Assessment of remaining insulation life of high voltage induction motors

Various insulation tests were performed on the aged polyester/mica insulation of 3-6 kV rating motors. About 100 machine samples have been retrieved after 11-22 years service in the field and tested before rewinding. The tests include measurements of insulation resistance, polarization index, dissipation factors, partial discharges, and AC breakdown voltage. The insulation resistance and polarization index tend to decrease with operating time, while the dissipation factor tip-up and partial discharges show no clear tendency. The breakdown voltage shows gradual decrease with operating time. A good correlation is found between the remaining breakdown voltage and a new index log (R _d/R_w), where R_d and R_w are the insulation resistances measured under dry and wet conditions, respectively. Using these test data and a modern software tool, the authors have developed a prototype expert system for a diagnostic consultation