Variable speed AC motor drives are widely used for industrial applications and a majority of the industrial drives employ induction motors. In most of the drives, variable speed is achieved by V/f control strategy, where the air-gap flux in the motor can be maintained constant at all frequencies. The V/f control can be achieved by various modulation schemes in inverter-fed drives. -- This thesis analyzes the advantages and limitations of the existing modulation schemes for V/f control and presents an improved modulation scheme called the unified modulation scheme (UMS). This scheme combines the advantages of two popular modulation schemes (the delta and sine-PWM modulation schemes), at the same time overcoming their disadvantages. The development, simulation results and experimental implementation of the proposed scheme are presented. Comparison between the different modulation schemes shows the advantages of the unified modulation scheme. -- This thesis also presents simulation results of a SIMULINK implementation of an induction motor dr.ve using the unified modulation scheme. These results are compared with the results of the delta-modulated inverter-fed induction motor drive. The results show that the unified modulation scheme provides improved performance in terms of lower-order harmonic attenuation and sub-harmonic elimination. Finally, a modified unified modulation scheme is presented using the conventional sine-PWM scheme, for performance enhancement.
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