Energy-Efficient Motion Control

DN Staff

April 12, 2010

9 Min Read
Energy-Efficient Motion Control

Energy efficiency is the goal and a prime target is the 63-65 percent of a typical manufacturing plant's monthly electric bill (according to the U.S. Dept. of Energy) that goes to pay for all the electricity consumed by the electric motors operating in the plant. But achieving higher levels of energy-efficient motion control involves a more complicated process than simply installing higher-efficiency motors and drives.

Industry experts say the focus needs to be on a complete machine design and engineering analysis, and using energy monitoring and software modeling tools to make decisions on the best investments to reduce energy costs.

"What's happening in the engineering community, in addition to more energy-efficient drives and motors, is a move toward mechatronic solutions," says Sal Spada, an industry analyst for ARC Advisory Group. "Engineers are taking a step back, reducing component count and understanding better how to improve mechanical efficiencies as well as overall electromechanical efficiency. The goal is to improve energy efficiency not only by reducing weight and sources of friction, but also by sizing motors and drives differently."

Spada says that the general approach to sizing in the past has been relatively simplistic and focused on worst case acceleration rates and inertia to size a motor and drive combination. But now there is more attention to overall duty cycles, peak torque issues and how to minimize energy consumption, not just worst case component selection.

"It's easy to look at the efficiency of a motor and compare one motor that is 85-percent efficient to another one that is 90 percent. But our perspective is there are often inefficiencies in the implementation of the motion control system, and a need to look at the whole system and not just one part," says Rich Mintz, product marketing for SEW Eurodrive. "With energy-efficient motion control, there are many factors to think about. The motor is an important component but there are other components to look at as well."

Mintz says the truth about energy-efficient motors is that they are only part of the efficiency puzzle. He says motors are at best one-sixth of the total energy loss potential for an electromechanical drivetrain and, by revamping the drivetrain, engineers may actually be able to use a smaller motor and save more.

In real-world applications and especially with older designs, applications tend to use more belts, chains and additional parts in the process that don't need to be included, and that reduce system efficiency.

One example of how system optimization can dramatically impact efficiency is the typical configuration of a conveyor driven by a worm gearbox with a chain sprocket drive connecting the two. The input to the gearbox uses a belted connection to the motor and when you add up the efficiencies of all the components, the overall efficiency totals 56.1 percent.

To optimize the design, a helical bevel gearbox is mounted directly to the shaft that drives the conveyor. Reducing the number of system components and using a more efficient gearbox and motor increases the system efficiency to 88 percent.

"If you look at it as the percentage increase versus the old method, that's a 57 percent increase in efficiency," says Will McFarland, corporate mechanical engineer for SEW Eurodrive. "When you take the energy saved, times real-world dollars per kilowatt-hour, times the number of hours you run the system per year, times the number of those situations you have in the factory, the numbers compound very quickly. It points out the attractiveness of an energy-efficient system versus just utilizing a more energy-efficient motor."

"Probably the most popular gear reducer style in conveyor applications is the worm gear reducer, which is a perfectly good reducer for many applications," Mintz says. "But for a continuous single-direction application, it is not the best selection from an efficiency standpoint."

He adds that efficiency is not always the number one goal, and there are times that a worm gear is the best gearbox to use and applications where they should be used. But if efficiency is the top priority, engineers need to look harder at how they are driving the system. Sometimes a V-belt or chain drive can give another benefit such as having the ability for the belt to slip if that is good for the application.

"In general, the application of worm gear reducers and belts has become common practice because they are inexpensive and have become the way everybody does it. Our thinking is to challenge engineers that if you are trying to save energy, just because you have always done it that way, can you do it a different way?"

"The basic philosophy is that the overall system efficiency is more important," says Himanshu Shah, an industry analyst at ARC Advisory Group who specializes in motors and drives. He says engineers can reduce the energy at the drive or motor, but often there's a need to make mechanical design changes in the entire machine to reduce inertia and losses that typically occur in the transducers or transmission gears.

"If you have an entire mechatronic design, then you can optimize the system. That way you can reduce the energy rather than looking at one component," Shah says.

Spada says that tools supplied by the major automation suppliers are also enabling machine builders to go through a process of identifying energy-efficient solutions. The tools take a more comprehensive look at mechatronic solutions and provide analytical tools for evaluating potential efficiency improvements.

Rockwell's Motion Analyzer recently added an efficiency analysis tool which helps engineers understand how the energy flows in and out of a machine, the efficiency of the process and system losses in specific devices.

"One thing we did which sounds simple, but is actually a complicated calculation, is to express energy in dollar terms," says John Pritchard, product marketing manager for Rockwell's Kinetics Motion Control Business.

"If an engineer enters all of the machine parameters into the tool including the unit energy cost, hours per day, days per week and weeks per year of operation, the tool provides energy costs in dollar terms and energy lost as heat due to shunting."

The advantage of modeling is that it helps engineers with design decisions on energy costs, and when is the right time to invest in a regen power supply, for example, over a conventional power supply.

The Motion Analyzer Tool allows engineers to choose multiple solution concepts, rank them in a list and sort by cost or other parameters. Users can look at hundreds of solutions for a given problem and rank them according to energy usage.

"What surprised us is that we have seen a difference as much as 4:1 or 5:1 between the most efficient solution and the least efficient solution," says Pritchard. "The most efficient design uses 1kw of energy, and the least efficient solution uses 5kw of electrical energy to achieve identical end goals. At first we thought that's not possible because all of the systems are doing the same thing, but of course it comes down to efficiency and losses."

The tool identifies losses, starting with the motor and all the way through the mechatronic chain down to the load. It reports gear box losses, transmission losses and losses in linear slides and other devices in the system. It is clear that servo drive technology, since it has adopted IGBTs and high-efficiency, high-frequency switching, is an extremely efficient way to convert electrical energy into mechanical energy.

"The real win is now how can we get rid of some of the inefficiencies in the mechanical piece," says Pritchard. "Drive solutions where we can eliminate a timing belt, or remove gear boxes and other mechanical components, can be combined with factors such as material selection to offer huge opportunities for savings."

To effectively attack energy costs, the tools provide an analytical view and the ability to know how much effort should be put into specific parts of the design, plus the potential return on investment. Tying the analysis back to dollars is critical, along with a more mechatronic approach to understanding how the whole system works, specific power losses in the system and how to reduce them.

Spada believes that more engineers are targeting energy efficiency as a major priority in new machinery designs. The focus on efficiency is often being driven, not by the machine builders or automation suppliers, but by end users looking at ways to reduce energy consumption in their manufacturing processes. These companies are looking not only for new technology but also ways to put intelligence into their operations and, for example, how to turn systems off when they are not in use.

"Everybody thinks there is a technological solution to solve all problems. But often engineers are analyzing the cycle of the machine and looking for ways to reduce energy consumption," says Spada. "Often the solution is applying better strategy, not improving energy consumption as a result of a better drive or motor design, but by becoming more intelligent about how to get a machine to do the same function with fewer process steps."

For a white paper on "The Truth About Energy-Efficient Motors," click here.

Optimizing the mechanical configuration of a conveyor application can increase efficiency by 57 percent and result in saving 23.6 MW-hr of energy and $2,360 per year. This example assumes a 20 hp motor operating 16 hours a day, 250 days a year, and the application requires 9.1kW be delivered to the conveyor head drum. The cost of energy is $0.10/kW-hr.

Energy-Efficient Motion Control diagram

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