A class of speed sensorless sliding mode observers for direct torque controlled induction motor drives

A family of speed-sensorless sliding-mode observers for induction motors has been developed. Three topologies have been investigated in order to determine their feasibility, parameter sensitivity and practical applicability, The salient feature of all schemes is that they do not require the rotor speed adaptation that is, they are inherently sensorless observers. The most versatile is a dual-reference-frame observer. The other two schemes, implemented in stator frame and rotor frame, respectively, are simpler and are insensitive to detuning of some of the motor parameters. Main theoretical aspects, results of parameter sensitivity analysis, and implementation details are given for each scheme. Experimental results with the dual-reference-frame observer, considered most adequate for practical applications, are presented and discussed. Sensorless operation at very low speeds is demonstrated. It is concluded that the proposed schemes represent a feasible alternative to classical speed-adaptive observers.