Marthinusen & Coutts 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 quite unique, especially in terms of the challenges of rewinding such a large unit.
“The 65 ton stator mass driving a squirrel cage induction motor very probably makes this the largest motor of its type in the world,” he says. “While there are many larger synchronous motors, it is highly unlikely that there are larger squirrel cage rotor machines.” He adds that the 6 mW, 3,3 Hz, eight-pole design of the stator makes it greater than a 90 mW, 50 Hz, 750 r/min equivalent, and that when one adds in the fact that these are driven by cycloconverters, the uniqueness of the installation is apparent.
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. “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 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.”
There are two identical motors mechanically coupled on the single winder so these have to share the load equally, requiring each of the cycloconverter variable speed drives to load the motors equally. The design change proposed by Marthinusen & Coutts required that they and the international drive OEM establish whether the load sharing would still be possible with the design modification so as to ensure that this would not impact on the reliable performance of the winders.
Testing was done at various set stages during the manufacturing process and included dimensional checks, high voltage checks, dissipation factor tests and impulse inter-turn tests. Despite the 1470 V rating of the motor, all coils were tested as if they were 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. Detailed measurements were taken to ensure absolute accuracy in coil manufacture, and these were manufactured to extremely stringent standards which were dedicated to this particular stator; they exceeded the requisite IEEE and IEC standards.
In total, 8000 kg of copper was used for the rewind of this stator, which had an assembled mass is 65 tons. The completed mass of the fully assembled motor is 98 tons. The new stator coils are also arguably the largest set diamond multi-turn coils manufactured in South Africa and on the continent. Melaia says that to add to this challenge, resin rich technology was used. “Resin rich technology is a challenge to implement even in small coils, so producing successful coils of these proportions is an achievement in itself.”
Marthinusen & Coutts has a long-standing association with Goldfields South Deep Twin Shafts and has successfully completed a number of repair and maintenance projects for this mine. “It is this depth of expertise that resides in Marthinusen & Coutts and our ability to undertake such a challenging project on a fast track schedule that underpins our reputation as the leading electrical rotating machinery repairer,” Melaia concludes.
For more information contact Richard Botton, Marthinusen & Coutts, +27 (0)11 607 1700, richardb@mandc.co.za, www.mandc.co.za
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