Adjustment, measurement and on-line detection of air gap asymmetry in AC machines

The incident of fault conditions, especially bearing defects, in modern drives has increased. Bearing defect and the resulting asymmetry of the airgap is considered most difficult to detect especially under inverter operation. In order to develop and test monitoring methods it is necessary to carry out extensive measurements at various kinds and levels of eccentricity. As the air gap length of modern induction machines is in the range of a few tenth millimeters a very accurate mechanism to adjust any kind of eccentricity is needed. In this paper a test and measurement setup is described to accurately adjust and verify airgap asymmetries as well as to detect asymmetries under inverter operation. It is based on a set of eccentric rings to be placed in the bearing housings as well as on the rotor shaft. It turned out during the measurements that any industrially manufactured machine has a build in asymmetry in at least one of the different parts construction. As a result, it is still absolutely necessary to calibrate each single combination of machine housing and rotor even using very sophisticated production techniques. This verification is done primarily by a digital camera placed at different spots along the circumference of the air gap. Once the inherent asymmetries of the housing, lamination, and rotor are identified, this time consuming measurement can be replaced by a set of laser sensors placed at both sides of the machine at the rotor shaft. The performance of this setup is demonstrated by a sensitivity analysis of an asymmetry monitoring method. The analysis is done on a two pole induction machine. The monitoring method is based on a exploitation of the machines transient current slope due to the switching of the voltage source inverter.