Complete skip drive solution offers superior control

WEG and Technidrive increase the safety and reliability of TH Moore quarrying operation

MOTION control is paramount in hazardous industries such as quarrying. If a skip laden with a tonne-and-a-half of product were to go hurtling off its rails at speed, the consequences could be devastating. To ensure thus never happens, motor and drive manufacturers WEG and their premier partner Technidrive developed a new skip drive with a brake control solution to upgrade TH Moore (Contracts) Ltd’s quarry equipment to improve reliability and safety on site.

TH Moore, a Gibson Bros Ltd associated company, are suppliers of quarry materials, high-grade aggregates, and CE marked bituminous products. Before the project, the quarry in question used an outdated skip drive system on a set of rails powered by a motor to move product on site. The system saw a skip drive up the rails to sit below a mixer or above three different bin positions – one used for loading directly into a lorry and two for storage.

The system’s usage was dependent on the product demand, with no set running hours. Plagued with a wide range of electrical problems and equipment breakdowns that led to multiple safety concerns, these faults were made harder to identify and rectify as the system was only operational for two to three months each year. The electrical contractor was the first point of contact for these breakdowns, but as the issues were getting increasingly difficult to diagnose, the contractor decided an entire system upgrade was necessary.

Technidrive were brought in to design and commission a new skip drive solution and control system with a programmable logic controller (PLC) and full touch-screen integration. They, in turn, turned to their long-term partner WEG to supply a motor and a variable-speed drive (VSD) for the skip upgrade project.

One of the main problems associated with the old system was excessive brake wear on the skip motor, as it was not equipped with a VSD. If an operator selected bin one and pressed start, the skip would first drive down to the mixer. As soon as it hit the limit switch the brake would slam on and the motor would disengage, resulting in a harsh stopping action. Once the system had sent the message that skip filling was complete, it would then engage the motor and the skip would accelerate harshly up to the bins.

For the upgrade, WEG provided a 22kW, four-pole, B5 cast iron brake motor with an integrated 1024 pulse encoder. With superior efficiency levels, this motor was selected as it offers cost effectiveness, greater productivity, and the durability required for quarrying applications. For the skip drive, WEG recommended the CFW700 VSD with integrated high-torque brake control. This integrated brake control, along with some intelligent programming of the onboard soft PLC, meant that Technidrive were able to virtually eliminate brake wear and significantly reduce motor maintenance.

The new skip system features an auto-slowdown function, which allows the motor to stop on the VSD before the brake is applied. Now the brake is used solely as a holding mechanism and the system no longer drives against an engaged brake. Moreover, virtual positions programmed for each location allow for a slow ramp-up using the VSD. Before the skip arrives in its final position, the new system leaves space for slowdown, allowing the skip to come to a stop from a slower speed and in the same position every time, which has further reduced wear and tear on the mechanical system.

Another glaring issue with the old system was that it used an outdated, unreliable, and obsolete PLC system. This meant that if a digital input or card went faulty, replacements were not readily available.

The original PLC system worked off a series of limit switches which were part of the control system and used as a safety interlock. Technidrive designed a new PLC system that was able to read the encoder. The new system features a 10in Hitachi human machine interface (HMI) instead of a series of push buttons and controls on a busy panel.

Thanks to the encoder feedback and the live skip position function, the operator knows exactly where the skip is at any point. This results in easier and safer control between the four positions, all visible on the touch-screen display. Monitoring can also be conducted from a tablet or mobile phone, so problems can be diagnosed remotely and quickly.

If for any reason the encoder fails, the operator also has full manual control. For example, if the encoder slips on the shaft of the motor and the skip is moving up, the encoder will not recognize it, and a belt-and-braces system is in place to prevent the skip driving over the end. To further increase safety, protection was built into the system to stop the skip motor if freewheeling.

‘The project was completed in April 2021 and ever since our plant has been operating at full efficiency,’ explained Mark Poots, quarry manager at Gibson Bros. ‘We used to be marred by electrical problems, but Technidrive and WEG provided a cost-effective way of upgrading the obsolete control system to an industry-leading alternative. This has not only reduced downtime, but if component failure does occur, replacements are more readily available. No one wants to see a skip rocket across the quarry and our new system is the safest way of operating this system.’

Gary Kernaghan, automation manager at Technidrive, added: ‘We chose WEG as the complete solutions provider for this application due to their renowned reputation in quarrying. This, paired with worldwide backup and engineering support, ensured WEG were our first-choice supplier for this skip drive project. Being able to offer the complete solution – brake motor and VSD – meant that we were able to extend the warranty to three years, giving our customer complete peace of mind. We have been working with WEG for more than 15 years and have built the relationship and product knowledge to allow us to offer industry-leading products and solutions.’

Safety was the primary concern when developing this control system and WEG and Technidrive were keen to give TH Moore as much control and reliability as possible. The new system has essentially reduced the number of potential failure points from eight to one.