Induction motor sensorless vector control in the field weakening region

Speed-sensorless control of induction motor drives has attracted considerable attention in recent past, with a vast amount of effort directed towards providing an accurate speed estimate in the vicinity of zero speed. The problem of speed estimation in the field weakening region has stayed to a large extent beyond the scope of interest. While some of the speed estimation algorithms completely fail in this region, the others exhibit a substantial speed estimation error, caused by de-fluxing of the machine and consequent variation in the magnetising inductance. This paper at first examines, by simulation, the speed estimation error that takes place in the field weakening region when a MRAS based speed estimator, of rotor flux type, is applied. Next, a possible solution to the problem of speed estimation under varying flux conditions is proposed. The speed estimator is modified in such a way that the variation in the instantaneous level of the flux during operation in the field weakening is recognised and properly adapted to at all times. Verification of the proposed scheme is provided by experimentation on an indirect feedforward rotor flux oriented induction machine.