Electrical Discharge Induced ESP Motor Bearing Failure

Motor bearings are a critical component in electric submersible pumping systems (ESP) motors. In many cases, dismantle analyses are unable to identify a definitive root cause for bearing failure. Several hypotheses have been studied to explain motor bearing failures. Some applications experience a higher bearing failure rate than other applications. In this paper, typical motor bearing failure modes will be reviewed, more specifically, the source of shaft voltage and the consequence of bearing electrical discharge failure will be discussed. During bearing failure root-cause analysis, mechanical components have been the primary focus. Critical thermal expansion coefficients have been verified. The bearing running stress, temperature, eccentricity, film thickness, and lubrication flow have been simulated using cutting-edge finite element analysis (FEA) and computational fluid dynamics (CFD). Results show that in certain material combinations, the incompatibility of the thermal growth of bearing and sleeve material could reduce the running clearance, which would then increase the oil shear loss, and the bearing rubbing. However, tiny pitting has been found in the outside diameters (OD) of cracked bearings returned from the field, with more pitting found on the sleeve ODs. This evidence indicates another bearing failure mode: shaft-induced voltage and bearing electrical discharge. The direct consequence of electrical discharge is generation of debris, vaporization of motor oil, quenching of bearing/sleeve surfaces, and increase in surface roughness. The debris size (0.001 in.) is larger than the hydrodynamic film thickness and can score the sleeve surfaces due to the loss of oil film protection. The chain of the motor bearing failure due to the electrical discharge is summarized, and this cannot be ruled out in other sizes, bearing systems, or material combinations in an ESP motor. By duplicating the poor power quality conditions, the pitting phenomenon on sleeve is confirmed. Future measurements have been planned to determine the correlation between the power quality and shaft voltage. This paper discusses the risk level for the bearing electrical discharge based the induced shaft voltage. Electrical discharge bearing damage is a widespread problem that has been studied in other industries since 1992. Electrical discharge / shaft voltage problems can occur on any variable-speed drive motor. The source of electrical discharge through the bearing comes from the voltage potential building from the shaft to the ground (motor housing). However, ESP industries, which produce small motors with multiple rotor sections, have not conducted sufficient analysis to understand this problem. This paper fills the gap to discuss detailed indication evidence and analysis that can be used as a toolbox for motor bearing failure analysis for the ESP industry.