Multilevel Inverter Fed Switched Reluctance Motor

Transcription

1 Multilevel Inverter Fed Switched Reluctance Motor 1,a* Mohd Ruddin Ab Ghani, 1,b Nabil Farah, 1 Nur Huda Mohd Amin, 1 Syariffah Othman, 2 Zanariah Jano 1 Faculty of Electrical Engineering (FKE), 2 Centre for Languages and Human Development, 1,2 Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 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. 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. 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) Frication (N.m.s) 0.01 Initial speed and position [0 0] Unaligned inductance(h) Aligned inductance (H) 0.67e e-4 Saturated aligned inductance(h) 0.15e-4 Maximum current (A) 250 Maximum flux linkage (V.s) 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

2 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. 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. 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 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 Figure 5. Half bridge converter for 8/6 SRM 10132

3 Figure 6 shows five-phase half bridge converter fed to 10/8 SRM motor. 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]: 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. 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. 1 2 Figure 8. The SPWM carriers signals of three-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. 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

4 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 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. 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 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 13. Static complete analysis for flux linkage Figure 11. Three-level inverter fed to 8/6 SRM 10134

5 By applying this converter to the SRM motor, the output flux of motor are shown in Figure 17. 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 %. 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. 1. Half bridge converter topology The half bridge converter with two IGBT and two freewheeling 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 18. Armature current of SRM fed by half bridge converter Figure 15. Output current of half bridge convert Figure 19. The output torque of SRM fed by half bridge converter Figure 16. THD of current of half bridge converter 10135

6 The multilevel inverter was fed to the SRM and the output flux of the motor are shown in Figure 23. 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 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 21. Three-level inverter output current Figure 25. Output torque of SRM fed by multilevel inverter Figure 22. Current THD of three-level inverter 10136

7 REFERENCES 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. [1] Miller, T., 1993, Switched reluctance motors and their control. Oxford, UK: Magna Physics Publishing/Clarendon Press. [2] Krishnan, R., 2001 Switched reluctance motor drives: modeling, simulation, analysis, design, and applications. Boca Raton, FL: CRC Press. [3] Miller, T. J. E., 2001, Electronic control of switched reluctance machines. UK: Newness. [4] Soares, F., and Costa Branco, P.J., 2001, "Simulation of a 6/4 switched reluctance motor based on Matlab/Simulink environment", Aerospace and Electronic Systems, IEEE Transactions on, Vol.37, No.3, pp [5] Uma, S., Kamalakannan, C., and Karthikeyan, R., 2013, "Static and Dynamic Characteristics of 8/6, 400W Switched Reluctance Motor", International Journal of Computer Applications, Vol.66, No.12. [6] Feyzi, M. R., and Ebrahimi, Y., "Direct torque control of 5-phase 10/8 switched reluctance motors", Iranian Journal of Electrical & Electronic Engineering, Vol.5, No.3, pp [7] Zhen Ye, Z., Martin, Terry W., and Balda, Juan C., 2001, "Control of a 10/8 SRM based on the Torque Ripple Minimization and Speed Feedback", Power Electronics Specialists Conference, PESC. IEEE 32nd Annual, Vol. 3, IEEE. [8] Mahmoud and Samia M., 2013, "Studying Different Types of Power Converters Fed Switched Reluctance Motor", International Journal of Electronics and Electrical Engineering, Vol.1, No.4, pp [9] Grbo, Željko, Slobodan Vukosavić, and Emil Levi, 2005, "A novel power inverter for switched reluctance motor drives", Facta Universitatis-Series: Electronics and Energetics, Vol.18, No.3, pp [10] Do-Hyun, J., 2001, "The converter topology with half bridge inverter for switched reluctance motor drives", Industrial Electronics, Proceedings ISIE IEEE International Symposium on, Vol. 2. [11] Mahavir Singh, N., Chauhan, D. S., and Singh, S. N, "Power Factor Improvement In Switched Reluctance Motor Drive Using PWM Converter", IJEET, Vol.4, pp [12] Keith Corzine, 2005, Operation and Design of Multilevel Inverters, University of Missouri - Rolla Developed for the Office of Naval Research December [13] Nursyafiqah binti Zahari, 2013, Cascaded H - Bridge Multilevel Inverter, B.Eng. Theses, Universiti Teknologi Malaysia. [14] McGrath, Brendan Peter, and Donald Grahame Holmes, 2002, "Multicarrier PWM strategies for multilevel inverters", Industrial Electronics, IEEE Transactions on, Vol.49, No.4, pp [15] Niklas Rueger, E., Harald Kuhn, and Axel Mertens, 2007, "Harmonic distortion of multicarrier PWM strategies in cascaded multilevel converters with unequal DC sources", Power Electronics and Applications, European Conference on, IEEE

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