Stability analysis and improvement of V/Hz controlled adjustable speed drives equipped with small DC-link thin film capacitors

Due to thin film capacitor's enhanced reliability and improved input current harmonic performance, adjustable speed drives (ASDs) without using electrolytic dc-link capacitors have been researched extensively in recent years. However, because of challenging stability and motor control issues associated with small thin film based dc-link capacitors, in depth studies on a V/Hz controlled ASD cannot be found in publications. In this paper, a state space model, taking into account the dc-link choke, the thin film dc-link capacitor as well as the inverter switching dead-time effect, is developed to analyze the stability of such a system. Instability is quantified analytically. A novel active damping method is proposed to improve the overall system stability. The accuracy of the model and the effectiveness of the active damping method are verified through experimentation using a 3% thin film capacitor on a 2.2kW, 380V, 50Hz setup.