Direct stator flux and torque control of an induction motor: theoretical analysis and experimental results

Direct stator flux and torque control (DSFTC) is a novel technique for the control of a voltage inverter-fed induction motor (IM). It carries out a precise and quick control of the stator flux and the electromagnetic torque of an IM without calling for coordinate transformation, speed measurement and stator current control. In principle, moreover, DSFTC operation requires only the knowledge of the stator resistance. The tutorial starts by reviewing the operation of an IM and of a voltage inverter using the space vector theory. The concept of DSFTC is then introduced, illustrating the action of an inverter supply on the stator flux and the electromagnetic torque of an IM. Afterwards, implementation of DSFTC according to the switching table (ST) strategy is focused. Different STs are considered and their influence on the torque response is shown by means of calculated and experimental data. The relationships between the drive performance, such as losses and switching frequency, and the amplitude of the hysteresis bands of the flux and torque controllers are given. Sensitivity to errors in the control system is also investigated. Solutions which improve the torque characteristic and the low-speed performance of the drive are presented. Finally, the DSFTC strategy, termed direct self control, and a scheme based on the space vector modulation are described, explaining how they can be interpreted as either a particular case or an extension of the ST strategy.