ENEL's experience with on-line diagnosis of large induction motors cage failures

The experience gained by ENEL on monitoring large induction motor's cage condition is reported in this paper. The diagnostic procedure is based on the motor current signature analysis (MCSA) and in particular on the two sideband current components near the frequency fundamental line that appear in the current power spectrum when a rotor bar/ring breakage occurs. According to the developed procedure, a diagnostic index obtained from these components is stored and its trend as a function of time allows detection of the occurrence of a failure in most cases. This event is clearly shown by the overcoming of a prefixed and triggered threshold. Moreover, machines with particular rotor magnetic structure are considered. In this case unexpectedly high sideband components appear even in the presence of healthy cages, and the test procedure was adapted to account for these conditions.