Switched Reluctance

Motor Drives
Switched Reluctance
Motor Drives
Fundamentals to Applications

Edited by
Berker Bilgin
James Weisheng Jiang
Ali Emadi
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Library of Congress Cataloging‑in‑Publication Data

Names: Bilgin, Berker, editor. | Jiang, James Weisheng, editor. | Emadi, Ali,
editor.
Title: Switched reluctance motor drives : fundamentals to applications /
[editors] Berker Bilgin, James Weisheng Jiang, and Ali Emadi.
Description: First edition. | Boca Raton, FL : CRC Press/Taylor & Francis
Group, 2018. | Includes bibliographical references and index.
Identifiers: LCCN 2018029898 | ISBN 9781138304598 (hardback : acid-free paper)
| ISBN 9780203729991 (ebook)
Subjects: LCSH: Reluctance motors.
Classification: LCC TK2785 .S84 2018 | DDC 621.46--dc23
LC record available at https://lccn.loc.gov/2018029898

Visit the Taylor & Francis Web site at

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 Co-authors of the Switched Reluctance Motor Drives: Fundamentals to Applications,
with our traction SRM drive at McMaster Automotive Resource Center (from left to right)
First row: James Weisheng Jiang and Jianbin Liang
Second row: Yinye Yang, Alan Dorneles Callegaro, Ali Emadi, Berker Bilgin,
Elizabeth Rowan, Brock Howey, Jianning (Joanna) Lin, and Haoding Li
Not pictured: Jianning Dong and Jin Ye
Photo credit: Ron Scheffler – Hamilton, Ontario, Canada
Contents

Preface ..............................................................................................................................................ix
Editors ..............................................................................................................................................xi
Contributors ................................................................................................................................. xiii
List of Symbols ..............................................................................................................................xv
List of Greeks ............................................................................................................................. xxiii
List of Abbreviations ............................................................................................................... xxvii

1. Electric Motor Industry and Switched Reluctance Machines .......................................1

Berker Bilgin and Ali Emadi

2. Electromagnetic Principles of Switched Reluctance Machines .................................. 35

Berker Bilgin

3. Derivation of Pole Configuration in Switched Reluctance Machines ....................... 91

Berker Bilgin

4. Operational Principles and Modeling of Switched Reluctance Machines............. 123

Brock Howey and Haoding Li

5. Switched Reluctance Machines in Generating Mode ................................................. 183

Berker Bilgin

6. Materials Used in Switched Reluctance Machines ..................................................... 197

Elizabeth Rowan and James Weisheng Jiang

7. Design Considerations for Switched Reluctance Machines ...................................... 253

James Weisheng Jiang

8. Mechanical Construction of Switched Reluctance Machines ................................... 315

Yinye Yang, James Weisheng Jiang, and Jianbin Liang

9. Control of Switched Reluctance Machines ................................................................... 371

Jin Ye, Haoding Li, and James Weisheng Jiang

10. Power Electronic Converters to Drive Switched Reluctance Machines ..................425

Jin Ye

11. Position Sensorless Control of Switched Reluctance Motor Drives ........................ 451
Jin Ye

12. Fundamentals of Vibrations and Acoustic Noise ........................................................ 473

James Weisheng Jiang and Jianbin Liang

vii
viii Contents

13. Noise and Vibration in Switched Reluctance Machines ............................................ 577

James Weisheng Jiang, Jianbin Liang, Jianning Dong, Brock Howey,
and Alan Dorneles Callegaro

14. Thermal Management of Switched Reluctance Machines ........................................ 705

Yinye Yang, Jianbin Liang, Elizabeth Rowan, and James Weisheng Jiang

15. Axial Flux Switched Reluctance Machines ................................................................... 735

Jianing (Joanna) Lin

16. Switched Reluctance Motor and Drive Design Examples.......................................... 755

Berker Bilgin, James Weisheng Jiang, and Alan Dorneles Callegaro

Index ............................................................................................................................................. 783

Preface

Electric motors impact various aspects of our lives. We use electric motors in numerous
applications, such as air conditioners, refrigerators, washing machines, fans, vacuum
cleaners, pools, beverage vending machines, and so on. Electric motors are the workhorse
in industrial applications as well, and we use them heavily in machining tools, cranes,
pumps, and compressors. With the emergence of more efficient electrified vehicles, electric
motors are also being used in transportation systems at an increasingly rapid rate.
Electric motors are the largest consumer of electrical energy; therefore, they play a critical
role in the growing market for electrification. Due to their simple construction, switched
reluctance motors (SRMs) are exceptionally attractive for the industry to respond to the
increasing demand for high-efficiency, high-performance, and low-cost electric motors
with a more secure supply chain.
This book is organized to provide a detailed discussion of the multidisciplinary aspects
of SRMs and help engineers design SRM drives for various applications. This book begins
with an overview of the electric motor market in Chapter 1. The use of electric motors in
industrial, residential, commercial, and transportation sectors are discussed, and the cur-
rent status and future trends in the industry are presented. Chapter 1 also highlights the
advantages and challenges of SRM technology compared to the other motor types.
Chapter 2 explains the operational principles of SRMs through fundamentals of electro-
magnetics and energy conversion. The selection of the number of stator and rotor poles of
an SRM is critical to satisfying the performance requirements of an application. Therefore,
Chapter 3 quantifies how the pole configuration of an SRM is calculated. Chapter 4 intro-
duces the modeling of an SRM, which is essential both for the motor design and control.
Chapter 5 presents the generating mode of operation in SRMs.
In Chapter 6, the considerations when selecting the materials for a switched reluctance
motor are explained. Chapter 7 presents the design considerations and describes how
defining the dimensions of a motor is related to the performance. Chapter 8 explains the
mechanical aspects and construction of a switched reluctance motor.
Chapter 9 presents the control of SRM and discusses how to define the control param-
eters to improve the performance and reduce torque ripple. In Chapter 10, power elec-
tronic converters to drive SRMs are introduced. Position sensorless control is discussed
in Chapter 11, which is important for the robust and low-cost operation of motor drives.
Acoustic noise and vibration is the most well-known issue in SRMs. However, it can be
reduced significantly by analyzing the noise source and by optimizing the motor geom-
etry and current control. In Chapter 12, fundamentals of vibrations and acoustic noise are
introduced first. Then, in Chapter 13, noise and vibration in SRMs are discussed in detail.
Chapter 14 introduces the thermal management aspects of SRMs. Chapter 15 presents
axial flux SRMs, which are attractive for selected applications including propulsion and
energy generation. Finally, Chapter 16 introduces two SRM designs, which are proposed to
replace the permanent magnet machines in a residential HVAC application and a hybrid-
electric propulsion system. The design of a high-power converter for switched reluctance
motor drives is also presented in Chapter 16.
Throughout this book, we aimed to cover major technical aspects of switched reluc-
tance motor drives and provide the tools that the engineers would need to design
SRMs. Therefore, the chapters include various practical examples, scripts, and detailed

ix
x Preface

illustrations. We hope that this book will be an easy-to-follow reference for the practic-
ing engineers, academicians, and students to understand, design, and analyze switched
reluctance motor drives.
We would like to acknowledge the efforts and assistance of the staff of Taylor & Francis/
CRC Press, especially Ms. Nora Konopka, Ms. Kyra Lindholm, and Ms. Vanessa Garrett,
and the help of the staff of Lumina Datamatics, especially Ms. Angela Graven and
Mr. Edward Curtis. We would also like to thank our colleagues at Cogent Power Inc. and
Canada Excellence Research Chair in Hybrid Powertrain Program at McMaster University
for their feedback on the chapters.

Berker Bilgin, James Weisheng Jiang, and Ali Emadi

May 2018

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Editors

Berker Bilgin (IEEE S’09-M’11-SM’16) received his PhD degree in electrical engineering
from Illinois Institute of Technology in Chicago, Illinois, USA. He also has an MBA degree
from DeGroote School of Business, McMaster University in Hamilton, Ontario.
He is the research program manager in Canada Excellence Research Chair in the Hybrid
Powertrain Program at McMaster Institute for Automotive Research and Technology,
McMaster University, Hamilton, ON, Canada. He is managing many multidisciplinary
projects on the design of electric machines, power electronics, electric motor drives, and
electrified powertrains.
Dr. Bilgin is the co-founder of Enedym, Inc., which is a spin-off company of McMaster
University. Enedym specializes in electric machines, electric motor drives, advanced con-
trols and software, and virtual engineering.
Dr. Bilgin was the general chair of the 2016 IEEE Transportation Electrification Conference
and Expo. He is also an associate editor for IEEE Transactions on Transportation Electrification.

James Weisheng Jiang received his bachelor’s degree in vehicle engineering from the
College of Automotive Engineering, Jilin University, China in 2009. He worked as a research
assistant at the Clean Energy Automotive Engineering Research Center, Tongji University,
China from 2009 to 2011. He got his PhD degree from McMaster University in 2016. He is
currently a principal research engineer at McMaster Automotive Resource Centre (MARC).
He has designed and implemented a 60 kW 24/16 switched reluctance motor for traction
purpose in HEV. He has also been involved in designs and implementations of traction
motors with interior permanent and ferrite magnets. He has also been working on NVH
analysis for switched reluctance motors and permanent magnet synchronous motors.

Ali Emadi (IEEE S’98-M’00-SM’03-F’13) received Bachelor of Science and Master of Science
degrees in electrical engineering with highest distinction from Sharif University of
Technology, Tehran, Iran, in 1995 and 1997, respectively, and a PhD degree in electrical engi-
neering from Texas A&M University, College Station, TX, USA, in 2000. He is the Canada
Excellence Research Chair in Hybrid Powertrain and a professor in the Departments of
Electrical and Computer Engineering and Mechanical Engineering at McMaster University
in Hamilton, Ontario, Canada. Before joining McMaster University, Dr. Emadi was the
Harris Perlstein Endowed Chair Professor of Engineering and director of the Electric Power
and Power Electronics Center and Grainger Laboratories at Illinois Institute of Technology
in Chicago, Illinois, USA, where he established research and teaching facilities as well
as courses in power electronics, motor drives, and vehicular power systems. He was the
founder, chairman, and president of Hybrid Electric Vehicle Technologies, Inc. (HEVT)—a
university spin-off company of Illinois Tech. He is the Founder, President, and CEO of
Enedym Inc.—a McMaster University spin-off company. Dr. Emadi has been the recipient
of numerous awards and recognitions. He was the advisor for the Formula Hybrid Teams
at Illinois Tech and McMaster University, which won the GM Best Engineered Hybrid
System Award at the 2010, 2013, and 2015 competitions. He is the principal author/coauthor
of over 450 journal and conference papers as well as several books including Vehicular
Electric Power Systems (2003), Energy Efficient Electric Motors (2004), Uninterruptible Power

xi
xii Editors

Supplies and Active Filters (2004), Modern Electric, Hybrid Electric, and Fuel Cell Vehicles (2nd
ed, 2009), and Integrated Power Electronic Converters and Digital Control (2009). He is also the
editor of the Handbook of Automotive Power Electronics and Motor Drives (2005) and Advanced
Electric Drive Vehicles (2014). Dr. Emadi was the Inaugural General Chair of the 2012 IEEE
Transportation Electrification Conference and Expo (ITEC) and has chaired several IEEE
and SAE conferences in the areas of vehicle power and propulsion. He is the founding
Editor-in-Chief of the IEEE Transactions on Transportation Electrification.
Contributors

Berker Bilgin Haoding Li

McMaster Institute for Automotive McMaster Institute for Automotive
Research and Technology (MacAUTO) Research and Technology (MacAUTO)
McMaster University McMaster University
Hamilton, Ontario, Canada Hamilton, Ontario, Canada

Alan Dorneles Callegaro Jianbin Liang

McMaster Institute for Automotive McMaster Institute for Automotive
Research and Technology (MacAUTO) Research and Technology (MacAUTO)
McMaster University McMaster University
Hamilton, Ontario, Canada Hamilton, Ontario, Canada

Jianning Dong Jianing (Joanna) Lin

Department of Electrical Sustainable McMaster Institute for Automotive
Energy, TU Delft Research and Technology (MacAUTO)
Delft, Netherlands McMaster University
Hamilton, Ontario, Canada
Ali Emadi
McMaster Institute for Automotive Elizabeth Rowan
Research and Technology (MacAUTO) McMaster Institute for Automotive
McMaster University Research and Technology (MacAUTO)
Hamilton, Ontario, Canada McMaster University
Hamilton, Ontario, Canada
Brock Howey
McMaster Institute for Automotive Yinye Yang
Research and Technology (MacAUTO) Magna Powertrain
McMaster University Concord, Ontario, Canada
Hamilton, Ontario, Canada
Jin Ye
James Weisheng Jiang San Francisco State University
McMaster Institute for Automotive San Francisco, California
Research and Technology (MacAUTO)
McMaster University
Hamilton, Ontario, Canada

xiii
List of Symbols

a m/s2 Acceleration
a m Radius of a shell (Chapter 13)
A m2 Area of the conducting loop (Chapter 4)
A m2 Area
A m Amplitude of the wave
Ac m2 Cross section are through which the flux flows
ae m/s2 Acceleration of a charge
As m2 Slot area
As m2 Area of the sound radiation surface (Chapter 13)
ax — Axial order (Chapters 12 and 13)
B T (Tesla) Magnetic flux density
Br T Magnetic flux density in radial direction
Bt T Magnetic flux density in tangential direction
c m/s Speed of sound (Chapter 13)
cB m/s Speed of bending sound wave (Chapter 13)
Ce s Electric time constant
circ — Circumferential order (Chapters 12 and 13)
cp J/(kg·K) Specific heat
Cr — Rotor geometric center (Chapter 7)
Cr — Circumference of the rotor (Chapter 7)
Csb — Shaft-bearing geometric center, also referred as
rotation center (Chapter 7)
Csc — Stator-case geometric center (Chapter 7)
D — Diode
D m or mm Amplitude for vibration at the steady state
(Chapter 12)
D mm Bore diameter (Chapter 7)
dc mm Diameter for bare copper (Chapter 7)
Dc mm Mean diameter of the stator yoke (Chapter 13)
Dr mm Rotor outer diameter (Chapter 7)
Ds mm Stator outer diameter (Chapter 7)
Dsh mm Rotor shaft diameter (Chapter 7)
dsum m or mm Sum of surface waves (Chapter 12)
dw mm Diameter for wire with insulation (Chapter 7)
e m Mass eccentricity
E Pa, MPa or GPa Modulus of elasticity or Young’s modulus
E’ Pa, MPa or GPa Real part of Young’s modulus
E’’ Pa, MPa or GPa Imaginary part of Young’s modulus
Et Pa, MPa or GPa Equivalent elasticity modulus (Chapter 13)

xv
xvi List of Symbols

f Hz Frequency
F N Force
F N External force (Chapter 12)
F0 N Amplitude for external force (Chapter 12)
FA — Sum for all individuals’ fitness values (Chapter 9)
felec Hz Electrical frequency (Chapter 13)
ffcopper — Bare copper slot fill factor
Fi — Fitness value for a particular individual (Chapter 9)
fm Hz Mechanical frequency (Chapter 7)
fmech Hz Mechanical frequency
fn Hz Natural frequency (Chapters 12 and 13)
Fr N Radial force wave over one mechanical cycle
(Chapter 13)
Fr N Amplitude of a harmonic for radial force
(Chapter 13)
fsamp Hz Sampling frequency (Chapter 4)
Fu N Unbalanced force resulting from unbalanced
mass
FV N Electromagnetic force acting on charges on
volume V (Chapter 2)
h W/(m2K) Heat transfer coefficient
H A/m Magnetic field strength
Hc A/m Magnetic field intensity in the core (Chapter 2)
hf mm Mean thickness of the frame (Chapter 13)
Hg A/m Magnetic field intensity in the airgap (Chapter 2)
hov mm One-sided axial overhang length of the winding
ends (Chapter 13)
hr mm Rotor pole height (Chapter 7)
hs mm Stator pole height (Chapter 7)
hs mm Tooth height (Chapter 13)
i A Electric current
I A Electric current
I kg·m2 Moment of inertia
Iamp A Amplitude of phase current (Chapter 4)
id A d-axis current (Chapter 4)
idq A Current in the dq frame (Chapter 4)
ik A Measured current
ik_low A Lower current references of the kth phase
ik_up A Upper current references of the kth phase
Ilower A Lower boundary of a hysteresis band for a
reference current
iLUT A Current lookup table (Chapter 4)
Imax A Maximum phase current
Imin A Minimum phase current
List of Symbols xvii

Ip A RMS value of the phase current

iph A Phase current
Iphase A Phase current array (Chapter 4)
iq A q-axis current (Chapter 4)
irated A Rated current of the machine (Chapter 4)
Iref A Reference current
IRMS_coil A RMS value of the current flowing through the
coil
Iupper A Upper boundary of a hysteresis band for a
reference current
iαβ A Current in the αβ frame (Chapter 4)
J A/m2 Current density in conductor
k — A lumped parameter term that takes into
account the mechanical construction of the
motor (Chapter 4)
k N/m Stiffness
k W/(m·K) Thermal conductivity
k N·s/m Spring coefficient (Chapter 12)
K(circ) N/m Lumped stiffness of the circumferential mode
circ (Chapter 13)
k0 — Acoustic waveform number (Chapter 13)
Ke — Eddy current loss coefficient (Chapter 14)
Kfb — Frictional loss coefficient (Chapter 14)
Kh — Hysteresis loss coefficient (Chapter 14)
kph — Array for phase shift factors
kr — Radial component of acoustic wave number
(Chapter 13)
krot — −1 for counter clockwise rotation and 1 for
clockwise rotation
kz — Axial component of acoustic wave number
(Chapter 13)
l m Length of a shell (Chapter 13)
l m Length
L H Inductance
La H Self-inductance at aligned position
LA dB Function of factor for A-weighting correction
lc m Length of a close-loop flux path (Chapter 2)
Lend m Height of end turn (Chapter 7)
Lf mm Frame length (Chapter 13)
lg m Length of the flux path in the air gap
Linc_k,k H Incremental self-inductance of the kth phase
Lk,k H Self-inductances of the kth phase
Lm H Self-inductance at midway position
Lmax H Max. Inductance
xviii List of Symbols

Lmin H Min. Inductance

lr m Rotor’s geometric center axis (Chapter 8)
LR m Stack length of the stator core (Chapter 7)
lrpa m Arc length of the rotor pole (Chapter 7)
Lrpa m Total of the arc lengths of the rotor poles
(Chapter 7)
LS m Stack length of the rotor core (Chapter 7)
lsb — Geometric axis for the shaft-bearing assembly
lsc — Geometric center axis for stator-case assembly
(Chapter 8)
lslot m Height of the slot
Ltotal m Total axial length of the active volume (Chapter 7)
Lu H Self-inductance at unaligned position (Chapter 7)
Lun m Total arc length available between the rotor
poles (Chapter 7)
lw m Length of wire (Chapter 2)
m kg Mass
m — Number of motor phases
M kg Lumped mass for the cylindrical shell
me g Mass of an electron
MF — Magnification factor (Chapter 12)
Mr kg Rotor mass
n rpm Revolutions per minute
N — Number of turns (Chapter 4)
ne — Number of charges per cubic meter (Chapter 2)
Nlam — Number of lamination sheets for either the stator
or the rotor (Chapter 7)
nm rpm Motor speed
Np — Number of poles (Chapter 7)
Nparallel — Number of parallel paths in a motor winding
(Chapter 7)
NPh — Number of phases (Chapter 7)
npulses — Number of encoder pulses at a time step
(Chapter 4)
Npulses — Number of pulses (Chapter 4)
Nr — Number of rotor poles
Nr#1mech degree Mechanical angle for Nr#1 (Chapter 3)
NRPM rpm Mechanical rotation speed (Chapter 4)
Ns — Number of stator poles (Chapter 7)
Ns#1elect degree Electrical angle for Ns#1 (Chapter 3)
Nr#1mech degree Electrical angle for Nr#1 (Chapter 3)
nsamp — Number of simulation time steps at each current
sampling period (Chapter 4)
List of Symbols xix

Nselect degree Electrical angle of a stator pole (Chapter 4)

Nseries — Number of coils connected in series per parallel
path in a winding (Chapter 7)
nslot — Slot number per pole
Nslot — Number of stator slots
Nsmech degree Mechanical angle of a stator pole (Chapter 4)
Nstr — Number of strands (Chapter 7)
Ntotal — Size of the population (Chapter 9)
Nturn — Number of turns (Chapter 7)
p Pa Pressure
Pcore W, kW Core loss
Pcu W, kW Copper loss
Peddy W, kW Eddy current loss
Pexcess W, kW Excess loss
Pfw W, kW Windage and friction losses (Chapter 14)
Phystersis W, kW Hysteresis loss
Pin W, kW Motor input power
Pmech W, kW Mechanical power
Pout W, kW Motor output power
pp — Number of pole pairs
pr Pa, GPa Analytical expression of a harmonic content for
pressure (Chapter 13)
Pr W, kW Rated power of motor
Pr Pa, GPa Amplitude of a harmonic content for radial
pressure (Chapter 13)
q — Temporal order (Chapters 12 and 13)
q C Charge
Q dB Peak power of the damped frequency (Chapter 13)
qenc C Total charge within a surface (Chapter 2)
r — Radial order (Chapter 12)
r m Radius
R Ω Electric resistance
R Ω Phase resistance
R H−1 Reluctance (Chapter 12)
Rcoil Ω Resistance of the coil
Rdc Ω DC resistance
Req Ω Equivalent reluctance (Chapter 2)
Rf mm Mean radius of the frame (Chapter 13)
RippleNormalized — Ratio between the net torque ripple and the
average torque
RipplePercentage — Normalized torque ripple multiplied by 100%
Rph Ω Phase resistance (Chapter 4)
xx List of Symbols

Rproximity Ω Proximity resistance

Rskin Ω Skin resistance
Rt mm Mean radius of the teeth-coil region (Chapter 13)
S — Total number of torque pulsations (Chapter 3)
S — Circuit switch
Sf — Stacking factor (Chapter 7)
T N·m Torque
T s Oscillating period
T °C or K Temperature
Tamb °C or K Ambient temperature (Chapter 7)
Tave Nm Average torque (Chapter 7)
Tave_r Nm Required torque (Chapter 9)
Tco Nm Co-energy torque (Chapter 4)
Td Nm Damping torque
Te Nm Electromagnetic torque
Te_ref Nm Total torque reference
Te_ref (k) Nm Reference torque for kth phase
tk_off — Time instant when the kth phase switching states
are OFF
tk_on — Time instant when the kth phase switching states
are ON
tlam mm Thickness for lamination sheets (Chapter 7)
Tlimit °C or K Maximum permitted temperature (Chapter 7)
TLUT Nm Torque lookup table (Chapter 4)
Tmax Nm Maximum torque
Tmin Nm Minimum torque
Tp Nm Peak torque
Tpeak-to-peak Nm Magnitude of the torque pulsation
Tphase Nm Phase torque array (Chapter 4)
Tpr degree Pole pitch for the rotor
Tps degree Pole pitch for the stator
Tripple — Periodic component of the instantaneous torque
waveform (Chapter 7)
Ts s Sampling time (Chapter 4)
Tsamp s Sampling period (Chapter 4)
u — Temporal order (Chapters 12 and 13)
ue — Temporal order in electrical position (Chapter 13)
un V Neutral point voltage (Chapter 10)
uw V Phase voltage (Chapter 10)
v — Spatial order (Chapters 12 and 13)
v m/s Vibration velocity on the surface (Chapter 13)
v m/s Velocity of a moving charge (Chapter 2)
V V Voltage
List of Symbols xxi

Va m3 Volume of the air gap (Chapter 2)

Vactive m3 Volume of active material (Chapter 7)
Vc m3 Volume of the core (Chapter 2)
vd m/s Drift velocity (Chapter 2)
Vdc V DC voltage
VDC V DC voltage
vk V Measured voltage of kth phase
vk V Measured voltage
vph V Phase voltage
Vt m3 Volume of the teeth-coil region (Chapter 13)
Wc J or kJ Co-energy (Chapter 2)
Wf J or kJ Magnetic energy
wslot mm Width of slot opening
x m or mm Displacement
X m or mm Amplitude of a vibrating wave (Chapter 12)
yr mm Rotor back iron thickness (Chapter 7)
ys mm Stator back iron thickness (Chapter 7)
List of Greeks

α rad/s2 Angular acceleration

α 1/°C or 1/K Thermal coefficient (Chapter 14)
α degree Air-gap spatial position (Chapter 13)
βr degree or radian Rotor pole arc angle (Chapter 7)
βs degree or radian Stator pole arc angle (Chapter 7)
γ degree Stator circumferential position (Chapter 7)
ΔTrms Nm Net RMS value of torque ripple
ΔTRMS Nm RMS value of net torque ripple
ε mm Required element size (Chapter 13)
ε mm/mm or μm/mm Strain (Chapter 13)
ε V Motional EMF (Chapter 9)
ε V Voltage induced across the armature (Chapter 4)
ϵ0 F/m Permittivity of free space
ζ — Damping ratio (Chapter 12 and 13)
ζa — Acoustic damping ratio (Chapter 13)
ζs — Structural damping ratio (Chapter 13)
η % Efficiency
θ degree Angle between normal vector of area and
direction of B field (Chapter 4)
θ degree Rotor position
θa degree Aligned position
θaligned degree Aligned position
θc degree Conduction angle
θoff degree Turn-off angle
θOFF degree Turn-off angle
θon degree Turn-on angle
θON degree Turn-on angle
θov degree Overlapping angle
θp degree Pole pitch angle
θr degree Mechanical position of the center axis of the
rotor poles (Chapter 3)
θrotor degree Machine position
θs degree Mechanical position of the center axis of the
stator poles (Chapter 3)
θun degree Unaligned position
θunaligned degree Unaligned position
λ m Sound wavelength
λa Wb Phase flux-linkage at unaligned position
λA Wb Flux linkage due to armature current
λax m Wavelength of axial mode, ax

xxiii
xxiv List of Greeks

λB m Wavelength of bending wave (Chapter 13)

λbase m Fundamental wavelength for the first (base)
circumferential or spatial order
λd Wb d-axis flux linkage (Chapter 4)
λdq Wb Flux in the dq frame (Chapter 4)
λfall Wb Decreasing flux linkage for the outgoing phase
λk Wb Estimated self-flux linkage
λLUT Wb Flux linkage lookup table
λph Wb Phase flux linkage
λq Wb q-axis flux linkage (Chapter 4)
λrise Wb Rising flux linkage for the incoming phase
λu Wb Phase flux-linkage at aligned position
λv m Wavelength of spatial order, v (Chapter 12)
λαβ Wb Flux in the αβ frame (Chapter 4)
μ N/A2 Permeability
μ0 N/A2 Permeability of free space (Chapter 2)
μeff N/A2 Effective permeability (Chapter 2)
μr N/A2 Relative permeability (Chapter 2)
ξbase cycles per meter Spatial frequency for the base circumferential or
spatial order (Chapter 12)
Π W, kW Sound power (Chapter 13)
Πref W Reference of sound power, 10-12 W (Chapter 13)
ρ kg/m3 Mass density
ρ C/m3 Charge density (Chapter 2)
ρ Ω·m Electric resistivity
ρ0 Ω Resistivity at the initial temperature (Chapter 14)
ρs kg/m2 Mass per unit surface area (Chapter 13)
σ N/m2 or Pa Stress (Chapter 13)
σ — Radiation ratio (Chapter 13)
τ s Time between the collusion of free electrons
with the atoms (Chapter 2)
τ N/m2 Shear stress
τ Nm Torque produced by the DC motor (Chapter 4)
τe s Electrical time constant
τr degree or radian Rotor taper angle (Chapter 7)
τs degree or radian Stator taper angle
φ degree or radian Phase angle (Chapters 12 and 13)
Φ Wb Magnetic flux
φind Wb Induced flux (Chapter 2)
ϕphase degree Angle of phase current (Chapter 4)
ϕs Wb Flux passing through the armature coil due to
the external field (Chapter 4)
ω rad/s Annular rotating speed
ω rad/s Oscillating angular frequency (Chapters 12 and 13)
List of Greeks xxv

Ω(circ) — Root of the characteristic equation of motion of

the cylindrical shells (Chapter 13)
ωd rad/s Damped circular frequency (Chapter 12)
ωelec rad/s Electrical speed
ωf rad/s Forcing angular frequency (Chapter 13)
ωm rad/s Mechanical angular speed (Chapter 15)
ωn rad/s Natural angular frequency
List of Abbreviations

AC Alternative current
AFM Axial flux machine
AFSRM Axial flux SRM
AM Amplitude modulation
ANN Artificial neural network
ARCFL Absolute value of the rate of change of flux linkage
ATF Automatic transmission fluid
AWG American wire gauge
BCC Body centered cubic
BDC Bottom dead center
BDRM Brushless double-rotor machine
BEV Battery electric vehicle
BLDC Brushless DC motor
BOF Basic oxygen furnace
CAC Central air conditioner
CAD Computer-aided design
CCW Counter clockwise
CDF Cumulative distribution function
CFD Computational fluid dynamics
CGO Conventional grain-oriented steel
CNC Computer numerical control
CSMO Current sliding mode observer
CTE Coefficient of thermal expansion
CUAC Commercial unitary air conditioners
CW Clockwise
DC Direct current
DDR Direct drive rotary
DIFC Direct instantaneous force control
DSP Digital signal processor
EAF Electric arc furnace
EMF Electromotive force
EMI Electromagnetic interference
EPR Ethylene-propylene
ETP Electrolytic tough pitch
FE Finite element
FEA Finite element analysis
FFT Fast Fourier transform
FNM Flow network modeling

xxvii
xxviii List of Abbreviations

FRF Frequency response function

FSMO Flux-linkage sliding mode observer
GA Genetic algorithm
GMO Gaussian mutation operator
GOES Grain-oriented electrical steel
HEV Hybrid electric vehicle
HGO High-permeability grain oriented
HP Horsepower
ICE Internal combustion engine
ICM Intermediate crossover method
ID Inner diameter
IEC The international electrotechnical commissions
IGBT Insulated gate bipolar transistor
IM Induction motor
IPM Interior permanent magnet
IPMSM Interior permanent magnet synchronous machine
ISO International Organization for Standardization
LB Lower boundary
LCM Linear crossover method
LED Light-emitting diode
LHM Linear hybrid motor
LPM Lumped parameter model
LPTN Lumped parameter thermal network
MF Magnification factor
MMF Magnetomotive force
MOSFET Metal–oxide–semiconductor field-effect transistor
MW Magnet wire
Nd-Fe-B Neodymium-iron-boron
NEMA National electrical manufacturers association
NOES Non-oriented electrical steel
NPC Neutral Point Diode Clamped, or Neutral Point Clamp
NTC Negative temperature coefficient
NVH Noise, vibration, and harshness
OD Outer diameter
OEM Original equipment manufacturer
PA Polyamide
PBT Polybutylene terephthalate
PC Polycarbonate
PCB Printed circuit board
PD Partial discharges
PD Specific power
PDIV Partial discharge inception voltage
PE Polyethylene
List of Abbreviations xxix

PEEK Polyetheretherketone
PHEV Plug-in hybrid electric vehicle
PI Proportional-Integral (controller)
PI Polarization index
PID Proportional–integral–derivative (controller)
PLL Phase locked loop
PM Permanent magnet
PM Phase modulation
PMMA Polymethylmetachrylate
PMSM Permanent magnet synchronous motor
PMW Pulse width modulation
PP Polypropylene
PPS Polyphenylene sulfide
PS Polystyrene
PSU Polysulfone
PTC Positive temperature coefficient
PTFE Polytetrafluoroethylene
PVC Polyvinyl chloride
PWM Pulse width modulation
RAC Room air conditioner
RBF Radial basis function
RFM Radial flux machines
RFSRM Radial flux SRM
RMO Random mutation operator
ROW Rest of the world
RTD Temperature detector
SED Sound energy density
SIL Sound intensity level
SMC Soft magnetic composites
SMO Sliding mode observer
SP Power density
SPL Sound pressure level
SRG Switched reluctance generator
SRM Switched reluctance machine
SWL Sound power level
TEFC Totally enclosed, fan-cooled
TENV Totally Enclosed, Non-ventilated
TEXP Totally Enclosed, Explosion-proof
THS Toyota hybrid system
TRFS Theoretical maximum ripple-free speed
TSF Torque sharing function
TSFF Temperature sensitive ferrofluids
UB Upper boundary
xxx List of Abbreviations

UDDS Urban dynamometer driving schedule

UMP Unbalanced magnetic pull
USGS United States geological survey
VA Volt-ampere
VPI Vacuum pressure impregnation
1
Electric Motor Industry and Switched
Reluctance Machines

Berker Bilgin and Ali Emadi

CONTENTS
1.1 Introduction ............................................................................................................................1
1.2 Overview of Energy Consumption of Electric Motors.....................................................2
1.3 The U.S. Motor Population ...................................................................................................5
1.4 Electric Motor Usage in the Industrial Sector....................................................................6
1.5 Electric Motor Usage in the Residential Sector .................................................................7
1.6 Electric Motor Usage in the Commercial Sector ............................................................. 10
1.7 Electric Motor Usage in the Transportation Sector ......................................................... 12
1.7.1 Vehicle Propulsion ................................................................................................... 12
1.7.2 e-Bike Motors ............................................................................................................ 15
1.8 Overview of Electric Motor Industry................................................................................ 17
1.9 Electric Motor Types ............................................................................................................ 18
1.9.1 Permanent Magnet Machines ................................................................................ 18
1.9.2 Rare-Earth Materials ............................................................................................... 21
1.9.3 Induction Machines ................................................................................................. 23
1.10 Switched Reluctance Machines.......................................................................................... 25
1.11 Switched Reluctance Motor Drive Applications ............................................................. 29
References....................................................................................................................................... 31

1.1 Introduction
Electric motors are electromechanical devices that convert electrical energy into mechani-
cal energy. During this conversion process, some of the electrical energy is lost and dis-
sipated as heat. Relative to alternatives, electric machines are efficient, cost effective and
can scale from very large applications to tiny applications; generally, not feasible for other
mechanical energy sources. These factors have driven a massive market demand for elec-
tric machines, making electric motors a major consumer of electrical energy.
Utilizing higher efficiency, lower cost, and variable speed electric motors will increase
the level of electrification, improve overall system efficiency, reduce operational costs,
and electricity consumption, while reducing emissions. The switched reluctance machine
(SRM) is a promising electric machine architecture that will definitely play a significant
role in future market expansion. One of the main advantages of SRM is its low-cost and
simple construction, which can provide reliable operation in a harsh environment, like
transportation. But, there are also challenges to solve in SRM including high torque ripples
and acoustic noise.

1
2 Switched Reluctance Motor Drives

Throughout this book, we will offer a microscopic view of all technical aspects of SRM.
Before that, we would like to provide a macroscopic view of the use of electric motors in
different sectors to explore the role of SRM in the electric motor industry. We will start by
looking at the energy consumption of electric motors. Then, we will analyze electric motor
usage in different sectors. After providing the big picture of the electric motor industry,
we will look at the potential and challenges in utilizing SRM technology in motor drive
applications.

1.2 Overview of Energy Consumption of Electric Motors

As the largest economy in the world, the United States is a major player in electricity gen-
eration, consumption, and use of electric motors in the industrial, commercial, residen-
tial, and transportation sectors. According to statistics from the U.S. Energy Information
Administration, total U.S. electricity consumption reached 9,627 million kilowatt-hours
(kWh) per day by November 2016 [1]. As shown in Figure 1.1, the majority of the elec-
trical energy has been produced from coal in the last decade. The share of coal-fired
energy generation in total electricity production continues to decline to limit the growth
of energy-related CO2 emissions. As a result, natural gas has become the predominant
source of power generation. In 2016, 34.21% of electricity was generated from natural gas.
Electricity generation from renewable sources increased from 6.26% in 2013 to 8.30% in

Coal Natural gas Petroleum Nuclear

Hydropower Renewables Others
2.38% 2.54% 3.07% 3.66% 4.07% 4.75% 5.41% 6.26% 6.84% 7.26% 8.30%
100%

80%
20.15%

21.50%
21.63%

23.38%

24.02%

60%
24.81%

30.39%

27.76%

27.61%

32.82%

34.21%

40%
49.13%

48.65%

48.34%

44.58%

44.92%

42.42%

42.42%

37.53%

39.02%

33.28%

30.53%

20%

0%
2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016

FIGURE 1.1
Energy resources for electricity generation in U.S. (From U.S. Energy Information Administration, Short-term
energy outlook. December 6, 2016, Available: http://www.eia.gov/.)
Electric Motor Industry and Switched Reluctance Machines 3

2016. The portion of electricity from renewables is expected to grow faster in the next few
decades due to declining costs of renewable energy sources, penetration of new energy
storage capabilities, and stricter targets in reducing energy-related CO2 emissions.
Figure 1.2 shows the percentage of energy use in different sectors [2–4]. Including the
electricity-related losses, the share of the industry sector in the total electrical energy
(a)

Residential
Transportation
21%1
29%1
Commercial

Industrial 18%1

32%1

Total energy consumption:

28.92 trillion kWh

(b) Transportation 39%

0.23%

Industrial
62.5% 22.0%
28.10%
Residential
36.74%

Commercial
34.94%

38.3%

percentage of motor-driven equipment energy

use in the electrical energy consumption

FIGURE 1.2
U.S. energy consumption by sectors in 2015: (a) percentage of total energy consumption, (b) percentage of
total electrical energy consumption. (From U.S. Energy Information Administration, Annual energy out-
look 2016 with projections to 2040, August 2016, Available: http://www/eia/gov/; Waide, P. and Brunner,
C.U., Energy-efficiency policy opportunities for electric motor-driven systems, International Energy Agency
Working Paper, 2011; U.S. Department of Energy, Advanced manufacturing office, premium efficiency motor
selection and application guide – A handbook for industry. February, 2014, Available: http://www.energy.gov/.)
4 Switched Reluctance Motor Drives

consumption is 28.10%, and 62.5% of the electrical energy consumed by the industrial sec-
tor was used to power electric motor driven applications, mostly for industrial handling
and processing. The highest percentage of the total electrical energy consumption was in
the residential sector at 36.74%. However, only 22% of the electrical energy in the residen-
tial sector was used to power electric motors. In residential applications, electric motors
are mostly employed in refrigerators, freezers, and heating, ventilation, and air condition-
ing (HVAC) systems. The second largest use of electric motors is in the commercial sec-
tor; 38.3% of the electrical energy consumed by the commercial sector is used for electric
motors, mostly for HVAC systems.
It should be noted that the industrial sector has the highest electrical energy usage for
electric motor driven equipment due to the use of a relatively large number of medium-
and large-size electric motors. However, as we will see in the next section, small size
motors make up the biggest portion of the U.S. motor population, and they are mostly
used in residential and commercial applications.
You must have already noticed that the portion of electrical energy consumption in the
transportation sector is small even though it has the second highest total energy consumption
in the United States. The reason is that the vast majority of the vehicles in the U.S. are powered
solely by internal combustion engines (ICEs) and require fossil fuels as their energy source
[5]. However, the portion of electrical energy is expected to increase in the transportation
sector with the penetration of more efficient electrified vehicles, including hybrid, plug-in
hybrid, and battery electric vehicles. Electric motors in the transportation system today
are mostly used in electric railways and low-power vehicular applications (e.g. windshield
wipers, seat adjustment, power windows, and sunroof), but electric motors used to power
vehicles are perhaps the fastest growing motor market in the transportation industry.
Electric motors are the biggest consumer of electrical energy in the United States. This is
also true globally. Figure 1.3 shows the global electricity demand by end-use. Around 46%
of the global electricity demand is from electric motors. Among the sectors, the highest

Transportation: 2.74%
Residential: 12.33%

Light
18%
Commercial: 20.55%

Electronics
10%

Electrolysis
Industry: 60.27% Heat 3%
Motors
46% 20%

Standby
3%

FIGURE 1.3
Global electrical energy demand by end use and electrical energy demand of motor applications by sector.
(From U.S. Department of Energy, Advanced manufacturing office, premium efficiency motor selection and
application guide – A handbook for industry. February, 2014, Available: http://www.energy.gov/.)
Electric Motor Industry and Switched Reluctance Machines 5

portion of electrical energy demand for motor applications is from the industrial sector
(60.27%), then the commercial sector follows at 20.55%, and residential sector at 12.33%.

1.3 The U.S. Motor Population

According to a 2011 report from International Energy Agency, the U.S. electric motor stock
was approximately 2,230 million [3]. Table 1.1 summarizes the U.S. motor population in
2011. The table shows that around 90% of these motors were less than 750 watts (W) and
they consumed 9% of the total electrical energy for all motors. Small size motors are mostly
used in mass-produced appliances in residential and commercial sectors (e.g. refrigerator
compressors, extractor fans). Most of the small size motors are either single-phase induc-
tion or shunt wound direct current (DC) motors with a maximum operating voltage of
240 volts (V). They operate 1,500 hours per year with a load factor of 40%, on average. They
have an average lifetime of 6.7 years and their nominal efficiency is around 40% [3].
Medium size motors are categorized from 0.75 kilowatts (kW) to 375 kW. These are
mostly general purpose motors used in pumps, fans, compressors, conveyors, and indus-
trial handling and processing. Medium size motors account for about 10% of all motor
population in the United States, but they consume 68% of motor-related electrical energy.
This portion increases to 75% on the global scale. The majority of the medium size motors
are three-phase induction motors with 2–8 poles. On average, they operate 3,000 hours per
year with a load factor of 60%, on average. The nominal efficiency is around 88%. They have
an average lifetime of 7.7 years [3].
Large size electric motors with more than 375 kW output power account for only 0.03%
of the electric motor population (around 0.6 million). However, they consume about 23%
of all motor-related electrical energy. These motors are manufactured in small quantities
(around 40,000 per year), and they are custom-designed for industrial and infrastructural
applications. The operating voltage can be between 1 kilovolts (kV) and 20 kV. Due to their
large size, they can achieve a nominal efficiency around 90%. On average, they operate
4,500 hours per year with a load factor of 70%. They have an average lifetime of 15 years [3].
It should be mentioned that in the industry, horsepower (HP) is often used to express the
output power of an electric motor. The unit of HP will be utilized in many places in this
chapter and 1 HP is 0.7457 kW.

TABLE 1.1
Summary of the U.S. Motor Population in 2011

Small-Size Medium-Size Large-Size

Output power <750 W 750 W–375 kW >375 kW

Population in 2011 2,000 million (90%) 230 million (10%) 0.6 million (0.03%)
Consumption of electrical energy for all motors 9% 68% 23%
Average operating hours 1,500 3,000 4,500
Average load factor 40% 60% 70%
Life time (years) 6.7 7.7 15
Nominal efficiency 40% 88% 90%
Source: Waide, P. and Brunner, C.U., Energy-efficiency policy opportunities for electric motor-driven systems,
International Energy Agency Working Paper, 2011.
6 Switched Reluctance Motor Drives

1.4 Electric Motor Usage in the Industrial Sector

As shown in Figure 1.2, the industrial sector in the United States consumes 32% of the total
energy and 28.10% of the total electrical energy. Within the industrial sector, motor drive
applications consume 62.5% of the electric power. This makes the U.S. industry sector the
largest consumer of electricity used by electric motors.
Figure 1.4 shows the motor applications and percentage of motor system energy used
in the industrial sector [6]. In the United States, material processing (e.g. mills, grinders,
lathes) and material handling applications (e.g. belts, conveyors, elevators, cranes) employ
34% of the general purpose industrial motors. Pump motors are mostly used for circulat-
ing water or other process fluids. In the industry sector, compressor motors are found in
HVAC and pneumatic power tools. Fan motors are used for ventilation and exhaust sys-
tems. Motors in refrigeration systems are mainly used in food industry, and in paper and
metal processing [6].
Figure 1.5 shows the motor system energy use by different manufacturing sectors in the
industry [7]. Chemical products industry is the largest process user of electrical energy for
motor applications. Paper industry follows it.

Refrigeration
7%

Material processing
Fans 22%
14%

Material handling
Compressors
12%
16%

Pumps
25%

Others
4%

FIGURE 1.4
Share of motor applications in the industrial sector. (From Lowe, M. et al., U.S. Adoption of high-efficiency
motors and drives: Lessons learned. February 25, 2010, Available: http://www.cggc.duke.edu/.)
Electric Motor Industry and Switched Reluctance Machines 7

Nonmetallic
minerals 5%
Primary
Food metals Plastics and
9% 9% rubber 6%

Balance of
manufacturing 6%
Petroleum and
coal 11% Non-energy intensive Fabricated
manufacturing metal 4%
industries 29% Transportation
Paper 14%
equipment 4%
Chemicals Wood products 3%
23% Machinery 2%
Textile mills 2%
Electronics 2%

FIGURE 1.5
Motor system energy use by different manufacturing sectors. (From U.S. Energy Information Administration,
Electricity use by machine drives varies significantly by manufacturing industry. October 8, 2013, Available:
http://www.eia.gov/.)

1.5 Electric Motor Usage in the Residential Sector

According to a 2013 report from U.S. Department of Energy Building Technologies Office,
HVAC applications account for 63% of the electric motor energy consumption in the
residential sector [8]. As shown in Figure 1.6, residential HVAC applications include cen-
tral air conditioners (CAC), heat pumps, furnace fans, room air conditioners (RAC), and
dehumidifiers. Refrigerators and freezers are the second largest consumers for motor-
related energy use. Electric motors in the range of 101 – 12 HP account for the biggest portion
of the motor population in the residential sector.
Clothes dryer 1%

Clothes washer 1% Misc 3%

Dishwasher 2%

Pool pump 2%

Central air
conditioner Heat pump
HVAC applications 30%
63% 19%
Refrigerator/
freezer
28% Furnace
fan
9%

Dehumidifier 2% Room air

conditioner 3%

FIGURE 1.6
Motor system energy consumption by end-use in the residential sector. (From U.S. Department of Energy,
Building technologies office, energy savings potential and opportunities for high-efficiency electric motors in
residential and commercial equipment. December 2013, Available: https://energy.gov/.)
8 Switched Reluctance Motor Drives

In residential HVAC systems, central air conditioners and heat pumps typically contain
a compressor, an outdoor fan, and an indoor blower motor. An air conditioner and a heat
pump operate based on similar principles, but unlike the air conditioner, a heat pump
can reverse the direction of the air flow and transfer heat from outside to increase indoor
temperatures [9]. 2–5 HP capacitor start single-phase induction motors are widely used for
residential HVAC compressors. Permanent magnet motors and variable speed capability is
being implemented in HVAC applications to improve the compressor efficiency. However,
since the initial purchase cost of the motor drive system is an important parameter in resi-
dential applications, use of permanent magnet motors might be an issue in the long run.
Low-noise operation is desirable to increase the comfort in residential HVAC compressors.
Fan motors commonly use permanent split capacitor single-phase induction motors. These
motors suffer from low efficiency. Today, these motors have been upgraded with brushless
DC (BLDC) motors to increase the system’s efficiency. Condenser fan motors are in the
range of 14 – 12 HP, while blower motors are in the range of 31 – 1 HP.
In window or through-wall room air conditioners, 90% of the electrical energy is used to
power the compressor motor, and the fans consume 10% of the total energy. Compressor
motors in room air conditioners are rated 12 – 2 HP and fan motors are rated 81 – 31 HP.
Condenser and blower are typically driven by a double-shafted single-phase induction
motor. Consumers are sensitive to the higher upfront purchase cost of room air condition-
ers. This is a major challenge in increasing the use of permanent magnet motors in this
application, especially for the condenser and blower fans.
Refrigerators and freezers (R/F) account for 28% of the electric motor energy consumption
in residential applications. Figure 1.7 shows the motor units used in different applications

R/F compressor Refrigerator: 8.50%;Freezer: 4.46% 12.97%

R/F condenser Refrigerator: 10.74%;Freezer: 2.23% 12.97%
R/F evaporator fan Refrigerator: 8.50%;Freezer: 4.46% 12.97%
Clothes washers Top loading: 5.10%;Front loading: 4.45% 9.86%
Clothes dryer 9.86%
Ceiling fan 7.57%
Dishwasher 6.33%
Furnace fan 5.58%
CAC compressor 5.38%
CAC outdoor fan 5.38%
RAC compressor 2.59%
RAC blower 2.59%
Dehumidifier compressor 1.35%
Dehumidifer fan 1.35%
HP compressor 1.33%
HP outdoor fan 1.33%
Pool pump 0.61%

FIGURE 1.7
Motor driven system units in the residential sector. (From U.S. Department of Energy, Building technolo-
gies office, energy savings potential and opportunities for high-efficiency electric motors in residential and
commercial equipment. December 2013, Available: https://energy.gov/.)
Electric Motor Industry and Switched Reluctance Machines 9

in the residential sector. It can be observed that R/F compressor, condenser, and evapo-
rator fan motors account for 39% of the total motor population in the residential sector.
Among them, compressor motors consume 90% of the R/F energy use. Residential R/F
compressors are typically 81 – 31 HP single-phase induction motors. These motors mostly
operate in on/off cycles, which cause significant power losses. Variable speed operation
could improve the compressor efficiency, but R/F applications are highly sensitive to com-
ponent purchasing costs. Condenser and evaporator fan motors are rated less than 100 1
HP.
Residential clothes washers and dryers account for 2% of the motor-related energy use;
however, as shown in Figure 1.7, they account for about 20% of the motor population in
residential applications. Clothes washers use a motor attached to the drum to provide
the power during the washing cycle (low speed-high torque) and spin cycle (high speed-
low torque). These motors range from 14 to 1 HP, and they spend 80% of their operation
in the washing cycle. Top loading washer motors account for 5.1% of the total residen-
tial motor population while front loading motor population account for 4.45%. Most top-
loading washers are sold at lower prices; therefore, this application is highly cost sensitive.
Low-efficiency capacitor-start induction motors dominate this market. In some high-end
products, BLDC motors are used to improve the system efficiency. Capacitor start single-
phase induction motors rated around 101 HP are widely used in clothes dryers. However,
dryer motors represent 5% of the total dryer energy consumption; therefore, the improved
overall efficiency with the utilization of a permanent magnet motor drive system might not
justify the higher cost. For other residential applications, typical motor ratings are listed in
Table 1.2 [10].

TABLE 1.2
Typical Motor Ratings for Other Residential Application
Application Typical Power (HP)

Vacuum cleaner 0.5–2

Dehumidifier 1
4

Attic fan 1
3

Window fan 1
20 – 51
Water well pump 1
2 –3
Mixer, food processor, Blender 1
50 – 101
Electric can opener 1
50

Trash compactor 1
4

Garbage disposal 1
4

Garage door opener 1

3 – 21
Large power tools 1–5
Hand held power tools 10 – 1
1

Electric lawn/garden tools 1

10 –2
PC fan < 100
1

Dishwasher pump 1
2

Source: U.S. Department of Energy, Opportunities for energy

savings in the residential and commercial sectors with
high-efficiency electric motors. December 1, 1999,
Available: https://energy.gov/.
10 Switched Reluctance Motor Drives

1.6 Electric Motor Usage in the Commercial Sector

In 2015, 28% of the total commercial sector energy was used for space heating, space
cooling, water heating, and ventilation [2]. As shown in Figure 1.8, HVAC applications
in the commercial sector (commercial air conditioners, air distribution, chiller compres-
sors, circulation pumps/water distribution) account for 74% of total motor-related energy
consumption.
Commercial air conditioning systems can be classified into two broad categories: decen-
tralized air conditioning systems and centralized air conditioning systems. Decentralized
air conditioning systems are employed in single and relatively smaller spaces. Centralized
air conditioning systems use mainly chilled water as the cooling medium. They are
found in large commercial buildings and require extensive ductwork and pipes for air
distribution [11].
In decentralized commercial air conditioning, the primary technologies are Packaged
Terminal Air Conditioners (PTAC) and Commercial Unitary Air Conditioners (CUAC).
PTAC is used in hotels, some apartments, and office buildings. They are designed to go
through the wall. These units operate similar to the room air conditioners in residential
applications, but they usually have a higher cooling capacity. Unlike room air conditioners
with double-shafted motors, they have separate motors for the condenser and evaporators.
The compressor motor of PTAC is rated at 12 – 3 HP, while the blower motors are rated at
10 – 4 HP. BLDC motors are being used more often in these types of units. However, the ini-
1 1

tial purchasing cost is still an important parameter that might limit the higher penetration
of permanent magnet-based motors in PTACs.
CUACs contain all the mechanical elements of the HVAC systems, including the com-
pressor, indoor blower, and outdoor blower. The rooftop units are suitable for single, flat

Circulation pumps/
water distribution
7%

Chiller
compressors Misc
17% Beverage
7%
vending
machines
Walk-in coolers 3%
Air distribution Refrigeration and freezers
Automatic
20% 19% 6%
commercial
ice makers
3%
Commercial air Central commercial
conditioners refrigeration
31% 7%

FIGURE 1.8
Motor system energy use by end-use in the commercial sector. (From U.S. Department of Energy, Building
technologies office, energy savings potential and opportunities for high-efficiency electric motors in residential
and commercial equipment. December 2013, Available: https://energy.gov/.)
Electric Motor Industry and Switched Reluctance Machines 11

buildings with large floor areas, such as motels, department stores, and movie theaters.
Permanent split capacitor type single-phase induction motors are typically used for
outdoor fans [8]. They range between 14 – 1 HP. Three-phase induction machines rated at
1.7–7.4 HP are used for indoor blowers. Depending on the capacity, one or multiple com-
pressors can be utilized in CUAC with individual three-phase induction motors running
at single-speed. For small size units, the compressor motor output power ranges between
2–5 HP, for medium size units 5–20 HP, and for large size units 20–100 HP.
Centralized air conditioning is mainly employed in large commercial buildings and
chilled water HVAC system is the primary solution. The chiller and auxiliary systems
are the biggest consumers of commercial sector electrical power. You can refer to [12] for
more information on the function of the chiller, air handling unit, and the water tower in
a centralized air conditioning system. Here, we will focus on the electric motors used in
chilled-water air conditioning systems.
Chillers usually run under partial-load conditions that might have a significant effect on
the efficiency of the system. For this reason, many chillers employ multiple compressors
and run them in on/off cycles to match the partial-load conditions. Large buildings often
use multiple chillers, air-handling units, and circulation pumps. Ratings of the motors and
circulation pumps depend on the type of the chiller. The compressor motor for the recip-
rocating chiller is rated between 7 12 and 150 HP, for a screw type chiller 40–750 HP, and
centrifugal chiller 50–1,000 HP. Large chillers can have motors greater than 1,000 HP [8].
In the cooling water loop, a centrifugal pump circulates the cooling water between the
chiller condenser and the cooling tower. In a reciprocating chiller, circulation pump motors
are rated at 5 HP, in screw-type chiller 15 HP, and in centrifugal chiller 20 HP. In the cool-
ing tower, motor driven fans assist the cooling process. Cooling tower fan/blower motors
are rated at 5–25 HP. Central air handling units distribute the cooling air to individual
spaces inside the commercial building. Depending on the size of the unit, they include
pump motors rated at 5–25 HP. Due to the high output power requirements, chilled water
HVAC systems employ three-phase induction motors [8].
Refrigeration applications use 19% of the commercial sector motor-related energy. As
shown in Figure 1.9, electric motors in self-contained refrigeration, central commercial

Commercial air conditioners Commercial unitary AC: 28.66% Packaged terminal AC: 10.78% 41.16%

Air distribution 32.97%

Beverage vending machines 7.97%

Central commercial refrigeration 6.68%

Automatic vending machines 4.31%

Walk-in coolers and freezers 4.31%

Circulation pumps/water distribution 1.72%

Chiller compressors 0.86%

FIGURE 1.9
Motor driven system units in the commercial sector. (From U.S. Department of Energy, Building technolo-
gies office, energy savings potential and opportunities for high-efficiency electric motors in residential and
commercial equipment. December 2013, Available: https://energy.gov/.)
12 Switched Reluctance Motor Drives

refrigeration, beverage vending machines, automatic vending machines, and walk-in cool-
ers account for 23.27% of the motor population in the commercial sector. Self-contained
commercial refrigeration equipment is employed for storing and displaying refrigerated
and frozen food in supermarkets and foodservice applications. In these applications,
motors drive compressors, evaporator, and condenser fans.
In self-contained refrigeration equipment, the compressor motor is rated less than
1 HP. Compressor motors in the reach-in refrigerators are rated at 31 – 12 HP, in reach-in
freezers 12 – 1 HP, and in beverage merchandisers 31 – 34 HP. Depending on the appli-
cation, single-phase capacitor starting induction motors and three-phase induction
motors are used for these types of refrigeration units. The efficiency of these compres-
sor motors is usually around 70% [10]. Condenser fan motors are typically 12 HP and
evaporator fan motors are less than 501 HP. The number of BLDC motors is increasing
as the condenser and evaporator fan motors in commercial refrigeration applications.
Central refrigeration systems in supermarkets and grocery stores can have compres-
sor racks employing some parallel-connected compressors. A typical store can have
10–20 compressors ranging between 3 and 15 HP. These are mostly single-speed, three-
phase induction motors. Due to the tight operating margins in supermarkets, a potential
investment in new motor technologies depends heavily on the lifetime cost of the motor,
which includes the upfront purchasing cost plus the cost of energy usage over the lifetime
of the system [10]. Electric motors in beverage vending machine compressors are rated
around 31 HP, and the fan motors are rated at 501 – 151 HP. Single-phase induction motors are
commonly used in these applications.

1.7 Electric Motor Usage in the Transportation Sector

1.7.1 Vehicle Propulsion
The transportation sector accounts for 23% of the global energy-related greenhouse gas
emission [13]. As shown in Figure 1.2, the transportation sector is the second largest energy
user in the United States, but the portion of electrical energy is low. This is because the
vast majority of the vehicles in the U.S. transportation system are powered solely by inter-
nal combustion engines and require fossil fuels as the energy source. In 2015, 96% of the
transportation sector energy was supplied from petroleum products, among which 63%
was motor gasoline (the remaining portion was mostly jet fuel and fuel oil). As shown in
Figure 1.10, light-duty vehicles consumed around 57% of the total transportation-related
energy in 2015 [2].
As shown in Figure 1.11, each year has seen an increase in the number of personal
cars and commercial vehicles in use. As of 2015, there were around 1.28 billion vehicles
in use globally [14]. The number of registered vehicles in the United States increased
from 193 million in 1990 to 264 million in 2015 [15]. U.S. auto sales were 17 million in
2016 [16].
Today, the majority of the U.S. vehicle stock is conventional gasoline and diesel powered
internal combustion engine vehicles. With the new regulations to reduce energy-related
greenhouse gas emissions and the decrease in the cost of electrified powertrains, the
percentage of hybrid, plug-in hybrid, and battery electric vehicles is expected to increase
significantly.
Electric Motor Industry and Switched Reluctance Machines 13

Pipeline Commercial light

Freight fuel trucks
trucks Air 3% 3%
20% 8%
Shipping,
international
3%
Other
8.93%
Military use
2%
Light-duty vehicles
57% Rail freight
2%
Bus transportation 1%
Recreational boats 1%
Lubricants ~0%

FIGURE 1.10
Energy demand by mode in U.S. transportation sector in 2015. (From U.S. Energy Information Administration,
Annual energy outlook 2016 with projections to 2040, August 2016, Available: http://www/eia/gov/.)

Europe United States China

1,400
Rest of Americas Rest of Asia/Ocenia Africa 1,282
1,235
1,185
1,200 1,142
1,097

273
1,056 263
992 1,020
252

1,000 960
242

927
230

892
Vehicle in use (in millions)

221

150
212

145
206

139
202

133
191

800
127
185

120

163
114

146
109

127
102

109
96

94
90

78

51 63
600 37 44
32
238

245

249

250

249

248

249

251

253

258

264

400

200
322

330

334

344

348

354

361

367

374

380

388

0
2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

FIGURE 1.11
Number vehicles in use worldwide (in millions). (From OICA, Motorization rate 2015 – Worldwide, Available:
http://www.oica.net/.)
14 Switched Reluctance Motor Drives

>100 kW
100
BEV

50-80 kW
Degree of electrification (%)

PHEV
20-50 kW

Full hybrid
12-20 kW

100%
8-15 kW

40-100%
HV mild
LV mild hybrid
3-10 kW
hybrid
20-50%

Micro
12-20%

3-7 kW hybrid
Start/stop
0
2-5%
3-10%

8-15%

Fuel efficiency improvement

FIGURE 1.12
Degree of electrification and electric traction motor power. (From Bilgin, B. et al., IEEE Trans. Transport. Electrific.,
1, 4–17, 2015.)

The degree of electrification for a vehicle defines the ratio of electrical power available
to the total power. As shown in Figure 1.12, as the degree of electrification increases, more
fuel efficiency improvement is achieved and the electrical power requirement increases.
Today, the majority of the hybrid, plug-in hybrid, and battery electric vehicles employ an
interior permanent magnet synchronous machine due to its high torque density and high
efficiency, especially at low and medium speed ranges. The Toyota Prius, the top sell-
ing hybrid-electric vehicle as of 2016, has a 60 kW V-shaped interior permanent magnet
traction motor with a maximum torque of 207 Newton meter (Nm) and a top speed of
13,500 rpm. The 2011 version of the Nissan Leaf electric automobile has 80 kW traction
motor with delta-shaped rotor magnets and it delivers a maximum torque of 280 Nm
and a maximum speed of 10,390 rpm. Another electric vehicle, the Chevrolet Spark, also
employs an interior permanent magnet traction motor with double-barrier rotor geom-
etry with bar windings and delivers a maximum power of 105 kW, a maximum torque of
540 Nm, and a maximum speed of 4,500 rpm [5]. Tesla is using 310 kW, 600 Nm, three-
phase induction machine with copper rotor bars in its Model S high-performance electric
vehicle [17].
As shown in Figure 1.12, the start/stop function provides 2%–5% fuel efficiency
improvement, and, today, this is mostly achieved by using the starter motor. The conventional
starter motors are brushed DC motors. Mild hybrids have an auto start/stop function and
regenerative braking capability. They also provide some use of electric power for propulsion.
Electric Motor Industry and Switched Reluctance Machines 15

48 V systems typically provide around 8%–15% fuel efficiency improvement. The majority
of the vehicles manufactured after 2017 will have start/stop capability [18]. Today, perma-
nent magnet and induction motors are employed in mild hybrid electric vehicles.
Electric motors are also utilized for many other uses in automotive applications, which are
not limited to hybrid and electric vehicles. Almost all vehicles include one or more electric
power steering motor, dual clutch transmission motor, seat adjustment motor, engine cool-
ing fan motor, electronic stability/anti-lock braking system motor, window lift motor, door
lock motor, hatch/tailgate motor, and sunroof motor. These are low-voltage, low-power
motors and they are designed for specific purposes. These motors are among the small-
size motors, which make up 90% of the motor population in the United States as shown
in Table 1.1. Therefore, it is important to notice the effect of car sales in the electric motor
industry, especially in the mature motor markets, such as in the United States and Western
Europe, where the growth in electric motor sales is driven by an increase in vehicle sales.

1.7.2 e-Bike Motors

With the trend towards improving energy efficiency and reducing energy-related green-
house gas emissions, electric bikes (e-bikes) are becoming a viable mode of transportation.
The total global market for e-bike sales was around 31.7 million units in 2014 [19]. China is
the global leader in the adoption and sales of e-bikes. Around 170 million consumers use
e-bikes every day in China. E-bike sales in China were 28.8 million, which corresponds
to 91% of the total global market. The second largest e-bike market is Western Europe
with 1.2 million e-bikes sold in 2014. Japan and the United States follow it by 440,000 and
185,000 units, respectively. According to the same research study, the e-bike market is
expected to reach 40.3 million units in 2023. China is projected to represent 85% of the total
market. Sales in North America, Western Europe, and Latin America will also grow signifi-
cantly. E-bike sales in Western Europe are expected to reach 3.3 million in 2023.
As of 2016, electric motor power for e-bikes is regulated, so that an e-bike can still be
categorized as a bicycle and used on roads and bike paths without requiring additional
licensing and registration. The electric drive system is usually activated through pedal-
ing, but in some classes, it is also possible to activate the electrical power with the throt-
tle. In Europe, motor power is limited to a top speed of 25 kilometers per hour (kph).
Continuous motor power is 250 W, and the peak motor power is 500 W. In the United
States, peak motor power can go up to 750 W with a top speed of 32 kph [20,21].
E-bikes today are mostly designed with either mid-drive or hub motors (see Figure 1.13).
As of 2016, mid-drive motors hold the majority of the market. Hub motors replace the regular
wheel hubs and connect the tire, rim, and spokes to the axle. The majority of the hub motors
are mounted on the rear wheel since the rear wheel usually has stronger support.
There are two types of hub motors. Geared hub motors include a planetary gear set.
The gear set enables higher motor speed leading to a reduction in motor dimensions.
To eliminate the resistance during coasting, geared hub motors might include a freewheel
mechanism, which limits the regenerative braking capability. In 2016, the majority of the
hub motors used in e-bikes are geared hub motors. Surface permanent magnet brushless
DC motors with concentrated windings are heavily used in geared hub motors.
The second type of hub motors is the direct drive or gearless hub motors. Since the
planetary gear set is eliminated, these motors provide higher torque at low speed.
The maximum speed of gearless hub motors is around 500 rpm. The planetary gear set is
the primary source of noise; therefore, gearless hub motors can deliver quieter operation.
On the other hand, they are usually heavier, and since they are located on the wheel, they
16 Switched Reluctance Motor Drives

(a) (b)

FIGURE 1.13
e-bike motors: (a) mid-drive unit and (b) hub motor.

position the weight towards the end of the bike. This may reduce the balance. Furthermore,
since the motor weight is built into the wheel, this increases the unsprung mass. It means
that the weight of the gearless hub motor cannot be sprung as a part of the frame [22].
Surface permanent magnet brushless DC motors with concentrated windings are the
preferred choice for gearless hub motors. Figure 1.14 shows the picture of a commercial
gearless hub motor from Crystallite. It is an exterior rotor machine, and the magnets are
located on the inner surface of the rotor.

FIGURE 1.14
Gearless rear hub motor (Crystallite HS3548).
Electric Motor Industry and Switched Reluctance Machines 17

FIGURE 1.15
Mid-drive motor of SHIMANO STEPS™ drive system.

The mid-drive unit is located at the bottom bracket, where the pedals are attached to
the frame. Hub motors drive the wheel itself whereas mid-drive motors drive the chain
forward. This way, they take advantage of the mechanical drive system. When the rider
changes the gear, the motor benefits from this, and it can run at relatively higher speed.
This makes mid-drive motors suitable for mountain biking and hill climbing.
Mid-drive motors also come with a much higher gear ratio. Planetary gear sets in hub
motors usually have a gear ratio around 5–11. With multi-level gear design, mid-drive
motors can achieve a higher gear ratio. Since the unit is attached to the bike frame, the
unsprung mass issue as in the gearless hub motors does not exist in mid-drive units.
However, mid-drive units require a torque sensor and speed sensor to control the output
power. This increases the cost of the motor and, hence, the e-bike.
Figure 1.15 shows the electric motor of SHIMANO STEPS™ mid-drive unit. It has three-
level gear system to achieve a high gear ratio. It has concentrated windings. This motor has
spoke-type magnets with tangential magnetization. This magnet configuration enables
high flux density in the air gap and maintains constant torque over a large speed range.
But the extended speed capability of the motor is limited. The motor is designed to provide
constant torque-assist up to the maximum pedaling speed. Spoke-type magnet rotor con-
figuration is usually more expensive to manufacture as compared to surface permanent
magnet motors.

1.8 Overview of Electric Motor Industry

The global electric motor market was valued at $122.5 billion in 2017, and it is estimated
to reach $164 billion in 2022. More than 60% of the demand and shipment will be in three
regions: United States, Western Europe and China. China’s share in the global demand is
18 Switched Reluctance Motor Drives

projected to increase to 40.91% by 2022 from 33.13% in 2012 [23]. This is due to the increas-
ing manufacturing and rising middle class in China.
Electric motor demand and shipment in the Asia/Pacific region (China, Japan, and
other Asia/Pacific) is expected to increase. In 2002, the Asia/Pacific region had 39.86% of
the global electric motor demand and 43.97% of global electric motor shipment. In 2022,
this region is expected to have 63.90% of the global demand and 69.21% of the global
shipment [23].
Leading electric motor suppliers in the North and South America are ABB, WEG, Nidec,
Regal Beloit, and Siemens. The suppliers are developing higher-efficiency motor drives.
One of the main targets in the industry is to achieve competitive prices. Increasing supply
chain cost has also been a concern [24]. Due to its low-cost and simple construction, SRMs
will definitely play a significant role in the price competition and market expansion of
high-efficiency motor drives.

1.9 Electric Motor Types

Induction and permanent magnet machines dominate in the motor drive systems used
in industrial, residential, commercial, and transportation applications. Figure 1.16 shows
the typical structure of these motors and compares them with that of a switched reluc-
tance motor. Here, the most significant advantage of SRM becomes apparent. All of these
motors have a stator, winding, and rotor. But SRM has one item less than permanent
magnet and induction machines. As shown in Figure 1.16a, permanent magnet machine
has magnets inserted in the rotor to provide the rotor excitation. As shown in Figure
1.16b, induction machine has rotor bars to achieve magnetic induction and create the
forces on the rotor.
Switched reluctance machine does not have permanent magnets or conductors on
the rotor. Also, its winding is much simpler than the permanent magnet and induction
machines. These features bring many advantages to the SRM architecture (e.g. low manu-
facturing cost, robust operation at high speed and harsh environment) and also many
challenges (e.g. nonlinear characteristics, torque ripples, and acoustic noise and vibration).
We will investigate these issues in more detail throughout this book. Now, we will have a
brief look at the other motor types, and address the advantages and challenges they have.

1.9.1 Permanent Magnet Machines

As shown in Figure 1.16a, rotor magnets in permanent magnet (PM) machines provide an
independent source of magnetic flux. In interior permanent magnets machines, the mag-
nets are embedded in the rotor. By applying a careful design for the location and arrange-
ment of the magnets, the output torque at higher speeds can be improved by utilizing the
reluctance torque. In residential and commercial applications, surface permanent magnet
machines are widely used, where the magnets are located on the surface of the rotor. These
motors usually come with concentrated windings, similar to an SRM [17].
The independent rotor excitation in permanent magnet machines can provide high
torque density and better efficiency especially at low and medium speed range [5]. This
is one of the main reasons why permanent magnet machines are preferred in applica-
tions with high-efficiency requirements. As we will see in the next chapter, there is a
Electric Motor Industry and Switched Reluctance Machines 19

Stator Winding Permanent

magnets
Rotor

(a)

Stator Winding Rotorbars

Rotor

(b)

Stator Winding Rotor

(c)

FIGURE 1.16
Typical structure of electric machine types: (a) interior permanent magnet synchronous machine, (b) induction
machine, and (c) switched reluctance machine. (From Bilgin, B. et al., IEEE Trans. Transport. Electrific., 1, 4–17, 2015.)

relationship between the strength of the magnetic field and forces acting on the rotor. If a
strong magnetic field is maintained in the air gap, higher forces can be generated, resulting
in higher torque density. Depending on the material, rotor magnets can provide a strong
magnetic field in the air gap without utilizing coils on the rotor. Therefore, copper losses
could be reduced leading to higher efficiency.
In permanent magnet machines, high torque density and high efficiency can be
achieved by using magnets that can provide high flux density. As the speed increases,
the back-electromotive force of a PM machine is likely to exceed the terminal voltage. If the
back-electromotive force is not reduced, current cannot be injected into the motor and
torque drops. To extend the speed range of the motor, field weakening is applied and the
phase angle between the current and voltage is adjusted so that the stator flux effectively
opposes the magnet flux. Hence, the excitation torque from permanent magnets is reduced
and the additional reluctance torque component from the rotor saliency helps to extend
the speed range [17]. Field weakening reduces the efficiency at high speeds. In addition,
the permanent magnet should have enough coercivity, so that it is not demagnetized
during the field weakening.
20 Switched Reluctance Motor Drives

High-energy rare-earth magnets (e.g. Neodymium Iron Boron and Samarium Cobalt)
are used in permanent magnet machines to achieve high torque density, high efficiency
and high resistance to demagnetization. The main disadvantage of permanent magnet
machines is the sensitivity of rare-earth magnets to temperature. When the temperature
of Neodymium Iron Boron (NdFeB) magnet increases to 160°C, the output torque can drop
by up to 46% [25]. Samarium Cobalt (SmCo) rare-earth magnet has the highest resistance
to demagnetization. It can handle continuous temperatures above 250°C and it is used in
aerospace and military applications.
However, permanent magnets have high cost, and they take up a significant portion of
the total cost of a permanent magnet machine, even though they represent a small portion
of the total weight of the motor. Figure 1.17 shows the distribution of mass and cost in an
80 kW interior permanent magnet motor designed for a traction application [26]. It can be
observed that rotor magnets take up 3% of the mass, but 53% of the cost, for this specific
application. The price of SmCo is even higher than NdFeB magnet.
Rare-earth permanent magnets are made from rare-earth elements. The most com-
monly used rare-earth elements in permanent magnets are neodymium and dysprosium.
Dysprosium is used to improve the resistance of the magnet towards demagnetization at
elevated temperatures. In a typical NdFeB magnet for a traction motor, neodymium takes
up around 31% of the total mass, whereas dysprosium takes up around 8.7% [17].
There are significant supply chain issues with rare-earth elements, which can cause
prices to spike from time to time. These events could adversely affect the cost of permanent
magnet motors as the demand for high-efficiency motors increase in industrial, residential,
commercial, and transportation applications and electricity generation. For this reason,
neodymium and dysprosium are defined as critical materials by the U.S. Department of
Energy [27].

Insulation Insulation
(a) mass ~0% (b) cost ~0%

Stator Stator
copper copper
Casting
10% Stator steel 10% 18%
Bearings
9%
Casting 1% Rotor steel
34% Shaft 3% 6%
Stator steel
27%

Rotor magnets
Rotor steel 53%
Shaft 18%
7%
Bearings
1%

Rotor
magnets
3%

FIGURE 1.17
Distribution of (a) mass and (b) cost in a permanent magnet machine for a traction application. (From Miller,
J.M., Electric Motor R&D, 2013 U.S. DOE Hydrogen and Fuel Cells Program and Vehicle Technologies Program
Annual Merit Review and Peer Evaluation Meeting, Oak Ridge National Laboratory. May 15, 2013, Available:
http://energy.gov/.)
Electric Motor Industry and Switched Reluctance Machines 21

Depending on the performance requirements and operating temperature, magnets other

than NdFeB and SmCo are used in electric motor applications. Ferrite magnets have low
energy product and they are usually used in low-cost applications, which do not require
high torque density. Aluminum nickel cobalt (AlNiCo) alloy magnets can operate at high
temperatures, but they have low coercivity. The biggest application for AlNiCo magnets is
watt-hour meters [28].

1.9.2 Rare-Earth Materials

The global market for permanent magnets is estimated to reach $15 billion by 2018.
The largest demand is in Asia-Pacific Region. The demand in this region is likely to
keep growing due to the increasing manufacturing and end-user applications in China.
The United States is an important manufacturer of high-performance magnets for
military and other strategic applications, but it still imports 60% of its permanent magnet
consumption [28]. China produces 76% of the permanent magnets in the world.
Rare-earth magnets have been dominating the global permanent magnet market and
they are heavily utilized in electric motor applications. Besides magnets, rare-earth mate-
rials are used in many critical applications such as in photovoltaic films, vehicle batteries,
and lighting. Neodymium and dysprosium are the primary rare-earth materials that are
most commonly employed in high-energy permanent magnets.
The rare-earth magnet supply chain can be divided into five stages: (i) mining, milling
and concentration of the ore, (ii) separation into individual rare-earth oxides, (iii) rare-earth
metal production, (iv) alloy or powder production, and (v) magnet manufacturing [27].
In the downstream portion of the supply chain, production is highly concentrated amongst
a few companies that dominate the market. China dominates the rare-earth production, even
though they hold only about half of the known world reserves [29]. According to United
States Geological Survey (USGS), the world rare-earth mine production was 125,000 metric
tons in 2014. As shown in Figure 1.18, China holds 84% of the world production. This value

United
States
4.32% Thailand
1.52%
Russia
China Other 2.08%
84.01% 5.26%

India
1.36%
Australia
6.40%

Malaysia
0.19% Kazakhstan
0.11%

FIGURE 1.18
Estimated world mine production of rare-earth materials by country (total output: 125,000 metric tons of rare-
earth oxide equivalent). (From United States Geological Survey (USGS), 2014 mineral yearbook, rare earths
[advance release], December 2016, Available: http://www.usgs.gov/.)
22 Switched Reluctance Motor Drives

500 3500
450
Neodymium Dysprosium 3000
400
350 2500
Neodymium [$/kg]

Dysprosium [$/kg]
300
2000
250
1500
200
150 1000
100
500
50
0 0

FIGURE 1.19
Price of neodymium and dysprosium between 2011 and 2016. (From Metal-Pages.com, 2018.)

is based on China’s Ministry of Land and Resources production quotas. Around 20% of
China’s rare earth oxide was produced illegally and the total production was well beyond
the 2015 quota. In the United States, the mining and processing of rare earth materials take
place in Mountain Pass Mine in California [30].
As it was shown in Figure 1.17, rare-earth magnets are the largest cost component of an
electric motor. This is related to the high price of rare-earth materials. Figure 1.19 illustrates
the change in price in neodymium and dysprosium between 2011 and 2016. Rare-earth prices
seem lowering from the peak in 2013. But it should be noted that in December 2001, the price
of neodymium was less than $20 per kg, and the price of dysprosium was less than $150 per
kg [27]. Being the largest producer of rare-earth materials, China has significant control over
the price and availability of these materials in the global market. China imposes export quo-
tas on rare-earth products. For example in 2010, China reduced the export quota for rare-
earth products by 40%. As a result of this, the price of neodymium jumped to $115 and the
price of dysprosium jumped to $400 per kg [27]. As of September 2016, the price of neodym-
ium and dysprosium were $49.89 per kg and $257.50 per kg, respectively.
With the increasing demand for high-efficiency motors in industrial, residential, com-
mercial, and transportation sectors, and in energy generation, the price volatility of rare-
earth elements and supply chain issues are significant concerns. As explained earlier, the
United States imports 60% of its permanent magnet consumption. China holds 76% of
permanent magnet production and 84% of rare-earth material production. Therefore, the
United States might end up being highly dependent on the external resources if it relies
heavily on electric machines using rare-earth permanent magnets. This is why rare-earth
elements are regarded as critical materials by U.S. Department of Energy [27].
There are also production and environmental issues for rare-earth materials. Rare-earth
elements are not rare as their name implies. The earth’s crust contains sufficient rare-earth
elements. However, they are difficult to mine and it is usually hard to find them in high
enough concentration so that the extraction process is economically viable. As depicted in
Figure 1.20, production of rare-earth materials is a small portion of the total metal produc-
tion. However, mining and refining of rare-earth materials require significant amount of
Electric Motor Industry and Switched Reluctance Machines 23

Zinc Boron 0.54%

0.73%
Manganese Chromium 0.53%
1.00%
Silicon 0.44%

Raw steel Other Copper Lead 0.28%

8.18% 1.09%
91.82% Nickel 0.13%
Aluminum
3.28% Magnesium 0.06%
Lithium 0.03%
Rare earths 0.007%
Other metals 0.072%

FIGURE 1.20
Breakdown of global metal production in 2015 - total metal production: 1.75 billion metric tones. (From United
States Geological Survey (USGS), Historical statistics for mineral and material commodities in the United States,
June 2017, Available: http://www.usgs.gov/.)

capital and expertise. Depending on the location and the production capacity, the extraction
and processing of rare-earth ore might need capital between $100 million to $1 billion [27].
The processing of rare-earth elements into high-purity rare-earth oxides is a highly-
specialized chemical process and it requires significant know-how in mineral processing.
Therefore, producing rare-earth materials is not a process that can be addressed just by
opening new mines. The same applies to the production of rare-earth permanent magnets.
Producing high-quality permanent magnets require expertise and know-how as well.
The Japanese organization, Hitachi, holds more than 600 patents on the production of
high-quality neodymium-iron-boron magnets, which makes it difficult for other corpo-
rations to produce these magnets [27]. Mining and extraction of rare-earth materials can
have adverse environmental effects, which also increase the cost of production. Besides the
political reasons, environmental concern is also a factor for the quota China is applying on
the rare-earth exports [31].
In summary, permanent magnet machines provide high torque density and high effi-
ciency, especially at low speeds. However, the price volatility, supply chain issues and
environmental concerns for the rare-earth materials, are significant concerns for the pro-
duction of high-energy rare-earth magnets. In the long run, these problems might prevent
the electric motor industry from responding to the increasing demand for high-efficiency,
high-performance, and low-cost electric motors. From an economic perspective, boosting
the supply and production of rare-earth metals might not solve the problem, because min-
ing and producing rare-earths is a capital intensive and environmentally delicate process.

1.9.3 Induction Machines

Induction machines (IMs) are the most widely used motors in many different applica-
tions in the industrial, commercial and residential sectors. The majority of the electric
motors in use today are IMs. Single-phase capacitor-start IMs are widely used in small
size motors.
There are primarily two types of induction machines. In wound-rotor IMs, the rotor
circuit is made up of three-phase windings. The rotor windings are then short circuited
24 Switched Reluctance Motor Drives

using slip rings. As shown in Figure 1.16b, in squirrel-cage IMs, the rotor bars are inserted
by die casting, where melted aluminum is molded in the rotor slots. End rings then short-
circuit the bars. Squirrel-cage IMs are commonly utilized in low- and medium-power
applications.
In an IM, the air gap magnetic field and the rotor rotate at different speeds. This is why
induction machines are referred as the asynchronous machine. When there is a differ-
ence between the speeds of the rotor and the air gap magnetic field, rotor conductors are
exposed to a time-changing magnetic field. The time-changing magnetic field induces
voltage across the rotor bars. Since the end-rings short circuit the rotor bars, current
flows through the conductors and this generates the rotor magnetic field. The interac-
tion between stator and rotor magnetic fields creates the torque. If the rotor rotates at
the same speed as the stator magnetic field, the rotor bars will not be exposed to a time-
changing field and torque will be zero. Then, the rotor speed is eventually balanced at
a speed lower than the synchronous speed, where the motor torque equals to the load
torque [17,25].
When compared to permanent magnet machines, IMs have lower cost construction,
because they don’t have permanent magnets. Die-casting of aluminum rotor bars is also
a low-cost process. The self-starting capability is one of the main reasons why IMs are
widely used in our industry. Both permanent magnet and switched reluctance machines
require a power converter, control algorithm, and position feedback or estimation. Three-
phase induction machines can start from the AC supply without a power converter, control
algorithm, or position feedback. Single-phase induction machines can also start directly
from a single phase AC supply. However, single-phase excitation generates two magnetic
fields, rotating in different directions. To maintain self-starting capability, they need a
means to create phase-lag between these two fields. This can be accomplished in many dif-
ferent ways, but capacitor start single-phase induction machine is the most popular type in
the residential, commercial and industrial sectors.
Today, the majority of IMs are still used in on/off cycles to match the partial load
requirements. Especially in commercial HVAC systems, there are sometimes three different
induction machines in one unit, which run three different compressors at different cycles.
This solution might look feasible from the cost point of view. There are well-established
low-cost manufacturing techniques available for induction machines. However, from
a systems perspective, this is not an efficient solution. With the increasing demand for
higher efficiencies – both in motor and system level – and decreasing cost of adjustable
speed drives, variable frequency operation becomes much more feasible.
As compared to permanent magnet machines, an IM-based motor drive system can
still have a significant cost advantage due to the lack of more costly permanent magnets.
However, it is harder to say the same when compared to a switched reluctance machine
based motor drive system. Lower manufacturing cost can be achieved with a switched reluc-
tance machine because it has much simpler windings and it doesn’t require rotor conductors.
Compared to permanent magnet machines, induction machines typically have lower
efficiency and power factor – especially single-phase induction machines typically run
at low efficiencies (around 70%). Since there is no direct excitation on the rotor, the excita-
tion current is drawn from the stator current. This leads to a low power factor. This is the
reason why induction machines usually require a small air gap to minimize the reactive
power requirement.
Due to induced currents on the rotor conductors, there are non-negligible rotor cop-
per losses in the induction machine. Especially for high-torque operation and applications
with high power density requirement, rotor copper losses present challenges in extracting
Electric Motor Industry and Switched Reluctance Machines 25

the heat out of the motor. For induction machines with die-casted aluminum rotor bars,
the rotor copper losses can be more significant, because the electrical conductivity of
aluminum is 60% that of copper.
For high torque and high power density applications, induction machines with copper
rotor bars can be utilized. The traction motor in Tesla Model S electric vehicle is an exam-
ple [5]. In this motor, copper is die-casted in the rotor slots to achieve high conductivity,
lower rotor resistance and, hence, higher efficiency.
Die casting is a metal casting process where molten metal is injected into the die at high
pressure. It is a quick and reliable process and enables low-cost mass manufacturing of
induction machines. However, copper die-casting is not a feasible solution for cost sensi-
tive applications. The melting temperature of copper (1,080°C) is much higher than alu-
minum (660°C). In copper die-casting, the dies should be preheated, and during the die
casting process, the temperature should be controlled. In addition, the density of copper is
higher than aluminum and this requires specialized tooling and higher tonnage presses
[32]. This makes copper die cast induction machines a lot more expensive than aluminum
die cast induction machines.
In summary, induction machine has been the low-cost solution in the industrial, residen-
tial, and commercial sectors for various direct-drive single-speed applications. However,
with the increasing demand for high system and motor efficiency, this advantage has been
diminishing. Higher efficiencies can be maintained by using variable frequency drives,
but this reduces the advantage from low-cost manufacturing and operation of self-starting
induction motors. In addition, aluminum die cast induction machine suffers from high
rotor losses. This also reduces the efficiency. Copper has much higher conductivity, but
copper die-cast induction motors are a lot more expensive. Therefore, the increasing
demand for higher efficiency and variable speed operation reduces the cost advantage of
induction machines and makes switched reluctance machine based motor drive systems
a more viable alternative.

1.10 Switched Reluctance Machines

As compared to the permanent magnet and induction motors, switched reluctance
machines have a simple, low-cost and robust construction. As shown in Figure 1.16, SRM
has one less component than other motor types. Permanent magnet machines have mag-
nets on the rotor, and induction machines have conductors on the rotor. The SRM stator
is made of a salient pole laminated core with concentrated windings, while the rotor also
has a salient pole structure without windings or permanent magnets. This enables reliable
operation at high speeds and high temperatures but also brings many challenges, such as
high torque ripples, and acoustic noise and vibration.
One of the most significant advantages of a switched reluctance machines is the simple
and low-cost construction. All mass-produced electric machines employ copper and steel.
Manufacturing of windings and laminated cores are well-established industry practices.
There are many companies with significant expertise in electrical steel production, punch-
ing or laser cutting laminations, lamination stacking, and winding automation. These are
the practices that are needed to manufacture an SRM. As compared to permanent magnet
machines, an SRM does not need magnet insertion and initial magnetization. As com-
pared to induction machines, an SRM does not need die-casting (in squirrel-cage IM) or
26 Switched Reluctance Motor Drives

additional winding process (in wound-rotor IM) for the rotor conductors. Therefore, any
motor manufacturer, which has the supply chain to manufacture permanent magnet or
induction machines, can easily make switched reluctance machines with much lower cost.
An SRM has a simple rotor construction without coils or permanent magnets giving it
two significant advantages of cost and supply chain security. As we discussed previously
in this chapter, rare-earth magnets, which provide high torque density and high efficiency
at low speeds in permanent magnet machines, suffer from price volatility, supply chain
issues, and environmental concerns. This can be a problem in the long run as the demand
for high-efficiency motors is increasing. Due to the lack of permanent magnets, the low-
speed efficiency in switched reluctance machines might end up lower. However, SRMs
can provide similar and even higher efficiencies at the medium and high-speed range. In
addition, SRMs can deliver comparable or superior efficiency over the entire operating or
drive cycle of the application.
As compared to other motor types, an SRM is more suitable to running at high-speed
and high-temperature conditions. This advantage also comes from the lack of rotor exci-
tation. At high speeds, the performance of the PM motors can be seriously limited by
the rotor displacement and rotational stress. When the rotor is rotating at high speeds,
centrifugal forces dominate and high stresses occur on the magnet slots and the bridges.
The bridges are the sections of the rotor core which are located between the magnet slot
and the air gap. They have to be carefully designed so that they can handle the centrifugal
forces and saturate quickly not to cause any flux leakage. Switched reluctance machine has
a simple and robust rotor core. It doesn’t have slots or bridges. This makes an SRM a better
candidate for high-speed operation.
As the speed increases, field weakening is applied in PM machines to extend the speed
range. Field weakening reduces the efficiency of PM machines at high speeds. As we will
see in the next few chapters, flux weakening naturally happens in SRMs at high speeds.
Compared to permanent magnet and induction machines, an SRM has the largest con-
stant-power speed range, which makes it suitable for high-speed operation.
When it comes to high-temperature operation, SRM has an advantage, as well. The mag-
netic properties of permanent magnets are highly dependent on the operating temper-
ature. In permanent magnet machines, the maximum operating temperature should be
defined to maintain stable operation of the magnet. As discussed previously in this chap-
ter, the flux density and coercivity of NdFeB magnet reduces as the temperature increases.
This affects the output torque and demagnetization. Permanent magnet machines using
NdFeB magnets are usually designed so that the magnet temperature stays around 100°C
during the continuous operation.
The SRM’s rotor is made of laminated steel only. The magnetic properties of steels change
drastically near the Curie temperature, which is 770°C for iron. In the case of non-oriented
steel, the permeability of electrical steel does not alter much below 500°C. Electrical steels
have surface insulation to reduce eddy current losses. The surface insulation chosen for
an SRM should generally handle the annealing temperature, which is a heat treatment
process to eliminate the stress on the laminations and helps to return the magnetic prop-
erties to the stress-free conditions [33]. Annealing temperature depends on many param-
eters such as the material, duration, and pressure. Surface insulation on electric steels can
handle temperatures in the range of 200°C–400°C.
In SRMs, the magnet wires used in the stator windings determine the thermal rating
of the machine. The insulation around the copper conductor enables the contact between
the wires without causing any electrical short circuit. Magnet wire insulation is made
of organic material, which softens at a lower temperature than copper or electrical steel.
Electric Motor Industry and Switched Reluctance Machines 27

However, thermal class of magnet wires can go up to 240°C, which is much higher than
permanent magnets. 200°C is a common standard for magnet wires.
When compared to induction machines, SRM designs are still advantageous for the
high-temperature operation. In induction machines, rotor conductors are conventionally
manufactured by die-casting aluminum in the rotor slots. Aluminum has lower conduc-
tivity than copper; therefore, rotor copper losses can be a significant constraint in the high-
temperature operation of induction machines.
As discussed previously, copper die-casted induction machines have lower rotor losses
and they are more suitable to operate at high temperatures. But their manufacturing pro-
cess is more challenging and expensive. When designing the thermal management sys-
tem for copper die-casted induction machines, the higher coefficient of thermal expansion
(CTE) of copper should also be taken into account. The difference between the CTE of
copper and steel would apply fatigue stress at the copper-steel interface and cause cracks
on the conducting bars when the motor goes through thermal cycling [33].
Another advantage of an SRM is its fault-tolerant operation capability. As it will be dis-
cussed in detail in the next few chapters, each phase of SRM can be considered electrically
isolated from each other. This means that when one phase is excited with current, the mag-
netic flux that links with other phase coils is negligible and, hence, the mutual coupling
can be ignored. Therefore, torque production of one phase is independent of the others. If
there is a fault in one phase, the other phases can generate torque and keep the motor run-
ning with reduced performance.
The salient-pole construction of SRMs enable fault-tolerant operation due to electri-
cally isolated phases. However, this is also a major source of high torque ripple. The
torque in each phase of an SRM is dependent on the relative position between the stator
and rotor poles, and the level of excitation current. During phase commutation, phase
torques are added up together, and the overall torque profile ends up with a pulsated
waveform. Torque ripple in an SRM is much higher than permanent magnet and induc-
tion machines.
Torque ripple should not be an obstacle in the adoption of SRMs in motor drive sys-
tems. Each particular application determines the need for torque ripple requirements. For
example, conventional internal-combustion engines have significant torque ripples due
to the firing of individual cylinders. In vehicular applications, large flywheels are used to
manage the torque ripples. Furthermore, the brackets connecting the engine to the chassis
are carefully designed to reduce the effect of torque ripple [34]. Torque ripples in SRM can
be reduced significantly by modifying the rotor geometry and shaping the phase current.
The torque ripple reduction techniques in SRM will be discussed in depth in different
chapters of the book.
Acoustic noise and vibration is the most well-known issue in switched reluctance
machines. Due to the salient pole construction of an SRM, when a phase is excited with
current, the flux penetrates into the rotor, mostly in the radial direction, and generates
large radial forces. These radial forces deform the stator core and the frame, which results
in vibrations and acoustic noise.
Acoustic noise and vibration cannot be eliminated in SRMs, but it can be reduced sig-
nificantly. Acoustic noise reduction in SRM requires a multidisciplinary approach. The
stator and rotor geometry, pole combination, materials, frame, shaft, and current control
should be optimized to match the acoustic noise specifications of the given application.
System level analysis plays a significant role in the design and optimization of switched
reluctance machines. It is very challenging to achieve low acoustic noise, low torque ripple,
low temperature rise, and high efficiency in a wide torque and speed range, by satisfying
28 Switched Reluctance Motor Drives

low-cost and high power density constraints. This is also true for permanent magnet and
induction machines. To better optimize a switched reluctance machine, the requirements
at different torque and speed points should be identified. It is important to know at which
operating points low acoustic noise is desired, and what the torque ripple and efficiency
requirements are at those points. Then the geometry, materials, and the current control
can be optimized to reduce the acoustic noise.
Single source of excitation in SRMs also causes some challenges. The stator current is
responsible for both excitation and torque generation. For this reason, an SRM is usually
designed with a smaller air gap, which requires tighter mechanical tolerances. Induction
machine also has a single source of excitation, and it is manufactured with a small air gap
as well. Permanent magnet machines can be designed with a larger air gap because the
permanent magnets independently generate the rotor field. A design with tighter mechan-
ical tolerances is not an issue to prevent utilization of switched reluctance machines. Our
industry has strong expertise in machining and manufacturing. If there is a small increase
in the cost of an SRM due to tighter tolerances, it can be justified by the material and
energy savings.
Another challenge for SRM is that it has low power factor at light load operation. As it
will be explained in detail in Chapter 2, at small currents, an SRM works in the linear
region of the magnetization curve. When operating in the linear region, an SRM has a lower
power factor and less than half of the total magnetic energy is converted into mechanical
work. The rest is stored in the magnetic circuit and supplied back to the source at the end
of the stroke or dissipated inside the motor. In practice, an SRM operates in the nonlinear
region of the magnetization curve to achieve a higher power factor and better utilization
of the converter.
The converter topology that is used in SRM drives is different from that of permanent
magnet and induction machines. As it will be addressed in the next few chapters, the
direction of the torque in an SRM is independent of the direction of the current. Therefore,
unipolar phase current is required. Asymmetric bridge converters are widely applied in
SRM drives. In permanent magnet and induction machines, three-phase full bridge con-
verters are used.
Utilizing a different converter is not a big challenge for the adoption and mass
manufacturing of SRM drives. The cost of permanent magnet and induction motor drives
might be cheaper today because semiconductor manufacturers are already manufacturing
three-phase modules where all the switches are in one package. However, the same can
be done for SRM drives. In fact, there are already modules available where the switches
and diodes for one phase are packaged in one module. This enables a more compact
converter design for SRM. A high-power traction SRM converter will be presented in
Chapter 16 utilizing customized modules.
It should also be noted that in the permanent magnet and induction motor drives, the
switches on the same phase leg cannot be activated at the same time. If that happens, the
DC link will be short-circuited. This is called a shoot-through fault. Since the phases in
SRMs are electrically isolated from each other, this is not the case. In fact, in an asymmetric
bridge converter, two switches are turned on at the same time. Therefore, SRM does not
have a shoot-through fault condition.
A challenge that has been attributed to SRM is its nonlinear characteristics, which
make the analysis and optimization harder. It is true that an SRM has nonlinear char-
acteristics. But, considering the advancements in modeling and computation tech-
niques, motor drives, and control algorithms, this is not a challenge any longer. In a
surface permanent magnet machine, since the permeability of permanent magnets is
Electric Motor Industry and Switched Reluctance Machines 29

very close to that of air, the effective air gap is large, and the magnetic circuit can be
assumed linear. Therefore, analytical expressions can be used to model and optimize
the surface permanent magnet motors.
Due to its nonlinear characteristics, numerical modeling techniques, such as finite ele-
ment analysis are needed to optimize SRMs. This is not a problem anymore, thanks to the
advanced software platforms (e.g. JMAG, ANSYS, Infolytica, COMSOL, Flux), which can
analyze and characterize an SRM quickly. As it will be introduced in Chapter 4, combining
analytical and finite element tools, it is possible to accurately model and optimize switched
reluctance machines.

1.11 Switched Reluctance Motor Drive Applications

Today, switched reluctance machines have a small share in the electric motor population.
SRMs are used in some industrial pumps, vacuum cleaners, and agricultural and mining
vehicles. The small market share of SRM is historically due to high acoustic noise and
vibration, torque ripples, challenges in design and optimization due to its nonlinear char-
acteristics, and the cost of the power converter due to the use of unconventional power
modules. As it was discussed in the previous section and will be addressed throughout
the book, these challenges can now be resolved and an SRM can now be designed to match
the requirements of a wide range of application.
Today, induction machines dominate in the electric motor driven applications. This
is mainly because induction machines don’t need an adjustable speed drive to start.
However, many mechanical systems in the industrial, residential, and commercial sectors
operate with variable loads in long operating hours. Significant efficiency gains can be
accomplished by adapting the motor speed and torque to the load conditions [3]. In many
applications, electric motors are oversized and they run continuously at partial load. This
is a significant loss in capital, efficiency, and performance. Adjustable speed drives can
eliminate the losses due to partial loads. They adjust the speed and torque of the motor
to the load and eliminate the need for mechanical components such as gears, transmis-
sions, and clutches. The motor size can also be optimized for the application requirements.
Besides, adjustable speed drives can enable regenerative braking capability, which leads to
higher system efficiency. The largest benefit of using adjustable speed drives comes with
pumps, fans, escalators, cranes, and air-conditioning system. Motor drive systems run-
ning with on/off cycles, such as air compressors, conveyors, and refrigerators can benefit
from adjustable speed drives.
Induction machines have been dominating the electric motor powered applications due
to their low-cost manufacturing and self-starting capability. However, with the increas-
ing demand for higher efficiencies, induction machines should be designed with smaller
losses, and they need to operate with adjustable speed drives. These factors diminish the
cost advantages of induction machines. Permanent magnet machines are being used more
often these days in various applications. They have lower losses as compared to induction
machines, and the system efficiency is improved with the use of adjustable speed drives.
However, as explained previously in this chapter, the price volatility, supply chain issues
and environmental concerns for the rare-earth materials, are significant issues for the per-
manent magnet machines as the demand increases.
Many motor drive applications are highly sensitive to initial purchasing cost. Industrial
pumps and compressors, commercial HVAC systems, crane, and conveyors are suitable
30 Switched Reluctance Motor Drives

applications for SRM technology. Most of these applications are not sensitive to acoustic
noise. SRM technology is also a good candidate for many of the residential applications
such as refrigerators and freezers, top-loading residential clothes washers, room air condi-
tioners, and swimming pool pumps.
Due to its wide constant power speed range, SRM is an excellent candidate for traction
applications. High torque ripples, acoustic noise, and low torque density at low speeds have
been the major obstacles in the adoption of SRMs in electrified drivetrain applications.
However, increasing demand for electrified vehicles will put more pressure on develop-
ing reduced cost motor drive systems, which will open further opportunities for switched
reluctance machines. Besides, especially for hybrid and plug-in hybrid electric vehicles,
the manufacturers are moving towards higher speed motors to reduce the motor volume
and improve the powertrain efficiency. This is an advantage for SRM since it provides
higher efficiency at high speeds.
E-bikes represent one potential opportunity for SRM technology. As discussed earlier,
the e-bike market is expected to grow drastically in the next few years. Today, permanent
magnet machines dominate the e-bike motor drives. E-bikes are extremely cost sensitive,
and manufacturers (mostly located in China) are putting significant effort into reduc-
ing the cost of motor drive systems. SRM could be a game changer in the e-bike market.
Powertrain costs can be reduced significantly with SRM, offering a potential cost advan-
tage to original equipment manufacturers (OEMs) to expand their market share.
Before we conclude this chapter, we would like to try to answer a major question: why
SRM technology is not widely applied in motor drive applications? The answer is quite
complicated, but we will try to explain it from a technology perspective and an indus-
try perspective. If you look at the history of electric motor technologies, you would see
that we’re moving from complicated motor structures and simple drives towards sim-
ple motor structures and complicated drives; because the cost of both power electron-
ics and digital control has been declining. In addition, government regulations have
been pushing markets toward more efficient designs to reduce energy consumption
and emissions. These forces place an emphasis on higher efficiency motors and better
controllability.
For example, the brushed DC motor, the very first technology we learn when we start
working on electric motors, has a complex structure. The armature circuit is on the rotor,
and mechanical brushes rectify the armature current generated by the alternating field
inside the motor. Hence, the brushed DC motor runs by applying DC voltage to the arma-
ture circuit. The motor drive systems have evolved to induction machines, brushless DC
and, then, interior permanent magnet machines. Among these electric machine types, SRM
has the simplest construction. But the controls and optimization are more complicated. As
we will see throughout the book, with the advancements in modeling, analysis, optimiza-
tion and control techniques, designing and implementing SRM based motor drives is no
longer a complex task. The increasing demand for higher-efficiency and lower-cost motor
drive system is already creating the need for switched reluctance motors.
From an industry perspective, the answer is more complicated. Besides the technical fac-
tors listed above, organizational, economic, and system level factors act as a barrier to the
adoption of SRM drives. Switched reluctance motors are still at the stage of product devel-
opment. This process has to go hand-in-hand with the system level analysis. Therefore, the
interaction between the motor manufacturers, steel producers, OEMs, and designers has
to be well established.
Today, electric motor wholesale sector prefers reducing types and number of electric
motors to a minimum to reduce the capital cost of manufacturing and inventory [3].
Electric Motor Industry and Switched Reluctance Machines 31

In many cases, they offer general-purpose motors, which are mass manufactured and usu-
ally oversized for the application. In critical industrial and commercial applications, the
customers keep multiple units in their inventory to prevent possible interruptions in their
processes in case the electric motor fails. The large inventory of old, never-used motors is
a factor causing delays and challenges in adopting new technologies.
Eighty percent of the electric motor sales are directly to wholesalers, distributors, and
OEMs [3]. The wholesalers get the majority portion of the profits. Motor manufacturers
are spending more effort and capital to reduce the cost of production of existing technolo-
gies rather than designing new technologies that could change the game. Therefore, there
haven’t been many efforts from the industry on developing low-cost SRM drives. But, it
is changing due to the increasing demand for lower-cost and high-efficiency motor drive
systems.
However, high volume applications, like washing machines and other consumer appli-
ances often use specially designed motors to meet increasingly strict cost and efficiency
targets. This process involves large volume contracts that the appliance manufacturers
made directly with the motor manufacturer, bypassing the distributor to achieve lower
costs. Automotive traction motors are also increasingly designed specifically to the auto-
motive product requirements and purchased directly from the manufacturer or build in-
house. As motor and drive design becomes easier and more cost effective, one can expect
the cost and efficiency to continue to favour simpler motor and more complex drive cus-
tom designed to a given application. These are the areas where SRM technology is likely
to become more prevalent in the future.
Last but not the least, SRM has not been well understood in our electric motor industry.
With this highly technical book, we expect to provide an in-depth discussion of the multi-
disciplinary aspects of switched reluctance machines and help engineers design switched
reluctance motor drives for various applications.

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