Multilevel Inverter Fed Switched Reluctance Motor · PDF fileThe torque is produced by...
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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
10131
Multilevel Inverter Fed Switched Reluctance Motor
1,a*Mohd Ruddin Ab Ghani, 1,bNabil Farah, 1 Nur Huda Mohd Amin, 1Syariffah Othman, 2Zanariah Jano
1Faculty of Electrical Engineering (FKE), 2Centre for Languages and Human Development, 1,2Universiti Teknikal Malaysia Melaka,
Hang Tuah Jaya, 76100 Durian Tunggal, Melaka, Malaysia.
Abstract This paper analyzed three-level inverter in order to control the
Switched Reluctance Motor (SRM). The inverter was
controlled using space vector modulation SVM. The
multilevel inverter was utilized to feed the SRM to reduce the
Total Harmonics Distortion THD and compare it with another
power electronic circuit. The simulation was conducted using
MATLAB/SIMULINK software. The findings yielded that a
half bridge converter produced more harmonics than the
multilevel inverter. Therefore, the multilevel inverter circuit
was the best drive circuit to be used compared to half bridge
converter.
Keywords: SRM, Multilevel inverter, SVM, THD, inverter
INTRODUCTION
The most important element in the electrical motors is to
design a driver circuit that can efficiently drive the motor to
produce the desired output. In the case of the Switched
Reluctance Motor (SRM), countless researchers have focused
on producing suitable drivers for the SRM [1]-[3]. Various
types and constructions of the SRM motor have been
proposed. The main difference is in the phases employed, the
number of stator/rotor poles used and torque production
mechanism [2]. In this paper, the SRM motor parameters has
been fixed for all types of SRM that will be used. Table 1
shows the SRM motors parameters. Three types of SRM
motor discussed here are subject to two different control
mechanisms namely the power converter and multilevel
inverter which are compared in terms of construction and type
of control mechanism being used.
Table 1: SRM motor parameters
Stator resistance (ohm) 0.01
Inertia (kg.m.m) 0.082
Frication (N.m.s) 0.01
Initial speed and position [0 0]
Unaligned inductance(H) 0.67e-4
Aligned inductance (H) 23.6e-4
Saturated aligned inductance(H) 0.15e-4
Maximum current (A) 250
Maximum flux linkage (V.s) 0.486
The basic and simplest type of SRM motor has 6 stator poles
and 4 rotor poles which reflects its name, 6/4. The motion of
this motor is produced due to the variable reluctance in the air
gap between the stator and rotor windings. A single magnetic
field is produced due to the rotor winding activation then the
torque is produced by the tendency of the rotor to move to its
reluctance position. Figure 1 shows the 6/4 SRM motor in the
case of a rotor pole is aligned with a stator pole; due to the
field lines are orthogonal to the surfaces, hence, no torque will
be produced [4]. The difference between this motor and other
type of motor construction is only the number of phases
employed as well as the shape of the configuration.
Figure 1. 6/4 SRM motor
a. 8/6 SRM motor
This type of motor is similar to the previous motor. The basic
difference is the number of phases employed increases
whereas the parameters are the same as the 6/4’s SRM motor.
The operation principle to produce motion does not differ as
well. The SRM is known as a double salient motor with sold
laminated rotor core and stator independent phase winding
connected in series to make one phase of the motor. Figure 2
shows the 8/6 SRM motor construction.
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 2. 8/6 SRM motor
The torque is produced by activating the stator phase which
will attract the most adjacent rotor pole pair, then, minimize
the reluctance of the magnetic path. As a result, constant
torque is developed due to the activation of consecutive
phases of stator in succession. Therefore, the SRM is an
electrical motor in which the torque is produced due to the
tendency of its movable parts to move till the reluctance of the
excited winding is maximized [5].
b. 10/8 SRM motor
This is another different construction of the SRM motor that
employs more phases than the previous SRM constructions.
However, the parameters are similar to the 6/4 and 8/6 SRM
motor. The name of this motor is derived from the number of
stator and rotor poles used. The principle of torque production
is the same as the previous SRM construction where the
torque production depends on the tendency of the movable
part of the motor to move to position where the inductance of
the excited windings is maximized [6][7]. Figure 3 shows the
10/6 SRM motor construction.
Figure 3. 10/8 SRM motor
SRM DRIVE TOPOLOGY
The SRM drives have received a great attention for the past
two decades’. Several studies have been conducted to develop
new and sufficient drives to drive the SRM motor efficiently
and produce the desired output. The power convers have been
highlighted in most research on the SRM motors as different
types of power converters have been designed to be fed to the
SRM motor [8]. A study [9] develops a new topology which
uses power inverter to control the SRM motor. However,
these drive topologies have their own drawbacks. The most
apparent drawback is the percentage of the Total Harmonics
Distortion (THD) that a driver produces. In this paper, power
converter and multilevel inverter to drive topology were
proposed.
a. Half bridge converter topology
There are numerous converter drives which have been
developed to be fed to the SRM motor. The most flexible and
common topology to be used for the SRM is the half bridge
converter. This converter is fed by 240 vdc sources and
requires two switches and two diodes per phase. The turn on
and off angles and reference current are kept constant at 45
deg and 75 deg, 200 A respectively. This type of converter is
advantageous over other converter type in terms of energy
efficiency where the energy returns from the motor to the
source after turn-off of phase switching. Moreover, the control
mechanism of this converter is done independently for each
phase [10]. As the aim of this paper was to investigate the
control techniques for three different SRM constructions, the
control circuit of half bridge converter has different
configuration for each SRM construction. Figure 4 shows the
three-phase half bridge converter to drive the 6/4 SRM Motor.
Figure 4. Half bridge converter for 6/4 SRM
Figure 5 shows the four-phase half bridge converter fed to 8/6
SRM motor.
Figure 5. Half bridge converter for 8/6 SRM
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 6 shows five-phase half bridge converter fed to 10/8
SRM motor.
Figure 6. Bridge converter for 10/8 SRM
During conduction periods, the active IGBTs apply positive
source voltage to the stator windings to drive positive currents
into the phase windings. During free-wheeling periods,
negative voltage is applied to the windings and the stored
energy is returned to the power DC source through the diodes
[11].
b. Multilevel inverter
The past SRM drives often use power converter. Several
attempts were made to develop new power electronic inverter
that utilizes the Voltage Source Inverter VSI [9]. In this paper,
new power electronics circuit was used to drive the SRM
motor using a multilevel inverter.
The concept of multilevel inverter is to produce multilevel
output voltages with less power switching loss and less
harmonic distortion.
A multilevel inverter can be constructed by connecting a set
of single full bridge inverter in series. Each bridge has its own
isolated dc sources which can solar cells or batteries. These
separated dc sources feeding the multilevel inverter can
generate almost sinusoidal waveform voltage. This type of
inverter can produce N level voltages (i.e for three level
inverter can generate three different voltage outputs +vdc, 0
and –vdc). Hence, the output voltage of an N-level multilevel
inverter is the sum of all the individual inverter outputs
[12][13]. Figure 7 shows the circuit of the 3-level inverter.
Figure 7. Circuit of the 3-level inverter
Modulation techniques of the multilevel inverter
In terms of the control strategy of the multilevel inverter,
numerous researchers in the power electronic field have
developed many modulation techniques. The famous and easy
modulation techniques are the Sinusoidal Pulse Width
Modulation (SPWM), and Space Vector Pulse Width
Modulation (SVPWM). The multilevel inverter switching
signal can be generated using these two methods with less
switching losses and harmonic distortion.
Sinusoidal Pulse Width Modulation (SPWM)
The multilevel inverter can be controlled by using the SPWM
where sinusoidal wave is compared with square waves to
generate the switching signal that will trigger the
semiconductors switches in time sequence considering the
phase between the phases shift three phase inverter legs. This
method uses N-1 level carrier signals to generate the N-level
inverter output voltage. In multilevel inverter, the frequency
modulation index, and the amplitude modulation index,
are defined as follows [14][15]:
1
2
Where =carrier signal frequency, = reference signal
frequency, = carrier signal amplitude, = reference signal
amplitude.
Figure 8 shows the SPWM carriers signal compared with
reference sinusoidal signal for three-level inverter.
Figure 8. The SPWM carriers signals of three-level inverter
Space Vector Pulse Width Modulation (SVPWM)
Space Vector Modulation (SVM) is an algorithm for the
control of pulse width modulation (PWM). The idea behind
SVPWM is to create a rotating space vector that will be used
for various applications. One method to be used in inverter is
the switching times [16].
The space vector of a three-level inverter is shown in Figure
9.
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 9. The three-level inverter voltage vector
In this paper three different multilevel circuit is used to drive
three different types of SRM motor. Figure 10 shows the
circuit of three-level inverter three-phase used to drive the 6/4
SRM motor.
Figure 10. Three-level inverter fed to 6/4 SRM
Figure 11 shows the four phase three-level inverter fed to
8/6 SRM motor.
Figure 11. Three-level inverter fed to 8/6 SRM
Figure 12 shows the five phase three-level inverter fed to
10/8 SRM motor.
Figure 12. Three-level inverter fed to 10/8 SRM
SIMULATION RESULTS
The aim of this paper was to study and control three different
constructions of switched reluctance motor namely 6/4, 8/6
and 10/8. First of all, these three types were constructed in
MOTOR SOLVE software with fixed parameters and then
constructed using MATLAB/Simulink. Two different drive
circuit were utilized; half bridge converter and multilevel
inverter.
a. Results of Motor Solve
Figure 13 shows the output flux of the SRM using motor solve
which is the same for the three construction types, 6/4, 8/6 and
10/8 because all the parameters are fixed. Figure 14 shows the
output of Torque VS rotor angle which are the same for the
three types of SRM.
Figure 13. Static complete analysis for flux linkage
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 14. SRM torque against rotor angle
b. Matlab/Simulink Results
First of all, the output of drive and control circuit are
presented and compared in terms of Total Harmonics
Distortion THD %.
1. Half bridge converter topology
The half bridge converter with two IGBT and two free-
wheeling diode was fed to the SRM motor. Figure 15 shows
the output current of one phase of half bridge converter and
Figure 16 shows the THD% of the half bridge converter one
phase.
Figure 15. Output current of half bridge convert
Figure 16. THD of current of half bridge converter
By applying this converter to the SRM motor, the output flux
of motor are shown in Figure 17.
Figure 17. The output flux of SRM motor
In addition, the output of armature current, torque and motor
speed are shown in Figures 18, 19 and 20 respectively.
Figure 18. Armature current of SRM fed by half bridge
converter
Figure 19. The output torque of SRM fed by half bridge
converter
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 20. The output speed of SRM fed by half bridge
converter
2. Multilevel inverter topology
In this paper, the three-level inverter was used, modulated by
the space vector. The three-level inverter had four IGBT in
each phase. Figure 21 shows the output current of one phase
of three-level inverter followed by THD current of the same
inverter in Figure 22.
Figure 21. Three-level inverter output current
Figure 22. Current THD of three-level inverter
The multilevel inverter was fed to the SRM and the output
flux of the motor are shown in Figure 23.
Figure 23. Output flux of SRM fed by multilevel inverter
Besides, the armature current, output torque and speed of the
SRM motor are presented in Figures 24, 25 and 26
respectively.
Figure 24. Armature current of SRM fed by multilevel
inverter
Figure 25. Output torque of SRM fed by multilevel inverter
International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 20 (2016) pp. 10131-10138
© Research India Publications. http://www.ripublication.com
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Figure 26. Output speed of SRM fed by multilevel inverter
DISCUSSION
Based on the simulation results, the multilevel inverter is best
suited for the SRM motor due to the best outputs results
produced and the less harmonics contents in output compared
to the half bridge converter. Figure 27 shows the comparison
between the half bridge converter and multilevel inverter in
terms of THD.
Figure 27. THD comparison of multilevel inverter VS half
bridge converter
CONCLUSION
This paper aims to develop multilevel inverter and use it as a
driver circuit for the switched reluctance motor SRM. A half
bridge converter has been compared to multilevel inverter.
The findings yield that the half bridge converter produces
more harmonics than the multilevel inverter. Therefore, the
multilevel inverter circuit is the best drive circuit to be used
compared to half bridge converter.
ACKNOWLEDGMENT
The authors would like to gratefully acknowledge the funding
support provided by Universiti Teknikal Malaysia Melaka
(UTeM) under the research grant No:
PJP/2016/FKE/HI5/S0148/Malaysia, UTeM.
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