Analytical Study of Affecting Characteristic of Voltage Vectors of a Three-level NPC Inverter on Torque and Flux of DTC controlled Drives

The direct torque control (DTC) is known as an effective control method for high performance drives in a wide variety of industrial applications. The objective of this paper is exploration of analytical equations describing the influence of voltage vectors of a space vector modulated neutral point clamped (NPC) Inverter on stator flux and electromagnetic torque variations in medium voltage induction motor drive applications. Using obtained expressions the paper shows that the degrees of freedom of DTC controller is seriously limited by changing the affecting characteristic of some voltage vectors inside the 60deg flux sectors. It proposes therefore the division of flux space vector hexagon into 12, 30deg sectors instead of conventional 60deg ones.