On speed control of induction motors

Espinosa and Ortega (1994) presented an output feedback globally stable speed tracking controller for induction motors. An important feature of the scheme is that it does not require state observers, thus reducing the computational burden, a critical limiting factor in physical implementations. The performance of the scheme is limited by the fact that the convergence rate of the speed tracking errors is determined by the natural mechanical damping of the motor. In this brief note the authors overcome this drawback and show that by simple linear filtering of the speed tracking error one can inject mechanical damping to the closed loop. This allows the authors to improve the transient performance of the scheme in position and speed tracking applications without significantly increasing the computational requirements, while preserving the global stability properties. Further, it is shown that if the inverter can be modelled as a current source and the desired speed is constant, the authors´ passivity-based scheme exactly reduces to the well known indirect field-oriented control scheme, hence providing a solid theoretical foundation to this popular control strategy