Constant V/f Control and Frequency Control of Isolated Winding Induction Motor Using Nine-Level Three-Phase Inverter

Abstract

Modern industrial applications deal with power conversion system with lesser harmonic content for variable voltage and variable frequency application. In this paper, a generalized multilevel inverter has been proposed with nearest level modulation technique. A nine-level asymmetrical inverter of 5 kW has been developed as laboratory prototype and tested with three-phase isolated winding induction motor of rating 1 HP, 200 V, 50 Hz. Unlike any other control technique, the output voltage levels remain fixed at different modulation indexes using nearest level modulation technique, thereby reducing the harmonic content, switching losses and total harmonic distortion. In this paper, a DC–DC converter stage is used to control the DC link voltage of inverter for variable speed application. AC link system (a multiwinding transformer–rectifier) and a closed-loop voltage control technique are used to obtain multiple variable DC voltage sources. A single DC source is used for constant V/f control and frequency control of induction motor, i.e. isolated or open-end winding at above base speed as well as below base speeds. Simulation results of phase voltage of stator terminal, phase current of stator terminal, speed and torque are shown at different speeds. The experimental results of output voltages at different speeds for constant V/f control and frequency control are recorded as well. The experiment is carried out at different stator voltages (or stator frequencies) below and above the rated speed, and a few experimental results are presented. The numbers of inverter voltage levels are always same, and hence, their total harmonic distortion also remains nearly constant at different speeds of operation.