Instability phenomena and remedies in sensorless indirect field oriented control

Sensorless indirect field oriented induction motor drives using information in the fundamental excitation for speed estimation are considered. It is shown that instability phenomena which are not present for sensored operation occur, both for nominal and low speeds. The problems at nominal speeds are remedied by adding a term to the standard slip relation, such that approximate "voltage model" characteristics are obtained. The low-speed problems are remedied by careful choice of the model machine parameters and the torque-producing stator current component. It is also shown how a speed estimation algorithm based on a voltage error can be designed. Its asymptotic properties are similar to those of a current error-based least squares estimator such as the extended Kalman filter. As no state observer is required, the sensorless control system is computationally very efficient, requiring only about 15 multiplications and two divisions per sample for a digital implementation.