The ability to implement enhancements during the repair or refurbishment of critical electrical rotating machinery is a major advantage to end users. Through the level of skills and expertise that resides within this leading electrical machinery service provider, Marthinusen & Coutts, a division of ACTOM (Pty) Ltd, has established a strong reference base and reputation for this.

A recent project which showcased this capability was the rewind and refurbishment of three gearless mill drive rotor poles for a platinum mine in the North West Province. The enormous 17.5 MW 4 220 Volt 2 531 Amp 12 r/min 5.8 Hz grinding mill motors, that are 15 metres in diameter, are driven by cycloconverters which supply the motors with a variable frequency to control their torque and also allow the speed to be controlled for optimum metallurgical processing.

Rob Melaia, engineering and technical executive at Marthinusen & Coutts, says that these are of the most critical large special high profile motors in the world. Interestingly, there are only two OEMs worldwide that manufacture these enormous machines which have an assembled mass of 1 000 tons each.

“The rotor poles were subjected to overheating due to an operational error, and we were approached by the mine to assist,” Melaia says. “While this contract could be seen as a simple procedure, we believe it is the immediate access to a large local service provider with OEM capabilities and backup that led the customer to award the contract to Marthinusen & Coutts instead of to the international OEM.”

“It is always critical for operations to have this level of support available, providing skilled technical assistance with very quick response times,” he says. 

The removal of the poles required a two day shutdown during which time Marthinusen & Coutts assisted the mine maintenance personnel with this task. The poles were delivered to Marthinusen & Coutts’ Cleveland facility where thorough investigative work was undertaken on the defective poles to determine the extent of the damage caused by the overheating.

“The second part of this activity was to investigate the best method to improve and partially rectify the surface insulation of the remaining poles of which there are 60 in total,” adds David Motloung, design engineer at Marthinusen & Coutts.

Marthinusen & Coutts made use of a two part epoxy spray treatment and repeated insulation resistance tests under extremely wet coil conditions.

David Motloung, of Marthinusen & Coutts says this was to simulate the worst case conditions on site with the ultimate purpose of evaluating a method devised by Marthinusen & Coutts to improve the insulation of the remaining poles still fitted to the machine. “This solution can be implemented in-situ negating the need to remove the poles to improve the insulation,” he explains.

Motloung says that after studying the design of the pole coil it was decided to use a different conductor for the rewind as this would increase the insulation integrity. Marthinusen & Coutts conducted detailed investigations and extensive tests, especially in terms of heat dissipation with the thicker insulation on the coils. Test results confirmed that the thicker coil insulation did not cause it to run at a higher temperature due to the reduced heat transfer, and there was a negligible impact from the reduced copper cross sectional area.

Upon the successful completion of the first coil, Marthinusen & Coutts was awarded the order for another two severely overheated coils. This was secured despite the pressure applied by the international OEM on the end user, who did not believe that a local service provider would be able to implement a quality refurbishment.

“The end user’s decision to award the additional rewind and refurbishment work to Marthinusen & Coutts is testimony to the trust in Marthinusen & Coutts which is due to providing innovative, high quality, technologically sound solutions for all the mine’s rotating machinery over many years,” Melaia says.

With increased pressure on mining operations and beneficiation plants, it is critical that suppliers remain focused on responsiveness and flexibility, and we believe this to have always been the case with our services,” Melaia concludes.