38-foot SAG mill with gearless drive system from Siemens. Throughput can be almost doubled with a 42-foot mill.

The 42-foot ring motor is the centerpiece of the gearless drive and wrapped around the tube mill. An increase in tube diameter means that the mass of the ore being ground rises dramatically, and hence also the strain on structural elements and the motor. During the design of the motor, therefore, all components were subjected to comprehensive and detailed risk analysis, for which Siemens relies on a combination of measurement-based and model-based techniques. A motor in ongoing operation, for example, was caused to vibrate with the aid of a shaker. More than 100 acceleration sensors recorded the resonance modes and frequencies.

The results of the measurements were used for the validation and optimization of a finite element model that Siemens has been using in the development of mill drives for twelve years now and is continuously improving. In conjunction with measured and empirical values, the finite element model allows all system components to be investigated with regard to anticipated loads, deformations, vibrations or material fatigue. In this way it is possible to ensure even at the design stage that the future mill drive will run safely and reliably in everyday operation.

A further advantage of this integrated design approach from Siemens is the application of proven system components. The fixation of the laminated iron core in the stator housing, for example, are identical to those that have provided fault-free service for decades. The insulation of the stator windings is made of Micalastic VPI, which Siemens has been using in all medium-voltage motors since 1966 and in all gearless mill drives since their market introduction in 1970.

As output volumes continue to rise in ore mines, the demands on mill drives are also growing. Whereas in the 1990s the most common types in service were 20- and 22-foot ball mills and 36-foot SAG mills, in the meantime ball mills with a diameter of 24 feet and SAG mills with a diameter of 38 feet are in widespread use. Since 1998 Siemens has been supplying gearless drive systems for 40-foot SAG mills. Over the past three years, Siemens has received ten orders for this type of mill, rated at up to 28 megawatts.

Further information about solutions for the mining industry is available at: http://www.siemens.com/mining