Influence of Rotor Eccentricity on the Reliability of the Pole Drop Test for Salient Pole Synchronous Machines

The pole drop test is the most common test for detecting inter-turn insulation failure in the rotor field winding of salient pole synchronous machines. Shorted field turns are detected by comparing the voltage drop across each rotor pole with ac voltage applied to the field winding terminals. It is a simple and effective test that is usually performed with the rotor inserted for convenience and high sensitivity. In this work, the influence of rotor eccentricity on the pole drop test is analyzed. It is shown that anomalies in the airgap can interfere with the pole drop test to cause false positive indications. Alternative test options immune to false indications are also suggested in this letter. The analysis and test options for screening false indications due to eccentricity are verified on 4 and 6 pole, 5, 5.5, and 30 kVA synchronous machines under emulated fault conditions. It is shown that eccentricity interferes with the pole drop test and that testing under dc excitation is immune to eccentricity although the sensitivity is lower, which are not reported elsewhere.