Compared with conventional solutions, a new drive concept with asynchronous squirrel-cage motors at the Lafarge Cement plant near Pécs, Hungary, boasts minimum service and maintenance costs. And if that was not enough, higher efficiency is also helping improve the bottom line. By Alf Banse, Siemens, Germany and Jozsfe Janovits, Lafarge Cement Hungary.
Normally, slip-ring motors with resistance or liquid starters in the rotor circuit are used in the main drives in vertical mills. The advantages of slip-ring motors include the low start-up current, which the starter limits to around twice that of nominal current, and the relatively high starting torque. In fact, the cement and mining industries are almost the only in which slip-ring motors are built into new installations – mainly due to experience gained in the past as well as special requirements like fixed-speed drives with high starting torque and the resulting high network stress. To ensure safe operations, intensive maintenance is necessary for the starter as well as for the required brushes and corresponding equipment.
During the tendering phase of the Lafarge Cement Hungary contract in 2007, project leaders were looking for a drive system that required lower maintenance. The solution: replace the commonly used slip-ring motors with squirrel-cage motors. One stipulation, though, was that performance had to remain the same. Siemens stepped up to the challenge.