Today’s Perfect
Package?
While we won’t be publicizing the environmental
Energy
Regeneration
for Small Drives
aspects of our switch to a top-load wraparound system to the public, we will be informing
our customers about it
By Atle Bjanes
Every industrial plant manager knows energy costs are digging deeper and
deeper into the bottom line. U.S. retail electricity rates jumped 25 percent
for industrial customers from 2002-2006, according to the Department of
Energy, and analysts see little relief in the years ahead.
Escalating industrial electricity costs—now topping 6 cents per
kilowatt-hour—are pushing manufacturers and packagers to become as
energy efficient as possible. From buildings to machinery to process flow,
businesses are seeking designs, materials and equipment that minimize
power use.
Now, in addition to the high cost of energy, the person who
ultimately pays the bills—the consumer—is growing “greener” than ever
before. People are looking for products that leave the smallest
environmental footprint, and they are looking all the way back to packaging
and manufacturing to determine that footprint.
And to top it all off, U.S. state and federal legislators are starting to
make serious noises about climate change legislation, which would likely
involve some type of program in which companies would purchase carbon
credits to offset emissions attributed to energy use.
As businesses seek ways to meet today’s environmental challenges
and address rising costs, some energy saving strategies previously
overlooked in certain applications are starting to generate renewed interest.
One of those strategies is the use of energy regeneration for small drives.
Drive options
Servo motors not only use electricity but have potential to generate
electricity as well. Motors convert DC electrical power into mechanical
power and deliver it to the load (the machine). But as that load decelerates,
the motor itself turns into a generator converting mechanical power back
into electrical power. Braking converts the mechanical energy into
electrical energy, and that energy needs somewhere to go.
There are three options for directing regenerated energy: bleeder
resistors, capacitors and line-regenerative drives.
* Bleeder resistors are used to dissipate the excess energy from
braking in the form of heat. They are often the most cost-effective solution
for handling regenerated energy, but with resistors, that energy essentially
goes to waste as heat. Furthermore, it creates a situation that demands
further energy use and, in turn, increases costs: The heat generated by the
resistor means the facility operator needs to use additional energy to cool
the electrical cabinet as well as the facility itself.
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Energy Regeneration for Small Drives
Resistors are able to handle peak loads well, but they become large
when handling continuous dissipation. In addition, over time they are a
cause of system failure due to the constant heating and cooling involved in
dissipating the energy.
* A capacitor receives energy (taken from the load and converted to
electrical power) and stores it like a battery, parceling it out again as the
motor needs power. Although energy efficient, capacitors have a very
limited capacity (proportional to voltage), and regenerated energy
exceeding this capacity can no longer be reused but must be dissipated as
heat across a bleeder resistor.
* Line-regeneration does not suffer from the limitations of either
resistors or capacitors. The energy generated when braking a load is sent
back to the electrical grid, effectively turning the plant into a power
generator (on a small scale). This power is then consumed elsewhere.
It’s the same concept as a home that uses solar cells or windmills to
generate a portion of its electricity. When fueled by the sun or wind, they
generate power, sending energy back to the electrical grid, in effect making
the residence’s meter run backwards.
The technology is similar to that used in the Toyota Prius and being
adapted by other automakers, including BMW. In the Prius, when the
driver steps on the brake, rather than heating up the brakes, the motor
becomes a generator and dumps the kinetic energy back into the battery for
later use. But unlike the Prius, instead of “filling up” the battery, the energy
goes back to the power grid.
AC line regeneration is the most efficient solution for handling
regenerated energy. It maximizes a drive’s output and, in directing energy
back to the grid, reduces a plant’s power consumption, saving considerable
money that would normally go to the electric utility.
Existing technology, new uses
Line regenerative drives are a necessity for applications where the
regenerated energy is so great that it is not feasible to bleed it off. The
resistor would be too large and the heat too great. Machines that cycle
frequently or provide constant breaking (such as a tension drive in a web
application) are natural candidates for regenerative drives. In such cases,
the money saved by power regeneration easily pays for the additional
equipment cost. Companies have been using AC line regeneration in such
applications for years.
But for small servo drives that start and stop in a relatively short
timeframe, regenerative drives have not made economic sense in the past,
since they add up to 5 percent to initial equipment costs. In such cases,
manufacturers generally opted for capacitor and bleeder resistor options.
But with the aforementioned emphasis on reducing energy use and costs,
regenerative solutions are now starting to show promise even on small
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Energy Regeneration for Small Drives
drives and other heretofore ignored applications, particularly since
technological advances are bringing costs down in the latest
regenerative modules.
The pure dollar calculation to determine the payback for a
drive goes something like this: If a drive produces average
regenerated power of 10kW per hour and operates 4,000 hours per
year, it translates to 40,000kWH. A large unwinder or web tensioner
that provides braking tension, a rotary transfer table that consistently
indexes parts, and most large robots fall into this power use category.
Although much depends on utility rates, at the national
average of 6 cents per kWH, that would result in a cost savings of
$2,400 per year. That makes for a return on investment of less than
one year. Using the same parameters for a machine producing an
average of only 2kW per hour results in a return on investment of a
little more than five years.
That return on investment may have been unacceptable in the
past and still borderline to many. But energy calculations are
changing as environmental issues come to the fore, as energy prices
rise, as talk of carbon taxes grows and as “green” becomes the
watchword of both consumers and businesses. Where regenerative
drives were not even considered in the past, these days it makes sense
to do the math and analyze the pros and cons in any motor
application—all the pros and cons.
Another element in the whole cost equation is the price of
cooling. Bleeder resistors generate heat and require cooling, which
increases air conditioning bills. A general rule of thumb is that a
typical enclosure air conditioner removes five times the amount of
heat as the energy it takes to remove the heat. So a machine
dissipating 2kW of heat requires 400W of power to cool—another
number that adds up over the course of a year and can be eliminated
from the budget with line-regenerative drives.
In the past, cost of ownership was not considered in
purchasing a machine. Now, the cost of regeneration may be more
worth it than any business would have imagined, especially as
manufacturers recognize the need to provide regenerative systems for
smaller machines and the cost of these modules decreases.
For More Information Contact:
Gradient Engineering
730 Webster Hill Rd.
Lititz, PA 17543
Telephone: 717-314-3777
Fax: 717-627-7717
Email: [email protected]
Web: www.gradientengineering.com