The new Cat® VFD-A800/W800 Variable Frequency Drive for longwalls is designed to work in conjunction with Cat UEL and Cat CST gearboxes to deliver variable speed to the armored face conveyor (AFC), very accurate load sharing between gearboxes, and better efficiency when operating in weak power networks. The advanced technology medium-voltage drive controls torque and speed to optimise longwall productivity, reduces wear and breakage of AFC components and makes most efficient use of electrical power.
The VFD delivers optimum start-ups, helps avoid overload stopping and fine tunes AFC speed to match the rate of production. At the same time, the system increases equipment life by minimising stress on drives and chains. Also, the system optimises power usage and even returns energy into the system when motors are used to brake drives.
The Cat VFD can be easily retrofitted to existing Cat face conveyor systems. Since the VFD operates on standard asynchronous motors, it does not require a change of the electrical network. In fact it will generate more effective power even from weaker sources.
Substantial changes to the load on the AFC as well as blockages always present the danger of chain breakage, and such events certainly cause chain stretching. The Cat VFD system monitors and shares the load between gearboxes with nearly 100% effectiveness to reduce stress on AFC chains and on plow chains. Extending chain life also reduces the need for chain tensioning.
The VFD adds a superior level of control to the proven line-up of Cat AFC drive technology. The Cat Controlled Start Transmission (CST), a planetary drive system with an integrated infinitely variable multi-disc clutch, has been a mainstay for nearly 20 years. It provides automatic and instant overload protection, soft starts and synchronized heavy-load startup. Caterpillar also offers a cost effective alternative that employs Cat UEL gearboxes rather than CST gearboxes.
Adjusting speed and torque by varying the frequencies of the alternating current enables precise and stepless control. The Cat VFD allows maximum torque at start-up and evenly distributes the load between the drives at both ends of the longwall. The system immediately reacts to a blockage or chain break with a controlled shutdown. In regular operation the speed of the AFC can be adjusted steplessly to load requirements when the plow or shearer are cutting the same direction or opposite direction of the AFC. Also, the complete longwall can easily adjust to the capacities of the subsequent coal clearance components such as the crusher, stage loader and belt conveyor.
Since all data on power input, effective power at the drives, speed and torque are instantly available through an external encoder interface, they can be analysed instantaneously. This again allows optimised production with reduced variability. Operating in the ‘green zone’ most of the time, dynamically adjusting to changing production needs, avoiding standstills, and facilitating fast start-ups are the strengths of the Cat VFD.
The Cat VFD will plug into existing systems. CatVFD drive systems are housed in separate explosion-proof cabinets, and, due to current source inverter technology (CSI), they can be installed as far as 4 km from motors. Compared to other systems that integrate VFD control into motors, they do not require additional space at the face ends. Also, unlike some other systems, the Cat VFD does not cause EMC effects such as over-voltages, reflection waves or other unwanted ground currents or network disturbances.
