The theory of reference frames and switching functions are effective in analyzing the performance of the induction motor fed from VSI (Voltage Source Inverter). In this work, mathematical model of Adjustable Speed Drive (ASD) is developed by taking synchronous reference frame equations for induction motor, switching function concept for VSI and non-switching concept for diode bridge rectifier. Simulation model of induction machine is implemented using dq0 axis transformations of the stator and rotor variables in the arbitrary reference frame. The corresponding equations are given in the beginning and then the developed model is implemented using MATLAB/Simulink. In this work, the proposed model is implemented using basic function blocks. The performance of induction motor is analysed for different frequencies. The developed model is tested for the steady state behavior of machine drive. The proposed mathematical model is validated by the simulation results. Keyword: Converters INTRODUCTIONIn many industrial applications Adjustable speed drives (ASD) are most commonly seen workhorses. In order to supply the motor with variable AC voltage or AC current with variable frequency Variable Frequency Drives (VFD) are employed. ASDs are used in pumping applications, in sugar cane industries, conveyor applications etc. The common VFD consists of a three phase diode bridge rectifier, dc link and a pulse width modulated inverter. It is necessary to develop a model for VFD for power system dynamic studies. In literature, for the three phase diode bridge rectifier dq impedance model is employed [1]. State-space averaging method is used for modeling a three phase four wire diode bridge rectifier [2]. Dynamic average value modeling methods are utilized for conventional three phase diode bridge rectifier and are validated [3]. This can capture the steady-state and transient characteristics of the diode bridge rectifier. An approximate switching function of the diode bridge rectifier is used in order to obtain the estimating function for the fundamental current harmonics [4].This method is proven to be effective in finding out the input current harmonic content. A switching function model for voltage source inverter is derived and also it is validated using MATLAB/Simulink [5].Modulation function theory is effectively utilized for deriving the Pulse Width Modulated (PWM) inverter which makes use of the Iterative Harmonic and Interharmonics Analysis (IHIA) [6]. Space vector pulse width modulation method is employed for inverter and the method is validated using MATLAB/Simulink [7]. A three phase boost dc-ac converter is used to supply the induction motor [8]. AC output voltage that is greater than the input dc voltage is obtained without the need of additional boost converter.

Multilevel inverter (MLI) is the recent advancement in power electronics which employed to synthesize a high-quality output voltage waveform. This paper focuses on developing a single phase modified nine-level inverter with the reduced number of switches for improving the quality of output voltage waveform. Single carrier and multireference Pulse Width Modulation (PWM) technique is utilized for generating the switching pulses. The proposed topology consists of a single DC source, level generation part (balancing capacitor and bidirectional switches) and polarity generation part (H-Bridge). The operation of the proposed topology is examined for generating 9-level output voltage using MATLAB/SIMULINK. The result of the proposed topology is verified through laboratory-based prototype model. For experimental setup, the switching pulses are generated by PIC16F877A microcontroller. This proposed lab experimental and simulation can be used to study the value of %THD at output side of the inverter.

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