Electricity and Control February 2020

PLANT MAINTENANCE, TEST + MEASUREMENT

At a glance ■  When a 6-tonne induction motor was sent to LH Marthinusen for a general overhaul, inspection revealed otherwise unseen damages that indicated material failure on the rotor.This meant the rotor core had to be unstacked and the rotor re-cored, as part of the repair and maintenance work. ■  From a comprehensive analysis of the rotor component materials, it was concluded that the failure had resulted from stress corrosion cracking.

composition are not publicised, no determination could be made on the lack of phosphate in the fractured material. Based on the analysis and test evidence it was concluded that the failure mechanism of the material that led to the cracking of the lamination plate was stress corrosion cracking (SCC). The three conditions required for stress corrosion cracking were evident in this investigation with (i) the stress caused by the outward radial force (centrifugal) due to the spinning rotor body; (ii) a corrosive environment (sodium found in the cracks) and (iii) susceptible material (deterioration of the coating exposing the alloy material to the environment). Finally, it is noted that the presence of sodium hydroxide in certain chemical plant processes creates a corrosive environment for certain materials and may result in SCC material failure mechanisms. The rotor was re-cored using the highest quality coated lamination steel available and returned to service.

ACTOM (Pty) Ltd offers manufacturing, service, repairs, maintenance, project management and distribution of electro- mechanical equipment and is one of the largest service providers in its sector in Africa.It operates through 35 outlets across southern Africa. ACTOM is also a major local supplier of electrical equipment, services and balance of plant to renewable energy projects and it holds numerous technology, distribution and value added reseller agreements with various partners, both locally and internationally.

Image 2: When the rotor core was unstacked, cracking of the sheet material was observed on both the ventilation finger plates (shown here) and the coated lamination steel (Image 3).

Image 1: It was observed that some of the ventilation finger plates were protruding from the surface.

Image 3: The coating of the lamination plate was discoloured in some areas (as shown in the top sample).

Image 4: The metallographic examination revealed that the cracks were intergranular, with a branched appearance.

Electricity + Control

FEBRUARY 2020

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