Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation
Stray ground currents caused by an induced magnetic field and running unchecked through a turbine generator set can damage the bearings. Know what signs to look for when inspecting your equipment.
Grounding currents that run through a shaft can damage the bearings. The ground current can be induced from a magnetic field other than the generator. The magnetic field can be caused by many things. Any time something that spins very fast gets magnetized currents are going to be generated. When there is enough magnetism in a rotating element, say a coupling flange, the current that is generated will try to go to ground. Many times that path to ground is through the bearings. A distinctive frosting pattern will be quite visible. The turbine shaft of a turbine generator set is shown to the left. If you look really hard you can see a washboard pattern that travels across the shaft. The pattern is quite noticeable to the left side of the shaft. A close up view of that pattern is shown in the picture below.
|The level of induced magnetism can be measured by a Gauss meter. The unit of measurement is the "Gauss". Typical levels in a machine may run between 5 and 10 gauss, with 10 being on the high side. The desirable level is 2 gauss or below. When the level gets up around 20 gauss there is too much magnetism in the unit. More than likely the grounding brushes or straps will be handling their maximum capacity. In the upper left picture the gaussmeter showed a level of 10 gauss several inches away from the shaft flange. A different meter measured over 20 gauss at the flange surface. The magnetic field can be eliminated or significantly reduced by inducing a counter magnetic field. The industry term is called "Degaussing", or simply put, de-magnetizing. In the upper right picture a coil is wrapped around the area needing treatment and a high amp (1000 to 6000 amps) low voltage (3 V) current is run through the coils. A close up is shown to the lower left. It doesn't take long to demagnetize the area.
|The picture in the upper right shows what the shaft looked like with a bad case of etching from the stray magnetic currents. If left unchecked the etching deteriorates the shaft surface. Needless to say the babbitt bearings deteriorate even faster. Grounding brushes and straps help to deplete the stray currents caused by the induced magnetic field. They should be the last resort. Periodic checks with a Gauss meter will help to prevent the magnetism from building up too high and damaging the bearings.
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