Concern for the environment may motivate private individuals and the general public, but it’s a poor reason for making business decisions. Companies buying and installing electric motors have a far better business reason for trying to save electricity— making their operations more cost-efficient. In a rare confluence of factors, reducing electricity usage by installing more energy-efficient electric motors promotes both of these goals. And, it might be a requirement as of Dec. 19, 2010, the next major U.S. motor efficiency compliance deadline.
When one talks about high-efficiency motors, it is important to note that this term really refers to traditionally architected electric motors consisting of armature and field windings. “Induction motors are available in standard-, high- and premium-efficiency models,” said David Hansen, global product manager, Kinetix Motion Control, Rockwell Automation, “whereas permanent magnet motors are not.”
The reason is that the permanent-magnet electric motor architecture is inherently more efficient, since no power is used to establish the stator magnetic field. John Malinowski, senior product manager – ac motors, Baldor Electric Company, pointed out: “ac induction motors have a family of motors that comply with NEMA Premium Efficiency per NEMA MG 1, tables 12-12 and 12-13, and IEC 60034- 30 [standards] for IE3 efficiency.”
For that reason, we limited this discussion to induction motors having stator coils wound on ferromagnetic cores, and save looking at the energy-efficiency characteristics of permanent magnet motors for a later discussion.
“Premium motors,” Malinowski continued, “are built to closer tolerances than older motors, run cooler, have less vibration, are quieter, and last longer.”
However, “high efficiency in today’s ac induction motors,” said Peter Fischbach, industry sector manager, Bosch Rexroth Corp., “is achieved by improving the energy conversion paths and physical properties with new slot and winding geometries, advanced magnet and core materials, and the use of copper rotors for ac induction motors.”
What makes a motor efficient?
“The key to higher efficiency is reducing losses,” Malinowski said. “More copper in the winding to reduce stator losses, and higher-grade electrical steel reduces iron-core losses. Lower losses mean fewer watts to cool so smaller fans can be used, [further] reducing losses.”
Fischbach added: “The majority of the losses are caused by conductive losses in the stator and rotor and core losses, also called iron or hysteric losses.”
Hansen listed a number of design features that give these motors their high efficien
- Winding Resistance—As winding resistance increases, efficiency decreases. To maximize motor efficiency, motor designers minimize resistance by maximizing slot fill (amount of copper windings in the stator slots) and minimizing end-turn radius (amount of copper windings outside the stator slots).
- Lamination Material—Core losses are directly influenced by the material properties and quality of the steel used in the stator laminations. In addition, thinner laminations will lead to lower core losses in the stator than thicker laminations.
- Lamination Tooth Geometry—Lamination tooth geometry impacts concentration of the magnetic flux inside the motor. Geometries that provide higher magnetic flux concentration will have lower stray losses and therefore higher efficiency.
It’s the system that counts
“The goal in most factory automation and industrial applications,” Fischbach suggesteded, “is efficient use of energy with the highest productivity. Therefore it is essential to analyze, model, and optimize the complete system before investing in individual components, like new motors.” Read More
