Cut Energy Costs Using Motion Control
| By: Kristin Lewotsky, Contributing Editor
There was a time cutting energy usage meant swapping out incandescent bulbs for LEDs on the production floor. Today, the motion control industry provides an array of options that enable end-users to realize substantial savings over the operating lifetime of a machine, which can be a decade or more. By combining smart components and integrated hardware with monitoring and analysis software, OEMs and end-users can gain significant advantages and fast return on investment. “The total cost of energy is about 25% of your operating cost for servo motor systems,” says Ariane Roberson, senior product marketing engineer at Mitsubishi Electric Automation, Inc. (Vernon Hills, Illinois). “Your peak and demand charges represent 30% to 60% so there is a lot of money to be saved.”
Roberson points to four ways to save: consumption monitoring, advanced energy visualization, efficiency optimization, and information collection. Motion control technology provides methods for monitoring consumption and optimizing efficiency. Combined with usage and performance analysis, the information can be used to maximize not just throughput but overall production performance.
The first step in managing energy is to understand the performance of your overall system. That starts with an external audit to reveal opportunities for improvement. Energy analysis can uncover the sources of peak energy usage to reveal problems. Instead of running a line at 10,000 parts an hour, around the clock, for example, it may be more cost-effective to cut the rate to 5000 parts during the high-rate morning and early evening periods and run faster after midnight. Alternatively, a manufacturer that normally runs a line at 10,000 parts per hour for an eight-hour shift might lower overall operating costs by running for 10 hours at 8000 parts per hour. Until you know what your line is doing, you can’t make an informed decision.
To be useful, this kind of energy monitoring can’t just take place once. It needs to occur on an ongoing basis. That means the system has to be equipped with sensors and hardware that can harvest consumption data and software that can analyze and present it. A decade ago, that would have involved external systems that entailed additional cost, integration and programming, as well as a fairly steep learning curve. Now, OEMs can leverage purpose-built energy monitoring systems designed to work with the equipment to not only gather information in real time but also deliver the results in a highly consumable form.
“The faster you can react, the quicker you can catch [consumption] peaks and reduce them, which leads to a large amount of savings,” says Roberson. “The interface is really important because you need people on the floor to be able to see what is happening and you need the decision-makers on the business side of things to be able to understand what’s going on with each individual machine and the combination of all the machines on the floor, then make decisions on the basis of it.”
The interface needs to not just provide information to the operators but deliver it from shop floor to the top floor.
Developing an energy management strategy is not enough. The system needs to be designed for optimal operation. This is where servo technology really shines.
One of the key approaches to cutting energy consumption is to share this among axes. A decelerating motor basically acts as a generator, transferring energy from the load to the motor and back into the electrical system, where it can be disposed of. The conventional approach has been to dissipate it as heat using a braking resistor. Unfortunately, that dissipated energy represents dissipated money. Worse, heat compromises electronics. Particularly if the drives are located in the cabinet, they will require active cooling, which consumes further power. The solution? Regenerative braking that allows it to be harvested for reuse.
Adding a capacitor to a single-axis drive, for example, lets the axis store energy during decelerations that it can apply to the next acceleration. The larger the capacitor, the greater the amount of energy that the drive can store. The approach has a practical limit, of course – capacitors add size and space is always at a premium in machines and on the production floor. Like everything else in engineering, it’s a matter of trade-offs.
A more nuanced version of the technique involves linking multiple drives on a common DC bus. Now, accelerating drives can harvest power from decelerating drives. The net result is a significant drop in peak demand (see figure 1). This can be effective for even a handful of drives. “If you have over three axes, that’s a good point to consider a common DC bus type of system,” says Craig Nelson, senior project manager with the motion control business unit of Siemens (Norcross, Georgia).
The approach provides big savings, even for a handful of drives. “We have a demo where we ran triple-axis amplifiers against single-axis amplifiers on four different real-life applications and we were able to save about 35% of the energy,” says Roberson.
In addition to power savings, the approach reduces the number of components involved. “Instead of each drive having its own contactor and reactor, they now share one larger contactor and reactor, “ Nelson says. “But it’s not sized to the addition of all of the individual loads, it’s sized to what the system requires.” Rather than six axes with six different line components, the system now has a single set of line components sized to the application. Fewer components means potentially lower cost in hardware and assembly, fewer points of failure and a smaller footprint. It can also reduce the spares inventory.
The biggest savings, however, show up on the electric bills. Utility companies base their charges on total energy consumption scaled by peak power demand in the previous quarter. Using fewer kilowatt hours of energy is good, but to be billed for those units at a lower price is even better.
Connecting multiple drives on a shared DC bus is not a trivial task. There was a time that the task fell to the OEM. Today, machine builders have a choice of drives designed to be wired together with ease, including multi-axis drives that feature multiple amplifiers enclosed in a shared housing, and ready drives that can act in a snap.
Faster settling drives
Today’s state-of-the-art drives provide other power saving benefits. Any time an axis moves, it is converting power into work, with the associated losses. When an axis tries to move a load and overshoots, it consumes power the entire time it is oscillating around the commanded position. Drives with real-time autotuning capabilities can minimize oscillations, delivering the load to the commanded position more quickly.
Perhaps easiest approach to saving energy is the most obvious –properly size the motors. Smaller motors need smaller drives, which require less energy to run, not to mention smaller enclosures with reduced cooling costs. For decades, the most common approach to motor sizing involved starting with the motor of the previous platform and scaling it by a safety factor. It wasn’t optimal but it was faster and less arduous than running through the calculations. These days, most vendors have motor sizing software that can do the calculations for you in a snap. Rather than presenting them with the parameters of your previous motor, talk to your vendor about the requirements of your application and get the right motor to accrue savings right off the bat.
In the aftermath of the recession, lean operations are on everyone’s mind. Significant savings – and a competitive advantage – await organizations willing to put in a certain amount of effort to harvest savings. “Customers who work on selecting an energy-efficient servo solution typically save 20% to 30% percent of their energy for that particular machine, so it can have a significant effect,” Robeson says. Getting those benefits takes some effort but the benefits are worthwhile. “There is money on the table waiting for you but you’re going to have to put in two or three hours to understand your consumption,” she comments. “We’re way beyond changing a light bulb now. There are a lot of initiatives right now to operate more lean and optimize our equipment. The more we do that, the more we will have to dig for improvements.”
Thanks go to Sergio Gama, director of market development for Power Control at Rockwell Automation for useful background information.
Want to know more details about how these processes take place? Register for “Power-saving Drives”, a free webcast from the MCMA on July 28 at 11 AM.