Real-Time Stability Index for Open Loop Controlled Synchronous Permanent Magnet Motor Drive

Open loop controlled AC motor drives incorporating permanent magnet (PM) machines have received a limited attention over the past years. This was mainly due to instability problems and loss of synchronism of the PM motor with sudden changes in the applied load or supply conditions. This paper investigates the stability of a previously-reported open loop drive system with the aim of obtaining a stability index that not only capable of detecting the system stability boundaries but also computationally efficient for real-time applications. A linearised model of the drive system is initially obtained and the stability boundaries of the drive system are then defined by applying Liapunov´s method to the resulting time-invariant linear system. The results from computer simulation indicate that the first principal minor of Liapunov´s function is of a distinct sign change whereas other principal minors remain positive in the region of interest. Such findings together with the relatively small computation time of the first principal minor of Liapunov´s function, offers the possibility of using it as a real-time stability index. The obtained results from computer simulation are found to be in agreement with the previously-reported observations and experiments on the stability boundaries of the drive system under study