An electrical impact study between variable speed dual pinion mills and gearless mill drives

Conventional mining process circuits use both Semi-Autogenous Grinding (SAG) and Ball mills. It is very common that the SAG mills are operated with adjustable frequency drives (AFDs) and the Ball mills are operated at fixed speed [1]. Recently, however, adjustability of Ball mill speed has been given more consideration. Also, to maximize throughput, the Ball mills are sized to be operated at 100% load. To achieve this target, efficiency (mechanical and electrical losses) and speed adjustability have become more crucial than ever in the selection of the Ball mill drive solution. When mill power is between 18 and 20 MW, which is the current manufacturing limit of ring geared mills [2], either a gearless mill drive (GMD) or a ring geared mill with dual pinion motors can be chosen. Selecting between gearless or ring geared design is mostly dependent on size of the mill and power and speed requirements [3]. This paper takes a closer look at the current manufacturing limitations of ring geared technology and compares the impacts of ring geared and gearless mills on mine power distribution systems. A case study using an 18 MW Ball mill will be presented and the power system considerations including different harmonic filtering and power factor correction requirements will be analyzed.