Marthinusen & Coutts, a division of Actom, recently demonstrated its ability to undertake challenging work on rotating machinery. The division was recently called upon to assist Goldfields’ South Deep Twin Shafts when the mine experienced a failure on one of the stators on its main winder installation.
Rob Melaia, engineering and technical executive at Marthinusen & Coutts, explains that the sheer physical size of the stator alone is unique – especially in terms of the challenges of rewinding such a large unit. “The 65 ton stator mass driving a squirrel cage induction motor makes this one of the largest of its type in the world,” he says.
Following a thorough assessment by Marthinusen & Coutts, it was ascertained that the stator insulation had failed prematurely after approximately eight years, when one would normally expect a 30 year service life from this machine. This failure was unusual in that there was no physical external damage and no signs of overheating. Furthermore, with two parallel-connected motors one would expect both to fail if a severe external transient was the root cause. The prevailing root cause remained cyclical stresses on the stator insulation due to thermal expansion and contraction cycles.
Melaia says that the identification of the proposed failure mechanism is both new and invaluable in that it identifies a possible weakness with stators, and this, he says, will allow owners to plan and manage any associated risk.
“As a result of the root cause of failure identification, the stator winding and insulation has been redesigned, with the objective of preventing a similar failure,” he says. “The modification made by Marthinusen & Coutts has resulted in a marginal but non-negligible difference to the performance and this, in turn, led to a detailed and very important technical assessment by the cycloconverter drive suppliers.”
Testing was done at various stages during the refurbishment process. Despite the 1470 V rating of the motor, all coils were tested well in excess of 3300 V nominal rating. This is because the cycloconverter driven stator produces voltage spikes. By overrating the insulation – the reliability and longevity of the winder installation has been improved.
The new stator coils are also arguably the largest set diamond multi-turn coils manufactured in South Africa. Melaia says to add to this resin rich technology was used. “Resin rich technology is a challenge to implement even in small size coils, so producing successful coils of these proportions is an achievement in itself.”
In total some 8000 kg of copper was used for the rewind of this stator whose assembled mass is 65 ton. The completed mass of the fully assembled motor is 98 ton.
For more information contact Richard Botton, Marthinusen & Coutts, +27 (0)11 607 1700, [email protected], www.mandc.co.za
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