GMDs: the driving force behind sustainable mill operations

Combined with digital solutions that optimise performance and reduce maintenance, gearless mill drives offer operators of grinding mills better power and efficiency.
GMDs: the driving force behind sustainable mill operations GMDs: the driving force behind sustainable mill operations GMDs: the driving force behind sustainable mill operations GMDs: the driving force behind sustainable mill operations GMDs: the driving force behind sustainable mill operations

Gearless mill drives can produce higher power for grinding

Marcelo Perrucci

Mining operators face a dilemma: how to optimise production, maintain quality control and reduce energy usage during high-volume projects that are increasingly characterised by poor ore grades, and therefore require larger mills with higher grinding capacities to achieve greater throughput. 

Gearless mill drives (GMDs) constitute a compelling alternative to traditional ring-geared mill drives (RMDs) in such environments. Both drive types can be used to power large semi-autogenous (SAG), ball and autogenous mills, but GMDs are proven to offer superior availability, uptime and efficiency. 

When the first GMD was developed by ABB in the late 1960s, a 5MW mill was considered powerful. Today, modern mines may have multiple 15-25MW GMDs running in tandem. By eliminating heavy, maintenance-intensive mechanical components, GMDs can improve efficiency by as much as 3% as well as reduce OPEX, energy usage and the mining operation's overall carbon emissions footprint. 

GMDs are also fitted with multiple sensors that relay a wealth of valuable data that can be collated, analysed and utilised using advanced digital solutions to improve maintenance, increase uptime, and allow mill operators to make smart, data-driven decisions that ultimately add value to the business. 

Gearless mill drives: better by design 

GMDs eliminate the need for a ring-gear, pinion, gearbox, coupling, motor shaft and motor bearings, which are used in a conventional mill drive system to transmit the torque from the motor to the mill. 

Instead, the torque is transmitted through the magnetic field in the air gap between the stator and rotor. By mounting the rotor poles directly onto the mill's flange, it becomes the rotor of the gearless motor. 

The drive system on ABB GMDs - including the gearless motor - does not require any lubrication, thus reducing maintenance costs. The GMD is designed to run the mill with variable speed, in both directions of rotation, meaning the grinding process can be optimised at any time during the mine's life. These optimisation strategies can be used to decrease the liner wear, resulting in less downtime. 

ABB gearless drives also contain a single-turn winding system to ensure that stator winding losses are minimised. In addition, the rise of motor temperature is relatively low, resulting in a low thermal expansion and less resistance of the copper windings, which translates into lower losses. A further reduction of losses is achieved by minimising the number of semiconductors in the cycloconverter. 

Comparatively, ring-geared mills can only achieve roughly 18MW in terms of power, meaning they are limited from a mechanical point of view. By eliminating components that limit torque transfer and achievable power, GMDs are now the preferred solution for large mills. ABB GMDs provide the power behind many of these mills and the global technology leader has even designed, manufactured and delivered the world's largest mill comprised of a 28MW, 42ft SAG. 

ABB Ability Predictive Maintenance for grinding 

Mills are by far the largest power consumers in a concentrator plant; choosing the correct drive is critical to ensure longer process uptime and sustainability. ABB partners with mill operators and engineering companies during the feasibility stage, providing them with trade-off and total cost of ownership calculations so they can decide if a GMD, RMD or a variable or fixed speed solution is right for their particular needs, which can significantly impact CAPEX and OPEX costs. 

Once installed and commissioned, ABB also offers a multitude of digital services to help mining companies keep their operational costs under control. These services form part of ABB Ability Predictive Maintenance, a digital remote diagnostic condition monitoring service that also enables ABB experts and mill operators to connect securely to the GMD from anywhere in the world, collect data from instrumentation, and stream this information to the ABB Ability cloud, where advanced algorithms can assess, and even predict, the condition of assets and equipment. 

In this way, ABB Ability Predictive Maintenance can be used to both create and manage key performance indicators (KPIs). ABB domain experts can carry out plant assessments and propose performance optimisation solutions - from a simple power upgrade to changing the nominal speed of the mill to advanced process control (APC) that can be extended beyond grinding to include other key processes such as flotation. 

As an example of ABB Ability Predictive Maintenance in action, a customer with GMDs from ABB and other suppliers was having issues with reduced ore throughput on several of their lines. ABB identified that the GMD motors could be pushed a little more in terms of their existing power; by doing so the client was able to gain more throughput than they were losing on the other lines that used non-ABB equipment. 

All of these services are backed by ABB Ability Remote Assistance for grinding, where ABB's experts are on call 24/7 to provide guided support to mining customers or troubleshoot directly using the remote secure connection. 

Future innovations 

ABB is currently working on updating its ABB Ability Predictive Maintenance platform to offer more freedom and flexibility to customers. A major step forward, the upgraded version will feature an improved user experience, customisable dashboards, more advanced analytics and predictive tools, and a host of new interactive features, all designed to add value for customers. 

ABB is also launching new mobile app with push notifications, allowing mill operators to access information on what is going on with the mill and the GMD via smartphones from anywhere in the world. The service will include status events, alarms, even the ability to write and forward actions on the mobile to others. The vision is to connect all operators with ABB wherever they are located. 

The ABB Ability Remote Insights is another application that improves interaction between remote experts and field personnel by enabling live instruction and guidance that can be overlaid on live video using augmented reality (AR) technology. This AR solution has the potential to simplify maintenance and improve safety in high-risk environments. 

Here again, ABB is not sitting still. The company is experimenting with mixed reality with a view to creating a collaborative, interactive virtual workplace to share documents. This extension could be used for personnel training and inspection services, for example. 

The drive towards sustainable operations

Electric motors consume more than 45% of the world's electricity. Adoption of high-efficiency motor systems could cut global electricity consumption by as much as 10%. In addition to developing a high efficiency GMD system, ABB is committed to limiting energy usage at its ring motor factory and is currently working on a new GMD sustainability strategy. 

These plans are embedded with the company's 2030 sustainability strategy, which includes working with its customers and suppliers to implement sustainable practices across the value chain and the lifecycle of its products and solutions. A key part of the strategy is to contribute to a low-carbon society, in line with the 1.5°C scenario of the Paris Agreement and following the guidelines of the Science Based Targets initiative. 

ABB delivered the world's first gearless mill drive to a cement plant back in 1969. More than 50 years on, GMDs are well-positioned to contribute to the building of larger, ever more powerful mills. When combined with 21st century digital solutions, they have the potential to improve efficiency and reduce both energy usage and carbon emissions in modern mining projects.