Influence of voltage vectors of a NPC inverter on torque and flux variations of a DTC drive considering different load and speed conditions

Direct torque control (DTC) is one of the most effective vector control methods proposed for high performance drives of a wide variety of industrial applications. This control method becomes more attractive when applied on the Neutral Point Clamped (NPC) Inverter driven drives. While exploring accurate analytical equations of torque and flux variations, this paper investigates analytically the affecting characteristic of voltage vectors of a NPC converter on variations of stator flux and electromagnetic torque (decreasing or increasing) of medium voltage induction motor drives. The analytically obtained results show that the affecting characteristic of some voltage vectors depends not only on the flux sector in which the stator flux vector is located but also on the exact position of flux vector inside that sector. Besides the load and speed conditions of a machine can change the affecting characteristic of voltage vectors undesirably. The paper studies application of DTC method in NPC inverter driven induction motors and considers different load and speed conditions. It proposes a set of appropriate voltage vectors for accurate control of torque and flux variations considering degrees of freedom in voltage vector selection provided by NPC Inverter.