A COMPARATIVE STUDY BETWEEN PMFSM AND HEFSM...

A COMPARATIVE STUDY BETWEEN PMFSM

AND HEFSM WITH IRON FLUX BRIDGES FOR

HEV APPLICATIONS

NURUL `AIN BINTI JAFAR

UNIVERSITI TUN HUSSEIN ONN MALAYSIA

i

A COMPARATIVE STUDY BETWEEN PMFSM AND HEFSM

WITH IRON FLUX BRIDGES IN HEV APPLICATIONS

NURUL „AIN BINTI JAFAR

A thesis submitted in

fulfillment of the requirement for award of the

Master of Electrical Engineering

Faculty of Electrical and Electronics Engineering

Unversiti Tun Hussein Onn Malaysia

March 2016

iii

For my beloved late mother and father

Che Anah bt Che Abdul Rahman and Jafar bin Samat,

My Siblings,

And my friends,

Thank you for your love, guidance and support

iv

AKNOWLEDGEMENT

First of all, I am grateful to The Almighty Allah for establishing me to complete my

research studies.

I would like to express my thanks my dedicated supervisor, Dr Erwan bin

Sulaiman. I am extremely grateful and indebted to him for his expert, dedication,

sincere and valuable guidance and encouragement extended to me.

I also would like to extend my gratitude to the Centre of Graduate Studies

UTHM for their financial support during my studies. Besides, I take this opportunity

to record my sincere thanks to all FSM members for their help and encouragement.

I also would like to acknowledge my parent and family members, for their

unceasing encouragement and support to complete my study.

I also place on record, my sense of gratitude to one and all who directly or

indirectly, have lent their helping hand in this venture.

vii

ABSTRACT

Nowadays, research and development of hybrid electric vehicles (HEVs) become

popular in effort to reduce the pollutant emissions that can cause global warming due

to demand of personal transportation. One example of successfully developed

electric machines for HEVs is interior permanent magnet synchronous machines

(IPMSMs). However, IPMSM design tends to be difficult because the permanent

magnet (PM) is embedded in the rotor core and also the distributed armature

winding. Therefore, a new machine known as flux switching machine (FSMs) which

categorized in three groups is proposed with advantage of robust rotor structure

which suitable for high speed applications. Thus, a new structure of 12S-10P

PMFSM is introduced and investigated. However, this machine has some drawbacks

and influence in term of performances, 12S-10P PMFSM have been transformed to

12S-10P and 12S-14P with additional FEC. In addition, iron flux bridges is

introduced, due to the developed C-Type HEFSMs has a problem of separated stator

core which lead to difficulty in manufacturing design as well as flux leakage to the

outer stator. In this research, performance of PMFSM and HEFSM are analyzed and

compared by 2D Finite Element Analysis (FEA) using JMAG Designer ver.13.0,

released by Japanese Research Institute (JRI). In order to achieve the target

performance, deterministic optimization method is applied to improve the initial

design. As a conclusion, HEFSM design has successfully achieved the target and

power similar with IPMSM used in Prius „07 specifications. This machine can also

be employed in motor with robust condition due to its high mechanical strength as

well as high revolution speed such as a hybrid electric vehicle.

viii

ABSTRAK

Pada masa kini, peyelidikan dan pembangunan terhadap kenderaan elektrik hibrid

(HEVs) menjadi semakin popular dalam usaha untuk mengurangkan pelepasan bahan

pencemar yang boleh mengakibatkan pemanasan global yang disebabkan oleh

permintaan pengangkutan persendirian. Satu contoh mesin elektrik yang telah

berjaya digunakan untuk kenderaan eletrik hibrid adalah mesin segerak magnet

dalaman kekal (IPMSMs). Walaubagaimanapun, rekabentuk IPMSM lebih

cenderung menghadapi kesukaran kerana kedudukan magnet kekal (PM) yang

terletak di dalam teras pemutar dan juga pengedaran angker penggulungan. Oleh

demikian, satu mesin baru diperkenalkan, dikenali sebagai pensuisan fluks (FSM),

dikategorikan kepada tiga kumpulan yang mempunyai kelebihan struktur pemutar

teguh yang sesuai untuk aplikasi berkelajuan tinggi. Oleh itu, satu struktur baru iaitu

12S-10P PMFSM telah diperkenalkan dan dianalisis. Oleh kerana mesin ini

mempunyai beberapa kelemahan dan ianya juga mempengaruhi dari segi

persembahan, struktur 12S-10P PMFSM telah ditukar kepada 12S-10P dan 12S-14P

dengan penambahan FEC. Di samping itu, jambatan fluks besi juga diperkenalkan,

kerana rekaan C-Type HEFSMs yang mempunyai masalah teras pemegun terpisah

justeru membawa kepada kesukaran dalam pembuatan rekabentuk dan juga

kebocoran fluks pada pemegun luar. Dalam kajian ini, prestasi antara PMFSM dan

HEFSM ini dijalankan oleh 2D-FEA iaitu JMAG Designer versi 13, dimana ianya

dikeluarkan oleh Institut Penyelidikan Jepun (JRI). Dalam usaha untuk mencapai

prestasi sasaran, kaedah pengoptimuman telah diguna pakai untuk penambahbaikan

struktur permulaan. Kesimpulannya, rekabentuk HEFSM telah berjaya mencapai

sasaran tork dan kuasa, iaitu menyamai IPMSM yang telah digunakan pada

spesifikasi Prius ‟07. Oleh yang demikian, mesin ini boleh digunakan kerana keadaan

motor yang mantap disebabkan oleh kekuatan mekanikal yang tinggi dan juga

kelajuan revolusi tinggi seperti kenderaan elektrik hibrid.

ix

TABLE OF CONTENT

TITLE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

CONTENTS

LIST OF TABLES

LIST OF FIGURE

i

ii

iii

iv

v

vi

vii

xi

xiii

LIST OF SYMBOL AND ABBREVIATIONS xvii

LIST OF APPENDICES

LIST OF PUBLICATIONS

xx

xxi

CHAPTER 1 INTRODUCTION 1

1.1 Introduction

Problem statement

1

1.2 3

1.3 Objectives

Project scopes

Thesis outline

4

1.4 4

1.5 5

CHAPTER 2 LITERATURE REVIEW 7

2.1 Introduction 7

2.2 Classification of traction motor 7

2.2.1 DC machines 7

2.2.2 Induction machines 9

2.2.3 Switch reluctance machines 9

2.2.4 Permanent magnet synchronous machines 10

x

2.3 IPMSM used in HEV

Introduction of flux switching machines (FSMs)

12

2.4 14

2.4.1 Permanent magnet flux switching machine

(PMFSM)

15

2.4.2 Field excitation flux switching machines

(FEFSM)

18

2.4.3 Hybrid excitation flux switching machines

(HEFSM)

22

2.5 Modes of flux switching machine 26

2.5.1 No load analysis 26

2.5.2 Load analysis 27

2.6 Selected HEFSM topology for HEV applications 28

2.7 Summary 29

CHAPTER 3 METHODOLOGY 30

3.1 Introduction 30

3.2 General methodology 30

3.2.1 Examine 12S-10P PMFSM 32

3.2.2 Design and investigation 12S-10P HEFSM with

iron flux bridges

33

3.2.3 Analysis of 12S-10P and 12S-14P HEFSMs 35

3.2.4 Improve 12S-10P and 12S-14P HEFSM and

sizing parameter

35

3.3 Summary 36

CHAPTER 4 RESULTS AND ANALYSIS 37

4.1 Introduction 37

4.2 Performances of 12S-10P C-Type PMFSM 37

4.2.1 Coil arrangement test 38

4.2.2 Flux line 39

4.2.3 Cogging torque under no load condition 40

4.2.4 Induced voltage under no load

condition

41

4.2.5 Torque and power versus various armature

xi

current density, Ja under on load condition 41

4.2.6 Torque and power versus speed characteristics

under on load condition

41

4.2.7 Magnet demagnetization analysis at high

temperature

42

4.3 Design and investigation of three phase 12S-10P C-Type

HEFSM with various iron flux bridges topologies

44

4.3.1 Flux line under no load condition 47

4.3.2 Flux strengthening under no load condition 48



condition

49

4.3.5 Torque versus field excitation current density, Je

at various armature current densities, Ja under on

load condition

50

4.3.6 Instantaneous torque characteristics 54

4.3.7 Torque and power versus speed characteristics 54

4.4 Analysis of 12S-10P and 12S-14P HEFSMs 55

4.4.1 Flux distribution under no load condition 57

4.4.2 Flux strengthening under no load condition 57


4.4.4 Induced voltage under no load condition 59



load condition

59

4.4.6 Instantaneous torque characteristics 61

4.4.7 Torque and power versus speed characteristics 62

4.5 Design Optimization via Empirical Study for 12S-10P

and 12S-14P of HEFSM

63

4.5.1 Flux distribution under no load condition 73



xii

condition 75



load condition

75

4.5.5 Torque and power versus speed characteristic 78

4.5.6 Motor loss and efficiency 79

4.5.7 Rotor mechanical stress 83

4.5.8 Magnet demagnetization analysis at high

temperature

84

4.5.9 Sizing parameter of 12S-14P HEFSM 86

4.6 Summary 89

CHAPTER 5 CONCLUSION AND RECOMMENDATIONS 93

5.1 Conclusion 93

5.2 Recommendations 94

REFERENCES 95

APPENDICES

xi

LIST OF TABLES

1.1 Design restrictions and target specifications of the proposed

PMFSM and HEFSM

5

2.1 Electric propulsion adopted in the automotive industry 11

2.2 Historical progress in power density of main traction motor

installed on Toyota Hybrid Electric Vehicles

13

4.1 Percentage of demagnetization of each step 43

4.2 Initial and final design parameters of 12S-10P and 12S-14P

HEFSM

73

4.3 Losses and efficiency of 12S-10P HEFSM 80

4.4 Loss and efficiency for operating points of 12S-14P

HEFSM

82



4.7 The performance for initial PMFSM and HEFSMs designs 91

4.5 The performance for final HEFSMs design 91

xii

LIST OF FIGURES

2.1 Types of electric traction 8

2.2 Classification of flux switching machines (FSMs) 14

2.3 Examples of PMFSM 16

2.4 Principle operation PMFSM 17

2.5 Examples of FEFSM 19

2.6 Principle operation of FEFSM 21

2.7 Examples of HEFSM 24

2.8 The operating principle of HEFSM 25

2.9 Initial design 29

3.1 General methodology 31

3.2 Flowchart for performance analysis of PMFSM and

HEFSM based on FEA

32

3.3 The initial dimension of 12S-10P PMFSM 33

3.4 The initial dimension of 12S-10P HEFSM (C1) 34

3.5 The initial dimension of 12S-10P HEFSM (C2) 34

3.6 The initial dimension of 12S-14P HEFSM 35

3.7 Flowchart of stack length reduction and PM weight

reduction

36

4.1 Initial design of 12S-10P PMFSM 38

4.2 UVW circuit 39

4.3 Flux linkage of phase U, V and W 39

4.4 Flux line of PM only 40

4.5 Cogging torque 40

4.6 Induced voltage at rated speed 1200 r/min 41

4.7 Torque and power versus various Ja 42

xiii

4.8 Torque and power versus speed characteristic 42

4.9 Flux density vector of each step 43

4.10 12S-10P HEFSM 44

4.11 Various bridges topologies of three-phase 12S-10P

HEFSM

45

4.12 Maximum torque and power for all configurations 46

4.13 Torque versus iron flux bridge width for C2 46

4.14 Flux line of PM only 47

4.15 Flux line of PM with maximum FEC of 30A/mm2 47

4.16 Flux strengthening for C1 and C2 design 48

4.17 Cogging torque 49

4.18 Induced voltage at rated speed 1200r/min 50

4.19 Torque versus field excitation current density, Je at

various armature current density, Ja for C1

50

4.20 Vector diagram for C1 51


various armature current density, Ja for C2

52

4.22 Vector diagram for C2 53

4.23 Instantaneous torque for C1 and C2 54

4.24 Torque and power versus speed characteristic for both

designs

55

4.25 Initial design of HEFSM with iron flux bridge 56

4.26 Flux distribution at condition PM only 57

4.27 Maximum flux linkages at various field excitation current

density, Je for 12S-10P and 12S-14P HEFSM

58

4.28 Cogging torque for both designs 58

4.29 Induced voltage at rated speed 1200r/min for both

designs

59


various armature current density, Ja for 12S-14P

60

4.31 Vector diagram of 12S-14P with iron flux bridges 61

4.32 Instantaneous torque of 12S-10P and 12S-14P 62

xiv

4.33 Torque and power versus speed curve for both designs 62

4.34 Design free parameter of HEFSM 63

4.35 Torque versus rotor radius (A1) 64

4.36 Torque versus rotor pole height (A2) 64

4.37 Torque versus rotor pole width (A3) 64

4.38 Torque versus PM height (A4) with respected to PM

width (A5) and FEC height (A6)

66

4.39 Torque versus FEC width (A7) 66

4.40 Torque versus stator back iron width (A8) 67

4.41 Torque versus stator teeth width (A9) 67

4.42 Deterministic optimization method 68

4.43 Torque and power versus cycle of optimization for 12S-

10P design

69

4.44 Torque and power versus cycle of optimization for 12S-

14P design

69

4.45 12S-10P HEFSM 70

4.46 Cross sectional of optimum 12S-10P HEFSM 70

4.47 Design of 12S-14P HEFSM 72

4.48 Dimension of optimum 12S-14P HEFSM 72

4.49 Flux distribution for 12S-10P HEFSM 73

4.50 Flux distribution for 12S-14P HEFSM 74

4.51 Cogging torque of initial and final for both designs under

PM with maximum Je of 30A/mm2

74

4.52 Induced voltage at rated speed of 1200r/min for both

designs under PM with maximum Je of 30A/mm2

75

4.53 Torque versus Je at various Ja for final 12S-10P HEFSM 76

4.54 Vector diagram for maximum Je and Ja of 30A/mm2 and

30Arms/mm2 for 12S-10P HEFSM

76

4.55 Torque versus Je at various Ja for final 12S-14P HEFSM 77

4.56 Vector diagram for maximum Je and Ja of 30A/mm2 and

30Arms/mm2 for 12S-14P HEFSM

77

4.57 Torque and power vs speed curve for initial and final of

xv

12S-10P HEFSM 78

4.58 Torque and power vs speed curve for initial and final of

12S-14P HEFSM

79

4.59 Frequent operating point of HEV for 12S-10P HEFSM 80

4.60 Iron losses and copper losses distribution for 12S-10P

HEFSM

81

4.61 Representative operating points for the target HEV drive

of 12S-14P HEFSM

82

4.62 Copper loss, iron loss and eddy current loss for 12S-14P

HEFSM

82

4.63 Principal stress distributions of rotor at 20,000r/min for

12S-10P HEFSM

83

4.64 Principal stress distributions of rotor at 20,000r/min for

12S-14P HEFSM

84

4.65 Flux density vector of each step for 12S-10P HEFSM 85

4.66 Flux density vector of each step for 12S-14P HEFSM 86

4.67 Torque versus stack length for case 1 87

4.68 3D-view of both designs 87

4.69 Torque vs PM weight by kept constant PM length (A4) 88

4.70 Torque vs PM weight by kept constant PM width (A5) 89

xvi

LIST OF SYMBOLS AND ABBREVIATIONS

A - Ampere

Arms - Ampere root mean square

fe - Electrical frequency

fm - Mechanical rotation frequency

Ia - armature current

Ja - Armature coil current density

Je - FEC current density

k - natural entity

kg - kilogram

kW - kilowatt

La - motor stack length

La-end - estimated average length

Le - motor stack length

Le-end - estimated average length of end coil

mm - milimeter

Nm - Newton meter

Na - number of turn of armature coil

Na-slot - total number of armature coil slot in the motor

Ne - number of turn of FEC

Ne-slot - total number of FEC slot in the motor

Nr - Number of rotor poles

Ns - Number of stator slots

ρ - copper resistivity

Pa armature coil copper loss

Pc - total copper loss

xvii

Pe - FEC copper loss

Pi - iron loss

Po - total output power

q - number of phases

Ag - Air gap

C1 - Configuration 1







emf - Electromotive force

DC - Direct current

V - Volt

EVs - Electric Vehicles

EMI - Electromagnetic-interference

FEC - Field Excitation Coil

FSM - Flux Switching Machine

HEFSM - Hybrid Excitation Flux Switching Machine

HEM - Hybrid Excitation Machine

HEVs - Hybrid Electric Vehicles

ICE - Internal Combustion Engine

IM - Induction Motor

IPMSM - Interior Permanent Magnet Synchronous Motor

JRI - Japanese Research Institute

mmf - Magnetomotive force

P - Pole

PM - Permanent magnet

PMFSM - Permanent Magnet Flux Switching Machine

PMSM - Permanent Magnet Synchronous Motor

S - Slot

xviii

SPMSM - Surface Permanent Magnet Synchronous Motor

SRM - Switch Reluctance Motor

2D - 2 Dimension

3D - 3 Dimension

xix

LIST OF APPENDICES

APPENDIX TITLE

A1 GANTT CHART

B1 TUTORIAL JMAG-DESIGNER

xx

LIST OF PUBLICATIONS

1. N.A. Jafar and E. Sulaiman, “Design Analysis of 12S-10P Hybrid-Excitation

Flux-Switching Permanent-Magnet Machines for Hybrid Electric Vehicle”,

2014 IEEE 8th International Power Engineering and Optimization Conference

(PEOCO2014), Langkawi, Malaysia, 24-25 March 2014, pp. 308-312.

2. Nurul „Ain Jafar, Siti Khalidah Rahimi, Siti Nur Umira Zakaria and Erwan

Sulaiman, “Performance Analysis of 12S-10P Hybrid-Excitation Flux

Switching Machines for HEV”, The 2nd

Power and Energy Conversion

Symposium (PECS 2014), Universiti Teknikal Malaysia,Melaka,12 May 2014,

pp 221-226.

3. Nurul „Ain Jafar, Erwan Sulaiman and Siti Khalidah Rahimi, “Premilinary

Studies of Iron Flux Bridges on Hybrid Excitation Flux Switching Machine for

HEV Applications”, J. Appl. Sci. & Agric., 9(18): 219-226, 2014.

4. Nurul „Ain Jafar, Siti Khalidah Rahimi, Erwan Sulaiman, “Performance

Comparison between Two Different Designs of 12S-10P HEFSM for High-

speed HEVs”, MUCET2014, Mahkota Hotel, Melaka.

5. Nurul „Ain Jafar, Erwan Sulaiman and Siti Khalidah Rahimi, “Performance

Analysis of 12S-10P HEFSM with Various Flux Bridges,” Applied Mechanics

and Materials Vols. 773-774 (2015), pp. 781-785.

6. Nurul „Ain Jafar, Erwan Sulaiman and Siti Khalidah Rahimi, “Parameter

Sensitivity of 12S-10P HEFSM with Iron Flux Bridges for HEV Applications”,

ARPN Journal of Engineering and Applied Sciences, ISSN 1819-6608 (2015).

CHAPTER 1

INTRODUCTION

1.1 Introduction

In over than hundred years, conventional vehicles operate on the principle of internal

combustion engine (ICE) which based on fossil fuels have been used for personal

transportation. Along with that, the demand of private vehicles has increased every

day due to the increasing rates of rapid world populations. Due to the high demand

and usage of personal vehicles, ICE automobile becomes a major source of the urban

pollutions. Hence, the problems associated with ICE automobiles are three fold,

environmental, economical, as well as political. One of the serious problems

regarding to the environmental issue is the emissions. The environmental concerns

can raise by the increasing pollution levels due to harmful emissions from

hydrocarbon fuelled power sources [1-2]. Besides air pollution, the other main

objection regarding ICE automobiles is the extremely low efficiency use of fossil

fuel. In addition, some of the major challenges currently faced in the automotive

industry are to reduce the dependency on fossil fuels, reduce the green house gases

emitted per km of travel [3]. Government agencies and organizations have developed

more stringent standards for the fuel consumption and emissions. Nevertheless, with

the ICE technology being matured over the past 100 years, although it will continue

to improve with the aid of automotive electronic technology, it will mainly rely on

alternative evolution approaches to significantly improve the fuel economy and

reduce emissions [4-5].

Therefore, in order to tackle these major issues, auto-manufacturers are

shifting towards new technologies such as electric vehicles (EVs) and hybrid electric

vehicles (HEVs). The concept of EV has been around since the early years of the

2

automotive industry. Since 1910, the EVs have been more numerous than vehicles

with ICE. New environmental concerns over the amount of pollutants spilled into the

atmosphere each day by ICE have become a powerful motivation for the use of EVs

once again. EVs are some of the alternatives for future transportation. However, the

major drawbacks of EVs are their limited range and high charging time. To

overcome these drawbacks, HEVs which combine battery-based electric machines

with an ICE is used. The concept of HEVs was conceived in the middle of the

previous century. HEVs offer the most promising solutions to reduce the emission

and the problem of limited range and inferior performance of EV [6-7].

Research in HEV development has focused on various aspects of design, such

as component architecture, engine efficiency, reduced fuel emissions, materials for

lighter components, power electronics, efficient motors, and high power density

batteries. The primary motivations of this research are to increase the fuel economy

and minimizing the harmful effects of the automobile to the environment. Except for

gains in the fuel economy, HEVs also demonstrated a potential of reducing exhaust

emission. For EVs and HEVs, the output characteristics of electric motors differ

from those of ICEs. Typically, the electric motor eliminates the necessity to be idle at

a stop, produce large torque at low speed, and offers a wide range of speed

variations. It may be possible to develop lighter, more compact, more efficient

systems by taking advantages of the characteristics of electric motors [8].

Selection of traction motors for hybrid propulsion systems is a very important

step that requires special attention. In fact, the automotive industry is still seeking for

the most appropriate electric-propulsion system for HEVs and even for EVs. In this

case, key features are efficiency, reliability, and cost. The process of selecting the

appropriate electric-propulsion systems is, however, difficult and should be carried

out at the system level. In fact, the choice of electric-propulsion systems for HEVs

mainly depends on three factors: driver expectation, vehicle constraint, and energy

source. With these considerations, it is obvious that the overall motor operating point

is not tightly defined [9]. Therefore, selecting the most appropriate electric

propulsion system for HEV is a challenging task. Several examples of traction motor

that have been applied for HEVs, such as direct current (DC) motor drives, induction

motor (IM), switch reluctance motor (SRM) and permanent magnet synchronous

3

motor (PFSM) are demonstrated in Figure 2.1. The basic characteristics of an electric

drive for HEVs are the following [10-11]:

(i) High torque and power density.

(ii) Very wide speed range, covering low-speed crawling and high-speed

cruising.

(iii) High efficiency over wide torque and speed ranges.

(iv) Wide constant-power operating capability.

(v) High torque capability for electric launch and hill climbing

(vi) High intermittent overload capability for overtaking.

(vii) High reliability and robustness for vehicular environment.

(viii) Low acoustic noise.

(ix) Reasonable cost.

(x) High-efficiency generation over a wide speed range.

(xi) Good voltage regulation over wide-speed generation.

1.2 Problem statement

Interior permanent magnet synchronous machine (IPMSMs) using rare-earth

permanent magnet is an example of successfully propulsion system employed for

HEVs. It has been selected by the automotives company due to the smaller size and

lighter weight providing with design freedom of the vehicles and its higher efficiency

contributing to less fuel consumption [12]. However, this machine also possess some

disadvantages such as uncontrolled flux due to the constant PM flux, distributed

armature winding resulting in much copper loss and high coil end length, and

difficult to perform the design optimization due to complex shape and structure of

IPMSM. As one option to overcome this problem, a new machine with flux

switching operating principle namely permanent magnet flux switching machine

(PMFSM) is introduced; which utilized permanent magnet as a flux source in stator

[13]. However, this design has low performances in terms of torque and power while

the flux source is difficult to regulate and also has a risk of high demagnetization.

Thus, 12S-10P hybrid excitation flux switching machine (HEFSM) becomes as one

of another alternative to increase the performances owing to the combination of flux

4

sources which consists of permanent magnet (PM) and field excitation coils (FECs)

[14].

Various topologies of HEFSMs have been developed and investigated. The

topology of 12S-10P C-Type HEFSM has an interest due to their simple and robust

rotor design and high torque density. However, the developed C-Type has a problem

of separated stator core which lead to difficulty in manufacturing design [15]. Hence,

the fabrication process also becomes difficult due to the consistency of the stator

shape. Moreover, the flux leakage will occur to the outer stator and extremity reduces

the total performance of the machine. Thus, investigation of the additional iron flux

bridge is introduced to overcome this problem.

1.3 Objectives

The objectives of this research are:

(i) To compare the electromagnetic performance between PMFSM and HEFSM

topologies of 12S-10P machine.

(ii) To investigate the influences of flux bridge on stator and pole number in 12S

HEFSM machine.

(iii) To obtain an optimum machine dimension for a desired rated performance.

1.4 Project scopes

The scopes of this research are:

(i) The design requirements, restrictions and specifications of the proposed

machine for HEV applications are listed in Table 1.1. The electrical

restrictions related with the inverter such as maximum 650V DC bus voltage

and maximum 360V inverter current are set. The armature coil current

density, Ja and the FEC current density, Je is limit to 30 Arms/mm2 and 30

A/mm2, respectively with the PM weight is set to 1.3kg. In addition, the

target torque and power is set to 303Nm and 123kW, correspondingly.

(ii) Seven arrangements of iron flux bridges are introduced.

(iii) JMAG-Designer software version 13 is used to analyze the performances

under no load analysis and load analysis.

5

(iv) Deterministic Optimization Method is used to improve the initial torque and

power. Hence, sizing parameter such as stack length reduction, PM volume

reduction with respects to PM width and PM height is introduced to the

optimum design of 12S-14P HEFSM.

Table 1.1: Design restrictions and target specifications of the proposed PMFSM and

HEFSM [12]

Descriptions Proposed

PMFSM and HEFSM

Max. DC-bus voltage inverter (V) 650

Max. inverter current (Arms) 360

Max. current density in armature coil, Ja (Arms/mm2) 30

Max. current density in FEC, Je (A/mm2) 30

Stator outer diameter (mm) 269

Motor stack length (mm) 84

Air gap length (mm) 0.7

PM weight (kg) 1.3

Maximum torque (Nm) 303

Maximum power (kW) 123

Iron flux bridge width (mm) 0.5

1.5 Thesis outline

This thesis deals with a comparative study between permanent magnet flux switching

machine (PMFSM) and hybrid excitation flux switching machine (HEFSM) for HEV

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

chapter are listed as follows:

(i) Chapter 1: Introduction

This chapter explains the introduction about the research, including the

research background and the problems of IPMSM used in HEV. Then

problems of current IPMSM used in HEV are highlighted and the research

objectives are set to solve the problems. Besides, the project scopes of the

research are included in this chapter.

6

(ii) Chapter 2: Literature review

In this chapter, the introduction of electric motors is discussed. Then, the

introduction and classifications of flux switching machine including previous

research and principle operation are discussed. In addition, the selected

topology of PMFSM and HEFSM also clearly explains in this chapter.

(iii) Chapter 3: Methodology

This part describes the process of completing the proposed machine. These

machines are examined using commercial 2D-FEA, JMAG-Studio ver. 13.0,

released by JSOL Corporation. The process of this research can be divided

into three phases including design process, performance analysis and

optimization method.

(iv) Chapter 4: Results

This chapter discusses the results of this research which divided into four

sections. Initially, the electromagnetic behaviour of 12S-10P C-Type

PMFSM and HEFSM are investigated. However, the C-Type design has a

problem of separated stator core which lead to difficulty in manufacturing

process. Hence, iron flux bridge is introduced to overcome this problem.

Then, the performances of 12S-10P HEFSM without and with iron flux

bridges are investigated and compared. The third section analyzes the

performance of various rotor pole studies i.e 12S-10P HEFSM and 12S-14P

HEFSM with iron flux bridge. Since the initial performances of torque and

power for both designs are far from target requirements of 303Nm and

123kW, respectively, design improvements using “deterministic optimization

method” to treat several design parameters are conducted until the target

performances are achieved. Then, the comparative performances of initial and

final design for both designs are discussed. Lastly, the sizing parameter for

12S-14P HEFSM is introduced due to the optimum torque and power that

obtained is more the target requirements. This study is divided into two cases,

i.e stack length reduction and PM weight reduction. All performances for this

study are discussed.

(v) Chapter 5: Conclusion and recommendations

The final chapter describes and concludes the summary of the research and

pointed out some future works for design improvements.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

This chapter clarifies the types of traction motor which has been used for HEVs. Flux

switching machines (FSMs) are introduced, that is categorized in three groups. The

operating principle of each FSMs is clearly discussed in this chapter. The overview

of selected topologies of PMFSM and HEFSM are introduced.

2.2 Classification of traction motor

Among the different types of electric drives, there are four major types that are viable

for HEVs, namely; DC machines, IMs, SRMs and PMSMs. They possess

fundamentally different machine topologies, as shown in Figure 2.1.

2.2.1 DC machines

DC drives used to be widely accepted for HEVs. DC motors is prominent in electric

propulsion, because their torque–speed characteristics suit the traction requirement.

DC drives take the definite advantage of simple speed control, because of the

orthogonal disposition of the field and armature MMFs. However, DC motor drives

have a bulky construction, low efficiency, low reliability, and higher need of

maintenance [16]. By replacing the field winding with PMs, the PM DC drives

permit a considerable reduction in stator diameter, due to the efficient use of radial

space.

8

Armature reaction is usually reduced, and commutation is improved, due to the low

permeability of PMs. Although, the commutators and brushes attract a problem of

DC drive, which makes them less reliable and unsuitable for a maintenance-free

operation. The commutator, if used in proper operation, is a very rugged “inverter”.

Therefore, the power electronics circuit can be kept relatively simple and at low cost.

This is the case of the French automaker, PSA Peugeot Citroën, who introduced the

HEV version of the well-known Berlingo, which is called Dynavolt, with a DC

motor as electric propulsion [17].

(a) (b)

(c) (d)

Figure 2.1: Types of electric traction

(a)DC motors [16] (b) IMs [18] (c) SRMs [22] (d) PMSM [24]

9

2.2.2 Induction machines

Today, an IM drive is the most mature technology among various brushless motor

drives [18]. Cage IMs are widely accepted as the most potential candidate for the

electric propulsion of HEVs, owing to their mature technology, reliability,

ruggedness. Not forgetting, their freedom of maintenance, low cost, high overall

efficiency and ability to operate in hostile environments. They are particularly well

suited for the rigors of industrial and traction drive environments. At a critical speed,

the breakdown torque is reached. However, conventional control of induction drives,

such as variable voltage variable frequency, cannot provide the desired performance.

Any attempt to operate the motor at the maximum current beyond this speed will

stall the motor. Moreover, efficiency at a high speed range may suffer, in addition to

the fact that IMs efficiency is inherently lower than that of PM motors; it is due to

the absence of rotor winding and rotor copper losses [19]. In addition, the

development of EV and HEV induction machines is continually fuelled by new

design approaches and advanced control strategies.

Generally, IM drives are having problems, such as high loss, low efficiency,

low power factor and low inverter-usage. These problems pushed them out from the

race of HEVs electric propulsion, which is more serious for the high speed and large

power motor. Fortunately, these drawbacks is considered according to the available

literature. Some researches propose takes these problems into action, in the design

step of the IM used for HEVs. To improve the IM drives efficiency, a new generation

of control techniques has been proposed. Some of the proposed techniques are

particularly devoted to HEV applications, which constitute a progress, compared to

the study made in [20-21].

2.2.3 Switch reluctance machines

SRMs are gaining much interest and have been recognized to have a potential for

HEV applications. They have the definite advantages of simple construction, low

manufacturing cost, fault-tolerant operation, and outstanding torque-speed

characteristics. Even though this machine has a simple construction however, their

design and control are difficult and subtle. Besides, this machine has disadvantages

10

of exhibit acoustic-noise problems torque ripple, special converter topology,

excessive bus current ripple, and electromagnetic-interference (EMI) noise

generation. Recently, fuzzy sliding mode control has been developed for the HEV

SRMs to handle the machine nonlinearities and minimize the control chattering [22].

Additionally, an active vibration cancellation technique for the SRMs has been

proposed, which induces an antiphase vibration to cancel a specified vibration mode.

Hence, it reduced the acoustic noise. Nevertheless, SRMs is a solution that is actually

envisaged for light and heavy HEV applications [23].

2.2.4 Permanent magnet synchronous machines

PMSM drives are becoming more and more attractive and can directly compete with

the induction machines for HEVs. The advantages of the PMSM machines are highly

efficiency, and have a high power density, and reliable. However, these motors

inherently have a short constant-power region, due to their rather limited field

weakening capability, resulting from the presence of the PM field (the fixed PM limit

their extended speed range). In order to increase the speed range and improve the

efficiency of PM brushless motors, the conduction angle of the power converter can

be controlled at above the base speed. However, at a high-speed range, the efficiency

may drop because of the risk of PM demagnetization [24]. Nevertheless, the key

problem is their relatively high cost, due to PM materials.

There are various configurations of the PMSMs, which can be classified as

surface PMSM (SPMSM) and interior PMSM (IPMSM). The latter is more

mechanically rugged, due to the embedded PM in the rotor. Although, the SPMSM

design uses fewer magnets than the IPMSM, both motors may achieve a higher air-

gap flux density. Another configuration of PMSM is the PM hybrid motor, where the

air-gap magnetic field is obtained from the combination of PM and DC FEC, as

mentioned previously. In the broader term, PM hybrid motor may also include the

motor whose configuration utilizes the combination of PMSM and SRM. Although

the PM hybrid motor offers a wide speed range and a high overall efficiency, the

construction of the motor is more complex than PMSM [25]. Table 2.1 briefly

reviews the electric propulsion, which is recently adopted in the automotive industry.

11

Table 2.1: Electric propulsion adopted in the automotive industry [26]

HEV Model Propulsion System

PSA Peugeot-Citroën/Berlingo (France)

DC Motor

Renault/Kangaroo (France)

Induction motor

Chevrolet/Silverado (USA)

Induction motor

DaimlerChrysler/Durango (Germany/USA)

Induction motor

BMW/X5 (Germany)

Induction motor

Holden/ECOmmodore (Australia)

Switch reluctance motor

12

Nissan/Tino (Japan)

Permanent magnet

synchronous motor

Honda/Insight (Japan)

Permanent magnet

synchronous motor

Toyota/Prius (Japan)

Permanent magnet

synchronous motor

2.3 IPMSM used in HEV

In the last decade, Interior Permanent Magnet Synchronous Motors (IPMSM) has

been extensively analyzed as feasible candidates for variable-speed HEV traction

applications [27]. The IPMSM exhibits high efficiency when operating at constant

speed in the constant-torque region, due to its lower copper losses. Besides, this

IPMSM has been employed, mainly to increase the power density of the machines

[28]. This can be proven by the historical progress in the power density of main

traction motor installed on Toyota HEVs, as listed in Table 2.2. The power density of

each motor employed in Lexus RX400h‟05 and GS450h‟06 have been improved

approximately five times and more, respectively, compared to that installed on

Prius‟97 [29]. Although the torque density of each motor has been hardly changed, a

reduction gear has been enabled, to elevate the axle torque. This is necessary for

propelling the large vehicles such as RX400h and GS450h. As one of the effective

strategies to increase the motor power density, the technological tendency to employ

the combination of a high-speed machine and a reduction gear, would be accelerated.

13

Table 2.2: Historical progress in power density of main traction motor installed on

Toyota Hybrid Electric Vehicles

Model Year Max. Speed of Main

Traction Motor (r/min)

Normalized Motor Power

Density (p.u)

Prius 2007 6000 1.0

Prius 2003 6700 1.75

RX400h 2005 12,400 4.86

GS450h 2006 14,400 5.61

Even though IPMSMs installed in HEV have good performances as well as

being operated, this machine also possess some drawbacks, such as:

(i) The three-phase armature windings are wounded in the form of distributed

windings, which results in much copper loss and high coil end length.

(ii) The mechanical stress of the rotor depends on the number of PM bridges.

High number of bridges not only increases the mechanically weak points, but

also causes much flux leakage between the PMs that will degrade the

performance of the machine.

(iii) The present IPMSM has a complex shape and structure, which is are

relatively difficult to perform the design optimization.

(iv) The constant flux from PM is difficult to control, especially at light load high

speed operating points.

(v) The volume of PM used in IPMSM is very high, more than 1.3kg, which

increases the cost of the machine.

In order to avoid these drawbacks, a new structure is proposed, to build up

the concentrated windings. It can reduce the copper loss and the coil end length, and

robust rotor structure, which is suitable for high speed applications. In addition, this

machine has a simple shape and ease for design optimization. Flux switching

machine (FSM) with less rare-earth magnet is identified, and selected as an

alternative candidate for HEV applications. FSM can be categorized into three

groups such as Permanent Magnet Flux Switching Machine (PMFSM), Field

Excitation Flux Switching Machine (FEFSM) and Hybrid Excitation Flux Switching

Machine (HEFSM).

14

With significant achievements and improvements of permanent magnet

materials and power electronics devices, the brushless machines excited by PM

associated with FEC, which can be developed. This type is known as hybrid

excitation machine (HEM) which combines both field excitation coil (FEC) and

permanent magnet (PM) as a flux sources. HEM has several unique characteristics

that are able to be applied in HEV drive system, which can be classified into four

types, according to location of FEC and PM. The first type consists both FEC and

PM are located at rotor side [30-32], the second type consists the FEC is in the stator,

while the PM is in the rotor [33], the third type consists the FEC in the machine end

while the PM is in the rotor [34-35] and the last category consists both FEC and PM

are located in the stator [36-38]. The HEMs stated in three earliest types consists of a

PM in the rotor and can be classified as “hybrid rotor-PM with FEC” whereas the

final machine can be referred as “hybrid stator-PM with FEC”. The fourth machine,

also known as HEFSM is getting more popular recently.

2.4 Introduction of flux switching machines (FSMs)

In the middle of 1950s, the concept of the FSM was founded and first published [39].

Over the last ten years or so, many novel and new FSM machine topologies have

been developed for various applications, ranging from low cost domestic appliances,

automotive, wind power, to aerospace. FSM has been seen as a potential candidate

for future variable speed applications because of its unique merits. FSM can be

categorized into three groups, which are Permanent Magnet Flux Switching Machine

(PMFSM), Field Excitation Flux Switching Machine (FEFSM) and Hybrid

Figure 2.2: Classification of flux switching machines (FSMs)

Flux Switching Machines

(FSMs)

Permanent Magnet

(PMFSM)

Field Excitation

(FEFSM)

Hybrid Excitation

(HEFSM)

15

Excitation Flux Switching Machine (HEFSM) as shown in Figure 2.2. Permanent

Magnet Flux Switching Machine (PMFSM), Field Excitation Flux Switching

Machine (FEFSM) and Hybrid Excitation Flux Switching Machine (HEFSM).

2.4.1 Permanent magnet flux switching machine (PMFSM)

For several decade, PM machine have been studied, based on the principle of flux

switching machine. Generally, such machines have a salient pole rotor and the PMs,

which are housed in the stator. A three-phase FSM based on the homopolar flux

principle and the bipolar flux principle have been described in [40-41] and [42-43],

respectively, while a new type of single phase and three-phase PMFSM in which a

pair of PMs is embedded on the stator were reported in [44] and [45], respectively. In

addition, the performance of Law‟s relay, a flux-switching type of limited angle

actuator was proposed in [46]. Various examples of three-phase PMFSM are

illustrated in Figure 2.3.

A three-phase of 12S-10P PMFSM is presented in Figure 2.3(a). This design

has a salient pole stator core that consists of modular U-shaped laminated segments

placed next to each other with circumferentially magnetized PMs placed in between

them. For the flux switching principles, the PM magnetization polarity is being

reversed from one magnet to another [42-43]. The stator armature winding consists

of concentrated coils and each coil being wound around the stator tooth formed by

two adjacent laminated segments and a magnet. In the same figure, all armature coil

phases such as A1, B1, C1, A2, B2 and C2 have the same winding configuration and

are placed in the stator core to form 12 slots of windings. The salient pole rotor is

similar to that of SRMs, which is more robust and suitable for high speed

applications. The difference in the number of rotor poles and stator teeth is two. In

contrast with conventional IPMSM, the slot area is reduced when the magnets are

moved from the rotor to the stator. The heat is easy to dissipate from the stator, and

the temperature rise in the magnet can be controlled by a proper cooling system.

Similar to the fractional-slot PM machine with none-overlapping windings,

alternate poles wound windings can also be employed by the three-phase FSPM

machine, in order to be fault-tolerant PMFSM [47] as shown in Figure 2.3(b).

16

Figure 2.3: Examples of PMFSM

(a) 12S-10P PMFSM (b) Fault Tolerance PMFSM (c) E-core PMFSM

(d) C-Core PMFSM (e) Outer rotor PMFSM

A1

A1

A2

A1

C1

B1

A2

B1

A1

B2

B2

C2

C1

C2

C1 B1

C1 B1

A1 A1

B1

B1

C1

C1

B1

B1

C1

C1

A1

(a) (b)

(c) (d)

A1

B1

B1

C1

C1

A2

A2

B2

B2

C2

C2

(e)

17

For this topology, the armature windings from Figure 2.3(a) are reduced by removing

A2, B2 and C2 to form a total of six armature windings. In addition, this design gives

the advantage of less copper loss, due the less usage of armature coil, but employed

the similar quantity of PM. Nevertheless, the conventional PMFSM for both designs

in Figure 2.3(a) and (b) have the demerit of high PM volume. The stator pole without

armature winding in the Figure 2.3(b) is replaced by a simple stator tooth, in order to

reduce the usage of PM, viz. the new E-core 12S-10P PMFSM is developed as in

Figure 2.3(c) [48]. Besides, to form E-Core stator, half of the PM volume in Figure

2.3(b) is removed and the stator core is attached together. Further, the middle E-

stator teeth can be removed to enlarge the slot area, and consequently the new C-core

6S-10P PMFSM is developed, as illustrated in Figure 2.3(d) [49]. It should be noted

that the rotor pole number is close to the stator pole number in the conventional 12S-

10P PMFSM, while it is close to twice of the stator pole number in the E- and C-core

PMFSM. The 12S-22P outer-rotor PMFSM configurations illustrated in Figure

2.3(e), has been presented for in-wheel traction applications [50-51].

The general operating principle of the PMFSM is represented in Figure 2.4.

As seen in figure, as example, the red arrows indicate the flux line of PM. Hence, at

the position in Figure 2.4(a), the rotor pole aligns with one of two stator teeth over

which a coil is wound, and the PM flux linked in the coil goes out of the stator and

into the rotor pole. When the rotor moves forward to align with the other stator tooth

belonging to the same coil shown in Figure 2.4(b), the flux goes out of the rotor pole

and into the stator tooth, realizing “flux switching”, from which the name of flux

switching of this motor is derived. Thus, as the rotor moves, the flux linkage in the

windings will change periodically.

Figure 2.4: Principle operation PMFSM

Stator

PM

Rotor

Stator

PM

Rotor

Armature Coil

(a) (b)

18

2.4.2 Field excitation flux switching machines (FEFSM)

In conventional PMFSM, the PM excitation on the stator can be easily replaced by

the DC excitation to form FEFSM as demonstrated in Figure 2.5. In other words, the

FEFSM having salient-rotor structure is a novel topology, merging the principles of

the inductor generator and the SRMs [52-53]. The idea of the FEFSM based on

switching the flux linking polarity with the armature winding following the rotor

position. All the motor windings are in stator slots and the rotor is a simple

reluctance rotor. Figure 2.5(a) shows the simplest example of single phase 4S-2P

FEFSM that employs with a DC FEC on the stator, a toothed-rotor structure and

fully-pitched windings on the stator. From the figure, it is clear that two armature coil

and FEC windings are placed in the stator which overlapped each other. This design

makes the FEFSM robust and easy to build due to the combination with SRM. As in

all doubly salient motors, torque is developed by the movement of the rotor, with

respect to the stator, to a position of minimum reluctance to the flux set up by current

in the motor windings [54]. The novelty of the invention was that the single-phase

AC configuration could be realized in the armature windings by the deployment of

DC FEC and armature winding, to give the required flux orientation for rotation.

In addition, another example of single phase is demonstrated in Figure 2.5(b), is 8S-

4P FEFSM with eight stator teeth and 4 rotor teeth. As seen in figure, it is obvious

that four pole magnetic field is established when direct current is applied to the FEC

winding in four of the slots. Meanwhile, the other four slots hold an armature

winding that also pitched over two stator teeth. The direction of the current in the

armature winding determines, so that a set of four stator poles carries flux and also

the position of the rotor. As the FEC is excited by unipolar current, it can be directly

connected in parallel, or in series with the DC-supply of power converter, which

feeds the bipolar current into the armature winding. The design principle is explained

in [55], and the single-phase 8S-4P FEFSM has achieved higher output power

density, than the equivalent induction motor. Besides, the machine also achieved

much higher efficiency, when compared with the IM. However, the single-phase

machine has problems of low starting torque, large torque ripple, fixed rotating

direction, and overlapped windings between armature coil and FEC.

19

Figure 2.5: Examples of FEFSM

(a)1-phase 4S-2P FEFSM (b) 1-phase 8S-4P FEFSM

(c) 3-phase 24S-10P FEFSM (d) 3-phase 12S-10P FEFSM

(e) 3-phase 12S-8P segmental rotor FEFSM

A1

A1

FEC

FEC

A1

A1

A1

A1

FEC

FEC

FEC

FEC

(a) (b)

FEC-1

FEC-2

FEC-1 FEC-2

FEC-1

FEC-2

A1

B1

C1 A2

B2

C2

A1

B1

C1 A2

B2

C2

(c)

FEC-1

FEC-1

FEC-1

FEC-2

FEC-2

FEC-2

A1

B1

C1 A2

B2

C2

A1

B1

C1 A2

B2

C2

(d)

FEC-1

FEC-2

FEC-1

FEC-2

FEC-1

FEC-2

A1

B1 C1

A2

B2 C2

(e)

20

The three phase 12S-10P is developed, as shown in Figure 2.5(c) and (d), due

to achieve the desired performance of single phase. The 12S-10P FEFSM has been

developed from 12S-10P PMFSM, in which the PM is removed from the stator and

half of the armature coil slots in the upper layer are placed with the FEC windings, as

explained in Figure 2.4(c) [56]. Similar to the PM polarity of 12S-10P PMFSM in

Figure 2.3(a), this design consists of two flux sources, the FEC1 and FEC2 that

arranged in alternate DC current source polarity to generate two flux polarities.

However, the total flux generation is limited due to the isolated and unused stator

teeth, and thus, affecting the performances of machine. Nonetheless, several

structures and performances of FEFSM for various applications have been

investigated in [57-58]. Regardless, another three phase machine of 12S-10P FEFSM

is shown in Figure 2.5(d) [59]. This combination has been chosen because it can be

considered as the best minimum combination of slot-pole. It is to avoid odd rotor

pole numbers, such as 6Slot-5Pole and 6Slot-7Pole machines, that yields unbalanced

pulling force. In addition, this combination is proposed to avoid high torque ripples,

in case of 6Slot-8Pole and 6Slot-4Pole machines. Besides, this machine has an

advantage to take good balance between rotor and stator pole widths for minimizing

inescapable torque pulsation.

Since all FEFSM explained above posses an overlap winding between FEC

winding and AC winding, which caused higher coil end length, a 12S-8P FEFSM

with segmental rotor is proposed as shown in Figure 2.5(e) [60]. Segmental rotor has

the ability to provide magnetic path for transmitting the field flux to nearby stator

armature coil, with respect to rotor position. In addition, this design presents shorter

end windings with non overlapping coil, when compares to salient rotor arrangement

having overlapping coils. It has considerable gains, due to the reason that it utilizes

less conductor materials, and has further improvement in overall machine efficiency

[61].

Figure 2.6 shows the operating principle of the FEFSM, in which a field flux

path generated by an mmf of FEC at different particular two rotor positions when the

FECs are energized with DC current. The line with arrows depicts the field flux path.

The direction of the FEC fluxes into the rotor is demonstrated in Figure 2.6(a) and

(b), respectively while the direction of FEC fluxes into the stator is represented in

Figure 2.6(c) and (d), correspondingly to produce a complete one cycle flux. The

21

flux linkage of FEC switches its polarity by following the movement of salient pole

rotor, which creates the word “flux switching” that is similar with PMFSM. Each

reversal of armature current shown by the transition between Figure 2.6(a) and (b),

causes the stator flux to switch between the alternate stator teeth. The flux does not

rotate, but shifts clockwise and counterclockwise with each armature-current

reversal. With rotor inertia and appropriate timing of the armature current reversal,

the reluctance rotor can rotate continuously at a speed controlled by the armature

current frequency. The armature winding requires an alternating current reversing in

polarity in synchronism with the rotor position. For automotive applications the cost

of the power electronic controller must be as low as possible. This is achieved by

placing two armature coils in every slot, so that the armature winding comprises a set

of closely coupled (bifilar) coils [62].

Figure 2.6: Principle operation of FEFSM

(a) θe=0° and (b) θe=180° flux moves from stator to rotor

(c) θe=0° and (d) θe=180° flux moves from rotor to stator

Stator

FEC

Stator

FEC

Stator

Rotor Rotor

Armature Coil

(a) (b)

FEC FEC FEC

Stator

Rotor

FEC

Rotor

FEC

Armature Coil

(c) (d)

22

2.4.3 Hybrid excitation flux switching machines (HEFSM)

PMFSM has attracted lot of attention over the last two decades, due to their high

torque density and outstanding robustness [63-64]. In order to improve the flux-

weakening capability of this kind of machine, and to extend their application range to

hybrid vehicles, various topologies of HEFSM have been developed [65-67].

HEFSM combines the advantages of PM excitation by PMs which utilize primary

excitation, whereas DC FEC as a secondary source. In order to get a high density

synchronous machine with good flux control, both excitations is combined.

Conventionally, PMFSMs can be operated beyond base speed in the flux weakening

region, by means of controlling the armature winding current. By applying negative

d-axis current, the PM flux can be counteracted, but with the disadvantage of

increase in copper loss and reduce the efficiency, reduce power capability, and also

possible irreversible demagnetization of the PMs. Such HEFSMs have the potential

to improve flux weakening performance, power and torque density, variable flux

capability, and efficiency, which have been researched extensively over many years

[68-70]. Because of the additional excitation windings, flux enhancing and

weakening operations become much easier, which makes the HEFSM a well-suitable

choice for HEVs [71].

Figure 2.7 presents the various combination of slot-pole for HEFSM that have

been developed. Figure 2.7(a) shows a 6S-4P HEFSM that was proposed in [72].

However, as seen in figure, this design has low torque density and long end winding

for FEC. In addition, both machines have overlapped armature windings, and can

increase copper loss. Meanwhile, another HEFSM is illustrated in Figure 2.7(c). This

machine is a 3 phase 12S-10P PMFSM, which slot in with the FEC at the outer

stator. Hence, this machine employing non overlapping field and armature windings

was proposed in [73]. However, the outer diameter of the machine is significantly

enlarged for the field winding, which significantly reduces torque density. In

addition, the magnets in the PMFSM can be partially replaced by the DC excitation

windings. Consequently, several hybrid-excited topologies were developed [74] as

shown in Figure 2.7(c), albeit they have overlapping armature and field windings,

with significantly reduced torque capability. The foregoing hybrid-excited machines

with magnets on the stator also suffer from one of these three disadvantages.

23

(i) The DC FEC is in series with the field excited by PMs, which limits the flux

adjusting capability due to low permeability of the PM, Figure 2.7(a).

(ii) The flux path of DC FEC significantly reduces the main flux excited by

magnets and even short circuits the magnet flux, Figure 2.7(c) and (d).

(iii) Torque density may be significantly reduced, due to less PM volume, Figure

2.7(c) and (d).

Therefore, a new 6S-10P E-core HEFSM is proposed, which is redesign from

12S-10P E-core PMFSM machine to eliminate the foregoing disadvantages, as

shown in Figure 2.7(d). As seen in figure, this machine is inserted with the DC FEC

on the middle teeth of the E-core stator. Additionally, it employs non overlapping

field and armature windings and exhibits a simple structure. Other than that, the

volume of magnet E-core HEFSM is kept maintain as same with the conventional E-

core PMFSM. Nevertheless, Figure 2.7(e) [75] presents a 12S-10P PMFSM with

additional excitations with theta direction, in order to provide further attractive

characteristics. However, this machine has a limitation of torque and power

production in high current condition, due to insufficient stator yoke width between

excitation coil and armature coil slots resulting in magnetic saturation and negative

torque production as discussed in [76-77]. Nonetheless, after some design

refinements and improvements especially on the yoke mentioned above and coil

area, this machine is capable of operating with desired performance. As the other

infirmity of this machine due to high pole-pair number, the PWM frequency required

for HEV application is very high, compared to the original frequency used in

IPMSM. Therefore, the original design has been transformed as 6-slot 5-pole

machine, to reduce PWM frequency similar to the implementation in conventional 8-

pole IPMSM drive.

However, to overcome the drawback in Figure 2.7(e), another topology of

12S-10 HEFSM with radial direction is proposed [78], as shown in Figure 2.7(f). The

FEC on this machine is wounded in radial direction on the stator to eliminate the flux

cancellation effect in HEM, thus can increase the total torque production. In addition,

the outer rotor version of the machine, Figure 2.7(g), has been proposed for direct

drive electric vehicle applications [79-80]. Although the outer-rotor configuration

machine is capable of providing higher torque density and efficiency, there is very

limited reports have been published for FSM, if compared to the inner-rotor.

24

Figure 2.7: Examples of HEFSM

(a) 6S-4P HEFSM (b) 12S-10P Outer FEC HEFSM (c) 12S-10P E-core HEFSM

(d) 12S-10P HEFSM Theta Direction (e) 12S-10P HEFSM Radial Direction

(f) 12S-10P Outer Rotor HEFSM

A1

A1

B1

B1

C1

C1

FEC

FEC

PM PM

(a)

FEC-2 FEC-1

FEC-2

FEC-2

FEC-2

FEC-2

FEC-2 FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

(b)

A1

B1

C1 A2

B2

C2

A1

B1

C1 A2

B2

C2

A1

B1 C1

A2

B2 C2

FEC-1 FEC-1

FEC-1

FEC-2

FEC-2 FEC-2

(c)

FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

FEC-2 FEC-2

FEC-2

FEC-2 FEC-2

FEC-2

A1

B1

C1 A2 B2

C2

A1

B1 C1 A2

B2

C2

(d)

FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

FEC-2

FEC-1

FEC-2

FEC-2 FEC-2

FEC-2

FEC-2

A1 A1

C2

B2 A2

C1

B1

A2 C1

B1

B2

C2

FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

FEC-1

FEC-2

FEC-2

FEC-2

FEC-2

FEC-2

FEC-2

A1

B1

C1

A2 B2

C2

A1

B1

C1

A2 B2

C2

(e) (f)

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A COMPARATIVE STUDY BETWEEN PMFSM AND HEFSM...

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