SRM Drives for Electric Traction

GAECE (Grup d’accionaments elèctrics amb commutació electrònica). The group of

edited by Pere Andrada

SRM Drives for Elecric Traction

electronically commutated electrical drives is a research team of Universitat Politècni-
ca de Catalunya (UPC BARCELONATECH), which conducts investigation in four areas:
electrical drives, power electronics, mechanics and energy and sustainability. Regarding
electrical drives, research focuses on the development of new reluctance, permanent
magnet and hybrid electrical drives. The main goal of those electrical drives is the in-
tegration of the power converter/controller and the mechanical transmission, being
specially intended for the traction of light electric vehicles. That research is carried out
by using the analysis of finite elements, taking into account eco-design criteria, consi-
dering new materials and new control strategies.

Pere Andrada
edited by

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 SRM Drives for Electric Traction
edited by Pere Andrada

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First Edition: December 2019

© The Authors, 2019

© Iniciativa Digital Politècnica, 2019
Oficina de Publicacions Acadèmiques Digitals de la UPC
Jordi Girona 1-3,
Edifici K2M - Planta S1, 08034 Barcelona
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E-mail: info.idp@upc.edu

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DL: B 29167-2019
ISBN: 978-84-9880-817-9

This work is subjet to copyright. All rights are reserved by de Publisher to reproduction, distribu-
tion, public communication or transformation.
 Contents

7 Preface

9 Contributors

13 Chapter 1. Trends in switched reluctance drives for electric traction

Pere Andrada. GAECE, DEE EPSEVG UPC, Spain.

29 Chapter 2. Soft Magnetic materials for switched reluctance machine

Pere Andrada. GAECE, DEE EPSEVG UPC, Spain.

41 Chapter 3. PM to RSM rotor replacement study on traction machines.

Avo Reinap. Industrial Electrical Engineering and Automation. Lund University,
Sweden.

57 Chapter 4. Variable reluctance motors for automotive applications. The mod-

ular construction approach
Loránd Szabó. Technical University of Cluj Napoca. Romania

87 Chapter 5. Double-stator switched reluctance motors

H. Torkaman and Majid Asgar. Shahid Beheshti University. Iran.

107 Chapter 6. Definition of a strategy for an axial flux switched reluctance ma-
chine torque control
A. Egea, G. Ugalde, J. Poza. Machines and Automatics Mondragon University-Fac-
ulty of Engineering Arrasate-Mondragon, Spain.

123 Chapter 7. Novel in-wheel double rotor axial-flux SRM drive for light electric
traction
Pere Andrada, Balduí Blanqué, Eusebi Martínez, José I. Perat, José A. Sánchez,
Marcel Torrent. GAECE, DEE EPSEVG UPC, Spain.

143 Chapter 8. Axial-flux twin-rotor segmented-stator high-speed SRM for high-

performance traction applications
Francisco J. Márquez-Fernández. Div. Industrial Electrical Engineering and Auto-
mation, LTH Faculty of Engineering, Lund University, Lund, Sweden. Johannes H.
J. Potgieter, Dyson Ltd, Malmesbury, UK. Malcolm D. McCulloch. Energy and
Power Group, Dept. of Engineering Science, University of Oxford, Oxford, UK.
Alexander G. Fraser. McLaren Automotive Ltd, Woking, UK.

5
 Preface

Current environmental concerns are driving the international community into

forcing the development of low-emission (hybrid electric) or zero-emission (bat-
tery electric) vehicles that will replace conventional internal combustion engine
vehicles, which now constitute one of the main causes of urban pollution. Alt-
hough the number of battery electric and hybrid electric vehicles has maintained
an upward growth in recent years, important technological advances are neces-
sary for improving the performance of these vehicles. This is true not only for
developing batteries that guarantee greater autonomy, but also electric power-
trains that demand higher torque, power densities, and efficiency. Nowadays,
most powertrains use permanent magnet synchronous motors (PMSM), but au-
tomakers are intensifying their research into finding alternatives with less mass
of permanent magnets or even without them.

This book presents a compilation of works carried out by a group of researchers

who believe that switched reluctance motor (SRM) drives can serve as a viable
alternative to PMSM drives. Its main goal is to disseminate knowledge about a
number of new contributions that reinforce the suitability of SRM drives for E-
Traction.

The book is organized in following order. Chapter 1 briefly explains the basics
of SRM drives and describes their developments and achievements in electric
traction. Chapter 2 provides a survey of the trends in soft magnetic materials for
SRM with a particular emphasis on their applications for traction motors in elec-
trical vehicles. Chapter 3 evaluates and compares two electrical machines when
replacing a permanent magnet rotor of a PMSM for a rotor of a SRM. Chapter 4
provides a survey of the main modular variable reluctance motors. The investi-
gation focuses especially on the machines proposed for diverse automotive ap-
plications, including traction drives and auxiliaries. Chapter 5 is devoted to

7
SRM DRIVES FOR ELECTRIC TRACTION

double-stator SRM drives and presents the motor structure, geometry, design,
electromagnetic characteristics and experimental tests. This work demonstrates
that double-stator SRMs can produce more torque and power density with less
radial force, vibration and acoustic noise. Chapter 6 presents the torque control
strategy for an axial-flux SRM with a Labak configuration. Chapter 7 presents a
summary of the work developed on axial-flux SRM drives using a particular dis-
tribution of stator and rotor poles that results in short flux paths without flux re-
versal. In addition, it also describes an SRM controller developed especially for
the propulsion of light electric vehicles. Chapter 8 summarizes the work done on
a novel SRM concept, featuring a 2-phase axial flux topology with a segmented
stator and twin segmented rotors. The proposed topology is designed for high-
speed operations that aim for power density values within the range of 7 kW/kg.

This work would not be possible without the efforts and generosity of all who
have contributed to its development.

Finally, we would like to extend our gratitude to Ana Latorre and Jordi Prats of
the Iniciativa Digital Politècnica for their contributions in editing this book.

Pere Andrada
May 2020

This book is supported by Spanish Ministry of Economy and Competitiveness

(DPI 2014-57086-R)

8
 Contributors

Pere Andrada was born in Barcelona (Spain) in 1957. He received the M.Sc.
and the PhD. degrees in Industrial Engineering from the Universitat Politècnica
de Catalunya (UPC), Barcelona, Spain, in 1980 and 1990 respectively. In 1980
he joined the Department of Electrical Engineering, Universitat Politècnica de
Catalunya UPC, where he is currently University School full professor in the Es-
cola Politècnica Superior d’Enginyeria de Vilanova i la Geltrú (EPSVG). He is
member of the Electronically Commutated Drive Group (GAECE). His teaching
activities and research interests include design, modelling and control of electri-
cal machines and drives.

Majid Asgar is currently an Assistant Professor at Technical and Vocational

University, Tehran, Iran. He has authored over 50 technical papers, and holds
three issued patents. He has co-authored three books. His main research interests
include power electronics, electrical machines, and renewable energies.

Balduí Blanqué was born in Reus (Tarragona, Spain) in 1970. He received the
B.S. degree in Telecommunications, the M.S. degree in Telecommunications, and
the Ph.D. degree from the Universitat Politècnica de Catalunya (UPC), in Barce-
lona, Spain, in 1996, 1999, and 2007, respectively. Since 1996, he has been with
the Department of Electrical Engineering, Universitat Politècnica de Catalunya
(UPC), where he is currently Associate Professor in the Escola Politècnica Supe-
rior d’Enginyeria de Vilanova i la Geltrú (EPSVG). He is member of the Electro-
nically Commutated Drives Group (GAECE). His teaching activities cover digital
design and electronics applications and his research interests include modelling,
simulation and control of electrical machines and drives.

Aritz Egea received the degree in electrical engineering from the University of
Mondragon, Mondragon, Spain, in 2009 and the Ph.D. degree in electrical engi-

9
SRM DRIVES FOR ELECTRIC TRACTION

neering in 2012. Currently he is an Associated Professor in the Faculty of Engi-

neering, University of Mondragon. His current research interests include electri-
cal machine design and control and electromagnetic actuators.

Alexander G. Fraser graduated with a B.Eng. degree in Automotive Engineer-

ing in 2006 and has since specialized in the field of transportation electrification.
Between 2007 and 2014 he was employed by Protean Electric Ltd fulfilling a
variety of mechanical design, research and business development roles. Since
2014 Alexander has been employed with McLaren Automotive Ltd where he is
currently principal research engineer, responsible for delivering motor research
projects and systems/vehicle integration activities related to traction e-machines.

Malcom D. McCullock received the B.Sc.(Eng.) and Ph.D. degrees in electrical

engineering from the University of Witwatersrand, Johannesburg, South Africa, in
1986 and 1990, respectively. In 1993, he moved to the University of Oxford to head
up the Energy and Power Group, where he is currently an associate professor at the
Department of Engineering Science. He is active in the areas of electrical machines,
transport, and smart grids. His work addresses transforming existing power net-
works, designing new power networks for the developing world, developing new
technology for electric vehicles and developing approaches to integrated mobility.
He is the Founder of three spin-out companies arising out of his research activity.
He has more than 100 journal and refereed conference papers, and has been an
Invited Speaker at the World Forum in Tianjin, China, 2010, and the Hindustan
Leadership Conference, Delhi, India, in 2013.

Francisco J. Márquez-Fernández was born in Huelva (Spain) in 1982. In 2006

he graduated as a M.Sc. on Industrial Engineering with a major in Industrial Elec-
tronics from the University of Seville (Spain). He received his Ph.D. in Electrical
Engineering in 2014 Lund University in Sweden. Between 2014 and 2016 he was
appointed as Post-doctoral Research Assistant at the Energy and Power Group,
University of Oxford, UK. Since 2016 he is a Post-doctoral Researcher at the In-
dustrial Electrical Engineering Group, Lund University, and the Swedish Electro-
mobility Centre. His research interests are design and control of new topologies of
electrical machines and power electronic drives, applied to either renewable energy
systems or electric vehicles, electrical machine characterization methods, energy
management in EVs and HEVs and their interaction with the power grid.

Eusebi Martínez-Piera was born in Barcelona (Spain) in 1960. He received the

Engineer degree in Industrial Engineering from Universitat Politècnica de Cata-
lunya in 1984. He is currently an Assistant Professor, in the Department of Elec-
trical Engineering, Universitat Politècnica de Catalunya, in the Escola Politècnica
Superior d’Enginyeria de Vilanova i la Geltrú (EPSVG). He is member of the
Electronically Commutated Drives Group (GAECE). His teaching activities and
research interests include design and finite element analysis of electrical machi-
nes.

10
 Contributors

José Ignacio Perat was born in Tamarite de Litera (Huesca, Spain), in 1965. He
received the B.Sc., M.Sc. and PhD. degrees in Industrial Engineering from the
Universitat Politècnica de Catalunya (UPC), in 1989, 1998 and 2006 respectively.
He is currently an Associate Professor in the Department of Electrical Engi-
neering, Universitat Politècnica de Catalunya, in the Escola Politècnica Superior
d’Enginyeria de Vilanova i la Geltrú (EPSVG). He is member of the Electroni-
cally Commutated Drives Group (GAECE). His teaching activities and research
interests include power electronics and control of electric machines and drives.

Johannes H. J. Potgieter received the B.Eng. degree in electrical and electronic

engineering in 2008 and the M.Sc. (Eng.) and the Ph.D. (Eng.) degrees in electri-
cal engineering from the University of Stellenbosch, Stellenbosch, South Africa
in 2011 and 2014 respectively. Between May 2014 and May 2016, he was em-
ployed as a Post-doctoral Research Assistant at the University of Oxford, United
Kingdom. Currently, he is appointed as advanced motor drives engineer at Dyson
UK. His research interests include wind power generation technologies, hybrid-
electrical automotive drive train solutions and the design and optimization of per-
manent magnet and switched reluctance electrical machines.

Javier Poza was born in Bergara, Spain, in June 1975. He received the degree in
electrical engineering from the University of Mondragon, Mondragon, Spain, in
1999, and the Ph.D. degree in electrical engineering from the Institut National
Polytechnique de Grenoble, Grenoble, France. In 2002 he joined the Department
of Electronics, Faculty of Engineering, University of Mondragon, where he is
currently an Associate Professor.

Avo Reinap was born in Estonia in 1973. He received the Diploma degree in
engineering and the M.Sc. degree in power engineering from Tallinn University
of Technology, Tallinn, Estonia, in 1998 and 2000, respectively. He received the
Ph.D. degree in technology from Lund University, Lund, Sweden, in 2005. From
2005 to 2010, he was an Associate Professor in the Department of Electrical
drives and Power Electronics, Tallinn University of Technology and from 2007
to 2011 a Postdoctoral Fellow and since 2012 Associate Professor in the Division
of Industrial Electrical Engineering and Automation, Faculty of Engineering,
Lund University.

José Antonio Sánchez was born in Vilanova i la Geltrú (Barcelona, Spain), in

1953. He received the B.Sc., M.Sc. degrees in Industrial Engineering from the
Universitat Politècnica de Catalunya (UPC), in 1976 and 1998 respectively. He
is currently an Assistant Professor in the Department of Electrical Engineering,
Universitat Politècnica de Catalunya, in the Escola Politècnica Superior
d’Enginyeria de Vilanova i la Geltrú (EPSVG). He is member of the Electroni-
cally Commutated Drives Group (GAECE). His teaching activities and research
interests include industrial automation and fault diagnostic techniques for electric
machines and drives.

11
SRM DRIVES FOR ELECTRIC TRACTION

Loránd Szabó was born in Oradea, Romania, in 1960. He received the B.Sc. and
Ph.D. degree from Technical University of Cluj-Napoca (Romania) in electrical
engineering in 1985 and 1995, respectively. He joined in 1990 the Technical Uni-
versity of Cluj-Napoca (Romania) as a research & design engineer. Since 1999
he is with the Department of Electrical Machines and Drives, where he was a
lecturer, associate professor, and by now is full professor. He is also director of
Centre of Applied Researches in Electrical Engineering and Sustainable Devel-
opment (CAREESD) in the frame of the same university. Prof. Szabó's current
research interests include linear electrical machines, variable reluctance ma-
chines, fault tolerant designs, fault detection and condition monitoring of electri-
cal machines, etc. He published more than 260 papers. He received the 2015
Premium Award for Best Paper in IET Electric Power Applications. He is mem-
ber of IEEE since 2005, and in 2017 he was elevated to Senior member.Prof.
Szabó's home page is: http://memm.utcluj.ro/szabo_lorand.htm.

Hossein Torkaman is currently an Associate Professor of electrical engineer-

ing at the Faculty of Electrical Engineering, Shahid Beheshti University, A.C.,
Tehran, Iran. He has authored over 130 technical papers, and holds three issued
patents. He has co-authored four books. He has been distinguished with numerous
awards from university and organizations. He was elected as a senior member of
IEEE. His main research interests include electrical machines, power electronics,
and renewable energies.

Marcel Torrent was born in Menàrguens (Lleida, Spain) in 1965. He received

the B.Sc., M.Sc. and PhD. degrees in Industrial Engineering from the Universitat
Politècnica de Catalunya (UPC), in 1988, 1997 and 2004 respectively. He is cur-
rently an Associate Professor in the Department of Electrical Engineering, Uni-
versitat Politècnica de Catalunya, in the Escola Politècnica Superior d’Enginyeria
de Vilanova i la Geltrú (EPSVG). He is member of the Electronically Commu-
tated Drives Group (GAECE). His teaching activities and research interests in-
clude design, modelling and testing of electric machines and drives.
Gaizka Ugalde received the B.S. and the Ph.D. degrees in electrical engineering
from the University of Mondragón, Mondragón, Spain, in 2006 and 2009, respec-
tively. Since 2009, he has been with the Department of Electronics, Faculty of
Engineering, University of Mondragón, where he is currently an Associate Pro-
fessor. His current research interests includepermanent-magnet machine design,
modeling, and control. He has participated in various research projects in the
fields of lift drives and railway traction.

12
 1
1. Trends in Switched Reluctance
Motor Drives for Electric Traction
Pere Andrada

1.1. Introduction
The electrification of road vehicles is one of the most consistent initiatives for
achieving a clean, environmentally friendly and efficient transport system. Now-
adays, the worldwide stock of electric passenger vehicles, plug-in hybrid electric
vehicles (PHEVs) and battery electric vehicles (BEVs) exceeds 5.1 million, 45%
of the vehicles in China followed by the EU and USA. So far, Norway represents
the country in which electric cars have the highest market share [1]. Several sce-
narios exist in regard to the expected growth of electric vehicles: the most mod-
erate foresees global electric vehicle sales reaching 23 million and a stock of over
130 million vehicles by 2030; the more optimistic view of the EV30@30 cam-
paign hopes that the market share will reach 30% by this same date (excluding
two- and three-wheel vehicles), with 43 million in sales and a stock of more than
250 million vehicles.

In order to further boost this growth, technological improvements and cost reduc-
tions must be made not only to electric storage systems (mainly the batteries), but
also to electric propulsion systems or traction drives. Currently, most traction
drives for HEVs and BEVs are permanent magnet synchronous motor (PMSM)
drives, which are more common due to their high power, torque density and effi-
ciency, as well as their wide range of speed. The main drawback to PMSM drives
is their use of rare-earth permanent magnets. Although rare-earth elements are
not very scarce, some noteworthy circumstances to consider are:
• 50% of reserves are located in China.
• China additionally controls most (85%) of the world’s rare-earth perma-
nent magnet production.
• Demand is growing for permanent magnets in green industries, such as
wind generation and electric vehicles.
• Historical volatility in the price of permanent magnets, as in 2011.

13
SRM DRIVES FOR ELECTRIC TRACTION

Given these circumstances, it is advisable to reduce the mass of permanent mag-

nets in electric machines and perhaps even to dispense of them completely. Thus,
there is presently great interest in developing permanent magnet-less (PM-less)
electric drives, with the most relevant drives in this category being: induction
motor (IM) drives, synchronous reluctance motor (SyncRM) drives and switched
reluctance motor (SRM) drives. Fig. 1 shows a basic cross-section of these mo-
tors; and their main advantages and drawbacks are listed, respectively, in Tables
I and II [2-5].

Fig. 1: Cross-section of the three most relevant PM-less motors: IM (left), SyncRM (center) and
SRM (right)

TABLE I. ADVANTAGES OF THE PM-LESS DRIVES UNDER CONSIDERATION

IM drives SyncRM drives SRM drives
Rotor losses can be significantly Simplification of rotor construc- The simplicity of the stator and
reduced by using a copper rather tion because there are no magnets rotor construction helps reduce
than aluminum squirrel cage. or embedded conductors that have the cost of SRM machine manu-
to be inserted. facturing.
The drive-cycle efficiency of a
well-designed induction machine Opportunity to achieve low rotor SRM rotors typically have low
can pose a challenge to that of an losses in the absence of magnets moments of inertia compared to
IPM machine in some applica- and conductors in the rotor. other machine types, thus im-
tions. proving acceleration and decel-
The machine’s saliency makes it eration.
The magnetic field strength of a an attractive candidate for rotor
machine can be lowered via stator position self-sensing. It achieves high CPSR values
current to reduce losses under due to low magnetic coupling
light load conditions, which is val- The absence of magnets reduces between phase windings. SRM
uable for high-speed cruising. the SyncRM machine’s vulnera- fault tolerances are best among
bility to damage from short circuit the evaluated machines.
The absence of magnets reduces faults.
an induction machine’s vulnera- SRM machine efficiency dur-
bility to damage from short circuit The electronic power converter is ing-high-speed, light-load oper-
faults. the usual three-phase bridge in- ations do not suffer from any
verter used in AC machines. need for a stator current to can-
The electronic power converter is cel flux linkage.
the usual three-phase inverter used
in AC machines. The number of SRM phases can
vary from two to higher num-
bers by changing the number
combinations of stator and rotor
poles.
CPSR Constant power speed range
PWM Pulse wide modulation

14
 Trends In Switched Reluctance Motor Drives for Electric Traction

TABLE II. DRAWBACKS OF THE CONSIDERED PM-LESS DRIVES

IM drives SyncRM drives SRM drives

There are rotor losses when- Limits on achievable sali- The pulsed nature of stator
ever torque is generated. ency ratio values reduce winding excitation tends to
machine torque density in result in high torque ripple
It requires a small air-gap to comparison to permanent when compared with other
minimize the reactive cur- machines. machines.
rent and maximize effi-
ciency. Increasing the inductance High frequency radial
saliency ratio tends to re- forces on stator poles can
It is not adequate for high duce mechanical strength result in undesirable acous-
pole numbers, thus making due to more flux barriers tic noise due to excitation of
them less attractive for and increase pressure to re- stator mechanical resonant
lower speeds and direct duce air-gap length. modes.
drive applications.
It requires very high sali- The torque density and
CPSR is more limited than ency ratios to achieve the rated efficiency of SRM
internal permanent synchro- CPSR values required in machines are lower than
nous machines. many traction applications. those of permanent magnet
Increasing CPSR requires synchronous machines.
Achievable saliency ratio
reducing leakage reactance, values result in a low power The large amplitude of
thus generating additional factor, thus increasing the magnetic flux variations in
PWM losses. inverter power rating. rotor poles can lead to high
It is not well suited to rotor rotor losses at high speeds.
Not a good candidate for
position self-sensing at low high pole number designs It requires a small air-gap to
speeds due to the absence of and, therefore, for high maximize torque density.
rotor saliency. torque direct-drive applica-
SRM performance depends
tions.
on the accurate sensing of
rotor position and tight con-
trol of current pulse timing.
Each SRM phase winding
requires two external termi-
nals, thus doubling the
number of excitation cables
and connector pins in com-
parison to AC machines.
The SRM electronic power
converter does not use the
standard three-phase in-
verter that is usually em-
ployed by all AC machines.
Control characteristics are
very non-linear due to the
high magnetic saturation of
SRM.
CPSR Constant power speed range
PWM Pulse wide modulation

15
SRM DRIVES FOR ELECTRIC TRACTION

1.2. SRM Basics

Although SRM drives present drawbacks such as large numbers of connections,
the inability to use phase-leg power modules developed for inverters in SRM
power converters, high torque ripple and acoustic noise; they do have advantages
such as torque-speed characteristics that match very well with the needs of elec-
tric vehicle traction as well as a simple and robust construction. From the point
of view of electromechanical conversion, SRM is a doubly salient pole device
with single excitation that usually works under strongly saturated conditions
[6,8]. Based on the air-gap flux direction, a rotary SRM can be classified as radial,
axial or transverse flux [9]. In all cases, torque is produced by the rotor’s tendency
to move to a position that maximizes the inductance of the excited phase winding,
i.e., it aligns with the stator and rotor poles. Therefore, a power converter with
solid-state switches is needed to generate the right sequence of phase commuta-
tions and this requires knowing the rotor position. Figure 2 shows a block diagram
of an SRM drive.

Fig. 2: Block diagram of an SRM drive

The electronic power converter in an SRM fulfills two functions. The first is to
switch phases in the order established by the controller, based on the rotational
position. The second is to ensure the rapid demagnetization of the SRM phases.
The phase current in an SRM is unipolar so that, in principle, the switching can
be performed sufficiently by only one switch per phase (unipolar electronic
power converter). Once the switch is open, the demagnetization of the phase is
carried out through a freewheeling diode with a series resistor that applies a re-
verse voltage to the phase terminals in order to force cancellation of the current.
Nevertheless, the so-called asymmetric bridge power converter or classic con-
verter is the most usual, Figure 3. It consists of two switches per phase – generally
IGBTs – and the demagnetization circuit is completed by means of two diodes
that, when the switches are opened, apply a negative voltage to the phase with the
same value as the supply voltage. Although this converter uses a high number of
power components and drive circuits, it maintains independence between phases,
has a high fault tolerance, and offers many control options.

16
 Trends In Switched Reluctance Motor Drives for Electric Traction

Fig. 3: Asymmetric bridge power electronic converter with two IGBTs and two diodes per phase

In variable speed applications, the SRM operates in one of the following three
control modes: current mode (Fig.4); voltage mode (Fig.5); and single-pulse
mode (Fig.6). Generally speaking, the SRM is controlled in the low speed range
either by current control (hysteresis control) or by voltage control. The former
sets a current reference (Iref) and maintains the current within a given hysteresis
band, while the latter uses PWM with a variable duty cycle (D). In both cases, the
turn-on and turn-off angles, respectively, θo and θc, are maintained constant unless
it is necessary to optimize torque ripple, efficiency or acoustic noise. At high
speeds, single-pulse control is used with the voltage at the rated value in order to
adapt the conduction period (θo - θc) to the torque and speed requirements.

Fig.4: Hysteresis control with unipolar modulation or soft switching.

17
SRM DRIVES FOR ELECTRIC TRACTION

Fig.5: PWM control with unipolar modulation or soft switching.

Fig.6. Single-pulse control

Fig.7 shows the torque speed characteristic of an SRM for electric traction with
three operational zones. In zone I, torque is constant at low speed and is controlled
by hysteresis or voltage PWM. In zone II, medium speeds, the single-pulse con-
trol is utilized, the voltage is maintained at its rated value and the conduction
period is increased (no more than half a rotor pole-pitch), which keeps a constant
power. In zone III, high speeds, voltage remains at its rated value and turn-on and
turn-off angles are also maintained at determined value following the natural
characteristic. In addition, the torque decreases more or less with the square of
the speed. The control strategies for the different zones of operation are summa-
rized in Table II.

18
 Trends In Switched Reluctance Motor Drives for Electric Traction

Fig. 7: Torque-speed characteristic of SRM

TABLE II. CONTROL STRATEGIES

Zone Control Operation Variables Constants

Zone I Hysteresis Iref

Constant θ0 and θc
PWM torque D

Zone II Single-pulse Constant θ0 and θc V

power

Zone III Single-pulse Natural θ0 and θc

V

1.3. SRM for electric traction

A. Early achievements

SRM has always been associated with electric traction, ever since the first docu-
mented switched reluctance motor appeared in the period 1837-1840 and one of
its earliest applications (such as Robert Davidson’s design) powered a locomotive
on a section of railway between Glasgow and Edinburgh [6, 10]. However, the
rapid development of D.C. motors in the second half of the 19th century eclipsed
switched reluctance motors. The appearance of solid-state controlled switches in-
itiated a renewed interest in switched reluctance motors, but the modern era of

19
SRM DRIVES FOR ELECTRIC TRACTION

SRM did not begin until the late 70s of the last century, thanks to a research pro-
ject on battery powered electric vehicles carried out by the Universities of Leeds
and Nottingham and sponsored by Chloride Technical Ltd. [11, 12]. Later, due to
the renewed interest in electric vehicles at the end of the last century and the
beginning of the current one, several major automobile manufacturers (Toyota
[13], Daimler-Chrysler [14-15], Volkswagen [16], General Motors [17]) and a
research agency (CSIRO) developed radial-flux switched reluctance motors as
drive trains for their prototype electric cars [18]. Although some of these initia-
tives proved to be successful in test vehicles, they were not used in commercial
cars. Nevertheless, at that time, the commercial electric motorcycle EMB-LEC-
TRA [19] with a two-phase variable reluctance motor was launched and four 400
kW SRMs were used as traction motors in a LeTourneau L-1350 electric wheel
loader [20]. Two other advances were also noteworthy: the motor/generator for a
mild hybrid-electric powertrain that was developed by the SR drives® for the EC-
funded project ELMAS [21]; and the hybrid electric bus constructed by Green
Propulsion and SR drives® [22]. In recent years, great concern has emerged re-
garding the research and development of SRM in radial flux, axial flux and trans-
verse flux for electric traction applications. The major trends in this research and
development are reported below.

B. Radial flux SRM

In radial flux SRM, the air-gap flux is mainly in the radial direction relative to
the axis of rotation (see Fig. 8). This type of SRM usually has a cylindrical shape
with a stator and rotor that can be either internal (the most common) or external.
Great efforts are being made to improve SRM power and torque density in order
to match the values of PMSM [23-28]. One attempt is to maintain the conven-
tional structure of the SRM and use special soft magnetic materials with high
performance [29]. Others are based on building structures that increase induct-
ance in the alignment position and reduce it in the non-alignment position.
Among these stands out the SRM with its segmented rotor, which has obtained
good results [30-33]. It is also interesting to note the structures with higher num-
bers of rotor poles than stator poles [34, 35] and those with segmented stators, of
which several variants have been presented [35-40]. Recently, alternative pro-
posals have been made, such as the SRM double stator [41] and the SRM double
rotor [42], both of which have delivered very promising results. Another option
is the multistack (or multilayer) SRM [43], in which each phase of the machine
is wound in independent parallel salient pole stators while the rotors correspond-
ing to each stator shift between them at a determined angle. In order to facilitate
the construction of the machine and increase its fault tolerance, some of these
proposals are modular, i.e., they are built with independent yet common parts that
are assembled to form the complete machine [44]. The presentation of Land
Rover, at the 2013 Geneva Motor Show, of a range of new Defender electric ve-
hicles powered by SRM units that were developed and built by Nidec SR Drives

20
 Trends In Switched Reluctance Motor Drives for Electric Traction

Ltd., generated great expectations for applying SRM drives to electric traction
[45].

Fig. 8: Flux paths in a radial 6/4 SRM: (left) aligned position; (right) unaligned position

C. Axial flux SRM

In axial flux SRM, the air-gap flux runs mostly parallel to the axis of rotation (see
Fig. 9). The stator and rotor are parallel plates arranged perpendicular to the axis
of rotation. Some studies carried out on axial flux SRM demonstrate that this type
of machine can obtain higher torque density than do radial flux switched reluc-
tance machines. These improved features of the axial flux SRM are due to the
increased air-gap area, which depends on the diameter of the machine; while the
air-gap area in the radial-type machine depends on the machine’s length. Alt-
hough, Unnewher and Koch reported the first variable reluctance axial flux motor
as early as 1973 [46], it was not until recently that many authors have made im-
portant contributions to developing axial-flux SRM, as reported by Torkman [47].
Nevertheless, it is relevant to point out the following contributions that are of
interest for electric traction. Arihara et al. have presented the basic design meth-
odology for the axial counterpart of the classic radial flux SRM [48]. Murakami
et al., have studied the optimization of an axial-flux 18/12 SRM [49]. Labak et
al. have proposed a novel multiphase pancake-shaped SRM with a stator com-
posed of a series of C-cores, each with an individually wound coil perpendicu-
larly disposed to a rotor that is made of aluminum in which a suitable number of
cubes, the rotor poles, of high permeability material have been added. The torque
production in this machine results from the tendency of these cubes to align with
the two poles of an energized C-core [50]. Madahvan et al. have contributed to
developing the axial counterpart of a segmented rotor SRM in a machine with
two rotors and a stator with toroidal-type winding [51]. Bo et al [52] designed an
axial field SRM with a single teeth stator and segmental rotor. Recently, Potgieter
et al. [53] have released a high-speed rotor two-phase axial-flux segmental SRM.

21
SRM DRIVES FOR ELECTRIC TRACTION

Andrada et al. have presented a new axial flux-switched reluctance machine with
a particular distribution of stator and rotor poles that results in short flux paths
without flux reversal. Furthermore, they propose two different types of rotors:
one conventional [54] and the other segmented [55]. Some of the contributions
cited above describe the manufacturing problems of these machines and propose
using soft magnetic composite materials for building their magnetic circuits [53-
56].

Fig.9: Flux path, aligned position, of an axial-flux SRM

D. Transverse flux SRM

In this class of machines, the flux is transverse to the direction of motion and is
generated by the flow of a current through a toroidal coil that results in a homo-
polar magneto-motive force [57-60] (see Fig. 10). The phases are independent
and disposed axially, with each one being formed basically by a U-shape stator
with a toroidal (or hoop) coil along it and a rotor with the same number of salient
poles as the stator. The torque production as in a conventional SRM is based on
the rotor’s tendency to find the minimum reluctance path when the winding is
energized. Despite their cumbersome construction, the main advantages of these
machines are that the number of poles can be increased without reducing the mag-
neto-motive force per phase. What is more, due to the particular type and dispo-
sition of the windings, the copper volume is minimized and thus copper losses

22
 Trends In Switched Reluctance Motor Drives for Electric Traction

are reduced [61]. Therefore, transverse flux SRM features high torque density
and high efficiency at low rotational speeds, which makes it a good candidate for
direct drive automotive applications [63-65].

Fig. 10: Flux paths: a) in a radial SRM; b) in a transverse flux SRM [60]

1.4. Conclusions
Despite the advantages indicated in Table I and the research efforts made in over-
coming their drawbacks (which are outlined in the previous sections), SRM
drives continue to be just one more option for the propulsion of BEV/PHEVs, as
evidenced by the fact that, to date, they have not been used in any of the commer-
cially available BEV/PHEVs [7,66]. The underlying justifications for this evi-
dence are:

• Using soft magnetic materials that are different from the current silicon-
iron laminations can improve the performance of SRM and make their
construction possible, but they greatly increase the cost.
• New SRM structures increase power and torque density in radial, axial
and transverse flux machines, but they clearly penalize the most relevant
advantage of SRMs, namely their simplicity and robustness of construc-
tion – circumstance that have a clear negative impact on cost.
• Despite the great progress made in researching the causes of torque ripple
and audible noise while seeking solutions that reduce and mitigate these
to acceptable levels in both the mechanical design and electronic control
of SRMs, these advances are, until now, insufficient in meeting the stand-
ards of the automotive industry [67].

23
SRM DRIVES FOR ELECTRIC TRACTION

• There are currently no commercial controllers on the market that are in-
tended for switched reluctance motors [68].
• SRM drives are little known among engineers working in automotive in-
dustry. Furthermore, academia dedicates hardly any attention to them in
courses on electric drives.

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actions on Energy Conversion. Year: 2015, Volume: 30, Issue: 1; pp. 175-182.

25
SRM DRIVES FOR ELECTRIC TRACTION

[30] B.C. Mecrow, J.W. Finch, E.A. El-Kharashi and A.G. Jack. “Switched reluctance
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[32] J.D. Widmer and B.C. Mecrow. “Optimized segmental rotor switched reluctance
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tance machine configuration with higher number of rotor poles than stator poles:
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[36] J.R. Hendershot. “Switched reluctance brushless DC motors with low loss magnetic
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[37] J.R. Hendershot. “Polyphase electronically commutated reluctance motor". US Pa-
tent 4883999, Filed: August 15, 1988.
[38] C. Hancock and J.R. Hendershot. "Electronically commutated reluctance motor".
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isolated flux path reluctance machine”. 2012 IEEE Energy Conversion Congress
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[40] S. R. Mousavi-Aghdam, M. Reza Feyzi, N. Bianchi, M. Morandin, "Design and
analysis of a novel high-torque stator-segmented SRM". IEEE Transactions on In-
dustrial Electronics, Vol. 63, No. 3, March 2016, pp. 1458-1466.
[41] M. Abbasian, M. Moallem, B. Fahimi. “Double-Stator Switched Reluctance Ma-
chines (DSSRM): Fundamentals and Magnetic Force Analysis”. IEEE Transactions
on Energy Conversion, Vol. 25, No. 3, September 2010, pp 589-597
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sign, simulations and validations”. IET Electrical System in Transportation Vol. 6,
Iss. 2, 2016, pp. 117-125.
[43] E.S. Afjei and H. A. Toliyat. “A Novel Multilayer Switched Reluctance Motor”.
IEEE Transactions on Energy Conversion, Vol. 17, No. 2, June 2002, pp 217-221
[44] M. Ruba, I. A. Viorel, L. Szabó. “Modular stator switched reluctance motor for
fault tolerant drive Systems”. IET Electr. Power Appl., 2013, Vol. 7, Iss. 3, pp. 159-
169.

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[45] Land Rover Defender electric. http://www.srdrives.com/land-rover.shtml.

[46] L. E. Unnewehr and W.H. Koch. “An axial air-gap reluctance motor for variable
speed applications”, Jan./Feb. 1974, IEEE Transactions on Power Apparatus and
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[47] H. Torkaman, A. Ghaheri, A. Keyhani. Axial flux switched reluctance Machines: a
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[50] A. Labak, N.C. Kar. “Designing and prototyping a novel five-phase pancake-shaped
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[51] R. Madhavan and B.G. Fernandes. “A novel axial flux segmented SRM for electric
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[53] H.J. Potgieter, F.M. Márquez-Fernández, A.G. Fraser, M.D. McCulloch. “Design
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2015 International Symposium on Advanced Electromechanical Motion Systems

(ELECTROMOTION), pp.269-273.
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[62] J. Doering, G. Steinborn, W. Hofmann, "Torque, power, losses, and heat calculation
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[63] Bolognesi, "Design and manufacturing of an unconventional variable reluctance
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P1-P11.

28
 2
Soft Magnetic Materials for
Switched Reluctance Machines
Pere Andrada

2.1. Introduction
In 2011, the sharp increase in raw material costs for rare-earth permanent magnets
resulted in automakers seriously considering reductions in the mass of magnets
and even eliminating them entirely from powertrains for electric and hybrid ve-
hicles (EV/HEVs). This led to increased research in permanent magnet-less mo-
tors, such as: induction motors (IM), synchronous reluctance motors (SyncRM)
and switched reluctance motors (SRM) [1]. Although IMs and SyncRMs, mainly
the first ones, are strong competitors for the substitution of permanent magnet
synchronous motors in electric traction, SRMs still offer viable options in meet-
ing the challenge, above all due to their simple construction.

Much work is currently being dedicated to improving conventional SRMs and in

seeking new SRM topologies. In this regard, materials – especially soft magnetic
materials – play a key role in developing SRMs with higher values of
power/torque density and efficiency for E-traction. Some authors have presented
up-to-date research in developing soft magnetic materials for electric machines
[2-6] and even for use in SRMs [7]. This chapter provides a survey of the trends
in soft magnetic materials for SRMs, with special attention to their applications
for traction motors in electrical vehicles.

2.2. Requirements of soft magnetic materials for SRM application in E-traction

This section first describes the requirements of soft magnetic materials for SRMs,
then considers the demands of these materials in traction motors for electric ve-
hicles.

29
SRM DRIVES FOR ELECTRIC TRACTION

A. Requirements of soft magnetic materials for SRMs

As is well known, the instantaneous electromagnetic torque of an SRM is given

by the partial derivative of co-energy, W’, versus position, keeping current con-
stant, that is
 ∂W ' 
T =  (2.1)
 ∂θ i = cte
The co-energy is given by
I
W ′ = ∫ ψ di (2.2)
0

With ψ = ψ ( i,θ ) the magnetization curve.

In Fig.1, the magnetization curve for the end of conduction, point B, is repre-
sented by two straight lines: OA and AB. The magnetization curve for the una-
ligned position is represented by the line OC.

Fig. 1: Magnetization curves and energy conversion loop

The SRM is a single electromechanical converter in which part of the absorbed

electrical energy is converted into mechanical energy. This mechanical energy is
the area, W (OABCO), enclosed by the energy loop between the magnetization
curve for the unaligned position, the evolution of the current during the period of
conduction of one phase and the magnetization curve for the end of conduction
(see Fig.1). Therefore, the average torque, TAV, will be the product of this energy
by the number of times the cycle is repeated throughout a revolution [8], that is:
mN r
TAV = W (2.3)
2π

30
 Soft Magnetic Materials for Switched Reluctance Machines

With
m being the number of phases and
Nr being the number of rotor poles

The energy ratio, E, is the relationship between energy converted in mechanical

work and the energy supplied [9], taking into account the energy returned by the
motor to the supply, R(OB’BAO), is given by
W
E= (2.4)
W +R
A high energy ratio requires that low energy be returned to the supply. Ideally,
most of the energy supplied has to be converted into mechanical work, which
implies that line OA’ overlaps with the ordinate axis, while line A’B maintains
the same slope as line AB. The coordinate axes of Fig. 1, can represent on another
scale, the ordinate axis the magnetic induction, B, and the abscissa axis the mag-
netic field strength, H. Therefore, in addition to low core losses, the magnetic
material used to channel the flux in an SRM should have high permeability values
and high magnetic flux density of saturation.

B. Requirements of soft magnetic materials used in E-traction.

The usual torque-speed characteristic of a traction motor for EV/HEVs is shown

in Fig. 2. In the case of direct drive traction, demands of speed are no so high
thus the requirements of torque are even higher.

Fig. 2: Torque-speed profile of an EV/HEV and properties required for soft magnetic materials [10]

31
SRM DRIVES FOR ELECTRIC TRACTION

From Fig. 2, it is clear that high torque is required at low speeds (starting, accel-
erating, hill climbing), with copper losses being dominant. Therefore, soft mag-
netic material requires high magnetic flux density and high magnetic flux density
of saturation. At high speeds, the iron losses are dominant and, hence, the soft
magnetic materials must have: low iron loss at high frequency; high mechanical
strength; high thermal conductivity; and low magnetostriction in order to reduce
noise. Obviously, the soft magnetic material must be efficient in the speed range
at which the vehicle is most frequently driven [10].

Fig. 2 outlines the properties required for the soft magnetic materials used in
EV/HEVs. In summary, the requirements of soft magnetic materials to be applied
in SRM for E-traction are:
• High permeability
• High magnetic flux density of saturation
• Low losses at driven and at high speeds
• High mechanical resistance
• High thermal conductivity
• Low magnetostriction

In addition, the material must be able to easily take the shape of the stator and
rotor of the SRM being built.

2.3. Laminated soft magnetic materials

In rotary electric machines, the magnetic core is usually made by stacking sheets
of soft magnetic materials that have been previously cut in the shape required by
the machine’s magnetic circuit. The most usual cutting process is stamping, alt-
hough laser cutting is suitable for prototypes. The laminated soft magnetic mate-
rials most commonly used are alloys made of silicon-iron (SiFe), nickel-iron
(NiFe) and cobalt-iron (CoFe), in this category can be included the new dual-
phase magnetic materials.

A. SiFe

The first choice and the most often used material in electric machines for chan-
neling the magnetic flux is iron alloyed with some content of silicon, usually be-
tween 0.5 and 3%. There is SiFe in grain-oriented state (GO steel) or anisotropic,
with better magnetic properties in the rolling direction, and non-oriented state
(non-GO steel) or isotropic with the same magnetic properties in all directions.
Isotropic SiFe is used mostly in the manufacturing of electric machines. Non-GO
steels are available as both semi-processed and fully processed products. Semi-
processed non-GO steels require annealing after stamping to remove stress and
excess carbon. Furthermore, they are delivered without any superficial coating in
order to electrically insulate laminations from each other once stacked. Fully

32
 Soft Magnetic Materials for Switched Reluctance Machines

processed steels are delivered annealed and with the desired coating so that they
are ready for use without any additional processing. Low loss standard grades of
non-GO steels are always presented as fully processed laminations, with thick-
nesses of 0.35, 0.5, 0.65 or 1 mm.

In the design of SRMs for electric traction with high power/torque density and
low loss standard grades (EN 10106), such as M 270-50 A (see Fig. 3), either
M250-50A or M235-35A [11] are usually used. Some electric steel manufactur-
ers provide specific non-Go steels with better properties. ArcelorMittal offers its
iCARe® range of electrical steels, which includes iCARe® Save with very low
losses, iCARe® Torque electric steels with high permeability and iCARe®Speed,
electric steels for high speed rotors [12]. JFE Steel introduced distinctive non-GO
steels such as its JNETM series for energy efficient motors, JNPTM series for
high torque motors, the JNEHTM series for high frequency motors, and the
JNTTM series for high speed rotors [13, 14].

Fig.3: SiFe (M 270-50 A) stator and rotor lamination of an 8/6 SRM

In order to reduce eddy current losses in high frequency machines, interest is cur-
rently increasing in thinner laminations of thin gauge non-GO steel (EN10303)
with thicknesses of between 0.1 to 0.3 mm.

In addition, alloys with higher silicon content of up to 6.5% are being used. Alt-
hough the increase in silicon content has some drawbacks such as the reduction
of saturation and permeability as well as complications in the manufacturing pro-
cess due to increased brittleness and hardness, eddy current losses are reduced as
a result of their high resistivity. Hiyashi et al. have demonstrated that an SRM
built using SiFe laminations with 6.5% silicon content and Super CoreTM
10JFX900 [15] can achieve efficiency values that are comparable to interior per-
manent magnet synchronous machines (IPMSM) [16, 17]. Furthermore, J.W.
Jiang built a three-phase 24/16 SRM using this same material for a hybrid electric
powertrain that delivered very good performance [18].

Although grain-oriented SiFe is used in transformers, an SRM was recently built

using GO steel in the stator and rotor poles and non-GO steel in the rest of its
magnetic circuit, and it reportedly provided higher torque and lower losses than
the same motor built exclusively with non-oriented SiFe [19].

33
SRM DRIVES FOR ELECTRIC TRACTION

B. NiFe

NiFe alloys, commonly named permalloy, typically have a Ni content of between

40% to 50%. These alloys present very low coercivity that results in very high
permeability, but their main drawbacks are their low saturation and small mag-
netic flux density.

C. CoFe

CoFe alloys, commercially known as permendur, exhibit the highest saturation

levels (2.43 T). Typically, they have a Co composition of 49%, Fe of 49% and V
of 2%. They also have high mechanical strength, for which they are recom-
mended in the construction of high-speed rotor cores. Due to their high cost, their
use is limited to high performance machines. Some SRMs have been built with
CoFe alloys, such as the high speed 250 kW starter/generator system used for
starting and secondary electrical power extraction from an aircraft propulsion gas
turbine [20]. Also notable is the comparative study of two 12/8 SRMs with the
same dimensions but comprising different core materials: one with conventional
non-GO steel and the other with permendur. The experimental results showed
that the SRM made of permendur achieved higher torque and efficiency than the
conventional SRM [21].

D. Dual-phase magnetic materials

Recently dual-phase magnetic materials have been developed [3, 22], with these
materials having intermixed first and second regions, the first a magnetic region
and the second a non-magnetic region. Both regions contain only limited amounts
of carbon: less than about 0.05 % of the total weight. The second region includes
greater than 0.4 % of the total nitrogen weight. The material is prepared according
to the method described in [22]. From a magnetic point of view, using this kind
of material can facilitate the construction of segmented SRMs and maintain the
rotor pole saliency in conventional SRMs, while the air-gap remains constant and
thus reduces windage losses and torque ripple (see Fig. 4).

Fig.4: Dual phase lamination of a 6/4 SRM rotor: magnetic region (in grey) and non-magnetic re-
gion (in black).

34
 Soft Magnetic Materials for Switched Reluctance Machines

2.4. Amorphous magnetic materials

Amorphous metals are non-crystalline materials that have a glass-like structure.
The boron, silicon, and phosphorus alloys, as well as other glass formers with
magnetic metals (iron, cobalt, and nickel) have high magnetic susceptibility, low
coercivity and high electrical resistance. The conductivity of metallic glass is of
the same low order of magnitude as molten metal at just above the melting point.
The high resistance leads to low losses from eddy currents when subjected to
alternating magnetic fields. The amorphous magnetic materials are provided as
thin metal sheets (25 μm) in the shape of a rolled ribbon. These materials are used
in wound core transformers in order to reduce iron losses. Due to their thinness
and brittleness, their use in electrical machines is more cumbersome and they are
thus limited to cores with simple geometries. The core is usually made by punch-
ing-out or by laser cutting a layered block made of ribbons of amorphous mag-
netic material that are held together by an adhesive. Some switched reluctance
machines have been built using amorphous magnetic materials that achieve
higher values of efficiency than those using conventional SiFe alloys [ 23, 24].

2.5. Soft magnetic composites

The use of powder metallurgical materials provides all the advantages of powder
metallurgical production, such as low cost, tight tolerances, complicated forms
and minimum material waste [25]. Soft magnetic composites (SMC) are powder
metallurgical materials that can provide magnetic properties comparable to SiFe
laminations at medium and high frequencies [26]. SMC materials are made of
iron powder particles coated with an electrically insulating layer and can be for-
med into complex shapes by means of powder metallurgy, thereby allowing for
three-dimensional magnetic circuit designs that open up the possibility of buil-
ding axial or transverse flux motors – which would be impossible or prohibitively
expensive if using laminated soft magnetic materials. Nevertheless, complex de-
signs are restricted by the compacting process in order to get a uniform pressure
and as a consequence density in the piece. The mechanical properties of SMC
are lower than laminated magnetic materials. SMC materials have very high re-
sistivity, which means low eddy current losses. The main drawbacks of SMC are
its low permeability, as well as the fact that its flux density for the same magnetic
field strength is lower than that of laminated magnetic materials. Manufacturing
SMC consists of mixing iron powder with a binder, compacting the material at
high pressures (500-800 MPa) in a mold and then curing it at relatively low tem-
peratures of 200-650ºC. Most SMC manufacturers provide specific material for
prototyping; these are machined blanks that suffer little change from subsequent
machining processes [27]. Vijayakumar et al. [28, 29] compared the torque and
made a dynamic analysis based on the geometry of a 6/4 SRM built first using
non-Go steel (M-19) and then two types of SMC (Somaloy 500 and 1000). Many
authors have used SMC to construct new types of axial-flux SRMs [30-32] that

35
SRM DRIVES FOR ELECTRIC TRACTION

would not have been possible using laminated soft magnet materials. Fig.5 shows
the stator and rotor pole pieces of an axial flux motor made using SMC.

Fig. 5: SMC pieces of an axial flux SRM [31]

2.6. Conclusions
None of the materials described in the previous sections meet all the soft magnetic
material requirements for applying SRM in E-traction. Nevertheless, each one
stands out in some way. For example, NiFe alloys have the highest permeability;
CoFe alloys achieve the highest magnetic flux saturation while also standing out
for their mechanical properties; amorphous metals have the lowest iron losses;
SMC allows making magnetic circuits in complicated shapes; and the different
SiFe alloys provide a cheap solution in most cases. Table I lists some of the phys-
ical and magnetic properties of the soft magnetic materials that are most com-
monly used in these kinds of applications, while table II compiles their
corresponding iron losses. Undoubtedly, a CoFe alloy would be the best alterna-
tive for a radial-flux SRM that can handle the same speed characteristics as those
illustrated in Fig. 2. However, its high cost may discourage its use in favor of
6.5% non-GO steel. In the case of a direct drive motor, it would be better to opt
for specific non-GO steel for high torque. Given the complex construction of a
medium or high frequency axial-flux motor, the best option would be to use SMC.
There presently remains much to investigate, not only in terms of materials that
improve on the properties of those currently in use, but also in regard to materials
that can be additively manufactured [33].

36
 Soft Magnetic Materials for Switched Reluctance Machines

TABLE I. COMPARISON OF MAGNETIC CHARACTERISTICS

Thickness Density Resistivity B at 0.8 B at 2,5

Alloy Type
(mm) (g/cm3) (μΩcm) kA/m kA/m
M250-35 0,35 7.6 55 - 1.53
M250-50 0,5 7.6 59 - 1.55

SiFe Thin Non-Go 0,1 7.65 52 1.15 1.63

Non-GO Si 6,5%
0,1 7.42 82 1.29 1.40
10JNEX900 [15]

49% Co, 49% Fe, 2% V

CoFe 0.35 8.12 42 2.1 2.23
VACOFLUX 48 [34]

Amor-
Fe-based amorphous 0.025 7.18 130 1.38 -
phous
SMC Somaloy 700 3P [35] solid 7.57 20000 0.74 1.25

TABLE II. COMPARISON OF SPECIFIC IRON LOSSES

Losses Losses Losses

Alloy Type W/kg W/kg W/kg
1T/50Hz 1T/400Hz 1T/1kHz
M250-35 0.98 17.1 72.6
M250-50 1.02 23.4 113

SiFe Thin Non-Go 0.8 8.5 27.1

Non-GO Si 6,5%
0.5 5.7 18.7
10JNEX900 [15]

1.6 31 147
49% Co, 49% Fe, 2% V
CoFe (1.5 T/ (1.5 T/ (1.5 T/
VACOFLUX 48 [34]
50 Hz) 400 Hz) 1kHz)

Amorphous Fe-based amorphous 0.1 1.5 5.5

SMC Somaloy 700 3P [35] 5.0/6.1 45 132

37
SRM DRIVES FOR ELECTRIC TRACTION

2.7. References
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many approaches being pursued to minimize or eliminate rare earth magnets from
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[2] M.D. Calin, M.C. Georgescu, E. Helerea. Magnetic Materials for Electric Machines
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[3] A.M. El-Refaie. “Role of advanced materials in electrical machines”. 20th Interna-
tional Conference on Electrical Machines and Systems (ICEMS), 2017.
[4] N. Fernando, F. Hanin. “Magnetic materials for electrical machine design and future
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[5] A. Krings, A. Boglietti, A. Cavagnino, S. Sprague. “Soft magnetic material status
and trends in electrical machines”. IEEE Transactions on Industry Electronics, Vol
64, No 3, March 2017, pp. 2405-2414.
[6] A. Krings, M. Cossale, A. Tenconi, J. Soulard, A. Cavagnino, A. Boglietti. “Mag-
netic Materials Used in Electrical Machines: A Comparison and Selection Guide for
Early Machine Design”. IEEE Industry Applications Magazine, 2017, Vol. 23, No 6,
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[7] F. Rebahi, A. Bentounsi, A. Lebair, T.Benamimour. “Soft magnetic materials for
switched reluctance machine: finite element analysis and perspective”. 2014 Interna-
tional Conference on Electrical Sciences and Technologies in Maghreb (CISTEM).
[8] T.J.E. Miller. “Converter volt-ampere requirements of the switched reluctance motor
drive”. IEEE Transactions on Industry Applications, Vol 21, No 5, September/Octo-
ber 1985, pp 1136-1144.
[9] T.J.E. Miller. Switched reluctance motors and their control. Magna Physics Publish-
ing Oxford Science Publications, 1993.
[10] T. Wakisaka, S. Arari, Y. Kurosaki. “Electrical steel sheet for traction motor of hy-
brid/electrical vehicles. Nippon Steel Technical report, No 103, May 2013, pp. 16-
120.
[11] M. Besharati ; K. R. Pullen ; J. D. Widmer ; G. Atkinson ; V. Pickert. “Investigation
of the mechanical constraints on the design of a super-high-speed switched reluc-
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Electronics, Machines and Drives (PEMD 2014) 2014, pp. 1-6.
[12] S. Jacobs, D. Hectors, F. Henrotte, M. Hafner, M. Herranz Gracia, K. Hameyer, P.
Goes, D. Ruiz Romera, E.Attrazic, S. Paonelinelli. A. “Magnetic material optimiza-
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Cell Electric Vehicle Symposium Stavanger, Norway, May 13 - 16, 2009, pp.1-9.
[13] O.Yoshihiko, O.Tomoyuki,T. Masaaki. “Recent Development of Non-Oriented
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[14] H. Toda, and K. Senda, "Influence of various non-oriented electrical steels on motor
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 Soft Magnetic Materials for Switched Reluctance Machines

[15] JFE Steel Corporation, “Super Core,” 2015. [Online]. Available: http://www.jfes-
teel.co.jp/en/products/electrical/catalog/f1e-002.pdf. [Accessed: 23- Nov- 2019].
[16] H. Hayashi, A. Chiba, T. Fukao. “Efficiency comparison of switched reluctance mo-
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ing, 2007, pp.1-6
[17] A. Chiba, K. Kiyota, N. Hoshi, M. Takemoto, S. Ogasawara. “Development of a rare-
earth-free sr motor with high torque density for hybrid vehicles”.IEEE Transactions
on Energy Conversion. Year: 2015, Vol. 30, Issue: 1; pp 175 – 182.
[18] J. W. Jiang. Three-phase 24/16 switched reluctance machine for hybrid electric
powertrains: design and optimization. PhD thesis McMaster University, December
2015.
[19] Y. Sugawara, and K. Akatsu, "Characteristics of Switched Reluctance Motor using
Grain-Oriented Electric Steel Sheet", 2013 IEEE ECCE Asia, 3-6 June 2013 Mel-
bourne, pp. 1105-1110.
[20] E. Richter, C. Ferreira. “Performance evaluation of a 250 kW switched reluctance
starter/ generator”. IAS '95. Conference Record of the 1995 IEEE Industry Applica-
tions Conference Thirtieth IAS Annual Meeting, 1995, pp. 434 -440.
[21] E. Y. Hasegawa, K. Nakamura, O. Ichinokura. “Experimental verification of perfor-
mance of a switched reluctance motor made of permendur”. The XIX International
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[22] L. Cerrully, R. DiDomizio, F. Johnson. Dual phase magnetic material component
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[23] A. Chiba, H. Hayashi, K. Nakamura, S. Ito, K. Tungpimolrut, T. Fukao, M. Azizur
Rahman, M.Yoshida. “Test results of an SRM made from a layered block of heat-
treated amorphous alloys”, IEEE Transactions on Industry Applications Vol. 44, No.
3, May/June 2008, pp. 699-706.
[24] S. Ito, A. Chiba, T. Fukao, M. Yoshida, “A Test Results of a SRM made of layered
Block of Heat-Treated Amorphous”. IAS2005, Vol. 4, pp.2698-2703.
[25] M.J. Dougan.”Powder metallurgical materials and processes for soft magnetic appli-
cations”. Workshop on SRM an alternative for E-traction. Vilanova i la Geltrú Bar-
celona Spain. February 2, 2018, pp. 11-18.
[26] A. Schoppa, P. Delarbre, A. Schaetz. “Optimal Use of Soft Magnetic Powder Com-
posites (SMC) in Electrical Machines”. [Online] Available: https://www.pmgsin-
ter.com/scripts/PowderMet_2013_Chicago.pdf. [Accessed: 23- Nov- 2019].
[27] Höganäs, Somaloy prototyping material, SPM. [Online] Available: https://www.ho-
ganas.com/globalassets/download-media/sharepoint/brochures-and-datasheets-all-
documents/somaloy-prototyping-material_march_2016_1334hog.pdf. [Accessed:23-
Nov- 2019].
[28] K. Vijayakumar, R. Karthikeyan and R. Arumugan. “Influence of soft magnetic com-
posite material on the electromagnetic torque characteristics of switched reluctance
motor”. 2008 Joint International Conference on Power System Technology and IEEE
Power India Conference, 12-15 Oct. 2008, New Delhi, India.

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[29] K. Vijayakumar, R. Karthikeyan, S. Kannan, G. Prem Sunder and R. Arumugam.

"Dynamic Analysis of Switched Reluctance Machine using Soft Magnetic Compo-
site Material", 2010 Joint International Conference on Power Electronics, Drives and
Energy Systems & 2010 Power India.
[30] T. Kellerer, O. Radler, T. Sattel, S. Purfürst, S. Uske. “Axial type switched reluctance
motor of soft magnetic composite” Innovative Small Drives and Micro-Motor Sys-
tems 19-20 september 2013, Nuremberg, Germany, pp. 29-34.
[31] P.Andrada, B.Blanqué, E. Martínez, J.A. Sánchez, M.Torrent. “In wheel-axial-flux
SRM drive for light electric vehicles”. Workshop on SRM an alternative for E-trac-
tion. Vilanova i la Geltrú Barcelona Spain. February 2, 2018, pp. 39-46.
[32] J. H.J-Potgeister, F. Márquez-Fernández, A.G.Fraser, M.D. McCulloch. “Effects ob-
served in the characterization of soft magnetic composite for high frequency, high
flux density applications”. IEEE Transactions on Industry Electronics, Vol 64, No 3,
March 2017, pp 2485-2493.
[33] F. Wu, A. M. El-Refaie. “Towards fully additively-manufactured permanent magnet
synchronous machines: opportunities and challenges. 2019 IEEE International Elec-
tric Machines & Drives Conference (IEMDC), pp. 2225-2232.
[34] Soft magnetic cobalt-iron alloys Vacoflux and Vacodur. VAC Vacuumschmelze.
[Online] Available: https://vacuumschmelze.com/Assets/Cobalt-Iron%20Alloys.pdf. [Acces-
sed:23- Nov-2019].
[35] Höganas. Somaloy® 3P Material data. [Online] Available https://www.hoga-
nas.com/globalassets/download-media/sharepoint/brochures-and-datasheets---all-
documents/somaloy-3p_material-data_june_2018_2273hog.pdf. [Accessed:23-
Nov-2019].

40
 3
PM to RSM Rotor Replacement
Study on Traction Machines
Avo Reinap

3.1. Introduction
Transport electrification gains uninterruptedly interest as this is apparently one
of the promising ways towards sustainable, energy efficient and environmen-
tally friendly future. No pain, no gain – all electric drive system components
are under continuous research and examination in order to increase the power
density, efficiency, reliability while seeking for suitable materials and produc-
tion technologies for the sake of more affordable solutions and sustainable prod-
ucts. Since the transportation means motion, mechanical energy needs to be
present and unquestionably the electrical machine or machines take place in this
system. Is it matter of taste, how the vehicle looks like or what components are
used to achieve the desired unfailing functionality? Well, usually the prosperity
advances living environment, which can be exemplified by the abrupt change
of raw material price in permanent magnet (PM) in 2011 that concern is clearly
seen in publications and research headlines as a cost issue. Inopportunely the
environmental issues related to material engineering and technology develop-
ment are seldom if ever reflected in context of electrical machine design. Posi-
tively, the cost concern of PM materials has triggered research towards PM-less
machine topologies, e.g. reluctance machines (RM), and judge different options
for system development instead of selecting hastily a PM-machine topology in
order to increase simultaneously torque density, power density, efficiency and
so forth and on. Electrical machine design and engineering is multidisciplinary
wherever competency should not be replaced by effectiveness. Diversity of
electric drive system requirements can take functional advantages of the spe-
cific features that an explicit type of electrical machine can provide. As a matter
of fact, the implication on high speed and temperature capability, higher relia-
bility over wider constant power speed range favor reluctance type of machines
to any other types. This objectionable exaggeration is based to facts that
switched reluctance type of machines (SRM) are used in aircraft as generators

41
SRM DRIVES FOR ELECTRIC TRACTION

[1][2][3] and auxiliary drives [4], turbo machines [5] where desirable high
power density is not gained from increasing the torque per unit of machine ra-
ther than increasing the speed. Fascinatingly, RM types of machines have found
a way into wind turbines [6][7] as this low speed application challenges ma-
chine designers when looking for direct gearless, reluctance generators as pro-
spectively pm-less and fault tolerant solutions. These prototypes were designed
and evaluated 100-115 rpm at output load partly or completely fulfilling the
targeted 10 kW [6] and 20 kW [7]. However, vehicular electric propulsion ma-
chines and drives is application area where the low price can be more valuable
than the weight and size [8]. As a matter of fact, the collected experience on
machine modelling and development of reluctance type of machines [9] com-
pared to various design challenges and practical realizations [10] encourages
researchers working on reluctance machines and drives. The challenging design
aspects of RM, which reduces their ranking [11], is torque ripple and noise
which is directly connected to torque production and torque capability of these
type of machines. The main target of this chapter is road bound transportation
and traction machines, which could take advantage of less permanent type of
machines. The disposition of this work is not restudying what previously is done
rather than providing evaluation example of a PM and its’ RM counterpart with
the focus on torque production capability and quality when exploring the mag-
nitude and force distribution in the air-gap. The study starts from general ma-
chine construction, moves into the air-gap of selected machines, focuses on the
magnetic shear stress distribution along the gap circumference and compares
the torque production.

The objective of this chapter is not praising reluctance machines nor do com-
paring alternative PM-less machines rather than putting RM into content of PM
traction machines. This contextualization is based to replacement of PM to RM
rotor and comparison of torque capability. However, since the PM machines
need to operate over relatively wide speed range the feature is achieved either
by rotor design or usage of distributed concentrated windings. Furthermore, as
the stator layout takes advantage of inserted windings or core segments the ma-
chine layout reminds apparently switched reluctance machines. As a matter of
fact, constructional modularity and simplicity are often awaited features that
not only production technology but also faultless drive can take advantage on
behalf of machine topology selection and design. Some efforts are made do
demonstrate the torque producing force distribution in the air-gap that is more
concentrated peaks in RM machines compared to PM machines. However, the
machine specification and design are driven by the system requirements to-
wards application and likely there are many options, solutions and valuable can-
didates that should not be related only to torque capability or ripple.

42
 PM to SRM Rotor Replacement Study on Traction Machines

3.2. Machine topologies

When considering that origin of torque in any electrical machine is based to ex-
citation and reluctance torque or in combination of these two then the machine
topologies can simply divided into non-excited, temporal (electrically) excited
and permanent excited machines, where the first category can take advantage
only of reluctance torque. This nomenclature of none-excited and temporary ex-
cited is more unconventional as terminology of single-fed and double-fed ma-
chines. The purpose of recalling the machine topologies differently is to
distinguish machine types that are naturally capable to higher torque and the oth-
ers which are better suited operating over wide speed range. Obviously electrical
machines for transportation need to provide both, and not only, reliability and
fault tolerant operation are some of the features where RM becomes beneficial.

The topology classification is made in two levels according to

A. electromagnetic and geometric arrangement – excitation and saliency

B. electromagnetic mechanic interaction – more focus on machines with no
rotor excitation

A. Excitation and saliency

The geometric electromagnetic arrangement of electrical machines can be sorted

among none, single, or double-salient and single or double-fed machines. Table
I demonstrates machine layouts that have coils in stator and rotor and formation
of stator and rotor saliency that result double salient structure used in variable and
switched reluctance machines.

TABLE I MACHINE CLASSIFICATION ACCORDING TO GEOMETRIC

ELECTROMAGNETIC ARRANGEMENT
ROTOR SALIENCY

STATOR
SALIENCY

43
SRM DRIVES FOR ELECTRIC TRACTION

The geometric arrangement of radial flux machines (Table I) can have even dual
rotor or stator but this does not provide any obvious benefits and common for all
topologies the saturation limit to magnetic loading and temperature limit to elec-
tric loading are the main constrains to torque capability. Ideally, the torque would
be generated all around rotor belt, but practically the force distribution between
the machine parts is rather local and concentrated. The machine layouts (Table I)
can reveal a subjective guesstimate of tangential forces behind torque generation
and structural loads behind noise generation.

B. Electromagnetic mechanic interaction

Starting from control involvement and maximizing torque product, the reluctance
type machines RM are mostly known as variable reluctance, switched reluctance
and synchronous reluctance. It is just unfair to locate or limit (us) in the UK or
few references but the RM knowledge established by T.J.E. Miller [12], A.G.
Jack, and B. Mecrow [13], the continuous contribution by Z.Q. Zhu [14] provides
rather adequate overview, outline and challenges of the development on RM type
of machine related topologies. As a matter of fact, there are considerably more
engineers and researches who have contributed to RM machine and drive system
development. When considering machine construction then the common nomi-
nator of all these contributions is based to a geometrically undemanding salient
rotor core that takes advantage of simplicity, fault tolerance, high speed and high
(rotor) temperature operation. Fig. 1 gives an example of research interest and
demonstrates rotor alternation from conventional (in the left), to unconventional
(in the middle) and further on to eccentric (in the right). The RM layouts that
were irrational or unthinkable in 1980s have received intention this century
[15][16]. The eccentric RM has been an effort to merge cycloid gear directly to
electromagnecally excited ring gear [17] and if nothing else so has this “inven-
tion” triggered modal analysis of forces in order to understand the behavior of the
actuator.

Fig. 1: 6-tooth stator 4-tooth rotor 6S4R (left), centric 6S5R (middle) and eccentric 6S5R (right)

44
 PM to SRM Rotor Replacement Study on Traction Machines

Electromagnetic mechanic interaction in magnetic field is important aspect when

comparing or developing RM type machines. Radial magnetic force is recognized
as main contributor to the noise [18] and satisfactory operation over the wide
power speed range including the unbalanced magnetic force requires powerful
models to understand the complex rotor dynamic interactions [19]. The electro-
magnetic forces applied to machine cores and analysis of structural response
modes facilitate understanding the origin of noise generation and propagation in
RM [20]. The RM can be operated into deep saturation for the sake of maximizing
torque and consequently the over-forced salient structures are more tend to audi-
ble noise than cylindrical medium forced machines. RM machines appear in var-
ious stator tooth to rotor tooth configurations from reluctance Vernier machine to
a conventional few tooth configuration (Fig. 1 right). Similarly, the coil arrange-
ments appear from single layer to double layer winding (coils sharing the same
slot) and from single-fed to double-fed machine when considering that armature
coil and field coils are both integrated into the stator core to facilitate flux switch-
ing and variable flux topologies. The core development contributes also to RM,
as it can take advantage of c-cores, have portioned cores and integrate machines
towards magnetic gears. However, PM integrated RM types, where PM is part of
stator structure has the majority but RM with dual stator windings and related
research is well presented.

3.3. Core material selection

Since the RM can operate to high saturation and the gap length is a few % of
magnetizing path the high flux density magnetic cores become stimulating design
option. Soft magnetic material development towards higher speed (frequency)
gives obviously satisfactory results of reduced power losses [21] while higher
magnetic flux and torque capability may not result to expected upgrading on spe-
cific torque [22]. In this evaluation 6 different grades are compared (Table II).

TABLE II SOFT MAGNETIC CORE MATERIALS

Density Thickness Saturation Losses @1kHz

Grade Material
[g/cm3] [mm] [T] 1.5T [W/kg]
Vacoflux 48 Fe Co 8.12 0.1 2.3 38.0
Vacodyr 49 Fe Co 8.12 0.15 2.3 100
Vacodur X1 Fe Co 7.9 0.2 2.1 146
Arnon-5 Fe Si 7.65 0.127 1.9 78.7
HI-LITE NO10 Fe Si 7.65 0.1 1.8 83.7
M250-35A Fe Si 7.6 0.35 1.8 191

45
SRM DRIVES FOR ELECTRIC TRACTION

Initially the selected machines are evaluated using FeSi core M250-35A, whereas
RM machines are also evaluated by using FeCo core Vacoflux 48 and (cost) op-
timized FeCo core Vacodur X1. Material data are provided by material develop-
ers and producers [23][24][25][26].

3.4. Torque generation

The primary purpose of an electrical machine is to produce the torque – driving
or braking, which means that the question for electrical machine engineer is
where (in design), when (in control) and what conditions (in physics) the con-
struction provides the desired forces for torque. The exploration of flux density
distribution and the resulting magnetic forces improves the understanding on the
origin of forces and the capabilities and design issues of different machine topol-
ogies rather than developing comparisons between apples and pineapples. Usu-
ally, the comparison condition is made simple as the criterion is which machine
provides most torque (consequently also power) and lowest price at given space.
This work provides the same type of unfair comparison as it evaluates two differ-
ent stator layouts with two different types of rotors. The initial machines are “op-
timized” PM machines for use in traction application and they take advantages of
3-phase vector control. Meanwhile, the proposed RM counterpart uses a simple
switching sequence with one-phase energized only and the proposed rotor geom-
etry, which replaces the PM rotor, is not optimized to take advantage of SRM
drive or use sinewave currents from conventional voltage source inverter as PM
machine [27]. The torque generation analysis is done as following

1. Flux density distribution and resulting forces at a single rotor position is

compared to demonstrate the conditions and outcomes for comparison

2. Static torque characteristics are obtained at different rotor position and

current levels in order to compare low-speed torque capability of selected
topologies

3.5. Case studies

The selection of electrical machines for comparison are shown in Fig. 2 and fur-
ther specified in TABLE III. One of the initial machines (12-tooth 8-pole Fig. 2),
which is initially with interior permanent magnet (IPM) rotor, are discussed in
[28] whereas the investigation on cooling integration and thereby improving the
torque capability of the machines are presented in [29] (12T8P) and [30]
(18T16P). The number of turns per phase and maximum reference current is cho-
sen so that the selection of machines becomes somehow comparable and place
the current level in content of previous thermal analyses. 0.7 mm air-gap is used
in counterpart RMs.

46
 PM to SRM Rotor Replacement Study on Traction Machines

-B +A -A+C -B +A -A+C

+B -C +B -C
-C +B -C +B

+C -B +C -B
-A +A -A +A

+A -A +A -A
-B +C -B +C

+B -C +B -C
-C +B -C +B
+C -A +A -B +C -A +A -B

+C -B +C -B
-C-C +B+B -C-C +B+B
+C -B +C -B
+C -B +C -B
-C +B -C +B
+A -A +A -A

-A +A -A +A
-A +A -A +A

+A -A +A -A
+A -A +A -A

-A +A -A +A
+B -C +B -C
-B +C -B +C
-B +C -B +C
+B+B -C +B+B -C
-B+C -C -B+C -C

Fig. 2: 12-tooth 8-pole, 12T8P (top) and 18-tooth 16-pole 18T16P (bottom) machines

TABLE III SPECIFICATION OF 12-TEETH 8-POLE (LEFT)

AND 18-TEETH 16-POLE (RIGHT) MACHINES

Quantity, symbol [Unit] 12-teeth 8-pole (12T8P) 18-teeth 16-pole (18T16P)

Outer diameter, Do [mm] 246 240

Active length, H [mm] 27 200

Number of turns, N [turns] 4x11 6x7

Reference current, I [A] 300 300

Active weight, m [kg] 12 45

The active length means that the axial length of overhangs is excluded and in case of weight the
housing, shaft and bearings are excluded. The weight approximation is made for PM type of ma-
chine.

47
SRM DRIVES FOR ELECTRIC TRACTION

3.6. 12-Tooth 8-pole machine

This machine is used mainly as generator for light vehicles with some evaluation
presented in [28] and research on development of direct cooled windings is pre-
sented in [29].

A. Magnetic loading

The purpose of magnetic analysis is to visualize flux density distribution of the

loaded machine with different rotors (Fig. 3) and investigate tangential force dis-
tribution around rotor in the air-gap (Fig. 4). As the PM machine have two coils
excited and RM only one then the current is doubled in RM machine in order to
show that the machines have the same MMF. The magnetic loading is shown with
FeSi core (M250-35A), optimized FeCo core (Vacodur X1) and high flux density
FeCo core (Vacoflux 48) in Fig. 3. Fig. 4 compares the machines with FeSi core.

Fig. 3: Flux density distribution of IPMSM at 150 A at two phases (left) and SRM at 300 A at sin-
gle phase

2 1
σ t-PM
Magnetic shear stress σ (θ), [N/mm2]

1.5
σ t-RM
1 0.5
flux density, B(θ) [T]

0.5

0 0

-0.5
Bn-PM
-1 Bt-PM -0.5

-1.5 Bn-RM
Bt-RM
-2 -1
0 20 40 60 80 100 120 140 160 180
position, θm [deg]

Fig. 4: Flux density distribution along the air-gap from 0 to 90 mechanical degrees and resulting
tangential magnetic stress component (σt) of 12T8P machine

48
 PM to SRM Rotor Replacement Study on Traction Machines

The resulting torque for these machines at given rotor position is 33.9 vs 59.8
Nm, respectively for SRM and IPMSM machine. As the instantaneous value of
torque contains ripple the torque values for specific position are not clearly com-
parable. However, this comparison demonstrates the tangential flux and tangen-
tial force distribution as a character of torque production.

B. Torque capability

The focus is on low-speed torque of the previously presented RM. Fig. 5 demon-
strates static torque as a function of rotor position from un-alignment to align-
ment, from unstable equilibrium position to stable equilibrium position and the
currents at different level are kept constant. If the excitation sequence that con-
sists of 60° constant current pulses from θe={40°-100°}(Fig. 5) then the (static)
torque capability can be compared for the machines (Fig. 6)
50 0.06
400A

300A
40 400A 0.05
300A

flux linkage, ψ [Vs]

400A
30 0.04
torque, T [Nm]

400A 200A
200A

20 300A300A 0.03

10 200A 100A100A 0.02

200A

100A100A
0 0.01

-10 0
0 20 40 60 80 100 120 140 160 180
position, θe [deg]

Fig. 5: Static torque characteristics of 12T8P machine at constant current

current, I [A]
0 100 200 300 400 500 600 700
120 24
[kW]

PM:FeSi-0.35
dc

100 RM:FeSi-0.35 20
dc power losses in the winding, P
maximum torque, T [Nm]

RM:FeSi-0.13
80 RM:FeSi-0.10 16
RM:FeCo-0.20
60 RM:FeCo-0.15 12
RM:FeCo-0.10
40 8

20 4

0 0
0 5 10 15 20 25 30 35 40 45 50
2
current density, J [Arms/mm ]

Fig. 6: Torque as function of current related to current density and power losses

49
SRM DRIVES FOR ELECTRIC TRACTION

Fig. 5 compares FeSi (M250-35A) core to FeCo (Vacoflux 48) core where lighter
curves with smaller labels belong to high flux density material. Fig. 6 extends
this comparison to other core materials. Different cores can be distinguished by
the materials used (FeSi or FeCo) and lamination thickness between 0.10 and
0.35 mm. The remaining challenge is to establish a fair comparison condition and
to interpret the equal amount of current in PM and RM. PM machine consumes
3-phase currents followed by maximum torque per ampere (MTPA) line. RM has
1/3 duty or rectangular wave excitation, which means that the dc current levels in
Fig. 5 are adjusted by √3 for the same heating power. As Fig. 6 shows in this
comparison, the RM is capable of 40-50 Nm depending on core type at 300A
while PM machine would be capable of 80 Nm.

3.7. 18-Tooth 16-pole machine

This machine with surface mounted magnets (SPM) is used for traction in an off-
road vehicle. The evaluation of power losses and integrated direct cooled wind-
ings of this machine is presented in [30].

A. Magnetic loading

Similar to previous machine example, the flux density distribution of the loaded
machine with different rotors (Fig. 7) and investigation of tangential force distri-
bution around rotor in the air-gap (Fig. 8) are shown. The core material is changed
and presented as previously in order to study the benefits of replacing core of RM
machines for higher torque capability.

Fig. 7: Flux density distribution of 18T16P SPMSM at 150 A at two phases (left) and SRM at 300
A at single phase

50
 PM to SRM Rotor Replacement Study on Traction Machines

2 1
σ t-PM

Magnetic shear stress σ (θ), [N/mm2]

1.5
σ t-RM
1 0.5
flux density, B(θ) [T]

0.5

0 0

-0.5
Bn-PM
-1 Bt-PM -0.5

-1.5 Bn-RM
Bt-RM
-2 -1
0 20 40 60 80 100 120 140 160 180
position, θm [deg]

Fig. 8: Flux density distribution along the air-gap from 0 to 180 mechanical degrees and resulting
tangential magnetic stress component (σt) of 18T16P machine

The resulting torque for these machines at given position is 91 Nm and 281 Nm,
respectively for SRM and SPMSM machine. The first impression is that the dif-
ference becomes larger than for the previous case. This is acceptable since if the
previous machine took advantage of PM and RM excitation to maintain high
speed torque then this purely surface mounted PM machine takes advantage of
stator reactance for high speed operation. However, this reactance is not favor for
RM counterpart and the discrepancy between the machines with different cores
becomes larger. Similar to Fig. 4, Fig. 8 demonstrates the significance of shear
stress, which is the tangential pressure that produces torque, and which becomes
thornier and less distributed in case of RM.
150
400A
0.26
300A
400A
200A 0.24

300A 400A 0.22

flux linkage, ψ [Vs]

100 300A
torque, T [Nm]

400A 0.2
200A
300A
200A
0.18
100A
200A
0.16
50 100A
0.14
100A100A 0.12

0.1
0
0 20 40 60 80 100 120 140 160 180
position, θe [deg]

Fig. 9: Static torque characteristics of 18T16P machine at constant current

51
 SRM DRIVES FOR ELECTRIC TRACTION

B. Torque capability

Even if Fig. 9 shows noticeable increase in torque when replacing FeSi material
to FeCo material the improvement is insignificant to the initial machine with PM
rotor (Fig. 10). The static torque for 18T16S RM as a function of rotor position
from un-alignment to alignment is proceeded identically to previous calculation
shown in Fig. 5.
current, I [A]
0 50 100 150 200 250 300
600 12

[kW]
PM:FeSi-0.35

dc
500 RM:FeSi-0.35 10

dc power losses in the winding, P

maximum torque, T [Nm]

RM:FeSi-0.13
400 RM:FeSi-0.10 8
RM:FeCo-0.20
300 RM:FeCo-0.15 6
RM:FeCo-0.10
200 4

100 2

0 0
0 5 10 15 20 25 30
current density, J [Arms/mm2]

Fig. 10: Torque as function of current related to current density and power losses

Fig. 10 reveals that simple replacement of PM rotor against RM rotor results vast
loss of torque, which due to machine layout that has rather large stator reactance
compared to reluctance change that the relatively small rotor is able to produce.
These outcomes convince that simple replacement of machine components such
as machine parts or materials when replacing the core does not provide direct
revenue in machine performance.

3.8. Conclusions
This chapter evaluates and compares two electrical machines when replacing PM
rotor with RM rotor. The comparison is rather limited and underestimates the
double salient reluctance machines as the machine type is not specifically de-
signed for the designated design space and mission. However, as the rotor re-
placement demonstrates of these examples the torque capability can be reduced
by half or by factor 5! The latter is a machine where surface mounted magnets
keep up the magnetization of the stator with distributed concentrated winding that
have already a large inherent reactance for wide speed range capability. This
weakness of torque capability in SRM machines is changed when considering the
advantageous features of SRM drives such as wide speed range and more hazard-
ous conditions and fault tolerant drive.

52
 PM to SRM Rotor Replacement Study on Traction Machines

3.9. References
[1] V. Madonna, P. Giangrande, M. Galea, "Electrical Power Generation in Aircraft:
Review, Challenges, and Opportunities", IEEE Transactions on Transportation
Electrification, V 4 n 3, Sep. 2018, pp.646-659
[2] E. Richter, C. Ferreira, "Performance evaluation of a 250 kW switched reluctance
starter generator", Conf. Rec. 1995 IEEE Industry Applications, vol. 1, pp. 434-
440, 1995.
[3] A. V. Radun, C. A. Ferreira, E. Richter, "Two-channel switched reluctance
starter/generator results", IEEE Transactions on Industry Applications, V 34 n 5,
Sep. 1998, pp.1026-1034
[4] A. Boglietti, A. Cavagnino, A. Tenconi, S. Vaschetto, P. di Torino, "The safety
critical electric machines and drives in the more electric aircraft: A survey", 2009
35th Annual Conference of IEEE Industrial Electronics, Nov 2009, pp.2587-2594
[5] Ma Xiaohe et al., "Review of high speed electrical machines in gas turbine elec-
trical power generation," TENCON 2015 - 2015 IEEE Region 10 Conference, Ma-
cao, 2015, pp. 1-9.
[6] M. A. Mueller, "Design and performance of a 20 kW, 100 rpm, switched reluc-
tance generator for a direct drive wind energy converter", IEEE International Con-
ference on Electric Machines and Drives, 2005., May 2005, pp.56-63
[7] Y. J. Bao, K. W. E. Cheng, X. D. Xue, J. Chan, Z. Zhang and J. K. Lin, "Research
on a novel switched reluctance generator for wind power generation," 2011 4th
International Conference on Power Electronics Systems and Applications, Hong
Kong, 2011, pp. 1-6.
[8] A. G. Jack, B. C. Mecrow, C. Weiner, "Switched reluctance and permanent mag-
net motors suitable for vehicle drives – a comparison", IEEE, pp. 505-507, Jul.
1999.
[9] E. Bostanci, M. Moallen, A. Parsapour, B. Fahimi, "Opportunities and challenges
of switched reluctance motor drives for electric propulsion: A comparative study",
IEEE Trans. Transportation Electrification, vol. 3, no. 1, pp. 58-75, March 2017.
[10] S. Li, S. Zhang, T.G. Habetler, R.G. Harley, "Modeling, design optimization, and
applications of switched reluctance machines – a review", IEEE Trans. Ind. Appl.,
vol. 55, no. 3, pp. 2660-2681, May/June 2019.
[11] E. Ganev, "Selecting the Best Electric Machines for Electrical Power-Generation
Systems: High-performance solutions for aerospace More electric architectures.,"
in IEEE Electrification Magazine, vol. 2, no. 4, pp. 13-22, Dec. 2014.
[12] T. J. E. Miller, "Optimal design of switched reluctance motors", IEEE Trans. Ind.
Electron., vol. 49, no. 1, pp. 15-27, Feb. 2002.
[13] A. G. Jack, B. C. Mecrow, J. A. Haylock, "A comparative study of permanent
magnet and switched reluctance motors for high-performance fault-tolerant appli-
cations", IEEE Trans. Ind. Appl., vol. 32, no. 4, pp. 889-895, Jul. 1996.

53
SRM DRIVES FOR ELECTRIC TRACTION

[14] X. Y. Ma, G. J. Li,G. W. Jewell and Z. Q. Zhu, "Recent development of reluctance

machines with different winding configurations, excitation methods, and machine
structures", CES Transactions on Electrical Machines and Systems, V 2 n 1, March
2018, pp.82-92
[15] P. C. Desai, M. Krishnamurthy, N. Schofield, A. Emadi, "Novel switched reluc-
tance machine configuration with higher number of rotor pole than stator poles:
Concept to implementation", IEEE Trans. Ind. Electron., vol. 57, no. 2, pp. 649-
659, Oct. 2009.
[16] X. Liu and Z. Q. Zhu, "Stator/Rotor Pole Combinations and Winding Configura-
tions of Variable Flux Reluctance Machines", IEEE Transactions on Industry Ap-
plications, V 50 n 6, Nov 2014, pp.3675-3684
[17] A. Reinap, M. Alaküla and G. D. Olavarria, "Direct electromagnetic actuation on
high ratio gears," 2016 XXII International Conference on Electrical Machines
(ICEM), Lausanne, 2016, pp. 743-749.
[18] D. H. Cameron, J. H. Lang, S. D. Umans, "The origin and reduction of acoustic
noise in doubly salient variable-reluctance motors", IEEE Trans. Ind. Appl., vol.
26, no. 6, pp. 1250-1255, Dec. 1992.
[19] N. R. Garrigan, W. L. Soong, C. M. Stephens, A. Storace, T. A. Lipo, "Radial
force characteristics of a switched reluctance machine", Proc. 34th IAS Annu.
Meeting Conf. Rec. IEEE Ind. Appl. Conf., pp. 2250-2258, Oct. 1999.
[20] A. Dorneles Callegaro and J. Liang and J. W. Jiang and B. Bilgin and A. Emadi,
"Radial Force Density Analysis of Switched Reluctance Machines: The Source of
Acoustic Noise", IEEE Transactions on Transportation Electrification, V 5 n 1,
March 2019, pp.93-106
[21] O. Bottauscio, G. Serra, M. Zucca, M. Chiampi, "Role of Magnetic Materials in a
Novel Electrical Motogenerator for the More Electric Aircraft", IEEE Transac-
tions on Magnetics, V 50 n 4, April 2014, pp.1-4
[22] N. Fernando, G. Vakil, P. Arumugam, E. Amankwah, C. Gerada, S. Bozhko, "Im-
pact of Soft Magnetic Material on Design of High-Speed Permanent-Magnet Ma-
chines", IEEE Transactions on Industrial Electronics, V 64 n 3, March 2017,
pp.2415-2423
[23] “Soft magnetic cobalt-iron alloys”, Vacuumschmelze (available online)
[24] “Lamination Stacks made of VACOFLUX X1”, Vacuumschmelze (available
online)
[25] “Energy Savings with Thin Gauge for Motors, Generators, Transformers and In-
ductors”, Arnold Magnetic Technologies, (available online)
[26] “Arnon thin and ultra-thin silicon iron datasheet”, Arnold Magnetic Technologies,
(available online)
[27] X. Y. Ma, G. J. Li, G. W. Jewell, Z. Q. Zhu, H. L. Zhan, "Performance comparison
of doubly salient reluctance machine topologies supplied by sinewave currents",
IEEE Trans. Ind. Electron., vol. 63, no. 7, pp. 4086-4096, Mar. 2016.

54
 PM to SRM Rotor Replacement Study on Traction Machines

[28] M. Olszewski, “Evaluation of the 2010 Toyota Prius hybrid synergy drive system”
Oak Ridge National Laboratory, U.S. Department of energy, 2011
[29] A. Reinap, F. J. Marquez-Fernandez, M. Alaküla, R. Deodhar and K. Mishima,
"Direct Conductor Cooling in Concentrated Windings," 2018 XIII International
Conference on Electrical Machines (ICEM), Alexandroupoli, 2018, pp. 2654-
2660.
[30] A. Reinap, C. Högmark, F. J. Marquez-Fernandez, M. Alaküla and M. Andersson,
"Power Losses and Heat Extraction in a Stator with Directly Air-Cooled Lami-
nated Windings," 2018 XIII International Conference on Electrical Machines
(ICEM), Alexandroupoli, 2018, pp. 2675-2681.

55
 4
Variable Reluctance Motors for
Automotive Applications. The
Modular Construction
Approach
Loránd Szabó

4.1. Introduction
Nowadays, the automotive industry faces significant technological changes,
mainly connected to the inherent transit to hybrid and full electric cars. Beside
the integration of new drivelines based on electrical machines, the electrical drive
systems gain more and more importance also for the car auxiliaries, basically due
to the spread of x-by-wire (XBW) systems proposed for diverse vehicle controls,
as throttle, braking, steering, shifting, clutch, etc. All for these purposes ad-
vanced, low cost, high efficiency, fault tolerant electrical machines are needed,
which can operate also at high temperatures [1], [2].

For such automotive applications, variable reluctance machines (VRMs) are well-
fitted due to their various advantages. Their construction is very simple, robust
and reliable, as they do not have in their rotor windings or cages, only possibly
in some special cases permanent magnets (PMs). The passive rotor is generating
low quantity of heat; therefore, the cooling is straightforward. Generally, they
have high efficiency and strong overload capacity. Supplementary, they can be
easily integrated in digital systems, as being driven in an advanced way by means
of electronic controllers, which usually implies the need of position sensors.

As most of VRMs are PM free, they do not face the numerous problems related
to the nowadays use of rare earth PMs. The demerits connected these magnetic
materials are connected to their high prices, lower maximum allowable tempera-
ture and weak resistance to corrosion. Furthermore, there are several discouraging
economy-related matters in relation with them, as unpredictable price due to the
market monopoly of China, rapid depletion of ore reserves, etc. [3].

57
SRM DRIVES FOR ELECTRIC TRACTION

The VRMs have two basic configurations, the simple salient type or the double
salient type. In both variants, the VRMs may have distributed, concentrated or
homopolar windings, and eventually PMs placed in the stator or in the rotor.

VRMs work upon the minimum magnetic reluctance principle. Their passive sa-
lient rotor moves in such an angular position where the minimum reluctance
(magnetic resistance) is achieved, i.e. when the poles on the two armatures are
aligned. In this situation, the axis of the magnetic field produced in the stator will
match the axis of the magnetic flux passing through the rotor. As a result, the
magnetic field in the active poles and the inductance of the supplied coils are at
their maximum value [4]. Another of their benefits is that they can be built up in
an advantageous modular way [5], [6].

Upon the definition of the modularity, different parts of a system comprise of

smaller subdivisions (modules), which can be designed and manufactured sepa-
rately, and only later connected together to work as a sole unit [7]. In the case of
electrical machines, the modular approach means that their main parts (the stator
and the rotor) can have a segmental construction. This design not only enables
easy manufacturing and maintenance, and increased efficiency, but assures in
several cases also a high fault tolerance of the electrical machines [8].

The "modular motor" concept originally came out as a response to increasing

safety and reliability requirements for the electrical machines. First time it was
used in 1977 when a modular motor was patented by C.R. Carlson Jr. and
M.J. Hillyer [9].

The chapter is a survey of the main modular VRMs. The investigation is focusing
especially on the machines proposed to be used in diverse automotive applica-
tions, including the traction drive and the auxiliaries.

The greatest part of the modular VRMs which were developed upon the modular
approach are switched reluctance machines (SRMs). They can have segmented
stator or rotor iron core, but also several variants having both armatures modu-
larized are cited in the literature. The survey also covers other modular VRMs, as
of transverse flux type or doubly salient ones.

The final part of the chapter is an overview of the advancements brought by the
iron core segmentation to the developments in the field of electrical machines.

4.2. Modular switched reluctance machines

Due to the simplicity of the SRMs, they can be easily constructed modularly. By
the segmentation of the iron core, the magnetic flux in the machines can be in-
creased, resulting in higher torque. Moreover, due to the optimized (shortened)
flux paths, the iron core losses can be diminished.

58
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

The main challenge in designing such machines is to find an efficient way of

joining together the modules in a way as to maintain the robustness of the classi-
cal construction SRMs.

A. Variants with modular stator

In the case of modular stator type SRMs, the stator is usually made of several, C-
or E-shaped electrical steel laminated segments.
C. Lee made significant efforts to reduce by 20% both the copper and iron re-
quirements of a two-phase SRM by applying E-shaped stator iron core modules
(see Fig. 1) [10].

Fig. 1: Two supplying sequences of the modular two-phased SRM [10]

Each stator segment has three poles, two at its ends, with concentrated coils
wounded around, and one in the center, having double cross section and lacking
windings. The two modules of the stator can be joined together either by prefabri-
cated plastic molding or by a sleeve-type fixture. The SRM variant enables a very
efficient use of the iron core, since the middle pole is passed thru all the time by
magnetic flux, as it can be seen in the two supplying sequences shown in Fig. 1
[10].

Fig. 2: 9/12 poles modular SRM [11]

59
SRM DRIVES FOR ELECTRIC TRACTION

The above presented structure was improved by H. Eskandari et al. by increasing

the number of stator modules from two to three (as it can be seen in Fig. 2),
mainly to increase the generated torque and the mechanical strength of the stator,
and also to reduce its vibrations, an essential issue in automotive applications
[11].

The segmented stator SRM can be built up also with external rotor. A such variant
has 6, essentially E-shaped stator iron core modules, each with four poles, and a
solid 22-poles outer rotor (as it can be seen in Fig. 3) [12]. It was demonstrated
in the paper, that this SRM, designed for hybrid electrical vehicle applications,
can produce over 20% higher torque as its classical SRM counterpart.

Fig. 3: Outer rotor segmented stator SRM topology [12]

A segmented stator SRM with 16 stator and 14 rotor poles given in Fig. 4 was
proposed by M. Ruba et al. [13], [14].

Fig. 4: The segmented stator SRM [13]

Its stator is built up of 8 U-shaped iron core modules, as that in Fig. 5. The coil
is wound round the yoke. Two adjacent modules are separated by a non-magnetic
spacer, which both assure the adequate shift of the modules, and a good magnetic
separation. The rotor is a usual salient pole passive one.

60
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Fig. 5: A stator segment of the SRM given Fig. 4

The main advantages of this SRM developed for automotive applications are the
easy and cheap manufacturing, high fault tolerance [15], less iron losses, and the
opportunity of a fast on-site replacement of a damaged module in case of a coil
failure.

B. Segmental rotor approach

Several SRMs were developed with modular rotors. One of the first such con-
structions was patented by G.E. Horst [16]. The rotor of this two-phase unidirec-
tional SRM is built up of independent iron core segments, as it can be seen in Fig.
6. The main advantage of this SRM variant is its great torque density.

Fig. 6: The iron cores of a segmental rotor [16]

A newer, three-phase variant of the modular rotor SRM was proposed by

D.C. Mecrow (see Fig. 7). The 8 rotor modules are attached to a non-magnetic
steel shaft, which disables the circulation of the magnetic flux between the mod-
ules. The rotor segments require both circumferential location and retention
against axial and radial forces, which is a common shortcoming of the segmental
rotor constructions [17].

61
SRM DRIVES FOR ELECTRIC TRACTION

Fig. 7: Segmented rotor SRM [17]

This 12/8 poles SRM variant, as most of the modular rotor variants, has an im-
portant advantage over the classical design, that the flux path through the rotor is
short. Each magnetic flux path only encloses a single stator slot, as it can be seen
in Fig. 8. Thus, lower iron losses were achieved.

a) aligned position b) unaligned position

Fig. 8: Flux lines in the segmental rotor SRM [17]

In [17] the authors compared the SRM given in Fig. 8 with both the axially lam-
inated and the usual salient pole design. It was shown that higher torque per unit
magnetic loading can be produced with the modular variant due to its better mag-
netic utilization. When one phase is supplied, 8 of the 12 stator poles are active
(as compared with 4 in the case of a classical SRM). Hence the motor carries
much more magnetic flux, more torque is generated.

J.D. Widmer et al. demonstrated in [18] that segmented rotor SRMs construction
having a greater number of rotor segments than stator poles can lead to substantial
performance increasing, mainly at low speeds and current densities. In the paper

62
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

also, useful relationships are given for the optimal calculation of the basic stator
pole per rotor segment combinations, both for the SRM variants with fully-
pitched and single-tooth windings.

a) 3D view b) cross section

Fig. 9: Modular rotor axial flux SRM [19]

Also, the axial flux SRMs were developed with segmented rotor. Diverse such
constructions are cited in the literature. An interesting variant was proposed by
W. Sun et al. It has a closed cylindric construction [19]. The basis of both twin-
stators, as it can be seen in Fig. 9, is an iron core disc having 6 poles on it. Each
pole has a concentrated coil wound around. The magnetic fluxes generated are
directed from one stator to another via the rotor by means of two conducting rings,
one in the exterior of the stator and another in its interior. The rotor of the SRM
comprises of 4 pie-shape cross section, equally shifted iron core segments. Due
to this compact construction the SRM is very robust, its coils being totally en-
closed inside the machine. Thus, they are well-protected, but their extra heating
can be expected, too

Fig. 10: Inner stator modular axial flux SRM [20]

63
SRM DRIVES FOR ELECTRIC TRACTION

A modular axial flux SRM with interior stator was proposed by R. Madhavan et
al. As it can be seen in Fig. 10, the stator of the machine has poles on both of its
faces, and the axis of the toroidal windings located on it are on the radial direc-
tion. The machine is closed on its both ends by two retaining disks. On the shaft,
between these disks and the inner stator, are placed the two rotors comprising of
a non-magnetic disc having embedded simple rotor iron core segments [20].

If the rotor segments of the axial flux SRM are fully inserted inside the non-mag-
netic material, high-speed applications are enabled. A simple such SRM with a
single stator is proposed in [21]. Another specially constructed axial flux SRM
variant was developed especially for automotive traction applications [22].

C. Doubly segmented construction

A doubly segmented SRM, having both segmented stator and rotor, was proposed
by M. Diko et al. (see it in Fig. 11) [23]. Its stator comprises of E-shaped modules
with a concentrated coil on their yoke, like that given in Fig. 5. The rotor has
segments as the SRM in Fig. 7.

Fig. 11: Doubly segmented SRM [23]

The three-phase machine has a 12/8 poles structure. It is water-cooled by means

of a spiral and is proposed to be used in an electric mini truck. It has very short
magnetic flux paths and increased fault tolerance.

In [24] X. Xue et al. proposed an axial flux SRM segmented on both armatures,
having a yokeless rotor (see it in Fig. 12). Its stator is built up of 8 C-shaped
modules, each having two coils. The 10 rotor poles are connected to the shaft by
simple support branches. Thus, the rotor had low mass and inertia, being well-
suited for applications requiring good dynamic behavior, as also some of the au-
tomotive applications.

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Fig. 12: Doubly segmented yokeless rotor Fig. 13: The pancake shaped doubly modular
SRM [24] SRM [25]

A. Labak and N.C. Kar developed a similar, but axial flux SRM. Its stator com-
prises of 16 C shaped iron core modules, each having a concentrated winding on
it. The active parts of the rotor are the so-called cubes, embedded in an aluminum
disc having the shaft in its middle, as it can be seen in Fig. 13. Due to its optimized
flux paths, it has high efficiency, and thus it is proposed for electrical vehicle
powertrain applications [25].

A very compact in wheel axial-flux segmented SRM was proposed by the re-
search team lead by P. Andrada (see Fig. 14) [25].

Fig. 14: The in wheel segmented rotor axial SRM [26]

As it can be seen in the figure, the stator is made of wedge-shaped iron core mod-
ules with concentrated coils wound round them. The segments are kept together

65
SRM DRIVES FOR ELECTRIC TRACTION

by a non-magnetic structural disk. The stator is sandwiched by two specially seg-

mented rotors, also placed on non-magnetic support plates. The iron core seg-
ments are made of soft magnetic composite (SMC). The SRM has very short
magnetic flux paths without flux reversal. It was particularly developed to direct
drive light scooters or motorcycles.

An axial flux doubly segmented SRM was also developed by Z. Pan et al. [27].
Each of the 12 C-shaped stator modules, placed directly on the housing, have two
together connected coils. The rotor comprises of 8 equally shifted active segment,
as it can be seen in Fig. 15. The highly compact machine has good efficiency due
to the low iron and copper losses.

Fig. 15: Doubly segmented axial SRM [27]

Researchers at University of Oxford developed a two-phase, axial flux, high

speed SRM with modular stator and twin segmented rotor [28]. The stator mod-
ules are of wedge-shape, and each one has wound round a concentrated coil. The
segmental rotor is given in Fig. 16. All its iron core segments are made of SMC.
As the machine can achieve high speed, it can be applied in hybrid electrical sport
car traction application.

Fig. 16: The rotor of the high-speed axial flux doubly segmented SRM [28]

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

More complicated, having a lot of extra iron core, but probably more robust, is
the SRM variant proposed by W. Ding in [29]. This axial flux doubly segmented
SRM is a further development of that presented in [30], which had simpler E-
shaped stator iron core modules. The SRM given in Fig. 17 has 6 E-shaped stator
segments, each with 2 concentrated coils wound on its yoke. The rotor is also
segmented in a similar way as the stator, as the poles cross section concerns (the
middle pole has double cross section area as those at the ends). In the paper, the
proposed SRM was compared with a classical construction SRM and with a sim-
ilar axial flux segmented stator variant but having non-segmented rotor. All of
them had the same external diameter and axial length. The variant in discussion
had the lowest iron mass, the highest torque, and the greatest power and torque
density [29]. Therefore, it can be a very good competitor for the electrical ma-
chines used in automotive applications.

Fig. 17: The axial flux doubly segmented SRM [29]

This machine, as also that proposed in [30], can be extended on the axial direction
by adding more poles or modules to fit the SRM to any power requirement.

D. Multilayer SRMs

Upon another approach, first proposed by E.S. Afjei et al., the segmenta-
tion of the SRM can be performed on the axial direction [31]. The sug-
gested multilayer SRM has three magnetically independent modules
(phases, or so-called layers), each of them being similar to a classical
SRM. A specific feature is that the machine has identical number of poles
(eight) on both armatures. All the coils on a module are connected together
and form a phase of the machine. The rotor modules are shifted by 15º
relatively to the neighbored modules (as it can be seen in Fig. 18).

67
SRM DRIVES FOR ELECTRIC TRACTION

Fig. 18: The iron cores of the three-layer SRM

The torque ripple of the machine is significantly reduced due the same effect as
in the case of skewed rotor EMs [32]. Supplementary, due to the achieved phase
independency, in its control larger phase advancements can be imposed, which
are enabling a faster phase current rise up, an important issue in high-speed auto-
motive applications.

Upon this approach, A. Salimi et al. developed a four-layer SRM [33]. Its stators
are placed in line, while the rotor ones are shifted by 11.25º, as it can be observed
in Fig. 19.

Fig. 19: The rotor of the four layers SRM [33]

In [34] a similar 3-phase, 7-layer SRM is proposed. Each 6-poles stator layers are
also placed un-shifted, while the seven 4-poles rotor layers are shifted by 12.7º.
Due to the great number of modules of this construction, at the same time three
layers are active (having supplied coils).

68
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

E. Hybrid modular SRMs

In the literature, there are several proposals to improve the performances of SRMs
by inserting PMs, mainly in their stators. The hybrid SRMs having stator modules
containing both coil and PM are combining the advantages of the PM excitation
and the simplicity of the VRMs construction.

One of the simplest ways to perform this was by replacing the non-magnetic spac-
ers from the modular SRM given in Fig. 5 with ceramic PMs, as shown in Fig. 20.

Fig. 20: The placement and magnetization of PMs [35]

As it can be seen in the figure, the PMs are magnetized alternatively, the magnet-
ization direction being perpendicular to their face in contact with the poles of the
stator segment. In [35] it was concluded, that the obtained mean torque of this
SRM is greater by near 27% as in the case of the modular SRM considered as
starting point (see Fig. 4). Unfortunately, also the torque ripples intensified with
near 25% [35].

The stator segments in the hybrid SRM developed by P. Andrada et al. are con-
taining two series connected concentrated coils wound round the poles and a PM
between the pole legs, as it can be seen in Fig. 21 [36]. The rotor of this SRM is
a usual passive one.

Fig. 21: The modular stator hybrid SRM [36]

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SRM DRIVES FOR ELECTRIC TRACTION

The working principle can be understood from Fig. 22. When rotor poles are to-
tally aligned with the poles of a stator module (as in the case of module A in
Fig. 16), the magnetic flux generated by the PM is passing thru both the iron core
and the rotor. When the rotor poles are in an un-aligned position, only a very
small amount of the PM flux can pass thru the rotor due to the great magnetic
reluctance. The magnetic flux generated by the coils can control the distribution
of the PM flux in a way to put in movement the SRM.

Fig. 22: Flux distribution in a modular stator hybrid SRM

Similar hybrid SRMs were also proposed by Ding et al. [37], [38].

Hybrid SRMs, beside their increased fault tolerance, have higher efficiency due
to its PMs, and can be rated at higher power than conventional SRMs of the same
size. Despite the PMs in their structure, such electrical machines lack cogging
torque due to the placement of the PMs. All these make them attractive also for
automotive applications [36].

4.3. Modular synchronous reluctance machines.

The synchronous reluctance machine (SyncRel) seem to be the most accepted
VRM both by the manufacturers and users. Several companies (as ABB and Sie-
mens) are series manufacturing such machines in a wide range of power. Their
popularity is due to their numerous advantages, mostly common to all the VRMs,
as cheap construction, high efficiency, good dynamics, low maintenance costs,
etc. Therefore, several of their developments are targeting the automotive appli-
cations, too [39].

The stator of the SyncRel is identical with that of any usual three-phase a.c. ma-
chines. Therefore, the stator segmentation approaches used in the case of the PM
synchronous machines (PMSMs) can be also applied for them.

A typical stator modularization of a three-phase a.c. machine (concretely pro-

posed for a PMSM) is given in Fig. 23 [40].

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Fig. 23: Stator modules with concentrated windings of an a.c. machine [40]

The stator iron core is designed in a way as to minimize the scrap of laminated
steel material, and to obtain large tooth width and small tooth tips. Three teeth
with the concentrated coils around them comprise a module. By using six such
modules a three phase 18-slot stator can be assembled. This segmented stator
construction has diverse advantages, as that the concentrated windings are rigidly
packed with a high slot fill factor. Supplementary, these a.c. windings have short
end-coils, which makes them superior to the common distributed windings [40].

SyncRel stators can be also made of salient pole segments of one pole pitch
length, having around non-overlapping fractional slot windings with significant
fill factor and short end windings, as it is shown in Fig. 24 [41]. This design con-
tributes to an increased power density and efficiency, strongly compulsory in all
the automotive applications.

Fig. 24: Modular stator SyncRel [41]

The salient poles of the SyncRel, shown in Fig. 25, can be also made of SMC
[42]

71
SRM DRIVES FOR ELECTRIC TRACTION

.
Fig. 25: Stator module made of SMC [42]

Since on each module a simple concentrated winding can be placed, its manufac-
turing is quite simple. This modular construction approach permits a high flexi-
bility in phase number and control strategy selection, too. Due to the flexible
SMC technology, special pole shoe and yoke shape designs can be applied. Also,
a very good air-gap space usage can be achieved, which can lead to a high torque
density, a basic requirement in automotive applications.

In the most widely spread variants of the SyncRels the laminations made rotor is
passive and have a strong distributed anisotropy due to flux barriers and bridges,
which must have optimized shapes for achieving high performances [4]. Even if
their rotor is the most sensitive part from mechanical point of view, there are
some developments considering the modular approach for them. As S. Taghavi
and P. Pillay proposed in [43], the SyncRel rotor is comprising of single-pole
laminations particularly bonded together as to have an adequate mechanical
strength. The segments of the machine are asymmetrically designed, since the
two sideway arms of the module do not have the same thickness (as it can be seen
in Fig. 26). As a consequence of the overlapping neighbored layers, this design
allows a strong lamination bonding.

Fig. 26: Segmented rotor SyncRel [43]

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

4.4. Other modular variable reluctance machines

Several other VRMs must or can be built upon the modular construction ap-
proach. Next some such machines are taken into survey.

A. Transverse flux electrical machines

The basic features of the transverse flux machines (TFMs) are radically differing
from those of usual electrical machines, due to the transverse direction of their flux
paths and the homopolar magnetomotive force (mmf) generated by their ring-type
coils. One of the major advantages of these machines is the high torque density.
Supplementary, despite of their complicated construction and low power factor, the
number of poles can be increased and meanwhile the synchronous speed dimin-
ished without reducing the mmf per pole [44]. All these features make the TFM a
promising candidate for automotive (especially in-wheel) applications.

Fundamentally, these machines due to their complex three-dimensional magnetic

flux path, can be constructed only upon the modular approach [45]. Usually their
iron core segments are made of SMCs and are embedded in light non-magnetic
materials, as glass or carbon fibre reinforced plastics, consequently reducing the
overall mass of the machine and improving its dynamics [46].

The simplest TFMs, without any PMs, are of reluctance type, having less power
density as those with PMs [47]. A typical such variant is given in Fig. 27a. The
machine has U-shaped stator iron core segments, with the two poles of each mod-
ule in the same plane. In the figure the ring-type armature winding, the segmented
passive rotor, and the zoom of two pole pitch length segments can be easily ob-
served, too.

a) reluctance type TFM b) flux concentrating PM TFM

Fig. 27: Two modular TFM variants [48]

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SRM DRIVES FOR ELECTRIC TRACTION

An example of the more typical, flux concentrating PM TFMs is shown in

Fig. 27b. A phase of an armature winding is comprising of two ring-shape coils
per phase. The armature flux direction has to be in accordance with the PM mag-
netic flux sense. Therefore, the two coils of the same phase are connected in series
in a way as to have their magnetic fluxes added.

In both cases, for a three-phase TFM three correctly shifted units must be attached
to a common shaft.

B. Modular doubly salient variable reluctance machines

The double salient PM machines (DSPMMs) have salient poles on their both ar-
matures. On their stator, beside the windings, also PMs are placed to improve
their performances. Their rotors are usually passive, like those of the SRMs.
DSPMM combine the advantages of the everlastingly available PM excitation
and the simplicity of the VRM structures.
One of its most typical variants is the flux switching machine (FSM) shown in
Fig. 28. The stator of the machine comprises of U-shaped iron core modules sep-
arated by alternatively magnetized PMs. The concentrated coils are placed around
one leg of two adjacent modules and the PM placed between them [49].

Fig. 28: Flux switching machine [49]

The FSM owns the advantages of the robustness and of having both mmf sources,
the windings and the PMs, on its stator. Due to its very good magnetic flux con-
centration capability, the FSM can achieve high torque density. In addition, it has
a passive, robust and easy to cool rotor [4].

74
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Inappropriately, the high flux concentration and power density of the FSMs is
supplemented by high iron core losses. These can be diminished without signifi-
cant output torque decrease by segmenting also its rotor, as it was proposed by
A. Thomas et al. in [50] As it is shown in Fig. 29, the rotor modules are lamination
stacks separated by flux barriers, made of non-magnetic material.

Fig. 29: Segmented rotor FSM [50]

The effect of the segmented rotor on the magnetic field distribution in FSM can
be clearly seen in Fig. 30. If the rotor is segmented, the flux lines passing it are a
little bit shorter than in the case of unsegmented rotor. As a consequence, better
flux concentration and less iron core losses are achieved. Meanwhile, also the
volume of the iron core is diminished.

a) conventional rotor b) segmented rotor

Fig. 30: Magnetic flux distribution in FSMs [50]

The FSMs can be also designed with an increased number of phases for better
fault tolerance. These can be a promising candidate in safety-critical applications,
as also the automotive ones [51].

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SRM DRIVES FOR ELECTRIC TRACTION

In a similar way also other DSPMMs, as the flux reversal (FR) PM machines can
be built up with segmented stators [52].

4.5. Advancements in electrical machines brought by the modular approach

The developments of the modular construction of the electrical machines resulted in
several enhancements in the field. Advancements had been achieved in the design,
manufacturing, maintenance, efficiency and fault tolerance increase, etc. [6].

A. Design related advances

The modular approach permits an increased flexibility for the electrical machine
designers. For example, more complex and efficient, even three-dimensional,
magnetic flux paths can be applied. Consequently, the designers can easier shape
the machines under development for any specific requirement imposed by the
automotive applications.

The segmented iron core electrical machines, due to their optimized design, have
iron core only where the magnetic flux must be directed. Therefore, significant
iron core savings can be achieved, and as a result the machines will have less iron
core losses.

Fig. 31: The integrated modular motor drive (IMMD) concept [53]

The segmentally constructed electrical machines can be easily combined with sim-
ilarly modular drive circuits. Upon the idea of N.R. Brown et al., within the so-
called integrated modular motor drive (IMMD) concept (given in Fig. 31), the
modules of the electrical machine and of the power converter can be integrated
together into a single unit to increase the compactness and easy reparability of the
machine-converter assembly [53].

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

B. Advantages Connected to Manufacturing and Maintenance.

The segmented iron cores can be manufactured of bonded, ready to wound lami-
nation stacks. This technology has various advantages, as high dimensional pre-
cision, great mechanical stiffness (which is permitting easy handling and further
machining if required), very good electrical insulation between the laminations
without any short cuts, no cushioning, less liquid absorption, etc. [54]. Supple-
mentary, the stamping tools required by the manufacturing process are smaller
and simpler, and accordingly they are much inexpensive.

By the iron core modularization, the magnetic material consumption is intensely

decreased. The stator or rotor laminations are not stamped as one piece but di-
vided into smaller segments. These modules can be stamped optimally from the
raw material strip to reduce material losses (scraps), as in the example shown in
Fig. 32 [55].

Fig. 32: The placement of iron core modules on the raw material strip [55]

Upon the modular iron core construction approach, the innovative bonding var-
nish technology can be applied for the efficient interconnection of the lamina-
tions. In contrast with the traditional technology of interlocking, this permits an
easy to automate manufacturing of mechanically stiff iron core parts, made of
evenly insulated sheets, as the poles of the electrical machines. Iron core seg-
ments produced by means of this technology can be seen in Fig. 33 [56].

Fig. 33: Iron core modules manufactured via the bonding varnish technology [56]

77
SRM DRIVES FOR ELECTRIC TRACTION

As the iron core laminations of the stator and rotor can be manufactured sepa-
rately, this technology also enables the fabrication of iron core stacks of the two
armatures from different raw materials, if different properties, as mechanical
strength, magnetizability, permeability, etc. are required for them [57].

The iron core modules can be also made of SMCs, which are ferromagnetic pow-
der particles embedded in an insulating film coating [58]. Such modules can be
formed of any three-dimensional shape and have very low eddy current losses
[42], [58]. The net shape fabrication approach (for example the uniaxial pressing)
can also lead to very low tolerances, and the material waste during manufacture
is minimal. Due to the use of precisely fabricated iron core pieces, the air-gap
length can be optimized, resulting in high torque density.

Iron cores made of SMC materials are very suitable mainly for the electrical ma-
chines working at high speeds [59]. As a wide range of SMCs are available on
the market, the designers can select the most appropriate material for the given
application, which can provide magnetic performances comparable to, or even
surpassing that of the classical laminated sheets.

The segmentation of the iron cores is frequently in conjunction with the use of
concentrated (non-overlapping) windings. The main reason is that they have
shorter end winding as that of the distributed windings (see Fig. 34), thus ena-
bling increased active length for the machine, in the case of automotive applica-
tions where often axial size limitations are imposed, as it can be This way they
are contributing to an increase of the power density of the electrical machine. It
must be also mentioned, that concentrated windings also facilitate higher winding
and filling factor [60].

a) distributed winding b) concentrated winding

Fig. 34: Distributed vs. concentrated windings [60]

Another advantage of the non-overlapping windings is their straightforward man-

ufacturing, which facilitates cheap high-series production. This advantage is mul-
tiplied by the simple winding of independent modules, as compared with that of
non-segmented iron cores, due to the greater place having at disposal of the wind-
ing nozzle during winding, as it is shown in Fig. 35 [61].

78
 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Fig. 35: Concentrated coils winding for different iron cores [61]

Fig. 36: Simple replacement of a faulted segment in a modular SRM [14]

The piece by piece assembly of the modular electrical machines also implies an
easier maintenance. The short-circuited or opened coils can be more straightfor-
wardly detected and replaced together with the iron core module round which are
wound, as it can be seen in Fig. 36. In some cases, this can be performed without
demounting the machine from the floor or pedestal [13].

The modular construction also facilitates a straightforward decommissioning and

recycling due to the rapid and easy disassembly of the modular parts.

C. Increased efficiency and lifetime.

As it was stated already, the iron cores segments of the modular electrical ma-
chines are designed in a way to have iron core only just where it is needed for
optimally guiding the magnetic flux. This leads to an iron core mass and volume

79
SRM DRIVES FOR ELECTRIC TRACTION

reduction, which is beneficial for the power density increase. Supplementary, the
optimized flux paths are much shorter than those in the classical variants of the
electrical machines, resulting in diminished iron core losses.

Due to their shorter end windings, the concentrated windings contain less copper,
and thus they have also less Joule losses. In addition, these windings are not so
strongly surrounded by the iron core as the distributed ones, enabling this way an
easier cooling. All these together contribute to the increase of efficiency. In ad-
dition, less heat is generated due to the smaller amount of losses, and the better
cooling of the machine leads to an increased lifetime. These both are basic re-
quirements in automotive applications.

D. Improved fault tolerance

One of the main advantages of the modular approach in the field of electrical
machines is the opportunity to increase their fault tolerance, a decisive issue in
safety and operationally critical applications, as those in automotive [62].

In most of the cases, one phase of the electrical machine comprises of one or more
independent modules. Consequently, the operation of a phase has an insignificant
influence on the other ones. If it is faulted, the other ones can continue to work in
an unaltered way. The phase separation can be beneficially extended also to the
power converter of the electrical machine [63], [64].

The iron core segmentation also makes easier the phase number increase of the
electrical machines. As they have more phases, a fault of a single phase has less
influence on the proper working of the machine. The phase separation can be best
observed in the case of the modular SRMs. Their phases are inherently well-in-
sulated, mainly because of the very small magnetic coupling between the stator
coils [65]. By the SRM iron core segmentation the phase separation gets better
[14].

E. Drawbacks of segmentation.

Unfortunately, the electrical machine segmentation has also some weaknesses.

These mainly are due to rigidity issues resulted from replacing a single, stiff body
by a structure made of more pieces. In many cases non-magnetic extra parts are
needed to link the separate modules. This can cause extra technological problems
and costs.

Because of the great electromagnetic forces acting on the less rigid iron core seg-
ments, a bending of the teeth or poles takes place (see Fig. 37).

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 Variable Reluctance Motors for Automotive Applications. The Modular Construction Approach

Fig. 37: Bending of the teeth in the case of a modular stator

The unsuppressed axial and radial forces can result in vibrations and noises, to-
tally annoying phenomena in all the automotive many applications.

4.6. Acknowledgement
This chapter was supported by the project "Advanced technologies for intelligent
urban electric vehicles – URBIVEL - Contract no. 11/01.09.2016", project co-
funded from the European Regional Development Fund through the Competitive-
ness Operational Program 2014-2020.

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86
 5
Double-Stator Switched
Reluctance Motors
Hossein Torkaman and Majid Asgar

5.1. Introduction
The advances in power electronic devices, simulation software and manufactu-
ring technology allow possibility of introducing SRMs as an attractive candidate
for a wide range of industrial applications such as automotive and domestic ap-
pliances [1-3]. The main advantages of SRMs are simple structure, high effi-
ciency, low maintenance cost, fault tolerance, high reliability, high torque-inertia
and low price-performance ratios [4-6]. The structure of SRM has enough flexi-
bility to produce distinctive characteristics for many required applications and
intentions. Operating in saturation condition leads to have complex relationship
and control in this machine. This reduces the total performance of the SRM and
its drive in comparison with commonly used induction and permanent magnet
machines [7]. Generally speaking, in SRM, the specific application determines
the motor geometry, the power converter topology and the applied control method
[8]. Hence, an enhancement in power density provides the opportunity to employ
this machine in high-performance applications [9].

In an SRM (Fig. 1), rotation of the rotor is due to the resultant radial and tangen-
tial force vectors (FR and FT), respectively. The structure of a rotary SRM dictates
produced propulsion force on the rotating part by the interaction between stator
and its corresponding coaxial rotor poles. The nonsymmetrical distribution of the
mentioned forces lead to torque ripple production that can disturb positioning ac-
curacy and thus causing vibrations. In this regard, monotonous distribution of
forces could avoid additional ripple.

However, there are many different methods to decrease the motor ripple and in-
crease the power and torque densities of SRM. These methods are categorized in
improved control methods and optimum design of motor topology. In this re-
gards, DSSRMs benefit from high torque density as well as low radial force level
in comparison with the conventional SRMs [10, 11].

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SRM DRIVES FOR ELECTRIC TRACTION

Stator

FR

FM
Rotor
FT

(a) (b)

Fig. 1: The produced forces in: (a) SRM, (b) DSSRM

5.2. DSSRM structure

The concept of DSSRM comes from double-sided linear SRM (DLSRM). Fig. 2
shows a DLSRM with segmented translator configuration.

Fig. 2: DLSRM with segmented translator

Unlike single-side LSRMs, DLSRM does not have a lot of freedom in the air gap
tolerance. One of the merits of DLSRM is high force density with lower induct-
ance as it has four air gaps in its flux path. DSSRM improves the power/torque
density and reduces the acoustic noise emission as compared with the conven-
tional SRM. Advantages of the DSSRM in comparison with the conventional
SRM are as follows [12]: 1) higher torque/power density; 2) higher force conver-
sion efficiency; 3) higher motional force while lower radial force; 4) larger effec-
tive air gap reluctance at unaligned position.

88
 Double-Stator Switched Reluctance Motors

In this type of machine, two inner and outer stators are employed (Fig. 3). These
stators are made of laminated ferromagnetic material and windings are wrapped
around each poles. The motor can be made in 3, 4 or 6 phases configurations.

Outer stator

Inner stator

Rotor
Coil

Shaft

(a) (b)

Fig. 3: (a) DSSRM, (b) Flux linkage path.

The rotor is located between the inner and outer stator. The rotor is formed by
segments of laminated, which is hold together using a bridge (in one piece) or by
a non-ferromagnetic cage (in segmented rotor). Because of position of the rotor
it would be lighter than the rotor in SRM. It means that the rotor has a low mo-
ment of inertia and a fast response time. There are two narrow air gaps between
rotor and two stators. There are different arrangements of the outer-stator/ro-
tor/inner-stator poles in DSSRM, in which the common arrangements are
12/8/12, 12/10/12 and 8/6/, that are shown in Fig.4.

(a) 12/8/12 full-pitched (FP) (b) 12/10/12 single-tooth win- (c) 8/6/8 DSSRM [14]
winding [13] ding [13]

Fig. 4: Different arrangements of outer-stator/rotor/inner-stator poles

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SRM DRIVES FOR ELECTRIC TRACTION

Based on the average torque equation ( Tavα ( La − Lu ) / (θ a − θu ) ), the instantaneous

torque profile of the motor depends on the phase inductance profile, which is heavily
influenced by the equivalent air gap between stator and rotor at various rotor posi-
tions. Therefore, the modification of rotor geometry could potentially shape the
torque profile of the DSSRM. Some of the rotor shapes are shown in Fig. 5. For
comparison of this rotor shapes, the length, width, tips geometry, arc angle, thickness
and number of steps should be optimized and then the motor characteristics would
be evaluated.

(a) (b) (c) (d)

Fig 5: (a) Solid rotor, (b) Rotor with arc edge, (c) Rotor with tips, (d) Rotor with stepped layer
[15]

The windings of the stators can be wrapped in form of single-tooth windings,

short-pitched, full-pitched, concentrated winding and hybrid combination coil.
The winding connections of the inner stator can be accessed through a hole placed
in the shaft of the inner stator. Fig. 6 shows some typical winding configuration
in DSSRM. The comparison among them does not result in a clear conclusion
that designates one winding typology to be the better one. It is dependent on var-
ious factors, including the chosen application of the DSSRM, operating point and
tolerable torque ripple. For fair comparison the copper, core and thermal losses,
also current density, torque density, torque ripple, flux leakage should be evalu-
ated for different configuration in each application. In addition to typical structure
of DSSRM, there are different type of DSSRM topologies such as: Bearing
less[16], linear[17, 18], yokeless linear[19], axial flux [20], that are used for var-
ious applications.

(a) (b) (c)

Fig. 6: (a) Full-pitched winding, (b) Concentrated windings-1, (c) Concentrated windings-2 [21]

90
 Double-Stator Switched Reluctance Motors

The torque production of DSSRM is determined such as SRM by variation of co-

energy versus rotor position or by the tendency of the magnetic circuit to mini-
mize the reluctance of the flux path. Because of two excitation windings in inner
stator and outer stator, the torque density would be higher in comparison to typi-
cal SRM with one excitation winding in. A counterclockwise excitation in stator
phases leads a clockwise rotation of the rotor.

As mentioned before, the vibration and acoustic noises are the main disadvantage
of SRMs, which overshadow their use in noise sensitive applications. DSSRMS
can be considered as an alternative to this issue. In this regard, firstly, a structural
analysis of the DSSRM should be done by multi-physic analysis, also FEM can
be used to evaluate radial, axial and tangential forces at various parts of the ma-
chines. After that the acceleration, deformation, and velocity of the vibration
should be assessed. In [14] compares structural analysis of a DSSRM and a con-
ventional SRM with the same outer diameter, yoke thickness, and output power.
It was shown that radial force is smaller in the DSSRM, and the force distribution
in this machine tends to offer a lower vibration level and acoustic noise. Results
show that the DSSRM machine experiences much lower vibration than the con-
ventional SRM as shown in Table I.
TABLE I: VIBRATION COMPARISON OF SRM AND DSSRM IN TIME DOMAIN [14]

Parameter 12/8 SRM 8/6/8 DSSRM

Maximum acceleration (m/s2) 150 25
Maximum velocity (mm/s) 9 1
Maximum deformation (nm) 425 60

To better understanding the behavior of the machine, a DSSRM will be designed

and different characteristics will be achieved and analyzed in the next section.

5.3. Design Procedure of the DSSRM

The design trend of the DSSRM is illustrated in Fig.7. Then the design of
DSSRM is based on division structure into two SRMs (inner rotor SRM and outer
rotor SRM). It should be considered that the same torque should be produced on
the rotor by each of the inner and outer stators. In fact, balanced condition of the
rotor is achieved by dividing the nominal torque of the DSSRM into two equal
parts between two SRMs. Determining the Din and Dout (of inner and outer stators
diameters) should be achieved or determined by application. Also, the specific
magnetic and electric loadings on two stators would create a balanced force on
the middle rotor.

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SRM DRIVES FOR ELECTRIC TRACTION

Fig. 7: Flowchart of design procedure of the DSSRM [10]

Fig. 8: Cross section view of the DSSRM with four parts: (1) Outer stator, (2) Rotor, (3) Inner-
stator, (4) Shaft.

The torque produced by a DSSRM is the result of torque vectors formed by the
inner and outer stators (Figure 8a). Therefore, by separating a DSSRM into two
independent SRMs, some parts such as rotor and shaft are shared between two
SRMs. The outer stator and shared rotor can form an independent interior rotor
SRM and the inner stator and shared rotor can form an independent exterior rotor
SRM. For design, the DSSRM is divided into four parts (Fig. 8). Parts 1, 2 and 4

92
 Double-Stator Switched Reluctance Motors

form the interior rotor SRM and parts 2, 3 and 4 form the exterior rotor SRM.
Therefore, parts 2 and 4 are common parts in the design of both SRMs

In this structure, the net centrifugal and centripetal forces on the rotor are the
vector sum of the torque produced by the two stators. This force causes rotational
motion that can be calculated as a function of rotating internal parameters:
F ≅ m × r ×ω2 (5.1)
where m is the mass turning around with the shaft, r is the radius of outside of the
stator and ω is the angular speed [rad/s]. As explained in the previous section, to
produce equivalent force on the rotor; due to inequality of the bore diameters,
both stators are forced to produce unequal power. The power capacity of the
DSSRM considering both inner and outer stators can be calculated as follows:
Pd =Pin + Pout =F × vm (5.2)
In DSSRM ⇒ Pin  Pout
where vm is maximum linear velocity [m/s]. Preceding the design of the SRM,
initial parameters of the SRM configurations appropriate to the specific applica-
tion are defined and listed in Table II. In this example, the D0 (outer diameter of
the DSSRM) are considered as initial value with specific value, in design trend.

TABLE II. INITIAL PARAMETERS OF THE DSSRM.

Parameter Description Value

Ps Number of Stator Poles 12 (dual)
Pr Number of Rotor Poles 8
q Number of Phases 3
D0 Stator Outside Diameter 260 mm
Pd Total Output Power 1.2 kW
Vbus Bus Voltage 80 V
IP Peak Phase Current 20 A
βs[max] Maximum Stator Pole Arc 7.5 Deg.

A. Inner rotor SRM design -Individually

The design variables considered for the analysis of inner rotor SRM are shown in
Fig. 9. Due to the structure of proposed segmented DSSRM, the rotor yoke has
been eliminated (bry = 0).
As seen in Fig. 9, the outer diameter of the SRM is as follows:
D0 = Dsh + 2bry + 2hr + 2lg + 2 ( hs − out + bsy − out ) (5.3)

93
SRM DRIVES FOR ELECTRIC TRACTION

Fig. 9: Dimension parameters of an inner rotor SRM.

It should be noted that, the process of SRM design has some assumptions and
constant coefficients which are detailed in Table III. The numerical value of the
frame size factor is considered with respect to the design of an SRM.

TABLE III ASSUMPTION OF DSSRM DESIGN.

Parameter Description Value

k Frame Size Factor 0.25
k1 Constant Factor 0.08225
k2 Constant Factor 0.7
ke Efficiency 40%
kd Duty Cycle 0.5
B Flux Density 1.4 T
Dsh Shaft Diameter 50 mm
lg Air Gap Length 0.5 mm
M Lamination Material M27 Steel
4π×10-7
µ0 Magnetic Permeability of Space
H.m-1
J Current Density 6 A/mm2

k factor is the ratio between bore diameter and stack length which is called frame
size factor. kl is a constant factor which is equal with π2/120. k2 is a constant factor
that is related to the ratio of aligned and unaligned inductances. kd is the ratio of
the on-time of the switch to the time period which is called duty cycle. The bore
diameter of an inner rotor SRM is obtained from the power output equation as

94
 Double-Stator Switched Reluctance Motors

Pout
Dout = 3 (5.4)
ke kd k1k2 kBAS − out N r

The outer stator electric loading of the rotary SRM is given by:
2 × i × m × Tph − out
AS − out = (5.5)
π × Dout
The output torque of inner rotor SRM can be explained by:
Tout = kd ke k2 k3 ( B × AS − out ) Dout
2
×L (5.6)

The stator yoke thickness is found as:

Dout × β s
bsy − out = (5.7)
2
The stator pole height is calculated as:
D D
hs − out = 0 − out − bsy − out (5.8)
2 2
The height of the rotor pole is defined as follows:
Dout
hr= − lg − bry (5.9)
2
The magnetic field intensity is given by:
Dout
H g − out = (5.10)
µ0
The number of turns per phase for peak phase current is:
H g − out × 2lg
TPH − out = (5.11)
IP

The area of cross-section of the conductor is calculated as:

IP
ac = (5.12)
J q

With calculating the number of coil winding turns completes the inner rotor SRM
design. Geometrical and electrical parameters of the designed inner rotor SRM
after required iterations are obtained and listed in Table IV.

95
SRM DRIVES FOR ELECTRIC TRACTION

TABLE IV GEOMETRICAL AND ELECTRICAL PARAMETERS OF THE DESIGNED INNER ROTOR SRM

Parameter Description Value

Pout Outer Stator Power 483 W
Dout Outer Stator Bore Diameter 200 mm
L Stack Length 40 mm
As-out Outer Stator Electric Loading 19100 A/m
hs-out Outer Stator Pole Height 20 mm
hr Rotor Pole Height 30 mm
bsy-out Outer Stator Yoke Thickness 10 mm
bry Rotor Yoke Thickness 0 mm
Wsp Outer Stator Pole Width 15 mm
Wrp-out Inner Stator Pole Width 28 mm
TPh-out Outer Stator Coil Turns 75
Tout Outer Stator Output Torque 5.88 Nm

B. Outer rotor SRM design individually

The design variables that are considered for the analysis of outer rotor SRM are
shown in Fig.10.

Fig.10: Dimension parameters of an outer rotor SRM.

The diameter of outer rotor SRM is obtained from the power output equation as:
Pin
Din = 3 (5.13)
ke kd k1k2 kBAS −in N r

96
 Double-Stator Switched Reluctance Motors

The output torque of outer rotor SRM can be explained by:

Tin = kd ke k2 k3 ( B × AS −in ) Din2 × L (5.14)
The stator yoke thickness is found as:
Din × β s
bsy −in = (5.15)
2
The stator pole height is given by:
D0 Din
hs −in = − − bsy −in (5.16)
2 2
The magnetic field intensity is defined as follows:
Din
H g −in = (5.17)
µ0
The number of turns per phase for peak phase current is:
H g −in × 2lg
TPH −in = (5.18)
IP

TABLE V GEOMETRICAL AND ELECTRICAL PARAMETERS OF THE DESIGNED OUTER ROTOR SRM.

Parameter Description Value

Pin Inner Stator Power 739 W
Din Inner Stator Bore Diameter 140 mm
L Stack Length 40 mm
As-in Inner Stator Electric Loading 36380 A/m
hs-in Inner Stator Pole Height 40 mm
hr Rotor Pole Height 30 mm
bsy-in Inner Stator Yoke Thickness 13 mm
bry Rotor Yoke Thickness 0 mm
Wsp Outer Stator Pole Width 15 mm
Wrp-in Rotor Pole Width 17 mm
TPh Inner Stator Coil Turns 100
Tin Inner Stator Output Torque 5.4 Nm

Comparing Tables IV with V shows that the power received from DC-link to the
inner stator is greater than the outer stator. Due to establish balancing condition
in this motor that has two different stator diameters, the number of turns in inner

97
SRM DRIVES FOR ELECTRIC TRACTION

stator should be increased. Generally, in a double-stator motor, the following re-

lationship is established (Dout >Din). Therefore, according to equations (6) and
(14), due to inequality of the stators bore diameters, the calculated torque in the
air gap between two stators and rotor poles, are not the same. In this case, the
rotor pulls toward the outer stator. To prevent this issue, the number of turns in
the inner stator windings increased to make the balancing condition. The results
which are listed in Tables III and IV refers to this point (α is inequality factor).

In DSSRM: Dout= α Din Tph-out = α2 Tph-in

2 × i × m × α Tph − out
=AS −in = α 2 AS − out (5.19)
π × Dout / α
Pin > α2Pout Tin ≅ Tout

C. Integration of the Designs

Based on the mentioned equations and specific application, the DSSRM has been
designed analytically. The common variable parameters are illustrated in Fig. 11.

Fig. 11: Pole shaping of the segmented DSSRM.

98
 Double-Stator Switched Reluctance Motors

TABLE VI GEOMETRICAL AND ELECTRICAL SPECIFICATIONS OF DESIGNED DSSRM.

Parameter Description Value

Pd Developed Output Power 1.2 kW
D0 Outer Diameter 260 mm
L Stack Length 40 mm
Ta Actual Output Torque 11.28 Nm
AST Total Electric Loading 55480 A/m
MT Total Magnetic Loading 0.175 T

5.4. Numerical Assessment of the Motor Characteristics

The overall performance of the designed DSSRM has been verified with static
and transient finite element analysis (FEA). Having knowledge about the mag-
netic flux distribution in the motor for various rotor angles and excitation current
is needed to predict motor efficiency. The 3D designed model using CAD pack-
age, is illustrated in Fig. 12. The designed DSSRM with the achieved dimensions
(Table VI) includes two coaxial stators and one segmented rotor.

Fig. 12: The coil numbering of the 3D model in DSSRM.

The FEM is a useful tool to provide a complete analysis of the SRM, as it is able
to model non-linear magnetic properties as well as static and dynamic studies. In
DSSRM, the relevance between dimensions of inner and outer stators and the
developed torque is not clear. Therefore, an adjustment of any device parameters
demands a new computational analysis by FEM.

In order to analysis the actual performance of the DSSRM, rotor starts from una-
ligned position -15 Deg. toward aligned position 0 Deg., and then, moves from

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SRM DRIVES FOR ELECTRIC TRACTION

aligned position 0 Deg. toward another unaligned position 15 Deg. in counter

clockwise direction. In this states the constant excitation current of phase “A” is
8 A. The values of flux linkage at all discrete position of the rotor movements are
computed. At aligned position, the magnetic flux density is obtained and shown
in Figure 13. The maximum flux density (Bmax) in 8 A excitation current is close
to 1.43 T. As seen in Fig. 13, four stator pole pairs are participated for one phase
energizing in DSSRM.

Fig. 13: Distribution of Flux density [Tesla].

Under 8A excitation current, the flux linkage vs. rotor position of the motor is
calculated and shown in Figure 14. In the unaligned position, minimum flux is
about 18.6 mWb and the peak flux in aligned position is 87 mWb. The inductance
is defined as the ratio of flux-linking in active phase to the exciting current (L=λ
/i). It means, with considering constant current excitation in the simulation, the
inductance shape is the same as flux-linkage that shown in Fig.14.

Fig. 14: Flux linkage profile.

100
 Double-Stator Switched Reluctance Motors

As shown in Fig.15, the generated torque by DSSRM follows minimum reluc-

tance path with the rotor rotation. The maximum values of the torque is obtained
about 5.9 Nm at fully unaligned positions 7.5 Deg. under 8 A constant current.

Fig. 15:Torque profile.

To verify the design strategy, the proposed flat-type DSSRM are fabricated. The
different parts are shown in Fig.16. The depicted three-phase motor parts include
a segmental rotor and two stators which the stators are located and centered inside
the aluminum hollow cylinder and the rotor is placed on a thick aluminum frame.
Also, the shaft is placed on one bearing that is ready to connect to the load. The
main controller is designed by DSPIC microcontroller.

5.5. Manufacturing and Experimental Test

The first task to obtain the best dynamic performance of the machine is to compute
turn ON/OFF angles at each current and speed working point. The main objective
is to get the maximum torque possible at each current and speed reference point. It
is also necessary to validate that the dynamic evolution of the phase currents is
good. This is a key point to assure that the maximum peaks values do not go above
the protection limits.

(a) (b)
Fig. 16: (a) Different parts of motor, (b) Assembled stators of the motor

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SRM DRIVES FOR ELECTRIC TRACTION

Different tests are done to verify the analytical results and FEM analysis [10].
The inductance values of the motor at aligned and unaligned positions are
achieved and are shown in Table IV in comparison with the mentioned analysis.
As seen in this table, the inductance value of aligned and unaligned positions are
about 47 and 16 mH, respectively. The appropriate difference between values of
these two positions proves the ideal operation of the motor in torque production.
Also, the difference between analytical, FEM and experimental results in the un-
aligned position is 1 and 2 mH which are in the minimum value. However, in the
aligned position, these differences are 5 and 1 mH.
TABLE IV. INDUCTANCE MAGNITUDE AT ALIGNED AND UNALIGNED POSITIONS.

Position Analytical FEM Experimental

Aligned 43mH 48mH 47mH
Unaligned 13mH 14mH 16mH

The experimental torque-speed characteristic is obtained and shown in Fig.17. In

low speeds (below the rated speeds) under 80 V DC-link power supply, the torque
is limited by the motor current. At 1000 rpm speed, the generated torque is ob-
tained about 8.5 Nm. At lower speeds (below 1200 rpm), the variation of the
current rises almost instantly. These specifications adequate to use the proposed
motor in special direct drive application.

Fig. 17: Torque vs. Speed profile in open loop mode.

The efficiency of a DSSRM are shown in Fig.18. The maximum efficiency of the
DSSRM is obtained about 78%. Also, in the medium and high speeds ranges, the
efficiency of the DSSRM is better than the conventional SRM.

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 Double-Stator Switched Reluctance Motors

Fig. 18: Efficiency vs. Speed profile.

5.6. Conclusions
This chapter introduced DSSRM. In this regards motor structure, geometry, de-
sign, electromagnetics characteristics, prototyping and test presented. The motor
characteristics show that the flux distribution, flux linkage, leakage inductance
and torque profiles have normal behavior such as conventional SRM. This con-
figuration can produce more torque and power density with lower radial force,
vibration and acoustic noises. In which make it good candidate for direct drive
applications. As a result, this analysis shows that this type of machine has poten-
tial to be used in different application such as electric vehicle, scooter, pump,
fans, washing machine and flywheel.

5.7. References
[1] H. Torkaman, A. Ghaheri, and A. Keyhani, “Axial flux switched reluctance ma-
chines: a comprehensive review of design and topologies,” IET Electric Power Ap-
plications, vol. 13, no. 3, pp. 310-321, 2019.
[2] O. Safdarzadeh, A. Mahmoudi, E. Afjei, and H. Torkaman, “Rotary-Linear
Switched Reluctance Motor: Analytical and Finite-Element Modeling,” IEEE
Transactions on Magnetics, vol. 55, no. 5, pp. 1-10, 2019.

103
SRM DRIVES FOR ELECTRIC TRACTION

[3] H. Torkaman, A. Ghaheri, and A. Keyhani, “Design of Rotor Excited Axial Flux-
Switching Permanent Magnet Machine,” IEEE Transactions on Energy Conversion,
vol. 33, no. 3, pp. 1175-1183, 2018.
[4] H. Torkaman, N. Faraji, and M. S. Toulabi, “Influence of Rotor Structure on Fault
Diagnosis Indices in Two-Phase Switched Reluctance Motors,” IEEE Transactions
on Magnetics, vol. 50, no. 3, pp. 136-143, 2014.
[5] H. Torkaman, “Rotor fault analysis and diagnosis in three-phase outer-rotor
switched reluctance motor,” in 4th Annual International Power Electronics, Drive
Systems and Technologies Conference, 2013, pp. 93-96.
[6] A. Miremadi, H. Torkaman, and A. Siadatan, “Maximum current point tracking for
stator winding short circuits diagnosis in Switched Reluctance Motor,” in 4th An-
nual International Power Electronics, Drive Systems and Technologies Conference,
2013, pp. 83-87.
[7] O. Safdarzadeh, E. Afjei, and H. Torkaman, “Effective Magnetic Decoupling Con-
trol Realization for Rotary-Linear Switched Reluctance Motors utilized in Drilling
Tools,” International Journal of Applied Electromagnetics and Mechanics, vol. 57,
no. 3, pp. 257-274, 2018.
[8] H. Khanbabaie Gardeshi, and H. Torkaman, “Improved Control of Switched Reluc-
tance Motor at High Speeds using Continuous Conduction Mode,” Tabriz Journal
of Electrical Engineering, vol. 47, no. 1, pp. 69-79, 2017.
[9] H. Torkaman, R. Moradi, A. Hajihosseinlu, and M. S. Toulabi, “A comprehensive
power loss evaluation for Switched Reluctance Motor in presence of rotor asym-
metry rotation: Theory, numerical analysis and experiments,” Energy Conversion
and Management, vol. 77, pp. 773-783, 2014.
[10] M. Asgar, E. Afjei, and H. Torkaman, “A New Strategy for Design and Analysis of
a Double-Stator Switched Reluctance Motor: Electromagnetics, FEM, and Experi-
ment”, IEEE Transactions on Magnetics, vol. 51, no. 12, pp. 1-8, 2015.
[11] M. Abbasian, M. Moallem, and B. Fahimi, “Double-Stator Switched Reluctance
Machines (DSSRM): Fundamentals and Magnetic Force Analysis,” IEEE Transac-
tions on Energy Conversion, vol. 25, no. 3, pp. 589-597, 2010.
[12] N. Arbab, W. Wang, C. Lin, J. Hearron, and B. Fahimi, “Thermal Modeling and
Analysis of a Double-Stator Switched Reluctance Motor,” IEEE Transactions on
Energy Conversion, vol. 30, no. 3, pp. 1209-1217, 2015.
[13] E. Cosoroaba, W. Wang, and B. Fahimi, “Comparative study of two winding con-
figurations for a double stator switched reluctance machine,” in 2014 International
Conference on Electrical Machines (ICEM), 2014, pp. 1013-1017.
[14] A. H. Isfahani, and B. Fahimi, “Comparison of Mechanical Vibration Between a
Double-Stator Switched Reluctance Machine and a Conventional Switched Reluc-
tance Machine,” IEEE Transactions on Magnetics, vol. 50, no. 2, pp. 293-296, 2014.
[15] W. Wang, M. Luo, E. Cosoroaba, B. Fahimi, and M. Kiani, “Rotor Shape Investi-
gation and Optimization of Double Stator Switched Reluctance Machine,” IEEE
Transactions on Magnetics, vol. 51, no. 3, pp. 1-4, 2015.

104
 Double-Stator Switched Reluctance Motors

[16] Y. Sun, F. Yang, Y. Yuan, F. Yu, Q. Xiang, and Z. Zhu, “Analysis of a hybrid double
stator bearingless switched reluctance motor,” Electronics Letters, vol. 54, no. 24,
pp. 1397-1399, 2018.
[17] D. H. Wang, X. H. Wang, C. L. Shao, and Z. L. Wang, “Analysis and design of an
annular winding dual side stator linear switch reluctance machine for ropless eleva-
tor driving system,” in 2015 IEEE International Conference on Applied Supercon-
ductivity and Electromagnetic Devices (ASEMD), 2015, pp. 322-323.
[18] D. Wang, X. Wang, and L. Xiong, “Performance analysis of a dual stator linear
switch reluctance machine with rectangular segments considering force ripples for
long stroke conveyor applications,” in 2015 18th International Conference on Elec-
trical Machines and Systems (ICEMS), 2015, pp. 295-298.
[19] D. H. Wang, X. H. Wang, C. L. Shao, and L. X. Xiong, “Design aspects of a high
thrust density dual stator yokeless Linear Switched Reluctance Machine for harsh
conditions,” in 2015 IEEE International Conference on Applied Superconductivity
and Electromagnetic Devices (ASEMD), 2015, pp. 468-469.
[20] H. Goto, “Double Stator Axial-Flux Switched Reluctance Motor for Electric City
Commuters,” in 2018 International Power Electronics Conference (IPEC-Niigata
2018 -ECCE Asia), 2018, pp. 3192-3196.
[21] E. Cosoroaba, E. Bostanci, Y. Li, W. Wang, and B. Fahimi, “Comparison of winding
configurations in double-stator switched reluctance machines,” IET Electric Power
Applications, vol. 11, no. 8, pp. 1407-1415, 2017.

105
 6
Definition of a Strategy for an
Axial Flux Switched Reluctance
Machine Torque Control
Aritz Egea, Gaizka Ugalde, Javier Poza

6.1. Introduction
In the transition towards a new era of low-carbon society and climate resilient
economy, one of the European Union’s key policy objectives for the upcoming
decades is cutting 60% of CO2 emissions from transport, where fossil fuel de-
pendence is around 96%. Electric vehicles (EV) are considered to be the most
plausible alternative to fossil fuel-based road transport. There is a source of un-
certainty related to the availability of reliable and diversified supply of metals to
produce the necessary permanent magnets (PM) to assure high efficiency and
high-power density electrical motors. The shift from a fossil fuel dependence
scenario to a permanent-magnet dependence scenario (even more critical as they
can only be found under single source monopolies) could limit significantly the
large-scale introduction of EVs as PM based motors could not be supplied in
adequate volumes at a competitive cost.

In this context, the development of high efficiency motors using magnet-free mo-
tor designs is crucial. A promising option for this new generation of electric mo-
tors could be reluctance technology: this kind of motors stands out for its wide
constant power capability, reliability and security. However, it has been left out
of the first line up to now due to its lower power density when compared to PM
motors.
On the other hand, the use of axial-flux configurations has proved recently in PM
motors that power density can be increased in a relatively cost-effective way.
VENUS European project Consortium is working in a design that combines both
approaches, reluctance motors in axial-flux configuration, Axial Flux Switched
Reluctance Motor (AFSR). The topology selected was first presented by Labak
[1, 2] and the designed motor for VENUS project will be constructed and inte-
grated in an electric vehicle in the future.

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SRM DRIVES FOR ELECTRIC TRACTION

Even if some authors proposed different analytical models for switched reluc-
tance machines [3, 4], the intrinsic nonlinearity of this type of machines makes
use of finite element almost unavoidable [5, 6]. Once the machine is character-
ized by FEM tools may be simulated using Matlab Simulink [7, 8].
The SRM machines by nature present a quite high torque ripple bringing a chal-
lenge for the definition of the control strategy in applications where this ripple
may be critical. Different strategies have been proposed in the literature. For ex-
ample, Dong-Hee Lee [9] presents an extended work about SRM control while
Husain [10], gives a general view of SRM machine torque ripple minimization.
Peter [11] and Lee [12] also give some notes for reduction of the torque ripple.
In this chapter, the torque control strategy for the designed machine is presented
[14]. This control is separated in two different actions, one for low speed opera-
tion and the other for higher speed operation. The aim of using two different ac-
tions is to improve the driving experience but reducing the computational load of
the control board.

a) b)
Fig.1: a) Rotor and stator arrangement by using axial-flux configuration and b) rotor detail [13]

6.2. Axial-Flux Switched Reluctant Motor model

A. Axial-Flux Switched Reluctant Motor

Fig.1 shows the geometry of the AFSRM used in this work. The motor comprises
a rotor disc carrying 8 rotor poles and 12 stator coil assemblies placed on the
periphery of the rotor disc.

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

B. Electrical Equations

Usually a simplified model of the SRM motor is used, as assuming the non-line-
arity of the inductance is not so easy analytically. The main electrical equation of
a SRM machine is the one described in (6.1) expression.
dψ (θ , i )
V ( t ) R ph i ( t ) +
= (6.1)
dt
The voltage in each phase (V) is the sum of the voltage drop in the phase re-
sistance Rph and the flux linkage derivative as a function of the position and the
current. Due to the non-linearity of the inductance, a simple mathematical ex-
pression cannot describe the current trajectory. Therefore, the flux linkage deriv-
ative can be split into two terms:

dψ (θ , i ) dψ (θ , i ) di dψ (θ , i ) dθ
= + (6.2)
dt di dt dθ dt
The first term in (6.2) assumes the current time derivative and the inductance that
is a function of the position and the current:

dψ (θ , i ) di di
= L (θ , i ) (6.3)
di dt dt
The second term in (6.2) is also known as the back emf. This emf is a func-
tion of the rotational speed, the current and the inductance change:

dψ (θ , i ) dθ dL (θ , i )
=emf = ωi ( t ) (6.4)
dθ dt dθ
So, expression (6.1) can be rewritten as follows:

di dL (θ , i )
V (t ) =R ph i ( t ) + L (θ , i ) + ωi ( t ) (6.5)
dt dθ
Equation (6.5) shows clearly that a mathematical model of a SRM machine is not
easy to develop. The evolution of the current is not an easy task to describe and
neither the inductance value, as it is dependent of the current and the position.

C. Torque production

In case of a linear machine, the inductance value has a trapezoidal shape as shown
in Fig. 2. Depending on the current activation moment, the machine may work at
motoring or regeneration. Ideally, the current is established instantaneously. If
the current is activated while the inductance is increasing, so when the rotor pole
is getting closer to the stator pole, the generated torque is positive. On contrary,

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SRM DRIVES FOR ELECTRIC TRACTION

when the rotor pole is moving from aligned to unaligned position the torque is
negative.
Un-aligned Aligned

Inductance
0

Phase current
0

Torque

0
Motor

Phase current
0
0

Torque θ

Generator

Fig.2: Inductance and torque profile [11]

This figure makes clear the importance of the current activation time. The maxi-
mum torque per phase is obtained if the current is activated just when the induct-
ance starts increasing (unaligned position) and deactivated at maximum value of
inductance (aligned position). In any other case, the average torque will be de-
creased. If the current activation time is shorter, the torque will decrease, while
if it is longer negative torque will be produced. This all shows the importance of
switching on and switching off the current at the right moment.

In case of a linear SRM machine, the electromagnetic torque, Te, can be expressed
as:
1 dL
Te = i 2 (6.6)
2 dθ
This expression is only valid with a linear inductance. A general expression valid
also in case of a non-linear inductance is when the electromagnetic torque is ob-
tained from the co-energy, W´:

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

dW ´
Te ( t ) = (6.7)
dt
The co-energy is the integration of the flux linkage along the current:
i
W ′ ( I , t ) = ∫ Ψ ( i, t ) di (6.8)
0

In Fig. 3 the energy and co-energy concept is explained in a graphical way. The
shown flux linkage follows a non-linear behavior, which results in a higher me-
chanical energy than the energy storage in the inductance. In case of a linear in-
ductance (flux linkage) both energies are equal making the expression (6.7) valid.

Fig.3: Energy and co-energy

D. Non-Linear model

At the proposed SRM model, the input of the machine is the stator voltage, while
the outputs are the torque, position and speed. Integrating the input voltage, the
flux in each phase is obtained. Having this flux and the rotor position, it is possi-
ble to get the instantaneous current in each phase from tables calibrated previ-
ously with FEM simulation, analytical model or experimental result. A torque
characteristic as function of the current and the rotor position must be also ob-
tained. The electromagnetic torque is the input of the mechanical model from
where the mechanical torque, speed and position are obtained.

In this project, the non-linear model is implemented in Matlab /Simulink. The

electromagnetic model is based in a torque and current map. FEM-3D simula-
tions are needed to get these torque vs. current and position, and current vs. flux
and position maps. Then including this maps in the electromagnetic model shown
in Fig. 4 it is possible to have a SRM machine model assuming the non-linearity.

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SRM DRIVES FOR ELECTRIC TRACTION

Fig.4: SRM Matlab-Simulink model [13]

Fig.5: Torque vs. Current and position curves (Each curve corresponds to a current level, starting
at 0A and reaching 910A) [13]

The modularity of this motor topology makes phases to be magnetically decou-

pled, thus motor performance can be extrapolated from the analysis of one pole
pair and considering the number of active phases and poles. As the motor has
three-phases, only one of them should be simultaneously active to generate torque
in one direction. With 12 stator poles in the motor, there are 4 poles per phase.
Therefore, motor parameters (torque, losses, etc.) are 4 times higher than in one
pole.

FEM-3D simulations get accurate curves of switched reluctance motor’s charac-

teristic (flux vs. current, and torque vs. position). In these FEM-3D magneto static
simulations a unique C-core together with three rotor poles is simulated. Three
rotor poles are simulated so the influence of the adjacent poles in the flux lines

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

can be considered. The coil in the C-core is fed with different current levels and
a movement from 0º to 45º is performed to take a complete torque cycle. From
these simulations torque (Fig. 5) and flux linkage curves are obtained so then
dynamic simulations may be performed in Matlab Simulink software.

6.3. Torque Control Strategy

The general view of the system is shown in Fig. 6. The control is based on two
main loops, the current and torque loops

ω ω

I* Sa Va
T* θon* Sb Vb
Torque control Current Control Converter SRM
θoff* Sc Vc

I θ Ia, Ib, Ic
T

Torque & Flux

Estimator

Fig. 6: Controlled SRM flow diagram [13].

The current loop gives gate signals to the converter considering the current con-
sign the actual current and the activation/deactivation angles. A simple on/off
control proves to be sufficient for the current loop.

The torque control generates the signals for the current control depending on the
rotational speed, the torque consigns and the torque estimator. The proposed
torque control strategy includes a direct torque control for higher speeds and a PI
control for the lower speeds as shown in Fig.7. The direct torque control is based
in a previous characterization of the machine to get a characteristic function,
which gives the current consign depending on the speed and the torque consign.
This control will give a constant current consign with which a main torque equal
to the consign torque will be generated. However, the torque ripple is not con-
trolled this way, and it could be quite dangerous when starting the vehicle in a
slope. Due to the ripple, a negative momentary negative torque may occur mak-
ing the vehicle move in reverse direction. To avoid this problem along with un-
comfortable vibrations a PI controller may be use at low speeds. The PI controller
generates a current consign from the error between the torque consign and the
real torque. As a torque sensor is not used the actual torque must be estimated

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SRM DRIVES FOR ELECTRIC TRACTION

using a torque estimator. This estimator is obtained when characterizing the ma-
chine. The torque is represented as a function of the currents and the position
(Fig.5).

I
Torque
θ estimator - PI
+
T* Ipi*
I*
ω <10rpm Switch

Ilut*

I(T,ω)

Fig. 7: Torque control strategy [13].

As explained before due to the impossibility to set the current instantaneously the
dynamic performance of the AFSRM varies from the static one.
In static mode or with ideal square form current the maximum torque is obtained
when the phase is activated from 22.5º to 45º. However, the current needs some
time to be established. The activation and deactivation signals must be given in
advance, so the maximum possible torque is given. In Table I and Table II the
optimal activation and deactivation angles are shown.

TABLE I: PHASE ACTIVATION ANGLE DEPENDING ON THE SPEED AND CURRENT

A\rpm 0 1000 2000 3000 4000 5000 6000 7000

50 22,5 22,0 21,6 21,1 20,6 20,2 19,7 19,2
150 22,5 21,0 19,5 18,0 16,5 15,0 13,5 12,0
250 22,5 19,9 17,3 14,6 12,0 12,0 12,0 12,0
350 22,5 19,5 16,5 13,5 12,0 12,0 12,0 12,0
450 22,5 19,0 15,5 12,0 12,0 12,0 12,0 12,0

TABLE II: PHASE DEACTIVATION ANGLE DEPENDING ON THE SPEED AND CURRENT

A\rpm 0 1000 2000 3000 4000 5000 6000 7000

50 45,0 44,2 43,4 42,6 41,8 41,0 40,2 39,4
150 45,0 43,2 41,4 39,6 37,8 36,0 36,0 36,0
250 45,0 42,8 40,5 38,3 36,0 36,0 36,0 36,0
350 45,0 42,4 39,9 37,3 36,0 36,0 36,0 36,0
450 45,0 42,0 39,0 36,0 36,0 36,0 36,0 36,0

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

As it can be deduced the higher the speed and the current are, more critical this
phenomenon is. In Fig. 8 the maximum torque the machine gives depending on
the speed and the current is shown. The torque generated by the machine is re-
duced markedly with increasing speed.

160

50A
100A
140
150A
200A
120 250A
300A
350A
100
400A
Torque [Nm]

80

60

40

20

0
0 1000 2000 3000 4000 5000 6000 7000

Speed [rpm]

Fig. 8: Dynamic torque performance [13]

Fig.9: Acceleration-deceleration speed profile [13]

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SRM DRIVES FOR ELECTRIC TRACTION

6.4. Machine Performance

A. Simulations

To check the validity of the proposed method by simulations the machine is ro-
tated at between 0 and 20 rpm speed as defined in Fig. 9 while a constant torque
consign of 48Nm is set. When the rotational speed is less than 10 rpm a PI torque
control is performed, while over this threshold the direct torque control is work-
ing obtaining the torque evolution shown in Fig. 10. It is easily noticeable how
the torque ripple of ±10Nm is eliminated when using the PI controller. This hap-
pens because the current consign is changing all the time when the PI is activated,
while a constant current consign is generated with the direct torque controller as
may be seen in Fig. 11.

Fig. 10: Torque time waveform [13]

Fig. 11: Three phase current waveforms [13]

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

The PI controller presents better performance in terms of torque ripple minimi-

zation and even following the torque consign. However, this is very dependent
on the current reading sample time capability and the needs of the control board.
Furthermore, using a PI controller the torque capability is reduced. Therefore, the
combination of control methods proposed in this work is considered a good op-
tion, where accuracy, behavior and computational speed are combined.

B. Test Bench

After simulation, and once having the motor prototype experimental tests were
carried on in the test bench shown in Fig. 12. First stall or static characteristic of
the machine was obtained supplying the machine with a DC current supply. Then
dynamic tests were done so both the machine dynamic performance and the con-
trol strategy were validated.

Fig. 12: Test Bench

Fig. 13: Experimental points of Torque VS current static characteristic (Phase maximum current
130 A DC)

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SRM DRIVES FOR ELECTRIC TRACTION

Fig. 13 shows the experimental test points of the Torque Vs current static char-
acteristic of the VENUS motor. Due to the power limits of the DC current supply
equipment, the maximum testing current level has been stablished at 130A.

Fig. 14: Resulting currents of each phase for the case of 100 A controlled at both control modes,
overlapping (up) and none overlapping (down)

Fig. 15: Torque @100A, Overlapping and none overlapping

118
 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

The first task to obtain the best dynamic performance of the machine is to com-
pute turn ON/OFF angles at each current and speed working point. The main
objective is to get the maximum torque possible at each current and speed refer-
ence point. It is also necessary to validate that the dynamic evolution of the phase
currents is good. This is a key point to assure that the maximum peaks values do
not go above the protection limits.

The optimization has been done for the two operation mode strategies, Fig. 14
and Fig. 15:
1. No overlapping Strategy: The phase currents do not have a common area of
constant current value at top set point level.
2. Overlapping Strategy: The phase currents have a common area of constant
current value at top set point level.

The optimization of the ON/OFF angles is based in an iterative process, for each
strategy (overlapping and no overlapping) and speed and current level:
1. The control is test with the theoretical ON/OFF values obtained from the sim-
ulations of the control design task.
2. An iterative adjustment of the ON/OFF angles is done around the initial values
searching the maximum torque value per ampere.
3. The current waveform is checked. In case of high currents peaks, these angles
are discarded going back to point 2.

Next, some examples of the current shape with the optimized points for the non-
overlapping strategy are shown in Fig. 16 and Fig.17. For the overlapping strat-
egy, similar dynamic performances are obtained.
400

350

300

250 Phase
Current [A]

Battery
200

150

100

50

0
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Time[s]

Fig. 16: Phase current waveform & battery current waveform (@Torque 1pu & Speed 100rpm)

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SRM DRIVES FOR ELECTRIC TRACTION

400
Phase
350 Battery

300

250
Current [A]

200

150

100

50

0
0 0.5 1 1.5 2 2.5 3 3.5 4
Time[s] -3
x 10

Fig. 17: Phase current waveform & battery current waveform (@Torque 0.6pu & Speed
4000rpm)

Once the on/off angles are optimized and the PI controller is set for low speeds,
the dynamic performance of the machine is obtained as shown in Table III. At
low speed / low torque demand, the system is able to give the desired torque but,
at higher speeds and torque demand the given torque is reduced as predicted.

TABLE III: MEASURED TORQUE (PU) AT DIFFERENT TORQUE REFERENCE SET-POINTS

PU\rpm 100 1000 2000 3000 4000

0.2 0.21 0.21 0.20 0.20 0.2
0.6 0.62 0.61 0.61 0.6 0.6
1 1 0.98 0.90 0.78 0.6

6.5. Conclusions
The results of the proposed torque control strategy, which combines a torque
feedback control for low speeds and a direct current, consign generator for higher
speeds show its validity.
At lower speeds where torque ripple may be critical, mostly when accelerating
from zero speed a prominent ripple can lead to an uncomfortable driving experi-
ence due to vibrations. Even more critical may be when starting in a slope; the
vehicle may go in reverse direction due to a negative torque caused by the ripple.
This all is avoided as shown in the results using a PI control at low speeds.

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 Definition of a Strategy for an Axial Flux Switched Reluctance Machine Torque Control

At higher speeds, the ripple vibration is not so noticeable by the vehicle occupants
as the inertia may absorb it, so a direct current consign generation show to be
enough.
The main difficulty of this control is that the characterization of the electrical
machine will define the effectivity of the control. However, his work has shown
that obtaining the torque and inductance characteristic of the machine is an easy
task, leading to a proper control.
The optimal on/off switching angles calculated in simulation give a good refer-
ence for the experimental setting of the actual optimal switching points. How-
ever, these points must be slightly changed so the torque per ampere ratio is
maximized.

6.6. References
[1] Labak and N. C. Kar, "A Novel Five-Phase Pancake Shaped Switched Reluctance
Motor for Hybrid Electric Vehicles," Vehicle Power and Propulsion Conference,
2009. VPPC '09. IEEE, Dearborn, MI, 2009, pp. 494-499.
[2] A. Labak and N. C. Kar, "Designing and Prototyping a Novel Five-Phase Pancake-
Shaped Axial-Flux SRM for Electric Vehicle Application Through Dynamic FEA
Incorporating Flux-Tube Modeling," IEEE Transactions On Industry Applications,
Vol. 49, No. 3, pp. 1276-1288, May-June 2013.
[3] Horacio Vasquez, Joey K Parker, "A New Simplified Mathematical Model for a
Switched Reluctance Motor in a Variable Speed Pumping Application". Mechatron-
ics, Volume 14, Issue 9, November 2004, Pages 1055-1068, ISSN 0957-4158
[4] S. Smaka, S. Masica and M. Cosovic, "Fast Analytical Model of Switched Reluc-
tance Machine," Power Electronics Conference (IPEc-Hiroshima 2014 - Ecce-
Asia), 2014 International, Hiroshima, 2014, pp. 1148-1154.
[5] P. Lobato, S. Rafael, P. Santos and A. J. Pires, "Magnetic Characteristics Modelling
for Regular Switched Reluctance Machines: Analytical and FEM Approaches,"
Power Engineering, Energy and Electrical Drives, 2009. POWERENG '09. Interna-
tional Conference on, Lisbon, 2009, pp 60-65.
[6] B. Parreira, S. Rafael, A. J. Pires And P. J. C. Branco, "Obtaining The Magnetic
Characteristics of an 8/6 Switched Reluctance Machine: from FEM Analysis to the
Experimental Tests," IEEE Transactions On Industrial Electronics, Vol. 52, No. 6,
pp. 1635-1643, Dec. 2005. DOI: 10.1109/Tie.2005.858709
[7] H. Le-Huy and P. Brunelle, "A Versatile Nonlinear Switched Reluctance Motor
Model in Simulink using Realistic and Analytical Magnetization Characteristics,"
Industrial Electronics Society, 2005. IECON 2005. 31st Annual Conference of
IEEE, 2005.

121
SRM DRIVES FOR ELECTRIC TRACTION

[8] F. Soares and P. J. Costa Branco, "Simulation of a 6/4 Switched Reluctance Motor
Based on Matlab/Simulink Environment," IEEE Transactions On Aerospace And
Electronic Systems, Vol. 37, No. 3, Pp. 989-1009, Jul 2001.
[9] Dong-Hee Lee (2011). "Advanced Torque Control Scheme for the High Speed
Switched Reluctance Motor". Advances in Motor Torque Control, Dr. Mukhtar Ah-
mad (Ed.), ISBN: 978-953-307-686-7, INTECH, DOI: 10.5772/20701.
[10] I. Husain, "Minimization of Torque Ripple in SRM Drives". IEEE Transactions on
Industrial Electronics, Vol. 49, No. 1, pp. 28-39, Feb 2002.
[11] J. Peter, “Modeling & Torque Ripple Minimization of Switched Reluctance Motor
for High Speed Applications” International Journal of Science and Modern Engi-
neering (IJISMe) ISSN: 2319-6386, Volume-1, Issue-10, September 2013
[12] Jin Woo Lee, Hong Seok Kim, Byung Il Kwon And Byung Taek Kim, "New Rotor
Shape Design for Minimum Torque Ripple of SRM Using FEM". IEEE Transac-
tions on Magnetics, Vol. 40, No. 2, pp. 754-757, March 2004.
[13] A. Egea, G. Ugalde and J. Poza, "Torque Control Strategy for an Axial Flux
Switched Reluctance Machine," 2016 XXII International Conference on Electrical
Machines (ICEM), Lausanne, 2016, pp. 1215-1220.
DOI: 10.1109/Icelmach.2016.7732679
[14] A.Egea, G. Ugalde, J.Poza. “Torque control strategy for an axial flux switched re-
luctance machine”. Workshop on SRM drives an alternative for E-Traction. Febru-
ary 2, 2018. Vilanova I la Geltrú (Spain)

122
 7
Novel In-Wheel Double Rotor
Axial-Flux SRM Drive for Light
Electric Traction
Pere Andrada, Balduí Blanqué, Eusebi Martínez,
José I. Perat, José A. Sánchez, Marcel Torrent

7.1. Introduction
Light electric vehicles (LEVs) can be categorized as L vehicles in accordance
with Directive 2007/46 /EC of the European Union (EU) [1]. This category of
vehicles comprises two-, three- and four-wheeled vehicles with limited weight
(maximum 550 kg) and power (maximum 15 kW). Light electric vehicles con-
tribute to improving mobility and reducing the emissions of pollution and
greenhouse gases, which is desired by both government authorities and citizens.
These vehicles are generally more affordable than cars and, given their smaller
size, they can be parked in small spaces while also helping to reduce traffic
congestion. As a result of these advantages, LEVs currently lead in global elec-
tric vehicle sales, a market that is expected to grow from 9.3 trillion dollars in
2017 to 23.9 trillion dollars in 2025. China and especially other countries in
Asia (Indonesia, Vietnam, Japan and India) are the markets where the highest
growth is expected [2]. More moderate growth is anticipated for Europe and the
USA, and it will depend not only on more attractive models emerging, but also
the enactment of government policies that will restrict the use of internal com-
bustion vehicles. LEVs are powered by an electric drive (motor + electronic
power converter + control) that ranges between 2-10 kW of power and they are
supplied by means of a battery pack (Pb-Acid, Li-Ion) with voltages of between
36 and 100 V. There are two different types of electric drives for LEVs: direct
drives, in which the motors are located inside the wheel or wheels (in-wheel
motor or hub motors) and drives with a mechanical transmission (gears, toothed
belts or chains) between the motor shaft and the wheel [3]. Although in-wheel
motor drives have some drawbacks, such as an increase in unsprung mass that

123
SRM DRIVES FOR ELECTRIC TRACTION

deteriorates ride comfort, some of their advantages are the elimination of a me-
chanical transmission and the provision of more useful space in the vehicle.

Nowadays, the most usual in-wheel motors are brushless DC motors and per-
manent magnet synchronous motors with external rotors. Nevertheless,
switched reluctance motors constitute a promising alternative because of their
rugged construction and lack of permanent magnets. Rotary switched reluctance
machines (SRM) are usually radial flux machines in which the air-gap flux
flows mainly in the radial direction relative to the axis of rotation. This type of
SRM usually has a cylindrical shape with a stator and rotor that are most com-
monly internal but can also be external. Less common among rotary switched
reluctance machines is the axial flux SRM, in which the air-gap flux flows
mostly parallel to the axis of rotation. The stator and rotor are arranged in paral-
lel plates perpendicular to the axis of rotation. Some studies on axial flux
switched reluctance motors demonstrate that this type of machine can obtain
higher torque density than radial flux switched reluctance machines. These su-
perior features of the axial flux switched reluctance machine are due to the in-
crease in air-gap area, which depends on the diameter of the machine; whereas
the air-gap area in radial type machines depends on the machine's length.

The first axial-flux variable reluctance motor was reported by Unnewher and
Koch in 1973 [4], while in recent years some authors have made important con-
tributions to developing axial-flux switched reluctance machines. Arihara et al.
have presented the basic design methodology for the axial counterpart of the
classic rotary SRM [5]. Murakami et al. have studied the optimization of an
axial-flux 18/12 SRM [6]. Madhavan el al. have contributed to developing the
axial counterpart to a rotor segment SRM in a machine comprising two rotors
and a stator with toroidal type winding [7]. Labak et. al. have proposed a novel
multiphase pancake shaped SRM with a stator composed of a series of C-cores,
each with an individually wound coil, perpendicularly disposed to an aluminum
rotor, in which a suitable number of cubes, the rotor poles, of high permeability
material have been added. Torque is produced in this machine by the tendency
of these cubes to align with the two poles of an energized C-core [8]. Some
authors have detailed the manufacturing problems of these machines and pro-
posed building its magnetic circuit with different materials such as grain orient-
ed electrical steel (Ma et al. [9]), soft magnetic composites (Kellerer et al. [10])
and sintered lamellar soft magnetic composites (Lambert et al. [11]).

This chapter presents a summary of the authors' work [12-15, 19] on a new
axial-flux switched reluctance machine (AFSRM) using a particular distribution
of stator and rotor poles, which results in short flux paths without flux reversal
and an electronic power controller specially intended for the propulsion of
LEVs.

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 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

7.2. Description of the proposed AFSRM

A. Basic considerations

In this chapter a novel axial-flux SRM (AFSRM) with a stator sandwiched be-
tween two rotors has been designed for LEVs. It is a particular case of a three-
phase machine, with multiplicity equal to two, of the family of axial-flux SRMs
presented in [12]. Fig.1 provides a schematic view of the proposed AFSRM.
The stator and rotors are arranged in parallel planes that are perpendicular to the
rotation axis. Each rotor is separated from the stator by an air-gap. Both rotors
are formed by a number of poles, NR, that are all joined together on the opposite
side of the air-gap by means of a flat annular piece. The rotor poles and flat
annular piece are made of ferromagnetic material. The stator is formed by a
number of poles, NS, protruding and with the same length at both ends of a sup-
porting disk that is attached to a hollow shaft. The cross section of the stator
pole is triangular, although other shapes are possible. The poles are made of
ferromagnetic material and the structural disk of a nonmagnetic material.

Fig.1: Schematic view of AFSRM

Two coils are wound around the opposite ends of the stator poles and are con-
nected in series. The coils of two adjacent stator poles are also connected in
series forming a double electromagnet, Z. The numbers of double electromag-
nets, stators and rotor poles will be given according to the number of phases of
the machine, m, in agreement with the following relationships:

125
SRM DRIVES FOR ELECTRIC TRACTION

Z =km (7.1)
where k is an integer denominating multiplicity, that is, the number of working
stator pole pairs. In the case of k >1, the phase windings are obtained by proper-
ly connecting the k different double electromagnets of each phase. In any case,
the terminals of the phase windings are led through the hollow shaft and out of
the machine.

The number of stator poles, NS, is given by:

N=
S 2=
Z 2km (7.2)
Both rotors have a number of rotor poles, NR, that have to accomplish the fol-
lowing rule:
N R k (2m − 1)
= (7.3)
The angle, γ, between the axes of two consecutive double electromagnets in the
stator is given by:
360º
γ= (7.4)
Z
And the angle, α, between two rotor poles is equal to:

360º
α= (7.5)
NR

Therefore the angle, δ, between the axes of the stator poles of two consecutive
double electromagnets is:
360º ⋅ ( N R − (k ⋅ m) )
δ =γ − α = (7.6)
k ⋅ m ⋅ NR

In Fig. 2, the disposition of the aforementioned angles (α, γ and δ) in the stator
and rotor is depicted.

Fig. 2: Disposition of the double electromagnets in the proposed AFSRM: stator (left), rotor
(right)

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 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

Fig. 3 shows the flux path when a current flows through the coils of a double
electromagnet. The flux lines link the stator poles of both sides with the poles of
the two rotors, thus forcing the alignment of these poles without flux reversal.

Fig. 3: View of the flux path in one double electromagnet

Fig. 4: Comparison of B-H curves between M235-35A, Somaloy 700 3P and the SMC used in
this study.

B. Magnetic material considerations

The proposed AFSRM's magnetic circuit is difficult to construct using silicon

iron laminations, which is why this machine's magnetic parts for the stator and
rotors were made of SMC (Soft Magnetic Composites) [16-18]. SMC are iron
powder particles separated by an electrically insulated layer. The benefits of
SMC are: a unique combination of magnetic saturation; low eddy current loss-
es; 3D-flux-carrying capability; and cost-efficient production of 3D-net-shaped
components due to the powder metallurgy process. However, SMC has low

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SRM DRIVES FOR ELECTRIC TRACTION

induction of saturation and higher specific losses than the silicon iron steels that
are commonly used in high-performance electric machines, such as the M235-
35A. Somaloy 700 3 P was our first choice, due to its high magnetic character-
istics. However, in the end, practical and economic factors informed our deci-
sion to manufacture the stator and rotor pole pieces of the AFSRM by tooling
pre-fabricated SMC blanks with lower magnetic characteristics than Somaloy
700 3 P. Here, we refer to this material as "used SMC". Fig. 4 compares the B-
H curves of both SMC materials with the B-H curve of M235-35 A.

C. Description of the AFSRM prototype

A prototype of an in-wheel AFSRM with natural cooling was designed with the
goal of propelling a direct traction light electric scooter with 13-inch wheels
and no transmission. It required a maximum torque of 80 Nm between 0 to 30
km/h (~334 rpm) and a constant power of 2.8 kW between 30 km/h to 18 km/h
(~1200 rpm). Due to the machine being placed inside a 13-inch wheel, the max-
imum external dimensions equal the motor frame's diameter of 308 mm and an
axial length of 116 mm. Fig.5 shows an exploded view of the prototype: a two-
rotor three-phase AFSRM with multiplicity two, 12 stator poles and 10 rotor
poles.

Fig. 5: Exploded view of the AFSRM prototype

The stator poles were made of SMC and inserted into the supporting disk
(shown in red in Fig. 5). Moreover, the rotor poles and flat annular support
were built by a set of 10 independent SMC pieces glued to both covers of the
machine. The covers and supporting disk were made of aluminum. Fig. 6 shows
photographs of the stator and rotor pole pieces. A rated voltage of 96 V was

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 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

chosen to avoid high current values. Two different alternatives were considered
for the winding of the AFSRM prototype, both of which using insulation of ther-
mal class 180ºC. First, opposite double electromagnets were connected in series
(Fig. 7), with Nc turns per coil and each conductor having two subconductors in
parallel each with diameter d. Second, opposite double electromagnets were con-
nected in parallel (Fig. 8), with 2Nc turns per coil and each conductor having a
diameter d. Finally, in order to facilitate the construction of the winding, we
chose the second alternative. Each coil was wound with 32 turns and rectangular
rather than round wire was used (2 x 2.39 mm2). This solution resulted in a very
low skin factor, since the maximum expected frequency of the currents in the
machine is 250 Hz (corresponding to a maximum speed of 1500 rpm). Further-
more, this provided a filling factor of 56.4%. Fig. 9 shows photographs of the
stator and the stator poles inserted into the supporting disk, with the disposition of
two adjacent pre-wound coils. The measured total phase resistance (20ºC) of the
winding and the output cable (AWG7) was 60 mΩ.

Fig. 6: Stator pole (left) and rotor pole pieces (right), made of SMC

Fig. 7: Schematic drawing of one phase of the AFSRM showing the coil arrangement and the
series connection of the double electromagnets that are diametrically opposed

Fig. 8: Schematic drawing of one phase of the AFSRM showing the coil arrangement and parallel
connection of the double electromagnets that are diametrically opposed.

129
SRM DRIVES FOR ELECTRIC TRACTION

Fig. 9: Stator showing the inserted pole pieces and the disposition of two adjacent coils

Fig.10: Stator and two motor covers: the central part the SMC rotor pieces glued onto the cover
(left); the fully assembled AFSRM prototype (right).

TABLE I. MAIN DIMENSIONS OF THE AFSRM PROTOTYPE

Parameter Symbol Values

Number of phases m 3
Number of double elec-
Z 6
tromagnets

Number of stator poles NS 12

Number of rotor poles NR 10

Angle between rotor poles α 36

Angle between axes of

stator poles in the two
consecutive double elec- δ 24º
Stator tromagnets
Angle between axes of
consecutive double elec-
γ 60º
tromagnets
Output stator/
Do 260 mm
rotor diameter

130
 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

Parameter Symbol Values

Inner rotor diameter Di 117.9 mm
Inner stator diameter d 50 mm
Air-gap g 0.5 mm
Height of protruding
he 28.9 mm
stator pole
Thickness of suporting
hce 12 mm
disk
Height of rotor pole hr 10 mm
Thickness of rotor yoke hcr 8 mm
Rotor

Fig.10 provides a view of the complete stator, including the winding and the
two covers with the glued rotor pole pieces. Table I indicates the main dimen-
sions of the prototype: a three-phase AFSRM with multiplicity two.

7.3. Electronic power controller

Currently, no commercial electronic power controllers exist for light electric

traction SRMs. This is one of the causes slowing down the use of SRM as a
power traction unit for light electric traction vehicles. In order to overcome this
drawback an SRM controller for light electric vehicles was designed and built.
Specifically, this controller was conceived for powering any type of low voltage
three-phase SRM designed for propelling light electric vehicles [19]. Fig. 11
depicts a wiring diagram of the SRM controller showing the connection of the
power components (battery pack, SRM), of the sensors (speed/position, temper-
ature) and of the driving functions of the vehicle (key switch, throttle, brake,
forward/reverse). The controller must handle input voltages of between 36 V up
to 100 V from a pack of batteries and should be able to work with a peak cur-
rent of 400 A for 2 minutes and 150 A RMS for 60 minutes. It has to manage
the signals from the SRM sensors (speed/position, current and temperature) and
the control inputs/outputs, thus requiring that special attention be paid to com-
munications. It should allow installing external software for controlling the
SRM and even connecting it to a HIL (hardware in the loop) platform. In addi-
tion, it should be small, compact, safe and ready to work on board a light elec-
tric vehicle; therefore, all pertinent regulations should be taken into account.
The entire controller should be encased in housing that guarantees IP66 protec-
tion.

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SRM DRIVES FOR ELECTRIC TRACTION

Key Switch

BUS +
Line
contactor
Battery
Pack Throtle DC

BUS -
A+
A-

SRM B+
B- Brake
C+
C-
+ PT100
- PT100 Forward-
Reverse
Ch A
Ch A
Ch B
Enc Ch B
Forward
Index
Index Reverse
5V
GND

Fig.11: SRM controller wiring diagram.

In order to accomplish the abovementioned specifications, the controller's archi-

tecture was designed according to the block diagram shown in Fig.12. In the
following sections describe the different parts: electronic power converter, ca-
pacitor bank, ancillary parts, printed circuit board, DSP controller and HIL plat-
form.

Serial Communitations: External digital Signal

Bus CAN / i2c Input/Output
Auxiliary
Battery IP66 enclosure
Position
DSP Controller/HIL Platform (dSPACE) Sensor
Power Signal
12 V Supply
Conditioning
Control
Drivers Protection
Motor
Voltage Temperature
Current Sensor
Battery VDC Temperature
V IA D’A IB D’B IC D’C
Pack
DC Link T T T
Capacitor SRM
I I I
DA I’A DB I’B DC I’C

0V Classic Converter

Heat sink-air cooled

Fig.12: Block diagram of the SRM controller

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 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

A. Electronic power converter

The electronic power converter is the core of the controller. Even though a
number of phases greater than three can be a good choice for an SRM drive, as
this reduces torque ripple and increases fault tolerance. When minimizing the
number of power components, the best and most advisable option is usually a
three-phase SRM drive, as a consequence the number of phases of the power
converter is set to three. Although some efforts have been made to use common
three-phase inverters for AC machines (IM and PMSM), they are not entirely
suitable for SRM drives [20, 21] because the torque in SRM drives is independ-
ent of the current sign and therefore requires unipolar converters. There are
several types of power converters for SRM [22], but in this case we chose a
classical asymmetric power converter with two power switches and two diodes
per phase (Fig.12), because this ensures independence of phases and provides
fault tolerance.
Commercial phase-leg modules containing solid state power switches can be
used in SRM power converters, but they require twice as many modules as an
inverter does for alternating current machines (when they really uses only half
of the components in each module), implying a significantly higher cost for the
electronic power converter. In the case under study, the high electric require-
ments resulted in the choice of discrete components for the power switches and
diodes, which were, respectively: power MOSFETs and high voltage Schottky
diodes.
Taking into account the maximum voltage that the converter needs to work
(112 V for a 100 V battery pack), the power semiconductors were chosen with a
voltage margin that was at least 30% higher in order to ensure that the voltage
peaks caused by switching transients did not damage the power semiconductors.
The power MOSFET selected was Ixys IXFN360N15T2, because of its low
RDS,on, and the chosen diode was Microsemi APT2X101S20J (which includes
two diodes in parallel). Both with SOT-227 package due to its ability to electri-
cally isolate active parts and thus requires no additional insulation film between
the case and heat sink, i.e., thermal resistance is reduced. In addition, this pack-
age allows for semiconductors to be simply screwed onto a standard flat extrud-
ed heat sink. Finally, as a result of many simulations, each switch and diode in
Fig. 12 was built with, respectively, 4 Ixys IXFN360N15T2 modules in parallel
and 2 Microsemi APT2X101S20J modules in parallel.

B. Capacitor bank

The design of the capacitor bank took into account: the maximum voltage ripple
(7 V peak to peak), the resistance of the battery and connection cables (10 mΩ);
and the worst-case current conditions (150 A RMS). Furthermore, we sought to
minimize the surface area occupied by the capacitors and found the most compact
solution to be 66 cylindrical capacitors (Rubycon .160BXW820MEFR18X50: 82

133
SRM DRIVES FOR ELECTRIC TRACTION

μF, 160 V), which occupied a surface area of approximately 210 cm2 (each ca-
pacitor had a diameter of 18 mm and a height of 50 mm), with a total capacity of
54 mF.

C. Ancillary parts

Other important parts of the SRM controller that we have denominated "ancil-
lary parts" are: the control power supplies, the signal conditioning and the gate
drivers. The controller's electronics were powered by a 12 V rail that required
an external supply. This 12 V rail generated three other auxiliary voltages for
internal usage and supplied optional external elements: -12 V for the negative
supply of current sensors and conditioning the analog circuit; plus 3.3 V and 5
V for supplying other digital components. These auxiliary voltage supplies were
implemented with non-isolated step-down converters.
For measuring the current, we used four open loop Hall effect current sensors
(LEM HAS 300-P): one for each of the three phases and another for the DC bus
current. The measurement gain can be configured in two ways: by making one
or two turns with the power cable around the sensor or by choosing one of two
possible gains from the analog circuitry. For the DC bus voltage measurement,
a voltage divider was used with an analog adaptation circuit. This measurement
was also configurable with two possible gains.
The signal conditioning was implemented in two stages. The first adjusted the
range to within 0-10 V when the measurement is unipolar, or ±10 V when it is
bipolar. After this first stage, the signal is branched into two paths: the first one
to the dSPACE; and the second one to another analog stage that adapts to the 0-
3 V range required by the DSC analog-to-digital acquisition.
Each of the 4 parallel power MOSFETs were driven by a single gate-driver,
which was isolated in order to avoid high frequency interference between power
and control. The integrated circuit chosen for this purpose was the
ADUM3223BRZ and employed one for each of the 6 channels. The floating
secondary sides were supplied with 9 V by means of 6 standard isolated power
supplies (MTE1S1209MC).

D. Printed circuit board

Regarding the power converter, the capacitor bank, the isolated gate drives, the
voltage, current and temperature sensors, the signal conditioning, the control
power supplies, the power connectors and the input/output connectors: all of
these were incorporated onto a printed circuit board (PCB) of 6 layers with a
total of 70 µm copper thickness. The semiconductors (MOSFETs and diodes)
were mounted on an aluminum heat sink. The great challenge we faced in de-
signing the power part of the PCB was obtaining good thermal performance in
light of the high currents it had to handle. By correctly placing the components
and adequately routing the tracks and conduction planes, the current path's re-

134
 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

sistance and inductance are minimized, which helps in two ways: first, the Joule
conduction losses are reduced, and this facilitates thermal handling; second, it
reduces ringing due to the switching of high currents. The layout of the power
part was designed in order to minimize the current loops of the 6 switching cells
(formed by the capacitor, the MOSFET and the diode of each leg). This layout
allows the DC bus to be implemented with 4 continuous copper layers along the
6 legs (2 layers for positive and 2 for negative), thus minimizing the DC bus
inductance and enhancing the heat dissipation. The midpoints of each arm,
which are also the motor outputs, were routed across only the bottom layer for
contact with the power semiconductors and across the top layer, where they
were reinforced with copper bars that were screwed onto the PCB. To improve
the flow of the currents from the battery to the PCB (400 A during 2 min), addi-
tional copper bars were implemented to connect the legs of the 3 phases (see
bottom of Fig. 13).

Fig.13: Plan (left) and side (right) views of the controller for SRM.

E. DSP Controller and HIL Platform

The SRM controller included a DSP, TMS320F28335 control CARD from

Texas Instruments attached to the main PCB, denominated DSC. The SRM
controller was designed to also allow for control of the converter by means of
an external HIL platform, if required. Fig. 14 shows how the converter can be
controlled and monitored by both DSC and by an HIL platform, which in this
case is dSPACE.

Fig.14: Control of the power converter by DSC or by dSPACE.

135
SRM DRIVES FOR ELECTRIC TRACTION

7.4. Performance evaluation of the proposed AFSRM drive

Once the AFSRM and the controller were built, the drive was tested in a test rig
in order to assess its performance.

A. Test rig description

The AFSRM was mounted and coupled through a torque-meter (Kistler

4550A500S10N1KA0 500 Nm) to a DC motor brake (DC LOAD). In addition,
it was connected to a DC source that was variable in voltage and current (APS
DPS 100-450) by means of the electronic power controller described in section
7.3 and controlled by a HIL platform (dSPACE ACE 1006). The measurements
of torque, speed and power were monitored by means of testing power analyzer
system (eDrive package GEN2tA-6ch-2Ms/s, from HBM). Fig. 15 shows a
photograph of the complete test layout.

B. AFSRM drive control

Fig. 15: AFSRM drive test rig

136
 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

In order to meet the drive's requirements (Section 7.2), the control strategy for
the proposed AFSRM drive used: hysteresis control with variable turn-on and
turn-off angles at low and medium speeds; and a single pulse with variable turn-
on and turn-off angles for high values of speed. The values of reference current
and of the turn-on and turn-off angles for each operating point were determined
after many performed with Matlab-Simulink using the results of finite element
analysis on the AFSRM [12-13].

C. Experimental results of the AFSRM drive

The test rig enabled us to determine the experimental torque-speed curves. Fig.
16 shows these curves, including power isolines, which allows for better com-
parison with the drive's requirements. We also consider the behavior of the
drive at different operating points under the aforementioned control strategy.
Fig.17 shows: the waveforms of phase current, phase voltage and bus current
for 500 rpm, 60 Nm with hysteresis control (θON= -4º, θOFF= 12º). Fig. 18 de-
picts the waveforms of phase current, phase voltage and bus current for 900
rpm, 30 Nm with hysteresis control (θON= -7º, θOFF= 8º). Fig. 19 shows the
waveforms of phase current, phase voltage and bus current for 1400 rpm, 20
Nm with single pulse control (θON= -7º, θOFF= 8º).

Fig.16: Torque-speed curves experimentally obtained on the test rig.

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SRM DRIVES FOR ELECTRIC TRACTION

Phase Voltage [V]

100

50

-50

-100
Phase Current [A]
200

100

0
Bus Current [A]
200

150

100

50

0
0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
Time (s)

Fig. 17: Waveforms of phase current, phase voltage and bus current for 500 rpm, 60 Nm with
hysteresis control θON= -4º, θOFF= 12º.
Phase Voltage [V]
100

50

-50

-100
Phase Current [A]
150

100

50

0
Bus Current [A]

100

50

0
0 0.005 0.01 0.015 0.02 0.025
Time (s)

Fig. 18: Waveforms of phase current, phase voltage and bus current for 900 rpm, 30 Nm with
hysteresis control θON= -7º, θOFF= 8º.
Phase Voltage [V]
100

50

-50

-100
Phase Current [A]
120

100

50

0
Bus Current [A]

100

80

60

40
0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 0.021
Time (s)

Fig. 19: Waveforms of phase current, phase voltage and bus current for 1400 rpm, 20 Nm with
single pulse control θON= -7º, θOFF= 8º.

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 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

7.5. Conclusions
This chapter has presented a new in-wheel axial-flux switched reluctance motor
(AFSRM) drive designed especially for the propulsion of light electrical vehi-
cles. This motor comprises a stator sandwiched by two rotors, and the particular
arrangement of the stator and rotor poles generates short flux paths without flux
reversal. Due to the difficulty in manufacturing the magnetic circuit with lami-
nated silicon iron was made of soft magnetic composites. The complete elec-
tromagnetic analysis of the prototype was performed using 3D-FEA, and the
results obtained were used by Matlab-Simulink for the simulation of the whole
drive, taking into account the AFSRM, the power converter and the control
strategies. Simulations show that the performance of the drive matches very
well with the requirements of LEVs.

7.6. Acknowledgements
This research was supported by the Spanish Ministry of Economy and Competi-
tiveness (DPI 2014-57086-R) and Feder funds. The electronic power controller
was built by CITCEA. The authors would like to thank AMES S.A. for provid-
ing the SMC pieces, especially for the support given by Dr. Mark Dougan,
Chief Metallurgist in the Dept. of R&D AMES S.A., and Dr. José Antonio
Calero, R&D Manager of AMES S.A..

7.7. References
[1] Directive 2007/46/EC, consolidated version of 31 March 2018 (contains amend-
ments and corrections up to and including Commission Regulation 2017/2400).
[2] Navigant Research. Executive Summary: "Light Electric vehicles, Low
speed/Neighborhood EVs, Electrical Motorcycles, and Electric Scooters: Global
Market Analysis and Forecasts". Published 1Q 2017.
[3] Y. Tang, J. J. H. Paulides, I. J. M. Besselink, F. Gardner, E. A. Lomonova. "Indi-
rect Drive In-Wheel System for HEV/EV Traction". EVS27 Barcelona, Spain,
November 17 - 20, 2013, pp. 1-9.
[4] L. E. Unnewehr and W.H. Koch. "An axial air-gap reluctance motor for variable
speed applications", January/February 1974, IEEE Transactions on Power Appa-
ratus and Systems.
[5] Arihara and K. Akatsu. "Basic properties of an axial-type switched reluctance
motor". IEEE Transactions on Industry Applications, Vol 49, No 1, Janu-
ary/February 2013, pp. 59-65.

139
SRM DRIVES FOR ELECTRIC TRACTION

[6] Murakami, H. Goto, O. Ichinokura. "A study about optimum stator pole design of
axial-gap switched reluctance motor". ICEM 2014, 2-5 September, Berlin, pp.
975-980.
[7] R. Madhavan, B.G. Fernandes. "Axial flux segmented SRM with a higher number
of rotor segments for electric vehicle". IEEE Transactions on Energy Conversion,
Vol. 28, No 1, March 2013, pp.203-213.
[8] Labak, N.C. Kar. "Designing and prototyping a novel five-phase pancake-shaped
axial flux SRM for electric vehicle application through dynamic FEA incorporat-
ing flux-tube modeling". IEEE Transactions on Industry Applications, Vol. 49, No
3, May/June 2013, pp.1276-1288.
[9] Ma, R. Qu, J. Li. "Optimal design of an axial flux switched reluctance motor with
grain oriented electrical steel". 18th International Conference on Electrical Ma-
chines and Systems (ICEMs), 25-28 October 2015, Pattaya City, Thailand 2015,
pp. 2071-2077.
[10] T. Kellerer, O. Radler, T. Sattel, S. Purfürst. "Axial type switched reluctance mo-
tor of soft magnetic composite". Innovative Small Drives and Micro-Motor Sys-
tems, 19-20 September 2013, Nuremberg, Germany, pp.29-34.
[11] T. Lambert, M. Biglarbegian, S. Mahmud. "A novel approach to the design of
axial-flux switched reluctance motors". Machines 2015, 3, 27-54.
[12] P. Andrada, E. Martínez, B. Blanqué, M. Torrent, J.I. Perat, J.A. Sánchez. "New
axial-flux switched reluctance motor for E-scooter". ESARS ITEC Toulouse, 2-4
November 2016.
[13] P. Andrada, E.Martínez, M.Torrent, B.Blanqué. "Electromagnetic evaluation of an
in-wheel double rotor axial-flux switched reluctance motor for electric traction"
REPQJ. Vol 1, No 15, April 2017. pp. 671-675.
[14] P.Andrada, B.Blanqué, E. Martínez, J.A. Sánchez, M.Torrent. "In wheel-axial-flux
SRM drive for light electric vehicles". Workshop on SRM an alternative for E-
traction. Vilanova i la Geltrú Barcelona Spain. February 2, 2018, pp. 39-46.
[15] PCT/EP2017/076976, "An axial flux switched reluctance machine and an electric
vehicle comprising the machine".
[16] A.Kringe, A.Boglietti, A.Cavagnino, S.Sprague. "Soft magnetic material status
and trends in electric achines". IEEE Transactions on Industrial Electronics, Vol
64, No 3, March 2017, pp 2404-2414.
[17] A.Schoppa, P.Delarbe. "Soft magnetic powder composites and potential applica-
tions in modern electric machines and devices". IEEE Transactions on Magnetics,
Vol 50, No 4, April 2014.
[18] M.J.Dougan.“An introduction to powder metallurgy soft magnetic components:
materials and applications”. Powder Metallurgy Review, Autumn/Fall 2015, pp
41-49.

140
 Novel In-Wheel Double Rotor Axial-Flux SRM Drive for Light Electric

[19] P. Andrada, B. Blanqué, M. Capó, G. Gross, D. Montesinos. "Switched reluctance

motor controller for light electric vehicles". 20th European Conference on Power
Electronics and Applications (EPE'18 ECCE Europe), 2018. P1-P11.
[20] A.C. Clothier and B.C.Mecrow. "The use of three phase bridge inverter with SR
drives". Eight Int. Conference on Electric Machines and Drives 1997, pp. 351-355.
[21] T. Celik. Segmental rotor switched reluctance drives. Phd Thesis. University of
Newcastle upon Tyne. School of Electrical, Electronic and Computer Engineering.
August 2011.
[22] S. Vukosavic and V.R. Stefanovic. "SRM inverter topologies: a comparative eval-
uation". IEEE Transactions on Industry Applications, vol 27, No 6, 1991, pp.
1034-1047.

141
 8
Axial-Flux Twin-Rotor
Segmented-Stator High-Speed
SRM for High-Performance
Traction Applications
Francisco J. Márquez-Fernández; Johannes H. J.
Potgieter, Malcolm D. McCulloch, Alexander G. Fraser

8.1. Introduction
The following chapter presents a summary of the work carried out by the authors
within the project M2SRM at the Energy and Power Group, Department of En-
gineering Science, University of Oxford, UK, and McLaren Automotive Ltd,
Woking, UK. This summary was first presented at the workshop "SRM drives -
an alternative for E-Traction", organized by the Department of Electrical Engi-
neering at the Polytechnic University of Catalonia (UPC) in February 2018 [1].
The original publications as well as numerous supporting references can be found
in [2 – 8].

Electric and Hybrid-Electric powertrains are now common in the market and
nowadays virtually all OEMs (Original Equipment Manufacturers or vehicle
manufacturers) offer one or several of such models. The motivations for electri-
fication differ across the different vehicle segments. While for small passenger
cars the main motivation is the reduction in fuel consumption and emissions, in
the higher performance vehicle class electric traction machines are used for their
superior dynamic response and higher power density. High performance turbo-
charged combustion engines suffer from “turbo-lag”, a phenomenon that drasti-
cally slows the response of the engine to step increases in torque demand,
especially at low rpm until the turbocharger kicks in. In contrast, an electric drive
provides almost instantaneous changes in torque. Combining both in a hybrid
powertrain results in a vehicle with increased performance and driveability that
is ultimately much more fun to drive.

Most of the commercial EV and HEV solutions sport some form of PMSM (Per-
manent Magnet Synchronous Machine) mostly due to their higher power and
torque density. However, PMSMs also bring in a number of disadvantages: over

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SRM DRIVES FOR ELECTRIC TRACTION

80% of the rare earth material needed for the fabrication of the permanent mag-
nets used in traction machines (neodymium and dysprosium mostly) is concen-
trated in China; thus the market price of these materials is somewhat volatile and
a change in trade policy by the Chinese government could have a large impact in
the supply chain of the EV and HEV industry. Moreover, in a PMSM the mag-
netizing field cannot be switched off, hence field weakening currents need to be
applied whenever the machine is operating above its base speed even if no torque
is being produced. This field weakening currents result in increased winding
losses, reducing the overall energy efficiency of the PMSM.

SRMs (Switched Reluctance Machines) do not rely on permanent magnets,

which can be seen as an advantage for the aforementioned reasons, while retain-
ing relatively high torque and power density in comparison with other topologies
such as the synchronous reluctance machine and the induction machine. The
higher levels of noise and vibrations associated with SRM operation could be
problematic, especially in a pure EV, but they are not considered a hinder in a
hybrid sport application as the one in this study, since the ICE (Internal Combus-
tion Engine) will expectedly create much stronger noise and vibrations.

In order to reduce cost and increase performance levels, there is a large drive to
increase the torque and power density of traction motors. By increasing the rota-
tional speed, the power density can be increased, although care must be taken to
preserve the mechanical integrity of the rotating parts. In addition, the high speed
needed for high power density, together with the high pole count needed to keep
the magnetic paths as short as possible while boosting the torque density result
in high frequency operation. This, in turn, leads to magnetic losses both in the
iron core of the machine (iron losses) and in the copper windings (proximity
losses), potentially reducing the energy efficiency of the machine. Moreover, the
lack of permanent magnets leads to higher current levels in order to produce the
desired torque. For all these reasons, the thermal management of this kind of ma-
chines is not trivial, and it will determine to a large extent the success of any
particular machine design.

8.2. Topology Selection and Description

A. Topology selection process

In order to find the optimal topology for the application, an extensive design
space was explored. All machine concepts considered in this process were based
on reluctance torque production and therefore permanent magnet free. The aim
was to maximize power and torque density. Besides, the machine concept se-
lected should preferably be multi-rotor, since this was considered a good feature
increasing both torque density and reliability in the initial project proposal. Fig.
1 shows some of the geometries considered.

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

All the machines were analyzed using 3D FEA (Finite Element Analysis) to ob-
tain the basic electromagnetic characteristics, and those that seemed interesting
were investigated further. The outcome of this selection process is the machine
topology described in this work.

Fig. 1: Some of the machine concepts in the design space considered

B. Description of the SRM topology

The selection process described in the previous section resulted in the following
electrical machine topology: 2-phase, axial flux, segmented twin rotors, seg-
mented stator SRM. The stator features individual poles made of SMC (Soft
Magnetic Composite) to achieve the complex geometric shapes and reduce mag-
netic losses. These poles are wound independently and then assembled together
as shown in Fig. 2 for the first prototype [5,6,8].

Fig. 2: Stator of the first prototype machine. Figure based on [6].

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SRM DRIVES FOR ELECTRIC TRACTION

The stator coils are pre-wounded and vacuum-impregnated in a high thermal con-
ductivity epoxy prior to assembly. Due to the specific requirements of the cooling
concept - presented in a later section - the coils must be structurally stable, able
to retain their shape when mounted on the stator poles. In addition, the high elec-
trical frequency during operation requires the use of litz wire. For all these rea-
sons, the manufacturing of the coils has proved to be significantly more
challenging than initially expected. Fig. 3 shows a coil under manufacturing and
a terminated coil [6].

Fig. 3: Fully finished coil (left) and bespoke tool for coil manufacturing (right). Figure based on [6].

The twin-rotors are also segmented, consisting of triangular SMC poles mounted
together in a structure that ensures the rotor’s mechanical integrity at the maxi-
mum rotational speed. A CAD (Computer Aided Design) model of the described
SRM topology can be seen in Fig. 4, where only the active parts of the machine
are shown. A list of the most relevant target specifications for the design of the
SRM traction machine is compiled in Table I.

Fig. 4: CAD model of the proposed SRM topology. Based on [5].

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

TABLE I. MAIN DESIGN PARAMETERS FOR THE SRM

Parameter Value
Rated power 60 – 80 kW
Maximum rotational speed 20 000 rpm
Base rotational speed 10 000 rpm
Maximum fundamental electrical frequency 4 kHz
Maximum outer diameter 254 mm
Maximum axial length 170 mm
Decoupled deceleration > 10 000 rpm/s
Rotor pole outer pressure limit < 125/353 MPa
Maximum winding temperature 180 C
Maximum SMC temperature 250 C
Maximum coolant outlet temperature 125 C

C. Cooling concept

Due to the high current levels in the winding, high frequency and magnetic satu-
ration during operation, the losses in the proposed SRM stator are not concen-
trated in either the coils or the stator poles, but rather evenly distributed among
them [6]. For this reason, a cooling concept in which the temperatures of the
winding coils and iron poles are decoupled is highly desirable.

This thermal decoupling is achieved by circulating a liquid coolant, preferably

water based, through cooling channels formed between the winding and the stator
poles as shown in Fig. 5. In this way, the cooling effect will be approximately
the same for both parts and the only thermal coupling besides the coolant tem-
perature is due to the plastic spacers used to mount the winding coils onto the
iron cores [3].

Fig. 5: Cooling ducts between the winding coils and the stator poles. Based on [4].

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SRM DRIVES FOR ELECTRIC TRACTION

8.3. Topology Optimization

A. Optimization process description

In this section, a brief description of the optimization of the selected SRM topol-
ogy is presented. The optimization scheme used couples FE electromagnetic
analysis and a lumped parameter thermal model in order to evaluate the impact
of the different design parameters in the performance of the machine [3].

Fig. 6: Optimization process flow diagram. Figure based on [3].

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

Each machine geometry considered is analyzed according to the following pro-

cedure: first, all main parameters of the machine are set. Then, the algorithm pro-
ceeds with the peak speed iteration, finding the optimal number of turns for the
stator coils such that at peak speed and full voltage the winding current is max-
imized without exceeding the maximum temperature threshold. In this iteration,
a transient thermal model considering only the thermal masses of the components
and excluding all cooling actions is used, ensuring that the peak power can be
sustained for at least 10 seconds.

Then, the continuous power iteration is carried out. First, the current is reduced
to half of the current obtained in the peak speed iteration, the speed is set to base
speed and the algorithm checks whether the current can be controlled to the spec-
ified value or a fixed pulse voltage excitation should be used. Subsequently, the
algorithm iterates again in order to find the current level that can be sustained
indefinitely without exceeding the thermal limits. In this case a steady state ther-
mal model is used, taking into account the direct cooling of both winding coils
and stator poles.

Finally, the optimizer evaluates the objective function for the analyzed geometry
and checks if it corresponds to a minimum point, in which case the optimization
is finished. Otherwise, the next parameter set is proposed and the whole process
is repeated for the corresponding new geometry.

B. Electromagnetic modelling

Both a 3D FE model and a 2D FE linearized model have been developed to eval-

uate the performance of the machine. During the optimization process, the 2D
model is used due to its lower computational cost. Once a good machine candi-
date is identified, a more exhaustive FE simulation is conducted using the 3D
model [3,5].

Fig. 7: 3D and 2D linearized FE models in the aligned (left) and unaligned (right) position. Figure
based on [5].

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SRM DRIVES FOR ELECTRIC TRACTION

C. Coolant flow and thermal modelling

In order to assess the thermal performance of the machine for the different load-
ing cases two models are created: a coolant flow model to estimate the coolant
flow distribution, heat transfer coefficients and pressure drop in the cooling chan-
nels, and a thermal model to estimate the temperature in the different parts of the
machine. Both models are lumped parameter models because of the need for fast
execution [4].

The coolant flow model estimates the flow distribution in the different cooling
paths by calculating the pressure drop based on the Darcy-Weisbach equation
and experimental correlations for the different bends and fittings.

The thermal model uses the convection coefficients from the coolant flow model
and the losses from the FE analysis in order to estimate the temperature of the coils
and the stator poles. The thermal model of the coil (shown in Fig. 8) models each
turn individually with four nodes, and takes into account not only heat conduction
along the wires, but also axial heat conduction between consecutive turns [8].

Fig. 8: Lumped parameter thermal model of a coil. Based on [8].

8.4. Experimental results

The degree of novelty in the proposed topology resulted in a number of unex-
pected delays during manufacturing – particularly of the winding coils – and also
during the testing phase that could not be addressed completely within the project
time scope. For these reasons, only two types of tests were successfully carried
out: thermal tests of purposely designed motorettes and static tests of the first
version of the machine prototype. Some relevant experimental results are pre-
sented in this section.

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

A. Thermal experiments

In order to validate the performance of the proposed cooling approach and to

calibrate the thermal models developed, motorettes comprising one stator tooth
and one coil have been manufactured (see Fig. 9) [6,8].

Fig. 9: Single core test motorette. Figure based on [6].

Fig. 10: Experimental setup (left). Location of the thermistors in a coil (right). Figure based on [6].

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SRM DRIVES FOR ELECTRIC TRACTION

Fig. 11: Logged temperatures (top), coolant flow rate (middle) and coolant temperature increase
(bottom) for 180 A. Figure based on [8].

Five miniaturized thermistors installed in different locations over the winding

allow to characterize the temperature distribution over the coil. One motorette is
placed in the test bench shown in Fig. 10, supplied by a 5 kW DC power supply
capable of delivering 25 V and 200 A. An electric pump circulates the coolant
fluid (distilled water in these experiments) through the circuit. The coolant flow
as well as the inlet and outlet coolant temperatures are also recorded.

B. Static mechanical tests

The machine prototype was installed in a test-bench in the lab and static torque
measurements were performed locking the rotor at certain predefined positions.
The figures in this section show some of the measurement results obtained from
these tests and their comparison to FE results when relevant [5].

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

Fig. 12: Experimental SRM on the test bench interfaced with a static adjustment mechanism.
Figure based on [5].

.
Fig. 13: Voltage and current vs. electrical angle for the SRM prototype (from FE simulations).
Figure based on [5].

Fig. 13 to 15 show the FEA results obtained for the prototype SRM. Fig 13 shows
the voltage and current of the machine when in operation. Fig. 14 shows the
torque vs. electrical angle obtained if the machine is supplied with the voltage
and current in Fig. 13. Fig. 15 shows the predicted torque and power as a function
of motor speed. Peak speed is defined as 20 000 r/min and base speed as 10 000
r/min.

Fig. 14: Torque vs. electrical angle for one electrical period for the SRM prototype (from FE
simulations). Figure based on [5].

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SRM DRIVES FOR ELECTRIC TRACTION

Fig. 15: Peak and continuous torque and power vs. speed for the SRM prototype
(from FE simulations). Figure based on [5].

Fig. 16: Measured and predicted static torque waveforms at a fixed current of 30 A.
Figure based on [5].

Fig. 17: Measured static torque waveforms at three different current values.
Figure based on [5].

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 Axial-Flux Twin-Rotor Segmented-Stator High-Speed SRM for High-Performance Traction Applications

Fig. 16 shows the static measured and FE-predicted torque versus position at a
constant current of 30 A per inverter leg. Fig. 17 shows the measured static torque
versus electrical angle for three different current values. Fig. 18 shows the meas-
ured average torque per electrical cycle versus the FE predicted torque. It should
be noted that the results shown are not conclusive and more testing is required to
provide a more indicative view on the performance of the new concept SRM.

Fig. 18: Measured and FE predicted average torque vs. current per coil of the experimental SRM.
Figure based on [5].

8.5. Conclusions
This chapter summarizes the work done on a novel SRM concept, featuring a 2-
phase axial flux topology, with a segmented stator and twin segmented rotors.
The proposed topology is designed for high-speed operation, aiming for power
density values in the range of 7 kW/kg. This extreme operation conditions moti-
vate the development of a novel cooling concept, decoupling the temperatures of
the winding and the stator poles.

Mechanical and thermal models of the machine have been developed and inte-
grated in an optimization scheme. A prototype has also been built and tested.

The design methodology has proven to be useful, and the optimizer with inte-
grated mechanical and thermal models has provided meaningful results.

The results from the experimental tests on the single-core motorettes show great
potential for the cooling solution. However, a number of manufacturing problems
in the full machine prototype impossible to solve in the available time for the
project limited the number of tests significantly. The static tests conducted show
good agreement at low current values, however, further testing would be required
to understand the deviations as the current level increases.

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SRM DRIVES FOR ELECTRIC TRACTION

8.6. References
[1] Universitat Politècnica de Catalunya. GAECE - Grup d'Accionaments Elèctrics
amb Commutació Electrònica. "Workshop SRM Drives an Alternative for E‐Trac-
tion: Proceedings", 2018. http://hdl.handle.net/2117/116160
[2] J.H.J. Potgieter, F.J. Márquez-Fernández, A. G. Fraser and M.D. McCulloch, “Loss
coefficient characterisation for high frequency, high flux density, electrical machine
applications”, International Electric Machines and Drives Conference IEMDC
2015, Coeur d’Alene, US.
[3] J.H.J. Potgieter, F.J. Márquez-Fernández, A. G. Fraser and M.D. McCulloch, “De-
sign optimisation methodology of a high-speed switched reluctance motor for auto-
motive traction applications”, Conference on Power Electronics Machines and
Drives PEMD 2016, Glasgow, UK.
[4] F.J. Márquez-Fernández, J.H.J. Potgieter, A. G. Fraser and M.D. McCulloch, “Ther-
mal management in a high speed switched reluctance ma-chine for traction applica-
tions”, Conference on Power Electronics Machines and Drives PEMD 2016,
Glasgow, UK.
[5] J.H.J. Potgieter, F.J. Márquez-Fernández, A. G. Fraser and M.D. McCulloch, “Per-
formance evaluation of a high speed segmented rotor axial flux switched reluctance
traction motor”, International Conference in Electrical Machines ICEM 2016, Lau-
sanne, Switzerland.
[6] F.J. Márquez-Fernández, J.H.J. Potgieter, A. G. Fraser and M.D. McCulloch, “Ex-
perimental validation of a thermal model for high speed switched reluctance ma-
chines for traction applications”, International Conference in Electrical Machines
ICEM 2016, Lausanne, Switzerland.
[7] J.H.J. Potgieter, F.J. Márquez-Fernández, A. G. Fraser and M.D. McCulloch, “Ef-
fects Observed in the Characterization of Soft Magnetic Composite for High Fre-
quency, High Flux Density Applications”, in IEEE Transactions on Industrial
Electronics, vol. 64, no. 3, pp. 2486-2493, March 2017.
[8] F.J. Márquez-Fernández, J.H.J. Potgieter, A. G. Fraser and M.D. McCulloch, “Ex-
perimental validation of a thermal model for high speed switched reluctance ma-
chines for traction applications”, IEEE Transactions on Industrial Applications, vol.
54, no.4, pp.3235-3244, July-August 2018.

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