Drives and Controls January 2023

30 n VARIABLE-SPEED DRIVES January 2023 www.drivesncontrols.com When industrial shredders jam, VSDs come to the rescue I ndustrial shredders used for recycling can shred anything from waste and plastics to metals and cars. The way they operate is not unlike an office shredder – but on a much bigger scale. Shredders can play an important role in developing a circular economy. For example, about 85% of steel is recycled at the end of its first useful life, meaning that there is high demand for shredders that can handle cars and scrap metal. Large shredders can suffer similar problems to their office bretheren – objects can jam between their teeth and they can grind to a halt. If the mechanical drive does not have sufficient torque to get rid of the object, the blockages persist. Repeated jams can limit throughput and wear down parts, resulting in raised maintenance costs. If the shredder is powerful enough, it can simply drive the blockage through. This is the quickest fix and is achieved by reducing speed, to gain torque. An alternative is to reverse the shredder’s blades to rotate in the opposite direction to clear the jam. For large shredders, this reversal function is triggered automatically. But reversals takes more time and too many reversals can limit a shredder’s throughput If you can monitor the load on the shredder and control its speed, you can avoid unnecessary reversals and improve the process efficiency. That’s the main attraction of using VSDs in shredding applications, but further benefits include energy savings and reduced maintenance costs. Traditionally, shredders have reversed based on the current drawn by the motor when it is overloaded. However, current can be affected by power factor, voltage drop or other factors. Using a VSD provides a more accurate measure of the torque and the true load of the motor. ‘‘Most shredders don’t use VSDs,’’explains David Strain, technical director of the Craigavon-based systems integrator, Technidrive, which has supplied shredding systems. ‘‘By monitoring the torque more accurately you measure the true force that is on the shaft and therefore you can protect that shaft and reverse at the correct times.’’This, in turn, protects the motor and other equipment from damage, by ensuring reversals take place only when necessary. More importantly, it cuts out unnecessary reversals and controls the speed so the shredder can drive through when necessary, improving throughput. One shredder supplied by Technidrive needed to be able to shred different materials including plastics and metals, so flexibility was important. The company designed a bespoke system including some programming done specifically for the application. The company also supplied the mechanical drive system, including motors and gearboxes, to drive the two shredder shafts, as well as installing the system and provided training for the operators. For the motors, it choseWEG’s W22 IE3 machines. ‘‘We opted for this motor due to the robust cast-iron frame and high efficiency,’’ Strain explains. The motors were paired with WEG VSDs which gather real-time data on torque, current, voltage, DC bus voltage and frequency from the motor. Technidrive programmed the VSD to decide when a reversal of the shredder should take place, based on this data. An HMI touchscreen on the front panel connects to the VSD. A soft-PLC built into the drive avoids the need for an external PLC. The operator can alter the current and speed via the HMI, and vary these depending on real- time data. This also resulted in more efficient throughput, while offering the flexibility to adjust for different materials. “We could have engineered a similar result with a VSD and an external PLC, but we were able to use a standard off-the-shelf WEG inverter, which includes safe stop, soft PLC, and DC bus chokes, and engineer them into a specialised application,” says Strain. n Industrial shredders can become jammed if an object gets jammed in their teeth. There are two ways of dealing with this, one of which is to reverse the machine. VSDs can improve this process as well as saving energy. The grinder blades on an industrial shredding machine are designed to shred materials such as scrap metal, but they can become jammed. If this happens, a VSD can help.