Electricity + Control March 2018

round up

DRIVES, MOTORS + SWITCHGEAR

High-speed induction motor rotor refurbished

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ACTOM division LH Marthinusen (LHM) , a leading repairer of transformers and large rotating machines and manufacturer of specialised transformers, recently conducted extensive refur- bishment of a rotor from a 8 800 kW high-speed 6 000 rpm in- duction motor used in a power generation application. The ageing motor, which drives a pump feeding water to a boiler in the power plant, was brought in for assessment at LHM’s main repair facility in Denver, Johannesburg, earlier last year after it had failed. Full non-destructive tests were carried out on all the rotor’s components and it was found that the insulation had degraded, requiring a total rewind. The original mica-based insulation was replaced by more mod- ern, efficient and durable Nomex-based material. Additionally the rotor’s winding heads were compressed with resiglass binders to isolate the damper from the excitation winding whereupon the damper winding arrangement was refitted. The fan hub assemblies were replaced by new manufactured units as the original assemblies were found to have minute cracks. The original fan assemblies were first 3D-scanned and new units were manufactured from the scanned data using five-axis machining technology. In order to establish the integrity of the rotor body structure a complete Finite Element Analysis (FEA) was undertaken. Finally the rotor underwent a high-speed bal- ancing test to confirm its operational integrity. The contract was completed late last year. Throughout the refurbishment process the high precision de- sign specifications had to be adhered to ensure complete integ- rity of the finished product. All the tests, encompassing the initial non-destructive test with subsequent balancing and FEA procedures were essential to ensure that the repairs and refurbishments conducted, con- formed to the close tolerances required for this high speed ap- plication. Enquiries: Boris Breganski. Email borisb@lhm.co.za

12 Electricity + Control

MARCH 2018