ABSTRACT:          There are various methods for speed control of induction motors. This paper specifically describes one of the methods: speed control using variable frequency. The proposed system is a MATLAB Simulink model, which is a closed loop model designed to achieve desired speed control of a three-phase induction motor by varying its frequency. The Simulink model has four main blocks, namely the inverter, synchronous machine, proportional integral control and current hysteresis control. For accuracy of output results and simplicity, we have used dq to abc transformation block and sin function block. The inverter is comprised of six insulated-gate bipolar transistors (IGBTs), which are fired by gate pulses generated by current hysteresis control block. The inverter generates variable frequency and variable voltage output, which is given to motor terminals. The project presents the working principle of variable frequency drive (VFD), its performance, and the use of Pulse Width Modulation (PWM) in a three-phase inverter to control the frequency and thus the speed. The proposed method conformed to performance predictions and delivered the desired outputs.TABLE OF CONTENTSABSTRACT    iiACKNOWLEDGMENTS    iiiLIST OF FIGURES    vLIST OF ABBREVIATIONS    vi1.    INTRODUCTION    12.    BACKGROUND AND BASIC UNDERSTANDING    23.    VARIABLE FREQUENCY DRIVE OPERATING PRINCIPLE    84.    COMPONENTS OF PROJECT MODEL    125.    BLOCK DIAGRAM AND OUTPUT    196.    CONCLUSION AND FUTURE WORK    23REFERENCES    24LIST OF FIGURES1.    Motor speed-torque chart    52.    Basic block diagram of VFD    103.    Hysteresis control block    174.    Simulink model of VFD    215.    Graph of desired speed    22LIST OF ABBREVIATIONSµA    micro-AmperemA    milli -AmpereµV    micro-VoltmV    milli-VoltAC    Alternating CurrentBJT    Bipolar Junction TransistorCSI    Current Source InverterDC    Direct CurrentDTC    Direct Torque ControlEMF    Electro Motive ForceHP    Horse PowerIGBT    Insulated Gate Bipolar TransistorKW    Kilo WattKV    Kilo VoltkHz    kilo HertzLCI    Load Commutated InverterLV    Low VoltageMOSFET    Metal-Oxide Semiconductor Field Effect TransistorMOSIGT    Metal Oxide Semiconductor Insulated Gate TransistorMV    Medium VoltageMW    Mega WattPI    Proportional IntegralPWM    Pulse Width ModulationSVPWM    Space Vector Pulse Width ModulationV/Hz    Volts per HertzVFD    Variable Frequency DriveVSI    Voltage Source InverterCHAPTER ONE:      INTRODUCTION  -        A variable-frequency drive (VFD) is a type of adjustable speed drive used in electro- mechanical drive systems to control AC motor speed and torque by varying motor input frequency and voltage. VFDs are used in applications ranging from small appliances to the largest of mine mill drives and compressors. Over the last four decades, power electronics technology has reduced VFD cost and size and improved performance through advances in semiconductor switching devices, drive topologies, simulation, and control techniques, and control hardware and software. VFDs are available in several different low and mediumvoltage AC-AC and DC-AC topologies. Variable speed drives are used for two main reasons: to improve the efficiency of motor-driven equipment by matching speed with the changing load requirements and to allow accurate and continuous process control over a wide range of speeds. This project specifically puts emphasis on one of the various methods of speed control of induction motor that is by changing frequency. The simulation of this VFD consists of four main units namely inverter, induction motor, PI (Proportional Integral) controller and hysteresis controller. In this project model, pulse width modulation technique is used to generate variable frequency.





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