Electricity and Control February 2020

PLANT MAINTENANCE, TEST + MEASUREMENT

Stress corrosion cracking on induction motor rotor ACTOM division LH Marthinusen (LHM), a leading repairer of transformers and large rotating machines and manufacturer of specialised transformers, recently completed a failure analysis and repair on a large induction motor rotor. Boris Breganski, Electrical Engineer at LH Marthinusen, explains the findings of the analysis.

C onstruction materials do not have an infinite life span and in some cases fail prematurely when conditions conducive to this present themselves. One such material failure mechanism is stress corrosion cracking, which can occur in susceptible materials due to a combination of applied stress and a corrosive environment. LHM has recently repaired a 6-tonne induction motor rotor in which stress corrosion cracking had occurred on the laminations. The motor was sent to the LHM works for a general overhaul. On inspection, it was observed that some of the ventilation finger plates were protruding from the surface perimeter ( Image 1 ). The rotor had a core length of 1 850 mm stacked with individual, insulated, laminated steel sheets. The individual laminated sheets were stacked in batches, separated by ventilation finger plates. The laminations and finger plates were individually manufactured from a single sheet and not as a segmented configuration. Based on the manufactured single sheet configuration, the observed plate protrusions could only have occurred if the sheet material had fractured. As there was a high risk that these protruding plates would dislodge and damage the stator during future operation, an agreement was reached with the client to unstack the rotor core. Upon unstacking of the rotor core, cracking of the sheet material was observed on both the coated lamination steel ( Image 3 ) and the ventilation finger plates ( Image 2 ). It was noted that the laminated steel plates with the material fractures were adjacent to the ventilation finger plates. From this it was deduced that the ventilation air contained certain contaminants or gaseous compounds which had reacted with the sheet material, causing the observed corrosion. It was further noted that the coating of the lamination plate (electrical steel) was discoloured in areas below the bottom of the rotor bar slot and around the circumference where it fitted onto the shaft (top sample in Image 3 ). Samples of the fractured lamination material were sent to a metallurgical laboratory for analysis. The metallurgical laboratory undertook the following analyses on the provided samples: - Metallographic examination - Scanning electron microscopy examination

- Hardness test - Coating discolouration test under elevated heat conditions - Material chemical analysis. The hardness test results were compared to a sample of imported lamination coated steel and no abnormalities were found. In addition, the hardness test was done on both the discoloured and non-discoloured areas, with results showing that the discolouration did not affect the material’s properties. A section of the coated plate sample was heated to 150ºC for 30 minutes with no indication of discolouration. Thus it was concluded that temperature did not affect the coating and the discolouration was more a result of a chemical reaction. This was as expected as the coatings have a temperature capability, in air, ranging from 180ºC to 230ºC, depending on the manufacturer. A metallographic surface replica was lifted from one of the cracks and a cross-section was cut from one of the sample plates. The metallographic examination revealed that the cracks were intergranular in nature with a branched appearance ( Image 4 ). The discoloured cross- section revealed that heating had no visible effect on the microstructure which consisted of non-orientated, mostly polygonal, grains of ferrite. The scanning electron microscopy (SEM) examination on a section of material with cracks – and specifically the cracked areas – yielded interesting results. It revealed high levels of sodium (0.41 wt%) within the cracks, which is typically associated with caustic soda (NaOH). The coatings of the lamination plate showed a non-continuous flaky appearance. A chemical (wet) analysis was performed on each sample of the cracked and the imported lamination materials. Comparative results showed that the composition of elements for both the cracked and imported lamination materials were similar. However, the chemical analysis of the coating showed that the imported lamination steel had a phosphate base, whereas the cracked material showed no sign of phosphate. As lamination steel coatings are the proprietary intellectual property of the respective manufacturers and specifications of the chemical

22 Electricity + Control

FEBRUARY 2020