Gearless Drives for Ore Conveying and Grinding
Antapaccay is the first mine using gearless drives for ore conveying and grinding. Antapaccay is a brownfield expansion of the Tintaya copper mine. The deposit is scheduled to produce 160,000 tons of copper in concentrate per year during at least in the first few years of its minimum 20 years mine life.
THE MINE pertaining to Glencore Xstrata comprises new mine facilities, material transport equipment and a new 70,000 tpd concentrator. On time startup of the plant and first delivery of concentrate dates back to November 2012. In May 2013 the concentrator reached its nominal design capacity and is now operating 10% above its design value.
Antapaccay sets the benchmark with the utilization of advanced gearless drive systems on the overland conveyor as well as for the grinding mills. Both drive systems are provided by Siemens. Siemens has also supplied the main electrical distribution equipment, such as the 220 kV gas-insulated high-voltage switchgear, the 33 kV gas-insulated medium voltage switchgear and the low-voltage distribution equipment.
Gearless drive solutions in mining
– Mega power for Mega tasks The growing demand for raw materials means that its extraction, transportation, and processing requires higher throughput resulting in larger machines and higher drive powers. The requirements relating to reliability, efficiency and availability of the drive solutions are also on the rise. Gearless drives with low-speed synchronous motors are a well-proven solution when more megawatts of drive power are required for mining applications.
Belt conveyors with gearless drives
Gearless drives have been becoming an ever more relevant solution for conveyor belts the past few years. Where several megawatts of power are required at a drive pulley, gearless drives are a real alternative to conventional drive solutions with gearboxes. In Germany a conveyor belt is already in operation since 1985, in which the driving pulley uses a gearless system with two (2) synchronous motors, each with 3.1 MW of rated power. According to the operating company RAG, this belt conveyor running in Prosper Haniel offers high availability without any significant interruptions or failures attributable to the drive system (synchronous motor, converter).
At the end of 2012, two drives were commissioned, each with a nominal power of 3.8 MW, at the conveyor system in Antapaccay, Peru (operating company: XSTRATA; construction of the belt conveyor by ThyssenKrupp). Up to 5,260 tons of copper ore are transported per hour from the mine to a processing plant over a length of 6.5km on a 1,370 mm wide belt. In 2013, nine additional gearless drives (4x 4.4 MW and 5x 5.056 MW) were supplied – and are presently being installed and commissioned in mines in Peru and Chile.
Fig. 2. Drive pulley and gearless drive of the conveyor belt.
The drive system of a belt conveyor has to provide the required speed and torque at any point of operation. This is ensured by using the appropriate converter technology and considering the engineering of the system with an integrated approach. Moreover ThyssenKrupp as the supplier of the belt conveyor for Antapaccay specified that load peaks have to be avoided. Additionally a staged starting procedure was required. The objective was to minimize additional forces on the mechanical components, the steel construction and the conveyor belt itself, and to prevent possible oscillations during starting.
In the first stage of the starting procedure the belt conveyor is accelerated within 30 seconds to 10% of its rated speed (figure 1). When the over 12 km of conveyor belt has been set in motion, as it is the case for Antapaccay, after approximately 80 seconds the second stage starts – and the belt conveyor is accelerated up to its rated speed within 270 seconds. The load does not influence the starting of the belt conveyor. Special control software integrated in the converter controls and monitors starting and stopping as well as all other operation modes.
When stopping (Fig. 01), the belt speed is reduced along a defined ramp and the mechanical brakes are applied shortly before the system comes to a standstill.
The control software is also used to implement slip monitoring (between the drive pulley and the conveyor belt) and load balancing control between the two drive pulleys. This facilitates an operation with low stress and minimum wear on the mechanical system in all operating conditions.
Drive solutions must be reliable and efficient – this is the reason why ThyssenKrupp and Siemens prefer a straightforward drive train design with few components. The gearless drive for the belt conveyor is comprised of just the drive pulley, two bearings, the rotor and stator of the motor. The drive pulley and rotor are arranged on the same shaft. When installing the system, the stator is shifted over the rotor, creating the gearless drive.
Fig. 1. Example of starting and stopping behavior of gearless drives.
Comparison between drive solutions (Fig. 03) shows that the geared drive requires significantly more components, such as bearings and couplings. The advantage of the gearless drive is obvious as with an increasing number of components the availability and reliability decreases. At the same time obviously fewer spare parts are required for the gearless solution.
Something similar applies to the efficiency of both drive solutions. As a result of friction energy is converted into heat in the conventional components. This energy is lost and cannot be used to move the drive pulley. As a consequence for a gearbox generally 2-4% power loss must be taken into account (depending on the particular gearbox design). The gearless drive transmits the power directly to the drive pulley.
To ensure continual improvement, the experience of completed projects has been and will continue to be gathered to incorporate this information in future projects. Here, Siemens and ThyssenKrupp are working closely together with one another.
Fig. 3. Principle design of a geared drive and a gearless drive.
Fig. 4. Air gap monitoring system in a gearless drive.
Gearless Drives for grinding mills
Gearless Drives are well proven stateof- the-art technology for large grinding mills. Siemens, the company that introduced Gearless Mill Drives (GMD) to mining industry started up its first GMD on a ball mill in Norway in 1980. The mill and its drive system are still in service. Today, GMDs are mainly used for SAG mills between 36 and 40 feet (10.7 to 12.2 m) and ball mills from 24 to 28 feet (7.3 to 8.5 m). The current size and power limits for GMD’s are 42 feet and up to 35,000 kW.
Among the most outstanding GMD’s in operation are the ones for the Antapaccay mine. The SAG mill has an inner diameter (ID) of 40 feet, an effective grinding length (EGL) of 25 feet and 24,000kW rated power, whereas the two (2) ball mills have an ID of 26 feet, an EGL of 40.5 feet, and 16,400kW rated power. The GMD’s to drive these mills are currently the largest in the world in continuous operation at a comparable high altitude of above 4,000masl (meter above sea level).
The high altitude of the mine with the low atmospheric pressure and the high solar radiation brings in special requirements on the electrical equipment. Siemens verified the complete gearless drive system and confirmed its suitability up to an altitude of 5,000masl. The most critical parts to guarantee a long life time of the gearless drive system in the altitude are the insulation of the motor windings and an effective cooling system. Siemens’ answer to the challenges on the winding system is the application of Vacuum Pressure Impregnation (VPI) to the windings. Especially the winding system passed various altitude tests for the installation at such an altitude. Among others a pressure chamber test, simulating 5000masl, and field tests at other high altitude installations have been passed successfully. To meet the cooling requirements Siemens provides a homogeneous motor cooling system distributed around the stator to ensure uniform cooling over the entire circumference of the motor. This is confirmed by actual measurements with a temperature difference of less than 5°C around the stator.
Antapaccay is also making use of the inherent GMD capability to vary speed, as the mill speed can be adapted to the operation requirements at all times. The variable speed allows compensating for throughput changes as well as harder ore, and significantly reduces wear to the mill liners and ball consumption by optimizing the cascading angle of the mill charge. Reduced wear of the liners increases the maintenance intervals on the mill paying back as increased productivity.
Generally, increasing the production time and productivity are the guidance for Siemens design objectives. This applies to numerous design features like the fuse-less combined with short-circuit proof cycloconverter or the short circuit proof motor as well as to the mill specific operation modes of the gearless mill drive. The inching, creeping and balancing modes are provided to ease and optimize the normal maintenance measures of the mill. To avoid damages due to a frozen charge the Siemens GMDs feature Frozen Charge Protection that stops the mill safely before a frozen charge incident occurs. Optionally the frozen charge can be loosened automatically by applying the Frozen Charge Shaker™.
Siemens, as a partner for the entire lifecycle of the mining project, also supports the user with its wide range of services during the operation of the gearless systems This comprises the joint implementation of an individual and effective service concept including training of users operation and maintenance personnel, start-up accompany by field service engineers on site, predictive and preventive maintenance with remote data analysis, 24/7 on-call service assistance with remote diagnostics, monitoring of spare parts, and annual preventive maintenance visits by specialists from the factory.
As a summary the combination of the design features, the high manufacturing quality and the comprehensive service portfolio is leading to an incomparable availability of the gearless drives systems and the full satisfaction of the end-user with the equipment in operation.
