Regenerative converter unit

Published: February 6, 2014


Electric motors driven by variable frequency drives (VFDs) regenerate electric power when the motor decelerates. Typically this energy is dissipated with a braking resistor. A regenerative converter unit allows this energy to be returned to the grid.

Recovering wasted energy from drive systems can save money and energy not only by returning the energy to the electrical grid, but by reducing the need for additional cooling since the waste heat is eliminated. A regenerative converter unit can be a cost-effective solution that replaces the dynamic braking transistor and resistor network. It absorbs regenerative energy from the VFD and returns it to the power source. The RCU converts 95 percent or more of the regenerated energy into reclaimed power.



Implementation costs

Dependent on project.

Potential energy savings

Calculated based on the braking torque and braking duty cycle of the motor.

Market barrier(s)

lack of technical knowledge

Applications that are suitable for regenerative converter units include:

Overhauling loads
High inertia loads
Machines requiring rapid deceleration
Punch presses
Vacuum pumps
Hydraulic pumps
Pump jacks
Drums (kilns)
Cranes and hoists
Stamping machines
Die casting machines
Extrusion presses
Injection molding machines
Webs (converting)


Save energy and money: regenerative converter units return waste energy to the electrical grid reducing the need for additional cooling by eliminating waste heat.

Challenges and market barriers

Lack of information: other than some information on the use of regenerative converter units with elevators, there is little practical information available for the various applications for this technology.

State impact

We took a high-level look at the potential energy savings in Wisconsin from regenerative converter units. The estimate is meant to provide a sense of scale showing the impact this technology might have on Wisconsin energy customers.

To estimate statewide impacts, we assumed that controls would be a retrofit opportunity in industrial applications that have machine drives. The technical savings rate is estimated to be 25% and we assumed that 20% of machine drive applications can implement the regenerative converters.

All data used for these estimates are from the Wisconsin Energy Statistics (2012), and Department of Energy's Manufacturing Energy Consumption Survey (2010).

Available incentives

There are not currently financial incentives available for the regenerative converter units specifically. However, Focus on Energy offers custom incentives for Large Energy User (with average peak monthly demand greater than 1000 kW) and regenerative converter units may be applicable under that program.

Energy-Efficient Elevator Machines
The elevators at Hyatt Place, a 20-story hotel in Honolulu, HI, were modernized to improve reliability and ride quality, and to reduce electrical consumption. The two high-rise elevators were powered by motor generators and direct current hoist motors that were controlled by electromechanical relay controllers. In the modernization project, the DC motors were replaced with permanent-magnet hoist machines with regenerative drives. The relay logic controllers were replaced with micro-processor controllers. The result was a 48 percent reduction in electrical consumption of the elevators.

Energy-Efficient Elevator Machines
This case study presents the results of modernizing the passenger elevators at Hyatt Place in Honolulu, Hawaii.
citation. "Energy-Efficient Elevator Machines" by Brad Nemeth, ThyssenKrupp Elevator, 2011, accessed April 14, 2014.

Regenerative power units save energy
This article in Control Engineering asserts that spindle drives, decanter centrifuges, hoists, cranes, elevators, and torque dynamometer test rigs can save energy from frequent run and stop, deceleration with high inertia load, and overhauling torque by using a regenerative power unit. One application saves 54% of the power used, $1017 per year.
citation. "Regenerative power units save energy" in Control Engineering, March 13, 2013, accessed April 14, 2014.

Using Regenerative Converter Units to Recover Over Voltage Energy in AC Drive Systems
This short paper by an application specialist and a product manager at Hitachi, a manufacturer of RCUs, provides an overview of how RCUs work and the various applications where they are cost-effective.
citation. "Using Regenerative Converter Units to Recover Over Voltage in AC Drive Systems" by Sam Mirza and Paul Curtis, accessed April 14, 2014.

RCU Series Regenerative Converter Unit - Estimating Energy Savings
This short paper put out by Hitachi, describes how to estimate energy savings and a payback period from replacing a conventional dynamic braking system with a regenerative converter unit. They use an example of a passenger elevator to illustrate the calculations.
citation. "AC Inverter Application Note: RCU Series Regenerative Converter Unit - Estimating Energy Savings," 2010, accessed April 14, 2014.