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Case Study:

Corona Findings on A Stator Motor After Repair

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A 40 years old stator, used by an oil drilling company in Norway, was refurbished by a leading repair and overhaul workshop in Scandinavia. Extensive experience of working on the modification and upgrading of motors has shown that old repaired machines are often better than new machines, provided the repair is comprehensive and thorough. To guarantee the quality of repair, a new testing method was tried at the wrokshop, namely corona testing with DayCor® camera. The experts at the workshop were surprised to see the unexpected dishcarges that the DayCor® camera revealed moments before the stator left the floor. The DayCor® pinpointed the exact location of the discharges, and proved again that a second opinion of an expert, or in this case of an equipment, is worth implementing.

In April 2013 Ofil's representative visited a leading specialist on electro-mechanical machinery, company in Norway whose core business is engineering, installation, monitoring and maintenance of electro mechanical components, in order to introduce the DayCor® corona cameras. Coincidentally, a stator-motor that belonged to a drilling company was repaired, tested and ready to be shipped back to the customer.

Ofil's Representative suggested demonstrating the capabilities of DayCor® corona cameras by using a camera to look at that stator. It was claimed that if there is a discharge it will be detected, pinpointed and displayed clearly and instantaneously. Such findings can ascertain the need for corona cameras as non-destructive and non-intrusive faults detectors. The inspection was performed with Ofil's DayCor® Luminar showing discharge form certain windings, as displayed in the following video clip.

The inspected stator was made by Electromotors LTV in 1973 and though its estimated cost was 10M USD its real value was higher. Apparently, according to motors specialists, older machines have more robust mechanical construction, are generally made from cast iron, their coils are often wound using thicker earth insulation, and the cables from the coil are connected to the plugboard using a flexible cable. Furthermore, it is claimed that older machines often have a lower operating temperature and therefore a longer service life. Consequently, overhauling old machines is justified as this will extend their service life and reliability to even longer than new machines.

About the testing procedure

The motor was connected to HV power generator raising slowly the voltage from 0kV. The inspector was standing on a ladder holding the DayCor® Luminar bi-spectral corona camera looking down at the stator's coils. Corona was incepted at 4 kV. The camera pinpointed discharges in several locations with values that are considered as medium-severe.

Extending the lifetime of rotating machines

Please refer to our review in July 2012 on corona on rotating machines.
When corona in detected on rotating machines it indicates failures that are related to: insulation, coils defects, contamination on slots exit, slots sections, connections, end-windings. To extend the lifetime of rotating machines, companies that repair and overhaul rotating machines:

  • Attach the coil head and outgoing cables using strips or alternatively perform wire wrapping, because strips become brittle and loosen, causing damage to the machine
  • Dip varnish new machines or use vacuum pressure impregnations
  • Rewind coils

Ofil recommends a 4 step solution for proper maintenance and repair on rotating machines:

  • Corona inspection: check for corona with a DayCor® UV camera
  • Naked eye inspection: look at the locations of the emitted corona in order to find possible defects and suspicious failures
  • Repair
  • Document & Recheck for corona: after repair recheck for corona signs to ensure the problem is solved and document findings for trending and for supporting a predicitve plan

Companies try to focus on cost-effective solutions that will improve their performances. With critical components, such as rotating machines, unpredicted breakage and down time often result in disrupted production and significant financial losses. Ofil offers NDT monitoring technology to prevent critical rotating machines from braking down and assure stable performance. Adopting Ofil's suggested methods together with a systematic predictive maintenance plan will result in stable operations and major financial savings.