Rotor Flux Estimator Design with Offset Extractor for Sensorless Driven Induction Motors

In sensorless drive of induction motors (IMs), the design of a voltage model (VM) for flux estimation is challenging because of its inherent problems with DC saturation and drift. In the implementation of VM, various strategies have been studied in flux estimator design. The closed-loop (CL) style flux estimator is a popular scheme owing to its good performance in a wide speed range. However, CL style suffers from DC drift problems because of an offset at the back electromotive force. Based on the fact that DC drift always persists in CL style flux estimator, this study proposes an offset extractor (OE) using a parallel structure of two CL style flux estimator. Moreover, a crossover strategy is applied to the comparative flux in CL scheme. This strategy guarantees low speed operation including zero speed. When the proposed OE is applied to the flux estimator in a feedforward manner, the proposed flux estimator exhibits good performance in a wide speed range with robustness to the offset. The proposed OE and flux estimator are compared with conventional methods, and are verified in various situations by simulation and experiment in a sensorless driven IM.