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Stray currents manifest in rotating machinery as a result of specific abnormalities in machines related to flux imbalance, residual magnetism, electrostatic build-up and induced voltages. Discharges of these voltages in components can be very harmful and sometimes catastrophic. Typical damage include pit marks, spark tracks, frosting and electric erosion in components such as bearings, seals and gear teeth. The presentation summarize, typical damage caused by stray currents, diagnostic techniques to identify the problem, remedies to resolve the problem and highlights several case studies.
This is an excerpt from a case study titled, "Stray currents and their damaging effects on rotating machinery" presented by Dian J Hanekom at the 47th Turbomachinery and 34th Pump Symposium.
Stray voltages are the occurrence of electrical potential between two objects that ideally should not have any voltage difference between them;
These voltages can discharge as Stray Currents;
Stray currents manifest itself in rotating machinery as a result of specific abnormalities related to: -
residual magnetism in parts;
electrostatic build-up in machines; and
induced voltages in electrical motors.
The field levels due to residual magnetism in machinery occur not from design, but from manufacturing techniques (magnetic beds), inspection techniques (MPI inspections) or external influences like welding or lightning strikes.
Gauss levels in parts have been measured from 2 to 100’s of gauss. These levels can increase in machines during operation where the magnetic material provides a closed path and reduced air gaps.
Frosting forms a uniform, continuous, satin-like surface, similar to sand blasting or shot-peened surface. It looks very much like a piece of frosted or etched glass, hence the term “frosting” or “spark etching”. This kind of damage can be found on radial and thrust pads.
Spark tracks. May be very fine, like 50 - 125 μm deep, an appearance like if drawn with a sharp needle, or may be as much as 3 mm wide. Tracks vary in length typically 3 mm to several centimetres long. Tracks may also zigzag like lightning strikes. Shining melted surface can be found at the bottom of the track.
Electro-discharge-machining (EDM) also called “current etching”, or “spark etching”, or “spark coining”. The term refers to noticeable hard metal removal by the spark erosion process, example shaft frosting.
An example of EDM is the erosion of floating carbon ring seals, both between carbon and shaft and carbon and carrier ring. Stray currents may form a closed loop between the seal, shaft and bearing housing, which can result in welding marks.
Burned spots, welding, and arcing of contacting parts. These occur at contact areas, for example at the back of pivot points of thrust or tilting pad shoes, resulting in electric erosion and oil vanishing. Also, look for connecting parts on the shaft, example inner- and outer ring of timing gears, coupling gears and thrust collar sleeves.
It is important to note that these current loops cannot be grounded since they are shortcircuited within themselves, they don’t really care whether there is a ground or not. This makes careful de-magnetization of stator and rotor components imperative.
Demagnetization is imperative for new parts and machines with existing magnetism problems. This can be achieved by rapping a coil around the component and using full wave DC demagnetization techniques to reverse the magnetic fields.
Earth Brush Installation is essential for machines that have already developed a problem. The location of earth brushes is dependent on the layout of the machine, bearing type, coupling type, train length and accessibility.