COMPARISON OF DIRECT TORQUE CONTROL AND VECTOR CONTROL OF INDUCTION MOTOR Thesis submitted in the partial fulfilment of the requirements for the award of degree of MASTER OF ENGINEERING In POWER SYSTEMS & ELECTRIC DRIVES Submitted by: Manjeet Singh Reg. No: 800941017 Under Supervision of: Mr. S.S.S.R. Sarathbabu Duvvuri Lecturer, EIED Thapar University, Patiala ELECTRICAL AND INSTRUMENTATION ENGINEERING DEPARTMENT THAPAR UNIVERSITY PATIALA, PUNJAB-147001 June, 2011 ACKNOWLEDGEMENT I take this opportunity to express my profound sense of gratitude and respect to all those who helped me through the duration of this project work. First of all, I thank the Almighty, who gave me the opportunity and strength to carry out this work. My greatest thanks are to my parents who bestowed ability and strength in me to complete this work. I would like to express my gratitude to Mr.SSSR Sarathbabu Duvvuri, Lecturer EIED, Thapar University, Patiala, for guidance and support throughout this project work. They have been a constant source of inspiration to me throughout the period of this work. I consider myself extremely fortunate for having the opportunity to learn and work under there supervision over the entire period. I would also take this opportunity to express my gratitude and sincere thanks to Dr. Smarajit Ghosh, Prof. & Head, EIED, Thapar University, Patiala for his valuable support. I am also thankful to the entire faculty and staff of EIED, Thapar University, Patiala for their help, inspiration and moral support. My thanks to all my classmates for their encouragement and help. Manjeet Singh Regn. No. 800941017 I ABSTRACT The basic theory of operation for the control techniques is presented. A mathematical model for the proposed DTC of the IM topology is developed. A simulation model is developed in MATLAB/SIMULINK and is used to verify the basic operation (performance) of the DTC technique. In conventional Direct Torque Control (DTC), the selection of flux linkage and electromagnetic torque errors are made within the respective flux and torque hysteresis bands, in order to obtain fast torque response, low inverter switching frequency and low harmonic losses. However, DTC drives utilizing hysteresis comparators suffer from high torque ripple and variable switching frequency. Space Vector Modulation (SVM) is the strategy to minimize the torque ripple of induction motor in which, the stator flux level is selected in accordance with the efficiency optimized motor performance. SVM method is incorporated with direct torque control (DTCSVM) for induction motor drives. However, the basis of the DTC SVM strategy is the calculation of the required voltage space vector to compensate the flux and torque errors exactly by using a predictive technique and then its generation using the SVM at each sample period. In this thesis work, the simulation of DTC scheme has been carried out using MATLAB/SIMULINK and the results are compared with AC Vector Control drive using hardware setup. II List of figures Figure no. Description Page no. Fig. 2.1 D,Q axes superimposed onto a three-phase induction motor 11 Fig. 3.1 Eight possible voltage space vectors obtained from VSI. 18 Fig. 3.2 Stator flux, rotor flux & stator current vectors on ds-qs. 19 Fig. 3.3. Incremental stator flux linkage space vector representation in the DQ-plane. 20 Fig. 3.4. Representation of direct and indirect components of the stator flux vector. 21 Fig. 3.5. Voltage source inverter (VSI) connected to the R-L load. 23 Fig. 3.6. Voltage vector selection when the stator flux vector is located in sector i. 24 Fig. 3.7. DTC scheme. 26 Fig. 3.8. DTC sectors and reference frames. 28 Fig. 3.9. VSI vectors used when Ψdq is in sector 1 28 Fig. 3.10. 2-level hysteresis comparator. 29 Fig. 3.11. VSI and how the motor windings are connected. 30 Fig.4.1 Vector controlled induction motor 35 Fig.4.2 Stator current space vector and its components in (a,b,c). 37 Fig.4.3 Stator current space vector and its components in (α,β). 37 Fig.4.4 Stator current space vector and its component in (α,β) and in the d,q rotating reference frame. 38 Fig.4.5 Basic scheme of FOC for AC-motor. 39 Fig.4.6 Current, voltage and rotor flux space vectors in the d,q rotating reference frame and their relationship with a,b,c and , stationary reference frame. 41 Fig. 4.7 AC Vector Control Drive 43 Fig. 4.8 Voltage Vs Speed 44 Fig. 4.9 Graph Speed Vs Voltage. 45 Fig. 5.1 Simulink model of Induction Motor. 48 Fig. 5.2 Park’s Transformation block. 47 Fig. 5.3 DIRECT TORQUE CONTROL. 48 Fig. 5.4 Flux response of DTC for 220V. 49 Fig. 5.5 Speed response of DTC for 220V. 49 III Fig. 5.6 Torque response of DTC for 220V. 51 Fig. 5.7 Flux trajectory for 220V. 51 Fig. 5.8 Flux response of DTC for 100V. 52 Fig. 5.9 Speed response of DTC for 100V. 52 Fig. 5.10 Torque response of DTC for 100V. 53 Fig. 5.11 Flux trajectory for 100V. 53 Fig. 5.12 Flux response of DTC for 413.5V. 54 Fig. 5.13 Speed response of DTC for 413.5V. 54 Fig. 5.14 Torque response of DTC for 413.5V. 55 Fig. 5.15 Flux trajectory for 413.5V. 55 IV LIST OF SYMBOLS Symbol Description V d-axis stator voltage ds V q-axis stator voltage qs V d- axis rotor voltage dr V q- axis rotor voltage qr i d-axis stator current ds i q-axis stator current qs i d- axis rotor current dr i q- axis rotor current qr ψ d-axis stator flux linkages ds ψ q-axis stator flux linkages qs ψ d- axis rotor flux linkages dr ψ q- axis rotor flux linkages qr ψ mutual flux linkages qm Rs stator winding resistance Xls stator winding leakage reactance Rr rotor winding resistance Xlr rotor winding leakage reactance Xm stator to rotor mutual reactance. ω base speed in rad/sec b ω rotor speed in rad/sec r θ arbitrary reference angle in radiance. a K short circuit ratio. ψ stator resultant flux linkages s ψ rotor resultant flux linkages r V LIST OF TABLES Table no. Description Page no. Table 3.1. Switching table 25 Table 3.2. Voltage Vector Switching Table 29 Table 4.1 Hardware Vector Control Motor Drive Table 42 VI LIST OF ABBREVATIONS AC Alternating Current IM Induction Motor FOC Field Oriented Control DTC Direct Torque Control VC Vector Control PWM Pulse Width Modulation MATLAB MATrix LABoratary SVPWM Space Vector Pulse Width Modulation PWM VSI Pulse Width Modulated Voltage Source Inverter IGBT Insulated Gate Bipolar Transistor T Torque m.m.f. magneto motive force e.m.f. electro motive force d.c. direct current r.p.m. revolution per minute KW Killowatt MW Megawatt PID Proportional Integral and Differential PI Proportional Integral DSP Digital Signal Processing DFOC Direct Field Oriented Control IFOC Indirect Field Oriented Control PMSM Permanent Magnet Synchronous Motor A/D Analog to Digital converter VII Table of Contents Page no. Chapter 1: INTRODUCTION 1 1.1 Introduction 2 1.2 Literature survey 3 1.3 Overview of the thesis 6 Chapter 2: MATHEMATICAL MODELLING OF INDUCTION MOTOR 8 2.1 Introduction 9 2.2 Induction motor 9 2.3 Theory 11 2.4 Stationary reference frame 12 2.5 Summary 14 Chapter 3: DIRECT TORQUE CONTROL (DTC) OF IM DRIVE 15 3.1 Introduction 16 3.2 Torque control strategy in DTC of IM drive 18 3.3 Flux control strategy of IM drive 20 3.4 Voltage vector selection in DTC of IM drive 22 3.5 Conventional DTC control system 25 3.5.1 Current transform 26 3.5.2 Flux estimator 27 3.5.3 Torque estimator 27 3.5.4 Sector calculation 27 3.5.5 Torque and flux hysteresis comparator 28 3.5.6 Look up table 29 3.5.7 Voltage source inverter 29 3.6 Advantages and disadvantages of the DTC technique 30 3.7 Summary 32 Chapter 4: VECTOR CONTROL OF IM DRIVE 33 4.1 Introduction 34 4.2 Field Orientated Control 34 4.3 Space Vector definition and projection 36 VIII 4.3.1 The (a,b,c) to (,) projection (Clarke transformation) 37 4.3.2 The (, to (d,q) projection (Park transformation) 38 4.3.3 The (d,q) to (,) projection (inverse Park transformation) 39 4.3.4 Torque equation in vector control 40 4.4 Input for the FOC 41 4.4.1 Current sampling 41 4.4.2 Rotor flux position 41 4.5 Summary 43 Chapter 5: SIMULATION RESULTS 44 5.1 Introduction 45 5.2 MATLAB/Simulink model for IM 46 5.3 DTC TECHNIQUE 47 5.4 Summary 55 Chapter 6: CONCLUSION AND RECOMMENDATIONS 56 References IX Chapter 1 Introduction 1 Chapter 1 1.1 INTRODUCTION: In Industrial automation electric motors play crucial role as heart of the system. Therefore, the best performance motor control systems contribute to a great extent, to the desirable performance of automated sectors by enhancing the production rate and the quality of products. In fact the performance of today’s automated systems is defined in terms of efficiency, smoothness and accuracy. The recent developments of the power electronics industry leads to the considerable increase of the power that can be manipulated by semiconductor devices. Even though the maximum voltage supported by these devices remains the major obstacle in medium and high voltage applications. As in multilevel inverters, due to lower order harmonics there is distortion of the output voltages when compared to standard two-level inverters operating at the same switching frequency.