Design and Experimental Verification of Linear Switched Reluctance Motor with Skewed Poles

International Journal of Power Electronics and Drive System (IJPEDS)
Vol. 6, No. 1, March 2015, pp. 18~25
ISSN: 2088-8694  18
Journal homepage: http://iaesjournal.com/online/index.php/IJPEDS
Design and Experimental Verification of Linear Switched
Reluctance Motor with Skewed Poles
N. C. Lenin*, R. Arumugam**
*
VIT University, Chennai-600 127, Tamilnadu, India
**
SSN College of Engineering, Chennai -603 110, Tamilnadu, India
Article Info ABSTRACT
Article history:
Received Aug 5, 2014
Revised Nov 3, 2014
Accepted Nov 20, 2014
This paper presents the realization and design of a linear switched reluctance
motor (LSRM) with a new stator structure. One of the setbacks in the LSRM
family is the presence of high force ripple leads to vibration and acoustic
noise. The proposed structure provides a smooth force profile with reduced
force ripple. Finite element analysis (FEA) is used to predict the force and
other relevant parameters.A frequency spectrum analysis of the force profile
using the fast Fourier transform (FFT) is presented.The FEA and
experimental results of this paper prove that LSRMs are one of the strong
candidates for linear propulsion drives.
Keyword:
FFT
Finite element analysis
Force ripple
Switched reluctance motor
Skewed pole Copyright © 2015 Institute of Advanced Engineering and Science.
All rights reserved.
Corresponding Author:
N. C. Lenin,
Department of Electrical and Electronics Engineering,
Associate Professor, VIT University – Chennai Campus,
Chennai-600 127, Tamilnadu, India
Email: lenin.nc@vit.ac.in
1. INTRODUCTION
Linear switched reluctance motors (LSRMs) with different machine configurations have been
explored past in the literature [1]–[12]. They are an attractive alternative to linear induction and linear
synchronous machines due to lack of windings on either the stator or translator structure. However LSRM
has some disadvantages such as high force ripple, vibration, and acoustic noise because of doubly salient
structure. Moreover power electronic converters are required for their continuous operation. Efforts to reduce
or eliminate the torque ripple of the rotary switched reluctance motors (SRMs) are presented in literature
[13]-[17]. Multi phase excitation to reduce the force ripple in the LSRM has been explained in [18]. However
the previous method considerably increases the copper losses. LSRM with pole shoes and inter poles are
presented in [19]-[20]. In this paper a new stator structure [21] is proposed to reduce the force ripple.
Most of the limitations of analytical techniques can be overcome by using the numerical methods
such as finite element analysis (FEA). These tools provide accurate results but require significant
computational effort and numerical procedures [22]-[24]. The FEA tools are used in this study to predict the
force and inductance profile.
When the frequency of the exciting force is close or equal to any of the natural frequencies of the
machine, then resonance occur, which results in dangerous deformations and vibrations and a substantial
increase in noise [25]. FFT steps to analyze ripple in the force profile of aLSRM is presented. This
methodology is comparatively simpler than the most widely used finite-element vibration analysis procedure
for mode frequency identification.
The organization of the paper is as follows: Section 2 and 3 presents new stator geometry for
LSRMs that improves the force profile. In the new geometry, poles are skewed. Section 4 presents FEA

IJPEDS ISSN: 2088-8694 
Design and Experimental Verification of Linear Switched Reluctance Motor with Skewed Poles (N. C. Lenin)
19
results for conventional and proposed structures. Frequency spectrum analysis of force profile using the fast
Fourier transforms (FFT) is highlighted in section 5. Experimental results from the prototype machine and
their correlation with FEA results are presented in Section 6. Conclusions are summarized in Section 7.
2. LSRM TOPOLOGY
Figure 1 shows the two dimensional (2D) cross sectional view for the conventional machine
structure of a three phase LSRM. The LSRM has an active translator and a passive stator. It consists of six
translator poles and 120 stator poles. Figure 2 shows the stator pole alone for the conventional structure
whereas, Figure 3 shows the stator pole for the proposed structure used for this study. The poles are skewed
by an angle 1 degree to 10 degrees in steps of 1 degree for the purpose of optimization. Table 1 shows the
physical dimensions of the LSRM prototype.
Figure 1. 2D cross sectional model of conventional LSRM
Figure 2. Conventional stator pole Figure 3. Proposed stator pole
Table 1. Specifications of Prototype LSRM
Translator pole width=20mm Translator slot width=20mm Translator pole height=27mm
Translator back iron thickness=20 mm No. of turns/phase = 86 Translator stack length=30 mm
Stator pole width=20mm Stator slot width=26mm Stator pole height=15 mm
Stator back iron thickness=20mm Air gap length=2mm Stator stack length=30 mm
Rated voltage=36 V Rated current=4amps Velocity=1m/s
Stator pole skewed angle = 1 to 10 degrees
Stator pole skew angle = 1 to 10
degrees
Maximum force=3.21N
3. INTRODUCTION TO FORCE RIPPLE IN LINEAR SWITCHED RELUCTANCE MOTOR
One of the inherent problems in LSRM is the force ripple due to switched nature of the force
production. Force ripple may be determined from the variations in the output force. In order to predict the
amount of force ripple, static force characteristics should be considered. The force dip is the distance between
the peak value and the common point of overlap in the force angle characteristics of two consecutive LSRM
phases as illustrated in Figure 4. Assuming that the maximum value of the static force Fmax (peak static force)
and the minimum value that occurs at the intersection point of two consecutive phases as Fmin, the percentage
force ripple may be defined as:
max min
avg
F - F
%Force Ripple = ×100
F
(1)
The force dip is an indirect indicator of force ripple in the machine; the lesser the value of the force
dip, the lesser will be the force ripple. The force dip of both the machines has been computed by FEA.

 ISSN: 2088-8694
IJPEDS Vol. 6, No. 1, March 2015 : 18 – 25
20
Figure 4. Force vs Translator position showing force dip
4. TWO – DIMENSIONAL FINITE ELEMENT ANALYSIS
Three asymmetric bridge metal oxide semiconductor field effect transistor (MOSFET) inverters are
used to drive the LSRM shown in Figure 5. The translator position with respect to the stator position is
sensed by three highly sensitive optical sensors. The active translator of the LSRM is moved from the
unaligned position with respect to the stator to the aligned position for the excitation current of 4 amps.
Therefore, static force and inductance profiles are obtained as a function of position and current.
Figure 6 shows the flux distribution taken from FEA for the conventional machine at the aligned
position. The force and inductance profiles for the conventional and proposed LSRMs are depicted in Figure
7-10 respectively.
Figure 5. Three phase power converter for LSRM
Figure 6. Flux distribution of conventional LSRM at the aligned position
IRFP360
3
1
2
IRFP360
3
1
2
MUR3060
12
MUR3060
12
IRPF360
3
1
2
IRFP360
3
1
2
MUR3060
12
IRPF360
3
1
2
IRPF360
3
1
2
MUR3060
12
MUR3060
12
MUR3060
12
G6G2
G5
G4
G1 G3
Phase-A
Phase-B
Phase-C
-

21
Figure 7. Propulsion force for base motor Figure 8. Inductance Profile for base motor
Figure 9. Propulsion force for proposed motor Figure 10. Inductance Profile for proposed motor
The force in a given direction is obtained by differentiating the magnetic co-energy of the system
with respect to a virtual displacement of the translator. Based on this approach, the propulsion force with
respect to various translator positions is calculated. The peak force obtained is 3.05N and the force ripple is
44.85% for the conventional LSRM whereas the peak force obtained in proposed LSRM is 3.32 N with the
force ripple of 32.79%. The entire comparisons of the two structures are tabulated in Table 2.
Table 2. Summary of Comparison of the Two Structures
Type
Peak propulsion
force (N)
Minimum propulsion
force (N)
Average propulsion
force (N)
Force
ripple (%)
Inductance (H)
Aligned Unaligned
Conventional Stator 3.05 1.83 2.72 44.85 0.0020 0.001
Stator with skewed
pole(6 degrees)
3.52 2.5 3.11 32.79 0.0023 0.0012
5. FAST FOURIER TRANSFORM APPLICATION TO LSRM
From the results of 2-D finite-element field analysis performed earlier, force (N) versus translator
position (mm) will be known (Figure 6-10). A program is written in MATLAB environment which contains a
sequence of instructions to store the force parameter array of the three phases. FFT is applied to the net force
profile after the elimination of dc offset [26]. Since FFT transforms the available data in time domain into
frequency domain, the available force versus translator position profile must be converted to force versus
time profile. In MATLAB, the command fft(x,p), where ‘x’ is the force array and ‘p’ is 512, denoting 512
point fft, will solve the Equation (2) to produce a complex discrete fourier transform (DFT) of force. The
absolute value of the obtained complex DFT will form the magnitude axis.
T
2
0
0
-T
2
1 jω t
f(t) = F(jω ) e dt
2π  (2)

 ISSN: 2088-8694
22
The magnitude plot is obtained by plotting the magnitude versus frequency. Figure 11 shows the
results of the frequency spectrum analysis for the conventional structure of the stator. The frequency
corresponding to the decibel (dB) peaks can be identified from the plot. Table 3 shows the dominant
frequencies in hertz and its amplitude in dB.
Figure 11. FFT output: dB versus frequency, for the LSRM
Table 3. Dominant Ripple Frequencies and its Amplitude for the Stator
Predominant ripple frequencies
(Hz)
Amplitude (dB)
10,050 36
13,100 21
14,910 8
19,260 9.2
Figure 12. FFT output: dB versus frequency, for the LSRM with skewed poles
The result of FFT for the LSRM with skewed poles is depicted in Figure 12. It is observed that the
magnitude dB peaks occurs at the same frequencies in both cases. However the magnitude of the dB peaks is
reduced by a considerable margin, encouraging the design case of skewed stator poles (Table 4).

23
Table 4. Dominant Ripple Frequencies and its Amplitude for Stator with Skewed Poles
Predominant ripple frequencies
(Hz)
Amplitude (dB)
10,050 29
14,620 17
19,260 7.4
22,420 7
6. EXPERIMENTAL RESULTS
Figure 13 shows the experimental setup for the prototype LSRM used as a material carrying vehicle
in the laboratory. The experimental road is 0.5 m long and translator weight is 2.7kg. It should be noted that
the present setup is intended for development purposes only.
(a) (b)
Figure 13. Experimental setup of (a) LSRM and converter (b) Driver circuit
The inductance for the different positions at rated current is measured by locking the translator at
each position. A constant current is applied to a phase and is turned off and the falling current profile is
computed. The time constant is measured from the profile and hence the inductance is calculated. The
measured values of inductance and propulsion force are plotted alongside the FEA results in Figure 14 and
Figure 15 respectively. Figure 16 shows phase current and pulse waveforms of the LSRM.
Figure 14. Comparison of FEA and measured
inductance values at rated current
Figure 15. comparison of FEA and measured
propulsion force at rated current

 ISSN: 2088-8694
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(a) (b)
Figure 16. Experimental waveforms (a) Pulses of LSRM (b) Phase current of LSRM
7. CONCLUSION
Modification of the stator geometry by the provision of skewed poles has been presented in this
paper. A 2m long LSRM prototype has been constructed. Force and inductance profile has been obtained by
using FEA. There is a good agreement between measurement results and FEA values of inductance profile of
the motor. The proposed structure reduces the force ripple by approximately 27% compared to the
conventional machine. FFT methodology is comparatively simpler than the most widely used finite-element
vibration analysis procedure for mode frequency identification.
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25
[19] NC Lenin, R Arumugam, Analysis and characterisation of longitudinal flux single sided linear switched reluctance
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BIOGRAPHIES OF AUTHORS
N. C. Lenin (M’2010) completed his PhD in Anna University, India in the year 2012. He has
published 20 international journals and more than 30 international conferences. Currently he is
working as Associate professor in the School of Electrical in VIT University, Chennai, India.
His areas of interest are Design of Electrical Machines and Finite Element Analysis.
R. Arumugam received his Ph.D. in electrical engineering from Concordia University,
Montreal, Canada in the year 1987. He worked in various capacities at College of Engineering,
Guindy, Anna University from 1976 onwards. He was a consultant to Lucas TVS Ltd., for the
design of Switched Reluctance Motor and to Combat Vehicle Research and Development
Establishment (CVRDE), DRDO, for the design of a prototype Linear Induction Motor. He was
the recipient of Fellowship from Natural Sciences and Engineering Research Council (NSERC)
of Canada during the period September 1983- December 1987. One of his technical papers
presented in the IEEE International conference on Power, Control and Instrumentation Systems,
IECON – 2000, conducted at Nagoya, Japan, was awarded the Best Technical Paper Award. He
is presently working as the Professor in Electrical Engineering at SSN. His research interests are
in Computer Aided Design of Electrical Machines, Finite Element Analysis, Electric Motor
Drives and Power Electronics.

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