VFDs: Boosting efficiency in hoisting applications

Feb. 13, 2013
Improving the energy efficiency of industrial applications is top of mind for today’s design engineers. Variable-frequency drives (VFDs) can make a difference by regenerating power that would otherwise be lost as heat in hoisting and other motion-control scenarios.

Authored by:
Don Cameron
Senior Product Manager
Mitsubishi Electric Automation Inc.
Vernon Hills, Ill.

Mitsubishi Electric Automation Inc.
(847) 478- 2100

In what applications is power regeneration possible?
Many applications are a good fit for regen drives. Overhead crane-hoisting mechanisms, elevators and escalators, heavy conveyor loads in mining operations, large-inertia centrifuges, and stamping presses are a few examples.

What are the chief benefits of employing regenerative drives?
Regenerative drives save users electricity, which is often the main motivator for adoption. Due to their very nature, applications such as those listed above have a load that tends to override or overdrive the motor shaft. When overridden, the motor acts as a generator and feeds power back to the VFD. A regenerative drive then routes this power back to the line. Without such a drive, the potential to harness this free electrical energy is lost, whether it’s generated by gravity or the load itself. Instead, the power dissipates as heat in a resistor bank or brake-chopper unit. Frequently, these units are external to the VFD.

Safety is another concern. External resistors can get extremely hot, which not only introduces “offlimits” floor space, but also increases fire risk. Using a regenerative drive eliminates this concern.

Relying on resistors can also limit performance. For example, a machine using brake choppers may need to be throttled back to avoid a situation marked by glowing red-hot resistors — not the ideal way to determine a machine’s cycle time. If the energy is instead fed back to the line, throughput can be based on more relevant criteria.

How is a typical regenerative-power system configured?
One option is to use componentbased regeneration solutions, in which the converter and other components are assembled external to the drive. Pure brake choppers are another.

However, these setups pose signi cant risk compared to all-in-one designs. Any initial cost advantage of a componentbased solution can be negated by lengthy installation times or component failures. Regarding potential failures, an engineer must determine whether individual components will continue to be available.

If not, the end user will need to engage in “ x-it-now” engineering should a component fail — a suboptimal situation if the application is unloading ship cargo by the metric ton, for example.

Are there other ways to set up regenerative systems that might save space, cost, or energy?
Yes. Another option is to install a factory-engineered solution that integrates all necessary components into a single box. For example, some drives look and install just like a regular inverter, with no special jobsite engineering required. By integrating components including the power supply, reactors, braking unit, and converter module, the installation footprint is reduced by an average of 50% versus a component-based solution. No special motors are necessary to complete the system.

Any final advice for engineers planning to set up a power-regeneration system?
The tipping point for adopting an integrated regenerative drive solution often hinges on the ability to save on electricity costs, and the ability to document the expected savings. Unlike fan and pump applications, however, these estimates are not so easy to ballpark. Too many variables, including system inputs such as friction and gearbox ef ciency, must be considered. The only way to accurately predict the outcome is to set up the respective solutions on identical equipment and document net energy consumption. Some drive manufacturers are willing to facilitate a “try before you buy” scenario to ease the rationalization process. A two-to-three-year return on investment is a reasonable assumption in most cases.

© 2013 Penton Media, Inc.

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