Motor designs focus on energy efficiency
Of the total cost of ownership of an electric motor in an industrial or commercial application, approximately 2% is the amount paid to purchase the motor. The remaining 98% is the cost of powering the motor over its lifetime. Hence, the energy efficiency of the motor weighs heavily in the purchase decision, with motor designers and manufacturers focused on delivering increased efficiency in the most economical means possible.
Fewer losses with materials
Traditional ways to improve motor efficiency include using better materials or using more materials. Better materials include higher quality steel with lower core loss or thinner laminations for motor construction. Both of these approaches, however, involve higher motor manufacturing costs. Another approach is to use more steel, thereby lowering flux density and losses and allowing room for more winding material, which in turn reduces the resistance and conduction losses. Adding materials, of course, increases the cost of the motor.
Several more innovative approaches to improving efficiency are currently being attempted in the industry. For induction motors, the most notable change is the move from cast aluminum rotors to cast copper rotors. This can raise efficiency by about 1% at three horsepower (3 hp). However, the cost of cast copper rotors is a significant factor.
Another approach is to use an electronic drive to operate motor systems at variable speeds. For many applications, this has the greatest energy savings potential. In some systems, this can save 50% or more of the total energy that would be consumed by a fixed-speed system without an electronic drive. The cost of adding the electronic drive to achieve variable speed can quickly be recouped with the savings from reduced energy consumption.
An emerging trend in product design is to change from induction motors to using permanent magnet motors with electronic drives, even in some fixed-speed applications. This is because permanent magnet motors can have higher efficiency and smaller size for the same output power. This saves on energy costs and can reduce overall product size and weight. Slowing this trend have been the recent dramatic increase in the price of rare earth magnet material and the increasing uncertainty of its supply.
A new motor category introduced by NovaTorque Inc. takes advantage of inherent energy efficiency of a permanent magnet motor, but without the increased costs associated with rare earth magnets. This design is an axial interior permanent magnet motor. A conical rotor and stator geometry concentrate magnetic flux in the field poles, allowing the use of low-cost, readily available, ferrite magnets, with performance and efficiencies equivalent to or better than conventional rare-earth-based motors. In variable-speed, variable-torque applications, such as fans or pumps, energy savings can exceed 10% when compared to NEMA Premium efficiency ac induction motors.
– Alan Crapo is vice president–engineering, NovaTorque Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering.
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