Performance Evaluation of Three-Phase Induction Motor Drive Fed from Z-Source Inverter

Transcription

1 Performance Evaluation of Three-Phase Induction Motor Drive Fed from Z-Source Inverter Mahima Sharma 1, Mahendra Lalwani 2 # Research Scholar, * Associate Professor Department of Electrical Engineering, Rajasthan Technical University, Kota, India. Abstract The use of induction motor is growing day by day because of its superiority and high efficiency over DC machine. So for wide range of use in the industry, this machine requires an efficient drive circuit arrangement. Currently conventional voltage source inverter (VSI) or current source inverter (CSI) is dealing as key part in the field of induction motor drive circuit. These converters fail to perform at our desire level due to some crucial drawbacks like they can perform as either buck or boost operation and they contain considerable amount of harmonics as well as EMI noise. So replace these traditional inverters by pulse width modulation (PWM) control Z-source inverter (ZSI) which offers buck-boost operation capability by utilizing shoot-through state and provides less EMI noise. This paper presents an exhaustive analysis, design of impedance network, implementation of simple boost control PWM technique and simulation of ZSI for different values of modulation index. Keywords Z-source inverter, Induction motor, Pulse width modulation, Modulation index (MI), Switching frequency, Harmonic. INTRODUCTION In previous decades, the DC motors were extensively used for the industrial purposes due to the decoupled torque and flux that can be obtained by controlling field and armature current respectively [1]. Though the DC motor provides high starting torque, it has a number of drawbacks such as it requires high maintenance and is not suitable for environment [2, 3]. Induction motor has taken the place of a workhorse in industry instead of DC motors because of its robustness, less maintenance, high efficiency and low cost [4]. In the past induction motors were used essentially for constant speed applications as there was the unavailability of the variable frequency voltage supply. Output voltage of inverter can be adjusted by controlling the inverter by PWM method [5, 6]. PWM is the method to control modern power electronic circuit. The basic idea is to control the duty cycle of a switch load controllable average voltage. The input DC supply is either in the form of voltage or current, converted in to variable output AC voltage [7]. Two prediction based on input source are VSI and CSI used in industries for variable speed drive and many other applications [8]. Currently conventional VSI or CSI is dealing as key part in the field of induction motor drive circuit. These converters succeed to present at our desire level due to several essential drawbacks like they can execute as either buck or boost operation and they contain considerable amount of harmonic as well as electromagnetic interference (EMI) noise [9]. These inverters are replaced by PWM control ZSI which offers buck-boost operation capability by utilizing shoot-through state (ST) and provides less EMI noise. The PWM inverter is controlled by three different PWM techniques: Sinusoidal PWM (SPWM) technique, Selective harmonic elimination PWM (SHEPWM) technique and Space vector modulation (SVM) [10, 11, 12]. This paper describes a harmonics reduction technique which is based on PWM generator to supply the induction motor. A MATLAB simulation model of induction motor drive is arranged by using SPWM, SHEPWM, SVM and ZSI. Then some important motor characteristics such as rotor speed, PWM output, electromagnetic torque, rotor current and stator current are observed for three system models at various loading conditions. The model using ZSI, exhibits efficient performance in all cases compared to the other models. Harmonic distortions are harmful for induction motor so it is necessary to mitigate the harmonics and provide a harmonic less supply to the induction motor. The main objective of this paper is to find the improved technique for harmonic reduction for induction motor. This paper presents an enhanced ZSI, used to limit the speed of an induction motor. The ripple of Z-source elements, output voltage source, current are varied with modulation index and switching frequency with their harmonic profile. This paper is the analysis of all the possible states of harmonic reduction in induction motor. Section II describes the objective of THD reduction techniques, Section III defines the outline of different harmonic mitigation techniques and section IV is to design the impedance network. Method of designing impedance network is presented in section V. Simulations are used to authenticate the accuracy of the design process by considering an illustrative example in section VI. Finally, conclusions are presented in section VII. STATEMENT OF THE PROBLEM When the motor is working in healthy condition, the modulation index and switching frequency are coordinated by the electric power system. Here modulation index is m=a m/a c. Where modulating signal A m is a sinusoidal wave and A c is the triangular carrier wave. When a high frequency triangular carrier wave is compared with the sinusoidal 6098

2 reference wave then it determines the switching instant of output voltage [13, 14, 15]. Table 1 show the variation in modulation index and switching frequency of PWM generator Modulation index increases with the increase of fundamental line voltage while the total harmonic distortion (THD) decreases. Thus THD can be reduced by increasing modulation index. Here modulation index is fixed at m=0.8. Switching frequency increases, harmonic loss decreases but above a particular level of switching frequency THD again increases. So, a moderate value of switching frequency is required for PWM fed induction motor [16, 17]. Induction motors have THD, this drawback can be overcome by using PWM techniques and ZSI. THD reduction control strategies are applied in induction motor which decreases the switching angles of non-sinusoidal waveforms. Figure 1 shows the detailed description of ZSI, PWM techniques and THD reduction [18, 19, 20, 21]. Table 1. Variation of THD in modulation index and switching frequency Parameter Value THD (PWM output voltage) % Modulation Index Switching frequency Induction Motor SPWM SHEPWM SVM Z-source Variation in modulation index and switching frequency Effect on inverter output Variation in output Phase angle Effect on rotor and stator current Variation in switching angle Generating control signal PWM fed induction motor Comparison on between ZSI and VSI output Variable input fed to motor Effect on rotor and stator current Check the THD of the stator current and PWM output Output of inverter THD (entering into motor stator) THD calculation THD reduction Figure 1. Block diagram of THD reduction 6099

3 Three-phase electronic power converters are controlled by PWM and have a wide range of applications for DC to AC power supplies and AC machine drives. Harmonics Mitigation Techniques HARMONIC MITIGATION TECHNIQUES Harmonic distortion is one of the major concerns in the area of electrical power quality. Because of their potential negative impacts, the uncontrolled flow of harmonics in power systems is prevented by employing various mitigation methods [22, 23]. The harmonic contents can be reduced with the help of PWM techniques and ZSI. PWM techniques mitigate the harmonics in induction motor and Z-source to control the number of modulation index and minimize the shoot-through state (ST). Different approaches in harmonic mitigation techniques are shown in figure 2 [24, 25, 26]. Sinusoidal pulse width modulation Pulse width modulation technique Selected harmonic elimination pulse width modulation Space vector pulse width modulation Figure 2. Harmonics mitigation technique Z-source technique Various techniques have been devised by many researches for controlling the output current of a PWM voltage-fed inverter. A current control technique is also devised for three-phase PWM AC/DC converters. Table 2 shows the pros and cons of different PWM techniques [27, 28, 29]. Table 2. Pros and cons of PWM techniques PWM Techniques SPWM SHEPWM SVM Pros Control of inverter output without any additional components [30] Reduction of lower harmonic The elimination of specific low-order Compared to SPWM with the same harmonics from a given voltage/current modulation index, the THD of SVM is waveform achieved by SHEPWM slightly lower [32] technique [31] SVM can achieve 15% more basic SHEPWM method at fundamental component than SPWM frequency, for which transcendental Decrease of input current THD and equations characterizing harmonics are increase of power factor are desired [37] solved to compute switching angles [35, 36] Cons Reduction of available voltage Increase switching losses due to high PWM frequency [33] Complexity increase High cost No flexible too control [34] It is difficult to solve the SVM expressions as these are extremely nonlinear in nature and may produce simple, multiple, or even no solutions for a meticulous value of modulation index [38, 39] PROPOSED TECHNIQUE The ZSI rectifies the associated problems with the VSI and CSI. The symmetrical impedance network connects the source to the load through the inverter. The impedance network consists of two equal inductors and two equal capacitors as show in figure 3. Input source Es Ds C2 L1 C1 V S Load I L2 Impedance Network Figure 3. Equivalent circuit of ZSI 6100

4 This network acts as a second order filter. In this impedance network a constant impedance output voltage is fed to the three-phase inverter main circuit. Depending upon the gating signal, the inverter operates and this output is fed to the threephase AC load/motor. The number of control methods are used to control ZSI, which include the variation in modulation index and switching frequency. The closed loop speed control scheme utilized by slip regulation of combined inverter and induction machine improves the dynamic performance. The * speed loop error generates the slip speed command ( ) sl through the proportional-integral controller and limiter. The slip is added to the speed feedback signal ( r ) to generate the synchronous speed command ( * s ) [40, 41, 42]. The synchronous speed command generates the voltage command (Vs) through a Volts/Hz function generator as shows in figure 4. Dc Supply SIMULATION AND RESULTS In this section the various condition of harmonic reduction and controlling of THD are describes. This paper defines two different control method of motor drive: PWM and Z-source. PWM technique is connected to output link for controlling speed regulation and Z-source network is connected to DClink for controlling ST switching states. A. Three-phase induction motor without PWM The simulation model of induction motor without using any mitigation technique is show in figure 5. In this model, pulse generator is used for triggering purpose of inverter and a stepup transformer is used to step-up the voltage signals. A threephase induction motor is connected to three-phase sinusoidal source via a transformer. Stator current harmonics is analyzed with fast fourier transform (FFT) show in figure 6. In this model THD is % without using any mitigation technique. Table 4 shows parameters of three-phase induction motor. * r r PI Controller Slip Limiter * sl * s r Constant V/f Speed sensor 3-phase ZSI Figure 4. Closed-loop speed control scheme utilizing V/f and slip regulation The control strategy of the ZSI is an important issue and several feedback control strategies have been investigated in recent publications. There are four methods to control the ZSI DC-link voltage: simple ST boost control (SBC), maximum ST boost control (MBC), maximum constant ST boost control (MCBC) and modified space vector modulation ST control method (MSVM). Following control methods are show in table 3. Table 3. Comparison of different ST control methods ST boost control method IM SBC MBC MCBC MSVM Line voltage harmonic Phase current harmonic DC link voltage ripples Switch voltage stress Inductor current ripples Efficiency Obtainable ac voltage Total Here (+, 0 and -) represents the best, the moderate and the lowest performance, respectively Parameters Table 4: Three-phase induction motor parameters Nominal power Voltage applied Frequency R s and L s (stator resistance and inductance) R r and L r (rotor resistance and inductance) Value 3HP 220V 50 Hz Mutual inductance L m(h) H 1.115A rms & H 1.083A rms & H Inertia, friction factor pole pairs 0.02 J (kg.m 2 ), F (N.m.s) & 2p Synchronous speed 1800 rpm Angular frequency rad/s Figure 5. MATLAB Model of induction motor without using any technique 6101

5 Figure 6. Stator Current THD A.1 Three-phase induction motor with PWM PWM technique is to control output voltage and harmonic reduction. PWM is commonly used technique for controlling power in inertial electrical devices, made practical by modern electronic power switches. Here PWM techniques like SPWM, SHEPWM and SVM are applied to inverter and it also studies the performance of the induction motor. A.1.1 Sinusoidal Pulse Width Modulation The MATLAB simulation model is show in figure 7. A snubber circuit is used between rectifier and inverter. A PWM generator is used to trigger the inverter. As perfect output waveforms are basic requirement for the smooth running of the induction motor. PWM technique is a technique which is used to mitigate the harmonics and speed control of induction motors. Figure 8 show the stator current responses of IM when at time 0.4 sec. Stator current waveform attains a steady state at near 0.4 sec. For an input DC voltage of 600 V, the circuit produces three-phase AC voltage output. This load is Y-connected and purely inductive in nature. Figure 9 shows the stator current and PWM inverter voltage waveform. THD is controlled till 11.61% therefore in the proposed simulation model the accurate maximum frequency increases efficiency of the motor. Figure 10 shows the PWM output voltage waveforms and THD of inverter. Figure 7. Simulation model of induction motor with using SPWM Figure 8. Stator current Figure 9. FFT analysis in Stator current THD 6102

6 Figure 10. PWM voltage waveform and THD A.1.2 Selected Harmonic Elimination PWM Technique Compared with other PWM topologies, only the SHEPWM based techniques work effectively at low switching frequencies and theoretically provides the best output voltage and current quality. Figure 11, show the simulation model of SHEPWM fed induction motor. The output waveforms for rotor current and stator current were obtained. The following are rotor and stator currents in the induction motor showing figure 12 and 13. This output is not pure because of many harmonics. Both the stator and rotor currents exhibit high transients during the starting of the motor. Stator and rotor current settle down to low amplitude sinusoidal oscillations. Figure 14 shows the THD in SHEPWM fed induction motor. Figure 11. Simulation model of SHEPWM fed induction motor Figure 12. Rotor current in SHEPWM fed induction motor Figure 13. Stator current in SHEPWM fed induction motor 6103

7 Figure 14. Stator current THD in SHEPWM fed induction A.1.3 Space Vector Modulation The SVM control system model is used to generate the SVM control signal for inverter and verify the output results are shown in figure 15. The space vector V ref with magnitude V m rotates in a circular orbit at angular velocity u, where the direction of rotation depends on the phase sequence of voltages are show in figure 16. With the sinusoidal threephase command voltage the composite PWM fabrication at the inverter output should be such that the average voltage follows these command voltage with minimum amount of harmonic distortion. Figure 15. Simulation model of SVM fed induction motor switching frequency to spread the harmonics continuously to a wide band area so that the peak harmonics can be reduced. Simulation has been carried out by varying the modulation index between 0 and 1. Figure 19 shows the voltage waveform of phase to neutral SVM inverter. Two popular modulation strategies have been implemented in the SVM: alternating zero vector strategy and symmetrical modulation strategy. Figure 20 shows the harmonics of rotor current of induction motor fed by SVM inverter with the help of fourier block present in simulink. Figure 16. Space vector trajectory Figure 17 shows the stator current of SVM is an advanced technique used for variable frequency drive applications. It utilizes DC bus voltage more effectively and generates less THD in the three-phase VSI. Figure 18 shows the control signal generated by SVM. SVM utilize a disordered changing Figure 17. Stator current of induction motor fed by SVM inverter 6104

8 SVM output waveforms have less harmonics, therefore to achieve better performance for inverter SVM algorithm in the ZSI is preferred. THD of stator and rotor current is shown figure 21 and 22. Figure 18. Control signal generated by space vector subsystem block Figure 21. Stator current THD in SVM fed induction motor Figure 19. Phase to neutral voltage of SVM inverter Figure 20. Rotor current of induction motor fed by SVM inverter Figure 22. PWM output current waveform and THD Table 7 shows the decrement in harmonics magnitude with respect to fundamental component of rotor current. Table 7: Harmonic Contents in Rotor Current Harmonics order Harmonic magnitude fundamental Z-source Fed Induction Motor In this model an induction motor drive system with the use of ZSI is implemented. Figure 23 show simulation model to verify the Z-source results, the simulations are conducted with initial system parameters. Here, inductance is 4mh and capacitance is 300mf. 6105

9 Figure 23. Simulation model of ZSI fed induction motor The block diagram of three-phase ZSI fed induction motor is shown in figure 24. It is frequently essential to control the output voltage of inverter from the constant V/f control of an induction motor [43]. PWM based firing of inverter provides the best constant V/f and controlling of the speed of induction motor [44, 45, 46]. Power supply Converter Z-source impedance network Inverter IM Figure 25. Stator current in ZSI fed induction motors PWM setting V/f control Speed setting Figure 27. Capacitor voltage in ZSI impedance network Figure 24. Block diagram of ZSI fed induction motor In this ZSI conventional control method modulation index are kept high so that minimum stress on the device are obtained. Simulation model shows that motor output line current is reduced to large extent. Three-phase stator current waveforms and stator voltage for a given load condition is shown in the figure 25 and 26 respectively. The ZSI can be improved by controlling capacitor voltage. The purpose of the capacitor is to absorb the current ripple and maintain a fairly constant voltage so as to keep the output voltage sinusoidal. The proposed control method generates the ST duty ratio by controlling both the inductor current and the capacitor voltage of the ZSI as shown in figure 27. Figure 26. ZSI voltage output waveform Harmonic analysis of ZSI The simulink block diagram is designed to obtain the different waveforms. Figure 22 describes the satisfactory performance of a drive, output should be ripple free so that the input taken from the inverter is analyzed for harmonics. The component values of ZSI depend on switching frequency only. Figure 28 show the stator current THD waveforms to be very smooth 6106

10 sinusoidal waveform as compared to the traditional PWM inverter. Due to this operational behaviour ZSI can boost the output voltage to any value greater than input voltage as shows in figure 29. be any value between zero and infinity regardless of the input DC voltage [14]. That is, ZSI is a buck-boost inverter that has a extensive range of accessible voltage. The traditional VSI and CSI cannot provide such features. CONCLUSIONS Figure 28. THD analysis of stator current in induction motor fed by ZSI Induction motor drive system with the use of PWM and ZSI technique describe new approaches towards control of motor drive over traditional control methods. PWM technique in induction motor investigates changes in THD and shows the effectiveness of harmonics. PWM parameter variation shows that modulation index and switching frequency are inversely proposition to each other and Z-source inverter take place in the field for better performance of induction motor. In this field one step ahead technique is used where VSI and ZSI comparison shows that Z-source impedance network increases the range of output voltage of inverter and reduces THD in stator current. This paper presents an enhanced ZSI, used to limit the speed of an induction motor. REFERENCES [1] F. Z. Peng, Z-Source Inverter, IEEE transactions on industry applications, Vol. 39, No. 2, pp , [2] Y. Tang, S. J. Xie, C. H. Zhang and Z. G. Xu, Improved Z-source inverter with reduced Z-source capacitor voltage stress and soft-start capability, IEEE transactions power electronics, Vol. 24,Nno. 2, pp , Figure 29. THD analysis of Z-source PWM inverter output voltage The speed control of such motors can be achieved by controlling the applied voltage on the control operation of ZSI. Table 8 represents the changes in THD of ZSI and VSI according to different modulation index. Table 8: Comparison of ZSI and VSI fed induction motor in terms of THD Modulation Index THD(ZSI)% THD(VSI)% The THD in stator current is lower in case of Z-source fed induction motor. The ZSI is analyzed using VSI. The distinctive feature of the ZSI is that the output AC voltage can [3] P. C. Loh, F. Gao and F. Blaabjerg, Embedded Z- source inverters, IEEE transactions industry applications, Vol. 46, No. 1, pp , [4] Y. Tang, S. Xie and C. Zhang, Single-phase Z-source inverter, IEEE transaction power electronics, Vol. 26, No. 12, pp , [5] D. Vinnikov and I. Roasto, Quasi-Z-source based isolated DC/DC converters for distributed power generation, IEEE transactions industry electronics, Vol. 58, No. 1, pp , [6] Y. Tang, S. J. Xie and C. H. Zhang, Z-source AC-AC converters solving commutation problem, IEEE transactions power electronics, Vol. 22, No. 6, pp , [7] P. C. Loh, D. M. Vilathgamuwa, Y. S. Lai, G. T. Chua and Y. Li, Pulsewidth modulation of Z-source inverters, IEEE transactions power electronics, Vol. 20, No. 6, pp , [8] Y. Tang, S. Xie, and C. Zhang, Single-phase Z-source inverter, IEEE transactions on power electronics, Vol. 26, No. 12, pp , [9] Yu Tang, Shaojun Xie, and Jiudong Ding Pulsewidth modulation of Z-source inverters with minimum 6107

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