Flux observer enhanced with low-frequency signal injection allowing sensorless zero-frequency operation of induction motors

In sensorless induction motor drives, flux estimators based only on the fundamental-wave motor model work well at sufficiently high stator frequencies, but they fail at speeds close to zero. To solve this problem, a new observer structure is proposed, combining a speed-adaptive full-order flux observer with a low-frequency signal injection method. An error signal obtained from the signal injection method is used as an additional feedback signal in the speed-adaptation law of the observer, resulting in a wide speed range, excellent dynamic properties, and zero-frequency operation capability. The enhanced observer is also robust against parameter errors. Experimental results are shown, including very slow speed reversals and long-term zero-frequency operation under rated load torque, as well as rated load torque steps and fast speed reversals under rated load torque.