applications, ranging from low-cost domestic

appliances, automotive, wind power, and aerospace.

2.2 Operation and classification of the Flux Switching Machine

In the mid-1950s, the term ‘flux switching’ was used to explain an inductor

alternator with bipolar armature flux linkages [25]. Figure 2.1(a) and (b) illustrate the

simple mechanism of flux switching alternator. A pair of stator windings, a dual set

of laminated yolks, and a pair of PMs were located on the stator, while the rotor was

shown as a two salient pole stacks of laminations on the shaft. The flux paths shown

by arrows in Figure 2.1(a) indicate the flow of the flux from left to right in both

windings. When the rotor position was moved by a half-electrical cycle, as in Figure

2.1(b), the flux linkage had the same magnitude but the direction had been reversed

as in Figure 2.1(a). A complete reversal of flux was attained by each revolution of

the rotor. Consequently, the salient pole of the stationary part and stator operated in a

conventional pulsating flux manner.

The PM of inductor alternator was replaced by appropriate field windings

without altering the basic performance [32]. The required resultant flux orientation

was supplied by a field winding with unipolar current and armature windings with

single-phase current. The operation principle of flux switching motor without

(a) (b)

Figure 2.1: A flux switching inductor alternator [25]

11

magnets could be described with the aid of Figure 2.2. Two possible aligned

positions of the rotor were shown by cross-sectional views of the motor.

Conventional cross and dot notations were used to indicate the path of current flow

in the armature and field windings.

The combined resultant flux created by field and armature windings travelled

in the northerly vertical direction; aligned the rotor with vertical stator poles, as

illustrated in Figure 2.2(a). Considering Figure 2.2(b), the current in the armature

winding was reversed for the next half cycle, while the field winding remained

excited in the same direction causing the resultant flux to flow in the westerly

direction, which tended to align the rotor with horizontal stator poles. Overlap

armature and field windings has been caused copper losses in this motor and thus

reduced the efficiency.

There are three classes of FSMs, namely permanent magnet FSM (PMFSM),

field excitation FSM (FEFSM), and hybrid excitation FSM (HEFSM). They are

broadly defined and differentiated by field flux source i.e PM or field winding and

both PM and field winding, as illustrated in Figure 2.3.

(a) (b)

Figure 2.2: Basic operation principle with constant field winding excitation and single-phase armature current, (a) Armature flux linkage-positive and

(b) Armature flux linkage-negative [32]

×

×

××

F

F

F

F

A

A

A

A

××

××

F

F

F

F

A

A

A

A

2.3 Permanent Magnet Flux Switching Motor (PMFSM)

Over the

various

expensive to fabricate. Furthermore, the torque ripple must be considered

caused by severe magnetic saturation and irregular variation of winding inductances

[33]. The PMFSM with PM embedded in the rotor has less robust structure and

cannot be used for high

practically applied in industrial and domestic applications.

2.3.1 Various Configurations of PMFSMs

Various configurations of PMFSMs have been proposed by researchers worldwide to

attain better characteristics in term

development and various structures have been overviewed in [34]. For low energy

axial fan applications, inexpensive ferrite magnet 4S

cheap solution in [35]. Numerous PMFSM topologies [36

recently, they are mentioned as follows:

(i)

(ii)

(iii)

(iv)

Permanent Magnet Flux Switching Motor (PMFSM)

Over the past decade, PMFSM has been attracting a lot of research interest for

applications. The PMFSM has high

expensive to fabricate. Furthermore, the torque ripple must be considered

caused by severe magnetic saturation and irregular variation of winding inductances

[33]. The PMFSM with PM embedded in the rotor has less robust structure and

cannot be used for high-speed applications [24].

Due to the aforementioned demerits,

practically applied in industrial and domestic applications.

.1 Various Configurations of PMFSMs

Various configurations of PMFSMs have been proposed by researchers worldwide to

attain better characteristics in terms

development and various structures have been overviewed in [34]. For low energy

axial fan applications, inexpensive ferrite magnet 4S

cheap solution in [35]. Numerous PMFSM topologies [36

recently, they are mentioned as follows:

(i) Single-phase, three-phase, and multiphase PMFSMs [43], [34]

(ii) Fault tolerant PMFSMs [38]

(iii) Tabular PMFSM and Transverse flux PMFSM, [39],

(iv) Outer and Inner rotor PMFSMs [41]

Figure 2.3: Classifications of

Flux Switching Motors (FSMs)

Permanent magnet(PM) FSM

Field Excitation (FE) FSM

Permanent Magnet Flux Switching Motor (PMFSM)

ast decade, PMFSM has been attracting a lot of research interest for

applications. The PMFSM has high magnet-consuming complex stator and

expensive to fabricate. Furthermore, the torque ripple must be considered

caused by severe magnetic saturation and irregular variation of winding inductances

[33]. The PMFSM with PM embedded in the rotor has less robust structure and

speed applications [24].

Due to the aforementioned demerits, the PMFSMs have not

practically applied in industrial and domestic applications.

.1 Various Configurations of PMFSMs

Various configurations of PMFSMs have been proposed by researchers worldwide to

of torque, speed, power, and efficiency. The

development and various structures have been overviewed in [34]. For low energy

axial fan applications, inexpensive ferrite magnet 4S-4P PMFSM was proposed as

cheap solution in [35]. Numerous PMFSM topologies [36-42] have been developed

recently, they are mentioned as follows:

and multiphase PMFSMs [43], [34]

Fault tolerant PMFSMs [38]

Tabular PMFSM and Transverse flux PMFSM, [39], [40]

Outer and Inner rotor PMFSMs [41]

Figure 2.3: Classifications of FSM

Flux Switching Motors (FSMs)

Field Excitation (FE) FSM

Hybrid Excitation(HE) FSM

12

ast decade, PMFSM has been attracting a lot of research interest for

consuming complex stator and is

expensive to fabricate. Furthermore, the torque ripple must be considered, which is

caused by severe magnetic saturation and irregular variation of winding inductances

[33]. The PMFSM with PM embedded in the rotor has less robust structure and

the PMFSMs have not yet been

Various configurations of PMFSMs have been proposed by researchers worldwide to

and efficiency. The

development and various structures have been overviewed in [34]. For low energy

4P PMFSM was proposed as a

42] have been developed

and multiphase PMFSMs [43], [34]

Hybrid Excitation(HE) FSM

13

(v) PMFSMs with single-tooth or multi-tooth per pole [42]

(vi) E-core and C-core PMFSMs [44], [45]

(vii) Segmental rotor PMFSM [46]

Figure 2.4(a) illustrates a typical three-phase 12S-10P PMFSM, where the

stator consists of concentrated armature winding, and each tooth is being wounded

by a coil. A PM was placed between U-shaped laminated segments and the polarity

of PM was reversed from one magnet to another [43]. For three-phase PMFSM,

alternate poles wound windings were investigated to be fault tolerant, as shown in

Figure 2.4(b) [38]. However, these PMFSMs had the disadvantage of high magnet

volume. To reduce the volume of PM, a new structure of E-core PMFSM was

developed by replacing the alternate wound pole with a simple stator tooth, as

illustrated in Figure 2.4(c) [44]. Furthermore, to enhance the characteristics of E-core

PMFSM, the stator tooth was removed to enlarge the slot area and as a result C-core

PMFSM was developed, as illustrated in Figure 2.4(d). Additionally, the multi-tooth

structure of PMFSM, discussed in [42], was utilised to improve the torque density

and reduce magnet usage, as in Figure 2.4(e). A three-phase segmental rotor PMFSM

was also reported, as shown in Figure 2.4(f). These kinds of machines have less

robust structure, which need to be considered if they are applied for high-speed

applications.

2.3.2 Modelling Techniques of PMFSMs

Numerous modelling techniques have been developed for PMFSMs and can be

generally divided into two categories: analytical and numerical. The analytical

approaches that are usually developed for specific geometry and topology are not as

versatile as the numerical ones and might lead to inaccuracy. In contrast, the

numerical measures, which are analysed with 2-D or 3-D FEA, present more precise

evaluations for even complex geometry. However, 3-D FEA that involves several

repetitive computations is extremely time-consuming for optimisation. Sizing-

designing procedure of the PMFSM based on simplified analytical model is

presented in [47]. Furthermore, the magnetic equivalent circuit (MEC) methods have

been widely applied to model the PMFSM more accurately compared with analytical

method. 12S-10P PMFSM was optimised and analysed by a new non-linear adaptive

14

(a) (b)

(c) (d)

(e) (f)

Figure 2.4: Topologies of PMFSMs. (a) 12S-10P PMFSM with all poles wound, (b) PMFSM with alternate poles wound, (c) E-core PMFSM, (d) C-core PMFSM,

(e) Multi-tooth PMFSM, (f) Segmental rotor PMFSM with all poles wound

A1 A1

C1

A2

A2

A1

B1

B1

B1

C1 C1

B1

C1

B2

C2

B2

C2

A1

C1

A1

B1

B1 C1

C1

A1

B1

B1 C1

A2

C2

A2 B2

B2

C2

C1

A1

B1

B1

C1

A1 A1

C1 B1

B1 C1

15

lumped parameter MEC model [48]. Additionally, the PMFSM model was patched to

analyse with iron bypass bridges [49]. For the radial flux PMFSM, 2-D FEA had

been routinely employed as it could predict the performance with short

computational time [50]. However, the 2-D FEA results are less precise due to

assumption of zero axial flux.

2.4 Hybrid Excitation Flux Switching Motor (HEFSM)

A non-exhaustive review of HEFSM literature was presented in this section.

HEFSMs are those that utilise two excitation flux sources: PM as well as FEC

source, as shown in Figure 2.5 [51], [52]. HEFSMs have been investigated widely

over many years and have the potential to provide high torque and power density,

high efficiency, and variable flux capability [11], [53-55].

A 6S-4P HEFSM is shown in Figure 2.5(a), which consisted of armature

winding, field winding, and PM, arranged in three layers in the stator [56]. However,

this configuration had long end DC windings, which increased copper loss and

reduces efficiency. A novel 12S-10P HEFSM was discussed in [53], in which the PM

was placed between the stator segments leaving enough space for DC FEC, as

depicted in Figure 2.5(b). The flux regulation capability of HEFSM can be controlled

by adjusting the dimensions of PM in radial direction.

Non-overlapping armature and field windings were employed in the new

structure of E-core HEFSM, as shown in Figure 2.5(c) [55]. The slot area for both

FEC and armature winding were equal with same number of turns. Recently, three-

phase E-core HEFSM with non-overlap windings and reduced PM size, as depicted

in Figure 2.5(d), was analysed in [57]. The performance of E-Core HEFSM in terms

of flux capability, torque, and power versus speed curve was also been examined.

The proposed machine prides itself on the merits of lower cost and less copper.

The aforementioned HEFSMs with active components on the stator suffered

from these disadvantages:

(i) Both the PM and FEC were in series with each other in Figure 2.5(a), which

limited the flux-adjusting capacity owing to low permeability of PM.

(ii) The main flux generated by PM for maximum torque production was reduced

by the flux path of DC FEC, Figure 2.5(b).

16

(iii) In Figures 2.5(b), (c), and (d), the PM was placed at the outer surface on the

stator. Hence, the flux of PM acted as a leakage flux and had no involvement

in torque generation.

(iv) The HEFSM was also difficult to manufacture due to segmented stator core,

Figure 2.5(b) and (c).

(v) Torque density might be reduced because of less PM volume, Figure 2.5(d).

(a) (b)

(c) (d)

Figure 2.5: HEFSM examples, (a) 6S-4P HEFSM, (b) Three-phase 12S-10P HEFSM, (c) 12S-10P E-core HEFSM and (d)6S-8Pole E-core HEFSM

FEC

FEC

PM PM

A1

A1

B1

B1

C1

C1 FEC-1

FEC-2

A1 A1

A2

A2

B1

C1

B1

C1

B2

C2

B2

C2

A1 A1

B1 C1

B1 C1

FEC-1

FEC-2

FEC-1

FEC-2

FEC-1

FEC-2

B1

C1 B1

A1

C1 FEC-2

FEC-1

FEC-2

FEC-1

17

2.5 Field Excitation Flux Switching Motor (FEFSM)

The quest for low-cost, high efficiency and simple configuration electric machines is

ongoing process. As the annual consumption and the cost of rare-earth material have

increased over the past decade, the prices of Neodymium (Nd) and Dysprosium (Dy)

used in PMFSM and HEFSM rise markedly which creates problems of supply

shortages and security issues. To overcome this issue, the excitation on the stator of

conventional PMFSM can be easily replaced by DC FEC to form field excitation flux

switching motor (FEFSM). In other words, the FEFSM is a form of salient-rotor

reluctance machine with a new topology. The concept of the FEFSM involves

changing the polarity of the flux linking with the armature winding, with respect to

the rotor position. The FEFSM has advantages of low cost, simple construction,

magnet-less machine, and variable flux control capabilities suitable for various

performances when compared with others FSMs. Many topologies of FEFSM have

been studied and documented [1], [5], [15], [20], [21], [32], [58-61].

2.5.1 Structure review of FEFSM

Early examples of single-phase 4S-2P FEFSM that employ a DC FEC on the stator, a

toothed-rotor structure, and fully-pitched windings on the stator were discussed [59].

Both the armature and field windings were placed in the stator, which overlapped

each other and produced high copper losses. This design is practically used in

various applications that require high power densities [60]. Single-phase AC and DC

were applied to armature and field winding of this machine, respectively. The

maximum flux interaction required for the rotation of rotor was supplied by armature

coil at zero rotor position and the torque was produced by the variable mutual

inductance of windings. The single-phase FEFSM can be considered as a simple

machine that required two powered electronic controllers for armature and DC FEC.

The main advantages of this design are low cost for high volume applications and

precise control of position, torque, and speed.

Another example of FEFSM is depicted in Figure 2.6(a) with eight stator

slots (four slots for armature coil and four for FEC) and four rotor poles. The design

specifications and operation of 8S-4P FEFSM were explained in [61]. When

18

compared with induction machine, the FEFSM had achieved high torque and

efficiency but the main problems concerned with this design are low starting torque,

high torque ripples, overlapped windings, and fixed rotating direction. Two single-

phase FEFSMs topologies with DC field and AC armature windings with the same

coil-pitch of two slot-pitches and different coil-pitches of one and three slot-pitches

respectively were discussed [6]. It is shown that the iron loss of FEFSM had been

reduced and thus increased the efficiency. These machines shared the same operating

principle as discussed in [61], but 12S-6P explained in [6] had much shorter end

windings, and therefore, much better copper usage efficiency. These topologies had

problems of overlap windings and high copper losses. The structure of single-phase

(a) (b)

(c) (d)

Figure 2.6: Single-phase FEFSMs, (a) Single-phase 8S-4P FEFSM, (b) single-phase 12S-6P FEFSM with salient rotor, (c) single-phase 8S-8P FEFSM with overlapping windings, (d) single-phase 12S-6P FEFSM with segmental rotor

A1

A1

A1

FEC

FEC-2

A1

A1 A1

FEC FEC

FEC

FEC-1

A1

A1

19

FEFSM is illustrated in Figure 2.6(b). The performance analysis of single-phase 8S-

8P FEFSM, depicted in Figure 2.6(c), was discussed in [62]. The proposed motor has

high cogging torque and overlap armature and field windings which made it

unsuitable to be applied for any electrical equipment. A single-phase 12S-6P FEFSM

for high density air conditioner with segmental rotor was designed and discussed in

[63]. The proposed motor had less copper losses due to non-overlap armature and

field windings as shown in Figure 2.6(d). Although 12S-6P had high average power,

the rotor was not robust due to segments, hence could not be used for high-speed

applications.

The PM of 24S-10P PMFSM was replaced by FEC to design a three-phase

FEFSM, as illustrated in Figure 2.7(a). The total flux generation was limited due to

the isolation of DC FEC and thus reducing the performances [14]. To improve the

drawbacks of FEFSM, 12S-8P FEFSM with segmental rotor was designed and

experimentally tested [20]. Concentrated winding arrangement was used in this

design, which gave shorter end windings of DC FEC and armature coil when

compared with the tooth rotor structure with distributed windings. This design had

significant gains over other designs as it utilised less conductor materials, thus

improving the overall efficiency due to the reduction of copper loss. FEFSMs with

single and dual polarity FEC were discussed in [21], [5], [64]. The advantages of

FEFSM with single polarity FEC are low copper loss due to less volume of FEC, less

leakage flux when compared with dual FEC windings. The field-weakening

capability of FEFSM was improved using toroidal DC field winding [22]. The

proposed machine had limited torque generation due to the field flux produced by

DC field conductors close to the outer surface of the stator, which went through the

outside of the stator instead of the rotor as well as high copper losses owing to

overlap armature and field windings.

In recent years, research on in-wheel motor for EV drive train system has

become more popular due to their several advantages of independent wheel

controllability and higher efficiency. In addition, it provides more cabin space caused

by the elimination of mechanical transmission gears as conventionally used in most

existing EVs with single motor propulsion system configuration. Since the outer-

rotor configuration was more suitable for direct drive, the PMFSM with outer-rotor

had been proposed only for light EV applications [65]. It provided essentially

sinusoidal back-EMF and high torque at low speed. Nonetheless, constant PM flux of

20

PMFSM made it difficult to control, which required field weakening flux at high

speed conditions. 12S-10P outer rotor FEFSM had been proposed and discussed in

[66], as illustrated in Figure 2.7(b). Robust rotor structure with high mechanical

strength and no PM were clear advantages of this machine while overlap windings

will create problem of high copper losses. Three types of low-cost FEFSMs with DC

field and AC armature windings with the same coil pitch of one slot pitch (12S-8P),

two slot pitches (12S-5P and 12S-7P machines), and different coil pitches of one and

three slot pitches (9S-5P), respectively, were analysed and compared in [67]. It was

concluded that halving stator slots and rotor poles number can be an effective

approach to increase density for FEFSMs.

2.5.2 Design restrictions and specifications for FEFSMs

The design specifications of FEFSMs are illustrated in Table 2.1 in which three

major parameters; stator radius, rotor radius and torque density were chosen to

differentiate FEFSMs from one another and be further explained in terms of their

characteristics. The operating principle of flux switching was obeyed by all these

designs. According to the rotor rotation, the flux flows generated by PMs and mmf of

FECs were switched and linked with the armature coil alternately while for FEFSM

there was no PM, the armature winding and the field excitation coil both were

stationary but the magnetic flux linkage in the armature winding could either be

(a) (b)

Figure 2.7: Three-phase FEFSM, (a) Three-phase 24S-10P FEFSM, (b) three-phase outer rotor FEFSM

A1

A2 C1 B1 B2

C2

A1

A2 C1 B1 B2

C2

FEC-2 FEC-1

FEC-1 FEC-2

FEC-1 FEC-2

FEC-1

FEC-2 A1

B1

C1 A2

B2

C2

A3

B3

C3 A4

B4

C4

21

positive or negative depending on the position of the mobile part. The flux generated

by field excitation coil flowed from stator to rotor and then from rotor to stator to

produce a complete cycle.

2.5.3 Flux linkages of various FEFSMs

A key factor for all developed FEFSM models was the phase flux linkage

calculation, which created a significant challenge, since the stator had salient poles

and the iron core saturation had a significant influence on the motor’s operation.

Many research works were published in the past decade in this domain, as [68] that

introduced analytical models to calculate the flux linkage, [69] that developed

models based on magnetic equivalent circuits, or [70] where the analytical model

was created using FEM analysis results. Deterministic optimisation and GA were

usually applied for the improvement of flux linkage. Additionally, swarm

optimisation [71] could be used to treat several parameters of motor to improve the

flux linkage. Although the flux linkage strength of segmental rotor FEFSM was high

due to short flux path as compared to salient rotor, the rotor was not robust due to

segments, as illustrated in Figure 2.8. All the FEFSMs with salient rotor were the

best candidates to be applied for high-speed applications.

Table 2.1: FEFSMs Comparison

Types of machines Stator radius (mm)

Rotor radius (mm)

Torque density

8S-4P FEFSM 45 25 Not mentioned

Single-phase FEFSM with salient rotor

45 25 Not mentioned

Single-phase FEFSM with segmental rotor

75 44.5 Low torque density due to saturation of rotor segments

Three-phase 24S-10P FEFSM

45 29.25 Less torque density compared to PMFSM

Three-phase 12S-8P FEFSM (segmental rotor)

75 45 Low torque density due to saturation of rotor segments

Three-phase 24S-10P (dual FEC)

132 97.2 8.09Nm/kg

Three-phase 24S-10P (single FEC)

132 97.2 Potentially good due to less FEC windings

Three-phase 12S-14P FEFSM (toroidal DC

winding)

240.3 158 Improvement in field weakening, High torque density compared with

PMFSM Three-phase outer rotor

FEFSM 109.5 132 2.63Nm/kg

22

2.5.4 Cogging torque of FEFSMs

Cogging torque did not add to electro-magnetic output torque. It only effects in

torque pulsations that corresponded to undesirable vibration and acoustic noise. The

number of periods, Np of the cogging torque waveform over a rotation of one stator

tooth pitch was given by [20];

],[ rotorst

rotorp NNHCF

NN (2.1)

where the denominator is defined as the highest common factor (HCF) between

stator slots, Nst and number of poles of the rotor, Nrotor. Using the index Np, the

electrical angle of rotation, αcog for each period of the cogging torque and the number

of periods of cogging torque, Nelec per electrical cycle are therefore,

stpcog NN

0360

(2.2)

rotor

stpelec N

NNN (2.3)

(a) (b)

Figure 2.8: Flux distribution, (a) Flux lines of segmental rotor and (b) salient rotor

Long flux path Short flux path

B[T] 2.8T

2.1T

1.4T

0.7T

0T

23

Various techniques had been published to reduce the cogging torque on both

radial type [72-74] and axial type PMFSM [75], left vacant space in the area of

FEFSMs, as all the designs discussed above had cogging torque especially various

topologies of three-phase FEFSM, which had high cogging torque, as shown in

Figure 2.9 [64]. Rotors with stepped skew [76, 77] could be adopted to reduce the

cogging torque and torque ripple under various load conditions. Different kinds of

notching schemes [78, 79] were also available for cogging torque reduction.

2.5.5 Average torque and efficiency

The electromagnetic torque, Te developed in an electrical machine generally

consisted of a reluctance torque component, Trel and an excitation torque component,

Texc, and might be expressed in a polyphase arrangement as [20],

k

kexck

krele TTT ,,

(2.4)

dd

iddL

iT exck

kk

k ke 221

(2.5)

Figure 2.9: Cogging torque of three-phase FEFSM [64]

-40

-30

-20

-10

0

10

20

30

40

0 60 120 180 240 300 360

T [Nm]

Electric cycle [°]

12S-10P 12S-14P 12S-16P12S-20P 12S-22P

24

where Lk is the phase winding inductance, ψexc is the flux linkage due to field

excitation, ik is the phase current, θ is the electrical angular position of the rotor, and

k= a, b, c is the phase designation. As the operation of the FSM with sinusoidal

current was with the current placed in phase with the back-EMF, the principal torque

was due to Texc and the contribution of Trel was negligible, so that Te can be expressed

as

k

kexcr

kkk

r

ie T

titetP

T ,

)()()(

(2.6)

with Pi being the instantaneous power, ωr the rotational speed, and ek the

phase back EMF. The flexible characteristic of the average torque, controllable by

both the field and armature current was one of the major advantages of FEFSMs

when compared with PMFSM. The first FEFSM developed a torque of just under 1.2

Nm at a speed of 9,500 r/min corresponded to a peak power output of 1170 W,

examined in [59]. Peak efficiency of the complete drive was measured at just less

than 50%, which was lower than anticipated. The maximum efficiency obtained for

8S-4P FEFSM drive was about 75% at various torque level but the average torque

needed improvement at approximately 2 Nm. In [14], the electromagnetic

performance of the 12S-10P, 12S-11P, 12S-13P, and 12S-14P FEFSMs were

investigated by 2D-FEA analysis and validated experimentally. The average torque-

current characteristics of various rotor pole configurations are illustrated in

Figure 2.10. The experimental results illustrated that slightly larger torque was

produced by 24S-10P FEFSM when compared with 24S-10P PMFSM at lower field

current. As the field current increased, at certain levels the stator teeth were saturated

and the FEFSM exhibited much lower torque.

A few investigations were undertaken for segmental rotor FEFSM but

researchers agreed that this topology was particularly attractive for low-cost

operation due to non-overlapping windings and no permanent magnet [20]. The

mean torque output increased proportionally with the armature current and field

current. At rated field and armature current, the output mechanical torque was

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