The current paper looks at evaluating the merits of large geared mills versus gearless driven mills by evaluating the following factors:
- The capability of currently available systems in terms of power and mill diameter
- Efficiency in terms of overall electrical power consumed vs mechanical output power
- Capital cost
- Running costs
- Cooling requirements in terms of air versus water cooling
- Installation and delivery time
- World’s largest geared drive – 2x8500kW – Wushan, China (operating)
Among the many reasons that geared drives on large SAG mills are not favoured, is due to not having an efficient variable speed drive option (60 to 80% critical speed), which is required for process reasons. As technological advances have occurred quite rapidly in recent years, this paper uses the latest data in applying these comparisons and attempts to dispel some of the myths regarding Slip Energy Recovery (SER) and variable frequency drive technology on twin pinion drive systems. Early attempts to provide load sharing on twin pinion drives used primitive electronics which have since been developed to eliminate the problems encountered in earlier attempts. In particular, torque pulsations between the twin motors have been overcome using a higher number of devices (pulses) and better electronics. With successful operation of 2 x 6.5MW (Phu Kham, Laos) and three 2 x 7MW units under manufacture (Pascu Lama) and a further 40 projects worldwide, energy efficient Hyper Synchronous SER drives are now mature and ready for use on 2 X 10MW dual drive mills. Speed variation required is in the range of +/- 15% of synchronous motor speeds. VVVF drive packages from ABB and Siemens are also available for 2 x 10MW in low and high speed options. Dual hydraulic inching drives provide a much safer and convenient operation with the added advantage of locked charge detection.
This paper was presented and published as an SME Preprint at the 2013 SME Annual Meeting & Exhibit, February 24-27, Denver, Colorado. It is provided with the generous permission of the SME.