Electricity + Control July 2016

ROUND UP

DRIVES, MOTORS + SWITCHGEAR

Completion of technically challenging project

Marthinusen & Coutts , a division of Actom, was recently called upon to assist Goldfields’ South DeepTwin Shafts when the mine experienced a failure on one of the stators on its main winder installation. The 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. While there are many larger synchronous motors, it is highly unlikely that there are larger squirrel cage rotor machines. The six megawatt, 3,3 Hz, eight pole design of the stator makes it greater than a 90 megawatt, 50 Hz, 750 r/min equivalent, and when one adds in the fact that these are driven by cycloconverters, the uniqueness of the installation is apparent. Following a thorough assessment, 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. The identification of the proposed failure mechanism is both new and invaluable in that it identifies a possible weakness with stators.This 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.The modificationmade 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 im- portant technical assessment by the cycloconverter drive suppliers. There are two identical motors mechanically coupled on the sin- gle 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 required that Marthinusen & Coutts 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. Once this redesign was carried out and verified the way was paved for the design change to be done.Testing was done at various set stages during themanufacturing process and included dimensional checks, high voltage checks, dissipation factor tests, impulse inter- turn tests. Despite the 1 470 volt rating of the motor, all coils were tested as if they were well in excess of 3 300 volts 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 some 8 000 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. The new stator coils are also arguably the largest set diamond multi-turn coils manufactured in SouthAfrica and on the continent. To add to this challenge resin rich technology was used. Resin rich technology is a challenge to implement even in small size a coils, so producing successful coils of these proportions is an achieve- ment in itself. Enquiries: Richard Botton.Tel. +27 (0) 11 607 1700 or email richardb@mandc.co.za

The 65 ton stator mass driving a squirrel cage induction motor very probably makes this the largest motor of its type in the world.

Richard Botton, divisional chief executive officer at Marthinusen & Coutts, Michael Olivier, engineering manager at South Deep, Chris van Heeswijk, electrical consultant to South Deep and Rob Melaia, engineering executive at Marthinusen & Coutts.

Electricity+Control July ‘16

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