Offline Monitoring of Airgap Eccentricity for Inverter-Fed Induction Motors Based on the Differential Inductance

The monitoring of airgap eccentricity for induction motors is important for guaranteeing motor reliability, since eccentricity can eventually lead to motor failure due to stator-rotor contact. However, there are many limitations to applying existing test methods such as the motor current signature analysis in the field, particularly for variable frequency applications. In this paper, a new inverter-embedded test approach for detecting eccentricity for induction motors is proposed. The main concept is to use the inverter to perform a standstill test whenever the motor is stopped, to extract information on motor eccentricity. The motor is excited with a low-frequency pulsating ac field superimposed on different levels of dc fields, and the variation in the differential inductance pattern due to the change in the degree of magnetic saturation caused by eccentricity is observed. An experimental study on a 7.5-Hp induction motor under controlled eccentricity conditions verifies that the sensitivity of eccentricity detection can be significantly improved compared with existing methods.