1. Abstract—Motivated by the newly emerging field of
electric/hybrid vehicles in the consumer industry, we look into the
characteristics of variable switch reluctance motors (VSRM). We
identify the characteristics and components of a VSRM and how
it operates.
This paper describes how other motors measure up to in design
and performance in comparison to a VSRM. Topics discussed
deal with the similarities and differences that can be accounted
for in different motors types when compared to variable switch
reluctance motors.
The mechanics of a VSRM are discussed in detail. The
extreme accuracy due to the commutation with respect to the
rotor angle is discussed. We go into their ability to be
programmed to match the exact load they serve and their low
material cost.
We look into the advantages and disadvantages that are
inherent to a VSRM and how the motor can be adjusted to ensure
better performance. The noise and vibration effects are discussed
along with the limitation due to the torque ripple. The paper
includes the mechanical factors that can create a dilemma with a
VSRM.
I. INTRODUCTION
N the most recent years, variable switch reluctance motors
(VSRMs) have become increasingly popular. Their
comparable performance and low material cost set them on top
of their category for motors of choice in industry. The
efficiency, torque per volume, and inverter power rating over a
reasonable power range of the permanent magnet synchronous
motors are comparable as well. Due to the wide range of
operating characteristics, VSRMs are desirable for many
variable speed applications.
This paper presents various aspects of a VSRM and begins
to explain them in detail. First, a general layout of the motor’s
parts and operation is given along with the basic torque and
electrical characteristics. Second, an explanation of switching
requirements and advanced DSP control schemes commonly
used for VSRM are described.
At the same time, this paper explains the advantages and
disadvantages you would expect to encounter with the
operation of the motor. Along with every application, losses
occur; therefore the typical losses found in VSRMs are
outlined. Finally, applications to which variable switched
reluctance motors are commonly applied are presented.
II. PARTS AND OPERATION
In this section the basic parts of a variable switched
reluctance motor will be outlined and discussed. Also, an
overview of how the motor operates will be described,
including the presentation of the primary torque and phase
voltage equations.
A. Components of a Variable Switched Reluctance Motor
Switched reluctance motors are perhaps the simplest motors
to construct. Both the rotor and the stator have salient poles,
but only the stator has excitation windings [1]. For proper
operation, every VRSM will have one more pair of poles on
the rotor than the stator. Fig.1 shows the basic layout of a
VSRM with six rotor poles and eight stator poles.
The rotor is generally constructed out of a laminated
permeable material to maximize its inductance [2]. Since it
has no windings there is no need for mechanical brushes or
slip rings to energize the rotor, this gives the VSRM a major
advantage over other DC motors. The lack of rotor windings
also makes the motor inherently resistant to overload and
immune to single point failure [2].
As shown in fig.1 each opposing pole pair on the stator
shares a common excitation winding. These windings are
activated in a very precise sequence (as will be discussed in
the following section) to produce rotor rotation and torque.
Fig.1. Basic layout of a variable switched reluctance motor [1]. Note the
differing number of poles on the rotor and stator.
Variable Switched Reluctance Motors
(March 2006)
STEVEN G. ERNST, IMELDA SAPUTRA, JOSHUA R. BEACHY, and JOHNNY S. TRUMPS JR.
I

2. Every VSRM must also have some sort of control module to
regulate the winding excitation sequence and timing. Before
the advent of control electronics, this was done by mechanical
means of sensing the rotor position and activating the
appropriate winding. Now, this can be done much more
efficiently and precisely using power electronics and control
circuitry in conjunction with sensors to determine the rotor
position. However, as will be discussed later, this control
circuitry is often complex and expensive and has been one of
the major limiting factors preventing the widespread use of
VSRMs.
B. Operation Techniques
When a set of windings is activated in a VSRM a flux path
is formed through the corresponding rotor poles, the rotor
experiences a torque that drives it in line with the coils thereby
maximizing the inductance [2]. In fig.1 the flux path can be
viewed as running between poles A and A’ on the stator and
through a and a’ on the rotor when the corresponding coils are
energized. To continue rotor movement once the point of
maximum inductance has been reached, the next set of coils
must be activated and the first shut off. Depending on the
direction of rotation desired, this would either be the B pair or
the D pair of stator poles in fig.1. It is important to note that if
the winding pairs are excited in a clockwise sequence, the
rotor will rotate in the opposite direction (i.e. counter
clockwise).
(i) Torque: The rotor torque produced by a single set of
phase windings depends on both the winding current and the
variance of the mutual inductance with the rotor angle. The
equation describing this relationship is given in (1).
(1)
It can easily be seen that the maximum torque will be
produced when the inductance has the greatest change with
respect to rotor angle (i.e. when its slope of the inductance
function is greatest). Fig.2 gives the general inductance profile
of a VSRM as a function of rotor angle. The inductance
variance is essentially a triangle function with maximums
when stator and rotor coils line up and minimums when they
are completely out of alignment. Therefore, since the slope is
constant, as the rotor moves into alignment with an energized
coil it will experience a constant torque until near alignment is
reached (the torque would then become negative if the winding
was kept energized). Once the rotor is near alignment, the
next set of windings can be energized, thereby providing near
constant torque (ideally) throughout rotation.
It is important to note that the motor torque of a VSRM does
not inherently depend on the rotational speed of the motor.
This implies that with the appropriate switching and activation
of the windings, the rotor can be made to rotate at any torque
and speed while maintaining efficiency [2].
(ii) Phase Voltage: The phase voltage equation is given by
(2).
Fig.2. Variation of inductance with rotor position [1].
This is a function of the inductance and the current, both of
which will vary with time [1].
(2)
This equation deserves some analysis to better understand
the variation in phase current with relation to rotor speed.
With a constant phase voltage and low rotor speed, the change
in current with respect to time will be large. This means the
rated current will be reached very quickly. Therefore, to limit
the current, a voltage modulation scheme must be employed at
low speeds. However, at large rotor speeds, the change in
current with respect to time will be small. This will allow the
current to be self-limiting and a simple pulsed voltage scheme
to be used in order to control the winding excitation.
III. VSRM CONTROL
Unlike induction, synchronous and DC motors the variable
switched reluctance motor will not operate by just energizing
the phases with AC or DC. Correct operation requires
switches to energize each phase of the VSRM individually in
specific sequences [3]. The following section discusses basic
switching of a VSRM and then discusses a few advanced DSP
switching schemes used to control a VSRM.
A. Basic Switching
As previously explained, energizing a phase in a VSRM will
cause the rotor to experience a torque that will rotate it
towards that particular phase. If the same phase is continually
energized, the rotor will eventually align itself with that phase
causing it to cease rotation. However, if the phase that is
causing the rotor to spin is switched off before the rotor aligns
itself, then the next phase is switched on, the rotor will
experience a torque causing it to rotate toward the newly
switched on phase. The rotor will rotate when this specific
switching pattern takes place (switching on a phase, switching
off that phase, and switching on the next phase) [3]. For a
standard variable switched reluctance motor, the on-time of
each switch is equal to the number of phases divided by 360
degrees (or 2π radians).
The switching on and off of each phase in a VSRM is
usually done by IGBT’s that are connected from each phase to
the power source. These switches work by turning on and off
when the appropriate control signal is sent to the gate of the
switch, therefore enabling control of the on and off time of
each phase. For a greater level of control, additional switches
can be added from the negative terminal of each phase to
θd
dL
IT 2
2
1
=
dt
di
Li
dt
d
d
dL
RV ++= )(
θ
θ

3. Fig.3. Schematic of a four phase SRM with a control switch at the positive
and negative terminal of each phase [3].
ground. The described control scheme also requires
freewheeling diodes to be connected across the motor for each
switch. The freewheeling diodes allow the stored inductive
current to follow out of each phase when a switch is turned off.
A diagram of this control scheme is shown in fig.3 [3].
When the VSRM was original designed switching was
controlled by elaborate mechanical systems [3]. Advances in
digital technology now allow digital signal processors to
control the control signal into the gate of an IGBT. DSP’s also
have enabled engineers to develop control schemes which
optimize torque, efficiency and innovative techniques which
eliminate the need for a rotor sensor. The advantages brought
by DSP’s have greatly increased the capability of a VSRM but
at the same time have greatly increased the complexity of its
design process [4]. Two complex control schemes are
summarized below.
B. Current Controlled Switching
Current controlled switching maintains a constant current in
the energized phase of the VSRM until the next phase needs to
be switched on. This switching scheme begins by measuring
the current in each phase and comparing it to the desired value.
The result of this comparison is fed in to a DSP which
calculates the necessary adjustments to maintain a constant
current for the energized phase [4].
This switching scheme requires an angle measuring sensor
on the rotor. This sensor not only enables the DSP to know
which phase needs to be energized, but it also allows the rotor
speed to be calculated. The rotor speed is important since it is
imperative in the calculation of the desired reference phase
currents [4].
The problem with this switching scheme is that the output
torque will have significant ripples. These ripples in the
torque produce the unwanted side effects of noise and
vibration. These side effects can shorten the life of the motor
and lead to velocity errors at the rotor. The following
switching scheme, torque controlled switching, attempts to
minimize the torque ripples [4].
C. Torque Controlled Switching
Torque controlled switching varies the current into each
phase in order to maintain a constant torque at the rotor. The
theory is that the torque produced at the rotor will be measured
and then compared to the desired torque. The error produced
by this comparison could then be fed into a DSP which
computes the current that must be delivered to each phase at
that particular time. With an appropriate DSP and torque
measuring device, the output torque should be able to be kept
constant [4].
The problem with this scheme is the fact that torque sensors
that precisely measure the torque at the rotor do not exist. In
real life applications of this scheme, each VSRM motor is
tested to determine the torque produced per phase at a given
rotor angle and given phase currents. This data is used to
produce a digital signal processing system which when given
the rotor angle and the phase currents can calculate the
expected torque at the rotor. This DSP system will try to
maintain the torque it calculates constant by varying the
current into each phase [4].
This real life application of the torque controlled switching
scheme does not succeed in maintaining a constant torque.
However, the torque ripple seen in this application is
significantly lower than the torque ripple seen in the current
controlled switching scheme [4].
IV. ADVANTAGES AND DISADVANTAGES
The variable switched reluctance motor has a number of
inherent advantages and disadvantages to be considered before
choosing the motor for a particular application. Fig.4 displays
a summary of the main advantages and disadvantages of the
variable switched reluctance motor [5]. Fig.5 compares four
commonly used motors (including VSRMs) based upon ten
key factors [1] .
Variable switched reluctance motors have several distinct
advantages when compared with other electric motors,. In six
to eight of the ten categories, it can be seen that VSRMs are
ADVANTAGES DISADVANTAGES
Low-cut Need for position
measurement
Robust construction Higher torque ripple than
other machine types
Absence of brushes Higher noise than other
machine types
No short-circuit fault Nonlinear and complex
characteristics
No shoot-through faults Large vibrations occur
Ability to operate with faulted
conditions
Precise mechanical gap is
difficult to achieve
High torque-to-inertia ratio Complex control circuitry
Unidirectional currents
High efficiency
High reliability (no brush
wear), failsafe for inverter
Driven by multi-phase inverter
controllers
Sensor less speed control
possible
Fig.4. Advantages and disadvantages of VSRMs.

4. DCM IM PMM VSRM
Controllability 1 4 3 2
Ruggedness 4 2 3 1
High power/ weight 4 3 1 2
High torque/ inertia 4 3 2 1
High speed capability 4 1 3 1
Low noise 3 2 1 4
Low maintenance 4 1 1 1
Low cost 3 2 4 1
Low EMI 4 1 1 1
Low torque pulses 2 1 2 4
Fig.5. Comparison table for HEV motors. (Note: DCM – DC motor, IM –
Induction motor, PMM – Permanent magnet motor, VSRM – Variable
switched reluctance motor) [1].
rated as the best, if not close to the best, in performance when
compared to other common electric motor types. The
maintenance, low cost, low electro-mechanical interference,
high torque/inertia, high speed capability and ruggedness of
the VSRM are the key factors that set this particular motor
apart from other in industry today.
Variable switched reluctance motors are suited for several
different applications, including the automotive and aerospace
industries. For automotive applications, such as the newly
emerging HEV industy, the variable switched reluctance motor
has the benefit of robustness and fault tolerance. In addition to
being able to withstand harsh conditions for a prolonged
period, the motors have the ability to continue operating in the
event of a phase failure. Another advantage of VSRMs is their
ability to be controlled for constant speed, acceleration and
regenerative braking applications. In aerospace applications,
the VSRM motor’s ability to run under faulted conditions and
its suitability for operation under harsh environments are
critical [5].
In contrast to induction motors, the VSRM motor is meant
to operate in deep magnetic saturation to raise the output
power density [6]. Due to saturation effects and variation of
magnetic reluctance and flux-linkage, the inductance and
torque of the machine are highly non-linear functions of both
rotor position and phase current.
In comparison between permanent magnet motor and the
VSRM, the PMM is the smallest, lightest motor but the magnet
costs are very high and the variable switched reluctance motor
has a considerably lower cost [6].
V. LOSSES
One problem that occurs in every motor design is the energy
losses. The process of converting mechanical energy to electric
energy and vise versa has the potential to create significant
losses. This loss in energy is different depending on several
factors, such as the motor type, materials used, and switching
scheme employed. For a variable switched reluctance motor,
the losses can be split into four main sections; copper losses,
iron losses, eddy-current losses and hysteresis losses [7].
A. Copper Losses
The first step in calculating the copper losses involves the
calculation of the resistance of each phase winding of the
VSRM. The mean length of a winding turn is given by (3).
( ) mDWLl STM
3
10*2sin(242 −
++= β (3)
The resistance of a single phase is calculated by (4).
( ) ΩCPHMS aTlR **0177.0= (4)
The copper losses at the rated current is given by (5).
WRiP SCU P
2
= (5)
B. Iron Losses
Iron losses or core losses can be can be demonstrated by two
major portions, hysteresis and eddy-current losses. In most
machines, the iron losses can be calculated using the Steinmetz
equation [7]. However, the non-sinusoidal flux waveforms of a
variable switched reluctance motor requires a different
method. The two major works in this field are by Materu, and
Hayashi [8].
The first work done observed the harmonic analysis of the
flux density at different parts of the magnetic circuit in order to
study iron losses. This method was effective and accurate,
however, there is no separation of hysteresis and eddy-current
losses. Another downside to this method is that the method
assumes that the core-loss data is valid in the presence of
arbitrary combinations of harmonics [7].
The second work describes a method to clearly separate the
hysteresis and eddy-current losses. Amongst the vast array of
hysteresis models, the Preisach model appears to be the most
practical due to its easy parameter identification and
considerable accuracy. It describes the hysteresis of a
magnetic material via an infinite set of magnetic dipoles,
which have rectangular hysteresis loops, as shown in Fig. 6
[8].
Fig.6. Rectangular hysteresis loop of dipoles.
The flux density B corresponding to the field strength H is
expressed by (6),
(6)

5. C. Eddy-Current Losses
The first step in calculating the eddy-current losses is to
obtain the plot of flux vs. time at rated speed [7]. Although,
theoretically this is an extremely simple concept, the procedure
to obtain this is elaborate.
The total eddy-current losses of the machine at rated speed
and power output is given by (7),
rsesyerpespee PPPPP +++= (7)
Where the number of strokes per revolution is given by
(NSNR) / 2, two poles are energized at each stroke and there is
no overlap between flux pulses, the eddy-current losses for the
stator poles Pspe and rotor poles Prpe are given by (8) and (9),
(8)
(9)
The equation for calculating the eddy-current loss in the
rotor core section is given by (10),
(10)
The total eddy current loss in the stator yoke is given by
(11),
(11)
D. Hysteresis Losses
The hysteresis losses can be calculated from the process
described by equations (12) thru (17). The classical equation
describing the hysteresis losses was given by [7] as,
6.1
MHH fBCP = (12)
Where Ch is the hysteresis coefficient, f is the frequency and
Bm is the maximum flux density. In [HAYASHI95], the total
hysteresis losses are given by (13),
(13)
Where the flux in the stator poles is unipolar and can be
calculated as,
(14)
The hysteresis losses in the stator yoke can be calculated by
(15),
(15)
The hysteresis losses in the rotor pole can be calculated as
(16),
(16)
The hysteresis losses in the rotor core can be calculated as
(17),
7)
VI. APPLICATIONS
There are several applications to where variable switched
reluctance motors become the ideal case. The most common
place a variable switched reluctance motor is found is in
smaller scale applications. In many appliances today, the
flexibility of motor speed adjustment is becoming an important
issue. Variable switched reluctance motors are a key element
in the role of adjustable speed applications. Because of their
lack of costly permanent magnets, these motors are slowly
working themselves into applications where equipment cost is
an issue.
If you were to take apart various small appliances found at
home that are commercially sold today, there would be a high
probability of encountering a switch reluctance motor within.
Blenders, household fans, and microwaves are just a few of
these appliances that commonly use these motors.
White goods are also a popular location to find switched
reluctance motors. They are commonly used in the production
of vacuum cleaners, washing machines, dryers, ranges, water
heaters, refrigerators, dishwashers, and freezers.
The application of a variable switched reluctance motor
doesn’t end within the household. There are several other
applications where they are used. In the laboratory
environment, such testing equipment as a centrifuge requires
the use of a variable switched reluctance motor for operation.
The technology is being applied to a wide variety of
automotive applications from small engine auxiliaries to large
hybrid traction schemes with electrical supply voltages from
12V up to several hundred volts at higher power. Applications
include:
• Starter generators
• Hybrid power train systems
• Full electric traction
• Transmission Systems
• Turbo machinery
• Various actuators
These motors also have been shown to have large potential
in future automotive applications. They are especially well
Fig.7. Image of a typical centrifuge commonly found in laboratory
environments.
esyesyesysylesye PPPPP 432 +++=

6. suited for electric/hybrid vehicles. Some of the important
characteristics include ruggedness, controllability and weight
to power ratio.
The reliability of an electric/hybrid vehicle motor is a
consumer demand that must be met. Variable switched
reluctance motors can continue operating even with the failure
of one phase, allowing the vehicle to have limp-home
capabilities. Also due to an absolute minimum of moving parts
and the lack of rotor windings, these motors have especially
long lifetimes and extreme reliability.
The speed and torque characteristics can be controlled
precisely, unlike induction motors, making them particularly
suited for the wide range of speeds and power requirements
inherent in highway and city driving. In the design of any
vehicle, weight is an important issue that cannot be
overlooked. Due to their high power to weight ratio VSRMs
are well suited as drives in electric/hybrid vehicles.
VII. CONCLUSION
This paper explains the components found in a variable
switched reluctance motor. Other motors commonly used have
similar and different characteristics which are compared
throughout the paper. The advantages, such as the high
efficiency and ability to be programmed to operate ideally with
the load applied, and reviewed and examined. The
disadvantages, such as the acoustic noise and large vibration
effects, are also examined.
ACKNOWLEDGMENTS
The paper is primarily based upon the references listed
below in the references section. All other information on used
throughout the paper was taken from general knowledge
obtained by one of the four authors thorough out their
educational and industrial careers.
REFERENCES
[1] P.C Sen, Principles of Electric Machines and Power Electronics. USA:
John Wiley & Sons Inc., 1997
[2] American Society of Mechanical Engineers. (1998). The Rise of VSR
Motors. Available: http://www.memagazine.org/backissues/february98
/features/risevsr/risevsr.html
[3] T.J.E. Miller, Switched Reluctance Motors and Their Control.
Hillsboro, OH: Magna Physics Publishing, 1993.
[4] Texas Instruments. (1997, July). Digital Signal Processing solutions for
the Switched Reluctance Motor. Available: http://focus.ti.com/lit/an
/bpra058 /bpra058.pdf
[5] Rashid, Muhammad H. Power Electronics. Stanford: CA: Elsevier
Science & Technology Books, 2001.
[6] W Wu, H C Lovatt, J B Dunlop. Optimization of Switched Reluctance
Motors for Hybrid Electric Vehicle. Australia: CSIRO
Telecommunications & Industrial Physics, 2006. http://www.cip.csiro.
au/Machines/papers/OptimizationOfSwitchedReluctanceMotorsForHybr
idElectricVehicles.pdf
[7] Vijayraghavan, Praveen. (2001) Design of Switched Reluctance Motors
and Development of a Universal Controller for Switched Reluctance
and Permanent Magnet Brushless DC Motor Drives. http://scholar.lib.
vt.edu/theses/available/etd-11302001-160101/unrestricted/praveen_
dissertation.pdf
[8] Ramsden, V.S. Discrete modeling of magnetic cores including eddy
current. New York, NY: Browns Publishing, 2005.
Steven G. Ernst Born in Salem, Oregon in
1984. Currently pursuing a bachelor’s of
science degree in Electrical Engineering at
Oregon State University in Corvallis, Oregon.
The date of graduation is expected to be in
2007.
He has worked for Jack Palmer Inc. as a
TECHNICION SPECIALIST for the years of
2003 – 2004. He has worked for Siltronic
Corporation as a FACILITIES ENGINEER in
2005. He currently is working for the Business
Solutions Group as a STUDENT TEST
ENGINEER in Corvallis, Oregon since 2004.
Beginning June of 2006, he will begin working
for Intel as a PASD BOARD DESIGN ENGINEER. Currently, he is
researching the effects of flow control when applied to networking devices.
Mr. Ernst has received the Ritter Scholarship Achievement for Electrical
Engineering as well as the McDougall Scholarship Award for Electrical
Engineering. Mr. Ernst regularly attends the IEEE meetings held at Oregon
State and provides thoughtful in-site to the congregations.
Johnny S. Trumps Jr. Born in Las Vegas,
Nevada February 29 1984. Currently pursuing
a bachelor’s of science degree in Electrical
Engineering and a minor in Business
Management at Oregon State University in
Corvallis, Oregon. The expected date of
graduate is June 2007.
He has worked for College Pro Painters as
a FRANCHISE MANAGER for the year of
2004. He currently is not employed but he is a
full time student at Oregon State University.
Mr. Trumps is a member of Eta Kappa Nu
Engineering Honor Society and the National
Honor Society. Mr. Trumps has received the
Zimmerman Engineering scholarship as well as the Wininger Gaylord
scholarship, , Earnheart Electrical and Computer Engineering scholarship, L.
Fisher memorial scholarship and E & G Kird Endowment for electrical and
computer engineers.
Imelda Saputra (M’04) She became a
Member (M) of IEEE in 2004. She was born
in Cirebon, Indonesia in 1983. Currently
pursuing Bachelor’s of science degree in
Electrical Engineering at Oregon State
University in Corvallis, Oregon. She will be
graduating in June 2006.
She is presently working with Business
Solution Group in Corvallis, Oregon, as a
NETWORK VALIDATION ENGINEER since
2005. She worked for Kwaplah International,
Inc. in Consultant Services as
INTERNATIONAL PROCUREMENT
COORDINATOR in the years of 2003 - 2006. She also worked at Lonnie B.
Harris Black Cultural Center at Oregon State University as an OFFICE
ASSISTANT in 2002 - 2003. She is Vice President of IEEE – Oregon State
Chapter since 2005 and was Head Publicity of IEEE – Oregon State Chapter
in 2004 - 2005. She is also working with many international students from
different countries through Every Nation Campus Ministries. She is a member
of Grace Christian Fellowship, a member of Indonesian Student Association
and a member of International Student of Oregon State University.
Ms. Saputra has received an Excellent World Bank Proposals Award from
Kwaplah International, Inc., John Fulton Scholarship, Educational
Opportunity Program Award and Immagia Foundation Scholarship.

7. Joshua R. Beachy Born in Portland,
Oregon in 1983.Currently pursuing a
bachelor’s of science degree in Electrical
Engineering at Oregon State University in
Corvallis, Oregon. The date of graduation is
expected to be in spring 2006.
He has no prior Engineering work
experience.
Mr. Beachy has received the John Engle
Electrical Engineering Scholarship as well
as the L. Fisher memorial scholarship,
Earnheart Engineering Scholarship and Rex
Miller scholarship.