Robust Stability Improvement for Speed Sensorless Induction Motor Drive at Low Speed Range by Virtual Voltage Injection

The unstable problem of speed sensorless induction motor drives (SSIMDs) at low synchronous speed range has not been solved yet. In this paper, the reason of this unstable problem is concluded as two key points: 1) there are poles of the transfer function of the SSIMD system owning positive real part at low speed range; and 2) the rotor speed is unobservable at zero synchronous speed. Correspondingly, to guarantee the stability of the SSIMD system at low speed range, the system has to achieve two conditions simultaneously: the transfer function of the SSIMD system is stable (transfer function stability (TFS) condition) and the rotor speed is observable (rotor speed observability (RSO) condition). To satisfy these two conditions, virtual voltage injection is proposed. The unstable problems can be solved by this proposed method. In addition, the robustness of the virtual voltage injection method is analyzed. When motor parameters are inaccurate, the stability of the system will not be influenced. Finally, the feasibility of the proposed method is verified by experiments.