MULTIPHASE MULTILEVEL INVERTER FOR INDUCTION MOTOR DRIVE

Abstract

In the field of variable speed drives, Induction motors are very popular because of its ruggedness, cost effectiveness, and ability to operate in contaminated environment. Nowadays, Pulse Width Modulated inverters are used to control induction motor drives which operates at high switching frequency to match fundamental reference as close as possible. For medium voltage high power drives and medium power drives, operation of single switch at high frequency poses several problems. Hence, switches are connected in series and/or parallel to match required ratings. However, it cannot avoid higher dv/dt and dynamic sharing problems associated with device due to mismatch in characteristic of the device. Multilevel inverter technology has recently emerged as a very important alternative in the area of high-power medium-voltage energy control as it provides less THD, reduced switching losses and avoids dynamic sharing problem for devices operating at higher voltage levels. Additionally, when used with induction motor drives it provides certain advantages like better dynamic response, reduced stator copper losses, and reduced common mode voltages. There are various multilevel inverter topologies available in literature out of which diode clamped inverter also known as Neutral Point Clamped (NPC) inverter is considered in the work. Out of the total demand of variable frequency drives, almost 85% applications do not require dynamic control. Hence, only open loop and closed loop variable frequency drives are considered in the work. Since Space Vector PWM method is popular and the best available method for variable frequency drive, it is considered for implementation. Acceleration and deceleration of drive, fluctuating load and operation at lower modulation index causes fluctuation in neutral point voltage; to avoid this, self-balancing space vector pulse width modulated algorithm is implemented. Digital Signal Processors/Controllers are widely used for the development of the control algorithm of the drive. In the current work, open loop variable frequency drive algorithm for three phase three level system is developed using TMS320F2812 development board. Due to the inherent benefits provided by FPGAs in development of PWM algorithm, implementation of the SVPWM for open loop variable frequency drive is explored. A new modular structure based simplified SVPWM algorithm is proposed.