A new torque calculator for AC induction motor drives that improves accuracy and dynamic behaviour

This paper describes a control structure for induction motors based on stator flux observer to compute the torque with an error of less than 1%. The observer includes a voltage model, an inverse current model, a decoupling network and a master feedback control. This structure considers different requirements of practical interest (standstill, flux weakening). The torque proves to be the most important controlled variable in electrical drive systems apart from the mechanical velocity. In basic industry for example the iron and steel industry or the paper and textile industry the technical quantities tensile stress and tensile force are controlled by the torque of the used electrical drives. In contrast to the torque of electrical drives for the controlled variable mechanical velocity a big quantity of universal and inexpensive measured devices are available. By using torque transducers, the mechanical transfer system has to be separated. A computation of the torque based on a machine model and the electrical supply values is proved to be a favourable alternative in contrast to the use of mechanical torque transducers. By means of a new computation method based on a stator flux and torque observer a dynamical high-grade and stationary precise actual torque value for control assignments and for closed control systems can be obtained