A survey on speed estimation for ensorless control of induction motors

The speed controlled AC drives without mechanical speed sensors have the attraction of low cost and high reliability. The sensorless control techniques require of the estimation of internal state variables of the machine like the linkage fluxes and the rotor speed. Generally, the estimation of the rotor speed is achieved through only the measurement of voltages and currents. Robustness of the open loop speed estimators is poor due the parametric variation therefore, to improve the robustness against the parameters and signal noise, the researches employ closed loop observers for the estimation of the state variables as well as the on-line identification of the motor parameters. However, it is well know in the literature on sensorless control that operating the motor at low frequency is challenging and that the design for such case will have to exploit parasitic effects of the machine, that furnish additional information on the state of the motor, like the rotor slot harmonic voltages. This paper presents a digest of some solutions recently reported to improve the performance in the speed estimation and some results about the observability of the induction motor in sensorless conditions