Zim gold mine gets a mill motor repair

Freda Rebecca gold mine has the largest AC electric motor operating in Zimbabwe which drives a reduction mill; this synchronous motor was recently refurbished by Marthinusen & Coutts.

Estimated to be about 30 years old, the 2 150kW synchronous mill motor suffered a failure caused by its age and metal fatigue. The main hub of the electrical rotor component had sheared off.

Following preliminary repair work undertaken by the mine, Marthinusen & Coutts was called to site by the mine to investigate further and to provide a long-term solution. A full assessment undertaken by the Marthinusen & Coutts team resulted in the motor being brought back to the division’s facility in South Africa.

Image credit: Marthinusen & Coutts

It was decided, following negotiations with the mine, to do a design modification instead of replacing the hub. Richard Botton, divisional CEO of Marthinusen & Coutts, explains that this option was chosen as it will save costs and reduce the lead time without sacrificing the performance of the refurbished motor.

“Our team’s experience and understanding of large rotating machinery enabled us to offer an alternative to the mine that would ensure optimum reliability going forward. One of the solutions is to use the existing rotor superstructure with a bolt-on hub on the motor,” Botton says.

In addition, 18 main pole coils were replaced; these were manufactured by Marthinusen & Coutts at its ISO 9001 accredited facility in Cleveland, Gauteng.

A complete rewind of the stator was necessary due to the extensive damage caused during the failure at the mine. Fortunately, the mine store had a spare set of coils, manufactured 20 years ago, and these were rewound at Marthinusen & Coutts in South Africa.

A compete upgrade and modification to the sleeve bearings also brought these back to OEM specifications.

“The ability to implement enhancements during the repair of critical electrical rotating machinery such as this motor is a major advantage to the mine as it allows for the latest technology and enhancements to be included in the final product solution,” Botton says.

The scope of work included the on-site commissioning of the motor which was facilitated by the Marthinusen & Coutts Zimbabwe operation with technical support from the division’s centre of excellence in Johannesburg.


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