Steady-state oscillation and stabilisation of variable-frequency invertor-fed induction-motor drives

Abstract
The steady-state stability of induction motors fed from variable-frequency invertors is investigated by a linearised analysis which is an extension of that given by Rogers, and experimentally from light-load tests on 30hp and 50hp machines fed from d.c.-link invertors over a frequency range of 0–40Hz. Attention is centred on stability boundaries in terms of the d.c.-link voltage and the invertor frequency—the control variables of the drive. Computational procedures are outlined, and necessary and sufficient conditions for stability at a given frequency are established. The results of the analysis allow a choice of machine parameter for stable operation. However, this can lead to special designs, and a preferable method for stabilisation which allows the use of standard 50 Hz induction motors is the use of feedback. Two simple types are investigated, one employing feedback to the d.c.-link voltage and the other to the invertor frequency. The latter is found to be more satisfactory. Theoretical results are obtained by recasting the system equations and employing the methods used previously to test stability. Experimental results are given demonstrating that the previously unstable 30hp and 50hp drives can be stabilised successfully by this means.