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Regenerative Power Units Save Energy

Mar 13, 2013

When an electric motor is driven by a variable frequency drive (VFD), electric power delivered to the motor is regenerated while the motor decelerates by applying negative torque to the motor shaft. Usually energy storage capacity inside the VFD is very limited so regenerative energy should be returned to the grid or quickly dissipated by a braking resistor. Otherwise, the dc bus will be overcharged and an over-voltage fault can occur. Dynamic braking resistors have been widely used to convert regenerated energy into heat loss because of simplicity and low installation cost, as illustrated in Figure 1. But a regenerative power unit provides a significant energy cost saving opportunity, especially in applications that require frequent run and stop, deceleration with high inertia load, and overhauling torque. Such applications include spindle drives, decanter centrifuges, hoists, cranes, elevators, and torque dynamometer test rigs. Electricity cost per kilowatt hour is getting more expensive. So it will be worthwhile to review the basics of regenerative units and understand estimated energy cost saving. Three types of regenerative power units are available in the U.S. market. Basic features and power topologies are explained and compared.

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