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A. Jagenbrein:
"Investigations of bearing failures due to electric current passage";
Bericht für SKF Science Report, Development Centre Steyr; 2005; 11 S.

Bearing damages due to electric current passage, such as circulating currents and electrostatic
discharges, through rolling bearings in induction motors and driven machines are known for a long
time. But the use of frequency converters for the control of the rotational speed has additionally
given rise to bearing failures. Depending on the electrical set-up of a system comprising a frequency
converter and a driven induction motor, additional effects, such as high-frequency dv/dt-currents,
currents in the protective earth (PE) wire and electric discharge machining (EDM) may also be
responsible for bearing damages. Electric current passage through rolling bearings happens in the
contact areas between the rolling elements (e.g., bearing balls or cylindrical rolling elements) and the
inner rings and outer rings, respectively, where it may damage the surfaces of the metallic
components of the bearings as well as the lubricants. Although the study of these phenomena is of
great interest, they have not always been distinguished clearly in the literature. An overview of
parasitic currents, especially those generated in frequency converter driven motors and machines,
and their sources according to recent literature and own research work is given in this work. To avoid
machinery breakdown due to bearing failure, several counter measures against electric current
passage through bearings have been developed, but most of them often allow to suppress, avoid or
interrupt only a part of the existing processes, which may cause electric current passage through
bearings. Therefore, the investigation of electric current passage through bearings and the effects
causing these currents, especially those in frequency converter driven motors, were within the scope
of these work.