On speed control of induction motors

In a recent paper [(Espinosa-Perez and Ortega, 1994). State observers are unnecessary for induction motor control. Syst. di Control Lett., 23, 315-323] Espinosa and Ortega presented an output feedback globally stable speed tracking controller for induction motors. 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 we overcome this drawback and show that by simple linear filtering of the speed tracking error we can inject mechanical damping to the closed loop. This allows us to improve the transient performance of the scheme in Espinosa-Perez and Ortega (1994) 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, our passivity-based scheme exactly reduces to the well-known indirect held-oriented control scheme, hence providing a solid theoretical foundation to this popular control strategy.