Performance improvement using a multi-level converter in a DTC based induction motor drive

Direct Torque Control (DTC) is the state-of-the-art control strategy for 3-phase AC motor drives which allows direct control of torque and flux without involving very intensive computations that are inevitable for field oriented control (FOC). It is well known that DTC has more torque and flux ripples as compared to FOC as the former is based on voltage vector selection from a 3-phase 2-level inverter that offers only 6 non-zero voltage vectors as feasible choices. Increasing the number of levels in a 3-phase inverter will allow more choices of voltage vectors and hence torque pulsations can be minimized in high performance drives; however, this also increases the complexity of computations and power circuit. The three-level inverter used in this work involves only three additional bi-directional switches as compared to the number of switches in two level inverter and hence the cost increase is insignificant as compared to a two-level voltage source inverter (VSI). The major advantages of DTC such as no requirements of current controllers, axes transformation and rotor position sensors-are still preserved when we use a three-level VSI. DTC's lesser parameter dependency and fast torque response has made it popular in industrial motor control. The objective of this research work is to evaluate and compare the performances of a DTC based three-phase induction motor drive while it is being fed by a two-level and three-level VSIs. A two-level hysteresis band controller is used for the flux comparator and a three-level and five level hysteresis controller is used for the torque comparators in the two cases. The performances of the drive in these two cases are compared in terms of starting current, starting torque, response time, and responses during speed and load disturbances. The amount of pulsations in the torque and current are estimated for the two-level and three-level VSI fed cases. From the obtained results it is clearly seen that the three-level VSI fed DTC drive yields a better overall performance as compared to the two-level case.