Larry Berardinis,
Editor Elisabeth Eitel,
Senior Editor

Motion systems, in the most basic sense, convert energy from one mode or form to another. In the process, some energy is rendered unusable, typically lost as heat. In some cases, as in mobile or battery-powered systems, concern for this sort of inefficiency often drives the design process. In other cases — industrial equipment, for example — efficiency may not seem to matter as much; that is, until you consider the relationship between excess heat and imprecision, vibration and premature wear, thermal cycles and lifetime, energy consumption and profitability.

Efficiency matters, and in a motion system, it is a collective property, resulting from many components working together. It may start with the motor, but also encompasses interface, drive, and control dynamics. Like any design variable, there is a price associated with efficiency, but the greatest costs are usually incurred when efficiency is overlooked.

Start with the right motor

Motors convert electrical power to an electromagnetic field that, in turn, produces mechanical energy. Making them more efficient requires decreasing losses at each point in the conversion process. "Basically this means reducing-iron losses and resistance losses," says John Malinowski of Baldor Electric Co., Fort Smith, Ark.

"Iron losses are largely determined by the type of steel used in the stator and rotor laminations, as well as the annealing process the steel undergoes," says Chris Medinger of Leeson Electric Corp., Grafton, Wis. Other motor design variables linked to efficiency include insulation materials, winding methods, and cooling fan parameters. "Cooling fans consume energy, and must be optimized to reduce friction and windage losses," explains Medinger.

Perhaps the biggest bang for the buck related to motor efficiency, however, is to "right size" the motor for the application. In a recent survey, the New York State Energy Research and Development Authority ( NYSERDA) found that 55% of all installed motors are oversized. The "sweet spot" for peak motor efficiency, says Malinowski, is about 80 to 100% of full load. "Below this, efficiency falls off, as does power factor." And the problem is compounded further. "An underutilized motor has a high amount of 'lost watts,' which turns into higher operating temperature that could lead to premature failure," adds Medinger.

Besides selecting a motor designed to run at its rated load, designers should also look at speed. "Some applications, such as pumps and fans, are sensitive to speed variations," says Medinger. "Motors with an rpm rating higher than the pump or fan's cause higher flow, reduced efficiency, and increased energy consumption." Along those lines, another energy saving idea is to use an adjustablespeed drive instead of a fixed-speed, on-off drive. "For variable-torque loads, this can reduce energy consumption by 50% or more," says Malinowski.

Designers can also save energy by ensuring that motor-drive combinations operate at the most efficient point along the speed-torque curve. "The efficiency of a motor and drive package is not a fixed number," says John Chandler of Automotion Inc., Ann Arbor, Mich. "Rather, package efficiency is a curve starting and ending at 0%." The curve peaks, says Chandler, where the motor-drive package operates very close to the mid-point of the speed-torque range. One way to lock in there is by adding power factor correction to the drive's ac input, thus eliminating adverse effects of variable ac-line conditions.