Total Cost of Ownership Can Be Less With Energy-Efficient Motors
ControlDesign.com recently celebrated their 15th birthday by re-posting 15 of their most popular articles from the past 15 years.
Although written three years ago, this is an excellent overview of just how effective high efficiency electric motors are:
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 said reducing eddy current and hysteritic lamination losses, generally called core loses, will rely on advance lamination alloys and production processes as technology allows.
“New stator-winding processes and copper rotor bars instead of aluminum reduce Joule-effect losses, the most significant losses in AC motors 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.”[highlight color=”light”] For permanent-magnet motors, materials are very important. [/highlight] The material used in the magnets themselves tends to be most critical. 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. With high-efficiency AC motors, the efficiency reach 90%, 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.
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.