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Electrical fluting

Electrical fluting – Bearing failure

Electrical erosion refers to the removal of material from bearing contact surfaces by unintended electric current or voltage. There are two distinct types of electrical erosion: fluting and pitting. Electrical fluting is often called current leakage due to the common cause of current ‘leaking’ from a generator. Stray current is common in motors and generators controlled by a variable frequency drive system. This leads to generator bearings, both rolling elements and raceways, being particularly prone to fluting. Small craters, caused by current flowing through the bearing, develop into the dark brown/grey equally spaced flutes. This happens due to the mechanical resonance vibration caused by the mechanical stress as the Hertzian contact zone passes over the small craters. Ensuring adequate grounding and using insulated bearings where necessary can help protect bearings from electrical erosion. Although electrical erosion is a severe failure mode, it is not common in wind turbine bearings.

Also referred to as: Current Leakage, Electric Current Damage, Current Erosion, Spark Erosion, Washboarding

Causes

Electrical erosion is caused by unintended currents flowing through Hertzian contact points between the rolling elements and raceways of a bearing. The unintended currents are often generated due to asymmetries in the magnetic field of an electric generator causing a potential difference. Rotating parts other than the rotor may also unintentionally intersect with the magnetic fields. As the current flows between bearing components, sparks may create many small craters by removing material. The material is removed due to localised melting before being vaporised or washed away. Over time, the initial micro-craters progress into fluting due to the mechanical stress as the Hertzian contact passes over the micro-craters. The subsequent mechanical resonance vibration occurs which causes the fluting marks. The properties of the material around the localised melting zone are often affected due to tempering/rehardening which can cause points of high stress concentration leading to micropitting or crack initiation.

Appearance

Shallow craters, positioned closely together on the rolling element or raceway are the initial signs of electrical erosion. Over time, equally spaced flutes will develop from the craters running transversely to the direction of travel. The flutes are generally a dark brown or grey colour. Ball bearings typically don’t show signs of fluting but instead experience a dark discoloration, similar in colour to the bottom of the craters.

Progression

All forms of electrical erosion are considered severe, however there can still be some progression. Over time, the initial micro-craters progress into fluting due to the mechanical stress as the Hertzian contact area passes over the micro-craters. The subsequent mechanical resonance vibration occurs which causes the grey fluting marks. This can progress into severe adhesive wear, micropitting and macropitting due to the stress concentration points created by the craters.

Detectability

MethodDetection EfficiencyNotes
Visual inspectionDifficult to observe visually due to access limitations on generator bearings. If the micro-craters are large enough and there is access to other bearings then it may be possible to observe visually.
Borescope inspection✓✓Difficult to observe with a borescope due to access limitations on generator bearings. However, if accessible a borescope will be able to detect electrical erosion.
Vibration analysis✓✓Noise and vibration of high intensity can be an early indication of electrical erosion so it can usually be detected using vibration data.
SCADA dataSCADA data does not usually aid detection of electrical erosion.
Oil debris sensorElectrical erosion does not shed much debris so cannot usually be detected using oil debris sensors.
Oil sample analysisElectrical erosion does not shed much debris so cannot usually be detected using oil sample analysis.

Discussion

Electrical erosion can be a very severe failure mode if found in a bearing due to the removal of material from the critical contacting surfaces and the high intensity vibration usually experienced. Also, the current discharge can damage the lubricant properties causing further damage to components.

To prevent electrical erosion of bearings and other issues, generator manufacturers incorporate a slip ring and brush system to provide a low resistance path to ground for any charge that may build on the rotor shaft. This grounding system may degrade over time if the ground ring becomes oxidized, the brushes become worn or the brush holder spring does not have enough pressure to hold the brush firmly against the ring. Maintenance of the system is critical and often prescribed bi-annually.

Several solutions exist for generators where electrical erosion is a persistent problem. Solutions are listed in order of increasing expense and effectiveness:

  • Improving grounding system maintenance practices and frequency
  • Resurface ground ring
  • Upgrade grounding brush and/or brush holder
  • Replace bearings with aluminium oxide outer or inner ring coating to increase electrical resistance
  • Replace deep grove ball bearings with hybrid bearings that use non-conductive ceramic balls

Severity Rating

RankDescriptionDetectionRecommended Action
1N/AN/AN/A
2N/AN/AN/A
3Equally spaced grey-brown flutes in the transverse direction to travel. Microscopically, densely spaced small craters of dark appearance in the flutes.Borescope, vibrationMonitor regularly for signs of progression or violent vibration and consider scheduling replacement soon. If persistent, improve mitigation against electrical erosion.
4N/AN/AN/A
1
Not applicable
2
Not applicable
3
Example of rank 3 electrical fluting (a bearing failure)
4
Progresses to other failure modes
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