Design of a MRAS-Based Estimator for the Speed and Rotor Time Constant of the Induction Motor Using Sliding Mode

The paper presents a method to estimate the fluxes, speed and rotor time constant of the induction motor (IM) drive. The estimation process is based on the model of the motor in the stationary reference frame. Estimation is done using successive observers that also involve the Model Reference Adaptive System (MRAS) principle. The fluxes are obtained first using a Voltage Model Observer - they are used to calculate the rotor flux angle of the drive. With known fluxes, the paper attempts to design a sliding mode observer for simultaneous estimation of the speed, load torque and rotor time constant. The mathematics of the observer is presented; it is shown that, due to singularity in the required gain design, an asymptotically stable observer cannot be obtained. Then, the paper presents an observer for the rotor time constant - this takes the speed signal as an input. The properties of this observer are investigated and it is shown how to make it less sensitive to the potential inaccuracy in the speed input. The method can be used in a sensorless IM control implementation where the fluxes and speed are required and where knowledge of the rotor time constant improves the control performance. The theoretical claims are supported by the simulation results shown.