Towards Low-Voltage, High-Current a Pioneering Drive Concept for Battery Electric Vehicles

2021 Towards© Low-Voltage,High-Current A pioneering drive concept forbattery electric vehicles

Stefan Haller by Main supervisor: Prof. Kent Bertilsson Co-supervisor: Prof. Bengt Oelmann Dr. Peng Cheng Stefan

Faculty of Science, Technology and Media Department ofHaller Electronics Design Thesis for the Degree of Doctor of Philosophy Mid Sweden University, Sundsvall, Sweden, 2020 Akademisk avhandling som med tillstånd av Mittuniversitetet i Sundsvall framläggs till offentlig granskning för avläggande av teknologie doktorsexamen i Elektronik måndagen den 11 januari 2021, klockan 8.30 i sal C306, Mittuniversitetet Sundsvall. Seminariet kommer© att hållas2021 på engelska. by Towards Low-Voltage, High-Current A pioneering drive concept for battery electric vehicles

“The most dangerous phrase in the language is ‘We’ve always done it this way.’ ” by – Grace Hopper

This thesis has been typeset using LATEX. Stefan Haller ABSTRACT

The first electric low-voltage vehicles were constructed in the mid-19th century, but by the early 20th century they were progressively replaced by successors with internal2021 combustion engines. As the consequences of using fossil fuels© are better understood, our society is now transitioning back. The strong driving force towards electric transportation can be traced to several events and trends. The foremost of these is perhaps the rising awareness of climate change and the necessary reduction of the environmental footprint, as well associated political will for change. Alongside this, the pioneering automotive company Tesla, Inc. showed what electric cars are capable of and how to easily charge them along the road. The diesel gate unearthed in 2015, also played a major role. This transition is not withoutby challenges, however. An electric car is expected to be reasonable priced, sustainable, environmentally friendly and electrically safe, even in case of an accident. Overnight charging at home should be possible, as well as the ability to quickly charge while in transit. While the industry has long experience with high-voltage electrical machines, the required battery technology is quite new and low-voltage in nature. Currently, the battery is the most costly part of an electric drivetrain and it has the highest environmental impact. Efficient battery use is therefore key for sustainability and a responsible consumption of the resources available. Nonetheless,Stefan most electric vehicles today use lethal high-voltage traction drives which require a considerable isolation effort and complex battery pack. Previous research results showed that a 48 V drivetrain compared to a high-voltage one, increases the drive-cycle efficiency. Hence, similar driving range can be reached with a smaller battery. This thesis provides an introduction to low-voltage, high-current, battery-powered traction drives. With the aim of increasing efficiency, safety and redundancy while reducing cost, a solution that breaks with century-old electric machine design principles is proposed and investigated. An overview and motivation to further investigate 48 V drivetrains withHaller intrinsically safe and redundant machines is provided. The main focus of this work is the practical implementation of multiphase low-voltage but high-current machines with integrated power

page | v Abstract

electronics as well as components for a 48 V drivetrain. With this work, it is confirmed that today’s MOSFETs are not the limiting factor towards low-voltage, high-current drives. In the first part of this work, two small-scale prototype machines were constructed and tested. The air-cooled, small-scale 1.2 kW prototype reached a copper fill-factor of 0.84. The machine’s low terminal- to-terminal resistance of 0.23 mΩ, including the MOSFET-based power electronics, allowed continuous2021 driving currents up to 600 A. The resistive MOSFET© losses stayed below 21 W. The second part focuses on the key components for a 48 V high- power drivetrain. A W-shaped coil for a multiphase 48 V machine with direct in-conductor cooling was designed and tested. With glycol water, it reached a current density of 49.5 A/mm2 with 0.312 l/min flow rate. Furthermore, a reconfigurable battery pack for 48 V driving and high-voltage charging wasby investigated.

Stefan Haller

page | vi SAMMANFATTNING

De första elektriska lågspänningsbilarna konstruerades i mitten av 1800-talet men ersattes succesivt med förbränningsmotorer i början av 1900-talet. Nu när konsekvenserna2021 av storskalig användning av fossila bränslen börjar© synas blir elektriska fordon återigen aktuella. Den starka drivkraften mot elektrifierad transport kan kopplas till ett flertal händelser och trender. Främst är troligen den ökade medve- tenheten kring klimatförändringar och den nödvändiga minskningen av samhällets klimatavtryck, tillsammans med den politiska föränd- ringsviljan. Samtidigt visade den banbrytande biltillverkaren Tesla, Inc. vilken prestanda elektriska bilar kunde förmå samt hur lätt de kunde laddas ute på vägarna. Dieselskandalen som uppdagades 2015 bidrog också på ett betydande sätt. Övergången sker dock inteby utan utmaningar och en elbil förväntas att vara prisvärd, hållbar, miljövänlig och elektriskt säker, även vid en olycka. Den ska kunna laddas över natten hemma samt snabbt utefter vägen. Medan industrin har lång erfarenhet av elektriska motorer som arbetar med hög spänning så är den erforderliga batteritekniken ganska ny och lågspänd till sin natur. För närvarande är batteriet den dyraste delen av en elektrisk drivlina och dessutom har den en stor negativ miljöpåverkan. För ett hållbart och ansvarsfullt utnyttjande av våra resurser måste batteriet användas så optimalt som möjligt. Trots detta,använder de flesta elfordon idag en dödligt hög spänning för sin drivlinaStefan vilket ger en komplex konstruktion av batteripaketet och många säkerhetsaspekter måste uppfyllas. Tidigare forskning visar dock att en 48 V drivlina kan uppnå högre effektivitet jämfört med en högspänd drivlina. Denna avhandling ger en introduktion till lågspända elektriska drivlinor för elektrifierade fordon. Målet är att öka verkningsgrad, säker- het, tillförlitlighet och samtidigt reducera kostnader, vilket är en lösning som bryter mot de traditionella konstruktioner som använts i över ett århundrade. Avhandlingen syftar till att ge en översikt och motivation för att ytterligare undersöka denna typ av elektriskt ofarliga och redun- danta motor förHaller elfordon. Arbetet fokuserar på en praktisk konstruktion av en mångfasig elektrisk motor med integrerad kraftelektronik för höga strömmar. Avhandlingen visar att dagens MOSFETs inte är den

page | vii Sammanfattning

begränsande faktorn i en lågspänd drivlina med höga strömmar. I den första delen av detta arbete har två småskaliga prototypma- skiner byggts och testats. Den luftkylda småskaliga 1,2 kW-prototypen hade en kopparfyllningsfaktor på 0,84 och det låga terminal-till-terminal motståndet på 0,23 mΩ, inklusive den MOSFET-baserade kraftelektro- niken, tillät en kontinuerlig driftström på upp till 600 A. Dom resistiva MOSFET-förlusterna var mindre än 21 W. Den andra delen fokuserar2021 på nödvändiga komponenter som krävs till en 48V© drivlina för höga effekter. En W-formad direktkyld ro- torlindning, för en flerfasig 48V-maskin, har designats och testats. Med flödande etylenglykolvatten genom lindningen uppnåddes en ström- täthet av 49,5 A/mm2 vid 0,312 l/min flödeshastighet. Utöver detta utforskades ett omkonfigurerbart batteripaket för 48 V vid körning och hög spänning vid laddning.by

Stefan Haller

page | viii CONTENTS Abstractv 2021 Sammanfattning© vii

Contents ix

List of Figures xv

List of Tables xvi List of Publications by xvii I Introduction1

1 Overview3 1.1 Traction Drives for EVs ...... 4 1.1.1 Resistive Losses...... 4 1.1.2 Machine Cooling ...... 5 1.1.3 Machine Faults...... 6 1.2 MOSFETs or IGBTs for Traction Drives...... 7 1.3 EnergyStefan Storage and Delivery...... 8 1.3.1 Lithium-Ion Technology...... 9 1.3.2 Risk of Li-Ion-based Energy Storage Systems 10 1.3.3 Cell Configurations...... 11 1.4 Battery Cost and Drivetrain Efficiency...... 12 1.5 Energy vs. Power Density...... 13 1.5.1 Environmentally Friendly Supercapacitor . . 14 1.6 Low-Voltage Charging...... 15 1.7 RelatedHaller Low-Voltage Drives...... 16 2 Problem Formulation and Objective 19 2.1 Towards Low-Voltage, High-Current...... 20

page | ix Contents

2.2 Problem Formulation ...... 21 2.2.1 LVHC Machines with Novel MOSFETs . . . . 22 2.2.2 Improvement and Limits of LVHC Machines 23 2.2.3 Components for 48 V LVHC Machines . . . . 23 2.3 Objective...... 23 2.3.1 Limitations...... 24 3 Publications© Included2021 in this Thesis 27 3.1 Air-Gap Flux Density Measurement System for Verifi- cation of PM Motor FEM Model ...... 28 3.2 Investigation of a 2 V 1.1 kW MOSFET Commutated DC Motor...... 29 3.3 Initial Characterization of a 2 V 1.1 kW MOSFET Com- mutated DC Motor...... 29 3.4 A 2.5 V 600 A MOSFET-Based DC Traction Motor . . 30 3.5 Phase Current Shift in Multiphase Single-Turn Con- centrated Windingsby ...... 31 3.6 CPLD and dsPIC Hybrid-Controller for Converter Prototyping...... 33 3.7 Multi-Phase Winding with In-Conductor Direct Cool- ing Capability for a 48 V Traction Drive Design . . . . 33 3.8 Reconf. Battery for Charging 48 V EVs in High-Voltage Infrastructure...... 35

4 Theory 37 4.1 Introduction to Electrical Machines ...... 37 4.1.1Stefan Synchronous Machines...... 38 4.1.2 Asynchronous Machines...... 39 4.2 EMF Generation ...... 40 4.3 Torque Production...... 41 4.3.1 Magnetic and Reluctance Torque ...... 42

5 Methods 43 5.1 Paper I to Paper III, Prototype Machine 1 ...... 43 5.2 Paper VI and Paper V, Prototype Machine 2 ...... 44 5.3 PaperHaller VI and Paper VII, Next Generation 48 V . . . . 46 6 Discussion 47 page | x Contents

6.1 Machine Prototypes ...... 48 6.1.1 1st Prototype...... 49 6.1.2 2nd Prototype...... 50 6.2 Integrated Power Electronics...... 51 6.3 Armature Reaction causing Current Unbalance . . . 53 6.4 Direct Winding Cooling...... 54 6.5 Next Generation2021 48 V Drive...... 54 6.6 Social© and Ethical Considerations ...... 55 7 Outlook and Conclusion 57 7.1 Conclusion ...... 57 7.2 Future Work...... 60

Acronyms 61 Bibliography by 63 II Included Papers 73

Paper I — Air-Gap Flux Density Measurement System for Verification of PM Motor FEM Model 75 i.1 Introduction...... 77 i.2 Existing Measurement Methods...... 79 i.3 Design and Implementation...... 80 i.3.1 Sensor Board...... 80 i.3.2 StefanAmplifier Board...... 81 i.4 Experimental Setup ...... 81 i.4.1 Transducer Normalization...... 84 i.5 Measurement Results ...... 84 i.6 Conclusion ...... 85 i.6.1 Future Work...... 88 i.7 References...... 89

Paper II — Investigation of a 2V 1.1kW MOSFET CommutatedHaller DC Motor 91 ii.1 Introduction...... 93 ii.2 Proposed Architecture...... 95

page | xi Contents

ii.3 Prototype Design...... 96 ii.3.1 Electronic Commutator Design ...... 96 ii.3.2 Outrunner Rotor Design...... 99 ii.3.3 Stator Design...... 99 ii.4 Experimental Setup ...... 101 ii.5 Measurement Procedure and Results ...... 102 ii.5.1 Stator Coil2021 Resistance ...... 102 ii.5.2©Locked Rotor Torque Measurement ...... 103 ii.5.3 Motor Constants kE and kT ...... 104 ii.5.4 Determine Motor Constant kT ...... 106 ii.5.5 Determine Motor Constant kE ...... 106 ii.5.6 No-Load Current ...... 108 ii.5.7 No-Load Power Consumption ...... 109 ii.6 Conclusion and Future Work ...... 109 ii.7 References...... by 110 Paper III — Initial Characterization of a 2V 1.1kW MOSFET Commutated DC Motor 113 iii.1 Introduction...... 115 iii.2 Proposed Drive System ...... 117 iii.3 Experimental Setup ...... 119 iii.4 Measurement Procedure and Results ...... 120 iii.4.1 Stator Coil Resistance Measurement . . . . . 120 iii.4.2 Locked Rotor Torque Measurement ...... 121 iii.4.3 Steady State Efficiency Measurement . . . . . 122 iii.4.4StefanEfficiency and Loss Map ...... 123 iii.5 Conclusion ...... 126 iii.6 References...... 127

Paper IV — A 2.5V 600A MOSFET-Based DC Traction Motor 129 iv.1 Introduction...... 131 iv.2 Prototype Design...... 133 iv.3 Experimental Setup ...... 136 iv.4 MeasurementHaller Procedure and Results ...... 138 iv.4.1 Stator Phase Resistance Measurement . . . . 138 iv.4.2 Efficiency Measurement ...... 140 page | xii Contents

iv.4.3 Torque-Speed Efficiency Map ...... 140 iv.4.4 Loss Distribution Map ...... 140 iv.5 Conclusion ...... 144 iv.6 References...... 145

Paper V — Rebalancing of Phase Current Shift Caused by Armature Reaction2021 in Multiphase Single-Turn Con- centrated© Winding Machines 147 v.1 Introduction...... 149 v.1.1 Resistive Losses and Fill-Factor ...... 150 v.1.2 Armature Reaction on Single-Turn Windings 150 v.1.3 Focus of this Study ...... 151 v.2 LVHC Machine...... 151 v.2.1 Machine Design ...... 152 v.2.2 Winding Design ...... 152 v.2.3 Power Electronicsby ...... 152 v.2.4 Commutation ...... 155 v.2.5 Commutation Strategy ...... 157 v.3 Simulation...... 158 v.4 Experimental Setup ...... 159 v.5 Results and Discussion ...... 162 v.5.1 Measurements...... 163 v.5.2 Resistive and Switching Losses ...... 163 v.5.3 LVHC Machine Improvements ...... 166 v.6 Conclusion and Future Work ...... 168 v.6.1 StefanConclusion...... 168 v.6.2 Future Work...... 169 v.7 References...... 169

Paper VI — CPLD and dsPIC Hybrid-Controller for Con- verter Prototyping 173 vi.1 Introduction...... 175 vi.2 Proposed Controller...... 177 vi.2.1 Signal Delay...... 178 vi.3 ExperimentalHaller Setup ...... 179 vi.3.1 Configurable Transformer PSFB ...... 179 vi.4 Experimental Results...... 180

page | xiii Contents

vi.5 Simulation Results...... 185 vi.6 Conclusion ...... 187 vi.7 References...... 187

Paper VII — Multi-Phase Winding with In-Conductor Direct Cooling Capability for a 48V Traction Drive Design 189 vii.1 Introduction©...... 2021 191 vii.2 Winding Design ...... 192 vii.3 Analytical Calculation...... 194 vii.4 FEM/CFD Simulation Setup ...... 197 vii.5 Measurement Setup ...... 198 vii.6 Results...... 201 vii.7 Conclusion ...... 205 vii.8 References...... 206 vii.9 Biographies...... by 207 Paper VIII — Reconfigurable Battery for Charging 48V EVs in High-Voltage Infrastructure 209 viii.1 Introduction...... 211 viii.2 Battery Reconfiguration ...... 214 viii.2.1 Prototype Design ...... 214 viii.2.2 Battery Pack Balancing ...... 218 viii.3 Experimental Results...... 218 viii.3.1 Charging...... 218 viii.3.2 Discharging ...... 219 viii.4 SystemStefan Analysis ...... 222 viii.4.1 System Performance ...... 222 viii.4.2 Cost Estimation ...... 224 viii.5 Conclusion ...... 225 viii.6 References...... 225 Haller

page | xiv LIST OF FIGURES

1.1 Simulated efficiency map of the next generation 48-phase low-voltage high-current2021 (LVHC) motor with peak efficiency at© the lower to medium power region...... 13 1.2 Ragone plot comparison of various energy storage technologies with specific power versus energy density on cell level...... 14 1.3 Prototype of a reconfigurable battery pack with parallel to series connection switching for 48 V discharging and 400 V charging...... 16 2.1 Overview of a principleby low-voltage drivetrain...... 25 3.1 Overview of included publications and progression towards a 48 volt high-current machine...... 27

4.1 Square-wave-like EMF of the first prototype machine at 300 rpm with 3 excited coils under no-load condition. . . 41

6.1 Sectional view of prototype II with mounted power electronics and VCC ring in the test-stand configuration with support bearing...... 50 6.2 One of 13 identical preformed stator coils of prototype II. 51 6.3 13-phaseStefan MOSFET commutator, tested up to 600 A continuous current...... 52 6.4 Example of a 13-phase high-current PCB design with aluminum core for a lab-winding machine...... 53 6.5 Sectional view of the current design stage of the next generation 48 V high-power machine with mounted power electronics.Haller...... 55

page | xv LIST OF TABLES

2.1 Comparison of low-voltage and high-voltage drive system properties.©...... 2021 22 6.1 Design guideline comparison of the first prototype and the second improved version, based on the results of the first prototype...... 49 by

Stefan Haller

page | xvi LIST OF PUBLICATIONS

This thesis is based© on the2021 following publications, attached in Part II. Paper I Air-Gap Flux Density Measurement System for Verification of Permanent Magnet Motor FEM Model S. Haller, P. Cheng, B. Oelmann IEEE Proceedings of Industrial Electronics 2015 Vol. 41, pp. 445–450 ...... 75

Paper II Investigation of a 2 Vby 1.1 kW MOSFET Commutated DC Motor S. Haller, P. Cheng, B. Oelmann IEEE Proceedings of Power Electronics and Motion Control 2016 Vol. 17, pp. 586–593 ...... 91

Paper III Initial Characterization of a 2 V 1.1 kW MOSFET Commutated DC Motor S. Haller, P. Cheng, B. Oelmann IEEE ProceedingsStefan of Industrial Electronics 2016 Vol. 42, pp. 4287–4292 ...... 113

Paper IV A 2.5 V 600 A MOSFET-Based DC Traction Motor S. Haller, P. Cheng, B. Oelmann IEEE Proceedings of Industrial Technology 2019 Vol. 20, pp.Haller 213–218 ...... 129

page | xvii List of Publications

Paper V Rebalancing of Phase Current Shift Caused by Armature Reaction in Multiphase Single-Turn Concentrated Winding Machines S. Haller, P. Cheng, K. Bertilsson Submitted to IEEE Access ...... 147 Paper VI ©2021 CPLD and dsPIC Hybrid-Controller for Converter Prototyping driving a Reconfigurable Transformer Phase-Shifted Full- Bridge S. Haller, M. Abu Bakar, K. Bertilsson Proceedings of Int. Conference PCIM Europe digital days 2020 pp. 1552–1558 ...... 173

Paper VII Multi-Phase Windingby with In-Conductor Direct Cooling Ca- pability for a 48 V Traction Drive Design S. Haller, J. Persson, P. Cheng, K. Bertilsson IEEE Proceedings of Int. Conference on Electrical Machines 2020 Vol. 1, pp. 2118–2124 ...... 189

Paper VIII Reconfigurable Battery for Charging 48 V EVs in High-Voltage Infrastructure S. Haller, F. Alam, K. Bertilsson In manuscriptStefan...... 209 Haller

page | xviii List of Publications

Publications not Included

The following publications by the author are not included in this thesis. Contribution of leakage flux to the total losses in transformers with magnetic shunt M. Abu Bakar, S. Haller, K.2021 Bertilsson International Journal© of Electronics, 2020 DOI 10.1080/00207217.2020.1793404

State of Art of Designing Power Electronics Converter for Low Voltage Motor Drives for Electric Vehicle M. Das, S. Barg, S. Haller, M. Abu Bakar, A. Rezaee, K. Bertilsson IEEE International Conference on Power Electronics, Smart Grid and Re- newable Energy (PESGRE 2020), pp. 1–6 DOI 10.1109/PESGRE45664.2020.9070438by Assembling surface mounted components on ink-jet printed double sided paper circuit board H. Andersson, A. Manuilskiy, S. Haller, M. Hummelgård, J. Sidén, C. Hummelgård, H. Olin and H. Nilsson Nanotechnology, Vol. 25, ss. Art. no. 094002, 2014 DOI 10.1088/0957-4484/25/9/094002

A ZVS Half Bridge DC-DC Converter in MHz Frequency Region using Novel Hybrid Power Transformer H.B. Kotte, R. Ambatipudi,Stefan S. Haller and K. Bertilsson International Conference for Power Electronics, Intelligent Motion, Renew- able Energy and Energy Management (PCIM 2012), pp. 399–406 ISBN 978-3-8007-3431-3

System of nano-silver inkjet printed memory cards and PC card reader and programmer H. Andersson,A. Rusu,A. Manuilskiy,S. Haller,S. Ayöz and H. Nilsson Microelectronics Journal, Vol. 42: 1, ss. 21–27, 2011 DOI 10.1016/j.mejo.2010.09.008Haller

page | xix List of Publications

A study of IGBT rupture phenomenon in medium frequency resistance welding machine J. Saleem, A. Majid, S. Haller and K. Bertilsson International Aegean Conference on Electrical Machines and Power Elec- tronics and Electromotion, Joint Conference (ACEMP 2011), pp. 236–239 DOI 10.1109/ACEMP.2011.64906022021 Analysis© of feedback in converter using coreless printed circuit board transformer A. Majid, J. Saleem, H.B. Kotte, R. Ambatipudi, S. Haller and K. Bertils- son International Aegean Conference on Electrical Machines and Power Elec- tronics and Electromotion, Joint Conference (ACEMP 2011), pp. 601–604 DOI 10.1109/ACEMP.2011.6490667 High Frequency Half-Bridgeby Converter using Multilayered Core- less Printed Circuit Board Step-Down Power Transformer A. Majid, H.B. Kotte, J. Saleem, R. Ambatipudi, S. Haller, and K. Bertils- son 8th International Conference on Power Electronics - ECCE Asia (ICPE 2011), pp. 1177–1181 DOI 10.1109/ICPE.2011.5944712 Stefan Haller

Part I

Introductionby

Stefan Haller

Stefan Haller Chapter 1 OVERVIEW

The transition to e-mobility is a big challenge in terms of energy storage, fast energy delivery at charging stations, cost and drivetrain efficiency. Rarely discussed,2021 but of major importance, is system safety. In automotive© engineering, voltages up to 60 V DC and 30 V AC are classified as low-voltage (LV) and handled according to the safety extra-low voltage (SELV) regulations. Voltages above these limits are classified as high-voltage (HV), requiring additional precautions [1] to avoid injury or even lethal accidents. To fulfill the regulations, double isolation of the high-voltage components and cables, for example, is mandatory. The orange double-isolated high-voltage cables may even require shielding. Additionally,isolation monitoring and 2-pole safety circuit breakers are neededby to disconnect the high-voltage source [2]. TheHV source is usually a battery. The nominal voltage level of current battery electric vehicles (BEVs) is higher than 320 V [3,4] and therefore classified as high-voltage. In addition to the common 400 V class, manufacturers are moving towards even higher voltages. The world’s first production car with an 800 V system was the Koenigsegg Regera, a high-performance sports car, which became available in 2017 [5]. In 2019, the first regular sports car, the Porsche Taycan, was launched [6]. It is the first mass production electric vehicle (EV) in the 800 V class. In contrast,Stefan SELV rated systems do not require double isolation, and the interruption of a single pole is sufficient to disconnect the battery. SELV systems even allow the use of structural parts for energy transfer, like the well-established utilization of the vehicle’s chassis as ground. The energy storage devices for e-mobility applications are currently the most sensitive and costly part of the drivetrain. Efficient use of this resource by drivetrains is key. By increasing system efficiency, driving rangeHaller can be extended for a given battery capacity [4]. To find novel solutions for these challenges, pioneering ideas that break with principles applied for more than a century have to be

page | 3 Chapter 1. Overview openly discussed. Recent developments in modern power electronics allow totally different drive concepts than those widely used onlya decade ago. This thesis describes the journey towards a low-voltage, high-current DC drive system. It is an inherently safe, efficient and redundant pioneering concept for 48 V battery electric vehicles. 1.1 Traction© Drives for2021 EVs Traction drives for EVs have to fulfill certain requirements. They need to be robust to survive in harsh environments where they are exposed to vibrations and both fast and slow temperature changes. Traction drives should deliver peak torque for acceleration and deceleration and thus need to handle fast current transients throughout their entire life span. They should furthermore be efficient, maintenance free and safe in case of failure or damage. The following sections discuss some novel ideas on how a LVHCby drive can achieve this. 1.1.1 Resistive Losses

More than 50 % of the losses in today’s most frequently used electric motors, the squirrel cage induction (SCI) and permanent magnet (PM) motor, are resistive losses [7], [8]. The exact distribution of the losses depends on the operating conditions and motor design. A considerable amount of research on well-known drive architectures has been dedicated to minimizing the losses for a given application, such as drivingStefan cycles of EVs[9–11]. The resistive losses nonetheless dominate beyond a certain load of the electrical machine [12]. The winding layout and configuration is of major importance in minimizing resistive losses, also known as copper losses or 퐼2푅 losses. End-windings, also called end-turns or winding head, do not produce any considerable amount of net-torque and thus mainly contribute to the copper losses. The larger the usable cross-sectional area of the conductor and the more compact the end-winding layout, the lower the resistive losses. This also applies to the stator winding part inside the slots.Haller Here, the winding layout and copper fill-factor is of importance. The copper fill factor is the ratio between the slot opening area of the machine’s electrical steel lamination and the area page | 4 1.1.2. Machine Cooling of copper or aluminum conductor, which is fitted inside the slot. The general aim is to increase the fill factor as much as is practically and economically possible. The conductor isolation, fixating assembly and resin to increase the thermal conductivity are considered passive material. The electrical steel and the conductor inside the slot to build the machine’s winding are active material, as they actively contribute to the machine’s torque generation.2021 In terms ofPM machines, the magnets are also© counted as active material. The random-winding, by far the most common winding configuration, allows a copper fill factor of up to 0.4–0.5. Special winding techniques, such as needle windings, allow copper fill factors of up to 0.65 [13]. More complex winding techniques, such as concentrated pre-formed windings, further increase the copper fill factor. Fill factors of up to 0.8 have been reported for larger machines, such as a 25 kW 50 V PM machine [14]. In general, the larger the electrical machine, the easier it is to achieveby a high fill factor. This becomes more of a challenge for compact high-power traction machines with small slots. For these machines, the hairpin winding technology is more frequently used [15]. Pre-formed rectangular hairpin-shaped wires are inserted in the slots and welded together. These windings provide a high fill factor and low thermal resistance to the stator core. The prototype machine presented in Papers IV and V use similarly shaped coils. 1.1.2 MachineStefan Cooling Both the first and second LVHC prototype machines are air cooled, like most industrial motors. Traction machines instead require a higher power density to reduce weight and size. A major portion of the machine’s losses are generated inside the machine’s stator winding. Heat extraction close to or inside the winding is therefore a key factor in achieving high-power density. To cool these machines, various forced liquid or air-cooling techniques can be used. Stator housings with a water jacket are easy to integrate but extract the heat throughHaller the machine’s stator lamination. Cooling channels embedded in the stator lamination can increase heat extraction capability, but still depend on low thermal resistance of the winding to

page | 5 Chapter 1. Overview the stator. Potting the winding in the stator slots increases mechanical stability and reduces thermal resistance. With indirect slot cooling, the cooling channels are embedded in the slot, together with the winding [16–18]. This further reduces the thermal path between the winding and the cooling medium. The larger the cross-sectional area of the coil conductors, the easier it is to use hollow or pre-formed2021 conductors, which allow direct inner cooling. Direct© conductor cooling is prevalent in very high power electrical machines, for example electrical generators at power plants. Interest is growing in this form of cooling, to increase the power density of traction machines. For high-voltage machines, oil-cooled conducts are used [19]. With laminated windings, forced in-stator air cooling was demonstrated by Reinap et al. [20]. Compressed air or oil cooling with hollow or profiled conductors have been investigated by Reinap et al. [21]. They have reported current densities of up to 50 A/mm2 with oil coolingby at 40° inlet temperature [22]. For a multi-phase, low-voltage W-shaped winding with direct in- conductor cooling, using ethylene glycol water 50/50 (EGW50/50) at 65° inlet temperature,a current density of 49.5 A/mm2 was reached,as shown in PaperVII. The use of a parallel hydraulic connection provides increased cooling capability in combination with low pressure. In- conductor cooling also eliminates the need for additional end-winding cooling. Gai et al. [23] provide a comprehensive overview of common cooling solutions for automotive traction drives. They include indirect rotor cooling through the shaft, as used in Tesla’s induction machine in the early modelStefan S, and wet cooling with a fully flooded machine, as in Tesla’s model 3. Spray cooling of rotor and end-winding are also discussed.

1.1.3 Machine Faults

Considerable research has been conducted in different areas of electrical machines to determine fault mechanisms and develop fault prediction methods.Haller According to studies on industrial motors [24], the most common induction motor (IM) faults were bearing faults followed by stator faults [25, 26]. Almost 37% of all faults on industrial page | 6 1.2. MOSFETs or IGBTs for Traction Drives machines were stator faults. More than 90 % of these stator faults were related to stator winding damage, of which the most prevalent was the stator inter-turn fault (SITF). The authors in [25] point out that traction motors are prone to SITF more than the industrial motors mentioned above. This is due to their high power density, very compact construction and extreme operating conditions. These drives are expected to work in harsh2021 environments, are prone to fast as well as slow temperature© changes, external as well as internal vibrations and fast current transients. Personal safety is furthermore the most important factor in vehicles and therefore a measure for traction drives. Electrical failures, such as SITF, cause large circulating currents inside the damaged winding, which could cause very rapid deceleration of the vehicle. These circulating currents lead to local hot spots, which further degrade the isolation. Ultimately, faults that started as SITF result in complete failureby of the winding as turn-to-ground and phase-to-phase faults as well as open-circuit faults [26, 27]. PM machines are favored for traction drives due to their higher efficiency compared to SCI motors. Electrical failures with these type of machines are, however, more severe compared to SCI motor failures. This is caused by the same effect, which makes them more efficient: the nature of magnets to maintain a permanent magnetic flux. Once the rotor is spinning, an electromotive force (EMF) is generated that can hardly be counteracted. In case of a SITF, a spinningPM rotor will generate a high level of circulating currents in the shorted winding. This generatesStefan a reverse magnetic field, which opposes the rotor’s field. In combination with high magnet temperatures, this could lead to permanent demagnetization and destruction of the rotor [28, 29].

1.2 MOSFETs or IGBTs for Traction Drives

Until now,commercial high-voltage traction drive inverters commonly use insulated-gate bipolar transistors (IGBTs), as in the Nissan Leaf, Toyota Prius orHaller Tesla Model S [30–32]. The development of the MOS- FET technology in recent years will lead to a progressive replacement of the IGBTs in traction drive applications.

page | 7 Chapter 1. Overview

MOSFET-based inverters have the advantage of significant loss reduction, and can thus increase the overall energy efficiency of the drivetrain [32]. The impact on the inverter losses by replacing commercial IGBTs-based traction drive inverters with silicon carbide (SIC) MOSFET-based ones has been discussed by Kim et al. [33]. A loss reduction on the inverter by approximately a factor of two was reached. The technology2021 replacement has already started: to the author’s knowledge,© the Tesla Model 3 is the first mass-produced BEV using SIC-MOSFETs in the traction drive inverter. More specifically, 24 SIC-MOSFETs from ST Microelectronics with 650 V rating are used [34]. While SIC-MOSFETs are designed for high voltages, commonly 1200 V, progress is also being made in the development of low-voltage silicon (SI)-MOSFETs. For example, the used 25 V IRF6718L2 MOSFET has a typical 푅퐷푆 푂푁 of 0.5 mΩ and can handle up to 270 A continuous ( ) drain current. The recentby IRL7472L1 can handle up to 40 V with decreased typical 푅퐷푆 푂푁 of 0.34 mΩ and up to 375 A continuous ( ) drain current in the same package of just 9.1 mm 7 mm 0.7 mm. × × The increased efficiency of the MOSFET technology compared to IGBTs, along with lower device costs and reduced cooling requirements,enables new machine designs. The aforementioned low-voltage MOSFETs are off-the-shelf components and have a good price performance ratio due to economy of scale. As a result, efficient low-voltage drives that use a large number of switches are feasible, as in the presented LVHC drive or related Intelligent Stator Cage Drive (ISCAD) drive [35, 36]. The advantages compared to high-voltageStefan drives are the inherent electrical safety and reliability due to the use of many parallel current paths. A low-voltage drivetrain provides increased system efficiency at reduced complexity of the battery pack. This reduces cost and increases driving range.

1.3 Energy Storage and Delivery

While there is good understanding ofHV motors and drives, energy storage technologyHaller is still the limiting factor of EVs. Batteries are by far the most common energy storage device in these vehicles. While the knowledge of and experience with lead-acid battery usage in page | 8 1.3.1. Lithium-Ion Technology vehicles is considerable, their energy density is significantly lower compared to lithium-ion (Li-Ion)-based batteries [37, 38]. As space and permitted weight are limited, battery technologies with a higher energy density need to be used in cars, such as nickel–metal hydride (NiMH) and today’s Li-Ion-based batteries. The use of the Li-Ion technology in EVs is fairly new though, and many challenges remain that require further research.2021 Some of these are briefly discussed in the following sections.© 1.3.1 Lithium-Ion Technology

It is worth mentioning a few properties among others that are chal- lenging to the current lithium-ion battery (LIB) technology when used inEV applications. The term Li-Ion battery encompasses a group of different types of Li-Ion-based battery cells and packaging. The exact material combination andby layout determines the battery properties in terms of power, energy, high- and low-temperature performance, safety and lifetime. The mechanical packaging of the cells also needs to be considered, in addition to the selected active material. There is considerable experience in production of cylindrical cells, such as the 18650, which is 18 mm in diameter and 65 mm long. This cell type has been used for more than a decade in laptop batteries. The recent 21700 cylindrical cell type provides about one third more capacity and is just slightly larger. It is used in the Tesla Model 3, for example. The cylindrical housing is solid, can withstand some internal pressure and providesStefan mechanical protection of the cells. The lower packaging density of cylindrical cells compared to semi-soft pouch cells can be considered a disadvantage. While pouch cells provide a slightly higher energy density due to the usage of a case foil instead of a cylindrical can, they are mechanically more fragile. A third type, commonly used in EVs, is the prismatic cell. Its assembly process is similar to that of the cylindrical cell, but has a higher packaging density. Li-Ion batteries cells swell slightly while they are cycled [39]. This mechanical displacementHaller needs to be considered when choosing the type of battery cell, package and assembly of the pack. Prismatic and cylindrical cells with a dense containment have the advantage of

page | 9 Chapter 1. Overview providing clearance for swelling [38]. In contrast to lead-acid batteries, the performance of Li-Ion-based batteries drastically decreases at extremely cold temperatures [40]. The usable energy density of lithium iron phosphate (LFP) batteries, for example, is reduced by more than 40 % at −20 °C [41]. In addition to the capacity reduction, the cell resistance will also rise due to the slower Li-Ion diffusion2021 rate. While this reduces the performance of BEVs with a cold© battery, it limits the use of LIBs to cold-crank internal combustion engines (ICEs). Thus, mild-hybrid vehicles often carry a 12 V lead-acid battery which delivers sufficient cold cranking amps (CCA). While discharging at a limited C-rate is still possible, charging capability is limited or may even need to be prevented to avoid plating, which constantly degrades the usable cell capacity [42, 43]. Different pre-heating techniques can be used to heat the cells to a more optimal operating temperature. Aby promising technique involves internal cell heating by applying alternating currents [44, 45]. Within a couple of minutes, the cells can be uniformly pre-heated to temperatures above 0°.

1.3.2 Risk of Li-Ion-based Energy Storage Systems

Depending on the chemistry used, every series-connected LIB cell adds approximately 3.3 V to 3.7 V to the battery pack’s voltage. Parallel- connected cells increase the pack’s capacity and current handling capability. All parallel connected cells sharing the same voltage level inside the packStefan are further denoted as cell stack. The high energy density ofEV battery cells, with more than 680 Wh/L or 260 Wh/kg [37], require that safety precautions are taken to protect the battery from mechanical and electrical damage. Battery safety precautions can be passive on the cell and pack levels, as well as active via the battery management system (BMS). The composition of the cell chemistry directly influences the cell’s passive safety. Increased passive cell safety lowers the voltage and thus energy density. Haller In addition, the material combination of today’s Li-Ion-based cells does not provide an integrated overcharge protection mechanism [38]. page | 10 1.3.3. Cell Configurations

Consequently,an external BMS that monitors the temperature, voltage and state of charge (SOC) of every cell stack is required. The properties of BMS systems range from simple to more advanced features, from just passively protecting the cells from overcharge or deep discharge to active cell level balancing, SOC monitoring, control of active cooling or heating, among others. EVs usually use an advanced BMS to ensure safety as well as active thermal2021 and endurance management for the costly battery pack.© 1.3.3 Cell Configurations

The current 400 V class of BEVs often use 96s푁p cell configurations [46], in other words 96 series connected battery cells, leading to a pack voltage of approximately 350 V. The pack’s capacity is either adjusted by the amount 푁 of parallel connected cells, or by slightly altering the number of seriesby connected cells. For example, a Tesla Model S with a 85 kWh battery uses a 96s74p configuration [46] with 96 series and 74 parallel connected cells. For Volkswagen’s Modularer E-Antriebs-Baukasten (MEB), it is assumed that 96s2p, 108s2p and 96s3p configurations with 24 cells in each module are initially used [47]. The resulting nominal voltage is between 350 V to 410 V, which is classified as high-voltage. Every series connected cell stack requires a dedicated channel on the BMS. The higher the battery pack’s voltage, the more channels on the BMS are required. Furthermore, due to voltages above the SELV limit of 60 V, the BMS requires more complex voltage and current sensing techniques.Stefan Battery cells deviate slightly from their nominal capacity as result of the production spread. This spread increases during cell aging. The string of series connected battery cells behaves like a chain. It is as capable as their weakest link. Each cell faces the same current and the cell with the lowest capacity determines the string’s capacity. To strengthen the link, additional cells can be added in parallel to each link. The more parallel connections are used, the greater the robustness ofHaller the string against single cell faults and production spreads. By reconfiguring a 400 V class battery pack from 96s74p to 48 V

page | 11 Chapter 1. Overview with 13s547p, Baumgard et al. [48] have shown that the pack’s capacity and lifetime could be slightly increased by approximately 1 % and 1.5 %, respectively. It should be noted that the number of cells and individual cell current within the battery pack does not alter. The benefits of such a 48 V battery pack are increased redundancy and intrinsic safety due to SELV level. Furthermore, the cost of the battery pack is reduced by limited2021 isolation effort, simpler cell connection schema and a© simplified BMS, which requires 86 % fewer channels.

1.4 Battery Cost and Drivetrain Efficiency

In 2012, the cell price ranged from 200 e/kWh to 500 e/kWh [38] and was estimated as 130 e/kWh to 170 e/kWh in 2017 [49]. By the end of 2019, the battery pack’s average price had fallen below 160 $/kWh [50]. by Even though cell and pack prices are constantly decreasing, the battery remains the most costly part of a BEV drivetrain. This fact emphasizes the importance of efficient use of the installed battery capacity to increase the BEV’s range. The drivetrain’s peak power is only occasionally required on the road. In fact, under normal driving conditions below top speed, it is only used during heavy acceleration and recuperation phases. To maintain the vehicle’s speed on a flat road, only a fraction of the peak power is required [4, 12]. A drivetrain with a peak efficiency at lower to medium power regions therefore maximizes the BEV’s range. A declining efficiencyStefan at high to peak power operation is acceptable due to its minor impact on the range. Figure 1.1 shows a simulated torque-speed efficiency map of the current state of development of the next generation 48 V traction motor. The peak efficiency is reached at the lower to medium power range. A comparison of a 360 V high-voltage drivetrain and a 48 V low- voltage drivetrain was presented by Patzak et al. [4, 36] in a simulation- based study conducted with the new european driving cycle (NEDC) and worldwideHaller harmonized light vehicles test cycle (WLTC). It was shown that an equivalent low-voltage system outperforms a high- voltage system at every operational point in terms of efficiency. The page | 12 1.5. Energy vs. Power Density ©2021

Figure 1.1: Simulated efficiency map of the next generation 48-phase LVHC motor with peak efficiency at the lower to medium power region. efficiency in low to mediumby power demand regions improved by more than 10 %.

1.5 Energy vs. Power Density – Battery and Supercapacitor

Today’s Li-Ion batteries provide among the highest energy density of batteries used for automotive applications. While a high energy density is crucial for the BEV’s range, it limits the batteries’ power density, charge and discharge rate and performance of the vehicle. Li-Ion batteries are offered as two major types, high-energy and high- power, as shownStefan in the ragone plot in Figure 1.2. By changing the cells’ internal layout and battery chemistry, their properties can be shifted between high power and high energy density. An overview of automotive battery technology was published by Budde-Meiwes et al. [38], while Schmuch et al. [37] have comprehensively summarized performance, energy density and material costs of lithium-based automotive batteries of current EVs. In addition to the good range ensured by high energy density batteries, BEVs require high power to deliver peak torque and allow efficient energyHaller recuperation during braking. The regular acceleration and recuperation phases lead to constant cycling of energy storage. Supercapacitors can be used to reduce cycling and peak power stress

page | 13 Chapter 1. Overview

Supercapacitors 10000

Li-Ion High Power Pb (spiral wound) 1000

NiMH 100 NiCd

NaNiCl Pb 2 (Zebra) Li-Ion Very High Energy 10 2021 Specific Power at Cell Level (W/kg) 0 © 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Specific Energy at Cell Level (Wh/kg)

Figure 1.2: Ragone plot comparison of various energy storage technologies with specific power versus energy density on cell level. Modified figure from[51]. on the battery. These provide among the highest power density and pulse efficiency, although their energy density is fairly low asshown in Figure 1.2. The combinationby of a high energy density battery as long-term storage and a high power density supercapacitor as short-term storage results in a hybrid energy storage system. Since the energy stored in a supercapacitor is quadratic to the voltage, a DC/DC converter between battery and capacitor is required to utilize it in an efficient way. This additional DC/DC converter allows precise control of the capacitor current and SOC. The concept of a hybridized battery was presented by Carter et al. [52]. A resonant DC/DC converter design for this application was presented by Arazi et al. [53]. It was found to be an efficient method to reduce peak battery currents and thus extend batteryStefan life. 1.5.1 Environmentally Friendly Supercapacitor

Commercial supercapacitors are generally expensive, flammable and possibly toxic. This is due to the use of organic electrolytes in their composition, which allows an operation voltage of approximately 2.7 V. As an alternative, low-cost environmentally friendly nanographite coated paper-based supercapacitors can be used [54]. Their use of aqueous electrolytes limitsHaller the operating voltage to approximately 1 V. To reduce the balancing effort of the stacked low-voltage supercapacitors, it is favorable to use a low-voltage drivetrain. page | 14 1.6. Low-Voltage Charging

1.6 Low-Voltage Charging

Many different charging connectors and power ratings are currently used forEV charging. They can be split into two main groups: AC and DC charging systems. Some connectors and charging systems support both of them, like the IEC Type 2 or combined charging system (CCS) connector. 2021 AC systems© require an on-board charger that can be tailored to the battery pack’s capabilities. DC systems that are mainly used for fast charging have the converter integrated into the charging station. TheEV’s battery is directly charged via DC from the station, under supervision of the BMS. To charge a 48 V drivetrain battery pack with 11 kW, a charge current of approximately 220 A is required. Since AC systems use an on-board charger, it can be closely mounted to the 48 V battery pack. Greifelt et al. [55] presentedby such a 11 kW bidirectional SIC MOSFET charger. Additional effort is required to handle the high currents associated with DC fast-charging for low-voltage EVs. Investigations of the usage of high-temperature superconductors instead of copper or aluminum cabling at the charging station have been conducted by Greifelt et al. [56]. Their solution would theoretically support low-voltage charging up to 60 kW. Instead of using superconductors, the author suggests using the existing high-voltage DC charging system. This could be achieved by on-board DC/DC point of load (POL) converters, integrated directly into the batteryStefan pack. An alternative concept with less power conversion is a dynamic reconfigurable battery pack, investigated in Paper VIII. Battery packs for EVs are commonly constructed with multiple modules made of individual cells. During charging these modules can be connected in series to match the charger’s high-voltage. Once the charger is disconnected, the modules are switched to parallel connection to provide low-voltage to the drive. The measurement setup is currently being prepared to test the concept for series charging and parallel dischargingHaller with eight 48 V batteries. The batteries with the first printed circuit board (PCB) generation mounted on top for series to parallel switching are shown in Figure 1.3.

Figure 1.3: Prototype of a reconfigurable battery pack with parallel to series connection switching for 48 V dischargingby and 400 V charging.

Another fast charging solution may be derived from the concept of a contactless magnetic plug presented by Beddingfield et al. [57]. The authors suggest splitting the transformer into two parts, with the primary winding using 3.5 kV inside a magnetic plug and a 400 V secondary winding mounted inside the vehicle. The plug’s size should be comparable to a CHAdeMO 1.0 plug. With the secondary winding mounted insideStefan theEV, it may be tailored to match a 48 V battery. 1.7 Related Low-Voltage Drives

The ISCAD from Molabo [36, 48, 58, 59] is the most similar drive concept to the traction drive presented in this thesis. Both concepts propose a high copper or aluminum fill-factor, a redundant multiphase electrical machine and MOSFET-based power electronics. The high fill-factor of the presented machines is achieved by single-turn windings, andHaller for the next generation 48 V machine a winding with W- shaped coils. The ISCAD instead uses a stator cage, which can be seen as a half-turn winding design with common star connection. Both page | 16 1.7. Related Low-Voltage Drives drives use on-stator mounted power electronics, which individually powers each stator phase. The prototype machines presented in this thesis arePM machines with a square wave-like air-gap magnetomotive force (MMF). The commutation is done block-wise in today’s power electronics. The machines allow up to six simultaneously powered phases with unidirectional coil current. The2021 armature reaction induces a phase current shift and jitter© in the rotor position detection, as described in Paper V. The machines were therefore always operated with five of 13 phases at a time. The unidirectional coil current simplified the power electronics design and enabled a terminal resistance of 0.23 mΩ. When powering only a fraction of the coils, however, the utilization of the machine’s active material is limited. The ISCAD concept instead uses an SCI rotor and is driven in such a way that a sinusoidal air-gap MMF is generated. The stator current is bidirectional due to the useby of a half-bridge configuration per phase. A 48 V 110 kW 42-phase ISCAD machine in a fully functional car is shown by Runde et al. [59]. The first series produced ISCAD V50 [58] is a permanent magnet assisted synchronous reluctance (PMaSR) machine delivering up to 80 kW. The concept of a 48 V concentrated single-turn bar windings machine reaching a high fill-factor is already described by Endert etal. [14, 60]. The machine uses a classic 3-phase stator winding instead of one independent coil per phase, however, and thus lacks redundancy in its design inStefan case of a single phase failure. Haller

Stefan Haller Chapter 2 PROBLEM FORMULATION AND OBJECTIVE As described in© the overview2021 in Chapter1 a fully low-voltage drivetrain seems beneficial in many aspects. It is intrinsically safe, no high voltage is created at any point and SELV limits are always maintained. Current BEV traction drives favor the 400 V to 800 V class. By regulation, this requires specially trained service personnel, and in case of a car accident, additional safety precautions are needed. Low-voltage drivetrains do not require these efforts. Low-voltage drives have potentially greater efficiency due to the high copper fill-factor and the use of MOSFET-based power electronics. Redundancy against singleby phase faults is achieved via the massive multi-phase design of the electrical machine and power electronics. The most common electrical machine fault, the SITF, can be eliminated by a single-turn or half-turn winding. The stator side cooling of such windings is greatly enhanced by the high copper fill-factor and thinner coil isolation. Due to the larger cross-sectional area of the coil wires or bars, hollow conductors for direct inner cooling can be used. This drastically increases the continuous power density of the machine. The low-voltage coils are connected electrically and hydraulically in parallel, which reduces the required coil coolant flow rate and absolute pressure.Stefan The battery pack’s complexity and cost is reduced by using a low- voltage massive parallel cell configuration instead of a high-voltage massive series configuration. A reconfigurable battery pack for low- voltage discharging and high-voltage fast charging is currently being investigated. To reduce short-term storage cycles and current spikes to the battery pack, supercapacitors can be added that are low-voltage in nature. When thisHaller work was initiated prior to the Diesel scandal in 2015, there was no major interest in low-voltage electrical machines for battery-powered traction applications. Research activity on electrical

page | 19 Chapter 2. Problem Formulation and Objective transportation has drastically increased since then, however.

2.1 Towards Low-Voltage, High-Current

To generate a rotating air-gap MMF of desired strength, a certain number of stator coil ampere-turns is required. Ampere-turns are a linear function of the coil current and number of turns. More simply explained, a coil© with 1 A2021and 100 turns as well as one with 100 A and 1 turn generate an air-gap MMF of similar strength. For more than a century, electrical machines were designed to operate with higher voltage and less current. This mindset is derived from Ohm’s law, since the power 푃 scales with the square of the current 퐼, 푃 = 퐼2 푅 (2.1) · with 푅 as opposing resistance. Nevertheless, it can be beneficial to use low-voltage, depending onby the power source, sink and application. To maintain the same power, the current needs to be consequently increased. To use an analogy: In many parts of the world, residential build- ings are supplied with up to 400 V, while most home appliances with external power supplies stay below 60 V. Most battery-powered devices use voltages below 60 V as well, and for good reasons: safety, costs, efficiency and usability. Most people would agree that it hardly makes sense to construct a 230 V pocket light to be able to use off- the-shelf household light sources just to reduce the conduction losses due to decreasedStefan current. While this is an extreme example, the same principle should apply to BEVs and hybrid electric vehicles (HEVs) as well. In the author’s opinion, it does not make sense to construct a high-voltage electric vehicle utilizing the same low-voltage battery technology as that used in mobile phones. The motive for using high-voltage might be derived from the existence and knowledge of conventional industry-approved 400 V class drive systems and time to market-related pressures. Yet without considering the complete system, an optimalHaller solution can hardly be found. First attempts to raise the voltage level above 12 V for passenger vehicles were discussed in the late 1990s. This 42 V system actually page | 20 2.2. Problem Formulation used 36 V batteries and was introduced to replace the existing 12 V board grid [61]. For various reasons it never gained market share and was only used in two car models. A few years ago, a new attempt at a 48 V system was made, this time as an addition to the existing 12 V board grid. The voltage range for the 48 V system was standardized through the VDA320 [61] in 2014. The stationary voltage is always kept within the SELV limits,2021 below 60 V DC. To be able© to meet the upcoming requirements on CO2 emission reduction, more and more manufacturers are equipping their ICE- based cars with an additional 48 V mild hybrid system. In addition to emission reduction, additional comfort features like boosting and e-creeping are possible. P0 belt starter generator (BSG) machines with 10 kW and 48 V are commonly available. Original Equipment Manufacturers (OEMs) are already offering integration support of their 48 V drives with power levels of 20 kW to 30by kW [62], ranging from belt-connected P0 to electric all-wheel drive P4 solutions. Some OEMs have stated that up to 50 kW will be available in future generations of automotive 48 V-based machines. Recently, Molabo released a 48 V drive with 80 kW peak and 50 kW continuous power for boats [58]. They also demonstrated a 48 V 110 kW 42-phase ISCAD machine in a fully functional car [59]. All this raises the question: why should a third, high-voltage system be used for EVs if the task can already be accomplished by a 48 V system?

2.2 ProblemStefan Formulation

A low-voltage drivetrain has many advantages but also several challenges compared to a high-voltage system. Table 2.1 compares some key properties of the proposed drivetrain against aHV one. Over the last century, a solid understanding of, and extensive experience with high-voltage machines has been obtained. For multiphase LVHC machines, however, experience and research is very limited. This thesis providesHaller an introduction into the field of low-voltage high- current machines, their advantages and limitations. The following research questions were raised and have been answered by this work.

page | 21 Chapter 2. Problem Formulation and Objective

Table 2.1: Comparison of low-voltage and high-voltage drive system properties.

Property HV System LV System battery cell setup few parallel many parallel BMS channels many isolated 13 unisolated ≤ drive origin industrial HV LV battery redundancy©limited/none2021redundant, modular electrical hazard lethal safe drivetrain efficiency good better safety low resistance challenges isolation LV fast charging redundancy key-component de- by velopment 2.2.1 LVHC Machines with Novel MOSFETs

When considering the recent advances made in high-current MOSFETs, is it possible to use these switches to construct a compact multi-phase LVHC machine with integrated power electronics? What would be the advantages and limitations of such a design? These main research questions are addressed by the studies presented in Papers I–III. To answer them, a first prototype machine was constructed, and in doing so the following design-specific questions have been addressed:Stefan Which MOSFETs can be used to achieve minimal 푅퐷푆 푂푁 ? • ( ) What is the simplest machine topology for an initial proof-of- • concept?

What power electronics topology can be integrated with suitable • effort, and what trade-offs need to be made?

What controller should be used? • Haller How should machine and power electronics be assembled to • provide a detachable, low resistance connection? page | 22 2.2.2. Improvement and Limits of LVHC Machines

How should the FEM model be verified for further design • improvements?

2.2.2 Improvement and Limits of LVHC Machines

The first prototype demonstrated that LVHC machines are feasible with today’s power electronics.2021 This leads to© the question: what efficiency improvements can be made while keeping the previous machine topology and power electronics? What limits exist and what issues arise that require further investigation? To answer these questions, a second prototype was constructed, and is discussed in Papers VI and V.

2.2.3 Components for 48 V LVHC Machines Both prototype machines servedby to validate the concept of multi-phase LVHC machines. As one of their limitations, the lag of individual phase current control and unidirectional current flow was identified. This led to the question of what type of multi-phase pulse-width modulation (PWM) controller can be used. Paper VI presents the study of a potential multi-phase PWM controller. Another obstacle for a 48 V traction drive is the limited cooling capability of the presented prototype machines. The question of how the stator winding can be cooled efficiently is answered in the study published in Paper VII. To investigate a 48 V fast-charging concept for traction batteries, a reconfigurableStefan battery pack for high-voltage charging and low-voltage driving is studied in Paper VIII.

2.3 Objective

The main objective of this work is to determine if multi-phase LVHC drives with integrated power electronics can be built by using today’s high-current MOSFETs. This type of machine breaks with the convention of reducingHaller the current by increasing the voltage to reduce conduction losses. These machines can theoretically be built and would offer many advantages if sufficiently low path resistance could

page | 23 Chapter 2. Problem Formulation and Objective be achieved to compensate for the 퐼2푅 losses that scale quadratic with the current. Many practical details have to be considered, however, and there are inevitable compromises when building such a machine. This work focuses on the practical implementation of multi-phase LVHC machines with integrated power electronics. It elucidates the solutions and limitations of low-voltage high-current, which are intrinsically safe and redundant2021 by design. The first proof-of-concept machine confirmed© that today’s high-current MOSFETs are not the limiting factor. To investigate the weaknesses of the concept and to develop an understanding of this type of machine and high-current power electronics, a second prototype was constructed. With the promising results and experience gained through this small-scale 600 A machine, the key components for a 48 V high-current fully electric drivetrain are now being investigated. 2.3.1 Limitations by The block diagram of a principle low-voltage drivetrain is shown in Figure 2.1. This thesis focuses on the LVHC machine with a MOSFET-based drive (green boxes). The reconfigurable battery is currently being investigated and constitutes future work. The LV and HV charging, as well as the LV supercapacitor and adjacent DC/DC converter are not included in this thesis. This work does not claim to provide an in-depth analysis of electrical machines. Rather, this thesis provides an overview of, and motives for further investigation of low-voltage, high-current, battery- powered drives.Stefan Haller

Reconfigurable Included in Li-Ion Battery this thesis

HV fast HV mode Charger by LV MOSFET LVHC LV mode Charger drive machine

LV Super- DC/DC capacitors converter

Figure 2.1: Overview of a principle low-voltage drivetrain. This thesis focuses on the LVHC machine and MOSFET drive (green boxes). A reconfigurable Li-Ion battery pack (blue boxes)Stefan is currently under development and constitutes future work. The parts in the yellow boxes are not included in this thesis. Haller

Stefan Haller Chapter 3 PUBLICATIONS INCLUDED IN THIS2021 THESIS The thesis includes© eight papers, found in Part II. Six of them have been published at the time of the thesis’ publication and one is in manuscript. Five of the papers are IEEE peer-reviewed publications. An overview of the publications is illustrated in Figure 3.1.

Papers I–III focus on the LVHC proof-of-concept by designing and constructing the first prototype machine. Papers IV and V discuss the second prototype machine, its torque-speed efficiency measurement and armature reaction to itsby concentrated single-turn windings. With the LVHC concept proven to work, the next drivetrain generation aims to implement a 48 V system. The investigation of the required components has been initiated with studies for Papers VI to VIII. A short summary of all papers and their contribution to the thesis, including the the author’s contribution, is found in this chapter.

Prototype Prototype Next Gen- Machine 1 Machine 2 eration 48 V Stefan Paper 6 Controller

Paper 1 Paper 2 Paper 3 Paper 4 Paper 5 Paper 7 Air-gap Motor In- Characte- 2.5V 600A Armature Cooling flux density vestigation rization motor Reaction Paper 8 Battery

Figure 3.1: Overview of included publications and progression towards a 48 volt high-current machine.Haller Paper VIII is currently in manuscript and constitutes future work.

page | 27 Chapter 3. Publications Included in this Thesis

3.1 Paper I — Air-Gap Flux Density Measurement Sys- tem for Verification of Permanent Magnet Motor FEM Model

Peer-reviewed and published in IEEE Proceedings of Industrial Electronics 2015 [63] Author’s contribution: principle2021 author, idea, development and construction of prototype© electronics and software, measurements, analysis, writing and presentation of the paper

Contribution to the thesis

Paper I describes the design of an air-gap flux density measurement system for electrical machines. The measurement system plots the flux density distributionby as function of the angular machine position. It was designed to measure the air-gap flux density of a LVHC drive prototype. Due to the modular design and simple hardware, it can be easily adjusted to match other applications. This was demonstrated in Paper V, where the differential amplifier board was used to capture the voltage drop across the MOSFET’s 푅퐷푆 푂푁 . ( ) The measured air-gap flux density distribution was used to verify the FEM simulation model of the first prototype machine. Fig. 6 in Paper I provides a 3-D inside view into the machine’s air-gap flux density distribution along the machine circumference. Theflux density distributionStefan reveals the position of the PMs inside and allows statements about the flux densities’ uniformity. Fig. 9 in Paper I shows the measured and the simulated mean air-gap flux density. It was concluded that the simulation model provided a good match to the prototype machine. However, it also revealed the nonuniform flux density between thePM sections. The slightly larger gaps between two adjacent PMs on each pole are particularly visible. These results have been taken into account when developing the second prototype machine. It used buried instead of inset mounted PMs to smooth the air-gap fluxHaller density. The construction details of the first prototype are found in Paper II. The improved, second prototype is described in Paper IV and V. page | 28 3.2. Investigation of a 2 V 1.1 kW MOSFET Commutated DC Motor

3.2 Paper II — Investigation of a 2 V 1.1 kW MOSFET Com- mutated DC Motor

Peer-reviewed and published in IEEE Proceedings of Power Electronics and Motion Control 2016 [64] Author’s contribution: principle author, development and construction of prototype electronics2021 and software, measurements, analysis, writing and presentation© of the paper

Contribution to the thesis

Paper II introduces an unconventional drive concept for battery- powered traction machines. Instead of using high voltage and low current in combination with IGBT drives, this paper introduces a low-voltage but high-current design with MOSFETs instead of IGBTs. A first prototype machineby with a single-turn concentrated multiphase winding was designed and constructed. This technique enabled a copper fill factor approaching one. The MOSFET-based drive was directly attached to the machine and tested up to 180 A. The locked rotor-torque and no-load power consumption, as well as the EMF measurements are presented. The experimental results from the first prototype show great potential for cost-effective LVHC MOSFET-based drive systems. The air-gap flux density of this prototype was measured and compared with the FEM simulation in Paper I. To evaluate the limits of this prototype,Stefan the test-setup was slightly modified, the machine’s software improved, contact and winding resistances reduced and a 500 A power supply utilized. The measurements and results of this modification are reported in Paper III.

3.3 Paper III — Initial Characterization of a 2 V 1.1 kW MOS- FET Commutated DC Motor Peer-reviewedHaller and published in IEEE Proceedings of Industrial Electronics 2016 [65] Author’s contribution: principle author, idea, development and con-

page | 29 Chapter 3. Publications Included in this Thesis struction of prototype electronics and software, measurements, analysis, writing and presentation of the paper

Contribution to the thesis

Paper III describes the measurements of the torque-speed efficiency map of a slightly improved2021 version of the first prototype, introduced in Paper II. In addition© to software improvements, the winding resistance was greatly reduced by shortening the connections to the terminal rings and MOSFET-based drive. A machined copper VCC ring and larger cross-sectional area of the supply cables were also used, as shown in Fig. 4 in Paper III. While the prototype was tested with currents up to just 180 A in Paper II, it was operated with up to 520 A after the modifications described in this paper. The machine could be operated continuously with currents up to 400 A andby allowed peak currents up to 520 A. Most of the obtained resistive losses were contact losses due to the flexible winding connections. Less than 6 % of these losses are dedicated to the MOSFETs. Due to the use of stranded wire and flexible coil connections, the non-MOSFET related losses dominated. The coil resistance could easily be reduced by more than 50 % by using a solid wire, which provided a copper fill factor larger than 0.8. The results show that such a high-current drive is feasible and has great potential for further improvement. Thus, a second prototype machine was designed and constructed. Its design details and efficiencyStefan measurements are published in Paper IV. An analysis of the current distribution in the stator coils is provided in Paper V.

3.4 Paper IV — A 2.5 V 600 A MOSFET-Based DC Traction Motor

Peer-reviewed and published in IEEE Proceedings of Industrial Technology 2019 [66] Author’s contribution:Hallerprinciple author, idea, development and construction of prototype electronics and software, measurements, analysis, writing and presentation of the paper page | 30 3.5. Phase Current Shift in Multiphase Single-Turn Concentrated Windings

Contribution to the thesis

Paper IV presents the design and efficiency measurements of the second prototype of the low-voltage high-currentPM DC drive. The results and experience gained from the previous work published in Papers I–III were used to design this machine. As a result, the copper2021 fill factor could be increased by 97 % compared to© the first prototype and reached 0.84 in this 1.2 kW version. The air-gap flux density could be increased while reducing the rare earth magnetic material by 20 %. The resistive losses were reduced by more than 62 %. While they reached 170 W at 520 A and 1100 rpm at the measurements reported in Paper III, they could be reduced to 63.4 W in the design reported in this paper. The presented machine was successfully operated with drive currents up to 600 A under continuous operation. The observed temperature stayed below 70by°C with forced air cooling. The hot spot was found to be at the connection between power electronics and stator coils, as shown in Fig. 10 in Paper IV. This observation led to the design of a direct winding cooling to extract heat from the winding and connections. The investigation of a prototype of this direct in-conductor cooling is presented in Paper VII. The measured torque-efficiency map shows that such a high- current drive with voltages below 60 V is feasible using present-day LVHC semiconductors. There is great potential for further improvements in this concept. This is supported by the rapid development of low-voltage high-currentStefan switches.

3.5 Paper V — Rebalancing of Phase Current Shift Caused by Armature Reaction in Multiphase Single-Turn Con- centrated Winding Machines

Submitted to IEEE Access Author’s contribution:Hallerprinciple author, idea, development and construction of prototype electronics and software, measurements, analysis and writing of the paper

page | 31 Chapter 3. Publications Included in this Thesis

Contribution to the thesis

Paper V provides a deeper analysis of the behavior of the second prototype machine under load. It showed the limitations of the power electronics used and investigated the machine’s armature reaction to further optimize the design. Under increasing stator current and load, it could© be shown that2021 the current was unevenly distributed in the parallel connected stator windings, which led to peak currents in the coils and power electronics phases. With some limitations of the existing power electronics, a PWM-based current control could be implemented. The peak phase current was thus reduced to counteract the armature reaction. Since the sum stator current was kept constant, the current was distributed more equally in all phases. In this paper, it was shown that individual phase current measurement and control is requiredby for a LVHC machine using a parallel connected concentrated winding with block commutation. By meeting this requirement, peak phase and switch currents could be reduced and the machine’s state of health could furthermore be constantly monitored. In case of implausible phase currents, these could be dis- abled to prevent a catastrophic machine failure and keep the machine operational with reduced performance. This built-in redundancy is a beneficial feature for BEVs. As result, the design of new power electronics with half-bridge configuration and current control to fully utilize the machine’s active material andStefan counteract the armature reaction is suggested. This requires a more sophisticated controller and thus led to the design and investigation of the hybrid-controller presented in Paper VI. From the machine’s perspective, the current and power density must be increased to meet the requirements of traction drives. The air-cooled second prototype reached an RMS current-density of 6.7 A/mm2 for continuous operation. Forced liquid-cooled machines, which are used as traction drives in BEVs, have a current density of 10 A/mm2 toHaller30 A/mm2 [67]. Thus, a winding design with direct in-conductor cooling that could reach a current density of up to 49.5 A/mm2 was investigated in Paper VII. page | 32 3.6. CPLD and dsPIC Hybrid-Controller for Converter Prototyping

3.6 Paper VI — CPLD and dsPIC Hybrid-Controller for Converter Prototyping driving a Reconfigurable Trans- former Phase-Shifted Full-Bridge

Published in Proceedings of Int. Conference PCIM Europe digital days 2020 [68] Author’s contribution: principle2021 author, idea, development of the software, measurements,© analysis, writing and presentation of the paper

Contribution to the thesis

Paper VI presents a PWM controller for multi-switch converter and machine topologies. This hybrid-controller uses a combination of a digital signal controller (DSC) with high resolution PWM and complex programmable logicby device (CPLD). The design is derived from the power electronics used on both prototype machines. Instead of a general purpose microcontroller, a dual-core DSC was used. It allowed execution of time-critical machine and converter control on a dedicated core, isolated from the user interface. Its multi-channel high-resolution PWM provided precise duty cycle and dead-time control. With the CPLD, a non-standard PWM pattern and additional fast protection mechanism could be implemented. The hybrid-controller is sufficient to drive a 13-phase power electronics with half-bridge configuration for the presented prototype machines. WithStefan an increased number of phases, however, a different approach with an field-programmable gate array (FPGA)-based solution is more favorable, similar to the ISCAD power electronics.

3.7 Paper VII — Multi-Phase Winding with In-Conductor Direct Cooling Capability for a 48 V Traction Drive Design Peer-reviewedHaller and published in IEEE Proceedings of Int. Conference on Electrical Machines 2020 [69] Author’s contribution: principle author, development and construc-

page | 33 Chapter 3. Publications Included in this Thesis tion of the experimental setup, measurements, analysis, writing and presenting of the paper

Contribution to the thesis Paper VII is© an initial investigation2021 of the cooling capability of a multi-phase stator winding for a 48 V traction drive. The hot-spot of both prototype machines was detected at the winding and power electronics connection, as shown in Paper III and IV. To extract the heat from the winding and connections, a liquid cooling method was investigated. Due to the unique properties of LVHC machines, direct in-conductor cooling – a decades-old method – could be employed. This solution is common in very high power electrical machines and transformers, often used in electrical generators at power plants. It is now gaining interest [by19] as a means of increasing the traction machine’s power and current density. For the next generation 48 V machine, a LVHC winding with direct in-conductor cooling was designed. The experimental results of a straight version of the coil with water and up to 500 A conductor current were used to verify the simulation model. Finally, the simulation was performed with a W-shaped coil using 700 A coil current with 65 °C at the inlet and EGW50/50 as coolant. This translated to a current density of 49.5 A/mm2 with a power dissipation of 710 W at a maximum coil temperature rise of only 56.9 °C. The flow rate required for aStefan single coil was 0.312 l/min at a pressure of 140 kPa. The results illustrated exceptional performance of the direct in- conductor cooling, even with hot coolant at the inlet. Beyond a certain load of the machine, the resistive winding losses dominate [12]. Other machine losses, such as eddy current and iron losses in the stator and rotor need to be considered as well, though. The suggested winding reaches a high copper fill-factor in the slot and thus provides alow thermal connection to the stator. This enables use of the stator winding itself to extractHaller heat from the machine as well. The achievable current density in practice is therefore expected to be lower, to the benefit of increased machine cooling through the stator winding. page | 34 3.8. Reconf. Battery for Charging 48 V EVs in High-Voltage Infrastructure

3.8 Paper VIII — Reconfigurable Battery for Charging 48V EVs in High-Voltage Infrastructure

In Manuscript Author’s contribution: corresponding author, idea, measurements, writing of the manuscript2021 Contribution to© the thesis Paper VIII proposes a reconfigurable battery pack for low-voltage driving and high-voltage charging. TheEV charging infrastructure currently being build up is designed for high-voltage batteries. Fast charging of low-voltage traction batteries requires high-currents that exceed the limits of today’s charging connectors. Being able to use this high-voltage infrastructure, is key for the acceptance of high-power 48 V drivetrains in BEVs. by Battery packs for most BEVs are assembled from modules con- taining the battery cells, like the Volkswagen’s MEB [47]. Instead of a static configuration, these modules can be dynamically reconfigured to switch between low-voltage and high-voltage operation. To test the concept of a reconfigurable battery pack, a small-scale proof-of- concept system for 48 V driving and 400 V charging is constructed. The presented initial measurements validate the concept and justify further researchStefan on the topic. Haller

Stefan Haller Chapter 4 THEORY

4.1 Introduction to Electrical Machines Electrical machines© are used2021 to convert electrical energy into mechanical energy, and vice versa. A machine converting electrical energy to mechanical is called a motor. A generator on the other hand converts the energy in the opposite direction. The electric motor and generator are technically very similar, with a principle of operation that can be described by Faraday’s law. Essentially, each electrical machine can be operated in motoring or generation mode [70]. In generation mode, the produced terminal voltage should generally be kept constant overby time. In motoring mode, the available voltage is mostly constant, while adjustable speed and torque is requested. To achieve speed and torque control on the mechanical side, as well as voltage and current control on the electrical side, a drive system is required. Various drive systems can be used depending on the type of electrical machine and its configuration. Electrical machines have historically been split into two main groups: AC-powered and DC-powered machines. AC motors can be operated directly from an AC source without any commutation. DC motors instead require a commutator to create an alternating magnetic field from a DC source. Fundamentally, all electrical machines areAC machines. Furthermore,Stefan if controlled by power electronics, they are essentially DC-powered AC machines. This is true for machines fed by a converter with a DC link, which are often low- to medium-power machines. The electrical machine operates through interaction of the stator and rotor magnetic field. The stator generally carries the machine’s main winding, also called stator winding. This winding is responsible for creating the stator’s magnetic field, which propagates through the machine’sHaller air gap. Other techniques are used to either create the magnetic field on the rotor side, or to facilitate the stator magnetic field. Depending on the rotor design, the electrical machine canbe

page | 37 Chapter 4. Theory either of synchronous or asynchronous type, or even both.

4.1.1 Synchronous Machines

The synchronous machine’s revolving speed is synchronized to the rotating air-gap field produced by the stator. Depending onthe number of poles, the machine’s2021 rotational speed 푛 in rpm is obtained by © 60 푓푙 푛 = · (4.1) 푝 where 푓푙 is the feeding frequency in Hz and 푝 the number of pole pairs. The rotational speed, for example, is 3000 rpm for a two-pole and 1500 rpm for a four-pole machine at 50 Hz. External Excitation Differentby techniques can be used to generate the rotor flux. External excitation requires energy transfer to the rotor by brushes or brush-less exciters. This is mainly used in generators, such as car generators or even large generators in power plants, for example. External excitation allows control of the rotor’s magnetic field and thereby influence over the stator current. For AC generators, itis not only possible to control the amount of reactive power produced, but even to decouple the mechanical from the electrical frequency by superimposing a frequency on the rotor. As the rotor current is just a fraction of the stator current, this provides significant savings for the converter. This technique is used in wind mills, for example. A disadvantageStefan of using brushes is increased wear and need for maintenance.

Permanent Magnets PM magnets, mostly made of neodymium iron boron (NdFeB) are often used for low- to medium-power applications and due to their higher efficiency are a competitor to speed-controlled IMs. The efficiency increase due to the permanent available rotor magnetization without additional energy also brings certain disad- vantages. TheHaller rare earth magnets are costly and the price varies greatly on account of economic factors. PMs are also sensitive to high temperatures and complicate the machine’s assembly. page | 38 4.1.2. Asynchronous Machines

For passenger vehicles, security is a very important factor. Due to the permanent rotor flux,PM machines permanently induce a voltage in the stator winding once the rotor is revolving. Unlike externally excited machines, the stator current cannot be controlled easily in case of a machine or inverter fault. This aspect needs to be considered during the machine and drivetrain design phase. Asynchronous machines offer some advantage2021 in this case, as they do not produce a significant ©terminal voltage without external stator current. Some effects ofPM machine faults are further described in subsection 1.1.3.

4.1.2 Asynchronous Machines

The SCI motor is currently most common in industrial applications due to its high reliability, good efficiency and low production cost [71, 72]. It is an asynchronous ACIM that does not require any additional electrical contacts to the rotor,by unlike externally excited machines. The magnetic field in the rotor is created by the rotating stator field itself. The stator winding of a 3-phase SCI motor, for example, creates a rotating magnetic field in the air gap. This alternating field induces a voltage in the rotor winding. To produce a current in the rotor and thus a magnetic field, the rotor winding is short-circuited. The rotor’s magnetic field interacts with the rotating field in the air gap and thus produced torque in the direction of the field rotation. To allow energy transfer from the stator on the primary side to the rotor on the secondary side, the rotor needs to be exposed to a flux change.Stefan A speed difference – the slip – between the airgap’s magnetic field and rotor’s speed ensures energy transfer. Since the rotor is lagging behind the field produced by the stator, its speed is asynchronous. The IMs are therefore also referred to as asynchronous machines. Speed and torque control for motoring as well as generation mode can easily be achieved by using a variable-speed drive (VSD) to control theIM. A stator current is required to maintain the rotor field. This is advantageous compared toPM machines if the inverter, a machine or isolation failureHaller occurs. By disabling the stator current, theIM does not produce a considerable amount of back EMF, reducing the risk of fault propagation and unwanted braking torque.

page | 39 Chapter 4. Theory

4.2 EMF Generation

The machine’s EMF can be expressed as the sum of the transformer EMF and the motional EMF. The transformer EMF can be expressed by Faraday’s law of electromagnetic inductance. The law states that a current is induced in a conductive loop by the change of the magnetic flux. The rate of change2021 of flux determines the value of the induced EMF. The motional© EMF is caused by the Lorentz force, pushing the freely movable electrons in a moving conductor towards one end. The left side of Equation (4.2) represents the EMF induced in the machine. The first term on the right side represents the transformer EMF, while the second term represents the motional EMF.

∫ 휕퐵 ∮ 퐸 = 푑푠 푢 퐵 푑푙 (4.2) − by푠 휕푡 · + ( × )· Which EMF effects to use depends on the chosen reference frame: rotor, stator or even something in between. The EMF of a wire loop, however, is always equal to the rate of change of the linked magnetic flux. The no-load EMF generated by the presented permanent magnet DC (PMDC) motor, shown in Figure 4.1, is almost a square wave. Once the coil entered the constant flux density region under eachPM pole, the generated back EMF was nearly constant. As the selection of the reference system and thus the share of both EMF components can be chosenStefan freely, the motional EMF was chosen. It represents a moving wire in a magnetic field and is considered more intuitive in describing the prototype machine. The machine’s EMF 퐸 can thereby be approximated by applying the Lorentz force law as

퐵 푙푎 휋 푑 푛 퐸 = · · · · (4.3) 60 where 퐵 is theHaller flux density inside the air gap, 푙푎 is the active length of the coil within the armature, 푑 is the center to center distance of the coil within the armature and 푛 the rotor speed in rpm. page | 40 4.3. Torque Production

0.2 E+U Rs 0.15 E 0.1

0.05

−0.05 EMF voltage [V] −0.1 2021 −0.15 © E −0.2

Figure 4.1: Square-wave-like EMF of the first prototype machine at 300 rpm with3 excited coils under no-load condition. 4.3 Torque Production by The torque produced in an electric motor is the result of the interaction between the stator’s and the rotor’s magnetic field. The force 퐹 on a wire in a static, homogenous magnetic field can be expressed as

퐹 = 퐼 푙 퐵 (4.4) ì ·(ì× ì) where 퐼 is the current in the wire, 푙 is length of the wire and 퐵 represents the magnetic flux density. Based on the motor design, some simplifications can be made. The rotor flux is assumed to be constant as it is generated by PMs. The stator windingStefan consists of individual, fully-pitched, single-turn concentrated coils. Due to the use of segmented magnets in the design, the flux is assumed to be perpendicular to each coil. Thus, (4.4) can be simplified to 퐹 = 퐵 퐼 2푙푎 (4.5) · · where 푙푎 is the active machine length. The motor’s torque 푇 can be estimated as

푑 푇 = 퐹 (4.6) Haller· 2 where 푑 is the center-to-center distance of the coil in the armature.

page | 41 Chapter 4. Theory

Substituting (4.5) into (4.6) allows estimation of the torque generation as a function of the armature current 퐼 accordingly

푇 = 퐵 퐼 푙푎 푑. (4.7) · · · 4.3.1 Magnetic and Reluctance Torque The torque production© of2021 an electric motor can be achieved in two different ways: via magnetic torque and via reluctance torque. The investigated prototype primarily used magnetic torque as described above. The reluctance torque can be neglected. Machines relying on reluctance torque have a special rotor ge- ometry that provides strong saliency and thus guides the stator flux through the rotor. Due to the saliency, the rotor locks to the air-gap field produced by the stator winding and is synchronously pulled by the field. This principle is fully utilized by switched reluctance motors (SRMs) for example. Essentially,by the ratio between magnetic and reluctance torque can be controlled by the rotor design as described by Morimoto [73]. Stefan Haller

page | 42 Chapter 5 METHODS

Theoretically, LVHC machines with integrated power electronics can be built and would offer many advantages. Still, many practical details have to be considered and2021 an optimal balance between machine and power electronics© design needs to be found. A compact integration of the power electronics as well as very low winding and connections resistances are critical for these types of machines, and this is not easily simulated. An experimental approach is necessary to determine if multiphase single-turn concentrated winding LVHC machines can be built using today’s high-current MOSFETs. To limit the machines’ power and size, as well as test-stand and equipment requirements, a small-scale version with a relatively low EMF of approximately 3 Vbyat 3000 rpm was constructed. Due to the low voltage, drive currents of up to 600 A were successfully tested with the 1.2 kW prototype. A PMDC type machine topology was chosen. It is a synchronous machine and does not require additional power transfer to the rotor. The control algorithm could also be simplified in comparison to induction machines. After confirming that modern high-current MOSFETs are not the limiting factor and that LVHC machines are feasible, research on component level and design details began. Simulations on key components for a 48 V high-current fully electric drivetrain were conducted in advanceStefan and confirmed by experimental results. 5.1 Paper I to Paper III, Prototype Machine 1

At first, the air-gap flux density was measured by a self-designed hall sensor-based setup. The flux density inside the machine’s air gap was plotted against its circumference. The setup and comparison to the machine’s FEM simulation is further described in Paper I. The measurements of the first prototype, presented in Papers II and III, were conductedHaller on a self-built test stand. The principle of reaction-torque measurement was used, further described in Paper II. It should be noted that the reaction-torque method, which measures

page | 43 Chapter 5. Methods the counter-force needed to keep the machine’s stator in balance, has a level of uncertainty due to the massive cable assembly. Precautions were taken to minimize the influence of the cable assembly onthe measurements. Due to the imperfect mechanical setup of the out- runner, vibrations were induced at the rotational measurements in Paper III. These measurements should therefore be conservatively interpreted. 2021 The initial© measurements of the first prototype included stationary torque, EMF, coil loop resistances, no-load motor current as well as motor constants 푘푇 and 푘퐸, described in Paper II. A peak stator current of 180 A was used throughout these measurements. After the initial tests indicated a feasible design, the coil connections to the power electronics and terminals were improved to reduce the resistance. This was achieved by shortening the coil connection wires to a minimum, using a massive copper VCC ring and increasing the cross-sectional area ofby the feeding cable assembly. The laboratory DC power supply was upgraded to allow peak currents of 520 A. The torque-efficiency map of the slightly improved first prototype was measured by the reaction-torque method, and presented in Paper III. The machine was tested up to 520 A and could be constantly operated at 400 A with forced air cooling.

5.2 Paper VI and Paper V, Prototype Machine 2

A second prototype machine with improved mechanics but the same power electronicsStefan was designed and measured on a calibrated test- stand at the Chair of Electrical Drives and Actuators at the Bundeswehr University Munich. A DC load-machine that could be operated in motoring or generation mode was used. A much more precise rotational torque transducer was used instead of the reaction-torque. The load machine was operated in constant speed mode while the machine under test was operated in motoring mode with constant current. The rotational speed, shaft torque, power in, power out as well as the machine’s and power electronics’ temperature was logged. Each speed and current stepHaller was recorded for one minute with 10 samples per second. The mean of the sampled data was processed and presented as a torque-speed efficiency map in Paper IV. To measure the out-runner page | 44 5.2. Paper VI and Paper V, Prototype Machine 2 with a straight shaft configuration, an additional support bearing was used. This bearing is not part of the motor design and its influence should ideally be removed from the measurement results. Due to the lack of data describing the bearing losses under various speed and load conditions, however, all measurements published included the additional support bearing. The presented torque-efficiency measurements are thus very conservative.2021 The actual machine efficiency is estimated to be© up to 5 % higher. An increased jitter of the hall sensor-based rotor position detection was observed during the torque-efficiency measurements. False commutation due to increased jitter in the rotor position detection would lead to excessive coil currents that the switches could not handle. This would result in a latch-up state for the MOSFETs due to excessive current, where they could no longer turn off. The silicone die of the used CAN-FETs would in fact rupture and delaminate from the case. Thus, it was not possible toby measure the machine’s efficiency up to 600 A at 3000 rpm. The jitter is caused by a shift in the machine’s winding current in conditions where load and speed are high. This was simulated and confirmed by measurements presented in Paper V. The shift incoil current was measured in one of the 13 coils, assuming similar behavior in the remaining coils. Due to mechanical restrictions of the power electronics, a current clamp not exceeding 30 A could be used, and even then just on a few coils. Only an indirect method of measuring coil currents above 30 A was possible, by calculating the current from the voltage drop across the MOSFETs 푅퐷푆 푂푁 . A differential amplifier Stefan( ) with common mode rejection was used to measure the voltage drop. The voltage during the MOSFETs on state was extracted from the recorded measurements and low-pass filtered. As the MOSFETs resistance depends on many parameters, including Drain current, Gate voltage, temperature and production spread, a calibration was needed. This was achieved by matching the calculated to the measured current at 30 A with cold and hot power electronics. For the 100 A calibration, the mean coil current of the commutation window was calculated andHaller matched to 100 A. It should be noted that the latter calibration is less precise, although it is sufficiently accurate for the purpose.

page | 45 Chapter 5. Methods

5.3 Paper VI and Paper VII, Next Generation 48 V

After demonstrating that LVHC machines can be built using today’s high-current MOSFETs, key components for a scaled-up 48 V traction drive were investigated. In Paper VI, a hybrid PWM controller for converters with a large number of switches was built and investigated. The concept is based2021 on the power electronics used for both presented machines.© Instead of a general purpose microcontroller, a DSC with dedicated PWM module was used. The CPLD was kept as a configurable multiplexer. The next generation 48 V drive will use a multiphase half-bridge configuration to allow bidirectional stator current flow and phase current control. Depending on the numberof phases required, the hybrid-controller or an FPGA solution can be used. Liquid cooling is necessary to increase the power and current density of the machine. Single-turnby windings with large cross-sectional area conductors allow the use of direct inner cooling. In Paper VII, a W-shaped coil with direct inner cooling is presented. A straight and a W-shaped version of the coil of the same length and diameter were produced. The flow rate versus pressure drop was measured with water on both versions to check the accuracy of the simulation model. Turbulent flow was used to increase the heat transfer into the coolant. The straight version was tested with currents up to 520 A while the coil and coolant temperature rise was monitored. A good match to the simulation model was identified, allowing estimation of the cooling capability of the W-shaped conductor. The temperatureStefan along the straight coil conductor was measured as well. Surprisingly,the temperature did not show a linear gradient from the coolant inlet to the outlet under all conditions. It seems that this occurs more often with reduced coolant pressure. It is assumed that laminar flow regions can arise under certain conditions, likely caused by the coolant inlet’s shape. Further research into this phenomenon is required to determine how turbulent flow can be maintained inside the entire conductor.Haller

page | 46 Chapter 6 DISCUSSION

The key enabling components for LVHC drives are the power switches on the machine’s power electronics. These high-current MOSFETs can handle currents© of a few hundred2021 ampere per device. Combined with a low-resistance design of the connections, battery pack and machine, electrical drivetrains with unique properties can be built.

These drivetrains are electrically intrinsically safe, as high voltage is not created at any point while driving. The use of multiple phases and single-turn or W-shaped windings in combination with the low system voltage allows direct in-conductor cooling using EGW50/50, offering outstanding cooling performance compared to oil-based coolants. by While the automotive industry initially used many different voltage levels to electrify the drivetrain, there is a clear trend towards two standards these days. One is the high-voltage system, exemplified by the 400 V class and the 800 V class [5,6, 74] in high-performance sports cars, such as the Koenigsegg Regera or Porsche Tycan. The other is the low-voltage system, with 48 V used in mild-hybrid systems. 48 V drives with power ratings of 30 kW to 80 kW [58, 62, 75] are already available. A 48 V 110 kW 42-phase ISCAD machine in a fully functional carStefan is demonstrated by Molabo [59]. The findings described in this work, together with the ongoing low- voltage research and increasing power level for mild-hybrid systems, show that LVHC motors are an alternative solution to high-voltage traction machines for future BEVs. The author expects two voltage levels as standard for future EVs: a 800 V high-voltage system for high- performance cars targeting the MW-range, and a 48 V low-voltage system for regular vehicles with a few hundred kW peak power, where cost-to-performanceHaller ratio is important. The major findings of this work are summarized and discussed in the following sections.

page | 47 Chapter 6. Discussion

6.1 Machine Prototypes

To test the concept of high-current but low-voltage multiphase machines, two extremely low-voltage prototypes have been built and tested. To limit the design and test-stand requirements, a shaft power of approximately 1 kW at 3000 rpm was targeted. In combination with the low EMF of less than20213 V at 3000 rpm, a stator current of up to 600 A could be© used. Table 6.1 summarizes the key properties of both built prototypes. The second prototype, shown in Figure 6.1, was an improved version based on the findings of the first version. Both machines used the same power electronics. The concentrated winding was constructed of 13 single-turn coils, driven by individual phases of the power electronics. The power electronics allowed unidirectional stator current flow with up to five simultaneous active phases. The commutation pattern is described in detail in Paper V. Eachby stator slot carried a single coil wire which allowed a high copper fill-factor and provided low thermal resistance. A PMDC design in an out-runner configuration was the chosen machine type. The PM design simplified the control algorithm in comparison to anIM design. The main focus was the integration of motor, power electronics and connections with a sufficient low resistance. In the first prototype, cumbersome inset-mounted magnets were used, locked in place by their slightly spherical shape and wedges. The second prototype used buried magnets that were much simpler to mount. The magnets nearly pulled themselves into the machine’s slot and wereStefan fixed in place using end plates. A drawback of theout- runner design was the single-side bearing. The machines vibrated due to slight unbalances and mechanical play in the bearings. The bearing setup was improved on the second prototype but still showed some undesired mechanical play. The experience gained from both extremely-low voltage and low- power out-runner prototypes enabled continued work on the next machine generation. It will be a high-power 48 V PM in-runner design with direct in-conductorHaller cooling of the multiphase W-shape coil winding. The half-bridge-based inverter should be able to measure the individual phase currents and control each coil individually. page | 48 6.1.1. 1st Prototype

Table 6.1: Design guideline comparison of the first prototype and the second improved version, based on the results of the first prototype.

Property 1st Prototype 2nd Prototype type 2-pole PMDC out-runner coil configuration single-turn, full pitch cogging torque reduction2021 skewed stator coil, 1 slot power electronics© IRF6718L2 MOSFET-based magnet mounting insert buried rated speed 3000 rpm 3000 rpm individual phases 13 13 copper fill factor 0.425 0.84 coil loop resistance 1.59/1.28 mΩ 0.59 mΩ phase loop resistance 3 mΩ 1.14 mΩ ≈ terminal-to-terminal resistanceby0.6 mΩ 0.23 mΩ ≈ power at rated speed 1 kW 1.2 kW EMF at rated speed 2 V 2.5 V ≤ ≤ peak motor current 520 A 600 A ≤ ≥ continuous motor current 400 A 550 A ≤ ≥

6.1.1 1st PrototypeStefan

The first prototype allowed continuous drive currents of up to 400A. A stranded flexible wire with a copper fill factor of just 0.43 andcrimp contacts were used to allow easy maintenance of the machine. The total resistive losses of this machine were up to 170 W at 520 A. The winding losses contributed 42 % and the MOSFETs less than 6 % to the machine’s losses. More than 50 % were various contact losses. This shows that theHaller MOSFETs could easily handle the machine current and that further optimization on the winding, and especially the contacts, is required.

page | 49 Chapter 6. Discussion

6.1.2 2nd Prototype

The second prototype, shown in Figure 6.1, was improved based the findings in the first version. A solid wire winding with pre-formed equal length coils was used, shown in Figure 6.2. This increased the copper fill-factor by 97 % to 0.84. Instead of crimp contacts, the coils were directly connected to2021 the PCB by copper nuts. The end-winding length could© also be reduced by using pre-formed coils. Together with mechanical and magnetic optimization, the total resistive losses of this version could be reduced by more than 62 % to 63 W at 520 A. The winding losses, including the VCC terminal and adjacent contacts accounted for 52 % and the MOSFETs for 22 % of the resistive losses. The remaining losses occurred at the PCB and GND terminal. This version was tested with continuous drive currents of up to 600 A. The power limit was set by the limited cooling capability of this air-cooled design and the coil currentby unbalance at increasing stator current, as shown in Paper V. The most complex part of the machine design was a compact Stefan

Figure 6.1: SectionalHaller view of prototype II with mounted power electronics and VCC ring in the test-stand configuration with support bearing. page | 50 6.2. Integrated Power Electronics

Figure 6.2:© One of 13 identical2021 preformed stator coils of prototype II. detachable construction with a low resistance connection between coils, PCB and VCC rings. Writing and tuning the machine’s software turned out to be far more time consuming than expected. The low resistance design of the second prototype did not tolerate a jitter in the commutation timing. Due to the block commutation used, a slight miss of the commutation window led to excessive circulating currents latching-up the switchesby and finally destroyed them almost immediately.

6.2 Integrated Power Electronics

The key components of the power electronics are the LVHC switches. The IRF6718L2 LVHC MOSFET, introduced to the market in 2009 [76], was used in the extremely-low voltage design of the presented prototype machines. With these switches, phase currents of more than 100 A at voltages below 20 V could already be reached with the air-cooling secondStefan prototype machine, as shown in Papers IV and V. The power electronics for both machines, shown in Figure 6.3, used these MOSFETs with a typical 푅퐷푆 푂푁 of 0.5 mΩ. Two of them ( ) were connected in parallel for each of the 13 phases, resulting in a typical switch resistance of 0.25 mΩ per phase. During all measurements, the machines were commutated using five of 13 simultaneous parallel connected coils. The machine’s position was tracked by hall sensors to calculate the commutation sequence. WhileHaller this worked fairly well in the first prototype, this approach was found to be unstable in the second version. Due to the mechanical design, the commutation window with 5 simultaneous

Figure 6.3: 13-phase MOSFET commutator, tested up to 600 A continuous current. active coils is very narrow, and the timing needs to be precise. With increased drive currents,by the detected machine position is slightly shifted from the real one. This increases the stress on the switches due to early or delayed commutation. Furthermore, the jitter of the hall sensors increases with larger drive currents. Another non-magnetic position detection mechanism should therefore be used in future designs, for example an optical encoder. A detailed description and limitations of the commutation pattern used are presented in Paper V. The recent generation of LVHC MOSFETs, such as the IRL7472L1, was released in 2016. It uses the same package and dimensions while the typical 푅퐷푆 푂푁 resistance is reduced by 32 % to 0.34 mΩ. At the ( ) same time, theStefan Drain-to-Source breakdown voltage was increased by 60 % to 40 V and the maximum Drain current by 39 % to 375 A. A prototype of a high-current power electronics using these MOSFETs in a half-bridge configuration is shown in Figure 6.4. It was intended to be used for a 10 kW 12 V prototype machine using a lab-winding. For an initial 48 V prototype, the automotive-graded AUIRF7749L2 released in 2015 can be used. It uses the same package and dimensions and can handle up to 345 A of Drain current. With a Drain-to-Source breakdown voltage of 60 V, it has a typical 푅퐷푆 푂푁 of 1.1 mΩ. While Haller( ) these switches may be used for a prototype, if the dynamic over- voltage is to be guaranteed below 60 V, later devices of at least 80 V page | 52 6.3. Armature Reaction causing Current Unbalance

Figure 6.4: Example of a 13-phase high-current PCB design with aluminum core for a lab-winding machine. by are required to comply with VDA320. All mentioned MOSFETs are mass-market devices that use a can-type package of just 9.1 mm 7 mm 0.7 mm. The performance × × improvement of LVHC semiconductors in recent years, together with the automotive industries decision to use 48 V for mild hybrids, shows the potential of LVHC designs. Drives with 30 kW are already offered for automotive integration [62]. For sport boats, a 48 V drive with 50 kW continuous power is available [58]. 6.3 ArmatureStefan Reaction causing Current Unbalance Under no-load conditions, the stator current is shared equally by all five parallel connected concentrated coils. With increasing stator current and machine load, the coil current distribution becomes distorted. This is caused by the machine’s armature reaction to the change in stator flux [77]. This effect was investigated in the second prototype and counteracted by PWM-based current control, as shown in Paper V. The armature reaction led to a flux density concentration towards the machine’sHaller direction of rotation and caused a change in the generated EMF. The coil current sharing is inversely proportional to the generated EMF and caused a current increase towards the

page | 53 Chapter 6. Discussion trailing coils of each commutation window. This unwanted effect is emphasized with increasing stator current and machine load. Unlike distributed windings, the layout of concentrated windings cannot compensate forthe armature reaction. Individual phase current measurement and control should be implemented to ensure a balanced coil current in the next 48 V prototype. The machine design can also be optimized to minimize2021 the effect of the armature reaction tothe coil current sharing,© by using W-shaped distributed coils, for example. 6.4 Direct Winding Cooling

The second prototype machine reached an RMS current density of 6.7 A/mm2 with air cooling. Beyond a certain machine load, the resistive losses in the stator winding dominate [12]. An efficient solution to extract the heat from the winding and machine is required to increase the current andby power density of the machine. Due to the unique properties of LVHC machines, a method that is used for electrical generators in power plants could be applied. In Paper VII, a direct in-conductor cooling solution was investigated for a W-shaped coil of a high-power 48 V multiphase machine that used EGW50/50 as coolant. At a coil current of 700 A, with 65 °C hot coolant at the inlet, a current density of 49.5 A/mm2 was reached with turbulent flow at flow rate of 0.312 l/min and a pressure drop of140kPa. The suggested winding reaches a high copper fill-factor in the slot and thus provides low thermal resistance to the stator. By that, with reduced coolant temperature or current density, additional machine cooling via theStefan stator winding becomes possible.

6.5 Next Generation 48 V Drive

A suitable voltage level for LV traction drives is the 48 V class as specified by VDA320 [61]. The voltage is as high as possible to still maintain the SELV limits. For the next generation LV traction drive, a 48 V system will be used. Figure 6.5 shows the current design stage of the 48 V machineHaller with integrated power electronics. The PMaSR machine uses W-shaped coils with direct in-conductor cooling. Due to the selected CAN-FETs, the power electronics with a multiphase page | 54 6.6. Social and Ethical Considerations ©2021 by

Figure 6.5: Sectional view of the current design stage of the next generation 48 V high-power machine with mounted power electronics.

half-bridge configuration is currently limited to 60 V. With these power electronics, individual phase current control and measurement is targeted to compensate for the machine’s armature reaction. Instead of block commutation,Stefan sinusoidal excitation is targeted. 6.6 Social and Ethical Considerations

There is strong evidence that climate change in modern times is a consequence of rising man-made 퐶푂2 emissions. Society is becoming more aware of its environmental footprint and there is an increasing need for, and interest in, sustainability. The automotive company Tesla, Inc. with their pioneering BEVs and simple chargingHaller concept gave a boost to fossil fuel-free personal transportation. In combination with strengthened political will, a shift to sustainable electric transportation is possible and in many

page | 55 Chapter 6. Discussion cases ongoing, as in Norway where already more than 60 % of new cars sold in September 2020 were electric. The environmental footprint of a BEV is nevertheless considerable and determined by the resources required during production, maintenance, usage and recycling. Efficient use of these resources by drivetrains is key. By increasing system efficiency, battery capacity can be reduced while maintaining2021 driving range. If novel low-voltage drivetrains can© offer increased efficiency with fewer resources, then the continued usage of high-voltage drivetrains forEV needs to be openly discussed. The results in this thesis justify further research into LVHC machines and drivetrains. While low-voltage systems are electrically intrinsically safe, high- voltage systems always involves the risk of a lethal electric shock for passengers and rescuing personal in case of an accident. Specially- trained personnel are required and strict safety guidelines need to be followed when carrying outby maintenance work onHV systems. Even dischargedHV batteries maintain lethal voltage, which means additional safety risks during handling and recycling must be managed. Stefan Haller

page | 56 Chapter 7 OUTLOOK AND CONCLUSION This thesis provides an introduction2021 into the field of battery-powered multiphase low-voltage© high-current machines, as well as their advantages and limitations. These machines are redundant and electrically intrinsically safe, as high voltage is not created at any point in the system during driving. High-power LVHC machines are a promising alternative for BEVs and HEVs, compared to high-voltage machines that require sophisticated and costly double isolation. This is indicated by this work and supported by the industry’s trend towards increased power levels using a 48 V system for automotive mild-hybrid applications [61, 62]. A 48 V 110 kWby42-phase machine in a fully functional car is demonstrated [59] and recently, a 48 V drive with a peak power of 80 kW for boats became available [58]. LVHC machines break with the century-old convention of reducing the current by increasing the voltage to reduce conduction losses. Instead, a high-current but low-voltage design has been investigated that aims to reduce the losses and costs while increasing safety. These machines can theoretically be built and would offer many advantages if a sufficiently low path resistance could be accomplished to compensate for the 퐼2푅 losses that scale quadratic with the current. Nonetheless, many practicalStefan details have to be considered and compromises will need to be made to construct such a machine.

7.1 Conclusion

This work shows that compact LVHC machines with integrated power electronics can be built. The focus of this work was the practical implementation of multiphase LVHC machines with integrated power electronics,withoutHaller providing an in-depth study of electrical machines. Instead, it aimed to show the possibilities and limitations of low- voltage high-current machines inEV applications.

page | 57 Chapter 7. Outlook and Conclusion

The first proof-of-concept machine confirmed that today’s high- current MOSFETs are no longer the limiting factor. The power electronics need to be integrated directly onto the machine to ensure a short end-winding. The historical choice of using an out-runner design complicated matters due to the limited space available. To cope with the space limit, the power electronics did not implement a half-bridge per phase.2021 Instead, two low-side CAN-FETs with a package of just 9.1 mm 7 mm 0.7 mm were used per phase. This ©× × solution only allows unidirectional stator current flow, but is sufficient as a proof-of-concept. To drive the CAN-FETs without current control in block commutation, a general purpose microcontroller and a CPLD for hardware-based monitoring and protection was used. To verify the machine’s FEM model, the air-gap flux density was measured with the hall-sensor setup designed in Paper I. A high-current design requires a complete system with very low resistance. Another machineby with a solid copper wire high fill-factor winding was therefore built. While the same power electronics was used, all high-current connections were now made from machined copper parts. To be able to detach the power electronics, the coil wires could not be soldered onto the PCB. Instead, the large cross- sectional area of the low-resistance coil wires enabled their use as copper connection bolts. Each coil wire was threaded, pushed through the PCB and locked by a copper nut. A low resistance detachable connection to the power electronics with a minimum number of contact interfaces was thus achieved. It was shown that a terminal- to-terminal resistance of 0.23 mΩ could easily be achieved for this small-scale 1.2Stefan kW 3 V machine, including power electronics. The air-cooled machine was successfully tested with drive currents up to 600 A under continuous operation on the test-stand. Concentrated windings are generally more sensitive to the armature reaction when compared to distributed windings. The properties and limits of the used power electronics topology at high speed and machine load resulted in a current shift in the parallel connected single-turn coils. A new power electronics with a half-bridge topology is suggestedHaller to rebalance the current shift, allow sinusoidal stator current and provide phase current control. This requires a more sophisticated PWM controller, like the investigated hybrid-controller page | 58 7.1. Conclusion with a DSC and CPLD. Depending on the number of PWM signals required, an FPGA implementation must be considered. Liquid cooling is required to increase the power and current density of the LVHC machine. Due to the low-voltage and large cross-sectional area of the stator winding, direct in-conductor cooling can be used. The parallel electrical connection of the coils simplifies a parallel hydraulic coolant2021 design. A W-shaped coil wire for the next generation© of a 48 V traction drive reached a continuous current density of 49.5 A/mm2. This is an increase by a factor of 7.4, compared to the second prototype. As coolant, EGW50/50 with 65 °C was used at the inlet, requiring a flow rate of 0.312 l/min at a pressure drop of 140 kPa. The W-shaped winding reaches a high copper fill-factor in the slot and thus provides low thermal resistance to the stator. In this manner it can also be used to extract heat from the machine’s stator. On the battery cell level, there is no difference between a high- voltage or low-voltage drive.by On pack level, a low-voltage system minimizes the isolation effort and increases reliability due to more parallel connected cells. While the connection from battery to drive is very short and allows a large cross-sectional area to handle currents of a few thousand amperes, fast charging is a challenge. A reconfigurable battery pack for 48 V discharging and 400 V charging could increase the acceptance of the LVHC drivetrain concept. The author’s research group is currently testing an initial prototype.

In conclusion, this work successfully demonstrates a pioneering machine designStefan thought to be disadvantageous or perhaps impossible a decade ago. The key enabling components are novel, cost-efficient high-current MOSFETs in combination with a low resistance design and in-conductor direct cooling. With the progress made in 48 V systems, the author expects two voltage levels as standard for future EVs: a 800 V HV system for high- performance cars targeting the MW-range, and a 48 V LV system for regular vehicles with a few hundred kW peak power, where the cost- to-performance ratio is important. For EVs, where battery and drive are at a very closeHaller distance, safety and efficiency are major concerns and redundancy is requested, the traditional use of high-voltage electrical machines needs to be reconsidered.

page | 59 Chapter 7. Outlook and Conclusion

7.2 Future Work

The LVHC prototype machines presented in this work serve as small- scale proof-of-concept. For a BEV or HEV traction drive, the concept needs to be scaled up to a 48 V design. Future work should investigate a next-generation high-power 48 V traction drive with2021 integrated power electronics. The current design stage© is shown in Figure 6.5. The PMaSR machine with an in-runner design should use the presented W-shaped coil with direct in-conductor cooling. A multiphase half-bridge-based power electronics should provide individual phase current measurement and control with bidirectional sinusoidal current flow. To increase the power density, liquid cooling techniques for the connections and PCB, such a boards with an aluminum core, should be further investigated. The use of EGW50/50by as coolant should also be verified for a direct in-conductor cooled low-voltage winding. To develop a better understanding of the observed sudden change in temperature gradient along the inner-cooled coil wire, further tests with the W- shaped conductor should be performed. Stefan Haller

page | 60 ACRONYMS

CCA cold cranking amps CCS combined charging system CPLD complex programmableby logic device DC direct current DSC digital signal controller

EGW50/50 ethylene glycol water 50/50 EMF electromotive force EV electric vehicle

FPGA field-programmable gate array HEVStefan hybrid electric vehicle HV high-voltage

ICE internal combustion engine IGBT insulated-gate bipolar transistor IM induction motor ISCAD Intelligent Stator Cage Drive

LFP lithium iron phosphate Li-IonHaller lithium-ion LIB lithium-ion battery

page | 61 Acronyms

LV low-voltage LVHC low-voltage high-current

MEB Modularer E-Antriebs-Baukasten MMF magnetomotive force MOSFET metal oxide2021 semiconductor field-effect transistor NdFeB© neodymium iron boron NEDC new european driving cycle NiMH nickel–metal hydride

OEM Original Equipment Manufacturer

PCB printed circuit board PM permanent magnet PMaSR permanent magnetby assisted synchronous reluctance PMDC permanent magnet DC POL point of load PWM pulse-width modulation

SCI squirrel cage induction SELV safety extra-low voltage SI silicon SIC silicon carbide SITF stator inter-turn fault SOC stateStefan of charge SRM switched reluctance motor

VSD variable-speed drive WLTCHaller worldwide harmonized light vehicles test cycle

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