Analytical investigation of torque and flux ripple in DTC schemes for induction motors

AC drives based on Direct Torque Control (DTC) of induction machines allow high dynamic performance to be obtained with very simple control schemes. The drive behaviour, in terms of flux and torque response, is dependent on the voltage vector selection strategy utilised and the operating conditions. In this paper a detailed discussion of DTC basic principles is carried out in order to determine analytically the relationships between the applied voltage vector and the corresponding torque and flux variations. The analysis allows the flux and the torque ripple to be easily determined. It has been observed that the effects produced by a voltage vector are strongly dependent on both the rotor speed and the voltage vector direction relative to the rotor flux. Some significant diagrams are presented showing the theoretical results obtained. The analysis can be useful in defining new voltage vector selection criteria and in optimising the dynamic behaviour with minimal torque and flux ripple.