The first project (the rubber mill feed project) included the design and implementation of a Rockwell Automation Allen Bradley PowerFlex AC variable speed drive (VSD) solution, forming part of the manufacturer’s in-house optimisation project. It involved the addition of a two roll rubber feed mill to aid in meeting the critical demand of downstream processes.
The second project (the Banbury mixer project) involved replacing obsolete drives for one of the plant’s Banbury mixer’s. Allen Bradley PowerFlex modular DC drives were implemented with a Rockwell Automation Connected Enterprise approach, which included making use of component selection and configuration tools such as Integrated Architecture Builder (IAB).
Rubber mill feed project
This project entailed the expansion of the rubber feeding process by the addition of a standalone two roll feed mill. The new feed mill is located adjacent to an existing MV motor-driven common shaft feed mills. The rationale – to have redundancy in the event of a drive or mechanical failure. The new mill furthermore allows for variable speed operation, optimally feeding the calendaring process.
The scope of work included the design, construction and commissioning of a VSD panel for the 150 kW, 400 V AC feed mill motor. Construction of the panel was carried out by a third party, overseen by Rockwell Automation. A PowerFlex 755 was chosen for this application. The AB PowerFlex 750 series drives allows expansion by installing (multiple) digital I/O, feedback and safety option cards, with the additional benefit of on-board programmable logic (DeviceLogix).
Rockwell Automation implemented the control philosophy provided by the machine-motor integrator for the project. This included programming software functionality in the VSD using the on-board DeviceLogix and integrating speed control encoders with an Allen Bradley safety relay.
The results of the factory acceptance testing performed in Gauteng, together with the pre-approved functional design specification documents, were reviewed on site. Site acceptance testing involved testing all functions, first without and then with the product. Project commissioning was completed on 9 November, in time and to budget.
“The novelty of this project,” says SJ Bergh, Project Engineer, Rockwell Automation, “is that we used the functionality provided by the embedded DeviceLogix technology to build the required safety functional intelligence without having to add another PLC.”
Banbury mixer project
The Banbury mixer project involved replacing four obsolete third party drives and their panels used for one of the manufacturer’s Banbury mixers. The mixers are used to mix the raw materials that form the rubber compound.
The Rockwell Automation Integrated Architecture-based solution, which allows for Ethernet enablement, replaced the older hard-wired technology. The migration of the safety and control of the mixer drives on to the newly installed GuardLogix safety PLC provides the ideal platform for future complete mixer control upgrades.
The scope of work included generating a hardware design specification, electrical design and draughting. Special attention was given to the packaging of equipment into customised modular electrical cabinets to conform to stringent space restrictions. For example, ensuring alignment with the fixed position of four 550V supply bus bars. The panels were constructed by a third party, guided by Rockwell Automation.
Two 1 000 HP DC (about 1.5 MW total) motors drive the mixer through a common gearbox. Essentially, four DC drives were replaced with a two-drive modular system using locally sourced Semikron SCR power modules interfacing to an Allen Bradley standalone DC Drive Regulator (SAR) module. This removes the requirement of having two drive interfaces per one motor, at the same time reducing system complexity, dependency on specific drive size spares, and minimised failure potential due to physical segregation and electrical isolation of power and control devices.
Each 1 000 HP DC motor is fed from a 30 degree phase shifted, 550V AC dual supply, which makes up a 12-pulse drive. The PowerFlex DC SAR module is scalable up to four parallel power units, either regenerative or non-regenerative. A major safety net is provided with a PowerFlex SAR -based 12-pulse arrangement in that cross conduction (plus to minus) cannot take place between two regenerative SCR bridges. This is a real possibility if a 12-pulse arrangement is implemented with the output of two separate drives connected together in a regenerative application. Even though the mixer application is non-regenerative, the SAR solution was still chosen due to the simpler and more logical drive to PLC interface.
The scaling of voltage and current feedback to the SAR is fully programmable. It is thus also ideal for retrofitting applications where existing SCR stacks are re-used.
In the case of the Banbury mixer, any of the two identical SAR interfaces can be configured to be the master drive for the system. This prevents the dependency on one specific drive to always be available, as was the case with the original four drive system. One motor can be shut down and the machine operated at reduced capacity. There are no direct power connections between the two new drive panels, which are separate. Auxiliary 110 V power makes fault-finding without the main 550 V power easier.
Interfacing with the third party PLC was done through an Allen Bradley networked Point IO, communicating to the safety PLC and drives via a Stratix 5700 managed switch. This permitted sufficient network integrity to handle critical communications like safety IO and load sharing between motors. Allen Bradley PanelView HMI-based diagnostic functionality was also installed to aid with system diagnostics.
Controlling all the electrical safety devices directly in the new drive panels resulted in an 80% field wiring reduction and also enabled pin-point fault-finding. “All components were selected in such a way that the same equipment part numbers can be used for any future Banbury mixer upgrades at this plant,” explains Nico Verhoef, Senior Engineer, Rockwell Automation, Sub-Saharan Africa and lead engineer on the Banbury project.
The order was received in mid-June 2017 and the project was completed on time before the start of the first 2018 production cycle. “A short turnaround time was accomplished with all results to spec,” says Verhoef.
“One of the challenges overcome was the tight time frame right through from order acceptance to project completion. Another was to remove the old and install and commission the new panels, working in a confined and raised electrical room together with the Rockwell Automation appointed installation company.”
The manufacturer’s plant now receives value-added support for both these projects via its Rockwell Automation onsite embedded engineer, who is responsible for parts and issues management, as well as maintenance forecasting and risk identification.