A sugar mill belonging to a major Mexican producer had been experiencing reliability problems with the drives for its sugar-separation centrifuges. New variable frequency drives (VFDs) not only cured the reliability problems, they improved productivity by reducing cycle time. One of the sweetest features of the drives was their regenerative capability. VFDs have become ubiquitous in food processing applications, driving all kinds of loads: mixers, conveyors, pumps, and the list goes on. Aside from their ability to provide precise control of speed and torque at high efficiency, VFDs offer the ability to recover energy from overhauling and high-inertia loads and feed it back to the source, saving costs. The process of extracting refined sugar from sugar cane involves four major steps: Shredding the cane, pressing it to extract the juice, clarifying and then concentrating the liquid into a mix of molasses and sugar crystals using a combination of heat and vacuum, and separating the sugar from the molasses by centrifugation (Figure 1). The resulting tan-colored “raw sugar” is then either processed further to make white sugar or dried and packaged for sale. Figure 1: The centrifuge, turning at about 200 rpm, is loaded with 1500 to 1750 kg (3300 to 3850lb) of hot concentrate; it then accelerates to 900 to 1000 rpm to spin out the molasses, leaving the sugar crystals behind. The centrifuge is then slowed to 50 rpm for unloading. The cycle takes about three minutes. The starting and stopping of the centrifuge made an ideal application for a VFD with regeneration. A brief tutorial is in order. Inside a Variable-Frequency Drive A VFD consists essentially of a rectifier, a DC link, and an inverter, as shown in Figure 2. The rectifier converts incoming AC power to DC to energize the DC link, while the inverter takes […]