Efficiency Cuts Cost of Electric Motor Operation

Feb. 18, 2009
Total Cost of Ownership Can Be Less With Energy-Efficient Motors
By Phil Burgert

Manufacturing requirements and energy consumption don’t have to compete with each other. Electric motors used in industrial equipment are increasingly efficient, and the factors that go into designing these energy-efficient motors include frequency of starting and stopping, rare earth and other permanent-magnet materials, torque density and size, and control techniques.

Peter Fischbach, manager of component sales for Bosch Rexroth says some key technology factors for electric motors include advanced lamination alloys and production processes to reduce eddy current and hysteretic lamination losses that generally are called core losses.

“New stator-winding processes and copper rotor bars instead of aluminum reduce Joule-effect losses, the most significant losses in ac motors,” says Fischbach. Low-friction grease, bearings, seals and optimized cooling fans also can lower mechanical friction losses, he says, as do new finite-element, magnetic-modeling tools that optimize magnetic properties, increasing power density and lowering stray losses.

Tim Schumann, corporate engineer and industry account manager for construction with SEW-Eurodrive, as well as the company’s National Electrical Manufacturers Assn. representative, says there are quite a few technology factors involved in the design of today’s energy-efficient motors used in discrete manufacturing. “Changing the slot design, the number of slots and other basic design elements of the motor also are looked at, changed and reviewed to see how more efficiency can be squeezed out of the motors,” he says.

“An energy-efficient motor generally will cost more, says Schumann, and depending on the size of the motor there will be a one-to-three-year payback period compared to more standard squirrel-cage induction motors. “But there will be gain on the total cost of ownership,” he says.

Jay Schultz, product manager for rotary and linear servo motors with the electromechanical automation division of Parker Hannifin, says “the highest efficiencies are generated by using permanent magnet ac motor technology. They inherently have a much higher efficiency—93% or more. In addition, torque density is improved by 250%. Weight is decreased 80% for the same torque and speed output.”

For permanent-magnet motors, materials are very important, Schultz says. “The material used in the magnets themselves tends to be most critical,” he states. “The density of the magnet field greatly influences performance. Recycled material typically is not used due to a reduction in material quality for the laminations.”

Because efficiencies of standard ac induction motors hover around 50-70%, the cost of energy can be significant, especially for continuous and high-dynamic applications, says Schultz. “With high-efficiency ac motors, the efficiency reach 90%,” he says. “So the energy costs only add up when frequent starting and stopping are part of the application.”

Richard Halstead, president of Empire Magnetics, says the vast majority of electric motors and alternators manufactured today are iron-based designs, “by this, I mean that magnetic iron, typically in thin laminations made so that copper wire can be wound into the iron to form magnetic poles,” he says.

One of the reasons behind the use of iron was the lack of powerful magnets, says Halstead, noting that iron structures were required to focus the magnetic flux. The availability of low-cost, very powerful rare earth magnets radically expanded the limits of technical feasibility, he adds, noting that motors that don’t require iron to provide good performance are not only feasible, but are becoming available in the marketplace.

An assumption behind the iron-based motor designs was the availability of cheap energy. “To the customer buying the product the purchase price of the item was the only selection criteria,” says Halstead, “but as the price of energy increases this equation also changes.”

Adam Shively, product manager for Kinetix motion control with Rockwell Automation says that while modern servo motors and high-efficiency induction motors generally use similar materials, the use of permanent magnets on the rotor eliminates magnetizing current inefficiencies. “In addition, because of the dynamic qualities of servo motors and the typically cyclic duty cycles of a servo system, a smaller motor can often be selected relative to a traditional asynchronous motor,” he says. “This further enhances the efficiency of the application.”

Particular care must be taken with high internal temperatures on components unique to servo motors, such as permanent magnets and encoders, says Shively, noting that servo-motor manufacturers have a lot of experience with the application of these components.

“Materials are critical in energy-efficient designs, and trade-offs between performance and cost must be considered when selecting materials,” says David Beckstoffer, product specialist with Portescap, a Danaher Motion company. “Proper material selection must be accomplished at the design stage of a new motor. Waiting until motor design is completed will result in lower-than-expected efficiency.”

David Marshall, business development manager for motor controls at Schneider Electric, puts materials used in motor design fourth or fifth on a list of 10 factors that are most important to efficiency of a motor in an application. “Many things affect the energy consumption more than purely the materials of the design,” he argues, including frame design, control techniques and the number of motor starts and stops per hour as top factors.

Marshall credits much of the focus on efficiency to NEMA’s introduction of the Design E motor responding to federal energy policy requirements for energy-efficient labeling in the 1990s.The most important factors in energy consumption involve being aware of the torque curve that the motor runs, revolutions per minute, temperature and number of starts and stops, he says. “Probably the biggest influencer on energy consumption of a motor has to do with the frame design and the match of the motor against the application,” he says.

Paul Webster, computer numerical controls product manager for GE Fanuc Intelligent Platforms says having drive amplifiers and software features matched to the motors and mechanical system selected also contributes to a direct reduction in electrical energy usage. “With an induction motor, an advanced control system can optimize the firing angles so optimum torque is produced for a given current,” he says.

Matching the current contribution of torque and magnetization will reduce heat losses in the motor but requires dynamic control, he adds. Using fast switching transistors and an increased pulse-width-modulated rate will lower iron losses in both permanent magnet and induction motors, says Webster.

He adds that, to reduce the cycle time and indirect energy consumption, the motor core shape must be optimized, inertia reduced and motor control matched closely to the needs of the system. “Using permanent-magnet synchronous motors provides high power density and excellent acceleration,” he says. “They are best implemented in high-torque, high-acceleration, but lower-rpm, applications.”

Fischbach of Bosch Rexroth says the total impact of energy-efficient motors on the cost of ownership will directly depend on the life of the motor, the energy saved and the initial cost difference. “That sounds like a simple economical calculation, but in a real production environment, most factors like working point and run time are not easy to quantify,” he says, adding that motor name plate data only shows “the sweet spot.” Losses can vary significantly with operational speed and power demand, he adds.

Phil Burgert is a freelance writer, specializing in the technical trade media.