Output feedback control of current-fed induction motors with unknown rotor resistance

On the basis of a third-order reduced model of an induction motor (current-fed) the authors design an output feedback control (from rotor speed measurements) which guarantees global exponential tracking of speed and rotor flux modulus reference signals. An adaptive version is designed when load torque is constant and unknown. The rotor resistance, which is a crucial parameter for the control, is updated by a seventh-order dynamic estimator designed on the basis of speed, current, and voltage signals. The estimator provides exponentially convergent estimates in physical operating conditions. A good performance of the adaptive control algorithm using a sampling time of 0.5 ms is documented by experimental tests. Experiments show that the main advantage of the proposed control with respect to the classical field oriented control algorithm is the decoupling of speed and flux tracking; in addition, efficiency is improved in presence of rotor resistance variations