Why is an electric motor’s energy efficiency (defined by the National Electrical Manufacturers Association (NEMA) as the ratio of its useful power output to its total power input) such a big deal? Because motors use approximately 2/3 of all the electricity consumed by industry, according to the International Electrotechnical Commission.
The importance of motor efficiency to energy conservation efforts has not escaped the notice of government bodies. Both U.S. and European officials have mandated the sale of more efficient motors and more new efficiency requirements are on the way.
In response, motor manufacturers have made a number of efficiency-boosting changes to their product designs. One common practice has been to increase the amount of so-called active material in the motor's core to reduce total losses, which translates into higher efficiency.
“A rule of thumb is that if you want to reduce a motor’s losses by 10%, you will have to add about 10% more active material,” says Brent McManis, industry engineering manager for Baldor Electric Co., a motor manufacturer based in Fort Smith, Ark.
Active materials include copper and electrical steel. Additional copper lowers the resistance in the motor windings. And adding thinner-gauge, lower-loss electrical steel in the core reduces eddy current losses, says Kay Cabaniss, Baldor’s industry business manager for energy efficiency.
Thanks to lower losses resulting from more active material, motors also run cooler. This means the motors need smaller cooling fans, which lowers windage losses. Another advantage of cooler-running motors is that they last longer.
“If you can make a motor run 10 degrees cooler, you will double the life of the insulation of the motor,” says Jerry Brown, chief engineer for electromagnetics at Kollmorgen, a motor maker based in Radford, Va.
But a downside exists to adding increased amounts of active material to motor designs. The technology has reached a point where “you can’t squeeze any more active material into a standard-size motor,” McManis says. This means that a more efficient motor with more active material may have to be larger in size. As a result, the motor may not be able to replace a similar, less-efficient motor installed in a space-constrained setting.
Another factor impacting the efficiency of permanent magnet (PM) motors is the choice of magnets, which also are considered part of a motor’s active material. Rare-earth magnets are “the best and most powerful magnets we have today, so they can make motors more efficient,” says Brown. On the down side, they can be expensive compared to ferrite magnets, another common choice.
PM motor efficiency also can get a boost from so-called single-tooth winding. In conventional motors, coils are wound around multiple lamination teeth. But in a single-tooth winding design, the coil is wound around a single tooth. Brown says this reduces the coil length and the coil material resistance, while it also reduces losses and increases motor efficiency.
In the U.S., replacing older motors with new, more energy efficient models may result in efficiency gains of 1.5 percentage points, unless the old motors predate first-generation efficiency rules that date from 1997. If the motors being replaced were made before 1997, McMannis says the efficiency gains could be about 5 percentage points.
Brown puts the efficiency gains offered by new induction motors at about 3 percentage points (representing an increase in motor efficiency from about 86% to 89%). Gains from the latest PM motors could be around 1.5 percentage points (an efficiency improvement from roughly 92.9% to 93.5%). He says that the efficiency gains for PM motors would be closer to 3 percentage points if the rated output torque of the latest PM motors hadn’t been increased compared to that of older, similarly sized packages. Some of the improved efficiency is “spent” as improved rated torque. He says that if these efficiency gains sound small, consider the broader energy-consumption picture.
“When you think of the number of these machines that are out there in industry, the differences are significant,” Brown says. Many motors operate 24/7, so energy savings can more than make up for the cost of a higher-efficiency motor in a relatively short time.
According to Baldor’s Cabaniss, so-called premium efficient motors can cost 10-20% more than standard efficient motors dating from the 1980s. Typically, she says, users will earn a payback on their efficiency investment in less than 2 years. The payback can be as short as a few months or as long as 10 years, depending on what the user is paying for energy.
Motor users can take other steps to cut energy use and costs besides buying a more efficient motor. One is to vary the speed of the motor as the needs of the application change, instead of running it at the same speed all the time. Operating a pump or fan at less than full speed, for example, can make a big dent in energy consumption.
The key to these speed changes is pairing the motor with a drive. Drive options include relatively simple variable frequency drives, as well as servo drives. Brown says that both types allow users to vary the motor speed and power to match the load requirement.
In addition to matching motor speed to the requirements, motor users also can save energy and money by better matching the motor to the application. Motor right-sizing means not being overly conservative, a practice than can lead to buying much more motor power than is needed for a given task.
When it comes to energy consumption, McManis says, overly conservative motor buyers may be penalized because most motor losses are essentially fixed losses: they don’t vary with the load. Losses associated with the motor’s fan, for example, are always about the same because the fan spins at close to the same speed whether the motor is unloaded or at 100% load. Due to fixed losses like these, McManis says, “if you are not running a motor at 75-100% load, you are wasting energy.”