April 2013 Electrical Apparatus

April 2013 Electrical Apparatus

This is a summary of the Electrical Apparatus April 2013 featured technical article,  by Richard L. Nailen, P.E.    Receive this article with your subscription to Electrical Apparatus

The International Electrotechnical Commission has proposed adding another class of higher a-c motor efficiencies to those already adopted. In the U.S., the National Electrical Manufacturers Association has repeatedly raised its efficiency standards and has joined with energy conservation organizations to seek higher legislated levels of efficiency.

The first steps in designing higher-efficiency standard motors were relatively easy. Much was achieved at affordable cost. However, the law of diminishing returns soon asserts itself as the process is repeated. Each successive redesign results in a lesser improvement at greater cost.

Other limitations apply as well. The tendency in the U.S. has been to add slower-speed, higher-voltage, and higher-horsepower ratings to existing high-efficiency requirements. Some standards now aim at ratings up through 5,000 volts and 2,500 hp (1,866 kW). Many such motors are subject to starting current limits, difficult or frequent acceleration, low temperature rise, or other application restrictions that eliminate many options for increasing efficiency. Seldom produced more than one at a time, such motors lack the benefit of repeated tests to verify performance, unlike smaller mass-produced motors.

For any motor, higher efficiency demands more than simply “adding copper and iron.” Some engineers cite the benefits of advanced technology such as computer design and improved lamination steel. But these developments have been in common use for decades. One newer development is the cast copper rotor, but high tooling cost has prevented almost all manufacturers from offering it.

Changes in winding configuration to reduce stray load loss were known 50 years ago and were in wide use during the 1980’s. Modest redesign of motor ventilating systems is possible when reduced heat losses require less cooling. For high-speed motors, the greatest improvement in windage comes from unidirectional fan design, which isn’t suitable for smaller machines that may need to operate in either direction. Adding copper to the stator winding typically requires larger (or fewer) stator slots, which may result in higher core loss, noise, or winding problems.

Thus, compromises become increasingly complex as mandated efficiency levels continue to rise. Ever smaller improvements become more difficult to evaluate by test. Such a progression cannot continue indefinitely.

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