Three-level inverter fed high performance induction motor drive system for high power application

In this paper, a three-level inverter fed induction motor drive operating under Direct Torque and Flux Control (DTFC) is presented. A diode clamped multilevel inverter is used for driving induction motor operating under DTFC scheme. Problems associated with application of DTFC methods to medium and high power drives in low motor speed range are discussed. With the proposed techniques the DTFC principle can be applied to medium and high power drives in the entire motor speed range. The proposed control strategies are verified by experimental results. The DTFC of induction motor using three-level inverter is realized in two methods. Initially, only half and full voltage vectors are used for torque and flux control. Later, a modified look-up table is used to take care of the demagnetizing effect of flux vector near sector boundary. The torque control under low speed operation has been successfully achieved with neutralization of the demagnetization effect due to stator resistance drop. An improved stator flux estimator is incorporated for low speed operation, to nullify offset problem of pure integrator. The experimental results of the complete system are presented here for forward and reverse motoring operations of the drive.