Gearless Mill Drive Sales Rev Up to Meet Demand
By Russell A. Carter, Managing Editor
Hidden away in cabinets, racks or in the motor unit itself are the brains behind intelligent motor behavior—the controllers or “drives” that provide a manual or automatic means for starting and stopping a motor, selecting forward or reverse rotation, regulating speed and torque, and protecting against overloads and faults—the latter a task rapidly gaining additional importance as mining operations spread to increasingly remote locations where maintenance may be subpar and the main power supply unreliable.
The relatively recent appearance of flexible, robust drives for AC induction motors has opened up new avenues of product development for a number of mine-equipment categories. These advances, which include the capability to control torque generation down to zero motor speed, have allowed polyphase AC induction motors to compete in areas where DC motors were long dominant, offering advantages in design robustness, lower cost, and reduced maintenance. This trend, coupled with the rapid forward pace of electronics technology in general and the current booming demand for mineral commodities— and for new mines and plants to produce them—has elevated motor drives and drive systems for all types of equipment to an unprecedented market- visibility level.
However, nothing in this sector is more noticeable than the flood of recent contract awards for gearless motor drives for grinding mills. These drives, which eliminate gearboxes or air clutches by transmitting torque between a “wraparound” motor and the mill by means of a magnetic field in an air gap between the motor stator and rotor (the mill body itself), have evolved into sophisticated control systems that include not just the motor and drive hardware but also power filtering, over/under current protection, control and supervision of hydraulic and lubrication systems for the mill bearings, braking and operator interface systems and more. In spite of their growing complexity, during the past several months two of the major suppliers in this sector have reported a steady stream of orders for units rated up to 28 megawatts (MW) to drive large SAG and ball mills. Siemens, for example, announced:
• A €16-million order to supply two
18.3-MW gearless drives for 36-ftdiameter
SAG mills at Minera Penasquito
• An order in November 2007 from CITIC Heavy Machinery Group, Luoyang, China, for gearless drive systems to power five 40-ft-diameter autogenous grinding (AG) mills that will be installed at the Sino Iron project in Western Australia. Siemens said the 28-MW output of these mill drives makes them the most powerful available in the market.
• An order announced on December 4, 2007, for gearless drive systems to be used on a 40-ft-diameter SAG mill and two 26-ft-diameter ball mills at Anglo American Chile’s Los Bronces Development project. The SAG mill is the third 40-ft unit to be driven by a Siemens system, the other two being the Sino Iron unit and another at Newcrest Mining’s Cadia Valley mine in Australia. The Los Bronces SAG mill drive is rated at 22 MW and the ball mill drives at 16.4 MW.
• A €20-million order on December 10, 2008 from Compania Aurifera Brisas del Cuyuni C.A., a subsidiary of Gold Reserve Inc., for gearless mill drives for two 36-ft-diameter SAG mills at the Brisas gold-copper project in Venezuela.
• A €30-million contract from Xstrata Copper in April 2008 for motors and drive system to be used on a SAG mill and two ball mills at one of its South American copper mines. The SAG mill drive package is rated at 21 MW; the ball mill drives at 16.4 MW.
• An order in May 2008 valued at more than €20 million from Aurox Resources Ltd. to supply gearless drives for a 34-ft-diameter, 14-MW SAG mill and a 26-ft-diameter, 17-MW ball mill for an iron ore project in Western Australia.
There are a number of factors driving this trend toward GMDs, particularly in applications involving large AG, SAG or ball mills. Near the top of the list is a GMD’s capability to easily tailor SAG mill speed to changing feed characteristics; or in the case of a ball mill, to control downstream circuit feed rates. The advantages of lower maintenance requirements, user-friendly man/machine interfaces, precise positioning capabilities and the prospect of longer service lives due to fewer mechanical components are also powerful incentives.
From a maintenance point of view, the low-speed positioning, anti-rocking and frozen charge protection capabilities of gearless drives are important features. Apart from the elimination of a separate inching drive to position the mill for liner replacement or other service, the inch/creep functionality of gearless systems reduces the problem of precise positioning to a few button pushes. For example, as explained by authors Norbert Becker and Kurt Tischler in Siemens’ Metals & Mining newsletter, with Siemens’ gearless mill drive “…inching for positioning of the mill to gain access to liners for replacement takes place at 1.2 rpm. Since it is a maintenance mode of the mill, inching is operated from the local control panel or MLCP. The operator indicates the angle to be turned on the MLCP (inching angle). To simplify selection of the correct angle, the selection is based on the number of bolts around the circumference that the mill is to be turned. Inching starts and stops with a balanced mill charge.”
The gearless drive turns the mill and lifts the material. The angle at which the material cascades the first time is measured and stored. The drive turns the mill by the requested angle, and then overturns the mill by the cascading angle. Upon reaching the sum of requested and cascading angle, the drive stops the mill and changes the direction of rotation. The gearless drive turns the mill back by the cascading angle, switching over to torque control. It turns the torque-controlled mill back until the torque is zero. With the torque at zero, the charge is balanced and there are no oscillations.
The authors also explain the system’s “frozen charge shaker” feature. Under certain conditions, a mill’s charge can solidify during a standstill and the “frozen” charge can stick to the mill body. This condition can damage the mill if the solidified charge is lifted during start and falls from the upper part of the mill.
The frozen charge shaker function can be initiated at the local control panel, where a key-operated switch must be activated to allow this mode of operation. When the start button is pressed, the drive turns the mill, lifts the charge up to the maximum safe angle for the mill, and moves the mill up and down. It then returns the mill to a balanced position, lifts the charge on the other side up to the maximum safe angle, and again moves the mill up and down. When this sequence is completed, the operator can verify whether there is still a frozen charge by starting the mill in any of the operation modes. According to the company, the total time for breaking a solidified charge with the shaker feature, including preparation and a test run afterwards, is about 30 minutes.
Not surprisingly, ABB’s gearless drive systems also have built-in lowspeed controls for inching, an antirocking software package, and a torque monitoring system to prevent damage from frozen mill charges. ABB also touts the robustness of its gearless drive design, emphasizing that “robustness means concentric” when referring to the air gap between the drive system’s rotor and stator. Maintaining the proper air gap—considered the most critical dimension for this type of drive—is dependent on rotor/stator roundness and concentricity. Deviation in concentricity is the source of magnetic pull imbalance around the air gap, and is consequently critical to force distribution throughout the integrated mill/GMD/foundation system. The air gap helps overcome roundness and concentricity variations that arise from heat, load and process conditions. For instance, with a nominal air gap of 16-17 mm for large SAG mills, the ABB design allows for considerable eccentricity. The gap is monitored by nine noncontact, capacitive sensors located inside the stator, which also inform the operator of air-gap status at standstill.
Sealing the air gap and rotor/stator surfaces against outside contaminants has always been a concern with gearless drive suppliers, and each GMD supplier has their own approach. ABB has developed an axial greaseless sealing system that employs Teflon to minimize friction. The spring-loaded design, according to the company, seals out particle and fluid contamination, and the Teflon-to-metal seal has a 16,000-20,000 hour service life. The sealing tracks can be inspected through access ports. Sensors monitor seal height and indicate end of seal life 1,500 hours in advance.
Comparing Drive System
Although GMD capabilities seem to present an unbeatable feature lineup, particularly for large AG/SAG or ball mills or plants that need process optimization or expect future throughput increases, there are a number of factors that come into play when deciding what type of drive system should be selected for a specific application. Not every operation will require the high power ratings and extended functionality offered by GMDs, and not every project owner can afford the cost or cope with the complexity of these systems.
A presentation at the 2007 Annual Meeting of the Society for Mining, Metallurgy and Exploration (SME) offered an excellent rundown of comparative issues when selecting a grinding mill drive system. The authors, Markus Ahrens and Johannes Gonser of ABB, explained that “The question of what drive system is optimal for SAG and ball mills is project specific and depends on the plant layout and the design of the grinding circuit. When drive systems are compared the main criteria are operating characteristics (fixed speed or variable speed, starting behavior, interaction with the network, harmonic distortion), maintenance aspects (reliability, wearing parts, downtime) and cost issues (capital expenditure, power factor and drive efficiency impacting energy cost, maintenance). In addition, drive systems show differences in other design and operational issues such as inching and creeping, load sharing (if dual pinion drives are used), frozen charge protection and space requirement.
“When determining the mill size and the drive type the required process power needs to be calculated based on design process specific energy (kWh/t), plant size (t/d) and total milling process power,” said the authors. “The required process power is divided into circuits and numbers of mills within a circuit, followed by the selection of the mill sizes to fulfill the requirements. The optimal drive type can only be selected after determining the mill size, the need for variable speed and the characteristics of the electrical system of the plant. There are a number of mill drive options that should be measured against the range of functions and capabilities required by a specific project.”
The universe of drive systems for grinding applications includes a halfdozen or so different technologies ranging from fixed-speed systems (slip-ring and synchronous motors) to variablespeed (slip energy recovery drives, load commutated inverter or LCI drives, cycloconverter drives and gearless mill drives). Briefly summarized by the authors, the characteristics of each type follow:
Slip-Ring Motors—High-speed drives generally used for smaller mills, but can be used on larger units as well. They offer low capital costs compared with other systems and are robust against voltage dips, but power factor is generally low and drops further in partial load conditions. A separate device is required for inching and creeping with these systems. Theoretically, speed regulation can be achieved with the starting resistor but this is an inefficient solution.
Synchronous Motors—Includes highspeed motors that drive the pinion through a main gearbox, or low-speed motors driving through an air-clutch coupling. A clutch allows the motor to be started uncoupled, but it can be a high-maintenance item. With weak power systems, reduced-voltage motor starting is possible, and on dual pinion drives each motor can be started separately, reducing the impact on the power system. Synchronous motors have a high inrush starting current—400% to 600% of nominal current. The starting torque depends on the motor design and can’t be adjusted later on.
Slip Energy Recovery Drives—These systems use slip ring motors and starting resistors, thus limiting inrush current. Speed is controlled by adjusting slip resistance, generally achieved by inserting resistance into the rotor circuit and dissipating this energy into the starting resistors—an inefficient approach. So, rather than using the starting resistor, the slip energy is converted to direct current, inverted to the frequency of the power system feeding the motor, and then fed back into the power system through a step-up transformer. The use of these systems has fallen sharply over the past decade, as better solutions are now available at comparable cost.
LCI Drives—Although these drives can be used with both low-speed and high-speed motors, the drives are highspeed in nature and thus their use for low-speed applications is limited. Continuous operation at low speeds (below 10% of normal speed) is not advisable due to high torque pulsations.
Voltage Source Inverter Drives— These drives can be used with both induction and synchronous motors, do not generate significant torque pulsations, have a very smooth starting behavior and are well-suited for weak power networks. The power factor to the network is high. They do not require inching or creeping auxiliary equipment.
Cycloconverter Drives—Can be used with both gearless drives and single- or dual-pinion geared drives, using lowspeed synchronous motors. Although the technical features of the cycloconverter remain the same when used with both types of drives, system efficiency is less with geared drives while maintenance costs are higher due to the ring gear.