On-line monitoring of mechanical unbalance/ misalignment troubles in ship alternators by direct measurement of split-phase currents

Synchronous generators (SGs) are vital ganglia of the ship. SGs supply survival/defense/attack systems, propulsion, etc., so deserving on-line monitoring. Rotor unbalances shorten bearing lifetime, with overheating and stator-rotor rubs. No technique is definitely established for on-line evaluation of rotor eccentricities (REs) in working SGs. Actual manual air gap checks are time-consuming, and require unit shut-down and dismounting. This paper shows a method based on measurement of split-phase currents in stator windings with parallel branches. A symmetrical component transformation reverts the split-phase currents in rotating space vectors. The (p±1)-order vectors provide an affordable estimation of REs in 2p-pole machines. Rogowski coils or current transformers can be installed in the terminal box, or on winding front-ends, for current measurement. A LabView virtual instrument (VI) was developed for method application. This paper also describes some tests carried out on board an Italian Navy's ship for on-line monitoring of shaft misalignments in a loaded 2MVA SG. Loads on board ships are usually unbalanced and may change suddenly. However, it is shown that the load-related unbalance can be separated from the eccentricity-related unbalance in the split-phase currents. Good assessment of air gap percentage reduction in the tested 2MVA machine was finally obtained.