Instability compensation of V/Hz PWM inverter-fed induction motor drives

Open loop V/Hz induction motor drives suffer severe instability problem usually under light load and at low frequency. This problem becomes even worse for large hp drives, high efficiency motors and high PWM frequency operation, and therefore limits the applications of these drives. In this paper, an extensive experimental investigation has been performed to characterize the instability phenomenon. Based on these results, a simple index signal which represents the degree of this instability has been derived by using phase current signals which are usually already available in the V/Hz PWM inverter for other purposes. A PI compensator has been developed to process this index signal and generate frequency and/or voltage adjustments to the existing V/Hz controller. Test results on a 10 hp drive and a 350 hp drive are presented to demonstrate the effectiveness of the proposed compensation algorithm, which has been shown robust to system changes and effective to improve the acceleration and deceleration performance