Danfoss drives keep Krupp on the rails

"Moving a 180-tonne Krupp reclaimer across a 45m bed of gravel, raking the gravel in a smooth and constant stream, and moving quickly on to the next pit requires enormous torque from high-performance drives. It is also critical that the drives on each side of the wide reclaimer operate in complete synchronization. The maximum offset allowable between left and right is less than 8cm otherwise the reclaimer would derail."

"Two modes of operation are involved. Material raking to the take-off conveyors is carried out between 0.13m/h and 3m/h, while moving from one gravel mound to another is at 9m/min. Previously a twin ac/dc-drive configuration was used, with the high-speed travel carried out on a mains-driven ac-geared motor unit with dual-shaft outputs. Slow-speed operation was carried out by a dc-geared motor, which was clutch-connected to the second shaft end. The ac motor would be switched off and the electrically operated clutch engaged, with slow-speed control provided by the dc drive. This system gave rise to crabbing problems with no real synchronization of the drives on each side of the machine."

To solve this problem Danfoss recommended 400kW VLT 5000 Flux ac open-loop vector drives equipped with SyncPos motion-controller cards and Profibus communications. Danfoss Bauer geared motors with wide ratios and high-service factors were also required.

"Two flux-vector drives mounted on the reclaimer now control ac-geared motor units on each side of the machine in master/slave mode. The SyncPos card installed within the master drive maintains the two drives in complete synchronization and crabbing is eliminated. The flux-vector drives, allied with the high-ratio geared motor units, provide high starting torque and close, smooth control of acceleration at start-up."

"The reclaimer’s drive system was rebuilt over a two-week maintenance window and no major problems were encountered during commissioning. Benefits of the new drive system include a reduction in spares provision and maintenance, improved reliability, and supervision of the plant from the control centre via the Profibus network."