January 2020

31 www.drivesncontrols.com January 2020 VARIABLE-SPEED DRIVES n Regen: a way to grab back ‘lost’ energy I n the past, if an application generated excess kinetic energy, this was usually dissipated electrically or mechanically using a braking resistor or friction. But in many industrial applications, regenerative (regen) drives can give this “lost” energy valuable potential by recirculating it, or feeding it back onto the mains supply, thus cutting electrical costs and eliminating the heating effects of braking resistors. The energy can also be put into “intermediate storage” and harvested later to cope with peak demands resulting from heavy loads or high accelerations. Generator load cycles where, traditionally, the energy was often “heated” in ballast circuits, can now take advantage of this technology too. Regen systems can feed the stored regenerated energy back to the supplying network either from a single inverter or, where a common DC-linked system is used, using several drive controllers. By cascading multiple drives, configurations can be designed to match the required power. In the supply mode, the systems can pre-charge the DC link to eliminate associated issues. The DC-link voltage corresponds to the rectified AC supply voltage. When energy is fed into a DC link by one or more drives in a deceleration or braking operation, the drive can feed the excess energy back into the mains power supply, allowing other consumers on the grid to use it. Depending on the system design, chokes or harmonic filters can be used to improve the THDi (lower harmonics). With standard mains chokes, most industrial needs can be met, including block-shaped regeneration. Using harmonic filters will result in near- sinusoidal current waveforms for supplying and regenerating energy, with a typical THDi of less than 8%. Suitable drives can either feed energy back into DC circuits, reducing the total electricity used, or onto the mains – provided that the installation complies with the electricity industry’s G98 regulations – thus recouping energy costs, or via intermediate storage. Compelling Regeneration becomes a compelling economic choice when the value of the recovered energy is large compared to the cost of the regenerative system. So if an application requires frequent braking and starting, or where there is speed control of overhauling loads, it is usually worth considering, as long as you have a clear understanding of the system dynamics. Typical applications include: Generators When connecting sources such as wind or hydroelectric power to a utility system, power quality standards can be met using harmonic filters. Centrifuges The kinetic energy of high centrifugal masses can be captured using regenerative braking. When multiple centrifuges are linked together, productivity can be improved by the short start-up and run-down times. Theatre technology Regenerative drives can help to achieve low-noise braking and to optimise energy use, without heating any resistors. Cranes Regen can be used where hoist motors stop and reverse frequently, and braking is needed to slow the load when lowering loads. Conveyor systems In production environments where conveyor belt drives or storage retrieval systems need to stop every few seconds or minutes for parts to be assembled correctly, before they are restarted, multiple DC-connected drives can support energy sharing, and peak energy can be returned to the mains supply, without needing any extra heat sources. Lifts In passenger and freight lifts, braking and stopping are frequent. Braking resistors can be replaced by regenerative drives, reducing fire hazards and delivering typical payback periods of two years. Supply and regeneration systems can replace braking resistors easily and can typically be used for supply voltages from 180–528V AC, 50/60Hz. They are usually compact and lightweight and are often supplied with a built-in pre-charging circuit along with a choke or harmonic filter. Because braking resistors are no longer needed, regenerative systems can reduce the risk of heat or fire in sensitive areas, as well as lowering the need for environmental cooling. Fieldbus interfaces can be incorporated as well as freely configurable inputs and outputs. In some installations, users fit energy meters to validate the energy savings they are achieving. n Many drive applications can benefit from the use of supply and regeneration systems that re-use kinetic energy from braking operations and feed it back into the DC circuit through the drive, or onto the mains power supply line. Mark Checkley of KEB Automation explains. By using regenerative drives in theatre applications, the use of energy is optimised and low-noise braking is possible. Regenerative drives can be used on cranes and lifting gear, especially where the hoist motor stops and reverses frequently..