Three-phase induction motors are by far the most common prime mover in industry today. Many of these motors, which may be as large as 25MW, are used in critical duties in industries such as oil, petrochemical, utilities, paper, mining and steel manufacture where sudden failure of a machine can have a drastic effect on production as well as safety implications. Prevention of such failures is vital. If a potential failure can be foreseen, the motor can be removed from service at a time which will minimise lost production and make the best use of maintenance facilities. This article describes the seven-year development of an online knowledge-based software program to detect rotor winding faults and mechanical load influences. This non-invasive technique is based on the analysis of the stator phase current spectra.