When a major player in the gold mining industry wanted to reduce the high cost of electricity required to run the shaft ventilation fans at a particular site – more than R90 million per annum – as well as reduce the cost of maintaining the existing slip ring motors, it approached Ana-Digi Systems for a solution. Management felt comfortable entrusting the project to Ana-Digi, as it had already implemented several similar conversions for the mining group. As a solution, Ana-Digi designed and implemented a new drive system based on LS Electric medium-voltage VFDs.
Solution design and implementation
The site has three 2,4 MW ventilation fans, two of which are duty fans, with the third being a hot standby unit. To drive the fans, the existing system utilised slip ring motors, which were run up to speed by using liquid starters. However, the slip rings needed regular brush maintenance and were prone to arcing across the brush-gear. In the new system, the existing motors were converted to operate as standard squirrel cage units by bypassing the slip rings. All three fans were then equipped with LS Electric VFDs, each with its own air-conditioned e-house, vacuum circuit breaker and fire detection system.
The reduction in energy consumption was immediately obvious. Maximum demand during starting was no longer a factor, with the mains current draw rising smoothly from zero to the reduced operating amps drawn by the motors. Underground air flow and air quality were then checked by the mine’s ventilation experts to ensure that all necessary standards were met.
Once the site acceptance tests had successfully been completed, a service level agreement was drawn up to cover routine cleaning and checking of the various system components. One spare power module was provided for each VFD to cover any possible failure, even though the drives have an MTBF of 100 000 hours and no immediate problems were envisaged. In addition, the motors themselves are now subjected to much less wear and tear due to the reduced speed and the drastically reduced starting currents.
Benefits to the customer
Expectations were that greater reliability and control of the fans could be achieved using 15-20% less energy, without compromising the ventilation performance. In fact, the new VFD system achieved better than 40% energy reduction with initial savings closer to 50%. The VFDs were turned up a bit after the 40% savings mark had been settled on.
As things stand, the new system has delivered outstanding improvements in energy consumption, promising to pay for itself in just over one year.
Ana-Digi Systems strove to design and implement a state-of-the-art drive system to match the client’s expectations of improved control and energy saving. Despite Covid-19 induced shipping delays and road transport disruption, which set the project back by about two months, an extremely successful upgrade was achieved at the mine. Delighted with the results, the client company is enjoying success in line with its stated energy-saving objectives and has been rewarded with a system that should pay for itself in just over a year. The success of this project has paved the way for more similar conversion projects in the future.
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