WEG Variable Speed Drives (VSDs) and WEG electric motors have provided the solution for ensuring optimum operation and control of a man-riding conveyor at Bafokeng Rasimone platinum mine in Rustenburg, South Africa. To boost cost-effectiveness, Anglo Platinum decided to sink two decline shafts at the mine for the handling of material and installed a man-riding conveyor for the transport of personnel and rock. The 1.6 km long belt is 1,050 mm wide and is designed to carry 480 t/h. It is also designed to transport between 2,000 and 3,000 people a day, and to operates 24/7. From the onset, for safety considerations the conveyor belt’s speed was reduced from 2.5 m/s to 2 m/s.
The reliability of operation and the ability to start smoothly and stop safely, especially under emergency circumstances, were critical. A range of operating speeds is also necessary for inspection purposes and different operating conditions. For instance, if personnel are not being transported and production needs to be increased then the running speed needs to be optimised. All these objectives were attained successfully by using WEG VSDs and WEG motors on each conveyor in the North and South shafts.
Designed for this specific application and incorporating all the safety mechanisms, the WEG VSDs are backed by mechanical and hydraulic safety systems. Three fully regenerative active front-end four-quadrant WEG VSDs and WEG motors were installed. These 330 kW, 4 pole motors are WEG Premium Efficiency motors which comply with Anglo Platinum’s energy saving drive. The operating speed differs from the standard 4 pole or 6 pole motor speed and this could only be achieved only through the use of the VSDs.
Starting and stopping control uses a customised ramp profile that minimises stresses in the belt, thus lengthening the belt lifecycle and reducing total cost of ownership over the lifespan of the conveying system itself.
The reason for the four quadrant and regenerative model is that, in practice, there is not always a balance of load between people and ore. So on some occasions, the conveyor will draw power from mains to move the load and, at other times, if there is more weight on the downward side of the conveyor, such as when people are going down when no ore is being transported out, the motors will regenerate power back into the mains.
It was also critical that three motors and a VSD per conveyor share the load equally, so they are run in a configuration known as master and follower.
The control requirements are far more stringent than normal conveyor control because people are being transported on the belt. Safety and reliability were primary concerns. In addition to this, smooth starting and stopping even under unbalanced load and supply voltage conditions were very important considerations.
The installation of the WEG VSDs and motors to reduce the speed to 2 metres per second has now made it possible for people to get on to the conveyor belt, remain seated for the journey and then stand and dismount safely on arrival without incident.