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Automation Technologies 2/2014

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Automation Technologies 2/2014

Control and drive

Control and drive technology Energy savings: stone by stone back to current issue Mircea Bogdan Balan The cement manufacturing process calls for drive solutions that perform reliably in tough operation conditions. When the motors driving limestone conveyors at the Carpatcement plant in Deva, Romania, had reached the end of their working lives, the company approached Eaton to help finding a solution that provides enhanced control and energy savings. Carpatcement Holding SA is a member of Heidelberg Cement Group which is one of the world’s largest manufacturers of building materials and one of the biggest investors in Romania. The company’s plant in Deva has a licensed production capacity of 1.65 m tonnes of cement per year. An essential part of the cement manufacturing process at Deva is the transport of limestone from the quarry to the manufacturing plant itself. Until recently, the conveyors were driven by 250 kW motors operating at 6 kV, but these were operated by fixed speed starters that provided only very basic control. The motors had also reached the end of their reliable working lives, as had the 6 kV switchgear used to control them. The decision was therefore taken to replace the motors and to fit drives that would offer enhanced control and would perform reliably in the tough operating conditions. Smooth starting and stopping To transport the limestone five conveyor belts are used at the plant ranging in length from 1 000 to 1 850 m. Three of the belts are sharply inclined while the remaining two conveyors are substantially level. The conveyors must be capable of smooth and controlled starting and stopping, even when they are fully loaded. Carpatcement approached Eaton for help with this project. Eaton’s engineers devised a solution based on replacing the old 6 kV motors with modern energyefficient 400 V motors. The inclined conveyors would have one motor each, while the horizontal conveyors would each be driven by two motors that would torque synchronised to ensure accurate load sharing. The five existing MV switchgear cells — one About Company: Eaton Headquarters: Dublin, Ireland Turnover: bn Employees: 102 000 Products: electrical, hydraulic and mechanical power management, ... AUTOMATION TECHNOLOGIES 2/2014

Control and drive technology for each conveyor — would also be scrapped, with each replaced by an Eaton Xiria vacuum switchgear assembly and a 6 kV/400 V transformer rated at 630 kVA for the inclined conveyors and 1.2 MVA for the horizontal conveyors. To control the new motors, Eaton SPA/SPI 9000 series variable speed ac drives (VSDs) were selected for the inclined conveyors, and Eaton SPX series variable speed ac drives for the horizontal conveyors. A local system integrator built the drives into control panels based on floor-standing cubicles from the Eaton xVTL range. The Xiria MV switchgear used for this project has a very long working life and requires almost no maintenance. It also incorporates a wide range of safety features, including inspection windows that allow instant visual confirmation of the on/off status of the vacuum interrupters. Xiria switchgear housings are also internal arc proof, offering additional protection for personnel. operation are not required for these. Instead, Eaton SPX drives equipped with a standard brake chopper arrangement were chosen. These units have low harmonic characteristics and excellent power factor, as well as provision for the torque synchronisation needed for the pairs of motors used on the horizontal conveyors. Currently, all of the equipment has been installed and two of the inclined conveyors are in regular use. They are performing faultlessly, and are providing accurate and smooth control over starting and stopping even when the conveyors are fully loaded. The regenerative operation of the drives is delivering valuable energy savings and, as anticipated, the power factor of the drives is almost unity, while the total harmonic distortion (THD) is less than 5%. All of Carpatcements objectives have been accomplished with the help of the solution provided by Eaton. Energy can be fed back The SPA/SPI 9000 VSDs used to control the motors on the three inclined conveyors are modular types with a common DC bus. In this application, SPA 9000 active front-end units are used to provide power to the DC bus, with the motors driven by SPI 9000 inverters supplied from the DC bus. The active frontend modules offer full four-quadrant operation. This is an important benefit because the motors need constant braking when the conveyors are fully loaded, as the weight of the limestone would otherwise lead to them running at excessive speed. Without four-quadrant operation, the energy generated by the motors while braking would have to be dissipated in resistors and, therefore, wasted. With four-quadrant operation, however, regeneration is possible, which means that this energy can be fed back into the supply network cutting energy costs and reducing the plant’s carbon footprint. The drives are also equipped with standard brake choppers, but these are used only in exceptional circumstances to ensure that the braking function remains available even if the connection between the drive and the mains supply is interrupted. Low harmonic distortion Since the motors that drive the horizontal conveyors need little or no braking, drives with four-quadrant 01 Limestone is transported from the quarry to the manufacturing plant by long conveyors 02 Eaton SPA/SPI 9000 series VSD were selected for the inclined conveyors, and Eaton SPX series VSD for the horizontal conveyors AUTOMATION TECHNOLOGIES 2/2014


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