Study on the speed observer and the rotor resistance identifier for an induction motor

This paper studies the use of only the stator current of an induction motor to observe its speed and rotor current and identify its rotor resistance. Based on the law of Lyapunov-stability, the observer and the identifier are realized by introducing a variable structure term, which will counteract the uncertain terms and ensure the good performance of the observer and the identifier. When the observer and the identifier work, alone, their errors will converge to zeros; when they are incorporated together, even their errors cannot be ensured to converge to zeros. The theoretical analysis demonstrates that the destruction of the good performance, the convergence without steady errors, results from the essential coupling of the speed and the rotor resistance; moreover it is impossible to design the incorporated observer and identifier whose steady errors all are zeros. The simulations show the validity of the theoretical analyses on observing the speed and identifying the rotor resistance