Sensitivity to Parameters' Variations in Sensorless Induction Motor Drives using a Reduced Order MRAS Observer

In recent years, the elimination of the speed transducer in vector controlled induction motor drives has assumed a particular importance and so several methods have been proposed to estimate the motor speed. One of the simplest approaches utilizes a Model Reference Adaptive System using a reduced order reference model. The main problem connected to this approach is represented by the inaccuracy of the speed estimation due to parameters’ errors. In particular, in the low speed region any error between the actual value of the stator resistance and that used within the speed estimator may lead not only to a substantial speed estimation error but also to drive instability. The paper takes into consideration the MRAS approach and presents a simple methodology to determine, in analytical way, the sensitivity of the speed estimation to electromagnetic parameters’ variations. The obtained sensitivity formulae were verified by a suitable simulation program, in Matlab-Simulink environment.