Control of a Linear Switched Reluctance Motor as a
Propulsion System for Autonomous Railway Vehicles
L. Kolomeitsev1, D. Kraynov1, S. Pakhomin1, F. Rednov1, E. Kallenbach2, V. Kireev2, T. Schneider3, J. Böcker3
Emetron Ltd., Novotcherkassk, Russian Federation, e-mail: email@example.com
Steinbeis Transferzentrum Mechatronik, Ilmenau, Germany, e-mail: firstname.lastname@example.org
University of Paderborn, Inst. of Power Electronics and Electrical Drives, Paderborn, Germany, e-mail: email@example.com
Abstract—1A linear switched reluctance drive with large switched reluctance motors. As a matter of fact, the motor
airgap was developed and manufactured for a test track of geometry, winding parameters, and the control should be
autonomous railway vehicles (RailCab). Control structure thoroughly designed using precise dynamic models
and current shaping algorithms are presented providing describing the nonlinear force generation with respect to
smooth transitions between the motoring and dynamic the position and current and taking into account the phase
braking operation modes at low switching losses. In both voltage and current limitations of the power converter as
modes low force and torque ripples are maintained with well.
reasonable efficiency. The control is tested on the rotational To enable relative motion between the vehicles of a
variant of the motor. Details of the test bench and convoy, the LSRM needs active translators below the
measurements of phase currents, force or torque,
undercarriage whereas the stator located between the rails
respectively, and efficiency are presented.
is the passive part. Therefore a catenary or a feeder rail is
Keywords—linear and rotational switched reluctance
necessary for energy supply of the vehicle.
drives, propulsion, control, dynamic braking, low ripple. To compare the proposed novel LSRM  with the
existing LDFM , the LSRM was designed within the
same geometrical spacing, i.e. with a scale of 1:2.5 of a
I. INTRODUCTION targeted system. The specifications of the test track which
The RailCab project was founded at the University of was built for real life validation of the RailCab concept
Paderborn in 1998. The focus of this project is to are:
investigate a novel railway system with individually • Maximum slope of 5.3%
operating autonomous vehicles called RailCabs. These • RailCab tare weight of 1.2t (incl. motor weight)
RailCabs can drive in convoy without the mechanical • Maximum speed of 10 m/s
couplings. • Mechanical airgap of 12 mm
For real life validation of convoy operation, a test track In order to meet the acceleration specification of 1 m/s²
in a scale of 1:2.5 was built at the University of the new LSRM should provide at least a force of 1500 N.
Paderborn . For building convoys, the propulsion Furthermore, the propulsion system has to provide not
and braking system must be independent of the weather only a smooth acceleration and cruising at constant speed,
conditions. For this reason a linear doubly-fed motor but also regenerative braking and reverse driving.
(LDFM) was chosen . The LDFM offers the The paper is organized as follows: Section II describes
opportunity to transmit energy contactless into the the structure and the parameters of a six-phase LSRM for
vehicle. Therefore a catenary or a feeder rail is not the RailCab test track. The results of transient finite
necessary for energy supply of the vehicle. But the element analyses are presented in Section III. The control
additional copper windings in the active stator increase for the presented LSRM in order to meet the force
the construction costs. That is the reason why two further specifications is covered in Section IV. Details of the
types of linear electric motors have been considered: The prototyped models and the experimental results for the
linear induction motor (LIM) and the linear switched torque and efficiency are presented in Section V and
reluctance motor (LSRM). Switched reluctance motors followed by conclusions in Section VI.
generally offer a very simple and robust design. Thus,
they are very suitable for highly reliable and fault-tolerant
II. SR MOTOR STRUCTURE AND PARAMETERS
applications . That is the reason why the LSRM was
discussed in  as an alternative propulsion concept for To study the LSRM characteristics experimentally,
RailCabs. two motor prototypes with drive control have been
An important issue with the switched reluctance developed . The first prototype (linear SRM with a
motors is the highly nonlinear magnetisation structure shown in Fig. 1) is intended for static
characteristic. If this issue is not addressed properly in measurements of the propulsion force characteristics and
motor design and control, the nonlinearity will of the normal force. The normal force (attraction force
considerably increase the force ripple and noise of between stator and translator) is an important issue of the
linear motor, because it implies additional load for the
track and the bogie.
This work was supported by the German Federal State of North Rhine- Because it is expensive to set up test facilities to run
Westphalia, the University of Paderborn, STZ Mechatronik Ilmenau, linear motors at constant speed, a second prototype was
Germany, and the Federal Agency of Science and Innovations, Russian built as a rotational SRM. The construction of this motor,
Federation, Grant No.02.516.11.6100
978-1-4244-1742-1/08/$25.00 c 2008 IEEE
in particular pole number, pole pitch, airgap, etc. are done of 1.5…1.6 higher than the average flux of other
as close as possible to that of the linear motor. segments.
Phase polarity Active Flux direction for three TECHNICAL DATA OF THE ROTATIONAL SRM
Airgap (12 mm) translator active phases
Supply voltage (RMS) 400 V
A B C D E F A` B` C` D` E` F`
Rated speed 406 rpm
Passive stator Rated power 15 kW
Fig. 1. Structure of a linear SRM. Maximal speed 812 rpm
Speed range (constant torque) 0…406 rpm
A. Design Procedure and Results Speed range (15 kW constant power) 406…812rpm
In the design phase, a combined approach of finite Outer diameter of the stator 698 mm
element analysis (FEA) together with an equivalent Inner diameter of the stator 469.7 mm
magnetic network model of the transient electromagnetic Airgap 12 mm
processes in switched reluctance motors with power Stacking length 140 mm
converters is used . The magnetic model is discussed Active phase resistance at 20°C 0.27 Ω
in detail in . Using the simplified model, power, Efficiency at the speed 406 (812) rpm 75 (83) %
efficiency as well as the DC-link and phase currents can
be predicted with an error less than 3...5%. The overall Rated phase current (amplitude) 70 A
computation time is much shorter compared to the Rated phase current (RMS) 32.3 A
complete transient finite element analysis of the motor Current density in the conductors 4.42 A/mm2
described below in Section III. Due to these advantages, Copper weight 83 kg
the geometry optimization and control synthesis tasks can Turns per phase 2×280
be solved more effectively with this simplified model.
The main parameters of the linear and rotational To maintain an acceptable level of flux density, the
motors determined during the design phase are presented yoke segments have been enlarged as shown in Fig. 1 and
in the Table I and Table II, respectively. Fig. 3.
TECHNICAL DATA OF THE LINEAR SRM
III. FINITE ELEMENT ANALYSIS
Supply voltage (RMS) 400 V To validate the initial motor design and to evaluate the
Rated speed 10 m/s force ripple characteristics of the linear and rotational
Rated power 15 kW SRM, complete finite element analyses of both rotational
and linear motors have been performed using
Maximal speed 20 m/s Maxwell 2D from ANSOFT. Some of the results for the
Speed range (constant force) 0…10 m/s static FEA have been presented in . The dynamic field
Speed range (15 kW constant power) 10…20 m/s distribution at rated speed and torque for the rotational
Stator height 103.5 mm and the linear SRM are shown in Fig. 2 and Fig. 3
Stator core length 1525 mm respectively.
Airgap 12 mm Due to the large airgap of 12 mm the windings should
Stacking length 140 mm provide a sufficient magnetomotive force (MMF) to
overcome high airgap reluctance and to produce
Active phase resistance at 20°C 0.27 Ω sufficient magnetic flux with a reasonable efficiency. The
Propulsion force : normal force ratio 1:4 peak value of the MMF is of 19600 A at rated speed of
Efficiency at the speed 10 (20) m/s 76 (84) % 406 rpm and rated current of 70 A. Therefore, the copper
Rated phase current (amplitude) 70 A loss is dominant in the overall power balance (the iron
Rated phase current (RMS) 32.3 A loss is only 5% of the total loss).
Current density in the conductors 4.42 A/mm2
Copper weight 83 kg
Turns per phase 2×280
In contrast to the rotational motor, the linear SRM with
an open magnetic circuit has some particular yoke cross-
sections that have to conduct more magnetic flux than
others. Depending on the direction of current
commutation (clockwise or counterclockwise) the
magnetic flux in these yoke segments can be by a factor of
1.5…2.5 higher in comparison to the weaker excited yoke
segments. From the analysis of some variants of winding
diagrams, a preferred combination of the magnetic pole
directions has been found which minimizes the magnetic
flux in the stator yoke (Fig. 1). In this manner, only two
yoke sections conduct a magnetic flux that is by a factor Fig. 2. Magnetic flux density and flux lines plot for the rotational SRM
at rated speed of 406 rpm and torque of about 350 Nm.
1622 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
very close by. Hence, it is assumed that the optimal
control algorithm for the rotational SRM is also adequate
for the LSRM.
The switched reluctance drive structure is presented in
Fig. 6. Only three of six phases are shown for means of
clarity. The motor phase windings are magnetized by a
Fig. 3. Magnetic flux density distribution and flux line plots for the power converter using six IGBT modules. Front end
linear SRM at rated speed of 10 m/s and force of about 1560N. converter is a three-phase bridge rectifier with a DC bus
The efficiency of the motor found by FEA is about capacitor. Each phase module consists of an
75% at 10 m/s (406 rpm) and approx. 82% at 20 m/s asymmetrical single phase half-bridge. A single break
(812 rpm). chopper and resistor are used for dumping the extra
The results found by transient FEA at rated speed of energy during braking operation.
10 m/s (Fig. 4) and at 20 m/s (Fig. 5) also confirm the The IGBT gate drivers comprise an integrated
initial design relation of the tangential to the normal force detection of the collector-emitter voltage desaturation and
Fx:Fy = 1:4 defined with an SRM design model. Because fault status feedback. Communication with the control
of a higher back-EMF level only phase currents of 52 A system is done via fiber optics (Fig. 6).
can be adjusted at 812 rpm.
3 of 6 phase currents
Phase currents (A), Forces/100 (N)
60 A+ A- B+ B- F+ F-
50 Braking Sensors
Normal force & IGBT
40 VTHA VTHB VTHF
30 Propulsion force
VTLA VTLB VTLF
0 Control System
0.01 0.015 0.02 0.025 0.03 0.035 0.04
Fig. 4. Current shapes for three of six phases, propulsion force Fx and HMI Control Panel
normal force Fy of linear SRM; phase currents and torque M/r of the
rotational SRM: FEA simulation at MMF of 19.6 kA, i.e. rated phase Fig. 6. SR drive structure
current amplitude of 70 A and rated speed of 10 m/s (406 rpm).
The control system is implemented using a 16-bit
55 Linear SRM Infineon XC161 microcontroller. The block diagram of
Rotational SRM the control system is presented in Fig. 7. The firing
3 of 6 phase currents
signals are generated by the microcontroller and two
Phase currents (A), Force/100 (N)
CPLDs (XILINX XC95144). Each CPLD is comprised of
eight 36V18 function blocks, providing 3,200 usable
gates with propagation delays of 7.5 ns.
30 Normal force
The control system is provided with a human-machine
interface (HMI) with LCD and keyboard panel for
20 Propulsion force communication with the operator.
The implementation of a two-quadrant phase current
10 control scheme for an SRM drive implies the
5 development of specialized control pulse sequences for
0 the converter IGBTs (Fig. 6) depending on the required
0.005 0.01 0.015 0.02 operating mode (motoring or dynamic braking). The
objectives of the control algorithm are to maximize the
Fig. 5. Current shapes for three of six phases, propulsion force Fx and
output power and efficiency. In motor mode, a phase
normal force Fy of linear SRM; phase currents and torque M/r of the current has to be switched on during the positive slope of
rotational SRM: FEAsimulation at a phase current amplitude of 53 A the phase inductance profile. For the highest power
and speed of 20 m/s (812 rpm). output, the current conduction angle should be nearly
180° (electrically) .
The diagrams in Fig. 4 and Fig. 5 also confirm that the
characteristics of the linear and of the rotational motor are
2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1623
From Current Sensors To IGBT Gates TRPS
A B ... F
Comparators ton Adv
IGBT to f f 1 " on " adv
Drivers "o f f 1
6 6 to f f 2
... "o f f 2
Voltage Ire f
6 6 6 Sensor
t p1 t pi tp j ik
XC161 tc0 tc t
Micro- 0 180 360 " , el.deg.
Current Reference Temperature
Generation Unit controller Sensor Fig. 8. Variable parameters of the control pulses.
The algorithm converts the calculated angles into the
commutation times ton, toff1, toff2 of the IGBTs for each
Sensor (RPS) motor phase.
LCD Display The regenerative braking mode is implemented
HMI through a change of a switching sequence and shift of a
Control current conducting interval by 180° (electrically) in
comparison to the motoring mode. The switching angles
αon, αoff1, αoff2 are varied in such a manner that the phase
currents are excited at negative slope of the inductance
Fig. 7. Block diagram of the control system
profile with respect to the rotor position.
The control algorithm includes control loops for the
phase currents, the speed and the DC link voltage.
For the generator mode or braking, the current Traditionally the SRM phase currents are limited with a
conduction interval should coincide with the negative hysteresis controller . The usual implementation
slope of the inductance profile. A smooth transition known from the literature requires two comparators and
between the motor and generator modes is of great results in a variable commutation frequency with a
importance for a robust drive system operation . relatively wide spectrum. In contrast to the standard
The current shape, amplitude, advance angle and the hysteresis controller, the implemented current limiting
pulse duration are controlled. The control algorithm has algorithm  generates an almost constant frequency
to adapt the control signals with respect to the reference (see Fig. 8), similar to the peak current mode control.
input Iref as shown in Fig. 8. The parameter set is For the motoring mode, this method requires just one
calculated in real time using a mathematical model of the comparator per phase. The positive comparator input is
motor implemented in the control algorithm. The vector connected with the current reference unit output (Iref), the
of the reference parameters includes absolute value and negative one with the phase current sensor. So, if the
direction of the speed and the torque (force). Additional current exceeds the Iref value, the comparator output
control objectives are to maintain maximum efficiency becomes active. The control system generates a command
with minimal torque ripple and acoustic noise. to switch the high and low side IGBTs of the active phase
The symbols in Fig. 8 are explained below: module in turn. The phase voltage becomes zero and the
- RPSk : rotor position sensor signal for phase k phase current decreases slowly. Using alternating
(k = 1..6); transistor switching, the converter losses are better
balanced between the high side and low side IGBTs in
- mVTLk, mVTHk : firing signals for low and high side comparison to the well known soft switching technique
IGBTs of the respective phase module; with choppering of a single (e.g. only high side) transistor
- Iref : current reference value; and another transistor being continuously closed.
- ik , uk : measured current and voltage of a SRM phase. With the presented algorithm, the current ripple
The period of the rotor position measuring system frequency is not exactly equal to the fc ref, but can be
(RPS) is used as a base value (TRPS) which is inversely maintained closely to this value. At high rotation speed
proportional to the instantaneous drive speed. The the phase current does not reach the Iref value due to a
calculated parameters are: higher back EMF voltage, which implies the
- αon: on-angle of the IGBTs, disappearance of the 8 kHz commutation current ripples.
- αoff1, αoff2 : off-angles of the high side and low side In order to implement the safe transition to dynamic
braking mode, the current feedback scheme has an
IGBTs, additional comparator, which has a higher switching level
- αadv : advance angle which is calculated as a linear (Iref”). The value of this level is approximately 3.4
function of the motor speed. higher than for the first comparator. This is less than 5%
of the rated phase current amplitude which is reasonable
for this application. The selected level difference makes
1624 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)
the converter more robust in the environment with strong forces with the linear test bed. The test bed of the
electromagnetic interference (EMI) due to high current equivalent rotational motor is used to evaluate the
switching. In the dynamic braking mode the back EMF is dynamic characteristics of the SRM and will be primarily
positive. After only one (high or low side) of the two discussed in the following.
IGBTs per phase is switched off, the phase current The rotational SRM is designed and built to validate
continues to increase. When it reaches the upper the control strategy and to investigate the dynamic
reference value Iref”, the upper comparator output characteristics of a six-phase SRM. The SRM is coupled
becomes active and the control system generates a to a DC motor to simulate the load. A torque sensor is
command to switch off both IGBTs. The phase voltage mounted on the coupling shaft. The arrangement of test
becomes negative and the phase current starts to decrease. bed is shown in following Fig. 10.
The speed controller is implemented as a proportional-
integral (PI) controller. In the motor mode the speed
controller adjusts the Iref value within the limits set by the
external analog signal. The speed controller task is SRM Torque DC motor (load)
executed at each RPS event. Therefore, the speed signal sensor
is updated with a higher frequency and the speed
dependent advance angle is changed more smoothly as
opposed to a control algorithm presented in .
Additionally, in contrast to a method described in , our
control implementation does not need any “mode
detector” neither in current control loop nor in the speed
control loop since the switching between motor and
braking modes is done automatically using two switching Fig. 10. Rotational SRM with DC motor and torque sensor
levels. Therefore, we suppose that the presented method
is more simple and reliable. As measurement signals, three of six phase currents,
The control loop for the DC link voltage is primarily the DC link current and the DC link voltage were
implemented to protect the power electronics. If the DC acquired in each operating point. Torque and current
link voltage rises above a certain critical level (e.g. while signals from measurement and simulation are shown in
braking), the voltage regulator reduces the Iref value in Fig. 11. The comparison of simulations and
order to limit an energy flow from the converter to measurements shows that the calculated torque can be
protect the DC link capacitors and the braking circuitry. achieved, which proves the combined approach of FEA
The control system has also an additional hysteresis together with the equivalent SRM magnetic network
voltage controller, which controls the braking IGBT. model used in the design phase. Furthermore it is possible
Fig. 9 depicts a smooth transition from motor to to validate the proper functionality of the control
regenerative braking mode and back to the motor mode. algorithm by comparing the simulated and measured
Rotor position sensor signal
Phase Currents [A], Torque/10 [Nm]
DC link voltage |Iref| value
phase current 50
0.01 0.015 0.02 0.025 0.03 0.035 0.04
Fig. 9. Smooth transition between motor and braking modes. Time [s]
The proportional and integral coefficients of both Fig. 11. Current shapes for three of six phases and torque at 406 rpm
speed and voltage controllers are set low for a better (10 m/s) and current amplitude of 68A.
visualization of the transitions. The operation of the brake –––– measurement (filtered); ------ transient FEM simulation
voltage hysteresis controller can be recognized by the
saw-tooth pulses of the DC link voltage caused by the
switching of the braking resistor. To validate the characteristics of the rotational SRM in
different operation points the average torque was
V. TEST BED AND EXPERIMENTAL RESULTS measured with different currents and rotational speed
To evaluate the analytical results, prototypes of both (Fig. 12). This was realized by limiting the current with
SRM types were built up. Although the LSRM is of the analog interface of the microcontroller.
particular interest, it is only possible to measure static
2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008) 1625
20 A The paper presents a study on model-based
200 30 A
40 A development and control of linear and rotational
180 50 A switched-reluctance motors. The models are validated
160 with static and transient finite element analysis and
measurements. Theoretical and practical results show
The optimization of the magnetic circuit geometry and
100 winding parameters with respect to the control limitations
(maximal voltage and current), provides a good
performance of the overall drive system.
The presented control scheme provides low force
40 ripple and smooth transition between the motor and
20 dynamic brake mode.
200 300 400 500 600 700 800
Speed [rpm] The initial design tasks have been solved, therefore the
linear SR motor can be supposed as a good candidate for
Fig. 12. Average torque characteristics for different phase currents use in an autonomous railway system.
The mechanical power (Fig. 13) is the product of the REFERENCES
measured torque and the rotational speed. The electrical  Henke, M.; Grotstollen, H.: Modelling and Control of a
power is calculated by the product of the measured DC Longstator-Linearmotor for a Mechatronic Railway Carriage.
Proc. of the 1st IFAC Symp. on Mechatronic Systems. Sept. 2000,
link current and voltage. The efficiency shown in Fig. 14 Darmstadt, Germany, pp. 353-357.
is defined as the mechanical power divided by the  Yang, B.; Grotstollen, H.: Control Strategy for a Novel Combined
electrical power. Operation of Long Stator and Short Stator Linear Drive System.
Proc. of the 10th European Conference on Power Electronis and
Applications (EPE 2003), Sept., 2003, Toulouse, France.
 Henke, C.; Fröhleke, N.; Böcker, J.: Advanced Convoy Control
14000 Strategy for Autonomously Driven Railway Vehicles. Proc. of
20 A IEEE Conf. on Intelligent Transportation Systems, Sept. 2006,
12000 Toronto, Kanada.
 Schramm, A.; Gerling, D.: Researches on the Suitability of
Switched Reluctance Machines and Permanent Magnet Machines
Mechanical power [W]
for Specific Aerospace Applications Demanding Fault Tolerance.
Int. Symp. on Power Electronics, Electrical Drives, Automation
and Motion (SPEEDAM), Taormina, Sicily, Italy, 2006.
 Miller, T.J.E.: Optimal Design of Switched Reluctance Motors. -
IEEE Transactions on Industrial Electronics, Vol. 49, No. 1,
4000  Kolomeitsev, L.; Kraynov, D.; Pakhomin, S.; Rednov, F.;
Kallenbach, E.; Kireev, V.; Schneider, T.; Böcker, J.: Linear
2000 Switched Reluctance Motor as a High Efficiency Propulsion
System for Railway Vehicles. Proc. Int. Symp. on Power
0 Electronics, Electrical Drives, Automation and Motion
200 300 400 500 600 700 800 (SPEEDAM), Ischia, Italy, 2008, pp. 155-160.
 Kolomeitsev, L.; Pahomin, S.; Krainov, D. et al.: Mathematical
Model for Calculation of the Electromagnetic Processes in a
Fig. 13. Shaft power vs. speed for different phase currents Multi-Phase Switched Reluctance Motor (in Russian). Izv. vuzov,
Electromechanika, No. 1, 1998, pp. 49-53.
90  Electronic control of switched reluctance machines: edited by
T.J.E. Miller, Oxford, U.K.: Newnes, 2001.
85  Kraynov, D.V.; Duvakin, A.V.; Kolomeytsev, V.L. et al.: Control
80 method of a switched reluctance motor (in Russian). Patent of
Russian Federation RU 2260243, IPC H02P 8/12, 6/00. -
75 2003136649/09; Priority 17.12.2003; Published by 10.09.2005,
 Mademlis, C.; Kioskeridis, I.: Smooth Transition between Optimal
65 Control Modes in Switched Reluctance Motoring and Generating
20 A Operation. Int. Conf. on Power Systems Transients (IPST’07),
30 A Lyon, France, June, 2007.
55 40 A  Gallegos-Lopez, G.; Walters, J. and Rajashekara, K.: Switched
50 A Reluctance Machine Control Strategies for Automotive
Applications. SAE 2001 World Congress, March 5-8, 2001.
45  Kraynov, D.V.; Duvakin, A.V.; Kolomeytsev, V.L. et al.: Method
of current shape synthesis in the phase winding of a switched
200 300 400 500 600 700 800 reluctance motor (in Russian). Patent of Russian Federation RU
Speed [rpm] 2229768, IPC 7H02P 8/12. - 2002101418/09; Priority 11.01.2002;
Published by 27.05.2004, Bulletin 15.
Fig. 14. SR-drive efficiency for different phase currents
1626 2008 13th International Power Electronics and Motion Control Conference (EPE-PEMC 2008)