Droop control based stabilizing method for V/f PWM inverter fed induction motor drive system

Sustained oscillations are often experienced when induction motor (IM) is fed with general-purpose PWM inverter operated in constant V/f mode, especially under light loaded and low frequency conditions. To stabilize the annoying oscillations and facilitate engineering practices of the simple control mode, an enhanced V/f control strategy is proposed following the analysis of the inherent mechanism of the phenomena. In the proposed control strategy, the frequency and amplitude of the output voltages of general-purpose inverter are dynamically dropped with respect to the oscillation components in the stator currents. To extract the right oscillation components, dual low pass filters with different cutoff frequencies are adopted to filter the active and reactive currents obtained by coordinate transformation on the sensed stator currents. The dynamic oscillation component out of the active current is used to drop the frequency and that out of the reactive current is used to drop the amplitude of the output voltages. By the proposed stabilizing method, the operating stability and drive system performances are improved as shown by small signal analysis, while the merits of parameter robustness and simplicity of V/f mode are still retained. The rationality of the analysis and the effectiveness of the proposed stabilizing method in this paper are demonstrated and validated by experiments.