Underground VFD Skid
Arrow Speed has supplied 18 VFD skids for a Canadian Potash mine to control their underground conveyors. Each skid has 2 x 300 hp VFDs and is designed to control multiple motors. Each conveyor will have either 2 or 4 x 250 hp motors and the VFDs are operating in a torque sharing mode. One VFD receives the speed command and the other VFDs follow it in torque mode to evening split the torque between all the motors.
The VFDs are mounted in a custom, severe duty skid enclosure with drag rails on the bottom, lifting eyes on each end and steel bumpers in key locations to protect the structure. The skid is designed to be lowered down the shaft on end and dragged several kilometers to the required location.
To minimize dust buildup in the VFDs, the skids utilize a cooling air plenum in the rear into which the VFD headsinks protrude. Cooling air is drawn through this plenum with a dedicated blower and is filtered on the way in with a series of replacaeble filters.
One of the advantages this system has for the mine is the increased starting torque compared to the traditional softstarts. Because a VFD has a much higher starting torque than a softstart, the mine can incread the ore load on the conveyors and be confident the conveyor will start after an unexpected shutdown.