Sensorless control of IM drives

This chapter addresses the fundamental issues of sensorless vector control of induction motor drives. Starting from an historical overview of induction motor control in general, we revisit the two archetypical flux estimators known as the current model and voltage model, as well as their combination into a reduced-order observer. It is demonstrated not only that how speed estimation can be added to the observer but also how the flux estimator can be made inherently sensorless, i.e. the rotor speed no longer appears in the estimator equations. It is shown that all inherently sensorless flux estimators that are based on the reduced-order observer resemble a variant of the voltage model, called the statically compensated voltage model. Finally, theory is developed whereby the coefficients of the inherently sensorless flux estimator can be selected so that stability is obtained for all operating conditions, called complete stability. This includes the low-speed regeneration region where often instability phenomena tend to occur.