The diagnosis and cause analysis of rolling-element bearing failure have been well studied and established in the literature. Failure of bearings due to unforeseen causes have been reported as: puncturing of bearing insulation; grease deterioration; grease pipe contacting the motor base frame; unshielded instrumentation cable; the bearing operating under the influence of magnetic flux, etc. These causes lead to the passage of electric current through the bearings of motors and alternators, which causes them to deteriorate in due course. Bearing failure due to localized electrical current between track surfaces of races and rolling-elements has not been hitherto diagnosed and analysed.
This study explains the cause of generation of localized current in the presence of shaft voltage. It also brings out the developed theoretical model to determine the value of localized current density depending on dimensional parameters, shaft voltage, contact resistance, frequency of rotation of shaft and rolling-elements of a bearing. Furthermore, failure caused by flow of localized current has been experimentally investigated.
Shaft voltages exist in electrical machines as a result of asymmetries of various faults, e.g. winding faults, unbalanced supplies, electrostatic effects, air-gap fields, magnetized shaft and asymmetries of magnetic fields. The causes of shaft voltage can be grouped into four categories:
external causes;
magnetic flux in the shaft;
homo-polar magnetic flux;
ring magnetic flux.
Furthermore, friction between the belt and pulley can set up an electrostatic voltage between the shaft and bearings. Accidental grounding of a part of the...
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