Robust rotor flux and speed control of induction motors using on-line time-varying rotor resistance adaptation

A novel scheme is proposed for direct speed and flux control of induction motors using on-line estimation of the rotor resistance. The stator current/voltage and the rotor mechanical angular speed are assumed to be measured. The design is based on the variable structure theories. The overall controller combined with on-line rotor resistance adaptation is implemented by using rotor flux and electromagnetic torque observers and the exponential convergence of these observers is demonstrated. The originality of the work is that it considers the unknown rotor resistance as time-varying. The convergence of the estimated rotor resistance is achieved using a Lyapunov-like technique and this guarantees the stability of the rotor speed and flux tracking. Simulation results show the quick convergence of both estimated rotor resistance and tracking of the rotor speed and flux. The robustness analysis revealed that the direct speed and flux tracking schemes are insensitive to the stator/rotor resistance variations (up to 50% for R-s and 100% for R-r) and to stator currents measurements noises. The proposed method can also be applied to failure and fault detection since it is easily implementable in real-time.