Stability analysis and improvements for combined voltage and current mode flux observer of induction machine

Accurate flux estimation is essential in rotor flux oriented speed sensorless induction machine control. The closed-loop of combined voltage and current (UI) mode flux observer obtains good flux estimation results in the full-speed range but has some drawbacks in the stability characteristics. This paper constructs the small signal model of the UI mode flux observer, derives the characteristic matrix to realize the stability analysis, and then calculates its stable range with the corresponding conditions. In addition, a cross-coupling feedback mechanism is proposed to expand the stable range, especially when the machine runs in the low-speed regeneration or plugging-in mode. Furthermore, the cross-coupling feedback coefficient is adjusted by the working torque to realize the stability in the full-speed range. The improvement strategy is well verified by both the simulation in MATLAB and corresponding experimental results.